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Advanced Bash-Scripting Guide

An in-depth exploration of the art of shell scripting
Mendel Cooper


6.4
30 Aug 2011
Revision History
Revision 6.2
'ROWANBERRY' release
Revision 6.3
'SWOZZLEBERRY' release
Revision 6.4
'VORTEXBERRY' release

17 Mar 2010

Revised by: mc

27 Apr 2011

Revised by: mc

30 Aug 2011

Revised by: mc

This tutorial assumes no previous knowledge of scripting or programming, but progresses rapidly toward an
intermediate/advanced level of instruction . . . all the while sneaking in little nuggets of UNIX® wisdom and
lore. It serves as a textbook, a manual for self-study, and a reference and source of knowledge on shell
scripting techniques. The exercises and heavily-commented examples invite active reader participation, under
the premise that the only way to really learn scripting is to write scripts.
This book is suitable for classroom use as a general introduction to programming concepts.

Dedication
For Anita, the source of all the magic

Advanced Bash-Scripting Guide

Table of Contents
Chapter 1. Shell Programming!.........................................................................................................................1
Chapter 2. Starting Off With a Sha-Bang........................................................................................................3
2.1. Invoking the script............................................................................................................................6
2.2. Preliminary Exercises.......................................................................................................................6
Part 2. Basics.......................................................................................................................................................7
Chapter 3. Special Characters...........................................................................................................................8
Chapter 4. Introduction to Variables and Parameters..................................................................................30
4.1. Variable Substitution......................................................................................................................30
4.2. Variable Assignment.......................................................................................................................33
4.3. Bash Variables Are Untyped..........................................................................................................34
4.4. Special Variable Types...................................................................................................................35
Chapter 5. Quoting...........................................................................................................................................41
5.1. Quoting Variables...........................................................................................................................41
5.2. Escaping..........................................................................................................................................43
Chapter 6. Exit and Exit Status.......................................................................................................................51
Chapter 7. Tests................................................................................................................................................54
7.1. Test Constructs...............................................................................................................................54
7.2. File test operators............................................................................................................................62
7.3. Other Comparison Operators..........................................................................................................65
7.4. Nested if/then Condition Tests.......................................................................................................70
7.5. Testing Your Knowledge of Tests..................................................................................................71
Chapter 8. Operations and Related Topics....................................................................................................72
8.1. Operators.........................................................................................................................................72
8.2. Numerical Constants.......................................................................................................................78
8.3. The Double-Parentheses Construct.................................................................................................80
8.4. Operator Precedence.......................................................................................................................81
Part 3. Beyond the Basics.................................................................................................................................84
Chapter 9. Another Look at Variables...........................................................................................................85
9.1. Internal Variables............................................................................................................................85
9.2. Typing variables: declare or typeset.............................................................................................104
9.2.1. Another use for declare.......................................................................................................106
9.3. $RANDOM: generate random integer..........................................................................................106
Chapter 10. Manipulating Variables.............................................................................................................119
10.1. Manipulating Strings...................................................................................................................119
10.1.1. Manipulating strings using awk........................................................................................126
10.1.2. Further Reference..............................................................................................................127
10.2. Parameter Substitution................................................................................................................127
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Advanced Bash-Scripting Guide

Table of Contents
Chapter 11. Loops and Branches..................................................................................................................138
11.1. Loops..........................................................................................................................................138
11.2. Nested Loops..............................................................................................................................151
11.3. Loop Control...............................................................................................................................152
11.4. Testing and Branching................................................................................................................156
Chapter 12. Command Substitution.............................................................................................................164
Chapter 13. Arithmetic Expansion................................................................................................................170
Chapter 14. Recess Time................................................................................................................................171
Part 4. Commands..........................................................................................................................................172
Chapter 15. Internal Commands and Builtins.............................................................................................180
15.1. Job Control Commands..............................................................................................................209
Chapter 16. External Filters, Programs and Commands...........................................................................214
16.1. Basic Commands........................................................................................................................214
16.2. Complex Commands...................................................................................................................219
16.3. Time / Date Commands..............................................................................................................230
16.4. Text Processing Commands........................................................................................................234
16.5. File and Archiving Commands...................................................................................................256
16.6. Communications Commands......................................................................................................274
16.7. Terminal Control Commands.....................................................................................................289
16.8. Math Commands.........................................................................................................................290
16.9. Miscellaneous Commands..........................................................................................................301
Chapter 17. System and Administrative Commands..................................................................................315
17.1. Analyzing a System Script..........................................................................................................344
Part 5. Advanced Topics.................................................................................................................................346
Chapter 18. Regular Expressions..................................................................................................................348
18.1. A Brief Introduction to Regular Expressions..............................................................................348
18.2. Globbing.....................................................................................................................................352
Chapter 19. Here Documents.........................................................................................................................354
19.1. Here Strings................................................................................................................................364
Chapter 20. I/O Redirection...........................................................................................................................368
20.1. Using exec...................................................................................................................................371
20.2. Redirecting Code Blocks............................................................................................................374
20.3. Applications................................................................................................................................379
Chapter 21. Subshells.....................................................................................................................................381

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Advanced Bash-Scripting Guide

Table of Contents
Chapter 22. Restricted Shells.........................................................................................................................386
Chapter 23. Process Substitution...................................................................................................................388
Chapter 24. Functions....................................................................................................................................393
24.1. Complex Functions and Function Complexities.........................................................................397
24.2. Local Variables...........................................................................................................................407
24.2.1. Local variables and recursion............................................................................................409
24.3. Recursion Without Local Variables............................................................................................411
Chapter 25. Aliases.........................................................................................................................................415
Chapter 26. List Constructs...........................................................................................................................418
Chapter 27. Arrays.........................................................................................................................................421
Chapter 28. Indirect References....................................................................................................................450
Chapter 29. /dev and /proc.............................................................................................................................454
29.1. /dev..............................................................................................................................................454
29.2. /proc............................................................................................................................................457
Chapter 30. Network Programming.............................................................................................................463
Chapter 31. Of Zeros and Nulls.....................................................................................................................466
Chapter 32. Debugging...................................................................................................................................470
Chapter 33. Options........................................................................................................................................481
Chapter 34. Gotchas.......................................................................................................................................484
Chapter 35. Scripting With Style..................................................................................................................493
35.1. Unofficial Shell Scripting Stylesheet..........................................................................................493
Chapter 36. Miscellany...................................................................................................................................496
36.1. Interactive and non-interactive shells and scripts.......................................................................496
36.2. Shell Wrappers............................................................................................................................497
36.3. Tests and Comparisons: Alternatives..........................................................................................502
36.4. Recursion: a script calling itself..................................................................................................503
36.5. "Colorizing" Scripts....................................................................................................................505
36.6. Optimizations..............................................................................................................................518
36.7. Assorted Tips..............................................................................................................................519
36.7.1. Ideas for more powerful scripts.........................................................................................519
36.7.2. Widgets..............................................................................................................................529
36.8. Security Issues............................................................................................................................531
36.8.1. Infected Shell Scripts.........................................................................................................531
36.8.2. Hiding Shell Script Source................................................................................................531
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Advanced Bash-Scripting Guide

Table of Contents
Chapter 36. Miscellany
36.8.3. Writing Secure Shell Scripts.............................................................................................532
36.9. Portability Issues.........................................................................................................................532
36.9.1. A Test Suite.......................................................................................................................533
36.10. Shell Scripting Under Windows...............................................................................................534
Chapter 37. Bash, versions 2, 3, and 4..........................................................................................................535
37.1. Bash, version 2............................................................................................................................535
37.2. Bash, version 3............................................................................................................................539
37.2.1. Bash, version 3.1...............................................................................................................542
37.2.2. Bash, version 3.2...............................................................................................................542
37.3. Bash, version 4............................................................................................................................543
37.3.1. Bash, version 4.1...............................................................................................................550
37.3.2. Bash, version 4.2...............................................................................................................551
Chapter 38. Endnotes.....................................................................................................................................555
38.1. Author's Note..............................................................................................................................555
38.2. About the Author........................................................................................................................555
38.3. Where to Go For Help.................................................................................................................555
38.4. Tools Used to Produce This Book..............................................................................................556
38.4.1. Hardware...........................................................................................................................556
38.4.2. Software and Printware.....................................................................................................556
38.5. Credits.........................................................................................................................................556
38.6. Disclaimer...................................................................................................................................558
Bibliography....................................................................................................................................................559
Appendix A. Contributed Scripts..................................................................................................................566
Appendix B. Reference Cards........................................................................................................................770
Appendix C. A Sed and Awk Micro-Primer................................................................................................775
C.1. Sed................................................................................................................................................775
C.2. Awk..............................................................................................................................................778
Appendix D. Exit Codes With Special Meanings.........................................................................................781
Appendix E. A Detailed Introduction to I/O and I/O Redirection.............................................................782
Appendix F. Command-Line Options...........................................................................................................784
F.1. Standard Command-Line Options................................................................................................784
F.2. Bash Command-Line Options......................................................................................................785
Appendix G. Important Files.........................................................................................................................787
Appendix H. Important System Directories.................................................................................................788

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Table of Contents
Appendix I. An Introduction to Programmable Completion.....................................................................790
Appendix J. Localization................................................................................................................................793
Appendix K. History Commands..................................................................................................................797
Appendix L. Sample .bashrc and .bash_profile Files..................................................................................798
Appendix M. Converting DOS Batch Files to Shell Scripts........................................................................813
Appendix N. Exercises....................................................................................................................................817
N.1. Analyzing Scripts.........................................................................................................................817
N.2. Writing Scripts.............................................................................................................................819
Appendix O. Revision History.......................................................................................................................829
Appendix P. Download and Mirror Sites.....................................................................................................832
Appendix Q. To Do List..................................................................................................................................833
Appendix R. Copyright..................................................................................................................................834
Appendix S. ASCII Table...............................................................................................................................837
Index....................................................................................................................................................838
Notes..............................................................................................................................................875

v

Chapter 1. Shell Programming!
No programming language is perfect. There is
not even a single best language; there are only
languages well suited or perhaps poorly suited
for particular purposes.
--Herbert Mayer
A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient at
system administration, even if they do not anticipate ever having to actually write a script. Consider that as a
Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and
set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a
system, and possibly modifying it.
The craft of scripting is not hard to master, since the scripts can be built in bite-sized sections and there is only
a fairly small set of shell-specific operators and options [1] to learn. The syntax is simple and straightforward,
similar to that of invoking and chaining together utilities at the command line, and there are only a few "rules"
governing their use. Most short scripts work right the first time, and debugging even the longer ones is
straightforward.

In the 1970s, the BASIC language enabled anyone reasonably
computer proficient to write programs on an early generation of
microcomputers. Decades later, the Bash scripting language enables
anyone with a rudimentary knowledge of Linux or UNIX to do the same
on much more powerful machines.

A shell script is a quick-and-dirty method of prototyping a complex application. Getting even a limited subset
of the functionality to work in a script is often a useful first stage in project development. In this way, the
structure of the application can be tested and tinkered with, and the major pitfalls found before proceeding to
the final coding in C, C++, Java, Perl, or Python.
Shell scripting hearkens back to the classic UNIX philosophy of breaking complex projects into simpler
subtasks, of chaining together components and utilities. Many consider this a better, or at least more
esthetically pleasing approach to problem solving than using one of the new generation of high powered
all-in-one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to
alter your thinking processes to fit the tool.
According to Herbert Mayer, "a useful language needs arrays, pointers, and a generic mechanism for building
data structures." By these criteria, shell scripting falls somewhat short of being "useful." Or, perhaps not. . . .

When not to use shell scripts
• Resource-intensive tasks, especially where speed is a factor (sorting, hashing, recursion [2] ...)
• Procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary
precision calculations, or complex numbers (use C++ or FORTRAN instead)
• Cross-platform portability required (use C or Java instead)

Chapter 1. Shell Programming!

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Advanced Bash-Scripting Guide
• Complex applications, where structured programming is a necessity (type-checking of variables,
function prototypes, etc.)
• Mission-critical applications upon which you are betting the future of the company
• Situations where security is important, where you need to guarantee the integrity of your system and
protect against intrusion, cracking, and vandalism
• Project consists of subcomponents with interlocking dependencies
• Extensive file operations required (Bash is limited to serial file access, and that only in a
particularly clumsy and inefficient line-by-line fashion.)
• Need native support for multi-dimensional arrays
• Need data structures, such as linked lists or trees
• Need to generate / manipulate graphics or GUIs
• Need direct access to system hardware or external peripherals
• Need port or socket I/O
• Need to use libraries or interface with legacy code
• Proprietary, closed-source applications (Shell scripts put the source code right out in the open for all
the world to see.)
If any of the above applies, consider a more powerful scripting language -- perhaps Perl, Tcl, Python, Ruby
-- or possibly a compiled language such as C, C++, or Java. Even then, prototyping the application as a
shell script might still be a useful development step.
We will be using Bash, an acronym [3] for "Bourne-Again shell" and a pun on Stephen Bourne's now classic
Bourne shell. Bash has become a de facto standard for shell scripting on most flavors of UNIX. Most of the
principles this book covers apply equally well to scripting with other shells, such as the Korn Shell, from
which Bash derives some of its features, [4] and the C Shell and its variants. (Note that C Shell programming
is not recommended due to certain inherent problems, as pointed out in an October, 1993 Usenet post by Tom
Christiansen.)
What follows is a tutorial on shell scripting. It relies heavily on examples to illustrate various features of the
shell. The example scripts work -- they've been tested, insofar as was possible -- and some of them are even
useful in real life. The reader can play with the actual working code of the examples in the source archive
(scriptname.sh or scriptname.bash), [5] give them execute permission (chmod u+rx
scriptname), then run them to see what happens. Should the source archive not be available, then
cut-and-paste from the HTML or pdf rendered versions. Be aware that some of the scripts presented here
introduce features before they are explained, and this may require the reader to temporarily skip ahead for
enlightenment.
Unless otherwise noted, the author of this book wrote the example scripts that follow.
His countenance was bold and bashed not.
--Edmund Spenser

Chapter 1. Shell Programming!

2

Chapter 2. Starting Off With a Sha-Bang
Shell programming is a 1950s juke box . . .
--Larry Wall
In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least,
this saves the effort of retyping that particular sequence of commands each time it is invoked.

Example 2-1. cleanup: A script to clean up log files in /var/log
# Cleanup
# Run as root, of course.
cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Log files cleaned up."

There is nothing unusual here, only a set of commands that could just as easily have been invoked one by one
from the command-line on the console or in a terminal window. The advantages of placing the commands in a
script go far beyond not having to retype them time and again. The script becomes a program -- a tool -- and it
can easily be modified or customized for a particular application.

Example 2-2. cleanup: An improved clean-up script
#!/bin/bash
# Proper header for a Bash script.
# Cleanup, version 2
# Run as root, of course.
# Insert code here to print error message and exit if not root.
LOG_DIR=/var/log
# Variables are better than hard-coded values.
cd $LOG_DIR
cat /dev/null > messages
cat /dev/null > wtmp

echo "Logs cleaned up."
exit # The right and proper method of "exiting" from a script.
# A bare "exit" (no parameter) returns the exit status
#+ of the preceding command.

Now that's beginning to look like a real script. But we can go even farther . . .

Example 2-3. cleanup: An enhanced and generalized version of above scripts.
#!/bin/bash
# Cleanup, version 3

Chapter 2. Starting Off With a Sha-Bang

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Advanced Bash-Scripting Guide
#
#
#
#+
#
#+

Warning:
------This script uses quite a number of features that will be explained
later on.
By the time you've finished the first half of the book,
there should be nothing mysterious about it.

LOG_DIR=/var/log
ROOT_UID=0
#
LINES=50
#
E_XCD=86
#
E_NOTROOT=87
#

Only users with $UID 0 have root privileges.
Default number of lines saved.
Can't change directory?
Non-root exit error.

# Run as root, of course.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
if [ -n "$1" ]
# Test whether command-line argument is present (non-empty).
then
lines=$1
else
lines=$LINES # Default, if not specified on command-line.
fi

#
#+
#+
#
#
#
#
#
#
#
#
#
#*

Stephane Chazelas suggests the following,
as a better way of checking command-line arguments,
but this is still a bit advanced for this stage of the tutorial.
E_WRONGARGS=85
case "$1"
""
)
*[!0-9]*)
*
)
esac

# Non-numerical argument (bad argument format).

in
lines=50;;
echo "Usage: `basename $0` file-to-cleanup"; exit $E_WRONGARGS;;
lines=$1;;

Skip ahead to "Loops" chapter to decipher all this.

cd $LOG_DIR
if [ `pwd` != "$LOG_DIR" ]

# or
if [ "$PWD" != "$LOG_DIR" ]
# Not in /var/log?

then
echo "Can't change to $LOG_DIR."
exit $E_XCD
fi # Doublecheck if in right directory before messing with log file.
# Far more efficient is:
#
# cd /var/log || {
#
echo "Cannot change to necessary directory." >&2
#
exit $E_XCD;
# }

Chapter 2. Starting Off With a Sha-Bang

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Advanced Bash-Scripting Guide

tail -n $lines messages > mesg.temp # Save last section of message log file.
mv mesg.temp messages
# Becomes new log directory.

# cat /dev/null > messages
#* No longer needed, as the above method is safer.
cat /dev/null > wtmp # ': > wtmp' and '> wtmp' have the same effect.
echo "Log files cleaned up."
# Note that there are other log files in /var/log not affected
#+ by this script.
exit 0
# A zero return value from the script upon exit indicates success
#+ to the shell.

Since you may not wish to wipe out the entire system log, this version of the script keeps the last section of
the message log intact. You will constantly discover ways of fine-tuning previously written scripts for
increased effectiveness.
***
The sha-bang ( #!) [6] at the head of a script tells your system that this file is a set of commands to be fed to
the command interpreter indicated. The #! is actually a two-byte [7] magic number, a special marker that
designates a file type, or in this case an executable shell script (type man magic for more details on this
fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that
interprets the commands in the script, whether it be a shell, a programming language, or a utility. This
command interpreter then executes the commands in the script, starting at the top (the line following the
sha-bang line), and ignoring comments. [8]
#!/bin/sh
#!/bin/bash
#!/usr/bin/perl
#!/usr/bin/tcl
#!/bin/sed -f
#!/bin/awk -f

Each of the above script header lines calls a different command interpreter, be it /bin/sh, the default shell
(bash in a Linux system) or otherwise. [9] Using #!/bin/sh, the default Bourne shell in most commercial
variants of UNIX, makes the script portable to non-Linux machines, though you sacrifice Bash-specific
features. The script will, however, conform to the POSIX [10] sh standard.
Note that the path given at the "sha-bang" must be correct, otherwise an error message -- usually "Command
not found." -- will be the only result of running the script. [11]
#! can be omitted if the script consists only of a set of generic system commands, using no internal shell
directives. The second example, above, requires the initial #!, since the variable assignment line, lines=50,
uses a shell-specific construct. [12] Note again that #!/bin/sh invokes the default shell interpreter, which
defaults to /bin/bash on a Linux machine.
This tutorial encourages a modular approach to constructing a script. Make note of and collect
"boilerplate" code snippets that might be useful in future scripts. Eventually you will build quite an
extensive library of nifty routines. As an example, the following script prolog tests whether the script has
Chapter 2. Starting Off With a Sha-Bang

5

Advanced Bash-Scripting Guide
been invoked with the correct number of parameters.
E_WRONG_ARGS=85
script_parameters="-a -h -m -z"
#
-a = all, -h = help, etc.
if [ $# -ne $Number_of_expected_args ]
then
echo "Usage: `basename $0` $script_parameters"
# `basename $0` is the script's filename.
exit $E_WRONG_ARGS
fi

Many times, you will write a script that carries out one particular task. The first script in this chapter is
an example. Later, it might occur to you to generalize the script to do other, similar tasks. Replacing the
literal ("hard-wired") constants by variables is a step in that direction, as is replacing repetitive code
blocks by functions.

2.1. Invoking the script
Having written the script, you can invoke it by sh scriptname, [13] or alternatively bash
scriptname. (Not recommended is using sh  redirection operator, truncates a file to zero length, without changing its
permissions. If the file did not previously exist, creates it.
: > data.xxx

# File "data.xxx" now empty.

# Same effect as
cat /dev/null >data.xxx
# However, this does not fork a new process, since ":" is a builtin.

See also Example 16-15.

Chapter 3. Special Characters

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Advanced Bash-Scripting Guide
In combination with the >> redirection operator, has no effect on a pre-existing target file (: >>
target_file). If the file did not previously exist, creates it.
This applies to regular files, not pipes, symlinks, and certain special files.
May be used to begin a comment line, although this is not recommended. Using # for a comment
turns off error checking for the remainder of that line, so almost anything may appear in a comment.
However, this is not the case with :.
: This is a comment that generates an error, ( if [ $x -eq 3] ).

The ":" serves as a field separator, in /etc/passwd, and in the $PATH variable.
bash$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games

A colon is acceptable as a function name.
:()
{
echo "The name of this function is "$FUNCNAME"
# Why use a colon as a function name?
# It's a way of obfuscating your code.
}
:
# The name of this function is :

This is not portable behavior, and therefore not a recommended practice.
!
reverse (or negate) the sense of a test or exit status [bang]. The ! operator inverts the exit status of
the command to which it is applied (see Example 6-2). It also inverts the meaning of a test operator.
This can, for example, change the sense of equal ( = ) to not-equal ( != ). The ! operator is a Bash
keyword.
In a different context, the ! also appears in indirect variable references.
In yet another context, from the command line, the ! invokes the Bash history mechanism (see
Appendix K). Note that within a script, the history mechanism is disabled.
*
wild card [asterisk]. The * character serves as a "wild card" for filename expansion in globbing. By
itself, it matches every filename in a given directory.
bash$ echo *
abs-book.sgml add-drive.sh agram.sh alias.sh

The * also represents any number (or zero) characters in a regular expression.
*
arithmetic operator. In the context of arithmetic operations, the * denotes multiplication.
** A double asterisk can represent the exponentiation operator or extended file-match globbing.
?
test operator. Within certain expressions, the ? indicates a test for a condition.
Chapter 3. Special Characters

12

Advanced Bash-Scripting Guide
In a double-parentheses construct, the ? can serve as an element of a C-style trinary operator.
condition?result-if-true:result-if-false
(( var0 = var1<98?9:21 ))
#
^ ^
#
#
#
#
#
#

if [ "$var1" -lt 98 ]
then
var0=9
else
var0=21
fi

In a parameter substitution expression, the ? tests whether a variable has been set.
?
wild card. The ? character serves as a single-character "wild card" for filename expansion in
globbing, as well as representing one character in an extended regular expression.
$
Variable substitution (contents of a variable).
var1=5
var2=23skidoo
echo $var1
echo $var2

# 5
# 23skidoo

A $ prefixing a variable name indicates the value the variable holds.
$
end-of-line. In a regular expression, a "$" addresses the end of a line of text.
${}
Parameter substitution.
$' ... '
Quoted string expansion. This construct expands single or multiple escaped octal or hex values into
ASCII [17] or Unicode characters.
$*, $@
positional parameters.
$?
exit status variable. The $? variable holds the exit status of a command, a function, or of the script
itself.
$$
process ID variable. The $$ variable holds the process ID [18] of the script in which it appears.
()
command group.
(a=hello; echo $a)

A listing of commands within parentheses starts a subshell.
Variables inside parentheses, within the subshell, are not visible to the rest of the
script. The parent process, the script, cannot read variables created in the child
process, the subshell.

Chapter 3. Special Characters

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Advanced Bash-Scripting Guide
a=123
( a=321; )
echo "a = $a"
# a = 123
# "a" within parentheses acts like a local variable.

array initialization.
Array=(element1 element2 element3)

{xxx,yyy,zzz,...}
Brace expansion.
echo \"{These,words,are,quoted}\"
# "These" "words" "are" "quoted"

# " prefix and suffix

cat {file1,file2,file3} > combined_file
# Concatenates the files file1, file2, and file3 into combined_file.
cp file22.{txt,backup}
# Copies "file22.txt" to "file22.backup"

A command may act upon a comma-separated list of file specs within braces. [19] Filename
expansion (globbing) applies to the file specs between the braces.
No spaces allowed within the braces unless the spaces are quoted or escaped.
echo {file1,file2}\ :{\ A," B",' C'}
file1 : A file1 : B file1 : C file2 : A file2 : B file2 :
C
{a..z}
Extended Brace expansion.
echo {a..z} # a b c d e f g h i j k l m n o p q r s t u v w x y z
# Echoes characters between a and z.
echo {0..3} # 0 1 2 3
# Echoes characters between 0 and 3.

base64_charset=( {A..Z} {a..z} {0..9} + / = )
# Initializing an array, using extended brace expansion.
# From vladz's "base64.sh" example script.

The {a..z} extended brace expansion construction is a feature introduced in version 3 of Bash.
{}
Block of code [curly brackets]. Also referred to as an inline group, this construct, in effect, creates
an anonymous function (a function without a name). However, unlike in a "standard" function, the
variables inside a code block remain visible to the remainder of the script.
bash$ { local a;
a=123; }
bash: local: can only be used in a
function

Chapter 3. Special Characters

14

Advanced Bash-Scripting Guide
a=123
{ a=321; }
echo "a = $a"

# a = 321

(value inside code block)

# Thanks, S.C.

The code block enclosed in braces may have I/O redirected to and from it.

Example 3-1. Code blocks and I/O redirection
#!/bin/bash
# Reading lines in /etc/fstab.
File=/etc/fstab
{
read line1
read line2
} < $File
echo
echo
echo
echo
echo

"First line in $File is:"
"$line1"
"Second line in $File is:"
"$line2"

exit 0
# Now, how do you parse the separate fields of each line?
# Hint: use awk, or . . .
# . . . Hans-Joerg Diers suggests using the "set" Bash builtin.

Example 3-2. Saving the output of a code block to a file
#!/bin/bash
# rpm-check.sh
#
#+
#
#
#

Queries an rpm file for description, listing,
and whether it can be installed.
Saves output to a file.
This script illustrates using a code block.

SUCCESS=0
E_NOARGS=65
if [ -z "$1" ]
then
echo "Usage: `basename $0` rpm-file"
exit $E_NOARGS
fi
{ # Begin code block.
echo
echo "Archive Description:"
rpm -qpi $1
# Query description.
echo

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echo "Archive Listing:"
rpm -qpl $1
# Query listing.
echo
rpm -i --test $1 # Query whether rpm file can be installed.
if [ "$?" -eq $SUCCESS ]
then
echo "$1 can be installed."
else
echo "$1 cannot be installed."
fi
echo
# End code block.
} > "$1.test"
# Redirects output of everything in block to file.
echo "Results of rpm test in file $1.test"
# See rpm man page for explanation of options.
exit 0

Unlike a command group within (parentheses), as above, a code block enclosed by
{braces} will not normally launch a subshell. [20]
{}
placeholder for text. Used after xargs -i (replace strings option). The {} double curly brackets are a
placeholder for output text.
ls . | xargs -i -t cp ./{} $1
#
^^
^^
# From "ex42.sh" (copydir.sh) example.

{} \;
pathname. Mostly used in find constructs. This is not a shell builtin.
The ";" ends the -exec option of a find command sequence. It needs to be escaped to
protect it from interpretation by the shell.
[]
test.
Test expression between [ ]. Note that [ is part of the shell builtin test (and a synonym for it), not a
link to the external command /usr/bin/test.
[[ ]]
test.
Test expression between [[ ]]. More flexible than the single-bracket [ ] test, this is a shell keyword.
See the discussion on the [[ ... ]] construct.
[]
array element.
In the context of an array, brackets set off the numbering of each element of that array.
Array[1]=slot_1
echo ${Array[1]}

[]
range of characters.
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Advanced Bash-Scripting Guide
As part of a regular expression, brackets delineate a range of characters to match.
$[ ... ]
integer expansion.
Evaluate integer expression between $[ ].
a=3
b=7
echo $[$a+$b]
echo $[$a*$b]

# 10
# 21

Note that this usage is deprecated, and has been replaced by the (( ... )) construct.
(( ))
integer expansion.
Expand and evaluate integer expression between (( )).
See the discussion on the (( ... )) construct.
> &> >& >> < <>
redirection.
scriptname >filename redirects the output of scriptname to file filename. Overwrite
filename if it already exists.

command &>filename redirects both the stdout and the stderr of command to filename.
This is useful for suppressing output when testing for a condition. For example, let us
test whether a certain command exists.
bash$ type bogus_command &>/dev/null

bash$ echo $?
1

Or in a script:
command_test () { type "$1" &>/dev/null; }
#
^
cmd=rmdir
# Legitimate command.
command_test $cmd; echo $?
# 0

cmd=bogus_command
# Illegitimate command
command_test $cmd; echo $?
# 1

command >&2 redirects stdout of command to stderr.
scriptname >>filename appends the output of scriptname to file filename. If
filename does not already exist, it is created.

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Advanced Bash-Scripting Guide
[i]<>filename opens file filename for reading and writing, and assigns file descriptor i to it. If
filename does not exist, it is created.
process substitution.
(command)>
<(command)
In a different context, the "<" and ">" characters act as string comparison operators.
In yet another context, the "<" and ">" characters act as integer comparison operators. See also
Example 16-9.
<<
redirection used in a here document.
<<<
redirection used in a here string.
<, >
ASCII comparison.
veg1=carrots
veg2=tomatoes
if [[ "$veg1" < "$veg2" ]]
then
echo "Although $veg1 precede $veg2 in the dictionary,"
echo -n "this does not necessarily imply anything "
echo "about my culinary preferences."
else
echo "What kind of dictionary are you using, anyhow?"
fi

\<, \>
word boundary in a regular expression.
bash$ grep '\' textfile
|
pipe. Passes the output (stdout of a previous command to the input (stdin) of the next one, or to
the shell. This is a method of chaining commands together.
echo ls -l | sh
# Passes the output of "echo ls -l" to the shell,
#+ with the same result as a simple "ls -l".

cat *.lst | sort | uniq
# Merges and sorts all ".lst" files, then deletes duplicate lines.

A pipe, as a classic method of interprocess communication, sends the stdout of one process to the
stdin of another. In a typical case, a command, such as cat or echo, pipes a stream of data to a
filter, a command that transforms its input for processing. [21]
cat $filename1 $filename2 | grep $search_word
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For an interesting note on the complexity of using UNIX pipes, see the UNIX FAQ, Part 3.
The output of a command or commands may be piped to a script.
#!/bin/bash
# uppercase.sh : Changes input to uppercase.
tr 'a-z' 'A-Z'
# Letter ranges must be quoted
#+ to prevent filename generation from single-letter filenames.
exit 0

Now, let us pipe the output of ls -l to this script.
bash$ ls -l | ./uppercase.sh
-RW-RW-R-1 BOZO BOZO
-RW-RW-R-1 BOZO BOZO
-RW-R--R-1 BOZO BOZO

109 APR 7 19:49 1.TXT
109 APR 14 16:48 2.TXT
725 APR 20 20:56 DATA-FILE

The stdout of each process in a pipe must be read as the stdin of the next. If this
is not the case, the data stream will block, and the pipe will not behave as expected.
cat file1 file2 | ls -l | sort
# The output from "cat file1 file2" disappears.

A pipe runs as a child process, and therefore cannot alter script variables.
variable="initial_value"
echo "new_value" | read variable
echo "variable = $variable"
# variable = initial_value

If one of the commands in the pipe aborts, this prematurely terminates execution of the
pipe. Called a broken pipe, this condition sends a SIGPIPE signal.
>|
force redirection (even if the noclobber option is set). This will forcibly overwrite an existing file.
||
OR logical operator. In a test construct, the || operator causes a return of 0 (success) if either of the
linked test conditions is true.
&
Run job in background. A command followed by an & will run in the background.
bash$ sleep 10 &
[1] 850
[1]+ Done

sleep 10

Within a script, commands and even loops may run in the background.

Example 3-3. Running a loop in the background
#!/bin/bash
# background-loop.sh
for i in 1 2 3 4 5 6 7 8 9 10
do
echo -n "$i "

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# First loop.

19

Advanced Bash-Scripting Guide
done & # Run this loop in background.
# Will sometimes execute after second loop.
echo

# This 'echo' sometimes will not display.

for i in 11 12 13 14 15 16 17 18 19 20
do
echo -n "$i "
done
echo

# Second loop.

# This 'echo' sometimes will not display.

# ======================================================
# The expected output from the script:
# 1 2 3 4 5 6 7 8 9 10
# 11 12 13 14 15 16 17 18 19 20
#
#
#
#

Sometimes, though, you get:
11 12 13 14 15 16 17 18 19 20
1 2 3 4 5 6 7 8 9 10 bozo $
(The second 'echo' doesn't execute. Why?)

# Occasionally also:
# 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
# (The first 'echo' doesn't execute. Why?)
# Very rarely something like:
# 11 12 13 1 2 3 4 5 6 7 8 9 10 14 15 16 17 18 19 20
# The foreground loop preempts the background one.
exit 0
# Nasimuddin Ansari suggests adding
sleep 1
#+ after the
echo -n "$i"
in lines 6 and 14,
#+ for some real fun.

A command run in the background within a script may cause the script to hang,
waiting for a keystroke. Fortunately, there is a remedy for this.
&&
AND logical operator. In a test construct, the && operator causes a return of 0 (success) only if both
the linked test conditions are true.
option, prefix. Option flag for a command or filter. Prefix for an operator. Prefix for a default
parameter in parameter substitution.
COMMAND -[Option1][Option2][...]
ls -al
sort -dfu $filename
if [ $file1 -ot $file2 ]
then #
^
echo "File $file1 is older than $file2."
fi
if [ "$a" -eq "$b" ]

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Advanced Bash-Scripting Guide
then #
^
echo "$a is equal to $b."
fi
if [ "$c" -eq 24 -a "$d" -eq 47 ]
then #
^
^
echo "$c equals 24 and $d equals 47."
fi

param2=${param1:-$DEFAULTVAL}
#
^

-The double-dash -- prefixes long (verbatim) options to commands.
sort --ignore-leading-blanks
Used with a Bash builtin, it means the end of options to that particular command.
This provides a handy means of removing files whose names begin with a dash.
bash$ ls -l
-rw-r--r-- 1 bozo bozo 0 Nov 25 12:29 -badname

bash$ rm -- -badname
bash$ ls -l
total 0

The double-dash is also used in conjunction with set.
set -- $variable (as in Example 15-18)
redirection from/to stdin or stdout [dash].
bash$ cat abc
abc
...
Ctl-D

As expected, cat - echoes stdin, in this case keyboarded user input, to stdout. But, does I/O
redirection using - have real-world applications?
(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)
# Move entire file tree from one directory to another
# [courtesy Alan Cox , with a minor change]
# 1) cd /source/directory
#
Source directory, where the files to be moved are.
# 2) &&
#
"And-list": if the 'cd' operation successful,
#
then execute the next command.

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Advanced Bash-Scripting Guide
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#

3) tar cf - .
The 'c' option 'tar' archiving command creates a new archive,
the 'f' (file) option, followed by '-' designates the target file
as stdout, and do it in current directory tree ('.').
4) |
Piped to ...
5) ( ... )
a subshell
6) cd /dest/directory
Change to the destination directory.
7) &&
"And-list", as above
8) tar xpvf Unarchive ('x'), preserve ownership and file permissions ('p'),
and send verbose messages to stdout ('v'),
reading data from stdin ('f' followed by '-').
Note that 'x' is a command, and 'p', 'v', 'f' are options.
Whew!

# More elegant than, but equivalent to:
#
cd source/directory
#
tar cf - . | (cd ../dest/directory; tar xpvf -)
#
#
Also having same effect:
# cp -a /source/directory/* /dest/directory
#
Or:
# cp -a /source/directory/* /source/directory/.[^.]* /dest/directory
#
If there are hidden files in /source/directory.
bunzip2 -c linux-2.6.16.tar.bz2 | tar xvf # --uncompress tar file-| --then pass it to "tar"-# If "tar" has not been patched to handle "bunzip2",
#+ this needs to be done in two discrete steps, using a pipe.
# The purpose of the exercise is to unarchive "bzipped" kernel source.

Note that in this context the "-" is not itself a Bash operator, but rather an option recognized by certain
UNIX utilities that write to stdout, such as tar, cat, etc.
bash$ echo "whatever" | cat whatever

Where a filename is expected, - redirects output to stdout (sometimes seen with tar cf), or
accepts input from stdin, rather than from a file. This is a method of using a file-oriented utility as
a filter in a pipe.
bash$ file
Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file...

By itself on the command-line, file fails with an error message.
Add a "-" for a more useful result. This causes the shell to await user input.
bash$ file abc
standard input:

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ASCII text

22

Advanced Bash-Scripting Guide

bash$ file #!/bin/bash
standard input:

Bourne-Again shell script text executable

Now the command accepts input from stdin and analyzes it.
The "-" can be used to pipe stdout to other commands. This permits such stunts as prepending lines
to a file.
Using diff to compare a file with a section of another:
grep Linux file1 | diff file2 Finally, a real-world example using - with tar.

Example 3-4. Backup of all files changed in last day
#!/bin/bash
# Backs up all files in current directory modified within last 24 hours
#+ in a "tarball" (tarred and gzipped file).
BACKUPFILE=backup-$(date +%m-%d-%Y)
#
Embeds date in backup filename.
#
Thanks, Joshua Tschida, for the idea.
archive=${1:-$BACKUPFILE}
# If no backup-archive filename specified on command-line,
#+ it will default to "backup-MM-DD-YYYY.tar.gz."
tar cvf - `find . -mtime -1 -type f -print` > $archive.tar
gzip $archive.tar
echo "Directory $PWD backed up in archive file \"$archive.tar.gz\"."

# Stephane Chazelas points out that the above code will fail
#+ if there are too many files found
#+ or if any filenames contain blank characters.
# He suggests the following alternatives:
# ------------------------------------------------------------------#
find . -mtime -1 -type f -print0 | xargs -0 tar rvf "$archive.tar"
#
using the GNU version of "find".

#
find . -mtime -1 -type f -exec tar rvf "$archive.tar" '{}' \;
#
portable to other UNIX flavors, but much slower.
# -------------------------------------------------------------------

exit 0

Filenames beginning with "-" may cause problems when coupled with the "-"
redirection operator. A script should check for this and add an appropriate prefix to
such filenames, for example ./-FILENAME, $PWD/-FILENAME, or
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23

Advanced Bash-Scripting Guide
$PATHNAME/-FILENAME.
If the value of a variable begins with a -, this may likewise create problems.
var="-n"
echo $var
# Has the effect of "echo -n", and outputs nothing.

previous working directory. A cd - command changes to the previous working directory. This uses
the $OLDPWD environmental variable.
Do not confuse the "-" used in this sense with the "-" redirection operator just
discussed. The interpretation of the "-" depends on the context in which it appears.
Minus. Minus sign in an arithmetic operation.
=
Equals. Assignment operator
a=28
echo $a

# 28

In a different context, the "=" is a string comparison operator.
+
Plus. Addition arithmetic operator.
In a different context, the + is a Regular Expression operator.
+
Option. Option flag for a command or filter.
Certain commands and builtins use the + to enable certain options and the - to disable them. In
parameter substitution, the + prefixes an alternate value that a variable expands to.
%
modulo. Modulo (remainder of a division) arithmetic operation.
let "z = 5 % 3"
echo $z # 2

In a different context, the % is a pattern matching operator.
~
home directory [tilde]. This corresponds to the $HOME internal variable. ~bozo is bozo's home
directory, and ls ~bozo lists the contents of it. ~/ is the current user's home directory, and ls ~/ lists the
contents of it.
bash$ echo ~bozo
/home/bozo
bash$ echo ~
/home/bozo
bash$ echo ~/
/home/bozo/
bash$ echo ~:
/home/bozo:

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Advanced Bash-Scripting Guide
bash$ echo ~nonexistent-user
~nonexistent-user

~+
current working directory. This corresponds to the $PWD internal variable.
~previous working directory. This corresponds to the $OLDPWD internal variable.
=~
regular expression match. This operator was introduced with version 3 of Bash.
^
beginning-of-line. In a regular expression, a "^" addresses the beginning of a line of text.
^, ^^
Uppercase conversion in parameter substitution (added in version 4 of Bash).
Control Characters
change the behavior of the terminal or text display. A control character is a CONTROL + key
combination (pressed simultaneously). A control character may also be written in octal or
hexadecimal notation, following an escape.
Control characters are not normally useful inside a script.
◊ Ctl-A
Moves cursor to beginning of line of text (on the command-line).
◊ Ctl-B
Backspace (nondestructive).
◊
Ctl-C
Break. Terminate a foreground job.
◊
Ctl-D
Log out from a shell (similar to exit).
EOF (end-of-file). This also terminates input from stdin.
When typing text on the console or in an xterm window, Ctl-D erases the character under
the cursor. When there are no characters present, Ctl-D logs out of the session, as expected.
In an xterm window, this has the effect of closing the window.
◊ Ctl-E
Moves cursor to end of line of text (on the command-line).
◊ Ctl-F
Moves cursor forward one character position (on the command-line).
◊
Ctl-G
BEL. On some old-time teletype terminals, this would actually ring a bell. In an xterm it
might beep.
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Advanced Bash-Scripting Guide
◊ Ctl-H
Rubout (destructive backspace). Erases characters the cursor backs over while backspacing.
#!/bin/bash
# Embedding Ctl-H in a string.
a="^H^H"
echo "abcdef"
echo
echo -n "abcdef$a "
# Space at end ^
echo
echo -n "abcdef$a"
# No space at end

# Two Ctl-H's -- backspaces
# ctl-V ctl-H, using vi/vim
# abcdef
# abcd f
^ Backspaces twice.
# abcdef
^ Doesn't backspace (why?).
# Results may not be quite as expected.

echo; echo
#
#
#
#
#

Constantin Hagemeier suggests trying:
a=$'\010\010'
a=$'\b\b'
a=$'\x08\x08'
But, this does not change the results.

########################################
# Now, try this.
rubout="^H^H^H^H^H"

# 5 x Ctl-H.

echo -n "12345678"
sleep 2
echo -n "$rubout"
sleep 2

◊ Ctl-I
Horizontal tab.
◊
Ctl-J
Newline (line feed). In a script, may also be expressed in octal notation -- '\012' or in
hexadecimal -- '\x0a'.
◊ Ctl-K
Vertical tab.
When typing text on the console or in an xterm window, Ctl-K erases from the character
under the cursor to end of line. Within a script, Ctl-K may behave differently, as in Lee Lee
Maschmeyer's example, below.
◊ Ctl-L
Formfeed (clear the terminal screen). In a terminal, this has the same effect as the clear
command. When sent to a printer, a Ctl-L causes an advance to end of the paper sheet.
◊
Ctl-M
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Advanced Bash-Scripting Guide
Carriage return.
#!/bin/bash
# Thank you, Lee Maschmeyer, for this example.
read -n 1 -s -p \
$'Control-M leaves cursor at beginning of this line. Press Enter. \x0d'
# Of course, '0d' is the hex equivalent of Control-M.
echo >&2
# The '-s' makes anything typed silent,
#+ so it is necessary to go to new line explicitly.
read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a'
# '0a' is the hex equivalent of Control-J, linefeed.
echo >&2
###
read -n 1 -s -p $'And Control-K\x0bgoes straight down.'
echo >&2
# Control-K is vertical tab.
# A better example of the effect of a vertical tab is:
var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a'
echo "$var"
# This works the same way as the above example. However:
echo "$var" | col
# This causes the right end of the line to be higher than the left end.
# It also explains why we started and ended with a line feed -#+ to avoid a garbled screen.
# As Lee Maschmeyer explains:
# -------------------------# In the [first vertical tab example] . . . the vertical tab
#+ makes the printing go straight down without a carriage return.
# This is true only on devices, such as the Linux console,
#+ that can't go "backward."
# The real purpose of VT is to go straight UP, not down.
# It can be used to print superscripts on a printer.
# The col utility can be used to emulate the proper behavior of VT.
exit 0

◊ Ctl-N
Erases a line of text recalled from history buffer [22] (on the command-line).
◊ Ctl-O
Issues a newline (on the command-line).
◊ Ctl-P
Recalls last command from history buffer (on the command-line).
◊ Ctl-Q
Resume (XON).
This resumes stdin in a terminal.
◊ Ctl-R
Backwards search for text in history buffer (on the command-line).
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Advanced Bash-Scripting Guide
◊ Ctl-S
Suspend (XOFF).
This freezes stdin in a terminal. (Use Ctl-Q to restore input.)
◊ Ctl-T
Reverses the position of the character the cursor is on with the previous character (on the
command-line).
◊ Ctl-U
Erase a line of input, from the cursor backward to beginning of line. In some settings, Ctl-U
erases the entire line of input, regardless of cursor position.
◊ Ctl-V
When inputting text, Ctl-V permits inserting control characters. For example, the following
two are equivalent:
echo -e '\x0a'
echo 

Ctl-V is primarily useful from within a text editor.
◊ Ctl-W
When typing text on the console or in an xterm window, Ctl-W erases from the character
under the cursor backwards to the first instance of whitespace. In some settings, Ctl-W
erases backwards to first non-alphanumeric character.
◊ Ctl-X
In certain word processing programs, Cuts highlighted text and copies to clipboard.
◊ Ctl-Y
Pastes back text previously erased (with Ctl-U or Ctl-W).
◊ Ctl-Z
Pauses a foreground job.
Substitute operation in certain word processing applications.
EOF (end-of-file) character in the MSDOS filesystem.
Whitespace
functions as a separator between commands and/or variables. Whitespace consists of either
spaces, tabs, blank lines, or any combination thereof. [23] In some contexts, such as variable
assignment, whitespace is not permitted, and results in a syntax error.
Blank lines have no effect on the action of a script, and are therefore useful for visually separating
functional sections.
$IFS, the special variable separating fields of input to certain commands. It defaults to whitespace.

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Advanced Bash-Scripting Guide
Definition: A field is a discrete chunk of data expressed as a string of consecutive characters.
Separating each field from adjacent fields is either whitespace or some other designated character
(often determined by the $IFS). In some contexts, a field may be called a record.
To preserve whitespace within a string or in a variable, use quoting.
UNIX filters can target and operate on whitespace using the POSIX character class [:space:].

Chapter 3. Special Characters

29

Chapter 4. Introduction to Variables and
Parameters
Variables are how programming and scripting languages represent data. A variable is nothing more than a
label, a name assigned to a location or set of locations in computer memory holding an item of data.
Variables appear in arithmetic operations and manipulation of quantities, and in string parsing.

4.1. Variable Substitution
The name of a variable is a placeholder for its value, the data it holds. Referencing (retrieving) its value is
called variable substitution.
$
Let us carefully distinguish between the name of a variable and its value. If variable1 is the name
of a variable, then $variable1 is a reference to its value, the data item it contains. [24]
bash$ variable1=23

bash$ echo variable1
variable1
bash$ echo $variable1
23

The only times a variable appears "naked" -- without the $ prefix -- is when declared or assigned,
when unset, when exported, in an arithmetic expression within double parentheses (( ... )), or in the
special case of a variable representing a signal (see Example 32-5). Assignment may be with an = (as
in var1=27), in a read statement, and at the head of a loop (for var2 in 1 2 3).
Enclosing a referenced value in double quotes (" ... ") does not interfere with variable substitution.
This is called partial quoting, sometimes referred to as "weak quoting." Using single quotes (' ... ')
causes the variable name to be used literally, and no substitution will take place. This is full quoting,
sometimes referred to as 'strong quoting.' See Chapter 5 for a detailed discussion.
Note that $variable is actually a simplified form of ${variable}. In contexts where the
$variable syntax causes an error, the longer form may work (see Section 10.2, below).

Example 4-1. Variable assignment and substitution
#!/bin/bash
# ex9.sh
# Variables: assignment and substitution
a=375
hello=$a

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Advanced Bash-Scripting Guide
#------------------------------------------------------------------------# No space permitted on either side of = sign when initializing variables.
# What happens if there is a space?
# "VARIABLE =value"
#
^
#% Script tries to run "VARIABLE" command with one argument, "=value".
# "VARIABLE= value"
#
^
#% Script tries to run "value" command with
#+ the environmental variable "VARIABLE" set to "".
#-------------------------------------------------------------------------

echo hello
# hello
# Not a variable reference, just the string "hello" . . .
echo $hello
# 375
#
^
This *is* a variable reference.
echo ${hello} # 375
# Also a variable reference, as above.
# Quoting . . .
echo "$hello"
echo "${hello}"

# 375
# 375

echo
hello="A B C
D"
echo $hello
# A B C D
echo "$hello" # A B C
D
# As you see, echo $hello
and
echo "$hello"
# Why?
# =======================================
# Quoting a variable preserves whitespace.
# =======================================

give different results.

echo
echo '$hello' # $hello
#
^
^
# Variable referencing disabled (escaped) by single quotes,
#+ which causes the "$" to be interpreted literally.
# Notice the effect of different types of quoting.

hello=
# Setting it to a null value.
echo "\$hello (null value) = $hello"
# Note that setting a variable to a null value is not the same as
#+ unsetting it, although the end result is the same (see below).
# -------------------------------------------------------------# It is permissible to set multiple variables on the same line,
#+ if separated by white space.
# Caution, this may reduce legibility, and may not be portable.
var1=21 var2=22
echo
echo "var1=$var1

var3=$V3
var2=$var2

var3=$var3"

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# May cause problems with older versions of "sh" . . .
# -------------------------------------------------------------echo; echo
numbers="one two three"
#
^
^
other_numbers="1 2 3"
#
^ ^
# If there is whitespace embedded within a variable,
#+ then quotes are necessary.
# other_numbers=1 2 3
# Gives an error message.
echo "numbers = $numbers"
echo "other_numbers = $other_numbers"
# other_numbers = 1 2 3
# Escaping the whitespace also works.
mixed_bag=2\ ---\ Whatever
#
^
^ Space after escape (\).
echo "$mixed_bag"

# 2 --- Whatever

echo; echo
echo "uninitialized_variable = $uninitialized_variable"
# Uninitialized variable has null value (no value at all!).
uninitialized_variable=
# Declaring, but not initializing it -#+ same as setting it to a null value, as above.
echo "uninitialized_variable = $uninitialized_variable"
# It still has a null value.
uninitialized_variable=23
# Set it.
unset uninitialized_variable
# Unset it.
echo "uninitialized_variable = $uninitialized_variable"
# It still has a null value.
echo
exit 0

An uninitialized variable has a "null" value -- no assigned value at all (not zero!).
if [ -z "$unassigned" ]
then
echo "\$unassigned is NULL."
fi
# $unassigned is NULL.

Using a variable before assigning a value to it may cause problems. It is nevertheless
possible to perform arithmetic operations on an uninitialized variable.
echo "$uninitialized"
let "uninitialized += 5"
echo "$uninitialized"

# (blank line)
# Add 5 to it.
# 5

# Conclusion:
# An uninitialized variable has no value,
#+ however it evaluates as 0 in an arithmetic operation.

See also Example 15-23.

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4.2. Variable Assignment
=
the assignment operator (no space before and after)
Do not confuse this with = and -eq, which test, rather than assign!
Note that = can be either an assignment or a test operator, depending on context.

Example 4-2. Plain Variable Assignment
#!/bin/bash
# Naked variables
echo
# When is a variable "naked", i.e., lacking the '$' in front?
# When it is being assigned, rather than referenced.
# Assignment
a=879
echo "The value of \"a\" is $a."
# Assignment using 'let'
let a=16+5
echo "The value of \"a\" is now $a."
echo
# In a 'for' loop (really, a type of disguised assignment):
echo -n "Values of \"a\" in the loop are: "
for a in 7 8 9 11
do
echo -n "$a "
done
echo
echo
# In
echo
read
echo

a 'read' statement (also a type of assignment):
-n "Enter \"a\" "
a
"The value of \"a\" is now $a."

echo
exit 0

Example 4-3. Variable Assignment, plain and fancy
#!/bin/bash
a=23
echo $a
b=$a

# Simple case

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echo $b
# Now, getting a little bit fancier (command substitution).
a=`echo Hello!`
# Assigns result of 'echo' command to 'a' ...
echo $a
# Note that including an exclamation mark (!) within a
#+ command substitution construct will not work from the command-line,
#+ since this triggers the Bash "history mechanism."
# Inside a script, however, the history functions are disabled.
a=`ls -l`
echo $a
echo
echo "$a"

# Assigns result of 'ls -l' command to 'a'
# Unquoted, however, it removes tabs and newlines.
# The quoted variable preserves whitespace.
# (See the chapter on "Quoting.")

exit 0

Variable assignment using the $(...) mechanism (a newer method than backquotes). This is likewise a
form of command substitution.
# From /etc/rc.d/rc.local
R=$(cat /etc/redhat-release)
arch=$(uname -m)

4.3. Bash Variables Are Untyped
Unlike many other programming languages, Bash does not segregate its variables by "type." Essentially, Bash
variables are character strings, but, depending on context, Bash permits arithmetic operations and
comparisons on variables. The determining factor is whether the value of a variable contains only digits.

Example 4-4. Integer or string?
#!/bin/bash
# int-or-string.sh
a=2334
let "a += 1"
echo "a = $a "
echo

# Integer.

b=${a/23/BB}
echo "b = $b"
declare -i b
echo "b = $b"

#
#
#
#
#

let "b += 1"
echo "b = $b"
echo

# BB35 + 1
# b = 1
# Bash sets the "integer value" of a string to 0.

c=BB34
echo "c = $c"

# c = BB34

# a = 2335
# Integer, still.

Substitute "BB" for "23".
This transforms $b into a string.
b = BB35
Declaring it an integer doesn't help.
b = BB35

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d=${c/BB/23}
echo "d = $d"
let "d += 1"
echo "d = $d"
echo

#
#
#
#
#

Substitute "23" for "BB".
This makes $d an integer.
d = 2334
2334 + 1
d = 2335

# What about null variables?
e=''
# ... Or e="" ... Or e=
echo "e = $e"
# e =
let "e += 1"
# Arithmetic operations allowed on a null variable?
echo "e = $e"
# e = 1
echo
# Null variable transformed into an integer.
# What about undeclared variables?
echo "f = $f"
# f =
let "f += 1"
# Arithmetic operations allowed?
echo "f = $f"
# f = 1
echo
# Undeclared variable transformed into an integer.
#
# However ...
let "f /= $undecl_var"
# Divide by zero?
#
let: f /= : syntax error: operand expected (error token is " ")
# Syntax error! Variable $undecl_var is not set to zero here!
#
# But still ...
let "f /= 0"
#
let: f /= 0: division by 0 (error token is "0")
# Expected behavior.

#
#+
#
#

Bash
when
But,
It's

(usually) sets the "integer value" of null to zero
performing an arithmetic operation.
don't try this at home, folks!
undocumented and probably non-portable behavior.

# Conclusion: Variables in Bash are untyped,
#+ with all attendant consequences.
exit $?

Untyped variables are both a blessing and a curse. They permit more flexibility in scripting and make it easier
to grind out lines of code (and give you enough rope to hang yourself!). However, they likewise permit subtle
errors to creep in and encourage sloppy programming habits.
To lighten the burden of keeping track of variable types in a script, Bash does permit declaring variables.

4.4. Special Variable Types
Local variables
Variables visible only within a code block or function (see also local variables in functions)
Environmental variables
Variables that affect the behavior of the shell and user interface
In a more general context, each process has an "environment", that is, a group of
variables that the process may reference. In this sense, the shell behaves like any other
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Advanced Bash-Scripting Guide
process.
Every time a shell starts, it creates shell variables that correspond to its own
environmental variables. Updating or adding new environmental variables causes the
shell to update its environment, and all the shell's child processes (the commands it
executes) inherit this environment.
The space allotted to the environment is limited. Creating too many environmental
variables or ones that use up excessive space may cause problems.
bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"
bash$ du
bash: /usr/bin/du: Argument list too long

Note: this "error" has been fixed, as of kernel version 2.6.23.
(Thank you, Stéphane Chazelas for the clarification, and for providing the above
example.)
If a script sets environmental variables, they need to be "exported," that is, reported to the
environment local to the script. This is the function of the export command.

A script can export variables only to child processes, that is, only to commands or
processes which that particular script initiates. A script invoked from the
command-line cannot export variables back to the command-line environment.
Child processes cannot export variables back to the parent processes that spawned
them.
Definition: A child process is a subprocess launched by another process, its
parent.
Positional parameters
Arguments passed to the script from the command line [25] : $0, $1, $2, $3 . . .
$0 is the name of the script itself, $1 is the first argument, $2 the second, $3 the third, and so forth.
[26] After $9, the arguments must be enclosed in brackets, for example, ${10}, ${11}, ${12}.
The special variables $* and $@ denote all the positional parameters.

Example 4-5. Positional Parameters
#!/bin/bash
# Call this script with at least 10 parameters, for example
# ./scriptname 1 2 3 4 5 6 7 8 9 10
MINPARAMS=10
echo
echo "The name of this script is \"$0\"."
# Adds ./ for current directory
echo "The name of this script is \"`basename $0`\"."

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Advanced Bash-Scripting Guide
# Strips out path name info (see 'basename')
echo
if [ -n "$1" ]
then
echo "Parameter #1 is $1"
fi

# Tested variable is quoted.
# Need quotes to escape #

if [ -n "$2" ]
then
echo "Parameter #2 is $2"
fi
if [ -n "$3" ]
then
echo "Parameter #3 is $3"
fi
# ...

if [ -n "${10}" ] # Parameters > $9 must be enclosed in {brackets}.
then
echo "Parameter #10 is ${10}"
fi
echo "-----------------------------------"
echo "All the command-line parameters are: "$*""
if [ $# -lt "$MINPARAMS" ]
then
echo
echo "This script needs at least $MINPARAMS command-line arguments!"
fi
echo
exit 0

Bracket notation for positional parameters leads to a fairly simple way of referencing the last
argument passed to a script on the command-line. This also requires indirect referencing.

args=$#
# Number of args passed.
lastarg=${!args}
# Note: This is an *indirect reference* to $args ...

# Or:
lastarg=${!#}
(Thanks, Chris Monson.)
# This is an *indirect reference* to the $# variable.
# Note that lastarg=${!$#} doesn't work.

Some scripts can perform different operations, depending on which name they are invoked with. For
this to work, the script needs to check $0, the name it was invoked by. [27] There must also exist
symbolic links to all the alternate names of the script. See Example 16-2.

If a script expects a command-line parameter but is invoked without one, this may
cause a null variable assignment, generally an undesirable result. One way to prevent
this is to append an extra character to both sides of the assignment statement using the
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Advanced Bash-Scripting Guide
expected positional parameter.
variable1_=$1_ # Rather than variable1=$1
# This will prevent an error, even if positional parameter is absent.
critical_argument01=$variable1_
# The extra character can be stripped off later, like so.
variable1=${variable1_/_/}
# Side effects only if $variable1_ begins with an underscore.
# This uses one of the parameter substitution templates discussed later.
# (Leaving out the replacement pattern results in a deletion.)
# A more straightforward way of dealing with this is
#+ to simply test whether expected positional parameters have been passed.
if [ -z $1 ]
then
exit $E_MISSING_POS_PARAM
fi

#
#+
#
#
#
#+

However, as Fabian Kreutz points out,
the above method may have unexpected side-effects.
A better method is parameter substitution:
${1:-$DefaultVal}
See the "Parameter Substition" section
in the "Variables Revisited" chapter.

---

Example 4-6. wh, whois domain name lookup
#!/bin/bash
# ex18.sh
# Does a 'whois domain-name' lookup on any of 3 alternate servers:
#
ripe.net, cw.net, radb.net
# Place this script -- renamed 'wh' -- in /usr/local/bin
#
#
#
#

Requires symbolic links:
ln -s /usr/local/bin/wh /usr/local/bin/wh-ripe
ln -s /usr/local/bin/wh /usr/local/bin/wh-apnic
ln -s /usr/local/bin/wh /usr/local/bin/wh-tucows

E_NOARGS=75

if [ -z "$1" ]
then
echo "Usage: `basename $0` [domain-name]"
exit $E_NOARGS
fi
# Check script
case `basename
"wh"
"wh-ripe"
"wh-apnic"
"wh-cw"
*

name and call proper server.
$0` in
# Or:
case ${0##*/} in
) whois $1@whois.tucows.com;;
) whois $1@whois.ripe.net;;
) whois $1@whois.apnic.net;;
) whois $1@whois.cw.net;;
) echo "Usage: `basename $0` [domain-name]";;

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esac
exit $?

---

The shift command reassigns the positional parameters, in effect shifting them to the left one notch.
$1 <--- $2, $2 <--- $3, $3 <--- $4, etc.
The old $1 disappears, but $0 (the script name) does not change. If you use a large number of
positional parameters to a script, shift lets you access those past 10, although {bracket} notation also
permits this.

Example 4-7. Using shift
#!/bin/bash
# shft.sh: Using 'shift' to step through all the positional parameters.
# Name this script something like shft.sh,
#+ and invoke it with some parameters.
#+ For example:
#
sh shft.sh a b c def 23 Skidoo
until [ -z "$1" ]
do
echo -n "$1 "
shift
done

# Until all parameters used up . . .

echo

# Extra linefeed.

# But, what happens to the "used-up" parameters?
echo "$2"
# Nothing echoes!
# When $2 shifts into $1 (and there is no $3 to shift into $2)
#+ then $2 remains empty.
# So, it is not a parameter *copy*, but a *move*.
exit
# See also the echo-params.sh script for a "shiftless"
#+ alternative method of stepping through the positional params.

The shift command can take a numerical parameter indicating how many positions to shift.
#!/bin/bash
# shift-past.sh
shift 3
# Shift 3 positions.
# n=3; shift $n
# Has the same effect.
echo "$1"
exit 0
# ======================== #

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$ sh shift-past.sh 1 2 3 4 5
4
# However, as Eleni Fragkiadaki, points out,
#+ attempting a 'shift' past the number of
#+ positional parameters ($#) returns an exit status of 1,
#+ and the positional parameters themselves do not change.
# This means possibly getting stuck in an endless loop. . . .
# For example:
#
until [ -z "$1" ]
#
do
#
echo -n "$1 "
#
shift 20
# If less than 20 pos params,
#
done
#+ then loop never ends!
#
# When in doubt, add a sanity check. . . .
#
shift 20 || break
#
^^^^^^^^

The shift command works in a similar fashion on parameters passed to a function. See
Example 36-16.

Chapter 4. Introduction to Variables and Parameters

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Chapter 5. Quoting
Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in
the string from reinterpretation or expansion by the shell or shell script. (A character is "special" if it has an
interpretation other than its literal meaning. For example, the asterisk * represents a wild card character in
globbing and Regular Expressions).
bash$ ls -l [Vv]*
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo

324 Apr 2 15:05 VIEWDATA.BAT
507 May 4 14:25 vartrace.sh
539 Apr 14 17:11 viewdata.sh

bash$ ls -l '[Vv]*'
ls: [Vv]*: No such file or directory

In everyday speech or writing, when we "quote" a phrase, we set it apart and give it special meaning. In a
Bash script, when we quote a string, we set it apart and protect its literal meaning.
Certain programs and utilities reinterpret or expand special characters in a quoted string. An important use of
quoting is protecting a command-line parameter from the shell, but still letting the calling program expand it.
bash$ grep '[Ff]irst' *.txt
file1.txt:This is the first line of file1.txt.
file2.txt:This is the First line of file2.txt.

Note that the unquoted grep [Ff]irst *.txt works under the Bash shell. [28]
Quoting can also suppress echo's "appetite" for newlines.
bash$ echo $(ls -l)
total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh

bash$ echo "$(ls -l)"
total 8
-rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh
-rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh

5.1. Quoting Variables
When referencing a variable, it is generally advisable to enclose its name in double quotes. This prevents
reinterpretation of all special characters within the quoted string -- except $, ` (backquote), and \ (escape). [29]
Keeping $ as a special character within double quotes permits referencing a quoted variable
("$variable"), that is, replacing the variable with its value (see Example 4-1, above).

Use double quotes to prevent word splitting. [30] An argument enclosed in double quotes presents itself as a
single word, even if it contains whitespace separators.

List="one two three"

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Advanced Bash-Scripting Guide
for a in $List
do
echo "$a"
done
# one
# two
# three

# Splits the variable in parts at whitespace.

echo "---"
for a in "$List"
do #
^
^
echo "$a"
done
# one two three

# Preserves whitespace in a single variable.

A more elaborate example:
variable1="a variable containing five words"
COMMAND This is $variable1
# Executes COMMAND with 7 arguments:
# "This" "is" "a" "variable" "containing" "five" "words"
COMMAND "This is $variable1" # Executes COMMAND with 1 argument:
# "This is a variable containing five words"

variable2=""

# Empty.

COMMAND $variable2 $variable2 $variable2
# Executes COMMAND with no arguments.
COMMAND "$variable2" "$variable2" "$variable2"
# Executes COMMAND with 3 empty arguments.
COMMAND "$variable2 $variable2 $variable2"
# Executes COMMAND with 1 argument (2 spaces).
# Thanks, Stéphane Chazelas.

Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting or
preservation of whitespace is an issue.
Example 5-1. Echoing Weird Variables
#!/bin/bash
# weirdvars.sh: Echoing weird variables.
echo
var="'(]\\{}\$\""
echo $var
# '(]\{}$"
echo "$var"
# '(]\{}$"

Doesn't make a difference.

echo
IFS='\'
echo $var
echo "$var"

# '(] {}$"
# '(]\{}$"

\ converted to space. Why?

# Examples above supplied by Stephane Chazelas.

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Advanced Bash-Scripting Guide
echo
var2="\\\\\""
echo $var2
#
"
echo "$var2"
# \\"
echo
# But ... var2="\\\\"" is illegal. Why?
var3='\\\\'
echo "$var3"
# \\\\
# Strong quoting works, though.
exit

Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special
meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally.
Consider single quotes ("full quoting") to be a stricter method of quoting than double quotes ("partial
quoting").
Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a
single quote within single quotes will not yield the expected result.
echo "Why can't I write 's between single quotes"
echo
# The roundabout method.
echo 'Why can'\''t I write '"'"'s between single quotes'
#
|-------| |----------|
|-----------------------|
# Three single-quoted strings, with escaped and quoted single quotes between.
# This example courtesy of Stéphane Chazelas.

5.2. Escaping
Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to
interpret that character literally.
With certain commands and utilities, such as echo and sed, escaping a character may have the opposite
effect - it can toggle on a special meaning for that character.
Special meanings of certain escaped characters
used with echo and sed
\n
means newline
\r
means return
\t
means tab
\v
means vertical tab
\b
means backspace
\a
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43

Advanced Bash-Scripting Guide
means alert (beep or flash)
\0xx
translates to the octal ASCII equivalent of 0nn, where nn is a string of digits

The $' ... ' quoted string-expansion construct is a mechanism that uses escaped
octal or hex values to assign ASCII characters to variables, e.g., quote=$'\042'.
Example 5-2. Escaped Characters
#!/bin/bash
# escaped.sh: escaped characters
#############################################################
### First, let's show some basic escaped-character usage. ###
#############################################################
# Escaping a newline.
# -----------------echo ""
echo "This will print
as two lines."
# This will print
# as two lines.
echo "This will print \
as one line."
# This will print as one line.
echo; echo
echo "============="

echo "\v\v\v\v"
# Prints \v\v\v\v literally.
# Use the -e option with 'echo' to print escaped characters.
echo "============="
echo "VERTICAL TABS"
echo -e "\v\v\v\v"
# Prints 4 vertical tabs.
echo "=============="
echo "QUOTATION MARK"
echo -e "\042"
# Prints " (quote, octal ASCII character 42).
echo "=============="

# The $'\X' construct makes the -e option unnecessary.
echo; echo "NEWLINE and (maybe) BEEP"
echo $'\n'
# Newline.
echo $'\a'
# Alert (beep).
# May only flash, not beep, depending on terminal.
# We have seen $'\nnn" string expansion, and now . . .
# =================================================================== #
# Version 2 of Bash introduced the $'\nnn' string expansion construct.

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Advanced Bash-Scripting Guide
# =================================================================== #
echo "Introducing the \$\' ... \' string-expansion construct . . . "
echo ". . . featuring more quotation marks."
echo $'\t \042 \t'
# Quote (") framed by tabs.
# Note that '\nnn' is an octal value.
# It also works with hexadecimal values, in an $'\xhhh' construct.
echo $'\t \x22 \t' # Quote (") framed by tabs.
# Thank you, Greg Keraunen, for pointing this out.
# Earlier Bash versions allowed '\x022'.
echo

# Assigning ASCII characters to a variable.
# ---------------------------------------quote=$'\042'
# " assigned to a variable.
echo "$quote Quoted string $quote and this lies outside the quotes."
echo
# Concatenating ASCII chars in a variable.
triple_underline=$'\137\137\137' # 137 is octal ASCII code for '_'.
echo "$triple_underline UNDERLINE $triple_underline"
echo
ABC=$'\101\102\103\010'
echo $ABC

# 101, 102, 103 are octal A, B, C.

echo
escape=$'\033'
# 033 is octal for escape.
echo "\"escape\" echoes as $escape"
#
no visible output.
echo
exit 0

A more elaborate example:

Example 5-3. Detecting key-presses
#!/bin/bash
# Author: Sigurd Solaas, 20 Apr 2011
# Used in ABS Guide with permission.
# Requires version 4.2+ of Bash.
key="no value yet"
while true; do
clear
echo "Bash Extra Keys Demo. Keys to try:"
echo
echo "* Insert, Delete, Home, End, Page_Up and Page_Down"
echo "* The four arrow keys"
echo "* Tab, enter, escape, and space key"
echo "* The letter and number keys, etc."
echo

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45

Advanced Bash-Scripting Guide
echo "
d = show date/time"
echo "
q = quit"
echo "================================"
echo
# Convert the separate home-key to home-key_num_7:
if [ "$key" = $'\x1b\x4f\x48' ]; then
key=$'\x1b\x5b\x31\x7e'
#
Quoted string-expansion construct.
fi
# Convert the separate end-key to end-key_num_1.
if [ "$key" = $'\x1b\x4f\x46' ]; then
key=$'\x1b\x5b\x34\x7e'
fi
case "$key" in
$'\x1b\x5b\x32\x7e') # Insert
echo Insert Key
;;
$'\x1b\x5b\x33\x7e') # Delete
echo Delete Key
;;
$'\x1b\x5b\x31\x7e') # Home_key_num_7
echo Home Key
;;
$'\x1b\x5b\x34\x7e') # End_key_num_1
echo End Key
;;
$'\x1b\x5b\x35\x7e') # Page_Up
echo Page_Up
;;
$'\x1b\x5b\x36\x7e') # Page_Down
echo Page_Down
;;
$'\x1b\x5b\x41') # Up_arrow
echo Up arrow
;;
$'\x1b\x5b\x42') # Down_arrow
echo Down arrow
;;
$'\x1b\x5b\x43') # Right_arrow
echo Right arrow
;;
$'\x1b\x5b\x44') # Left_arrow
echo Left arrow
;;
$'\x09') # Tab
echo Tab Key
;;
$'\x0a') # Enter
echo Enter Key
;;
$'\x1b') # Escape
echo Escape Key
;;
$'\x20') # Space
echo Space Key
;;
d)
date
;;

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46

Advanced Bash-Scripting Guide
q)
echo Time to quit...
echo
exit 0
;;
*)
echo You pressed: \'"$key"\'
;;
esac
echo
echo "================================"
unset K1 K2 K3
read -s -N1 -p "Press a key: "
K1="$REPLY"
read -s -N2 -t 0.001
K2="$REPLY"
read -s -N1 -t 0.001
K3="$REPLY"
key="$K1$K2$K3"
done
exit $?

See also Example 37-1.
\"
gives the quote its literal meaning
echo "Hello"
echo "\"Hello\" ... he said."

# Hello
# "Hello" ... he said.

\$
gives the dollar sign its literal meaning (variable name following \$ will not be referenced)
echo "\$variable01"
echo "The book cost \$7.98."

# $variable01
# The book cost $7.98.

\\
gives the backslash its literal meaning
echo "\\"

# Results in \

# Whereas . . .
echo "\"

# Invokes secondary prompt from the command-line.
# In a script, gives an error message.

# However . . .
echo '\'

# Results in \

The behavior of \ depends on whether it is escaped, strong-quoted, weak-quoted, or appearing within
command substitution or a here document.

echo
echo
echo
echo

\z
\\z
'\z'
'\\z'

Chapter 5. Quoting

# Simple escaping and quoting
# z
# \z
# \z
# \\z

47

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echo "\z"
echo "\\z"

echo
echo
echo
echo
echo
echo
echo
echo

`echo
`echo
`echo
`echo
`echo
`echo
`echo
`echo

# \z
# \z

\z`
\\z`
\\\z`
\\\\z`
\\\\\\z`
\\\\\\\z`
"\z"`
"\\z"`

#
#
#
#
#
#
#
#
#

Command substitution
z
z
\z
\z
\z
\\z
\z
\z

# Here document
cat < /dev/null # Suppress output.
then echo "Files a and b are identical."
else echo "Files a and b differ."
fi
# The very useful "if-grep" construct:
# ----------------------------------if grep -q Bash file
then echo "File contains at least one occurrence of Bash."
fi
word=Linux
letter_sequence=inu
if echo "$word" | grep -q "$letter_sequence"
# The "-q" option to grep suppresses output.
then
echo "$letter_sequence found in $word"
else
echo "$letter_sequence not found in $word"
fi

if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED
then echo "Command succeeded."
else echo "Command failed."
fi

• These last two examples courtesy of Stéphane Chazelas.

Example 7-1. What is truth?
#!/bin/bash
# Tip:
# If you're unsure of how a certain condition would evaluate,
#+ test it in an if-test.
echo
echo "Testing \"0\""
if [ 0 ]
# zero
then
echo "0 is true."
else
# Or else ...
echo "0 is false."
fi
# 0 is true.
echo

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echo "Testing \"1\""
if [ 1 ]
# one
then
echo "1 is true."
else
echo "1 is false."
fi
# 1 is true.
echo
echo "Testing
if [ -1 ]
then
echo "-1 is
else
echo "-1 is
fi

\"-1\""
# minus one
true."
false."
# -1 is true.

echo
echo "Testing \"NULL\""
if [ ]
# NULL (empty condition)
then
echo "NULL is true."
else
echo "NULL is false."
fi
# NULL is false.
echo
echo "Testing \"xyz\""
if [ xyz ]
# string
then
echo "Random string is true."
else
echo "Random string is false."
fi
# Random string is true.
echo
echo "Testing \"\$xyz\""
if [ $xyz ]
# Tests if $xyz is null, but...
# it's only an uninitialized variable.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi
# Uninitialized variable is false.
echo
echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]
# More pedantically correct.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi
# Uninitialized variable is false.
echo

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xyz=

# Initialized, but set to null value.

echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]
then
echo "Null variable is true."
else
echo "Null variable is false."
fi
# Null variable is false.

echo

# When is "false" true?
echo "Testing \"false\""
if [ "false" ]
# It seems that "false" is just a string.
then
echo "\"false\" is true." #+ and it tests true.
else
echo "\"false\" is false."
fi
# "false" is true.
echo
echo "Testing \"\$false\"" # Again, uninitialized variable.
if [ "$false" ]
then
echo "\"\$false\" is true."
else
echo "\"\$false\" is false."
fi
# "$false" is false.
# Now, we get the expected result.
#

What would happen if we tested the uninitialized variable "$true"?

echo
exit 0

Exercise. Explain the behavior of Example 7-1, above.

if [ condition-true ]
then
command 1
command 2
...
else # Or else ...
# Adds default code block executing if original condition tests false.
command 3
command 4
...
fi

When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both if
and then are keywords. Keywords (or commands) begin statements, and before a new statement on the
same line begins, the old one must terminate.

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if [ -x "$filename" ]; then

Else if and elif
elif
elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one.
if [ condition1 ]
then
command1
command2
command3
elif [ condition2 ]
# Same as else if
then
command4
command5
else
default-command
fi

The if test condition-true construct is the exact equivalent of if [ condition-true ]. As
it happens, the left bracket, [ , is a token [32] which invokes the test command. The closing right bracket, ] , in
an if/test should not therefore be strictly necessary, however newer versions of Bash require it.

The test command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash
script, test does not call the external /usr/bin/test binary, which is part of the sh-utils package.
Likewise, [ does not call /usr/bin/[, which is linked to /usr/bin/test.
bash$ type test
test is a shell builtin
bash$ type '['
[ is a shell builtin
bash$ type '[['
[[ is a shell keyword
bash$ type ']]'
]] is a shell keyword
bash$ type ']'
bash: type: ]: not found

If, for some reason, you wish to use /usr/bin/test in a Bash script, then specify it by full
pathname.
Example 7-2. Equivalence of test, /usr/bin/test, [ ], and /usr/bin/[
#!/bin/bash
echo
if test -z "$1"
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."

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fi
echo
if /usr/bin/test -z "$1"
# Equivalent to "test" builtin.
# ^^^^^^^^^^^^^
# Specifying full pathname.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
if [ -z "$1" ]
# Functionally identical to above code blocks.
#
if [ -z "$1"
should work, but...
#+ Bash responds to a missing close-bracket with an error message.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo

if /usr/bin/[ -z "$1" ]
# Again, functionally identical to above.
# if /usr/bin/[ -z "$1"
# Works, but gives an error message.
#
# Note:
#
This has been fixed in Bash, version 3.x.
then
echo "No command-line arguments."
else
echo "First command-line argument is $1."
fi
echo
exit 0

The [[ ]] construct is the more versatile Bash version of [ ]. This is the extended test command, adopted from
ksh88.
***
No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and
command substitution.
file=/etc/passwd
if [[ -e $file ]]
then
echo "Password file exists."
fi

Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, the
&&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.

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Arithmetic evaluation of octal / hexadecimal constants takes place automatically within a [[ ... ]] construct.
# [[ Octal and hexadecimal evaluation ]]
# Thank you, Moritz Gronbach, for pointing this out.

decimal=15
octal=017
hex=0x0f

# = 15 (decimal)
# = 15 (decimal)

if [ "$decimal" -eq "$octal" ]
then
echo "$decimal equals $octal"
else
echo "$decimal is not equal to $octal"
# 15 is not equal to 017
fi
# Doesn't evaluate within [ single brackets ]!

if [[ "$decimal" -eq "$octal" ]]
then
echo "$decimal equals $octal"
# 15 equals 017
else
echo "$decimal is not equal to $octal"
fi
# Evaluates within [[ double brackets ]]!
if [[ "$decimal" -eq "$hex" ]]
then
echo "$decimal equals $hex"
else
echo "$decimal is not equal to $hex"
fi
# [[ $hexadecimal ]] also evaluates!

# 15 equals 0x0f

Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary.
dir=/home/bozo
if cd "$dir" 2>/dev/null; then
echo "Now in $dir."
else
echo "Can't change to $dir."
fi

# "2>/dev/null" hides error message.

The "if COMMAND" construct returns the exit status of COMMAND.
Similarly, a condition within test brackets may stand alone without an if, when used in combination with
a list construct.
var1=20
var2=22
[ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2"
home=/home/bozo
[ -d "$home" ] || echo "$home directory does not exist."

The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it
returns an exit status of 1, or "false". A non-zero expression returns an exit status of 0, or "true". This is in
marked contrast to using the test and [ ] constructs previously discussed.

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Example 7-3. Arithmetic Tests using (( ))
#!/bin/bash
# arith-tests.sh
# Arithmetic tests.
# The (( ... )) construct evaluates and tests numerical expressions.
# Exit status opposite from [ ... ] construct!
(( 0 ))
echo "Exit status of \"(( 0 ))\" is $?."

# 1

(( 1 ))
echo "Exit status of \"(( 1 ))\" is $?."

# 0

(( 5 > 4 ))
echo "Exit status of \"(( 5 > 4 ))\" is $?."

# true
# 0

(( 5 > 9 ))
echo "Exit status of \"(( 5 > 9 ))\" is $?."

# false
# 1

(( 5 == 5 ))
echo "Exit status of \"(( 5 == 5 ))\" is $?."
# (( 5 = 5 )) gives an error message.

# true
# 0

(( 5 - 5 ))
echo "Exit status of \"(( 5 - 5 ))\" is $?."

# 0
# 1

(( 5 / 4 ))
echo "Exit status of \"(( 5 / 4 ))\" is $?."

# Division o.k.
# 0

(( 1 / 2 ))
echo "Exit status of \"(( 1 / 2 ))\" is $?."

# Division result < 1.
# Rounded off to 0.
# 1

(( 1 / 0 )) 2>/dev/null
#
^^^^^^^^^^^
echo "Exit status of \"(( 1 / 0 ))\" is $?."

# Illegal division by 0.
# 1

# What effect does the "2>/dev/null" have?
# What would happen if it were removed?
# Try removing it, then rerunning the script.
# ======================================= #
# (( ... )) also useful in an if-then test.
var1=5
var2=4
if (( var1 > var2 ))
then #^
^
Note: Not $var1, $var2. Why?
echo "$var1 is greater than $var2"
fi
# 5 is greater than 4
exit 0

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7.2. File test operators
Returns true if...
-e
file exists
-a
file exists
This is identical in effect to -e. It has been "deprecated," [33] and its use is discouraged.
-f
file is a regular file (not a directory or device file)
-s
file is not zero size
-d
file is a directory
-b
file is a block device
-c
file is a character device
device0="/dev/sda2"
# /
(root directory)
if [ -b "$device0" ]
then
echo "$device0 is a block device."
fi
# /dev/sda2 is a block device.

device1="/dev/ttyS1"
# PCMCIA modem card.
if [ -c "$device1" ]
then
echo "$device1 is a character device."
fi
# /dev/ttyS1 is a character device.

-p
file is a pipe
function show_input_type()
{
[ -p /dev/fd/0 ] && echo PIPE || echo STDIN
}
show_input_type "Input"
echo "Input" | show_input_type

# STDIN
# PIPE

# This example courtesy of Carl Anderson.

-h
file is a symbolic link
-L
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file is a symbolic link
-S
file is a socket
-t
file (descriptor) is associated with a terminal device
This test option may be used to check whether the stdin [ -t 0 ] or stdout [ -t 1 ] in a
given script is a terminal.
-r
file has read permission (for the user running the test)
-w
file has write permission (for the user running the test)
-x
file has execute permission (for the user running the test)
-g
set-group-id (sgid) flag set on file or directory
If a directory has the sgid flag set, then a file created within that directory belongs to the group that
owns the directory, not necessarily to the group of the user who created the file. This may be useful
for a directory shared by a workgroup.
-u
set-user-id (suid) flag set on file
A binary owned by root with set-user-id flag set runs with root privileges, even when an
ordinary user invokes it. [34] This is useful for executables (such as pppd and cdrecord) that need to
access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root
user.
-rwsr-xr-t

1 root

178236 Oct

2

2000 /usr/sbin/pppd

A file with the suid flag set shows an s in its permissions.
-k
sticky bit set
Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a
file has this flag set, that file will be kept in cache memory, for quicker access. [35] If set on a
directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or
directory listing.
drwxrwxrwt

7 root

1024 May 19 21:26 tmp/

If a user does not own a directory that has the sticky bit set, but has write permission in that directory,
she can only delete those files that she owns in it. This keeps users from inadvertently overwriting or
deleting each other's files in a publicly accessible directory, such as /tmp. (The owner of the
directory or root can, of course, delete or rename files there.)
-O
you are owner of file
-G
group-id of file same as yours
-N
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file modified since it was last read
f1 -nt f2
file f1 is newer than f2
f1 -ot f2
file f1 is older than f2
f1 -ef f2
files f1 and f2 are hard links to the same file
!
"not" -- reverses the sense of the tests above (returns true if condition absent).

Example 7-4. Testing for broken links
#!/bin/bash
# broken-link.sh
# Written by Lee bigelow 
# Used in ABS Guide with permission.
# A pure shell script to find dead symlinks and output them quoted
#+ so they can be fed to xargs and dealt with :)
#+ eg. sh broken-link.sh /somedir /someotherdir|xargs rm
#
# This, however, is a better method:
#
# find "somedir" -type l -print0|\
# xargs -r0 file|\
# grep "broken symbolic"|
# sed -e 's/^\|: *broken symbolic.*$/"/g'
#
#+ but that wouldn't be pure Bash, now would it.
# Caution: beware the /proc file system and any circular links!
################################################################

# If no args are passed to the script set directories-to-search
#+ to current directory. Otherwise set the directories-to-search
#+ to the args passed.
######################
[ $# -eq 0 ] && directorys=`pwd` || directorys=$@

# Setup the
#+ for files
# If one of
#+ send that
##########

function linkchk to check the directory it is passed
that are links and don't exist, then print them quoted.
the elements in the directory is a subdirectory then
subdirectory to the linkcheck function.

linkchk () {
for element in $1/*; do
[ -h "$element" -a ! -e "$element" ] && echo \"$element\"
[ -d "$element" ] && linkchk $element
# Of course, '-h' tests for symbolic link, '-d' for directory.
done
}
# Send each arg that was passed to the script to the linkchk() function
#+ if it is a valid directoy. If not, then print the error message
#+ and usage info.

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##################
for directory in $directorys; do
if [ -d $directory ]
then linkchk $directory
else
echo "$directory is not a directory"
echo "Usage: $0 dir1 dir2 ..."
fi
done
exit $?

Example 31-1, Example 11-7, Example 11-3, Example 31-3, and Example A-1 also illustrate uses of the file
test operators.

7.3. Other Comparison Operators
A binary comparison operator compares two variables or quantities. Note that integer and string comparison
use a different set of operators.
integer comparison
-eq
is equal to
if [ "$a" -eq "$b" ]
-ne
is not equal to
if [ "$a" -ne "$b" ]
-gt
is greater than
if [ "$a" -gt "$b" ]
-ge
is greater than or equal to
if [ "$a" -ge "$b" ]
-lt
is less than
if [ "$a" -lt "$b" ]
-le
is less than or equal to
if [ "$a" -le "$b" ]
<
is less than (within double parentheses)
(("$a" < "$b"))
<=
is less than or equal to (within double parentheses)

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(("$a" <= "$b"))
>
is greater than (within double parentheses)
(("$a" > "$b"))
>=
is greater than or equal to (within double parentheses)
(("$a" >= "$b"))
string comparison
=
is equal to
if [ "$a" = "$b" ]
Note the whitespace framing the =.
if [ "$a"="$b" ] is not equivalent to the above.
==
is equal to
if [ "$a" == "$b" ]
This is a synonym for =.
The == comparison operator behaves differently within a double-brackets test than
within single brackets.
[[ $a == z* ]]
# True if $a starts with an "z" (pattern matching).
[[ $a == "z*" ]] # True if $a is equal to z* (literal matching).
[ $a == z* ]
# File globbing and word splitting take place.
[ "$a" == "z*" ] # True if $a is equal to z* (literal matching).
# Thanks, Stéphane Chazelas

!=
is not equal to
if [ "$a" != "$b" ]
This operator uses pattern matching within a [[ ... ]] construct.
<
is less than, in ASCII alphabetical order
if [[ "$a" < "$b" ]]
if [ "$a" \< "$b" ]

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Note that the "<" needs to be escaped within a [ ] construct.
>
is greater than, in ASCII alphabetical order
if [[ "$a" > "$b" ]]
if [ "$a" \> "$b" ]
Note that the ">" needs to be escaped within a [ ] construct.
See Example 27-11 for an application of this comparison operator.
-z
string is null, that is, has zero length
String=''

# Zero-length ("null") string variable.

if [ -z "$String" ]
then
echo "\$String is null."
else
echo "\$String is NOT null."
fi
# $String is null.

-n
string is not null.
The -n test requires that the string be quoted within the test brackets. Using an
unquoted string with ! -z, or even just the unquoted string alone within test brackets
(see Example 7-6) normally works, however, this is an unsafe practice. Always quote a
tested string. [36]

Example 7-5. Arithmetic and string comparisons
#!/bin/bash
a=4
b=5
# Here "a" and "b" can be treated either as integers or strings.
# There is some blurring between the arithmetic and string comparisons,
#+ since Bash variables are not strongly typed.
# Bash permits integer operations and comparisons on variables
#+ whose value consists of all-integer characters.
# Caution advised, however.
echo
if [ "$a" -ne "$b" ]
then
echo "$a is not equal to $b"
echo "(arithmetic comparison)"
fi
echo

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if [ "$a" != "$b" ]
then
echo "$a is not equal to $b."
echo "(string comparison)"
#
"4" != "5"
# ASCII 52 != ASCII 53
fi
# In this particular instance, both "-ne" and "!=" work.
echo
exit 0

Example 7-6. Testing whether a string is null
#!/bin/bash
# str-test.sh: Testing null strings and unquoted strings,
#+ but not strings and sealing wax, not to mention cabbages and kings . . .
# Using

if [ ... ]

# If a string has not been initialized, it has no defined value.
# This state is called "null" (not the same as zero!).
if [ -n $string1 ]
# string1 has not been declared or initialized.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Wrong result.
# Shows $string1 as not null, although it was not initialized.
echo
# Let's try it again.
if [ -n "$string1" ] # This time, $string1 is quoted.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Quote strings within test brackets!
echo
if [ $string1 ]
# This time, $string1 stands naked.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# This works fine.
# The [ ... ] test operator alone detects whether the string is null.
# However it is good practice to quote it (if [ "$string1" ]).
#
# As Stephane Chazelas points out,
#
if [ $string1 ]
has one argument, "]"
#
if [ "$string1" ] has two arguments, the empty "$string1" and "]"

echo

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string1=initialized
if [ $string1 ]
# Again, $string1 stands unquoted.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Again, gives correct result.
# Still, it is better to quote it ("$string1"), because . . .

string1="a = b"
if [ $string1 ]
# Again, $string1 stands unquoted.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Not quoting "$string1" now gives wrong result!
exit 0

# Thank you, also, Florian Wisser, for the "heads-up".

Example 7-7. zmore
#!/bin/bash
# zmore
# View gzipped files with 'more' filter.
E_NOARGS=85
E_NOTFOUND=86
E_NOTGZIP=87
if [ $# -eq 0 ] # same effect as: if [ -z "$1" ]
# $1 can exist, but be empty: zmore "" arg2 arg3
then
echo "Usage: `basename $0` filename" >&2
# Error message to stderr.
exit $E_NOARGS
# Returns 65 as exit status of script (error code).
fi
filename=$1
if [ ! -f "$filename" ]
# Quoting $filename allows for possible spaces.
then
echo "File $filename not found!" >&2
# Error message to stderr.
exit $E_NOTFOUND
fi
if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
echo "File $1 is not a gzipped file!"
exit $E_NOTGZIP
fi
zcat $1 | more

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# Uses the 'more' filter.
# May substitute 'less' if desired.
exit $?
# Script returns exit status of pipe.
# Actually "exit $?" is unnecessary, as the script will, in any case,
#+ return the exit status of the last command executed.

compound comparison
-a
logical and
exp1 -a exp2 returns true if both exp1 and exp2 are true.
-o
logical or
exp1 -o exp2 returns true if either exp1 or exp2 is true.
These are similar to the Bash comparison operators && and ||, used within double brackets.
[[ condition1 && condition2 ]]

The -o and -a operators work with the test command or occur within single test brackets.
if [ "$expr1" -a "$expr2" ]
then
echo "Both expr1 and expr2 are true."
else
echo "Either expr1 or expr2 is false."
fi

But, as rihad points out:
[ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ]
# true
[ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ]
# (no output)
# ^^^^^^^ False condition. So far, everything as expected.
# However ...
[ 1 -eq 2 -a -n "`echo true 1>&2`" ]
# true
# ^^^^^^^ False condition. So, why "true" output?
# Is it because both condition clauses within brackets evaluate?
[[ 1 -eq 2 && -n "`echo true 1>&2`" ]]
# (no output)
# No, that's not it.
# Apparently && and || "short-circuit" while -a and -o do not.

Refer to Example 8-3, Example 27-17, and Example A-29 to see compound comparison operators in action.

7.4. Nested if/then Condition Tests
Condition tests using the if/then construct may be nested. The net result is equivalent to using the &&
compound comparison operator.
a=3

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if [ "$a" -gt 0 ]
then
if [ "$a" -lt 5 ]
then
echo "The value of \"a\" lies somewhere between 0 and 5."
fi
fi
# Same result as:
if [ "$a" -gt 0 ] && [ "$a" -lt 5 ]
then
echo "The value of \"a\" lies somewhere between 0 and 5."
fi

Example 37-4 demonstrates a nested if/then condition test.

7.5. Testing Your Knowledge of Tests
The systemwide xinitrc file can be used to launch the X server. This file contains quite a number of if/then
tests. The following is excerpted from an "ancient" version of xinitrc (Red Hat 7.1, or thereabouts).
if [ -f $HOME/.Xclients ]; then
exec $HOME/.Xclients
elif [ -f /etc/X11/xinit/Xclients ]; then
exec /etc/X11/xinit/Xclients
else
# failsafe settings. Although we should never get here
# (we provide fallbacks in Xclients as well) it can't hurt.
xclock -geometry 100x100-5+5 &
xterm -geometry 80x50-50+150 &
if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then
netscape /usr/share/doc/HTML/index.html &
fi
fi

Explain the test constructs in the above snippet, then examine an updated version of the file,
/etc/X11/xinit/xinitrc, and analyze the if/then test constructs there. You may need to refer ahead to
the discussions of grep, sed, and regular expressions.

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8.1. Operators
assignment
variable assignment
Initializing or changing the value of a variable
=
All-purpose assignment operator, which works for both arithmetic and string assignments.
var=27
category=minerals

# No spaces allowed after the "=".

Do not confuse the "=" assignment operator with the = test operator.
#

=

as a test operator

if [ "$string1" = "$string2" ]
then
command
fi
# if [ "X$string1" = "X$string2" ] is safer,
#+ to prevent an error message should one of the variables be empty.
# (The prepended "X" characters cancel out.)

arithmetic operators
+
plus
minus
*
multiplication
/
division
**
exponentiation
# Bash, version 2.02, introduced the "**" exponentiation operator.
let "z=5**3"
echo "z = $z"

# 5 * 5 * 5
# z = 125

%
modulo, or mod (returns the remainder of an integer division operation)
bash$ expr 5 % 3
2

5/3 = 1, with remainder 2

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This operator finds use in, among other things, generating numbers within a specific range (see
Example 9-11 and Example 9-15) and formatting program output (see Example 27-16 and Example
A-6). It can even be used to generate prime numbers, (see Example A-15). Modulo turns up
surprisingly often in numerical recipes.

Example 8-1. Greatest common divisor
#!/bin/bash
# gcd.sh: greatest common divisor
#
Uses Euclid's algorithm
# The "greatest common divisor" (gcd) of two integers
#+ is the largest integer that will divide both, leaving no remainder.
# Euclid's algorithm uses successive division.
#
In each pass,
#+
dividend <--- divisor
#+
divisor <--- remainder
#+
until remainder = 0.
#
The gcd = dividend, on the final pass.
#
# For an excellent discussion of Euclid's algorithm, see
#+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm.

# -----------------------------------------------------# Argument check
ARGS=2
E_BADARGS=85
if [ $# -ne "$ARGS" ]
then
echo "Usage: `basename $0` first-number second-number"
exit $E_BADARGS
fi
# ------------------------------------------------------

gcd ()
{
dividend=$1
divisor=$2

# Arbitrary assignment.
#! It doesn't matter which of the two is larger.
# Why not?

remainder=1

# If an uninitialized variable is used inside
#+ test brackets, an error message results.

until [ "$remainder" -eq 0 ]
do
# ^^^^^^^^^^ Must be previously initialized!
let "remainder = $dividend % $divisor"
dividend=$divisor
# Now repeat with 2 smallest numbers.
divisor=$remainder
done
# Euclid's algorithm
}

# Last $dividend is the gcd.

gcd $1 $2

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echo; echo "GCD of $1 and $2 = $dividend"; echo

# Exercises :
# --------# 1) Check command-line arguments to make sure they are integers,
#+
and exit the script with an appropriate error message if not.
# 2) Rewrite the gcd () function to use local variables.
exit 0

+=
plus-equal (increment variable by a constant) [37]
let "var += 5" results in var being incremented by 5.
-=
minus-equal (decrement variable by a constant)
*=
times-equal (multiply variable by a constant)
let "var *= 4" results in var being multiplied by 4.
/=
slash-equal (divide variable by a constant)
%=
mod-equal (remainder of dividing variable by a constant)
Arithmetic operators often occur in an expr or let expression.

Example 8-2. Using Arithmetic Operations
#!/bin/bash
# Counting to 11 in 10 different ways.
n=1; echo -n "$n "
let "n = $n + 1"
echo -n "$n "

# let "n = n + 1"

also works.

: $((n = $n + 1))
# ":" necessary because otherwise Bash attempts
#+ to interpret "$((n = $n + 1))" as a command.
echo -n "$n "
(( n = n + 1 ))
# A simpler alternative to the method above.
# Thanks, David Lombard, for pointing this out.
echo -n "$n "
n=$(($n + 1))
echo -n "$n "
: $[ n = $n + 1 ]
# ":" necessary because otherwise Bash attempts
#+ to interpret "$[ n = $n + 1 ]" as a command.
# Works even if "n" was initialized as a string.

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echo -n "$n "
n=$[ $n + 1 ]
# Works even if "n" was initialized as a string.
#* Avoid this type of construct, since it is obsolete and nonportable.
# Thanks, Stephane Chazelas.
echo -n "$n "
# Now for C-style increment operators.
# Thanks, Frank Wang, for pointing this out.
let "n++"
echo -n "$n "

# let "++n"

also works.

(( n++ ))
echo -n "$n "

# (( ++n ))

also works.

: $(( n++ ))
echo -n "$n "

# : $(( ++n )) also works.

: $[ n++ ]
echo -n "$n "

# : $[ ++n ] also works

echo
exit 0

Integer variables in older versions of Bash were signed long (32-bit) integers, in the range of
-2147483648 to 2147483647. An operation that took a variable outside these limits gave an erroneous
result.
echo $BASH_VERSION
a=2147483646
echo "a = $a"
let "a+=1"
echo "a = $a"
let "a+=1"
echo "a = $a"

# 1.14

#
#
#
#
#
#
#
#

a = 2147483646
Increment "a".
a = 2147483647
increment "a" again, past the limit.
a = -2147483648
ERROR: out of range,
+
and the leftmost bit, the sign bit,
+
has been set, making the result negative.

As of version >= 2.05b, Bash supports 64-bit integers.

Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as
strings.
a=1.5
let "b = $a + 1.3" # Error.
# t2.sh: let: b = 1.5 + 1.3: syntax error in expression
#
(error token is ".5 + 1.3")
echo "b = $b"

# b=1

Use bc in scripts that that need floating point calculations or math library functions.

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Advanced Bash-Scripting Guide
bitwise operators. The bitwise operators seldom make an appearance in shell scripts. Their chief use seems to
be manipulating and testing values read from ports or sockets. "Bit flipping" is more relevant to compiled
languages, such as C and C++, which provide direct access to system hardware. However, see vladz's
ingenious use of bitwise operators in his base64.sh (Example A-54) script.
bitwise operators
<<
bitwise left shift (multiplies by 2 for each shift position)
<<=
left-shift-equal
let "var <<= 2" results in var left-shifted 2 bits (multiplied by 4)
>>
bitwise right shift (divides by 2 for each shift position)
>>=
right-shift-equal (inverse of <<=)
&
bitwise AND
&=
bitwise AND-equal
|
bitwise OR
|=
bitwise OR-equal
~
bitwise NOT
^
bitwise XOR
^=
bitwise XOR-equal
logical (boolean) operators
!
NOT
if [ ! -f $FILENAME ]
then
...

&&
AND
if [ $condition1 ] && [ $condition2 ]
# Same as: if [ $condition1 -a $condition2 ]
# Returns true if both condition1 and condition2 hold true...
if [[ $condition1 && $condition2 ]]
# Also works.
# Note that && operator not permitted inside brackets
#+ of [ ... ] construct.

&& may also be used, depending on context, in an and list to concatenate commands.
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||
OR
if [ $condition1 ] || [ $condition2 ]
# Same as: if [ $condition1 -o $condition2 ]
# Returns true if either condition1 or condition2 holds true...
if [[ $condition1 || $condition2 ]]
# Also works.
# Note that || operator not permitted inside brackets
#+ of a [ ... ] construct.

Bash tests the exit status of each statement linked with a logical operator.
Example 8-3. Compound Condition Tests Using && and ||
#!/bin/bash
a=24
b=47
if [ "$a" -eq 24 ] && [ "$b" -eq 47 ]
then
echo "Test #1 succeeds."
else
echo "Test #1 fails."
fi
# ERROR:
if [ "$a" -eq 24 && "$b" -eq 47 ]
#+
attempts to execute ' [ "$a" -eq 24 '
#+
and fails to finding matching ']'.
#
# Note: if [[ $a -eq 24 && $b -eq 24 ]] works.
# The double-bracket if-test is more flexible
#+ than the single-bracket version.
#
(The "&&" has a different meaning in line 17 than in line 6.)
#
Thanks, Stephane Chazelas, for pointing this out.

if [ "$a" -eq 98 ] || [ "$b" -eq 47 ]
then
echo "Test #2 succeeds."
else
echo "Test #2 fails."
fi

# The -a and -o options provide
#+ an alternative compound condition test.
# Thanks to Patrick Callahan for pointing this out.

if [ "$a" -eq 24 -a "$b" -eq 47 ]
then
echo "Test #3 succeeds."
else
echo "Test #3 fails."
fi

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if [ "$a" -eq 98 -o "$b" -eq 47 ]
then
echo "Test #4 succeeds."
else
echo "Test #4 fails."
fi

a=rhino
b=crocodile
if [ "$a" = rhino ] && [ "$b" = crocodile ]
then
echo "Test #5 succeeds."
else
echo "Test #5 fails."
fi
exit 0

The && and || operators also find use in an arithmetic context.
bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0))
1 0 1 0

miscellaneous operators
,
Comma operator
The comma operator chains together two or more arithmetic operations. All the operations are
evaluated (with possible side effects. [38]
let "t1 = ((5 + 3, 7 - 1, 15 - 4))"
echo "t1 = $t1"
^^^^^^ # t1 = 11
# Here t1 is set to the result of the last operation. Why?
let "t2 = ((a = 9, 15 / 3))"
echo "t2 = $t2
a = $a"

# Set "a" and calculate "t2".
# t2 = 5
a = 9

The comma operator finds use mainly in for loops. See Example 11-12.

8.2. Numerical Constants
A shell script interprets a number as decimal (base 10), unless that number has a special prefix or notation. A
number preceded by a 0 is octal (base 8). A number preceded by 0x is hexadecimal (base 16). A
number with an embedded # evaluates as BASE#NUMBER (with range and notational restrictions).

Example 8-4. Representation of numerical constants
#!/bin/bash
# numbers.sh: Representation of numbers in different bases.
# Decimal: the default
let "dec = 32"
echo "decimal number = $dec"

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# 32

78

Advanced Bash-Scripting Guide
# Nothing out of the ordinary here.

# Octal: numbers preceded by '0' (zero)
let "oct = 032"
echo "octal number = $oct"
# Expresses result in decimal.
# --------- ------ -- -------

# 26

# Hexadecimal: numbers preceded by '0x' or '0X'
let "hex = 0x32"
echo "hexadecimal number = $hex"
# 50
echo $((0x9abc))
# 39612
#
^^
^^
double-parentheses arithmetic expansion/evaluation
# Expresses result in decimal.

# Other bases: BASE#NUMBER
# BASE between 2 and 64.
# NUMBER must use symbols within the BASE range, see below.

let "bin = 2#111100111001101"
echo "binary number = $bin"

# 31181

let "b32 = 32#77"
echo "base-32 number = $b32"

# 231

let "b64 = 64#@_"
echo "base-64 number = $b64"
# 4031
# This notation only works for a limited range (2 - 64) of ASCII characters.
# 10 digits + 26 lowercase characters + 26 uppercase characters + @ + _

echo
echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA))
# 1295 170 44822 3375

#
#
#
#+

Important note:
-------------Using a digit out of range of the specified base notation
gives an error message.

let "bad_oct = 081"
# (Partial) error message output:
# bad_oct = 081: value too great for base (error token is "081")
#
Octal numbers use only digits in the range 0 - 7.
exit $?

# Exit value = 1 (error)

# Thanks, Rich Bartell and Stephane Chazelas, for clarification.

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8.3. The Double-Parentheses Construct
Similar to the let command, the (( ... )) construct permits arithmetic expansion and evaluation. In its simplest
form, a=$(( 5 + 3 )) would set a to 5 + 3, or 8. However, this double-parentheses construct is also a
mechanism for allowing C-style manipulation of variables in Bash, for example, (( var++ )).

Example 8-5. C-style manipulation of variables
#!/bin/bash
# c-vars.sh
# Manipulating a variable, C-style, using the (( ... )) construct.

echo
(( a = 23 ))

# Setting a value, C-style,
#+ with spaces on both sides of the "=".
echo "a (initial value) = $a"
# 23
(( a++ ))
# Post-increment 'a', C-style.
echo "a (after a++) = $a"
# 24
(( a-- ))
# Post-decrement 'a', C-style.
echo "a (after a--) = $a"
# 23

(( ++a ))
# Pre-increment 'a', C-style.
echo "a (after ++a) = $a"
# 24
(( --a ))
# Pre-decrement 'a', C-style.
echo "a (after --a) = $a"
# 23
echo
########################################################
# Note that, as in C, pre- and post-decrement operators
#+ have different side-effects.
n=1; let --n && echo "True" || echo "False"
n=1; let n-- && echo "True" || echo "False"

# False
# True

# Thanks, Jeroen Domburg.
########################################################
echo
(( t = a<45?7:11 ))
# C-style trinary operator.
#
^ ^ ^
echo "If a < 45, then t = 7, else t = 11." # a = 23
echo "t = $t "
# t = 7
echo

# ----------------# Easter Egg alert!

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# ----------------# Chet Ramey seems to have snuck a bunch of undocumented C-style
#+ constructs into Bash (actually adapted from ksh, pretty much).
# In the Bash docs, Ramey calls (( ... )) shell arithmetic,
#+ but it goes far beyond that.
# Sorry, Chet, the secret is out.
# See also "for" and "while" loops using the (( ... )) construct.
# These work only with version 2.04 or later of Bash.
exit

See also Example 11-12 and Example 8-4.

8.4. Operator Precedence
In a script, operations execute in order of precedence: the higher precedence operations execute before the
lower precedence ones. [39]

Table 8-1. Operator Precedence
Operator

Meaning

var++ var-++var --var

post-increment, post-decrement
pre-increment, pre-decrement

! ~

negation

logical / bitwise, inverts sense of
following operator

**
* / %
+ -

exponentiation
multiplication, division, modulo
addition, subtraction

arithmetic operation
arithmetic operation
arithmetic operation

<< >>

left, right shift

bitwise

-z -n
unary comparison
-e -f -t -x, etc.
unary comparison
< -lt > -gt <= -le >= -ge compound comparison
-nt -ot -ef
compound comparison
== -eq != -ne
equality / inequality
&
^
|

AND
XOR
OR

Chapter 8. Operations and Related Topics

Comments
HIGHEST PRECEDENCE
C-style operators

string is/is-not null
file-test
string and integer
file-test
test operators, string and integer
bitwise
exclusive OR, bitwise
bitwise

81

Advanced Bash-Scripting Guide
&& -a
|| -o

AND
OR

logical, compound comparison
logical, compound comparison

?:
=

trinary operator
assignment

C-style
(do not confuse with equality
test)
times-equal, divide-equal,
mod-equal, etc.

*= /= %= += -= <<= >>= &= combination assignment

,

comma

links a sequence of operations
LOWEST PRECEDENCE

In practice, all you really need to remember is the following:
• The "My Dear Aunt Sally" mantra (multiply, divide, add, subtract) for the familiar arithmetic
operations.
• The compound logical operators, &&, ||, -a, and -o have low precedence.
• The order of evaluation of equal-precedence operators is usually left-to-right.
Now, let's utilize our knowledge of operator precedence to analyze a couple of lines from the
/etc/init.d/functions file, as found in the Fedora Core Linux distro.
while [ -n "$remaining" -a "$retry" -gt 0 ]; do
# This looks rather daunting at first glance.

# Separate the conditions:
while [ -n "$remaining" -a "$retry" -gt 0 ]; do
#
--condition 1-- ^^ --condition 2#
#+
#+
#+
#+
#+
#
#
#+
#+
#+

If variable "$remaining" is not zero length
AND (-a)
variable "$retry" is greater-than zero
then
the [ expresion-within-condition-brackets ] returns success (0)
and the while-loop executes an iteration.
==============================================================
Evaluate "condition 1" and "condition 2" ***before***
ANDing them. Why? Because the AND (-a) has a lower precedence
than the -n and -gt operators,
and therefore gets evaluated *last*.

#################################################################
if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then

# Again, separate the conditions:
if [ -f /etc/sysconfig/i18n -a -z "${NOLOCALE:-}" ] ; then
#
--condition 1--------- ^^ --condition 2----# If file "/etc/sysconfig/i18n" exists
#+
AND (-a)
#+ variable $NOLOCALE is zero length
#+ then

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#+
#+
#
#
#+
#+
#
#
#
#
#+
#

the [ test-expresion-within-condition-brackets ] returns success (0)
and the commands following execute.
As before, the AND (-a) gets evaluated *last*
because it has the lowest precedence of the operators within
the test brackets.
==============================================================
Note:
${NOLOCALE:-} is a parameter expansion that seems redundant.
But, if $NOLOCALE has not been declared, it gets set to *null*,
in effect declaring it.
This makes a difference in some contexts.

To avoid confusion or error in a complex sequence of test operators, break up the sequence into
bracketed sections.
if [ "$v1" -gt "$v2" -o "$v1" -lt "$v2"
# Unclear what's going on here...

-a

-e "$filename" ]

if [[ "$v1" -gt "$v2" ]] || [[ "$v1" -lt "$v2" ]] && [[ -e "$filename" ]]
# Much better -- the condition tests are grouped in logical sections.

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Part 3. Beyond the Basics
Table of Contents
9. Another Look at Variables
9.1. Internal Variables
9.2. Typing variables: declare or typeset
9.3. $RANDOM: generate random integer
10. Manipulating Variables
10.1. Manipulating Strings
10.2. Parameter Substitution
11. Loops and Branches
11.1. Loops
11.2. Nested Loops
11.3. Loop Control
11.4. Testing and Branching
12. Command Substitution
13. Arithmetic Expansion
14. Recess Time

Part 3. Beyond the Basics

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Chapter 9. Another Look at Variables
Used properly, variables can add power and flexibility to scripts. This requires learning their subtleties and
nuances.

9.1. Internal Variables
Builtin variables:
variables affecting bash script behavior
$BASH
The path to the Bash binary itself
bash$ echo $BASH
/bin/bash

$BASH_ENV
An environmental variable pointing to a Bash startup file to be read when a script is invoked
$BASH_SUBSHELL
A variable indicating the subshell level. This is a new addition to Bash, version 3.
See Example 21-1 for usage.
$BASHPID
Process ID of the current instance of Bash. This is not the same as the $$ variable, but it often gives
the same result.
bash4$ echo $$
11015

bash4$ echo $BASHPID
11015

bash4$ ps ax | grep bash4
11015 pts/2
R
0:00 bash4

But ...
#!/bin/bash4
echo "\$\$ outside of subshell = $$"
echo "\$BASH_SUBSHELL outside of subshell = $BASH_SUBSHELL"
echo "\$BASHPID outside of subshell = $BASHPID"

# 9602
# 0
# 9602

echo
( echo "\$\$ inside of subshell = $$"
echo "\$BASH_SUBSHELL inside of subshell = $BASH_SUBSHELL"
echo "\$BASHPID inside of subshell = $BASHPID" )
# Note that $$ returns PID of parent process.

# 9602
# 1
# 9603

$BASH_VERSINFO[n]
A 6-element array containing version information about the installed release of Bash. This is similar
to $BASH_VERSION, below, but a bit more detailed.

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Advanced Bash-Scripting Guide
# Bash version info:
for n in 0 1 2 3 4 5
do
echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
done
#
#
#
#
#
#

BASH_VERSINFO[0]
BASH_VERSINFO[1]
BASH_VERSINFO[2]
BASH_VERSINFO[3]
BASH_VERSINFO[4]
BASH_VERSINFO[5]

=
=
=
=
=
=

3
00
14
1
release
i386-redhat-linux-gnu

#
#
#
#
#
#
#

Major version no.
Minor version no.
Patch level.
Build version.
Release status.
Architecture
(same as $MACHTYPE).

$BASH_VERSION
The version of Bash installed on the system
bash$ echo $BASH_VERSION
3.2.25(1)-release

tcsh% echo $BASH_VERSION
BASH_VERSION: Undefined variable.

Checking $BASH_VERSION is a good method of determining which shell is running. $SHELL does
not necessarily give the correct answer.
$CDPATH
A colon-separated list of search paths available to the cd command, similar in function to the $PATH
variable for binaries. The $CDPATH variable may be set in the local ~/.bashrc file.
bash$ cd bash-doc
bash: cd: bash-doc: No such file or directory

bash$ CDPATH=/usr/share/doc
bash$ cd bash-doc
/usr/share/doc/bash-doc

bash$ echo $PWD
/usr/share/doc/bash-doc

$DIRSTACK
The top value in the directory stack [40] (affected by pushd and popd)
This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the
directory stack.
$EDITOR
The default editor invoked by a script, usually vi or emacs.
$EUID
"effective" user ID number
Identification number of whatever identity the current user has assumed, perhaps by means of su.
The $EUID is not necessarily the same as the $UID.

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$FUNCNAME
Name of the current function
xyz23 ()
{
echo "$FUNCNAME now executing."
}

# xyz23 now executing.

xyz23
echo "FUNCNAME = $FUNCNAME"

# FUNCNAME =
# Null value outside a function.

See also Example A-50.
$GLOBIGNORE
A list of filename patterns to be excluded from matching in globbing.
$GROUPS
Groups current user belongs to
This is a listing (array) of the group id numbers for current user, as recorded in /etc/passwd and
/etc/group.
root# echo $GROUPS
0

root# echo ${GROUPS[1]}
1

root# echo ${GROUPS[5]}
6

$HOME
Home directory of the user, usually /home/username (see Example 10-7)
$HOSTNAME
The hostname command assigns the system host name at bootup in an init script. However, the
gethostname() function sets the Bash internal variable $HOSTNAME. See also Example 10-7.
$HOSTTYPE
host type
Like $MACHTYPE, identifies the system hardware.
bash$ echo $HOSTTYPE
i686

$IFS
internal field separator
This variable determines how Bash recognizes fields, or word boundaries, when it interprets character
strings.

$IFS defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a
comma-separated data file. Note that $* uses the first character held in $IFS. See Example 5-1.

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Advanced Bash-Scripting Guide
bash$ echo "$IFS"
(With $IFS set to default, a blank line displays.)

bash$ echo "$IFS" | cat -vte
^I$
$
(Show whitespace: here a single space, ^I [horizontal tab],
and newline, and display "$" at end-of-line.)

bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"'
w:x:y:z
(Read commands from string and assign any arguments to pos params.)

$IFS does not handle whitespace the same as it does other characters.
Example 9-1. $IFS and whitespace
#!/bin/bash
# ifs.sh

var1="a+b+c"
var2="d-e-f"
var3="g,h,i"
IFS=+
# The plus sign will be interpreted as a separator.
echo $var1
# a b c
echo $var2
# d-e-f
echo $var3
# g,h,i
echo
IFS="-"
# The plus sign reverts to default interpretation.
# The minus sign will be interpreted as a separator.
echo $var1
# a+b+c
echo $var2
# d e f
echo $var3
# g,h,i
echo
IFS=","
# The comma will be interpreted as a separator.
# The minus sign reverts to default interpretation.
echo $var1
# a+b+c
echo $var2
# d-e-f
echo $var3
# g h i
echo
IFS=" "
# The space character will be interpreted as a separator.
# The comma reverts to default interpretation.

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Advanced Bash-Scripting Guide
echo $var1
echo $var2
echo $var3

# a+b+c
# d-e-f
# g,h,i

# ======================================================== #
# However ...
# $IFS treats whitespace differently than other characters.
output_args_one_per_line()
{
for arg
do
echo "[$arg]"
done # ^
^
Embed within brackets, for your viewing pleasure.
}
echo; echo "IFS=\" \""
echo "-------"
IFS=" "
var=" a b c
"
#
^ ^^
^^^
output_args_one_per_line $var
# [a]
# [b]
# [c]

# output_args_one_per_line `echo " a

b c

"`

echo; echo "IFS=:"
echo "-----"
IFS=:
var=":a::b:c:::"
#
^ ^^
^^^
output_args_one_per_line $var
# []
# [a]
# []
# [b]
# [c]
# []
# []

# Same pattern as above,
#+ but substituting ":" for " "

...

# Note "empty" brackets.
# The same thing happens with the "FS" field separator in awk.

echo
exit

(Many thanks, Stéphane Chazelas, for clarification and above examples.)
See also Example 16-41, Example 11-7, and Example 19-14 for instructive examples of using $IFS.
$IGNOREEOF
Ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out.
$LC_COLLATE
Often set in the .bashrc or /etc/profile files, this variable controls collation order in filename
expansion and pattern matching. If mishandled, LC_COLLATE can cause unexpected results in
filename globbing.
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As of version 2.05 of Bash, filename globbing no longer distinguishes between
lowercase and uppercase letters in a character range between brackets. For example, ls
[A-M]* would match both File1.txt and file1.txt. To revert to the customary
behavior of bracket matching, set LC_COLLATE to C by an export
LC_COLLATE=C in /etc/profile and/or ~/.bashrc.
$LC_CTYPE
This internal variable controls character interpretation in globbing and pattern matching.
$LINENO
This variable is the line number of the shell script in which this variable appears. It has significance
only within the script in which it appears, and is chiefly useful for debugging purposes.
# *** BEGIN DEBUG BLOCK ***
last_cmd_arg=$_ # Save it.
echo "At line number $LINENO, variable \"v1\" = $v1"
echo "Last command argument processed = $last_cmd_arg"
# *** END DEBUG BLOCK ***

$MACHTYPE
machine type
Identifies the system hardware.
bash$ echo $MACHTYPE
i686

$OLDPWD
Old working directory ("OLD-Print-Working-Directory", previous directory you were in).
$OSTYPE
operating system type
bash$ echo $OSTYPE
linux

$PATH
Path to binaries, usually /usr/bin/, /usr/X11R6/bin/, /usr/local/bin, etc.
When given a command, the shell automatically does a hash table search on the directories listed in
the path for the executable. The path is stored in the environmental variable, $PATH, a list of
directories, separated by colons. Normally, the system stores the $PATH definition in
/etc/profile and/or ~/.bashrc (see Appendix G).
bash$ echo $PATH
/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin

PATH=${PATH}:/opt/bin appends the /opt/bin directory to the current path. In a script, it
may be expedient to temporarily add a directory to the path in this way. When the script exits, this
restores the original $PATH (a child process, such as a script, may not change the environment of the
parent process, the shell).

The current "working directory", ./, is usually omitted from the $PATH as a security
measure.
$PIPESTATUS
Array variable holding exit status(es) of last executed foreground pipe.

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bash$ echo $PIPESTATUS
0
bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo ${PIPESTATUS[1]}
127
bash$ ls -al | bogus_command
bash: bogus_command: command not found
bash$ echo $?
127

The members of the $PIPESTATUS array hold the exit status of each respective command executed
in a pipe. $PIPESTATUS[0] holds the exit status of the first command in the pipe,
$PIPESTATUS[1] the exit status of the second command, and so on.
The $PIPESTATUS variable may contain an erroneous 0 value in a login shell (in
releases prior to 3.0 of Bash).
tcsh% bash
bash$ who | grep nobody | sort
bash$ echo ${PIPESTATUS[*]}
0

The above lines contained in a script would produce the expected 0 1 0 output.
Thank you, Wayne Pollock for pointing this out and supplying the above example.
The $PIPESTATUS variable gives unexpected results in some contexts.
bash$ echo $BASH_VERSION
3.00.14(1)-release
bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
0
0
0
bash$ echo ${PIPESTATUS[@]}
141 127 0

Chet Ramey attributes the above output to the behavior of ls. If ls writes to a pipe
whose output is not read, then SIGPIPE kills it, and its exit status is 141. Otherwise
its exit status is 0, as expected. This likewise is the case for tr.
$PIPESTATUS is a "volatile" variable. It needs to be captured immediately after the
pipe in question, before any other command intervenes.
bash$ $ ls | bogus_command | wc
bash: bogus_command: command not found
0
0
0
bash$ echo ${PIPESTATUS[@]}
0 127 0

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bash$ echo ${PIPESTATUS[@]}
0

The pipefail option may be useful in cases where $PIPESTATUS does not give the
desired information.
$PPID
The $PPID of a process is the process ID (pid) of its parent process. [41]
Compare this with the pidof command.
$PROMPT_COMMAND
A variable holding a command to be executed just before the primary prompt, $PS1 is to be
displayed.
$PS1
This is the main prompt, seen at the command-line.
$PS2
The secondary prompt, seen when additional input is expected. It displays as ">".
$PS3
The tertiary prompt, displayed in a select loop (see Example 11-29).
$PS4
The quartenary prompt, shown at the beginning of each line of output when invoking a script with the
-x option. It displays as "+".
$PWD
Working directory (directory you are in at the time)
This is the analog to the pwd builtin command.
#!/bin/bash
E_WRONG_DIRECTORY=65
clear # Clear the screen.
TargetDirectory=/home/bozo/projects/GreatAmericanNovel
cd $TargetDirectory
echo "Deleting stale files in $TargetDirectory."
if [ "$PWD" != "$TargetDirectory" ]
then
# Keep from wiping out wrong directory by accident.
echo "Wrong directory!"
echo "In $PWD, rather than $TargetDirectory!"
echo "Bailing out!"
exit $E_WRONG_DIRECTORY
fi
rm -rf *
rm .[A-Za-z0-9]*
# Delete dotfiles.
# rm -f .[^.]* ..?*
to remove filenames beginning with multiple dots.
# (shopt -s dotglob; rm -f *)
will also work.
# Thanks, S.C. for pointing this out.
# A filename (`basename`) may contain all characters in the 0 - 255 range,
#+ except "/".
# Deleting files beginning with weird characters, such as -

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#+ is left as an exercise. (Hint: rm ./-weirdname or rm -- -weirdname)
echo
ls -al
# Any files left?
echo "Done."
echo "Old files deleted in $TargetDirectory."
echo
# Various other operations here, as necessary.
exit $?

$REPLY
The default value when a variable is not supplied to read. Also applicable to select menus, but only
supplies the item number of the variable chosen, not the value of the variable itself.
#!/bin/bash
# reply.sh
# REPLY is the default value for a 'read' command.
echo
echo -n "What is your favorite vegetable? "
read
echo "Your favorite vegetable is $REPLY."
# REPLY holds the value of last "read" if and only if
#+ no variable supplied.
echo
echo -n "What is your favorite fruit? "
read fruit
echo "Your favorite fruit is $fruit."
echo "but..."
echo "Value of \$REPLY is still $REPLY."
# $REPLY is still set to its previous value because
#+ the variable $fruit absorbed the new "read" value.
echo
exit 0

$SECONDS
The number of seconds the script has been running.
#!/bin/bash
TIME_LIMIT=10
INTERVAL=1
echo
echo "Hit Control-C to exit before $TIME_LIMIT seconds."
echo
while [ "$SECONDS" -le "$TIME_LIMIT" ]
do
if [ "$SECONDS" -eq 1 ]
then
units=second
else
units=seconds
fi

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echo "This script has been running $SECONDS $units."
# On a slow or overburdened machine, the script may skip a count
#+ every once in a while.
sleep $INTERVAL
done
echo -e "\a"

# Beep!

exit 0

$SHELLOPTS
The list of enabled shell options, a readonly variable.
bash$ echo $SHELLOPTS
braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs

$SHLVL
Shell level, how deeply Bash is nested. [42] If, at the command-line, $SHLVL is 1, then in a script it
will increment to 2.
This variable is not affected by subshells. Use $BASH_SUBSHELL when you need
an indication of subshell nesting.
$TMOUT
If the $TMOUT environmental variable is set to a non-zero value time, then the shell prompt will
time out after $time seconds. This will cause a logout.
As of version 2.05b of Bash, it is now possible to use $TMOUT in a script in combination with read.
# Works in scripts for Bash, versions 2.05b and later.
TMOUT=3

# Prompt times out at three seconds.

echo "What is your favorite song?"
echo "Quickly now, you only have $TMOUT seconds to answer!"
read song
if [ -z "$song" ]
then
song="(no answer)"
# Default response.
fi
echo "Your favorite song is $song."

There are other, more complex, ways of implementing timed input in a script. One alternative is to set
up a timing loop to signal the script when it times out. This also requires a signal handling routine to
trap (see Example 32-5) the interrupt generated by the timing loop (whew!).

Example 9-2. Timed Input
#!/bin/bash
# timed-input.sh
# TMOUT=3

Also works, as of newer versions of Bash.

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TIMER_INTERRUPT=14
TIMELIMIT=3 # Three seconds in this instance.
# May be set to different value.
PrintAnswer()
{
if [ "$answer" = TIMEOUT ]
then
echo $answer
else
# Don't want to mix up the two instances.
echo "Your favorite veggie is $answer"
kill $! # Kills no-longer-needed TimerOn function
#+ running in background.
# $! is PID of last job running in background.
fi
}

TimerOn()
{
sleep $TIMELIMIT && kill -s 14 $$ &
# Waits 3 seconds, then sends sigalarm to script.
}

Int14Vector()
{
answer="TIMEOUT"
PrintAnswer
exit $TIMER_INTERRUPT
}
trap Int14Vector $TIMER_INTERRUPT
# Timer interrupt (14) subverted for our purposes.
echo "What is your favorite vegetable "
TimerOn
read answer
PrintAnswer

#
#
#
#
#+

Admittedly, this is a kludgy implementation of timed input.
However, the "-t" option to "read" simplifies this task.
See the "t-out.sh" script.
However, what about timing not just single user input,
but an entire script?

# If you need something really elegant ...
#+ consider writing the application in C or C++,
#+ using appropriate library functions, such as 'alarm' and 'setitimer.'
exit 0

An alternative is using stty.

Example 9-3. Once more, timed input
#!/bin/bash

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# timeout.sh
# Written by Stephane Chazelas,
#+ and modified by the document author.
INTERVAL=5

# timeout interval

timedout_read() {
timeout=$1
varname=$2
old_tty_settings=`stty -g`
stty -icanon min 0 time ${timeout}0
eval read $varname
# or just read $varname
stty "$old_tty_settings"
# See man page for "stty."
}
echo; echo -n "What's your name? Quick! "
timedout_read $INTERVAL your_name
# This may not work on every terminal type.
# The maximum timeout depends on the terminal.
#+ (it is often 25.5 seconds).
echo
if [ ! -z "$your_name" ] # If name input before timeout ...
then
echo "Your name is $your_name."
else
echo "Timed out."
fi
echo
# The behavior of this script differs somewhat from "timed-input.sh."
# At each keystroke, the counter resets.
exit 0

Perhaps the simplest method is using the -t option to read.

Example 9-4. Timed read
#!/bin/bash
# t-out.sh
# Inspired by a suggestion from "syngin seven" (thanks).

TIMELIMIT=4

# 4 seconds

read -t $TIMELIMIT variable <&1
#
^^^
# In this instance, "<&1" is needed for Bash 1.x and 2.x,
# but unnecessary for Bash 3.x.
echo
if [ -z "$variable" ] # Is null?
then
echo "Timed out, variable still unset."

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else
echo "variable = $variable"
fi
exit 0

$UID
User ID number
Current user's user identification number, as recorded in /etc/passwd
This is the current user's real id, even if she has temporarily assumed another identity through su.
$UID is a readonly variable, not subject to change from the command line or within a script, and is
the counterpart to the id builtin.

Example 9-5. Am I root?
#!/bin/bash
# am-i-root.sh:
ROOT_UID=0

Am I root or not?

# Root has $UID 0.

if [ "$UID" -eq "$ROOT_UID" ] # Will the real "root" please stand up?
then
echo "You are root."
else
echo "You are just an ordinary user (but mom loves you just the same)."
fi
exit 0

# ============================================================= #
# Code below will not execute, because the script already exited.
# An alternate method of getting to the root of matters:
ROOTUSER_NAME=root
username=`id -nu`
# Or...
username=`whoami`
if [ "$username" = "$ROOTUSER_NAME" ]
then
echo "Rooty, toot, toot. You are root."
else
echo "You are just a regular fella."
fi

See also Example 2-3.
The variables $ENV, $LOGNAME, $MAIL, $TERM, $USER, and $USERNAME are not
Bash builtins. These are, however, often set as environmental variables in one of the
Bash startup files. $SHELL, the name of the user's login shell, may be set from
/etc/passwd or in an "init" script, and it is likewise not a Bash builtin.
tcsh% echo $LOGNAME
bozo
tcsh% echo $SHELL
/bin/tcsh

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tcsh% echo $TERM
rxvt
bash$ echo $LOGNAME
bozo
bash$ echo $SHELL
/bin/tcsh
bash$ echo $TERM
rxvt

Positional Parameters
$0, $1, $2, etc.
Positional parameters, passed from command line to script, passed to a function, or set to a variable
(see Example 4-5 and Example 15-16)
$#
Number of command-line arguments [43] or positional parameters (see Example 36-2)
$*
All of the positional parameters, seen as a single word
"$*" must be quoted.
$@
Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without
interpretation or expansion. This means, among other things, that each parameter in the argument list
is seen as a separate word.
Of course, "$@" should be quoted.
Example 9-6. arglist: Listing arguments with $* and $@
#!/bin/bash
# arglist.sh
# Invoke this script with several arguments, such as "one two three".
E_BADARGS=65
if [ ! -n "$1" ]
then
echo "Usage: `basename $0` argument1 argument2 etc."
exit $E_BADARGS
fi
echo
index=1

# Initialize count.

echo "Listing args with \"\$*\":"
for arg in "$*" # Doesn't work properly if "$*" isn't quoted.
do
echo "Arg #$index = $arg"
let "index+=1"
done
# $* sees all arguments as single word.
echo "Entire arg list seen as single word."
echo

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index=1

# Reset count.
# What happens if you forget to do this?

echo "Listing args with \"\$@\":"
for arg in "$@"
do
echo "Arg #$index = $arg"
let "index+=1"
done
# $@ sees arguments as separate words.
echo "Arg list seen as separate words."
echo
index=1

# Reset count.

echo "Listing args with \$* (unquoted):"
for arg in $*
do
echo "Arg #$index = $arg"
let "index+=1"
done
# Unquoted $* sees arguments as separate words.
echo "Arg list seen as separate words."
exit 0

Following a shift, the $@ holds the remaining command-line parameters, lacking the previous $1,
which was lost.
#!/bin/bash
# Invoke with ./scriptname 1 2 3 4 5
echo "$@"
shift
echo "$@"
shift
echo "$@"

# 1 2 3 4 5
# 2 3 4 5
# 3 4 5

# Each "shift" loses parameter $1.
# "$@" then contains the remaining parameters.

The $@ special parameter finds use as a tool for filtering input into shell scripts. The cat "$@"
construction accepts input to a script either from stdin or from files given as parameters to the
script. See Example 16-24 and Example 16-25.
The $* and $@ parameters sometimes display inconsistent and puzzling behavior,
depending on the setting of $IFS.
Example 9-7. Inconsistent $* and $@ behavior
#!/bin/bash
# Erratic behavior of the "$*" and "$@" internal Bash variables,
#+ depending on whether they are quoted or not.
# Inconsistent handling of word splitting and linefeeds.

set -- "First one" "second" "third:one" "" "Fifth: :one"
# Setting the script arguments, $1, $2, etc.
echo

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echo 'IFS unchanged, using "$*"'
c=0
for i in "$*"
# quoted
do echo "$((c+=1)): [$i]"
# This line remains the same in every instance.
# Echo args.
done
echo --echo 'IFS unchanged, using $*'
c=0
for i in $*
# unquoted
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS unchanged, using "$@"'
c=0
for i in "$@"
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS unchanged, using $@'
c=0
for i in $@
do echo "$((c+=1)): [$i]"
done
echo --IFS=:
echo 'IFS=":", using "$*"'
c=0
for i in "$*"
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS=":", using $*'
c=0
for i in $*
do echo "$((c+=1)): [$i]"
done
echo --var=$*
echo 'IFS=":", using "$var" (var=$*)'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS=":", using $var (var=$*)'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo --var="$*"
echo 'IFS=":", using $var (var="$*")'

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c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS=":", using "$var" (var="$*")'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS=":", using "$@"'
c=0
for i in "$@"
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS=":", using $@'
c=0
for i in $@
do echo "$((c+=1)): [$i]"
done
echo --var=$@
echo 'IFS=":", using $var (var=$@)'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS=":", using "$var" (var=$@)'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo --var="$@"
echo 'IFS=":", using "$var" (var="$@")'
c=0
for i in "$var"
do echo "$((c+=1)): [$i]"
done
echo --echo 'IFS=":", using $var (var="$@")'
c=0
for i in $var
do echo "$((c+=1)): [$i]"
done
echo
# Try this script with ksh or zsh -y.
exit 0

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# This example script by Stephane Chazelas,
# and slightly modified by the document author.

The $@ and $* parameters differ only when between double quotes.
Example 9-8. $* and $@ when $IFS is empty
#!/bin/bash
# If $IFS set, but empty,
#+ then "$*" and "$@" do not echo positional params as expected.
mecho ()
# Echo positional parameters.
{
echo "$1,$2,$3";
}

IFS=""
set a b c

# Set, but empty.
# Positional parameters.

mecho "$*"
#
mecho $*

# abc,,
^^
# a,b,c

mecho $@
mecho "$@"

# a,b,c
# a,b,c

# The behavior of $* and $@ when $IFS is empty depends
#+ on which Bash or sh version being run.
# It is therefore inadvisable to depend on this "feature" in a script.

# Thanks, Stephane Chazelas.
exit

Other Special Parameters
$Flags passed to script (using set). See Example 15-16.
This was originally a ksh construct adopted into Bash, and unfortunately it does not
seem to work reliably in Bash scripts. One possible use for it is to have a script
self-test whether it is interactive.
$!
PID (process ID) of last job run in background
LOG=$0.log
COMMAND1="sleep 100"
echo "Logging PIDs background commands for script: $0" >> "$LOG"
# So they can be monitored, and killed as necessary.
echo >> "$LOG"
# Logging commands.

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echo -n "PID of \"$COMMAND1\":
${COMMAND1} &
echo $! >> "$LOG"
# PID of "sleep 100": 1506

" >> "$LOG"

# Thank you, Jacques Lederer, for suggesting this.

Using $! for job control:
possibly_hanging_job & { sleep ${TIMEOUT}; eval 'kill -9 $!' &> /dev/null; }
# Forces completion of an ill-behaved program.
# Useful, for example, in init scripts.
# Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.

Or, alternately:
# This example by Matthew Sage.
# Used with permission.
TIMEOUT=30
count=0

# Timeout value in seconds

possibly_hanging_job & {
while ((count < TIMEOUT )); do
eval '[ ! -d "/proc/$!" ] && ((count = TIMEOUT))'
# /proc is where information about running processes is found.
# "-d" tests whether it exists (whether directory exists).
# So, we're waiting for the job in question to show up.
((count++))
sleep 1
done
eval '[ -d "/proc/$!" ] && kill -15 $!'
# If the hanging job is running, kill it.
}

$_
Special variable set to final argument of previous command executed.

Example 9-9. Underscore variable
#!/bin/bash
echo $_

#
#
#
#+

/bin/bash
Just called /bin/bash to run the script.
Note that this will vary according to
how the script is invoked.

du >/dev/null
echo $_

#
#

So no output from command.
du

ls -al >/dev/null
echo $_

#
#

So no output from command.
-al (last argument)

:
echo $_

#

:

$?
Exit status of a command, function, or the script itself (see Example 24-7)
$$
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Process ID (PID) of the script itself. [44] The $$ variable often finds use in scripts to construct
"unique" temp file names (see Example 32-6, Example 16-31, and Example 15-27). This is usually
simpler than invoking mktemp.

9.2. Typing variables: declare or typeset
The declare or typeset builtins, which are exact synonyms, permit modifying the properties of variables. This
is a very weak form of the typing [45] available in certain programming languages. The declare command is
specific to version 2 or later of Bash. The typeset command also works in ksh scripts.
declare/typeset options
-r readonly
(declare -r var1 works the same as readonly var1)
This is the rough equivalent of the C const type qualifier. An attempt to change the value of a
readonly variable fails with an error message.
declare -r var1=1
echo "var1 = $var1"

# var1 = 1

(( var1++ ))

# x.sh: line 4: var1: readonly variable

-i integer
declare -i number
# The script will treat subsequent occurrences of "number" as an integer.
number=3
echo "Number = $number"

# Number = 3

number=three
echo "Number = $number"
# Number = 0
# Tries to evaluate the string "three" as an integer.

Certain arithmetic operations are permitted for declared integer variables without the need for expr or
let.
n=6/3
echo "n = $n"

# n = 6/3

declare -i n
n=6/3
echo "n = $n"

# n = 2

-a array
declare -a indices

The variable indices will be treated as an array.
-f function(s)
declare -f

A declare -f line with no arguments in a script causes a listing of all the functions previously
defined in that script.

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declare -f function_name

A declare -f function_name in a script lists just the function named.
-x export
declare -x var3

This declares a variable as available for exporting outside the environment of the script itself.
-x var=$value
declare -x var3=373

The declare command permits assigning a value to a variable in the same statement as setting its
properties.

Example 9-10. Using declare to type variables
#!/bin/bash
func1 ()
{
echo This is a function.
}
declare -f

# Lists the function above.

echo
declare -i var1
# var1 is an integer.
var1=2367
echo "var1 declared as $var1"
var1=var1+1
# Integer declaration eliminates the need for 'let'.
echo "var1 incremented by 1 is $var1."
# Attempt to change variable declared as integer.
echo "Attempting to change var1 to floating point value, 2367.1."
var1=2367.1
# Results in error message, with no change to variable.
echo "var1 is still $var1"
echo
declare -r var2=13.36

# 'declare' permits setting a variable property
#+ and simultaneously assigning it a value.
echo "var2 declared as $var2" # Attempt to change readonly variable.
var2=13.37
# Generates error message, and exit from script.
echo "var2 is still $var2"

# This line will not execute.

exit 0

# Script will not exit here.

Using the declare builtin restricts the scope of a variable.
foo ()
{
FOO="bar"
}
bar ()
{
foo

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echo $FOO
}
bar

# Prints bar.

However . . .
foo (){
declare FOO="bar"
}
bar ()
{
foo
echo $FOO
}
bar

# Prints nothing.

# Thank you, Michael Iatrou, for pointing this out.

9.2.1. Another use for declare
The declare command can be helpful in identifying variables, environmental or otherwise. This can be
especially useful with arrays.
bash$ declare | grep HOME
HOME=/home/bozo

bash$ zzy=68
bash$ declare | grep zzy
zzy=68

bash$ Colors=([0]="purple" [1]="reddish-orange" [2]="light green")
bash$ echo ${Colors[@]}
purple reddish-orange light green
bash$ declare | grep Colors
Colors=([0]="purple" [1]="reddish-orange" [2]="light green")

9.3. $RANDOM: generate random integer
Anyone who attempts to generate random
numbers by deterministic means is, of course,
living in a state of sin.
--John von Neumann
$RANDOM is an internal Bash function (not a constant) that returns a pseudorandom [46] integer in the range 0
- 32767. It should not be used to generate an encryption key.

Example 9-11. Generating random numbers
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#!/bin/bash
# $RANDOM returns a different random integer at each invocation.
# Nominal range: 0 - 32767 (signed 16-bit integer).
MAXCOUNT=10
count=1
echo
echo "$MAXCOUNT random numbers:"
echo "-----------------"
while [ "$count" -le $MAXCOUNT ]
# Generate 10 ($MAXCOUNT) random integers.
do
number=$RANDOM
echo $number
let "count += 1" # Increment count.
done
echo "-----------------"
# If you need a random int within a certain range, use the 'modulo' operator.
# This returns the remainder of a division operation.
RANGE=500
echo
number=$RANDOM
let "number %= $RANGE"
#
^^
echo "Random number less than $RANGE

---

$number"

echo

# If you need a random integer greater than a lower bound,
#+ then set up a test to discard all numbers below that.
FLOOR=200
number=0
#initialize
while [ "$number" -le $FLOOR ]
do
number=$RANDOM
done
echo "Random number greater than $FLOOR --echo

$number"

# Let's examine a simple alternative to the above loop, namely
#
let "number = $RANDOM + $FLOOR"
# That would eliminate the while-loop and run faster.
# But, there might be a problem with that. What is it?

# Combine above two techniques to retrieve random number between two limits.
number=0
#initialize
while [ "$number" -le $FLOOR ]
do
number=$RANDOM
let "number %= $RANGE" # Scales $number down within $RANGE.
done

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echo "Random number between $FLOOR and $RANGE --echo

$number"

# Generate binary choice, that is, "true" or "false" value.
BINARY=2
T=1
number=$RANDOM
let "number %= $BINARY"
# Note that
let "number >>= 14"
gives a better random distribution
#+ (right shifts out everything except last binary digit).
if [ "$number" -eq $T ]
then
echo "TRUE"
else
echo "FALSE"
fi
echo

# Generate a toss of the dice.
SPOTS=6
# Modulo 6 gives range 0 - 5.
# Incrementing by 1 gives desired range of 1 - 6.
# Thanks, Paulo Marcel Coelho Aragao, for the simplification.
die1=0
die2=0
# Would it be better to just set SPOTS=7 and not add 1? Why or why not?
# Tosses each die separately, and so gives correct odds.
let "die1 = $RANDOM % $SPOTS +1" # Roll first one.
let "die2 = $RANDOM % $SPOTS +1" # Roll second one.
# Which arithmetic operation, above, has greater precedence -#+ modulo (%) or addition (+)?

let "throw = $die1 + $die2"
echo "Throw of the dice = $throw"
echo

exit 0

Example 9-12. Picking a random card from a deck
#!/bin/bash
# pick-card.sh
# This is an example of choosing random elements of an array.

# Pick a card, any card.
Suites="Clubs
Diamonds
Hearts
Spades"

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Denominations="2
3
4
5
6
7
8
9
10
Jack
Queen
King
Ace"
# Note variables spread over multiple lines.

suite=($Suites)
denomination=($Denominations)

# Read into array variable.

num_suites=${#suite[*]}
# Count how many elements.
num_denominations=${#denomination[*]}
echo -n "${denomination[$((RANDOM%num_denominations))]} of "
echo ${suite[$((RANDOM%num_suites))]}

# $bozo sh pick-cards.sh
# Jack of Clubs

# Thank you, "jipe," for pointing out this use of $RANDOM.
exit 0

Example 9-13. Brownian Motion Simulation
#!/bin/bash
# brownian.sh
# Author: Mendel Cooper
# Reldate: 10/26/07
# License: GPL3
#
#
#+
#+
#+

---------------------------------------------------------------This script models Brownian motion:
the random wanderings of tiny particles in a fluid,
as they are buffeted by random currents and collisions.
This is colloquially known as the "Drunkard's Walk."

#
#+
#+
#+
#+
#
#
#+
#+
#
#+
#+

It can also be considered as a stripped-down simulation of a
Galton Board, a slanted board with a pattern of pegs,
down which rolls a succession of marbles, one at a time.
At the bottom is a row of slots or catch basins in which
the marbles come to rest at the end of their journey.
Think of it as a kind of bare-bones Pachinko game.
As you see by running the script,
most of the marbles cluster around the center slot.
This is consistent with the expected binomial distribution.
As a Galton Board simulation, the script
disregards such parameters as
board tilt-angle, rolling friction of the marbles,

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#+ angles of impact, and elasticity of the pegs.
# To what extent does this affect the accuracy of the simulation?
# ---------------------------------------------------------------PASSES=500
ROWS=10
RANGE=3
POS=0
RANDOM=$$

#
#
#
#
#
#+

Number of particle interactions / marbles.
Number of "collisions" (or horiz. peg rows).
0 - 2 output range from $RANDOM.
Left/right position.
Seeds the random number generator from PID
of script.

declare -a Slots
NUMSLOTS=21

# Array holding cumulative results of passes.
# Number of slots at bottom of board.

Initialize_Slots () { # Zero out all elements of the array.
for i in $( seq $NUMSLOTS )
do
Slots[$i]=0
done
echo
}

# Blank line at beginning of run.

Show_Slots () {
echo; echo
echo -n " "
for i in $( seq $NUMSLOTS )
do
printf "%3d" ${Slots[$i]}
done
echo
echo
echo
echo

# Pretty-print array elements.
# Allot three spaces per result.

# Row of slots:
" |__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|__|"
"
||"
# Note that if the count within any particular slot exceeds 99,
#+ it messes up the display.
# Running only(!) 500 passes usually avoids this.

}

Move () {
Move=$RANDOM
let "Move %= RANGE"
case "$Move" in
0 ) ;;
1 ) ((POS--));;
2 ) ((POS++));;
* ) echo -n "Error
esac
}

# Move one unit right / left, or stay put.
# How random is $RANDOM? Well, let's see ...
# Normalize into range of 0 - 2.

";;

Play () {
i=0
while [ "$i" -lt "$ROWS" ]
do
Move
((i++));
done

#
#
#
#

Do nothing, i.e., stay in place.
Left.
Right.
Anomaly! (Should never occur.)

# Single pass (inner loop).
# One event per row.

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SHIFT=11
let "POS += $SHIFT"
(( Slots[$POS]++ ))

# Why 11, and not 10?
# Shift "zero position" to center.
# DEBUG: echo $POS

# echo -n "$POS "
}

Run () {
# Outer loop.
p=0
while [ "$p" -lt "$PASSES" ]
do
Play
(( p++ ))
POS=0
# Reset to zero. Why?
done
}

# -------------# main ()
Initialize_Slots
Run
Show_Slots
# -------------exit $?
#
#
#
#+
#
#
#
#

Exercises:
--------1) Show the results in a vertical bar graph, or as an alternative,
a scattergram.
2) Alter the script to use /dev/urandom instead of $RANDOM.
Will this make the results more random?
3) Provide some sort of "animation" or graphic output
for each marble played.

Jipe points out a set of techniques for generating random numbers within a range.
#

Generate random number between 6 and 30.
rnumber=$((RANDOM%25+6))

# Generate random number in the same 6 - 30 range,
#+ but the number must be evenly divisible by 3.
rnumber=$(((RANDOM%30/3+1)*3))
#
#

Note that this will not work all the time.
It fails if $RANDOM%30 returns 0.

#

Frank Wang suggests the following alternative:
rnumber=$(( RANDOM%27/3*3+6 ))

Bill Gradwohl came up with an improved formula that works for positive numbers.
rnumber=$(((RANDOM%(max-min+divisibleBy))/divisibleBy*divisibleBy+min))

Here Bill presents a versatile function that returns a random number between two specified values.

Example 9-14. Random between values

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#!/bin/bash
# random-between.sh
# Random number between two specified values.
# Script by Bill Gradwohl, with minor modifications by the document author.
# Corrections in lines 187 and 189 by Anthony Le Clezio.
# Used with permission.

randomBetween() {
# Generates a positive or negative random number
#+ between $min and $max
#+ and divisible by $divisibleBy.
# Gives a "reasonably random" distribution of return values.
#
# Bill Gradwohl - Oct 1, 2003
syntax() {
# Function
echo
echo
echo
echo -n
echo
echo
echo
echo -n
echo
echo
echo
echo
echo -n
echo
echo
echo -n
echo
echo -n
echo
}

embedded within function.
"Syntax: randomBetween [min] [max] [multiple]"
"Expects up to 3 passed parameters, "
"but all are completely optional."
"min is the minimum value"
"max is the maximum value"
"multiple specifies that the answer must be "
"a multiple of this value."
"
i.e. answer must be evenly divisible by this number."
"If any value is missing, defaults area supplied as: 0 32767 1"
"Successful completion returns 0, "
"unsuccessful completion returns"
"function syntax and 1."
"The answer is returned in the global variable "
"randomBetweenAnswer"
"Negative values for any passed parameter are "
"handled correctly."

local min=${1:-0}
local max=${2:-32767}
local divisibleBy=${3:-1}
# Default values assigned, in case parameters not passed to function.
local x
local spread
# Let's make sure the divisibleBy value is positive.
[ ${divisibleBy} -lt 0 ] && divisibleBy=$((0-divisibleBy))
# Sanity check.
if [ $# -gt 3 -o ${divisibleBy} -eq 0 -o
syntax
return 1
fi

${min} -eq ${max} ]; then

# See if the min and max are reversed.
if [ ${min} -gt ${max} ]; then
# Swap them.
x=${min}
min=${max}
max=${x}

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fi
# If min is itself not evenly divisible by $divisibleBy,
#+ then fix the min to be within range.
if [ $((min/divisibleBy*divisibleBy)) -ne ${min} ]; then
if [ ${min} -lt 0 ]; then
min=$((min/divisibleBy*divisibleBy))
else
min=$((((min/divisibleBy)+1)*divisibleBy))
fi
fi
# If max is itself not evenly divisible by $divisibleBy,
#+ then fix the max to be within range.
if [ $((max/divisibleBy*divisibleBy)) -ne ${max} ]; then
if [ ${max} -lt 0 ]; then
max=$((((max/divisibleBy)-1)*divisibleBy))
else
max=$((max/divisibleBy*divisibleBy))
fi
fi
#
#

--------------------------------------------------------------------Now, to do the real work.

# Note that to get a proper distribution for the end points,
#+ the range of random values has to be allowed to go between
#+ 0 and abs(max-min)+divisibleBy, not just abs(max-min)+1.
# The slight increase will produce the proper distribution for the
#+ end points.
#
#+
#+
#+
#

Changing the formula to use abs(max-min)+1 will still produce
correct answers, but the randomness of those answers is faulty in
that the number of times the end points ($min and $max) are returned
is considerably lower than when the correct formula is used.
---------------------------------------------------------------------

spread=$((max-min))
# Omair Eshkenazi points out that this test is unnecessary,
#+ since max and min have already been switched around.
[ ${spread} -lt 0 ] && spread=$((0-spread))
let spread+=divisibleBy
randomBetweenAnswer=$(((RANDOM%spread)/divisibleBy*divisibleBy+min))
return 0
#
#+
#+
#
#
#

However, Paulo Marcel Coelho Aragao points out that
when $max and $min are not divisible by $divisibleBy,
the formula fails.
He suggests instead the following formula:
rnumber = $(((RANDOM%(max-min+1)+min)/divisibleBy*divisibleBy))

}
# Let's test the function.
min=-14
max=20
divisibleBy=3

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# Generate an array of expected answers and check to make sure we get
#+ at least one of each answer if we loop long enough.
declare -a answer
minimum=${min}
maximum=${max}
if [ $((minimum/divisibleBy*divisibleBy)) -ne ${minimum} ]; then
if [ ${minimum} -lt 0 ]; then
minimum=$((minimum/divisibleBy*divisibleBy))
else
minimum=$((((minimum/divisibleBy)+1)*divisibleBy))
fi
fi

# If max is itself not evenly divisible by $divisibleBy,
#+ then fix the max to be within range.
if [ $((maximum/divisibleBy*divisibleBy)) -ne ${maximum} ]; then
if [ ${maximum} -lt 0 ]; then
maximum=$((((maximum/divisibleBy)-1)*divisibleBy))
else
maximum=$((maximum/divisibleBy*divisibleBy))
fi
fi

# We need to generate only positive array subscripts,
#+ so we need a displacement that that will guarantee
#+ positive results.
disp=$((0-minimum))
for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
answer[i+disp]=0
done

# Now loop a large number of times to see what we get.
loopIt=1000
# The script author suggests 100000,
#+ but that takes a good long while.
for ((i=0; i<${loopIt}; ++i)); do
# Note that we are specifying min and max in reversed order here to
#+ make the function correct for this case.
randomBetween ${max} ${min} ${divisibleBy}
# Report an error if an answer is unexpected.
[ ${randomBetweenAnswer} -lt ${min} -o ${randomBetweenAnswer} -gt ${max} ] \
&& echo MIN or MAX error - ${randomBetweenAnswer}!
[ $((randomBetweenAnswer%${divisibleBy})) -ne 0 ] \
&& echo DIVISIBLE BY error - ${randomBetweenAnswer}!
# Store the answer away statistically.
answer[randomBetweenAnswer+disp]=$((answer[randomBetweenAnswer+disp]+1))
done

# Let's check the results

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for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
[ ${answer[i+disp]} -eq 0 ] \
&& echo "We never got an answer of $i." \
|| echo "${i} occurred ${answer[i+disp]} times."
done

exit 0

Just how random is $RANDOM? The best way to test this is to write a script that tracks the distribution of
"random" numbers generated by $RANDOM. Let's roll a $RANDOM die a few times . . .

Example 9-15. Rolling a single die with RANDOM
#!/bin/bash
# How random is RANDOM?
RANDOM=$$

# Reseed the random number generator using script process ID.

PIPS=6
MAXTHROWS=600
throw=0

# A die has 6 pips.
# Increase this if you have nothing better to do with your time.
# Throw count.

ones=0
twos=0
threes=0
fours=0
fives=0
sixes=0

# Must initialize counts to zero,
#+ since an uninitialized variable is null, not zero.

print_result ()
{
echo
echo "ones =
$ones"
echo "twos =
$twos"
echo "threes = $threes"
echo "fours = $fours"
echo "fives = $fives"
echo "sixes = $sixes"
echo
}
update_count()
{
case "$1" in
0) let "ones += 1";;
# Since die has no "zero", this corresponds to 1.
1) let "twos += 1";;
# And this to 2, etc.
2) let "threes += 1";;
3) let "fours += 1";;
4) let "fives += 1";;
5) let "sixes += 1";;
esac
}
echo

while [ "$throw" -lt "$MAXTHROWS" ]
do
let "die1 = RANDOM % $PIPS"

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update_count $die1
let "throw += 1"
done
print_result
exit 0
#
#
#
#
#+

The scores should distribute fairly evenly, assuming RANDOM is fairly random.
With $MAXTHROWS at 600, all should cluster around 100, plus-or-minus 20 or so.
Keep in mind that RANDOM is a pseudorandom generator,
and not a spectacularly good one at that.

# Randomness is a deep and complex subject.
# Sufficiently long "random" sequences may exhibit
#+ chaotic and other "non-random" behavior.
#
#
#
#

Exercise (easy):
--------------Rewrite this script to flip a coin 1000 times.
Choices are "HEADS" and "TAILS".

As we have seen in the last example, it is best to reseed the RANDOM generator each time it is invoked. Using
the same seed for RANDOM repeats the same series of numbers. [47] (This mirrors the behavior of the
random() function in C.)

Example 9-16. Reseeding RANDOM
#!/bin/bash
# seeding-random.sh: Seeding the RANDOM variable.
MAXCOUNT=25

# How many numbers to generate.

random_numbers ()
{
count=0
while [ "$count" -lt "$MAXCOUNT" ]
do
number=$RANDOM
echo -n "$number "
let "count += 1"
done
}
echo; echo
RANDOM=1
random_numbers

# Setting RANDOM seeds the random number generator.

echo; echo
RANDOM=1
random_numbers

# Same seed for RANDOM...
# ...reproduces the exact same number series.
#
# When is it useful to duplicate a "random" number series?

echo; echo
RANDOM=2

# Trying again, but with a different seed...

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random_numbers

# gives a different number series.

echo; echo
# RANDOM=$$ seeds RANDOM from process id of script.
# It is also possible to seed RANDOM from 'time' or 'date' commands.
# Getting fancy...
SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }')
# Pseudo-random output fetched
#+ from /dev/urandom (system pseudo-random device-file),
#+ then converted to line of printable (octal) numbers by "od",
#+ finally "awk" retrieves just one number for SEED.
RANDOM=$SEED
random_numbers
echo; echo
exit 0

The /dev/urandom pseudo-device file provides a method of generating much more "random"
pseudorandom numbers than the $RANDOM variable. dd if=/dev/urandom of=targetfile
bs=1 count=XX creates a file of well-scattered pseudorandom numbers. However, assigning these
numbers to a variable in a script requires a workaround, such as filtering through od (as in above
example, Example 16-14, and Example A-36), or even piping to md5sum (see Example 36-14).

There are also other ways to generate pseudorandom numbers in a script. Awk provides a convenient
means of doing this.

Example 9-17. Pseudorandom numbers, using awk
#!/bin/bash
# random2.sh: Returns a pseudorandom number in the range 0 - 1.
# Uses the awk rand() function.
AWKSCRIPT=' { srand(); print rand() } '
#
Command(s) / parameters passed to awk
# Note that srand() reseeds awk's random number generator.

echo -n "Random number between 0 and 1 = "
echo | awk "$AWKSCRIPT"
# What happens if you leave out the 'echo'?
exit 0

# Exercises:
# --------# 1) Using a loop construct, print out 10 different random numbers.
#
(Hint: you must reseed the "srand()" function with a different seed
#+
in each pass through the loop. What happens if you fail to do this?)
# 2) Using an integer multiplier as a scaling factor, generate random numbers

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#+

in the range between 10 and 100.

# 3) Same as exercise #2, above, but generate random integers this time.

The date command also lends itself to generating pseudorandom integer sequences.

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10.1. Manipulating Strings
Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified
focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr
command. This results in inconsistent command syntax and overlap of functionality, not to mention
confusion.
String Length
${#string}
expr length $string
These are the equivalent of strlen() in C.
expr "$string" : '.*'
stringZ=abcABC123ABCabc
echo ${#stringZ}
echo `expr length $stringZ`
echo `expr "$stringZ" : '.*'`

# 15
# 15
# 15

Example 10-1. Inserting a blank line between paragraphs in a text file
#!/bin/bash
# paragraph-space.sh
# Ver. 2.0, Reldate 05Aug08
# Inserts a blank line between paragraphs of a single-spaced text file.
# Usage: $0  "$filename.$SUFFIX"
# Redirect conversion to new filename.
rm -f $file
# Delete original files after converting.
echo "$filename.$SUFFIX" # Log what is happening to stdout.
done
exit 0
# Exercise:
# -------# As it stands, this script converts *all* the files in the current
#+ working directory.
# Modify it to work *only* on files with a ".mac" suffix.

Example 10-4. Converting streaming audio files to ogg
#!/bin/bash
# ra2ogg.sh: Convert streaming audio files (*.ra) to ogg.
#
#
#
#
#
#
#

Uses the "mplayer" media player program:
http://www.mplayerhq.hu/homepage
Uses the "ogg" library and "oggenc":
http://www.xiph.org/
This script may need appropriate codecs installed, such as sipr.so ...
Possibly also the compat-libstdc++ package.

OFILEPREF=${1%%ra}
# Strip off the "ra" suffix.
OFILESUFF=wav
# Suffix for wav file.
OUTFILE="$OFILEPREF""$OFILESUFF"
E_NOARGS=85
if [ -z "$1" ]
# Must specify a filename to convert.
then
echo "Usage: `basename $0` [filename]"
exit $E_NOARGS
fi

##########################################################################
mplayer "$1" -ao pcm:file=$OUTFILE
oggenc "$OUTFILE" # Correct file extension automatically added by oggenc.
##########################################################################
rm "$OUTFILE"

# Delete intermediate *.wav file.
# If you want to keep it, comment out above line.

exit $?
#
#
#
#+
#
#+

Note:
---On a Website, simply clicking on a *.ram streaming audio file
usually only downloads the URL of the actual *.ra audio file.
You can then use "wget" or something similar
to download the *.ra file itself.

#
#
#

Exercises:
--------As is, this script converts only *.ra filenames.

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#
#
#
#+
#
#+

Add flexibility by permitting use of *.ram and other filenames.
If you're really ambitious, expand the script
to do automatic downloads and conversions of streaming audio files.
Given a URL, batch download streaming audio files (using "wget")
and convert them on the fly.

A simple emulation of getopt using substring-extraction constructs.

Example 10-5. Emulating getopt
#!/bin/bash
# getopt-simple.sh
# Author: Chris Morgan
# Used in the ABS Guide with permission.

getopt_simple()
{
echo "getopt_simple()"
echo "Parameters are '$*'"
until [ -z "$1" ]
do
echo "Processing parameter of: '$1'"
if [ ${1:0:1} = '/' ]
then
tmp=${1:1}
# Strip off leading '/' . . .
parameter=${tmp%%=*}
# Extract name.
value=${tmp##*=}
# Extract value.
echo "Parameter: '$parameter', value: '$value'"
eval $parameter=$value
fi
shift
done
}
# Pass all options to getopt_simple().
getopt_simple $*
echo "test is '$test'"
echo "test2 is '$test2'"
exit 0

# See also, UseGetOpt.sh, a modified versio of this script.

--sh getopt_example.sh /test=value1 /test2=value2
Parameters are '/test=value1 /test2=value2'
Processing parameter of: '/test=value1'
Parameter: 'test', value: 'value1'
Processing parameter of: '/test2=value2'
Parameter: 'test2', value: 'value2'
test is 'value1'
test2 is 'value2'

Substring Replacement

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${string/substring/replacement}
Replace first match of $substring with $replacement. [49]
${string//substring/replacement}
Replace all matches of $substring with $replacement.
stringZ=abcABC123ABCabc
echo ${stringZ/abc/xyz}

# xyzABC123ABCabc
# Replaces first match of 'abc' with 'xyz'.

echo ${stringZ//abc/xyz}

# xyzABC123ABCxyz
# Replaces all matches of 'abc' with # 'xyz'.

echo --------------echo "$stringZ"
echo ---------------

# abcABC123ABCabc
# The string itself is not altered!

# Can the match and replacement
match=abc
repl=000
echo ${stringZ/$match/$repl} #
#
^
^
echo ${stringZ//$match/$repl} #
# Yes!
^
^

strings be parameterized?

000ABC123ABCabc
^^^
000ABC123ABC000
^^^
^^^

echo
# What happens if no $replacement string is supplied?
echo ${stringZ/abc}
# ABC123ABCabc
echo ${stringZ//abc}
# ABC123ABC
# A simple deletion takes place.

${string/#substring/replacement}
If $substring matches front end of $string, substitute $replacement for $substring.
${string/%substring/replacement}
If $substring matches back end of $string, substitute $replacement for $substring.
stringZ=abcABC123ABCabc
echo ${stringZ/#abc/XYZ}

# XYZABC123ABCabc
# Replaces front-end match of 'abc' with 'XYZ'.

echo ${stringZ/%abc/XYZ}

# abcABC123ABCXYZ
# Replaces back-end match of 'abc' with 'XYZ'.

10.1.1. Manipulating strings using awk
A Bash script may invoke the string manipulation facilities of awk as an alternative to using its built-in
operations.

Example 10-6. Alternate ways of extracting and locating substrings
#!/bin/bash
# substring-extraction.sh

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String=23skidoo1
#
012345678
Bash
#
123456789
awk
# Note different string indexing system:
# Bash numbers first character of string as 0.
# Awk numbers first character of string as 1.
echo ${String:2:4} # position 3 (0-1-2), 4 characters long
# skid
# The awk equivalent of ${string:pos:length} is substr(string,pos,length).
echo | awk '
{ print substr("'"${String}"'",3,4)
# skid
}
'
# Piping an empty "echo" to awk gives it dummy input,
#+ and thus makes it unnecessary to supply a filename.
echo "----"
# And likewise:
echo | awk '
{ print index("'"${String}"'", "skid")
# 3
}
# (skid starts at position 3)
'
# The awk equivalent of "expr index" ...
exit 0

10.1.2. Further Reference
For more on string manipulation in scripts, refer to Section 10.2 and the relevant section of the expr command
listing.
Script examples:
1. Example 16-9
2. Example 10-9
3. Example 10-10
4. Example 10-11
5. Example 10-13
6. Example A-36
7. Example A-41

10.2. Parameter Substitution
Manipulating and/or expanding variables
${parameter}
Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less
ambiguous ${parameter} form works.
May be used for concatenating variables with strings.
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your_id=${USER}-on-${HOSTNAME}
echo "$your_id"
#
echo "Old \$PATH = $PATH"
PATH=${PATH}:/opt/bin # Add /opt/bin to $PATH for duration of script.
echo "New \$PATH = $PATH"

${parameter-default}, ${parameter:-default}
If parameter not set, use default.
var1=1
var2=2
# var3 is unset.
echo ${var1-$var2}
echo ${var3-$var2}
#
^

# 1
# 2
Note the $ prefix.

echo ${username-`whoami`}
# Echoes the result of `whoami`, if variable $username is still unset.

${parameter-default} and ${parameter:-default} are almost
equivalent. The extra : makes a difference only when parameter has been declared,
but is null.
#!/bin/bash
# param-sub.sh
# Whether a variable has been declared
#+ affects triggering of the default option
#+ even if the variable is null.
username0=
echo "username0 has been declared, but is set to null."
echo "username0 = ${username0-`whoami`}"
# Will not echo.
echo
echo username1 has not been declared.
echo "username1 = ${username1-`whoami`}"
# Will echo.
username2=
echo "username2 has been declared, but is set to null."
echo "username2 = ${username2:-`whoami`}"
#
^
# Will echo because of :- rather than just - in condition test.
# Compare to first instance, above.

#
# Once again:
variable=
# variable has been declared, but is set to null.

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echo "${variable-0}"
echo "${variable:-1}"
#
^

# (no output)
# 1

unset variable
echo "${variable-2}"
echo "${variable:-3}"

# 2
# 3

exit 0

The default parameter construct finds use in providing "missing" command-line arguments in scripts.
DEFAULT_FILENAME=generic.data
filename=${1:-$DEFAULT_FILENAME}
# If not otherwise specified, the following command block operates
#+ on the file "generic.data".
# Begin-Command-Block
# ...
# ...
# ...
# End-Command-Block

# From "hanoi2.bash" example:
DISKS=${1:-E_NOPARAM}
# Must specify how many disks.
# Set $DISKS to $1 command-line-parameter,
#+ or to $E_NOPARAM if that is unset.

See also Example 3-4, Example 31-2, and Example A-6.
Compare this method with using an and list to supply a default command-line argument.
${parameter=default}, ${parameter:=default}
If parameter not set, set it to default.
Both forms nearly equivalent. The : makes a difference only when $parameter has been declared
and is null, [50] as above.
echo ${var=abc}
# abc
echo ${var=xyz}
# abc
# $var had already been set to abc, so it did not change.

${parameter+alt_value}, ${parameter:+alt_value}
If parameter set, use alt_value, else use null string.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared
and is null, see below.
echo "###### \${parameter+alt_value} ########"
echo
a=${param1+xyz}
echo "a = $a"

# a =

param2=
a=${param2+xyz}
echo "a = $a"

# a = xyz

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param3=123
a=${param3+xyz}
echo "a = $a"

# a = xyz

echo
echo "###### \${parameter:+alt_value} ########"
echo
a=${param4:+xyz}
echo "a = $a"

# a =

param5=
a=${param5:+xyz}
echo "a = $a"
# a =
# Different result from
param6=123
a=${param6:+xyz}
echo "a = $a"

a=${param5+xyz}

# a = xyz

${parameter?err_msg}, ${parameter:?err_msg}
If parameter set, use it, else print err_msg and abort the script with an exit status of 1.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared
and is null, as above.

Example 10-7. Using parameter substitution and error messages
#!/bin/bash
#
#
#
#+

Check some of the system's environmental variables.
This is good preventative maintenance.
If, for example, $USER, the name of the person at the console, is not set,
the machine will not recognize you.

: ${HOSTNAME?} ${USER?} ${HOME?} ${MAIL?}
echo
echo "Name of the machine is $HOSTNAME."
echo "You are $USER."
echo "Your home directory is $HOME."
echo "Your mail INBOX is located in $MAIL."
echo
echo "If you are reading this message,"
echo "critical environmental variables have been set."
echo
echo
# -----------------------------------------------------# The ${variablename?} construction can also check
#+ for variables set within the script.
ThisVariable=Value-of-ThisVariable
# Note, by the way, that string variables may be set
#+ to characters disallowed in their names.
: ${ThisVariable?}
echo "Value of ThisVariable is $ThisVariable".
echo; echo

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: ${ZZXy23AB?"ZZXy23AB has not been set."}
# Since ZZXy23AB has not been set,
#+ then the script terminates with an error message.
# You can specify the error message.
# : ${variablename?"ERROR MESSAGE"}

# Same result with:
#
#
#

dummy_variable=${ZZXy23AB?}
dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."}
echo ${ZZXy23AB?} >/dev/null

# Compare these methods of checking whether a variable has been set
#+ with "set -u" . . .

echo "You will not see this message, because script already terminated."
HERE=0
exit $HERE

# Will NOT exit here.

# In fact, this script will return an exit status (echo $?) of 1.

Example 10-8. Parameter substitution and "usage" messages
#!/bin/bash
# usage-message.sh
: ${1?"Usage: $0 ARGUMENT"}
# Script exits here if command-line parameter absent,
#+ with following error message.
#
usage-message.sh: 1: Usage: usage-message.sh ARGUMENT
echo "These two lines echo only if command-line parameter given."
echo "command-line parameter = \"$1\""
exit 0

# Will exit here only if command-line parameter present.

# Check the exit status, both with and without command-line parameter.
# If command-line parameter present, then "$?" is 0.
# If not, then "$?" is 1.

Parameter substitution and/or expansion. The following expressions are the complement to the match in
expr string operations (see Example 16-9). These particular ones are used mostly in parsing file path names.
Variable length / Substring removal
${#var}
String length (number of characters in $var). For an array, ${#array} is the length of the first
element in the array.
Exceptions:
◊
${#*} and ${#@} give the number of positional parameters.

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◊ For an array, ${#array[*]} and ${#array[@]} give the number of elements in
the array.
Example 10-9. Length of a variable
#!/bin/bash
# length.sh
E_NO_ARGS=65
if [ $# -eq 0 ] # Must have command-line args to demo script.
then
echo "Please invoke this script with one or more command-line arguments."
exit $E_NO_ARGS
fi
var01=abcdEFGH28ij
echo "var01 = ${var01}"
echo "Length of var01 = ${#var01}"
# Now, let's try embedding a space.
var02="abcd EFGH28ij"
echo "var02 = ${var02}"
echo "Length of var02 = ${#var02}"
echo "Number of command-line arguments passed to script = ${#@}"
echo "Number of command-line arguments passed to script = ${#*}"
exit 0

${var#Pattern}, ${var##Pattern}
${var#Pattern} Remove from $var the shortest part of $Pattern that matches the front end
of $var.

${var##Pattern} Remove from $var the longest part of $Pattern that matches the front end
of $var.
A usage illustration from Example A-7:
# Function from "days-between.sh" example.
# Strips leading zero(s) from argument passed.
strip_leading_zero () # Strip possible leading zero(s)
{
#+ from argument passed.
return=${1#0}
# The "1" refers to "$1" -- passed arg.
}
# The "0" is what to remove from "$1" -- strips zeros.

Manfred Schwarb's more elaborate variation of the above:
strip_leading_zero2 () # Strip possible leading zero(s), since otherwise
{
# Bash will interpret such numbers as octal values.
shopt -s extglob
# Turn on extended globbing.
local val=${1##+(0)} # Use local variable, longest matching series of 0's.
shopt -u extglob
# Turn off extended globbing.
_strip_leading_zero2=${val:-0}
# If input was 0, return 0 instead of "".
}

Another usage illustration:
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echo `basename $PWD`
echo "${PWD##*/}"
echo
echo `basename $0`
echo $0
echo "${0##*/}"
echo
filename=test.data
echo "${filename##*.}"

# Basename of current working directory.
# Basename of current working directory.
# Name of script.
# Name of script.
# Name of script.

# data
# Extension of filename.

${var%Pattern}, ${var%%Pattern}
${var%Pattern} Remove from $var the shortest part of $Pattern that matches the back end
of $var.

${var%%Pattern} Remove from $var the longest part of $Pattern that matches the back end
of $var.
Version 2 of Bash added additional options.

Example 10-10. Pattern matching in parameter substitution
#!/bin/bash
# patt-matching.sh
# Pattern matching

using the # ## % %% parameter substitution operators.

var1=abcd12345abc6789
pattern1=a*c # * (wild card) matches everything between a - c.
echo
echo "var1 = $var1"
echo "var1 = ${var1}"

# abcd12345abc6789
# abcd12345abc6789
# (alternate form)
echo "Number of characters in ${var1} = ${#var1}"
echo
echo "pattern1 = $pattern1"
# a*c (everything between 'a' and 'c')
echo "--------------"
echo '${var1#$pattern1} =' "${var1#$pattern1}"
#
d12345abc6789
# Shortest possible match, strips out first 3 characters abcd12345abc6789
#
^^^^^
|-|
echo '${var1##$pattern1} =' "${var1##$pattern1}"
#
6789
# Longest possible match, strips out first 12 characters abcd12345abc6789
#
^^^^^
|----------|
echo; echo; echo
pattern2=b*9
# everything between 'b' and '9'
echo "var1 = $var1"
# Still abcd12345abc6789
echo
echo "pattern2 = $pattern2"
echo "--------------"
echo '${var1%pattern2} =' "${var1%$pattern2}"
#
abcd12345a
# Shortest possible match, strips out last 6 characters abcd12345abc6789
#
^^^^
|----|
echo '${var1%%pattern2} =' "${var1%%$pattern2}"
#
a

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# Longest possible match, strips out last 12 characters
#
^^^^

abcd12345abc6789
|-------------|

# Remember, # and ## work from the left end (beginning) of string,
#
% and %% work from the right end.
echo
exit 0

Example 10-11. Renaming file extensions:
#!/bin/bash
# rfe.sh: Renaming file extensions.
#
#
rfe old_extension new_extension
#
# Example:
# To rename all *.gif files in working directory to *.jpg,
#
rfe gif jpg

E_BADARGS=65
case $# in
0|1)
# The vertical bar means "or" in this context.
echo "Usage: `basename $0` old_file_suffix new_file_suffix"
exit $E_BADARGS # If 0 or 1 arg, then bail out.
;;
esac

for filename in *.$1
# Traverse list of files ending with 1st argument.
do
mv $filename ${filename%$1}$2
# Strip off part of filename matching 1st argument,
#+ then append 2nd argument.
done
exit 0

Variable expansion / Substring replacement
These constructs have been adopted from ksh.
${var:pos}
Variable var expanded, starting from offset pos.
${var:pos:len}
Expansion to a max of len characters of variable var, from offset pos. See Example A-13 for an
example of the creative use of this operator.
${var/Pattern/Replacement}
First match of Pattern, within var replaced with Replacement.
If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is,
deleted.
${var//Pattern/Replacement}
Global replacement. All matches of Pattern, within var replaced with Replacement.

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As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing,
that is, deleted.

Example 10-12. Using pattern matching to parse arbitrary strings
#!/bin/bash
var1=abcd-1234-defg
echo "var1 = $var1"
t=${var1#*-*}
echo "var1 (with everything, up to and including first - stripped out) = $t"
# t=${var1#*-} works just the same,
#+ since # matches the shortest string,
#+ and * matches everything preceding, including an empty string.
# (Thanks, Stephane Chazelas, for pointing this out.)
t=${var1##*-*}
echo "If var1 contains a \"-\", returns empty string...

var1 = $t"

t=${var1%*-*}
echo "var1 (with everything from the last - on stripped out) = $t"
echo
# ------------------------------------------path_name=/home/bozo/ideas/thoughts.for.today
# ------------------------------------------echo "path_name = $path_name"
t=${path_name##/*/}
echo "path_name, stripped of prefixes = $t"
# Same effect as
t=`basename $path_name` in this particular case.
# t=${path_name%/}; t=${t##*/}
is a more general solution,
#+ but still fails sometimes.
# If $path_name ends with a newline, then `basename $path_name` will not work,
#+ but the above expression will.
# (Thanks, S.C.)
t=${path_name%/*.*}
# Same effect as
t=`dirname $path_name`
echo "path_name, stripped of suffixes = $t"
# These will fail in some cases, such as "../", "/foo////", # "foo/", "/".
# Removing suffixes, especially when the basename has no suffix,
#+ but the dirname does, also complicates matters.
# (Thanks, S.C.)
echo
t=${path_name:11}
echo "$path_name, with first 11 chars stripped off = $t"
t=${path_name:11:5}
echo "$path_name, with first 11 chars stripped off, length 5 = $t"
echo
t=${path_name/bozo/clown}
echo "$path_name with \"bozo\" replaced by \"clown\" = $t"
t=${path_name/today/}
echo "$path_name with \"today\" deleted = $t"

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t=${path_name//o/O}
echo "$path_name with all o's capitalized = $t"
t=${path_name//o/}
echo "$path_name with all o's deleted = $t"
exit 0

${var/#Pattern/Replacement}
If prefix of var matches Pattern, then substitute Replacement for Pattern.
${var/%Pattern/Replacement}
If suffix of var matches Pattern, then substitute Replacement for Pattern.

Example 10-13. Matching patterns at prefix or suffix of string
#!/bin/bash
# var-match.sh:
# Demo of pattern replacement at prefix / suffix of string.
v0=abc1234zip1234abc
echo "v0 = $v0"
echo

# Original variable.
# abc1234zip1234abc

# Match at prefix (beginning) of string.
v1=${v0/#abc/ABCDEF}
# abc1234zip1234abc
# |-|
echo "v1 = $v1"
# ABCDEF1234zip1234abc
# |----|
# Match at suffix (end) of string.
v2=${v0/%abc/ABCDEF}
# abc1234zip123abc
#
|-|
echo "v2 = $v2"
# abc1234zip1234ABCDEF
#
|----|
echo
# ---------------------------------------------------# Must match at beginning / end of string,
#+ otherwise no replacement results.
# ---------------------------------------------------v3=${v0/#123/000}
# Matches, but not at beginning.
echo "v3 = $v3"
# abc1234zip1234abc
# NO REPLACEMENT.
v4=${v0/%123/000}
# Matches, but not at end.
echo "v4 = $v4"
# abc1234zip1234abc
# NO REPLACEMENT.
exit 0

${!varprefix*}, ${!varprefix@}
Matches names of all previously declared variables beginning with varprefix.
# This is a variation on indirect reference, but with a * or @.
# Bash, version 2.04, adds this feature.
xyz23=whatever
xyz24=
a=${!xyz*}
# ^ ^
^

#

Expands to *names* of declared variables
+ beginning with "xyz".

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echo "a = $a"
a=${!xyz@}
echo "a = $a"

#
#
#

a = xyz23 xyz24
Same as above.
a = xyz23 xyz24

echo "---"
abc23=something_else
b=${!abc*}
echo "b = $b"
#
c=${!b}
#
echo $c
#

b = abc23
Now, the more familiar type of indirect reference.
something_else

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What needs this iteration, woman?
--Shakespeare, Othello
Operations on code blocks are the key to structured and organized shell scripts. Looping and branching
constructs provide the tools for accomplishing this.

11.1. Loops
A loop is a block of code that iterates [51] a list of commands as long as the loop control condition is true.
for loops
for arg in [list]
This is the basic looping construct. It differs significantly from its C counterpart.

for arg in [list]
do
command(s)...
done
During each pass through the loop, arg takes on the value of each successive variable
in the list.
for arg in "$var1"
# In pass 1 of the
# In pass 2 of the
# In pass 3 of the
# ...
# In pass N of the

"$var2" "$var3" ... "$varN"
loop, arg = $var1
loop, arg = $var2
loop, arg = $var3
loop, arg = $varN

# Arguments in [list] quoted to prevent possible word splitting.

The argument list may contain wild cards.

If do is on same line as for, there needs to be a semicolon after list.
for arg in [list] ; do

Example 11-1. Simple for loops
#!/bin/bash
# Listing the planets.
for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
do
echo $planet # Each planet on a separate line.

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done
echo; echo
for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"
# All planets on same line.
# Entire 'list' enclosed in quotes creates a single variable.
# Why? Whitespace incorporated into the variable.
do
echo $planet
done
echo; echo "Whoops! Pluto is no longer a planet!"
exit 0

Each [list] element may contain multiple parameters. This is useful when processing parameters
in groups. In such cases, use the set command (see Example 15-16) to force parsing of each [list]
element and assignment of each component to the positional parameters.

Example 11-2. for loop with two parameters in each [list] element
#!/bin/bash
# Planets revisited.
# Associate the name of each planet with its distance from the sun.
for planet in "Mercury 36" "Venus 67" "Earth 93" "Mars 142" "Jupiter 483"
do
set -- $planet # Parses variable "planet"
#+ and sets positional parameters.
# The "--" prevents nasty surprises if $planet is null or
#+ begins with a dash.
# May need to save original positional parameters,
#+ since they get overwritten.
# One way of doing this is to use an array,
#
original_params=("$@")
echo "$1
#-------two
done

$2,000,000 miles from the sun"
tabs---concatenate zeroes onto parameter $2

# (Thanks, S.C., for additional clarification.)
exit 0

A variable may supply the [list] in a for loop.

Example 11-3. Fileinfo: operating on a file list contained in a variable
#!/bin/bash
# fileinfo.sh
FILES="/usr/sbin/accept
/usr/sbin/pwck

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/usr/sbin/chroot
/usr/bin/fakefile
/sbin/badblocks
/sbin/ypbind"
# List of files you are curious about.
# Threw in a dummy file, /usr/bin/fakefile.
echo
for file in $FILES
do
if [ ! -e "$file" ]
# Check if file exists.
then
echo "$file does not exist."; echo
continue
# On to next.
fi
ls -l $file | awk '{ print $8 "
file size: " $5 }' # Print 2 fields.
whatis `basename $file`
# File info.
# Note that the whatis database needs to have been set up for this to work.
# To do this, as root run /usr/bin/makewhatis.
echo
done
exit 0

If the [list] in a for loop contains wild cards (* and ?) used in filename expansion, then globbing
takes place.

Example 11-4. Operating on files with a for loop
#!/bin/bash
# list-glob.sh: Generating [list] in a for-loop, using "globbing"
echo
for file in *
#
^ Bash performs filename expansion
#+
on expressions that globbing recognizes.
do
ls -l "$file" # Lists all files in $PWD (current directory).
# Recall that the wild card character "*" matches every filename,
#+ however, in "globbing," it doesn't match dot-files.
# If the pattern matches no file, it is expanded to itself.
# To prevent this, set the nullglob option
#+
(shopt -s nullglob).
# Thanks, S.C.
done
echo; echo
for file in [jx]*
do
rm -f $file
# Removes only files beginning with "j" or "x" in $PWD.
echo "Removed file \"$file\"".
done
echo

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exit 0

Omitting the in [list] part of a for loop causes the loop to operate on $@ -- the positional
parameters. A particularly clever illustration of this is Example A-15. See also Example 15-17.

Example 11-5. Missing in [list] in a for loop
#!/bin/bash
# Invoke this script both with and without arguments,
#+ and see what happens.
for a
do
echo -n "$a "
done
# The 'in list' missing, therefore the loop operates on '$@'
#+ (command-line argument list, including whitespace).
echo
exit 0

It is possible to use command substitution to generate the [list] in a for loop. See also Example
16-54, Example 11-10 and Example 16-48.

Example 11-6. Generating the [list] in a for loop with command substitution
#!/bin/bash
# for-loopcmd.sh: for-loop with [list]
#+ generated by command substitution.
NUMBERS="9 7 3 8 37.53"
for number in `echo $NUMBERS`
do
echo -n "$number "
done

# for number in 9 7 3 8 37.53

echo
exit 0

Here is a somewhat more complex example of using command substitution to create the [list].

Example 11-7. A grep replacement for binary files
#!/bin/bash
# bin-grep.sh: Locates matching strings in a binary file.
# A "grep" replacement for binary files.
# Similar effect to "grep -a"
E_BADARGS=65

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E_NOFILE=66
if [ $# -ne 2 ]
then
echo "Usage: `basename $0` search_string filename"
exit $E_BADARGS
fi
if [ ! -f "$2" ]
then
echo "File \"$2\" does not exist."
exit $E_NOFILE
fi

IFS=$'\012'

# Per suggestion of Anton Filippov.
# was: IFS="\n"
for word in $( strings "$2" | grep "$1" )
# The "strings" command lists strings in binary files.
# Output then piped to "grep", which tests for desired string.
do
echo $word
done
# As S.C. points out, lines 23 - 30 could be replaced with the simpler
#
strings "$2" | grep "$1" | tr -s "$IFS" '[\n*]'

# Try something like "./bin-grep.sh mem /bin/ls"
#+ to exercise this script.
exit 0

More of the same.

Example 11-8. Listing all users on the system
#!/bin/bash
# userlist.sh
PASSWORD_FILE=/etc/passwd
n=1
# User number
for name in $(awk 'BEGIN{FS=":"}{print $1}' < "$PASSWORD_FILE" )
# Field separator = :
^^^^^^
# Print first field
^^^^^^^^
# Get input from password file
^^^^^^^^^^^^^^^^^
do
echo "USER #$n = $name"
let "n += 1"
done

#
#
#
#
#

USER
USER
USER
...
USER

#1 = root
#2 = bin
#3 = daemon
#30 = bozo

exit $?

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#
#
#
#+
#

Discussion:
---------How is it that an ordinary user, or a script run by same,
can read /etc/passwd? (Hint: Check the /etc/passwd file permissions.)
Isn't this a security hole? Why or why not?

Yet another example of the [list] resulting from command substitution.

Example 11-9. Checking all the binaries in a directory for authorship
#!/bin/bash
# findstring.sh:
# Find a particular string in the binaries in a specified directory.
directory=/usr/bin/
fstring="Free Software Foundation"

# See which files come from the FSF.

for file in $( find $directory -type f -name '*' | sort )
do
strings -f $file | grep "$fstring" | sed -e "s%$directory%%"
# In the "sed" expression,
#+ it is necessary to substitute for the normal "/" delimiter
#+ because "/" happens to be one of the characters filtered out.
# Failure to do so gives an error message. (Try it.)
done
exit $?
#
#
#
#+

Exercise (easy):
--------------Convert this script to take command-line parameters
for $directory and $fstring.

A final example of [list] / command substitution, but this time the "command" is a function.
generate_list ()
{
echo "one two three"
}
for word in $(generate_list)
do
echo "$word"
done

# Let "word" grab output of function.

# one
# two
# three

The output of a for loop may be piped to a command or commands.

Example 11-10. Listing the symbolic links in a directory
#!/bin/bash
# symlinks.sh: Lists symbolic links in a directory.

directory=${1-`pwd`}

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# Defaults to current working directory,
#+ if not otherwise specified.
# Equivalent to code block below.
# ---------------------------------------------------------# ARGS=1
# Expect one command-line argument.
#
# if [ $# -ne "$ARGS" ] # If not 1 arg...
# then
#
directory=`pwd`
# current working directory
# else
#
directory=$1
# fi
# ---------------------------------------------------------echo "symbolic links in directory \"$directory\""
for file in "$( find $directory -type l )"
# -type l = symbolic links
do
echo "$file"
done | sort
# Otherwise file list is unsorted.
# Strictly speaking, a loop isn't really necessary here,
#+ since the output of the "find" command is expanded into a single word.
# However, it's easy to understand and illustrative this way.
#
#+
#+
#

As Dominik
failing to
will choke
containing

'Aeneas' Schnitzer points out,
quote $( find $directory -type l )
on filenames with embedded whitespace.
whitespace.

exit 0

# -------------------------------------------------------# Jean Helou proposes the following alternative:
echo "symbolic links in directory \"$directory\""
# Backup of the current IFS. One can never be too cautious.
OLDIFS=$IFS
IFS=:
for file in $(find $directory -type l -printf "%p$IFS")
do
#
^^^^^^^^^^^^^^^^
echo "$file"
done|sort
# And, James "Mike" Conley suggests modifying Helou's code thusly:
OLDIFS=$IFS
IFS='' # Null IFS means no word breaks
for file in $( find $directory -type l )
do
echo $file
done | sort
# This works in the "pathological" case of a directory name having
#+ an embedded colon.
# "This also fixes the pathological case of the directory name having
#+ a colon (or space in earlier example) as well."

The stdout of a loop may be redirected to a file, as this slight modification to the previous example
shows.

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Example 11-11. Symbolic links in a directory, saved to a file
#!/bin/bash
# symlinks.sh: Lists symbolic links in a directory.
OUTFILE=symlinks.list

# save file

directory=${1-`pwd`}
# Defaults to current working directory,
#+ if not otherwise specified.

echo "symbolic links in directory \"$directory\"" > "$OUTFILE"
echo "---------------------------" >> "$OUTFILE"
for file in "$( find $directory -type l )"
do
echo "$file"
done | sort >> "$OUTFILE"
#
^^^^^^^^^^^^^

# -type l = symbolic links

# stdout of loop
redirected to save file.

exit 0

There is an alternative syntax to a for loop that will look very familiar to C programmers. This
requires double parentheses.

Example 11-12. A C-style for loop
#!/bin/bash
# Multiple ways to count up to 10.
echo
# Standard syntax.
for a in 1 2 3 4 5 6 7 8 9 10
do
echo -n "$a "
done
echo; echo
# +==========================================+
# Using "seq" ...
for a in `seq 10`
do
echo -n "$a "
done
echo; echo
# +==========================================+
# Using brace expansion ...
# Bash, version 3+.
for a in {1..10}
do
echo -n "$a "
done

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echo; echo
# +==========================================+
# Now, let's do the same, using C-like syntax.
LIMIT=10
for ((a=1; a <= LIMIT ; a++))
do
echo -n "$a "
done

# Double parentheses, and "LIMIT" with no "$".

# A construct borrowed from 'ksh93'.

echo; echo
# +=========================================================================+
# Let's use the C "comma operator" to increment two variables simultaneously.
for ((a=1, b=1; a <= LIMIT ; a++, b++))
do # The comma chains together operations.
echo -n "$a-$b "
done
echo; echo
exit 0

See also Example 27-16, Example 27-17, and Example A-6.
--Now, a for loop used in a "real-life" context.

Example 11-13. Using efax in batch mode
#!/bin/bash
# Faxing (must have 'efax' package installed).
EXPECTED_ARGS=2
E_BADARGS=85
MODEM_PORT="/dev/ttyS2"
#
^^^^^
if [ $#
# Check
then
echo
exit
fi

# May be different on your machine.
PCMCIA modem card default port.

-ne $EXPECTED_ARGS ]
for proper number of command-line args.
"Usage: `basename $0` phone# text-file"
$E_BADARGS

if [ ! -f "$2" ]
then
echo "File $2 is not a text file."
#
File is not a regular file, or does not exist.
exit $E_BADARGS
fi

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fax make $2

#

Create fax-formatted files from text files.

for file in $(ls $2.0*)

# Concatenate the converted files.
# Uses wild card (filename "globbing")
#+ in variable list.

do
fil="$fil $file"
done
efax -d "$MODEM_PORT" -t "T$1" $fil
# Finally, do the work.
# Trying adding -o1 if above line fails.

# As S.C. points out, the for-loop can be eliminated with
#
efax -d /dev/ttyS2 -o1 -t "T$1" $2.0*
#+ but it's not quite as instructive [grin].
exit $?

# Also, efax sends diagnostic messages to stdout.

while
This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is
true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the
number of loop repetitions is not known beforehand.
while [ condition ]
do
command(s)...
done
The bracket construct in a while loop is nothing more than our old friend, the test brackets used in an
if/then test. In fact, a while loop can legally use the more versatile double-brackets construct (while [[
condition ]]).

As is the case with for loops, placing the do on the same line as the condition test requires a
semicolon.
while [ condition ] ; do
Note that the test brackets are not mandatory in a while loop. See, for example, the getopts construct.

Example 11-14. Simple while loop
#!/bin/bash
var0=0
LIMIT=10
while [ "$var0" -lt "$LIMIT" ]
#
^
^
# Spaces, because these are "test-brackets" . . .
do
echo -n "$var0 "
# -n suppresses newline.
#
^
Space, to separate printed out numbers.

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var0=`expr $var0 + 1`

done

#
#
#
#

var0=$(($var0+1)) also works.
var0=$((var0 + 1)) also works.
let "var0 += 1"
also works.
Various other methods also work.

echo
exit 0

Example 11-15. Another while loop
#!/bin/bash
echo
# Equivalent to:
while [ "$var1" != "end" ]
# while test "$var1" != "end"
do
echo "Input variable #1 (end to exit) "
read var1
# Not 'read $var1' (why?).
echo "variable #1 = $var1"
# Need quotes because of "#" . . .
# If input is 'end', echoes it here.
# Does not test for termination condition until top of loop.
echo
done
exit 0

A while loop may have multiple conditions. Only the final condition determines when the loop
terminates. This necessitates a slightly different loop syntax, however.

Example 11-16. while loop with multiple conditions
#!/bin/bash
var1=unset
previous=$var1
while echo "previous-variable = $previous"
echo
previous=$var1
[ "$var1" != end ] # Keeps track of what $var1 was previously.
# Four conditions on "while", but only last one controls loop.
# The *last* exit status is the one that counts.
do
echo "Input variable #1 (end to exit) "
read var1
echo "variable #1 = $var1"
done
# Try to figure out how this all works.
# It's a wee bit tricky.
exit 0

As with a for loop, a while loop may employ C-style syntax by using the double-parentheses construct
(see also Example 8-5).

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Example 11-17. C-style syntax in a while loop
#!/bin/bash
# wh-loopc.sh: Count to 10 in a "while" loop.
LIMIT=10
a=1

# 10 iterations.

while [ "$a" -le $LIMIT ]
do
echo -n "$a "
let "a+=1"
done
# No surprises, so far.
echo; echo
# +=================================================================+
# Now, we'll repeat with C-like syntax.
((a = 1))
# a=1
# Double parentheses permit space when setting a variable, as in C.
while (( a <= LIMIT ))
# Double parentheses,
do
#+ and no "$" preceding variables.
echo -n "$a "
((a += 1))
# let "a+=1"
# Yes, indeed.
# Double parentheses permit incrementing a variable with C-like syntax.
done
echo
# C and Java programmers can feel right at home in Bash.
exit 0

Inside its test brackets, a while loop can call a function.
t=0
condition ()
{
((t++))
if [ $t -lt 5 ]
then
return 0 # true
else
return 1 # false
fi
}
while condition
#
^^^^^^^^^
#
Function call -- four loop iterations.
do
echo "Still going: t = $t"
done
# Still going: t = 1

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# Still going: t = 2
# Still going: t = 3
# Still going: t = 4

Similar to the if-test construct, a while loop can omit the test brackets.
while condition
do
command(s) ...
done

By coupling the power of the read command with a while loop, we get the handy while read construct,
useful for reading and parsing files.
cat $filename |
while read line
do
...
done

# Supply input from a file.
# As long as there is another line to read ...

# =========== Snippet from "sd.sh" example script ========== #
while read value
# Read one data point at a time.
do
rt=$(echo "scale=$SC; $rt + $value" | bc)
(( ct++ ))
done
am=$(echo "scale=$SC; $rt / $ct" | bc)
echo $am; return $ct
# This function "returns" TWO values!
# Caution: This little trick will not work if $ct > 255!
# To handle a larger number of data points,
#+ simply comment out the "return $ct" above.
} <"$datafile"
# Feed in data file.

A while loop may have its stdin redirected to a file by a < at its end.
A while loop may have its stdin supplied by a pipe.
until
This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is
false (opposite of while loop).
until [ condition-is-true ]
do
command(s)...
done
Note that an until loop tests for the terminating condition at the top of the loop, differing from a
similar construct in some programming languages.
As is the case with for loops, placing the do on the same line as the condition test requires a
semicolon.
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until [ condition-is-true ] ; do

Example 11-18. until loop
#!/bin/bash
END_CONDITION=end
until [ "$var1" = "$END_CONDITION" ]
# Tests condition here, at top of loop.
do
echo "Input variable #1 "
echo "($END_CONDITION to exit)"
read var1
echo "variable #1 = $var1"
echo
done
# ------------------------------------------- #
# As with "for" and "while" loops,
#+ an "until" loop permits C-like test constructs.
LIMIT=10
var=0
until (( var > LIMIT ))
do # ^^ ^
^
^^
No brackets, no $ prefixing variables.
echo -n "$var "
(( var++ ))
done
# 0 1 2 3 4 5 6 7 8 9 10

exit 0

How to choose between a for loop or a while loop or until loop? In C, you would typically use a for loop
when the number of loop iterations is known beforehand. With Bash, however, the situation is fuzzier. The
Bash for loop is more loosely structured and more flexible than its equivalent in other languages. Therefore,
feel free to use whatever type of loop gets the job done in the simplest way.

11.2. Nested Loops
A nested loop is a loop within a loop, an inner loop within the body of an outer one. How this works is that
the first pass of the outer loop triggers the inner loop, which executes to completion. Then the second pass of
the outer loop triggers the inner loop again. This repeats until the outer loop finishes. Of course, a break
within either the inner or outer loop would interrupt this process.

Example 11-19. Nested Loop
#!/bin/bash
# nested-loop.sh: Nested "for" loops.
outer=1

# Set outer loop counter.

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# Beginning of outer loop.
for a in 1 2 3 4 5
do
echo "Pass $outer in outer loop."
echo "---------------------"
inner=1
# Reset inner loop counter.
# ===============================================
# Beginning of inner loop.
for b in 1 2 3 4 5
do
echo "Pass $inner in inner loop."
let "inner+=1" # Increment inner loop counter.
done
# End of inner loop.
# ===============================================
let "outer+=1"
# Increment outer loop counter.
echo
# Space between output blocks in pass of outer loop.
done
# End of outer loop.
exit 0

See Example 27-11 for an illustration of nested while loops, and Example 27-13 to see a while loop nested
inside an until loop.

11.3. Loop Control
Tournez cent tours, tournez mille tours,
Tournez souvent et tournez toujours . . .
--Verlaine, "Chevaux de bois"
Commands affecting loop behavior
break, continue
The break and continue loop control commands [52] correspond exactly to their counterparts in other
programming languages. The break command terminates the loop (breaks out of it), while continue
causes a jump to the next iteration of the loop, skipping all the remaining commands in that particular
loop cycle.

Example 11-20. Effects of break and continue in a loop
#!/bin/bash
LIMIT=19

# Upper limit

echo
echo "Printing Numbers 1 through 20 (but not 3 and 11)."
a=0
while [ $a -le "$LIMIT" ]

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do
a=$(($a+1))
if [ "$a" -eq 3 ] || [ "$a" -eq 11 ] # Excludes 3 and 11.
then
continue
# Skip rest of this particular loop iteration.
fi
echo -n "$a "
done

# This will not execute for 3 and 11.

# Exercise:
# Why does the loop print up to 20?
echo; echo
echo Printing Numbers 1 through 20, but something happens after 2.
##################################################################
# Same loop, but substituting 'break' for 'continue'.
a=0
while [ "$a" -le "$LIMIT" ]
do
a=$(($a+1))
if [ "$a" -gt 2 ]
then
break # Skip entire rest of loop.
fi
echo -n "$a "
done
echo; echo; echo
exit 0

The break command may optionally take a parameter. A plain break terminates only the innermost
loop in which it is embedded, but a break N breaks out of N levels of loop.

Example 11-21. Breaking out of multiple loop levels
#!/bin/bash
# break-levels.sh: Breaking out of loops.
# "break N" breaks out of N level loops.
for outerloop in 1 2 3 4 5
do
echo -n "Group $outerloop:

"

# -------------------------------------------------------for innerloop in 1 2 3 4 5
do
echo -n "$innerloop "

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if [ "$innerloop" -eq 3 ]
then
break # Try
break 2
to see what happens.
# ("Breaks" out of both inner and outer loops.)
fi
done
# -------------------------------------------------------echo
done
echo
exit 0

The continue command, similar to break, optionally takes a parameter. A plain continue cuts short
the current iteration within its loop and begins the next. A continue N terminates all remaining
iterations at its loop level and continues with the next iteration at the loop, N levels above.

Example 11-22. Continuing at a higher loop level
#!/bin/bash
# The "continue N" command, continuing at the Nth level loop.
for outer in I II III IV V
do
echo; echo -n "Group $outer: "

# outer loop

# -------------------------------------------------------------------for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop
do
if [[ "$inner" -eq 7 && "$outer" = "III" ]]
then
continue 2 # Continue at loop on 2nd level, that is "outer loop".
# Replace above line with a simple "continue"
# to see normal loop behavior.
fi
echo -n "$inner " # 7 8 9 10 will not echo on "Group III."
done
# -------------------------------------------------------------------done
echo; echo
# Exercise:
# Come up with a meaningful use for "continue N" in a script.
exit 0

Example 11-23. Using continue N in an actual task
# Albert Reiner gives an example of how to use "continue N":
# --------------------------------------------------------# Suppose I have a large number of jobs that need to be run, with
#+ any data that is to be treated in files of a given name pattern

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#+
#+
#+
#

in a directory. There are several machines that access
this directory, and I want to distribute the work over these
different boxen.
Then I usually nohup something like the following on every box:

while true
do
for n in .iso.*
do
[ "$n" = ".iso.opts" ] && continue
beta=${n#.iso.}
[ -r .Iso.$beta ] && continue
[ -r .lock.$beta ] && sleep 10 && continue
lockfile -r0 .lock.$beta || continue
echo -n "$beta: " `date`
run-isotherm $beta
date
ls -alF .Iso.$beta
[ -r .Iso.$beta ] && rm -f .lock.$beta
continue 2
done
break
done
exit 0
# The details, in particular the sleep N, are particular to my
#+ application, but the general pattern is:
while true
do
for job in {pattern}
do
{job already done or running} && continue
{mark job as running, do job, mark job as done}
continue 2
done
break
# Or something like `sleep 600' to avoid termination.
done
#
#+
#+
#+
#+
#+
#+
#+
#+
#+
#+

This way the script will stop only when there are no more jobs to do
(including jobs that were added during runtime). Through the use
of appropriate lockfiles it can be run on several machines
concurrently without duplication of calculations [which run a couple
of hours in my case, so I really want to avoid this]. Also, as search
always starts again from the beginning, one can encode priorities in
the file names. Of course, one could also do this without `continue 2',
but then one would have to actually check whether or not some job
was done (so that we should immediately look for the next job) or not
(in which case we terminate or sleep for a long time before checking
for a new job).

The continue N construct is difficult to understand and tricky to use in any
meaningful context. It is probably best avoided.

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11.4. Testing and Branching
The case and select constructs are technically not loops, since they do not iterate the execution of a code
block. Like loops, however, they direct program flow according to conditions at the top or bottom of the
block.
Controlling program flow in a code block
case (in) / esac
The case construct is the shell scripting analog to switch in C/C++. It permits branching to one of a
number of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple
if/then/else statements and is an appropriate tool for creating menus.
case "$variable" in
"$condition1" )
command...
;;
"$condition2" )
command...
;;

esac

◊ Quoting the variables is not mandatory, since word splitting does not take
place.
◊ Each test line ends with a right paren ). [53]
◊ Each condition block ends with a double semicolon ;;.
◊ If a condition tests true, then the associated commands execute and the case
block terminates.
◊ The entire case block ends with an esac (case spelled backwards).
Example 11-24. Using case
#!/bin/bash
# Testing ranges of characters.
echo; echo "Hit a key, then hit return."
read Keypress
case "$Keypress" in
[[:lower:]]
) echo "Lowercase letter";;
[[:upper:]]
) echo "Uppercase letter";;
[0-9]
) echo "Digit";;
*
) echo "Punctuation, whitespace, or other";;
esac
# Allows ranges of characters in [square brackets],
#+ or POSIX ranges in [[double square brackets.
# In the first version of this example,
#+ the tests for lowercase and uppercase characters were

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#+
#
#
#

[a-z] and [A-Z].
This no longer works in certain locales and/or Linux distros.
POSIX is more portable.
Thanks to Frank Wang for pointing this out.

#
#
#
#
#+
#

Exercise:
-------As the script stands, it accepts a single keystroke, then terminates.
Change the script so it accepts repeated input,
reports on each keystroke, and terminates only when "X" is hit.
Hint: enclose everything in a "while" loop.

exit 0

Example 11-25. Creating menus using case
#!/bin/bash
# Crude address database
clear # Clear the screen.
echo
echo
echo
echo
echo
echo
echo
echo
echo

"
Contact List"
"
------- ----"
"Choose one of the following persons:"
"[E]vans, Roland"
"[J]ones, Mildred"
"[S]mith, Julie"
"[Z]ane, Morris"

read person
case "$person" in
# Note variable is quoted.
"E" | "e" )
# Accept upper or lowercase input.
echo
echo "Roland Evans"
echo "4321 Flash Dr."
echo "Hardscrabble, CO 80753"
echo "(303) 734-9874"
echo "(303) 734-9892 fax"
echo "revans@zzy.net"
echo "Business partner & old friend"
;;
# Note double semicolon to terminate each option.
"J" | "j" )
echo
echo "Mildred Jones"
echo "249 E. 7th St., Apt. 19"
echo "New York, NY 10009"
echo "(212) 533-2814"
echo "(212) 533-9972 fax"
echo "milliej@loisaida.com"
echo "Ex-girlfriend"
echo "Birthday: Feb. 11"
;;

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# Add info for Smith & Zane later.
* )
# Default option.
# Empty input (hitting RETURN) fits here, too.
echo
echo "Not yet in database."
;;
esac
echo
#
#
#
#+

Exercise:
-------Change the script so it accepts multiple inputs,
instead of terminating after displaying just one address.

exit 0

An exceptionally clever use of case involves testing for command-line parameters.
#! /bin/bash
case "$1" in
"") echo "Usage: ${0##*/} "; exit $E_PARAM;;
# No command-line parameters,
# or first parameter empty.
# Note that ${0##*/} is ${var##pattern} param substitution.
# Net result is $0.
-*) FILENAME=./$1;;

#

If filename passed as argument ($1)
starts with a dash,
replace it with ./$1
so further commands don't interpret it
as an option.

#+
#+
#+
#+
* ) FILENAME=$1;;
esac

# Otherwise, $1.

Here is an more straightforward example of command-line parameter handling:
#! /bin/bash

while [ $# -gt 0 ]; do
# Until you run out of parameters . . .
case "$1" in
-d|--debug)
# "-d" or "--debug" parameter?
DEBUG=1
;;
-c|--conf)
CONFFILE="$2"
shift
if [ ! -f $CONFFILE ]; then
echo "Error: Supplied file doesn't exist!"
exit $E_CONFFILE
# File not found error.
fi
;;
esac

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shift
done

# Check next set of parameters.

# From Stefano Falsetto's "Log2Rot" script,
#+ part of his "rottlog" package.
# Used with permission.

Example 11-26. Using command substitution to generate the case variable
#!/bin/bash
# case-cmd.sh: Using command substitution to generate a "case" variable.
case $( arch ) in
i386
i486
i586
i686
*
esac

)
)
)
)
)

echo
echo
echo
echo
echo

# $( arch ) returns machine architecture.
# Equivalent to 'uname -m' ...
"80386-based machine";;
"80486-based machine";;
"Pentium-based machine";;
"Pentium2+-based machine";;
"Other type of machine";;

exit 0

A case construct can filter strings for globbing patterns.

Example 11-27. Simple string matching
#!/bin/bash
# match-string.sh: Simple string matching.
match_string ()
{ # Exact string match.
MATCH=0
E_NOMATCH=90
PARAMS=2
# Function requires 2 arguments.
E_BAD_PARAMS=91
[ $# -eq $PARAMS ] || return $E_BAD_PARAMS
case "$1" in
"$2") return $MATCH;;
*
) return $E_NOMATCH;;
esac
}

a=one
b=two
c=three
d=two

match_string $a
echo $?

# wrong number of parameters
# 91

match_string $a $b
echo $?

# no match
# 90

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match_string $b $d
echo $?

# match
# 0

exit 0

Example 11-28. Checking for alphabetic input
#!/bin/bash
# isalpha.sh: Using a "case" structure to filter a string.
SUCCESS=0
FAILURE=-1
isalpha () # Tests whether *first character* of input string is alphabetic.
{
if [ -z "$1" ]
# No argument passed?
then
return $FAILURE
fi
case "$1" in
[a-zA-Z]*) return $SUCCESS;; # Begins with a letter?
*
) return $FAILURE;;
esac
}
# Compare this with "isalpha ()" function in C.

isalpha2 ()
# Tests whether *entire string* is alphabetic.
{
[ $# -eq 1 ] || return $FAILURE
case $1 in
*[!a-zA-Z]*|"") return $FAILURE;;
*) return $SUCCESS;;
esac
}
isdigit ()
# Tests whether *entire string* is numerical.
{
# In other words, tests for integer variable.
[ $# -eq 1 ] || return $FAILURE
case $1 in
*[!0-9]*|"") return $FAILURE;;
*) return $SUCCESS;;
esac
}

check_var () # Front-end to isalpha ().
{
if isalpha "$@"
then
echo "\"$*\" begins with an alpha character."
if isalpha2 "$@"
then
# No point in testing if first char is non-alpha.
echo "\"$*\" contains only alpha characters."
else
echo "\"$*\" contains at least one non-alpha character."

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fi
else
echo "\"$*\" begins with a non-alpha character."
# Also "non-alpha" if no argument passed.
fi
echo
}
digit_check () # Front-end to isdigit ().
{
if isdigit "$@"
then
echo "\"$*\" contains only digits [0 - 9]."
else
echo "\"$*\" has at least one non-digit character."
fi
echo
}
a=23skidoo
b=H3llo
c=-What?
d=What?
e=`echo $b`
f=AbcDef
g=27234
h=27a34
i=27.34

# Command substitution.

check_var $a
check_var $b
check_var $c
check_var $d
check_var $e
check_var $f
check_var
# No argument passed, so what happens?
#
digit_check $g
digit_check $h
digit_check $i

exit 0

# Script improved by S.C.

# Exercise:
# -------# Write an 'isfloat ()' function that tests for floating point numbers.
# Hint: The function duplicates 'isdigit ()',
#+ but adds a test for a mandatory decimal point.

select
The select construct, adopted from the Korn Shell, is yet another tool for building menus.
select variable [in list]
do
command...
break
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done
This prompts the user to enter one of the choices presented in the variable list. Note that select uses
the $PS3 prompt (#? ) by default, but this may be changed.

Example 11-29. Creating menus using select
#!/bin/bash
PS3='Choose your favorite vegetable: ' # Sets the prompt string.
# Otherwise it defaults to #? .
echo
select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas"
do
echo
echo "Your favorite veggie is $vegetable."
echo "Yuck!"
echo
break # What happens if there is no 'break' here?
done
exit
# Exercise:
# -------# Fix this script to accept user input not specified in
#+ the "select" statement.
# For example, if the user inputs "peas,"
#+ the script would respond "Sorry. That is not on the menu."

If in list is omitted, then select uses the list of command line arguments ($@) passed to the script
or the function containing the select construct.
Compare this to the behavior of a
for variable [in list]
construct with the in list omitted.
Example 11-30. Creating menus using select in a function
#!/bin/bash
PS3='Choose your favorite vegetable: '
echo
choice_of()
{
select vegetable
# [in list] omitted, so 'select' uses arguments passed to function.
do
echo
echo "Your favorite veggie is $vegetable."

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echo "Yuck!"
echo
break
done
}
choice_of beans rice carrots radishes tomatoes spinach
#
$1
$2
$3
$4
$5
$6
#
passed to choice_of() function
exit 0

See also Example 37-3.

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Chapter 12. Command Substitution
Command substitution reassigns the output of a command [54] or even multiple commands; it literally plugs
the command output into another context. [55]
The classic form of command substitution uses backquotes (`...`). Commands within backquotes (backticks)
generate command-line text.
script_name=`basename $0`
echo "The name of this script is $script_name."

The output of commands can be used as arguments to another command, to set a variable, and even for
generating the argument list in a for loop.
rm `cat filename`
# "filename" contains a list of files to delete.
#
# S. C. points out that "arg list too long" error might result.
# Better is
xargs rm -- < filename
# ( -- covers those cases where "filename" begins with a "-" )
textfile_listing=`ls *.txt`
# Variable contains names of all *.txt files in current working directory.
echo $textfile_listing
textfile_listing2=$(ls *.txt)
echo $textfile_listing2
# Same result.
#
#
#
#
#
#
#
#

# The alternative form of command substitution.

A possible problem with putting a list of files into a single string
is that a newline may creep in.
A safer way to assign a list of files to a parameter is with an array.
shopt -s nullglob
# If no match, filename expands to nothing.
textfile_listing=( *.txt )
Thanks, S.C.

Command substitution invokes a subshell.
Command substitution may result in word splitting.
COMMAND `echo a b`

# 2 args: a and b

COMMAND "`echo a b`"

# 1 arg: "a b"

COMMAND `echo`

# no arg

COMMAND "`echo`"

# one empty arg

# Thanks, S.C.

Even when there is no word splitting, command substitution can remove trailing newlines.
# cd "`pwd`"
# However...

# This should always work.

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mkdir 'dir with trailing newline
'
cd 'dir with trailing newline
'
cd "`pwd`" # Error message:
# bash: cd: /tmp/file with trailing newline: No such file or directory
cd "$PWD"

# Works fine.

old_tty_setting=$(stty -g)
echo "Hit a key "
stty -icanon -echo

# Save old terminal setting.

# Disable "canonical" mode for terminal.
# Also, disable *local* echo.
key=$(dd bs=1 count=1 2> /dev/null)
# Using 'dd' to get a keypress.
stty "$old_tty_setting"
# Restore old setting.
echo "You hit ${#key} key." # ${#variable} = number of characters in $variable
#
# Hit any key except RETURN, and the output is "You hit 1 key."
# Hit RETURN, and it's "You hit 0 key."
# The newline gets eaten in the command substitution.
#Code snippet by Stéphane Chazelas.

Using echo to output an unquoted variable set with command substitution removes trailing newlines
characters from the output of the reassigned command(s). This can cause unpleasant surprises.
dir_listing=`ls -l`
echo $dir_listing

# unquoted

# Expecting a nicely ordered directory listing.
# However, what you get is:
# total 3 -rw-rw-r-- 1 bozo bozo 30 May 13 17:15 1.txt -rw-rw-r-- 1 bozo
# bozo 51 May 15 20:57 t2.sh -rwxr-xr-x 1 bozo bozo 217 Mar 5 21:13 wi.sh
# The newlines disappeared.

echo "$dir_listing"
# quoted
# -rw-rw-r-1 bozo
30 May 13 17:15 1.txt
# -rw-rw-r-1 bozo
51 May 15 20:57 t2.sh
# -rwxr-xr-x
1 bozo
217 Mar 5 21:13 wi.sh

Command substitution even permits setting a variable to the contents of a file, using either redirection or the
cat command.
variable1=`/dev/null|grep -E "^I.*Cls=03.*Prot=02"`
kbdoutput=`cat /proc/bus/usb/devices 2>/dev/null|grep -E "^I.*Cls=03.*Prot=01"`
...
fi

Do not set a variable to the contents of a long text file unless you have a very good reason for doing so.
Do not set a variable to the contents of a binary file, even as a joke.

Example 12-1. Stupid script tricks
#!/bin/bash
# stupid-script-tricks.sh: Don't try this at home, folks.
# From "Stupid Script Tricks," Volume I.

dangerous_variable=`cat /boot/vmlinuz`

# The compressed Linux kernel itself.

echo "string-length of \$dangerous_variable = ${#dangerous_variable}"
# string-length of $dangerous_variable = 794151
# (Does not give same count as 'wc -c /boot/vmlinuz'.)
# echo "$dangerous_variable"
# Don't try this! It would hang the script.

# The document author is aware of no useful applications for
#+ setting a variable to the contents of a binary file.
exit 0

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Notice that a buffer overrun does not occur. This is one instance where an interpreted language, such as
Bash, provides more protection from programmer mistakes than a compiled language.
Command substitution permits setting a variable to the output of a loop. The key to this is grabbing the output
of an echo command within the loop.

Example 12-2. Generating a variable from a loop
#!/bin/bash
# csubloop.sh: Setting a variable to the output of a loop.
variable1=`for i in 1 2 3 4 5
do
echo -n "$i"
done`

# The 'echo' command is critical
#+ to command substitution here.

echo "variable1 = $variable1"

# variable1 = 12345

i=0
variable2=`while [ "$i" -lt 10 ]
do
echo -n "$i"
# Again, the necessary 'echo'.
let "i += 1"
# Increment.
done`
echo "variable2 = $variable2"

# variable2 = 0123456789

# Demonstrates that it's possible to embed a loop
#+ within a variable declaration.
exit 0

Command substitution makes it possible to extend the toolset available to Bash. It is simply a matter of
writing a program or script that outputs to stdout (like a well-behaved UNIX tool should) and assigning
that output to a variable.
#include 
/*

"Hello, world." C program

*/

int main()
{
printf( "Hello, world.\n" );
return (0);
}
bash$ gcc -o hello hello.c

#!/bin/bash
# hello.sh
greeting=`./hello`
echo $greeting

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bash$ sh hello.sh
Hello, world.

The $(...) form has superseded backticks for command substitution.
output=$(sed -n /"$1"/p $file)

# From "grp.sh"

example.

# Setting a variable to the contents of a text file.
File_contents1=$(cat $file1)
File_contents2=$(<$file2)
# Bash permits this also.

The $(...) form of command substitution treats a double backslash in a different way than `...`.
bash$ echo `echo \\`

bash$ echo $(echo \\)
\

The $(...) form of command substitution permits nesting. [56]
word_count=$( wc -w $(echo * | awk '{print $8}') )

Or, for something a bit more elaborate . . .

Example 12-3. Finding anagrams
#!/bin/bash
# agram2.sh
# Example of nested command substitution.
#
#+
#
#

Uses "anagram" utility
that is part of the author's "yawl" word list package.
http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
http://bash.webofcrafts.net/yawl-0.3.2.tar.gz

E_NOARGS=66
E_BADARG=67
MINLEN=7
if [ -z "$1" ]
then
echo "Usage $0 LETTERSET"
exit $E_NOARGS
# Script needs a command-line argument.
elif [ ${#1} -lt $MINLEN ]
then
echo "Argument must have at least $MINLEN letters."
exit $E_BADARG
fi

FILTER='.......'
# Must have at least 7 letters.
#
1234567
Anagrams=( $(echo $(anagram $1 | grep $FILTER) ) )
#
$(
$( nested command sub.
) )

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#

(

echo
echo "${#Anagrams[*]}
echo
echo ${Anagrams[0]}
echo ${Anagrams[1]}

array assignment

)

# echo "${Anagrams[*]}"

7+ letter anagrams found"
# First anagram.
# Second anagram.
# Etc.
# To list all the anagrams in a single line . . .

# Look ahead to the "Arrays" chapter for enlightenment on
#+ what's going on here.
# See also the agram.sh script for an example of anagram finding.
exit $?

Examples of command substitution in shell scripts:
1. Example 11-7
2. Example 11-26
3. Example 9-16
4. Example 16-3
5. Example 16-22
6. Example 16-17
7. Example 16-54
8. Example 11-13
9. Example 11-10
10. Example 16-32
11. Example 20-8
12. Example A-16
13. Example 29-3
14. Example 16-47
15. Example 16-48
16. Example 16-49

Chapter 12. Command Substitution

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Chapter 13. Arithmetic Expansion
Arithmetic expansion provides a powerful tool for performing (integer) arithmetic operations in scripts.
Translating a string into a numerical expression is relatively straightforward using backticks, double
parentheses, or let.
Variations
Arithmetic expansion with backticks (often used in conjunction with expr)
z=`expr $z + 3`

# The 'expr' command performs the expansion.

Arithmetic expansion with double parentheses, and using let
The use of backticks (backquotes) in arithmetic expansion has been superseded by double parentheses
-- ((...)) and $((...)) -- and also by the very convenient let construction.
z=$(($z+3))
z=$((z+3))

#
#
#+
#+

# $((EXPRESSION)) is arithmetic expansion.

Also correct.
Within double parentheses,
parameter dereferencing
is optional.

# Not to be confused with
#+ command substitution.

# You may also use operations within double parentheses without assignment.
n=0
echo "n = $n"
(( n += 1 ))
# (( $n += 1 )) is incorrect!
echo "n = $n"

let z=z+3
let "z += 3"

# n = 0
# Increment.
# n = 1

# Quotes permit the use of spaces in variable assignment.
# The 'let' operator actually performs arithmetic evaluation,
#+ rather than expansion.

Examples of arithmetic expansion in scripts:
1. Example 16-9
2. Example 11-14
3. Example 27-1
4. Example 27-11
5. Example A-16

Chapter 13. Arithmetic Expansion

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Chapter 14. Recess Time
This bizarre little intermission gives the reader a chance to relax and maybe laugh a bit.

Fellow Linux user, greetings! You are reading something which
will bring you luck and good fortune. Just e-mail a copy of
this document to 10 of your friends. Before making the copies,
send a 100-line Bash script to the first person on the list
at the bottom of this letter. Then delete their name and add
yours to the bottom of the list.
Don't break the chain! Make the copies within 48 hours.
Wilfred P. of Brooklyn failed to send out his ten copies and
woke the next morning to find his job description changed
to "COBOL programmer." Howard L. of Newport News sent
out his ten copies and within a month had enough hardware
to build a 100-node Beowulf cluster dedicated to playing
Tuxracer. Amelia V. of Chicago laughed at this letter
and broke the chain. Shortly thereafter, a fire broke out
in her terminal and she now spends her days writing
documentation for MS Windows.
Don't break the chain!

Send out your ten copies today!

Courtesy 'NIX "fortune cookies", with some alterations and many apologies

Chapter 14. Recess Time

171

Part 4. Commands
Mastering the commands on your Linux machine is an indispensable prelude to writing effective shell scripts.
This section covers the following commands:
• . (See also source)
• ac
• adduser
• agetty
• agrep
• ar
• arch
• at
• autoload
• awk (See also Using awk for math operations)
• badblocks
• banner
• basename
• batch
• bc
• bg
• bind
• bison
• builtin
• bzgrep
• bzip2
• cal
• caller
• cat
• cd
• chattr
• chfn
• chgrp
• chkconfig
• chmod
• chown
• chroot
• cksum
• clear
• clock
• cmp
• col
• colrm
• column
• comm
• command
• compgen
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• complete
• compress
• coproc
• cp
• cpio
• cron
• crypt
• csplit
• cu
• cut
• date
• dc
• dd
• debugfs
• declare
• depmod
• df
• dialog
• diff
• diff3
• diffstat
• dig
• dirname
• dirs
• disown
• dmesg
• doexec
• dos2unix
• du
• dump
• dumpe2fs
• e2fsck
• echo
• egrep
• enable
• enscript
• env
• eqn
• eval
• exec
• exit (Related topic: exit status)
• expand
• export
• expr
• factor
• false
• fdformat
• fdisk
• fg
• fgrep
• file
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• find
• finger
• flex
• flock
• fmt
• fold
• free
• fsck
• ftp
• fuser
• getfacl
• getopt
• getopts
• gettext
• getty
• gnome-mount
• grep
• groff
• groupmod
• groups (Related topic: the $GROUPS variable)
• gs
• gzip
• halt
• hash
• hdparm
• head
• help
• hexdump
• host
• hostid
• hostname (Related topic: the $HOSTNAME variable)
• hwclock
• iconv
• id (Related topic: the $UID variable)
• ifconfig
• info
• infocmp
• init
• insmod
• install
• ip
• ipcalc
• iptables
• iwconfig
• jobs
• join
• jot
• kill
• killall
• last
• lastcomm
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• lastlog
• ldd
• less
• let
• lex
• lid
• ln
• locate
• lockfile
• logger
• logname
• logout
• logrotate
• look
• losetup
• lp
• ls
• lsdev
• lsmod
• lsof
• lspci
• lsusb
• ltrace
• lynx
• lzcat
• lzma
• m4
• mail
• mailstats
• mailto
• make
• MAKEDEV
• man
• mapfile
• mcookie
• md5sum
• merge
• mesg
• mimencode
• mkbootdisk
• mkdir
• mke2fs
• mkfifo
• mkisofs
• mknod
• mkswap
• mktemp
• mmencode
• modinfo
• modprobe
• more
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• mount
• msgfmt
• mv
• nc
• netconfig
• netstat
• newgrp
• nice
• nl
• nm
• nmap
• nohup
• nslookup
• objdump
• od
• openssl
• passwd
• paste
• patch (Related topic: diff)
• pathchk
• pax
• pgrep
• pidof
• ping
• pkill
• popd
• pr
• printenv
• printf
• procinfo
• ps
• pstree
• ptx
• pushd
• pwd (Related topic: the $PWD variable)
• quota
• rcp
• rdev
• rdist
• read
• readelf
• readlink
• readonly
• reboot
• recode
• renice
• reset
• resize
• restore
• rev
• rlogin
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• rm
• rmdir
• rmmod
• route
• rpm
• rpm2cpio
• rsh
• rsync
• runlevel
• run-parts
• rx
• rz
• sar
• scp
• script
• sdiff
• sed
• seq
• service
• set
• setfacl
• setquota
• setserial
• setterm
• sha1sum
• shar
• shopt
• shred
• shutdown
• size
• skill
• sleep
• slocate
• snice
• sort
• source
• sox
• split
• sq
• ssh
• stat
• strace
• strings
• strip
• stty
• su
• sudo
• sum
• suspend
• swapoff
• swapon
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• sx
• sync
• sz
• tac
• tail
• tar
• tbl
• tcpdump
• tee
• telinit
• telnet
• Tex
• texexec
• time
• times
• tmpwatch
• top
• touch
• tput
• tr
• traceroute
• true
• tset
• tsort
• tty
• tune2fs
• type
• typeset
• ulimit
• umask
• umount
• uname
• unarc
• unarj
• uncompress
• unexpand
• uniq
• units
• unlzma
• unrar
• unset
• unsq
• unzip
• uptime
• usbmodules
• useradd
• userdel
• usermod
• users
• usleep
• uucp
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• uudecode
• uuencode
• uux
• vacation
• vdir
• vmstat
• vrfy
•w
• wait
• wall
• watch
• wc
• wget
• whatis
• whereis
• which
• who
• whoami
• whois
• write
• xargs
• yacc
• yes
• zcat
• zdiff
• zdump
• zegrep
• zfgrep
• zgrep
• zip
Table of Contents
15. Internal Commands and Builtins
15.1. Job Control Commands
16. External Filters, Programs and Commands
16.1. Basic Commands
16.2. Complex Commands
16.3. Time / Date Commands
16.4. Text Processing Commands
16.5. File and Archiving Commands
16.6. Communications Commands
16.7. Terminal Control Commands
16.8. Math Commands
16.9. Miscellaneous Commands
17. System and Administrative Commands
17.1. Analyzing a System Script

Part 4. Commands

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Chapter 15. Internal Commands and Builtins
A builtin is a command contained within the Bash tool set, literally built in. This is either for performance
reasons -- builtins execute faster than external commands, which usually require forking off [57] a separate
process -- or because a particular builtin needs direct access to the shell internals.

When a command or the shell itself initiates (or spawns) a new subprocess to carry out a task, this is called
forking. This new process is the child, and the process that forked it off is the parent. While the child
process is doing its work, the parent process is still executing.
Note that while a parent process gets the process ID of the child process, and can thus pass arguments to it,
the reverse is not true. This can create problems that are subtle and hard to track down.

Example 15-1. A script that spawns multiple instances of itself
#!/bin/bash
# spawn.sh

PIDS=$(pidof sh $0) # Process IDs of the various instances of this script.
P_array=( $PIDS )
# Put them in an array (why?).
echo $PIDS
# Show process IDs of parent and child processes.
let "instances = ${#P_array[*]} - 1" # Count elements, less 1.
# Why subtract 1?
echo "$instances instance(s) of this script running."
echo "[Hit Ctl-C to exit.]"; echo

sleep 1
sh $0

# Wait.
# Play it again, Sam.

exit 0

# Not necessary; script will never get to here.
# Why not?

# After exiting with a Ctl-C,
#+ do all the spawned instances of the script die?
# If so, why?
#
#
#
#

Note:
---Be careful not to run this script too long.
It will eventually eat up too many system resources.

# Is having a script spawn multiple instances of itself
#+ an advisable scripting technique.
# Why or why not?

Generally, a Bash builtin does not fork a subprocess when it executes within a script. An external system
command or filter in a script usually will fork a subprocess.
A builtin may be a synonym to a system command of the same name, but Bash reimplements it internally. For
example, the Bash echo command is not the same as /bin/echo, although their behavior is almost
identical.
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#!/bin/bash
echo "This line uses the \"echo\" builtin."
/bin/echo "This line uses the /bin/echo system command."

A keyword is a reserved word, token or operator. Keywords have a special meaning to the shell, and indeed
are the building blocks of the shell's syntax. As examples, for, while, do, and ! are keywords. Similar to a
builtin, a keyword is hard-coded into Bash, but unlike a builtin, a keyword is not in itself a command, but a
subunit of a command construct. [58]
I/O
echo
prints (to stdout) an expression or variable (see Example 4-1).
echo Hello
echo $a

An echo requires the -e option to print escaped characters. See Example 5-2.
Normally, each echo command prints a terminal newline, but the -n option suppresses this.

An echo can be used to feed a sequence of commands down a pipe.
if echo "$VAR" | grep -q txt
# if [[ $VAR = *txt* ]]
then
echo "$VAR contains the substring sequence \"txt\""
fi

An echo, in combination with command substitution can set a variable.
a=`echo "HELLO" | tr A-Z a-z`
See also Example 16-22, Example 16-3, Example 16-47, and Example 16-48.
Be aware that echo `command` deletes any linefeeds that the output of command generates.
The $IFS (internal field separator) variable normally contains \n (linefeed) as one of its set of
whitespace characters. Bash therefore splits the output of command at linefeeds into arguments to
echo. Then echo outputs these arguments, separated by spaces.
bash$ ls -l /usr/share/apps/kjezz/sounds
-rw-r--r-1 root
root
1407 Nov 7 2000 reflect.au
-rw-r--r-1 root
root
362 Nov 7 2000 seconds.au

bash$ echo `ls -l /usr/share/apps/kjezz/sounds`
total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root root ...

So, how can we embed a linefeed within an echoed character string?
# Embedding a linefeed?
echo "Why doesn't this string \n split on two lines?"
# Doesn't split.

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# Let's try something else.
echo
echo $"A line of text containing
a linefeed."
# Prints as two distinct lines (embedded linefeed).
# But, is the "$" variable prefix really necessary?
echo
echo "This string splits
on two lines."
# No, the "$" is not needed.
echo
echo "---------------"
echo
echo -n $"Another line of text containing
a linefeed."
# Prints as two distinct lines (embedded linefeed).
# Even the -n option fails to suppress the linefeed here.
echo
echo
echo "---------------"
echo
echo
# However, the following doesn't work as expected.
# Why not? Hint: Assignment to a variable.
string1=$"Yet another line of text containing
a linefeed (maybe)."
echo $string1
# Yet another line of text containing a linefeed (maybe).
#
^
# Linefeed becomes a space.
# Thanks, Steve Parker, for pointing this out.

This command is a shell builtin, and not the same as /bin/echo, although its
behavior is similar.
bash$ type -a echo
echo is a shell builtin
echo is /bin/echo

printf
The printf, formatted print, command is an enhanced echo. It is a limited variant of the C language
printf() library function, and its syntax is somewhat different.
printf format-string... parameter...
This is the Bash builtin version of the /bin/printf or /usr/bin/printf command. See the
printf manpage (of the system command) for in-depth coverage.
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Older versions of Bash may not support printf.
Example 15-2. printf in action
#!/bin/bash
# printf demo
declare -r PI=3.14159265358979
declare -r DecimalConstant=31373

# Read-only variable, i.e., a constant.

Message1="Greetings,"
Message2="Earthling."
echo
printf "Pi to 2 decimal places = %1.2f" $PI
echo
printf "Pi to 9 decimal places = %1.9f" $PI

# It even rounds off correctly.

printf "\n"

# Prints a line feed,
# Equivalent to 'echo' . . .

printf "Constant = \t%d\n" $DecimalConstant

# Inserts tab (\t).

printf "%s %s \n" $Message1 $Message2
echo
# ==========================================#
# Simulation of C function, sprintf().
# Loading a variable with a formatted string.
echo
Pi12=$(printf "%1.12f" $PI)
echo "Pi to 12 decimal places = $Pi12"

# Roundoff error!

Msg=`printf "%s %s \n" $Message1 $Message2`
echo $Msg; echo $Msg
# As it happens, the 'sprintf' function can now be accessed
#+ as a loadable module to Bash,
#+ but this is not portable.
exit 0

Formatting error messages is a useful application of printf
E_BADDIR=85
var=nonexistent_directory
error()
{
printf "$@" >&2
# Formats positional params passed, and sends them to stderr.
echo
exit $E_BADDIR
}
cd $var || error $"Can't cd to %s." "$var"

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# Thanks, S.C.

See also Example 36-15.
read
"Reads" the value of a variable from stdin, that is, interactively fetches input from the keyboard.
The -a option lets read get array variables (see Example 27-6).

Example 15-3. Variable assignment, using read
#!/bin/bash
# "Reading" variables.
echo -n "Enter the value of variable 'var1': "
# The -n option to echo suppresses newline.
read var1
# Note no '$' in front of var1, since it is being set.
echo "var1 = $var1"

echo
# A single 'read' statement can set multiple variables.
echo -n "Enter the values of variables 'var2' and 'var3' "
echo =n "(separated by a space or tab): "
read var2 var3
echo "var2 = $var2
var3 = $var3"
# If you input only one value,
#+ the other variable(s) will remain unset (null).
exit 0

A read without an associated variable assigns its input to the dedicated variable $REPLY.

Example 15-4. What happens when read has no variable
#!/bin/bash
# read-novar.sh
echo
# -------------------------- #
echo -n "Enter a value: "
read var
echo "\"var\" = "$var""
# Everything as expected here.
# -------------------------- #
echo
# ------------------------------------------------------------------- #
echo -n "Enter another value: "
read
# No variable supplied for 'read', therefore...
#+ Input to 'read' assigned to default variable, $REPLY.
var="$REPLY"
echo "\"var\" = "$var""
# This is equivalent to the first code block.

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# ------------------------------------------------------------------- #
echo
echo "========================="
echo

# This example is similar to the "reply.sh" script.
# However, this one shows that $REPLY is available
#+ even after a 'read' to a variable in the conventional way.

# ================================================================= #
#
#

In some instances, you might wish to discard the first value read.
In such cases, simply ignore the $REPLY variable.

{ # Code block.
read
read line2
} <$0
echo "Line 2 of
echo "$line2"
echo

# Line 1, to be discarded.
# Line 2, saved in variable.
this script is:"
#
# read-novar.sh
#
#!/bin/bash line discarded.

# See also the soundcard-on.sh script.
exit 0

Normally, inputting a \ suppresses a newline during input to a read. The -r option causes an
inputted \ to be interpreted literally.

Example 15-5. Multi-line input to read
#!/bin/bash
echo
echo "Enter a string terminated by a \\, then press ."
echo "Then, enter a second string (no \\ this time), and again press ."
read var1

# The "\" suppresses the newline, when reading $var1.
#
first line \
#
second line

echo "var1 = $var1"
#
var1 = first line second line
# For each line terminated by a "\"
#+ you get a prompt on the next line to continue feeding characters into var1.
echo; echo
echo "Enter another string terminated by a \\ , then press ."
read -r var2 # The -r option causes the "\" to be read literally.
#
first line \
echo "var2 = $var2"
#
var2 = first line \

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# Data entry terminates with the first .
echo
exit 0

The read command has some interesting options that permit echoing a prompt and even reading
keystrokes without hitting ENTER.
# Read a keypress without hitting ENTER.
read -s -n1 -p "Hit a key " keypress
echo; echo "Keypress was "\"$keypress\""."
# -s option means do not echo input.
# -n N option means accept only N characters of input.
# -p option means echo the following prompt before reading input.
# Using these options is tricky, since they need to be in the correct order.

The -n option to read also allows detection of the arrow keys and certain of the other unusual keys.

Example 15-6. Detecting the arrow keys
#!/bin/bash
# arrow-detect.sh: Detects the arrow keys, and a few more.
# Thank you, Sandro Magi, for showing me how.
# -------------------------------------------# Character codes generated by the keypresses.
arrowup='\[A'
arrowdown='\[B'
arrowrt='\[C'
arrowleft='\[D'
insert='\[2'
delete='\[3'
# -------------------------------------------SUCCESS=0
OTHER=65
echo -n "Press a key... "
# May need to also press ENTER if a key not listed above pressed.
read -n3 key
# Read 3 characters.
echo -n "$key" | grep "$arrowup"
if [ "$?" -eq $SUCCESS ]
then
echo "Up-arrow key pressed."
exit $SUCCESS
fi

#Check if character code detected.

echo -n "$key" | grep "$arrowdown"
if [ "$?" -eq $SUCCESS ]
then
echo "Down-arrow key pressed."
exit $SUCCESS
fi

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echo -n "$key" | grep "$arrowrt"
if [ "$?" -eq $SUCCESS ]
then
echo "Right-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$arrowleft"
if [ "$?" -eq $SUCCESS ]
then
echo "Left-arrow key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$insert"
if [ "$?" -eq $SUCCESS ]
then
echo "\"Insert\" key pressed."
exit $SUCCESS
fi
echo -n "$key" | grep "$delete"
if [ "$?" -eq $SUCCESS ]
then
echo "\"Delete\" key pressed."
exit $SUCCESS
fi

echo " Some other key pressed."
exit $OTHER
# ========================================= #
# Mark Alexander came up with a simplified
#+ version of the above script (Thank you!).
# It eliminates the need for grep.
#!/bin/bash
uparrow=$'\x1b[A'
downarrow=$'\x1b[B'
leftarrow=$'\x1b[D'
rightarrow=$'\x1b[C'
read -s -n3 -p "Hit an arrow key: " x
case "$x" in
$uparrow)
echo "You
;;
$downarrow)
echo "You
;;
$leftarrow)
echo "You
;;
$rightarrow)
echo "You
;;

pressed up-arrow"

pressed down-arrow"

pressed left-arrow"

pressed right-arrow"

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esac
exit $?
# ========================================= #
# Antonio Macchi has a simpler alternative.
#!/bin/bash
while true
do
read -sn1 a
test "$a" == `echo -en "\e"` || continue
read -sn1 a
test "$a" == "[" || continue
read -sn1 a
case "$a" in
A) echo "up";;
B) echo "down";;
C) echo "right";;
D) echo "left";;
esac
done
# ========================================= #
#
#
#

Exercise:
-------1) Add detection of the "Home," "End," "PgUp," and "PgDn" keys.

The -n option to read will not detect the ENTER (newline) key.
The -t option to read permits timed input (see Example 9-4 and Example A-41).
The -u option takes the file descriptor of the target file.

The read command may also "read" its variable value from a file redirected to stdin. If the file
contains more than one line, only the first line is assigned to the variable. If read has more than one
parameter, then each of these variables gets assigned a successive whitespace-delineated string.
Caution!

Example 15-7. Using read with file redirection
#!/bin/bash
read var1 .

Example 15-21. Using getopts to read the options/arguments passed to a script
#!/bin/bash
# ex33.sh: Exercising getopts and OPTIND
#
Script modified 10/09/03 at the suggestion of Bill Gradwohl.

# Here we observe how 'getopts' processes command-line arguments to script.
# The arguments are parsed as "options" (flags) and associated arguments.
# Try invoking this script with:
#
'scriptname -mn'
#
'scriptname -oq qOption' (qOption can be some arbitrary string.)
#
'scriptname -qXXX -r'
#
#
'scriptname -qr'
#+
- Unexpected result, takes "r" as the argument to option "q"
#
'scriptname -q -r'
#+
- Unexpected result, same as above
#
'scriptname -mnop -mnop' - Unexpected result
#
(OPTIND is unreliable at stating where an option came from.)
#
# If an option expects an argument ("flag:"), then it will grab
#+ whatever is next on the command-line.
NO_ARGS=0
E_OPTERROR=85
if [ $# -eq "$NO_ARGS" ]
# Script invoked with no command-line args?
then
echo "Usage: `basename $0` options (-mnopqrs)"
exit $E_OPTERROR
# Exit and explain usage.
# Usage: scriptname -options
# Note: dash (-) necessary
fi

while getopts ":mnopq:rs" Option
do
case $Option in
m
) echo "Scenario #1: option -m[OPTIND=${OPTIND}]";;
n | o ) echo "Scenario #2: option -$Option[OPTIND=${OPTIND}]";;
p
) echo "Scenario #3: option -p[OPTIND=${OPTIND}]";;
q
) echo "Scenario #4: option -q-\
with argument \"$OPTARG\"
[OPTIND=${OPTIND}]";;
# Note that option 'q' must have an associated argument,
#+ otherwise it falls through to the default.
r | s ) echo "Scenario #5: option -$Option-";;
*
) echo "Unimplemented option chosen.";;
# Default.
esac
done
shift $(($OPTIND - 1))
# Decrements the argument pointer so it points to next argument.
# $1 now references the first non-option item supplied on the command-line
#+ if one exists.

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exit $?
#
As Bill Gradwohl states,
# "The getopts mechanism allows one to specify: scriptname -mnop -mnop
#+ but there is no reliable way to differentiate what came
#+ from where by using OPTIND."
# There are, however, workarounds.

Script Behavior
source, . (dot command)
This command, when invoked from the command-line, executes a script. Within a script, a source
file-name loads the file file-name. Sourcing a file (dot-command) imports code into the script,
appending to the script (same effect as the #include directive in a C program). The net result is the
same as if the "sourced" lines of code were physically present in the body of the script. This is useful
in situations when multiple scripts use a common data file or function library.

Example 15-22. "Including" a data file
#!/bin/bash
. data-file
# Load a data file.
# Same effect as "source data-file", but more portable.
# The file "data-file" must be present in current working directory,
#+ since it is referred to by its 'basename'.
# Now, reference some data from that file.
echo "variable1 (from data-file) = $variable1"
echo "variable3 (from data-file) = $variable3"
let "sum = $variable2 + $variable4"
echo "Sum of variable2 + variable4 (from data-file) = $sum"
echo "message1 (from data-file) is \"$message1\""
# Note:
escaped quotes
print_message This is the message-print function in the data-file.

exit 0

File data-file for Example 15-22, above. Must be present in same directory.
# This is a data file loaded by a script.
# Files of this type may contain variables, functions, etc.
# It may be loaded with a 'source' or '.' command by a shell script.
# Let's initialize some variables.
variable1=22
variable2=474
variable3=5
variable4=97
message1="Hello, how are you?"
message2="Enough for now. Goodbye."

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print_message ()
{
# Echoes any message passed to it.
if [ -z "$1" ]
then
return 1
# Error, if argument missing.
fi
echo
until [ -z "$1" ]
do
# Step through arguments passed to function.
echo -n "$1"
# Echo args one at a time, suppressing line feeds.
echo -n " "
# Insert spaces between words.
shift
# Next one.
done
echo
return 0
}

If the sourced file is itself an executable script, then it will run, then return control to the script that
called it. A sourced executable script may use a return for this purpose.

Arguments may be (optionally) passed to the sourced file as positional parameters.
source $filename $arg1 arg2

It is even possible for a script to source itself, though this does not seem to have any practical
applications.

Example 15-23. A (useless) script that sources itself
#!/bin/bash
# self-source.sh: a script sourcing itself "recursively."
# From "Stupid Script Tricks," Volume II.
MAXPASSCNT=100

# Maximum number of execution passes.

echo -n "$pass_count "
# At first execution pass, this just echoes two blank spaces,
#+ since $pass_count still uninitialized.
let "pass_count += 1"
# Assumes the uninitialized variable $pass_count
#+ can be incremented the first time around.
# This works with Bash and pdksh, but
#+ it relies on non-portable (and possibly dangerous) behavior.
# Better would be to initialize $pass_count to 0 before incrementing.
while [ "$pass_count" -le $MAXPASSCNT ]

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do
. $0

# Script "sources" itself, rather than calling itself.
# ./$0 (which would be true recursion) doesn't work here. Why?

done
#
#+
#+
#+
#
#
#
#+

What occurs here is not actually recursion,
since the script effectively "expands" itself, i.e.,
generates a new section of code
with each pass through the 'while' loop',
with each 'source' in line 20.
Of course, the script interprets each newly 'sourced' "#!" line
as a comment, and not as the start of a new script.

echo
exit 0

# The net effect is counting from 1 to 100.
# Very impressive.

# Exercise:
# -------# Write a script that uses this trick to actually do something useful.

exit
Unconditionally terminates a script. [62] The exit command may optionally take an integer argument,
which is returned to the shell as the exit status of the script. It is good practice to end all but the
simplest scripts with an exit 0, indicating a successful run.
If a script terminates with an exit lacking an argument, the exit status of the script is
the exit status of the last command executed in the script, not counting the exit. This is
equivalent to an exit $?.
An exit command may also be used to terminate a subshell.
exec
This shell builtin replaces the current process with a specified command. Normally, when the shell
encounters a command, it forks off a child process to actually execute the command. Using the exec
builtin, the shell does not fork, and the command exec'ed replaces the shell. When used in a script,
therefore, it forces an exit from the script when the exec'ed command terminates. [63]

Example 15-24. Effects of exec
#!/bin/bash
exec echo "Exiting \"$0\"."

# Exit from script here.

# ---------------------------------# The following lines never execute.
echo "This echo will never echo."
exit 99

#
#
#+
#

This script will not exit here.
Check exit value after script terminates
with an 'echo $?'.
It will *not* be 99.

Example 15-25. A script that exec's itself
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#!/bin/bash
# self-exec.sh
# Note: Set permissions on this script to 555 or 755,
#
then call it with ./self-exec.sh or sh ./self-exec.sh.
echo
echo "This line appears ONCE in the script, yet it keeps echoing."
echo "The PID of this instance of the script is still $$."
#
Demonstrates that a subshell is not forked off.
echo "==================== Hit Ctl-C to exit ===================="
sleep 1
exec $0

# Spawns another instance of this same script
#+ that replaces the previous one.

echo "This line will never echo!"

# Why not?

exit 99

# Will not exit here!
# Exit code will not be 99!

An exec also serves to reassign file descriptors. For example, exec  $IMAGE_DIRECTORY/$CONTENTSFILE
# The "l" option gives a "long" file listing.
# The "R" option makes the listing recursive.
# The "F" option marks the file types (directories get a trailing /).
echo "Creating table of contents."
# Create an image file preparatory to burning it onto the CDR.
mkisofs -r -o $IMAGEFILE $IMAGE_DIRECTORY
echo "Creating ISO9660 file system image ($IMAGEFILE)."
# Burn the CDR.
echo "Burning the disk."
echo "Please be patient, this will take a while."
wodim -v -isosize dev=$DEVICE $IMAGEFILE
# In newer Linux distros, the "wodim" utility assumes the
#+ functionality of "cdrecord."
exitcode=$?
echo "Exit code = $exitcode"
exit $exitcode

cat, tac
cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it
is commonly used to concatenate files.
# Uses of 'cat'
cat filename

# Lists the file.

cat file.1 file.2 file.3 > file.123

# Combines three files into one.

The -n option to cat inserts consecutive numbers before all lines of the target file(s). The -b option
numbers only the non-blank lines. The -v option echoes nonprintable characters, using ^ notation.
The -s option squeezes multiple consecutive blank lines into a single blank line.
See also Example 16-28 and Example 16-24.
In a pipe, it may be more efficient to redirect the stdin to a file, rather than to cat
the file.
cat filename | tr a-z A-Z
tr a-z A-Z < filename

# Same effect, but starts one less process,
#+ and also dispenses with the pipe.

tac, is the inverse of cat, listing a file backwards from its end.
rev
reverses each line of a file, and outputs to stdout. This does not have the same effect as tac, as it
preserves the order of the lines, but flips each one around (mirror image).
bash$ cat file1.txt
This is line 1.
This is line 2.

bash$ tac file1.txt

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This is line 2.
This is line 1.

bash$ rev file1.txt
.1 enil si sihT
.2 enil si sihT

cp
This is the file copy command. cp file1 file2 copies file1 to file2, overwriting file2 if
it already exists (see Example 16-6).
Particularly useful are the -a archive flag (for copying an entire directory tree), the
-u update flag (which prevents overwriting identically-named newer files), and the
-r and -R recursive flags.
cp -u source_dir/* dest_dir
# "Synchronize" dest_dir to source_dir
#+ by copying over all newer and not previously existing files.

mv
This is the file move command. It is equivalent to a combination of cp and rm. It may be used to
move multiple files to a directory, or even to rename a directory. For some examples of using mv in a
script, see Example 10-11 and Example A-2.
When used in a non-interactive script, mv takes the -f (force) option to bypass user
input.
When a directory is moved to a preexisting directory, it becomes a subdirectory of the
destination directory.
bash$ mv source_directory target_directory
bash$ ls -lF target_directory
total 1
drwxrwxr-x
2 bozo bozo

1024 May 28 19:20 source_directory/

rm
Delete (remove) a file or files. The -f option forces removal of even readonly files, and is useful for
bypassing user input in a script.

The rm command will, by itself, fail to remove filenames beginning with a dash.
Why? Because rm sees a dash-prefixed filename as an option.
bash$ rm -badname
rm: invalid option -- b
Try `rm --help' for more information.

One clever workaround is to precede the filename with a " -- " (the end-of-options
flag).
bash$ rm -- -badname

Another method to is to preface the filename to be removed with a dot-slash .

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bash$ rm ./-badname

When used with the recursive flag -r, this command removes files all the way down
the directory tree from the current directory. A careless rm -rf * can wipe out a big
chunk of a directory structure.
rmdir
Remove directory. The directory must be empty of all files -- including "invisible" dotfiles [69] -- for
this command to succeed.
mkdir
Make directory, creates a new directory. For example, mkdir -p
project/programs/December creates the named directory. The -p option automatically
creates any necessary parent directories.
chmod
Changes the attributes of an existing file or directory (see Example 15-14).
chmod +x filename
# Makes "filename" executable for all users.
chmod u+s filename
# Sets "suid" bit on "filename" permissions.
# An ordinary user may execute "filename" with same privileges as the file's owner.
# (This does not apply to shell scripts.)
chmod 644 filename
# Makes "filename" readable/writable to owner, readable to others
#+ (octal mode).
chmod 444 filename
# Makes "filename" read-only for all.
# Modifying the file (for example, with a text editor)
#+ not allowed for a user who does not own the file (except for root),
#+ and even the file owner must force a file-save
#+ if she modifies the file.
# Same restrictions apply for deleting the file.
chmod 1777 directory-name
# Gives everyone read, write, and execute permission in directory,
#+ however also sets the "sticky bit".
# This means that only the owner of the directory,
#+ owner of the file, and, of course, root
#+ can delete any particular file in that directory.
chmod 111 directory-name
# Gives everyone execute-only permission in a directory.
# This means that you can execute and READ the files in that directory
#+ (execute permission necessarily includes read permission
#+ because you can't execute a file without being able to read it).
# But you can't list the files or search for them with the "find" command.
# These restrictions do not apply to root.
chmod 000 directory-name
# No permissions at all for that directory.
# Can't read, write, or execute files in it.
# Can't even list files in it or "cd" to it.
# But, you can rename (mv) the directory
#+ or delete it (rmdir) if it is empty.
# You can even symlink to files in the directory,

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#+ but you can't read, write, or execute the symlinks.
# These restrictions do not apply to root.

chattr
Change file attributes. This is analogous to chmod above, but with different options and a different
invocation syntax, and it works only on ext2/ext3 filesystems.
One particularly interesting chattr option is i. A chattr +i filename marks the file as immutable.
The file cannot be modified, linked to, or deleted, not even by root. This file attribute can be set or
removed only by root. In a similar fashion, the a option marks the file as append only.
root# chattr +i file1.txt

root# rm file1.txt
rm: remove write-protected regular file `file1.txt'? y
rm: cannot remove `file1.txt': Operation not permitted

If a file has the s (secure) attribute set, then when it is deleted its block is overwritten with binary
zeroes. [70]
If a file has the u (undelete) attribute set, then when it is deleted, its contents can still be retrieved
(undeleted).
If a file has the c (compress) attribute set, then it will automatically be compressed on writes to disk,
and uncompressed on reads.
The file attributes set with chattr do not show in a file listing (ls -l).
ln
Creates links to pre-existings files. A "link" is a reference to a file, an alternate name for it. The ln
command permits referencing the linked file by more than one name and is a superior alternative to
aliasing (see Example 4-6).
The ln creates only a reference, a pointer to the file only a few bytes in size.

The ln command is most often used with the -s, symbolic or "soft" link flag. Advantages of using the
-s flag are that it permits linking across file systems or to directories.
The syntax of the command is a bit tricky. For example: ln -s oldfile newfile links the
previously existing oldfile to the newly created link, newfile.
If a file named newfile has previously existed, an error message will result.
Which type of link to use?
As John Macdonald explains it:
Both of these [types of links] provide a certain measure of dual reference -- if you edit the contents
of the file using any name, your changes will affect both the original name and either a hard or soft

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new name. The differences between them occurs when you work at a higher level. The advantage of
a hard link is that the new name is totally independent of the old name -- if you remove or rename
the old name, that does not affect the hard link, which continues to point to the data while it would
leave a soft link hanging pointing to the old name which is no longer there. The advantage of a soft
link is that it can refer to a different file system (since it is just a reference to a file name, not to
actual data). And, unlike a hard link, a symbolic link can refer to a directory.
Links give the ability to invoke a script (or any other type of executable) with multiple names, and
having that script behave according to how it was invoked.

Example 16-2. Hello or Good-bye
#!/bin/bash
# hello.sh: Saying "hello" or "goodbye"
#+
depending on how script is invoked.
#
#
#
#
#

Make a link in current working directory ($PWD) to this script:
ln -s hello.sh goodbye
Now, try invoking this script both ways:
./hello.sh
./goodbye

HELLO_CALL=65
GOODBYE_CALL=66
if [ $0 = "./goodbye" ]
then
echo "Good-bye!"
# Some other goodbye-type commands, as appropriate.
exit $GOODBYE_CALL
fi
echo "Hello!"
# Some other hello-type commands, as appropriate.
exit $HELLO_CALL

man, info
These commands access the manual and information pages on system commands and installed
utilities. When available, the info pages usually contain more detailed descriptions than do the man
pages.
There have been various attempts at "automating" the writing of man pages. For a script that makes a
tentative first step in that direction, see Example A-39.

16.2. Complex Commands
Commands for more advanced users
find
-exec COMMAND \;

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Carries out COMMAND on each file that find matches. The command sequence terminates with ; (the
";" is escaped to make certain the shell passes it to find literally, without interpreting it as a special
character).
bash$ find ~/ -name '*.txt'
/home/bozo/.kde/share/apps/karm/karmdata.txt
/home/bozo/misc/irmeyc.txt
/home/bozo/test-scripts/1.txt

If COMMAND contains {}, then find substitutes the full path name of the selected file for "{}".
find ~/ -name 'core*' -exec rm {} \;
# Removes all core dump files from user's home directory.
find /home/bozo/projects -mtime -1
#
^
Note minus sign!
# Lists all files in /home/bozo/projects directory tree
#+ that were modified within the last day (current_day - 1).
#
find /home/bozo/projects -mtime 1
# Same as above, but modified *exactly* one day ago.
#
# mtime = last modification time of the target file
# ctime = last status change time (via 'chmod' or otherwise)
# atime = last access time
DIR=/home/bozo/junk_files
find "$DIR" -type f -atime +5 -exec rm {} \;
#
^
^^
# Curly brackets are placeholder for the path name output by "find."
#
# Deletes all files in "/home/bozo/junk_files"
#+ that have not been accessed in *at least* 5 days (plus sign ... +5).
#
# "-type filetype", where
# f = regular file
# d = directory
# l = symbolic link, etc.
#
# (The 'find' manpage and info page have complete option listings.)
find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \;
# Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files.
# There a few extraneous hits. Can they be filtered out?
# Possibly by:
find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \
| grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$'
#
# [:digit:] is one of the character classes
#+ introduced with the POSIX 1003.2 standard.
# Thanks, Stéphane Chazelas.

The -exec option to find should not be confused with the exec shell builtin.
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Example 16-3. Badname, eliminate file names in current directory containing bad characters
and whitespace.
#!/bin/bash
# badname.sh
# Delete filenames in current directory containing bad characters.
for filename in *
do
badname=`echo "$filename" | sed -n /[\+\{\;\"\\\=\?~\(\)\<\>\&\*\|\$]/p`
# badname=`echo "$filename" | sed -n '/[+{;"\=?~()<>&*|$]/p'` also works.
# Deletes files containing these nasties:
+ { ; " \ = ? ~ ( ) < > & * | $
#
rm $badname 2>/dev/null
#
^^^^^^^^^^^ Error messages deep-sixed.
done
# Now, take care of files containing all manner of whitespace.
find . -name "* *" -exec rm -f {} \;
# The path name of the file that _find_ finds replaces the "{}".
# The '\' ensures that the ';' is interpreted literally, as end of command.
exit 0
#--------------------------------------------------------------------# Commands below this line will not execute because of _exit_ command.
# An alternative to the above script:
find . -name '*[+{;"\\=?~()<>&*|$ ]*' -maxdepth 0 \
-exec rm -f '{}' \;
# The "-maxdepth 0" option ensures that _find_ will not search
#+ subdirectories below $PWD.
# (Thanks, S.C.)

Example 16-4. Deleting a file by its inode number
#!/bin/bash
# idelete.sh: Deleting a file by its inode number.
# This is useful when a filename starts with an illegal character,
#+ such as ? or -.
ARGCOUNT=1
E_WRONGARGS=70
E_FILE_NOT_EXIST=71
E_CHANGED_MIND=72

# Filename arg must be passed to script.

if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` filename"
exit $E_WRONGARGS
fi
if [ ! -e "$1" ]
then
echo "File \""$1"\" does not exist."
exit $E_FILE_NOT_EXIST
fi

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inum=`ls -i | grep "$1" | awk '{print $1}'`
# inum = inode (index node) number of file
# ----------------------------------------------------------------------# Every file has an inode, a record that holds its physical address info.
# ----------------------------------------------------------------------echo; echo -n "Are you absolutely sure you want to delete \"$1\" (y/n)? "
# The '-v' option to 'rm' also asks this.
read answer
case "$answer" in
[nN]) echo "Changed your mind, huh?"
exit $E_CHANGED_MIND
;;
*)
echo "Deleting file \"$1\".";;
esac
find . -inum $inum -exec rm {} \;
#
^^
#
Curly brackets are placeholder
#+
for text output by "find."
echo "File "\"$1"\" deleted!"
exit 0

The find command also works without the -exec option.
#!/bin/bash
# Find suid root files.
# A strange suid file might indicate a security hole,
#+ or even a system intrusion.
directory="/usr/sbin"
# Might also try /sbin, /bin, /usr/bin, /usr/local/bin, etc.
permissions="+4000" # suid root (dangerous!)

for file in $( find "$directory" -perm "$permissions" )
do
ls -ltF --author "$file"
done

See Example 16-30, Example 3-4, and Example 11-9 for scripts using find. Its manpage provides
more detail on this complex and powerful command.
xargs
A filter for feeding arguments to a command, and also a tool for assembling the commands
themselves. It breaks a data stream into small enough chunks for filters and commands to process.
Consider it as a powerful replacement for backquotes. In situations where command substitution fails
with a too many arguments error, substituting xargs often works. [71] Normally, xargs reads from
stdin or from a pipe, but it can also be given the output of a file.
The default command for xargs is echo. This means that input piped to xargs may have linefeeds and
other whitespace characters stripped out.
bash$ ls -l
total 0
-rw-rw-r--rw-rw-r--

1 bozo
1 bozo

bozo
bozo

0 Jan 29 23:58 file1
0 Jan 29 23:58 file2

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bash$ ls -l | xargs
total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan...

bash$ find ~/mail -type f | xargs grep "Linux"
./misc:User-Agent: slrn/0.9.8.1 (Linux)
./sent-mail-jul-2005: hosted by the Linux Documentation Project.
./sent-mail-jul-2005: (Linux Documentation Project Site, rtf version)
./sent-mail-jul-2005: Subject: Criticism of Bozo's Windows/Linux article
./sent-mail-jul-2005: while mentioning that the Linux ext2/ext3 filesystem
. . .

ls | xargs -p -l gzip gzips every file in current directory, one at a time, prompting before
each operation.

Note that xargs processes the arguments passed to it sequentially, one at a time.
bash$ find /usr/bin | xargs file
/usr/bin:
directory
/usr/bin/foomatic-ppd-options:
. . .

perl script text executable

An interesting xargs option is -n NN, which limits to NN the number of arguments
passed.
ls | xargs -n 8 echo lists the files in the current directory in 8 columns.
Another useful option is -0, in combination with find -print0 or grep -lZ.
This allows handling arguments containing whitespace or quotes.
find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs
-0 rm -f
grep -rliwZ GUI / | xargs -0 rm -f
Either of the above will remove any file containing "GUI". (Thanks, S.C.)
Or:
cat /proc/"$pid"/"$OPTION" | xargs -0 echo
# Formats output:
^^^^^^^^^^^^^^^
# From Han Holl's fixup of "get-commandline.sh"
#+ script in "/dev and /proc" chapter.

The -P option to xargs permits running processes in parallel. This speeds up
execution in a machine with a multicore CPU.
#!/bin/bash
ls *gif | xargs -t -n1 -P2 gif2png

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# Converts all the gif images in current directory to png.
#
#
#
#
#

Options:
=======
-t
Print command to stderr.
-n1
At most 1 argument per command line.
-P2
Run up to 2 processes simultaneously.

# Thank you, Roberto Polli, for the inspiration.

Example 16-5. Logfile: Using xargs to monitor system log
#!/bin/bash
# Generates a log file in current directory
# from the tail end of /var/log/messages.
# Note: /var/log/messages must be world readable
# if this script invoked by an ordinary user.
#
#root chmod 644 /var/log/messages
LINES=5
( date; uname -a ) >>logfile
# Time and machine name
echo ---------------------------------------------------------- >>logfile
tail -n $LINES /var/log/messages | xargs | fmt -s >>logfile
echo >>logfile
echo >>logfile
exit 0
#
#
#
#+
#+
#
#
#

Note:
---As Frank Wang points out,
unmatched quotes (either single or double quotes) in the source file
may give xargs indigestion.

#
#
#
#+
#

Exercise:
-------Modify this script to track changes in /var/log/messages at intervals
of 20 minutes.
Hint: Use the "watch" command.

He suggests the following substitution for line 15:
tail -n $LINES /var/log/messages | tr -d "\"'" | xargs | fmt -s >>logfile

As in find, a curly bracket pair serves as a placeholder for replacement text.

Example 16-6. Copying files in current directory to another
#!/bin/bash
# copydir.sh
# Copy (verbose) all files in current directory ($PWD)
#+ to directory specified on command-line.

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E_NOARGS=85
if [ -z "$1" ]
# Exit if no argument given.
then
echo "Usage: `basename $0` directory-to-copy-to"
exit $E_NOARGS
fi
ls
#
#
#
#
#
#
#
#+
#+
#
#
#+
#+

. | xargs -i -t cp ./{} $1
^^ ^^
^^
-t is "verbose" (output command-line to stderr) option.
-i is "replace strings" option.
{} is a placeholder for output text.
This is similar to the use of a curly-bracket pair in "find."
List the files in current directory (ls .),
pass the output of "ls" as arguments to "xargs" (-i -t options),
then copy (cp) these arguments ({}) to new directory ($1).
The net result is the exact equivalent of
cp * $1
unless any of the filenames has embedded "whitespace" characters.

exit 0

Example 16-7. Killing processes by name
#!/bin/bash
# kill-byname.sh: Killing processes by name.
# Compare this script with kill-process.sh.
# For instance,
#+ try "./kill-byname.sh xterm" -#+ and watch all the xterms on your desktop disappear.
#
#
#
#
#+

Warning:
------This is a fairly dangerous script.
Running it carelessly (especially as root)
can cause data loss and other undesirable effects.

E_BADARGS=66
if test -z "$1" # No command-line arg supplied?
then
echo "Usage: `basename $0` Process(es)_to_kill"
exit $E_BADARGS
fi

PROCESS_NAME="$1"
ps ax | grep "$PROCESS_NAME" | awk '{print $1}' | xargs -i kill {} 2&>/dev/null
#
^^
^^
#
#
#
#
#
#

--------------------------------------------------------------Notes:
-i is the "replace strings" option to xargs.
The curly brackets are the placeholder for the replacement.
2&>/dev/null suppresses unwanted error messages.

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# Can grep "$PROCESS_NAME" be replaced by pidof "$PROCESS_NAME"?
# --------------------------------------------------------------exit $?
# The "killall" command has the same effect as this script,
#+ but using it is not quite as educational.

Example 16-8. Word frequency analysis using xargs
#!/bin/bash
# wf2.sh: Crude word frequency analysis on a text file.
# Uses 'xargs' to decompose lines of text into single words.
# Compare this example to the "wf.sh" script later on.

# Check for input file on command-line.
ARGS=1
E_BADARGS=85
E_NOFILE=86
if [ $# -ne "$ARGS" ]
# Correct number of arguments passed to script?
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ]
# Check if file exists.
then
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi

#####################################################
cat "$1" | xargs -n1 | \
# List the file, one word per line.
tr A-Z a-z | \
# Shift characters to lowercase.
sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\
/g' | \
# Filter out periods and commas, and
#+ change space between words to linefeed,
sort | uniq -c | sort -nr
# Finally remove duplicates, prefix occurrence count
#+ and sort numerically.
#####################################################
# This does the same job as the "wf.sh" example,
#+ but a bit more ponderously, and it runs more slowly (why?).
exit $?

expr
All-purpose expression evaluator: Concatenates and evaluates the arguments according to the
operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison,
string, or logical.

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expr 3 + 5
returns 8
expr 5 % 3
returns 2
expr 1 / 0
returns the error message, expr: division by zero
Illegal arithmetic operations not allowed.
expr 5 \* 3
returns 15
The multiplication operator must be escaped when used in an arithmetic expression with
expr.
y=`expr $y + 1`
Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)). This is an
example of arithmetic expansion.
z=`expr substr $string $position $length`
Extract substring of $length characters, starting at $position.
Example 16-9. Using expr
#!/bin/bash
# Demonstrating some of the uses of 'expr'
# =======================================
echo
# Arithmetic Operators
# ---------- --------echo "Arithmetic Operators"
echo
a=`expr 5 + 3`
echo "5 + 3 = $a"
a=`expr $a + 1`
echo
echo "a + 1 = $a"
echo "(incrementing a variable)"
a=`expr 5 % 3`
# modulo
echo
echo "5 mod 3 = $a"
echo
echo
# Logical Operators
# ------- --------# Returns 1 if true, 0 if false,
#+ opposite of normal Bash convention.
echo "Logical Operators"
echo

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x=24
y=25
b=`expr $x = $y`
echo "b = $b"
echo

# Test equality.
# 0 ( $x -ne $y )

a=3
b=`expr $a \> 10`
echo 'b=`expr $a \> 10`, therefore...'
echo "If a > 10, b = 0 (false)"
echo "b = $b"
# 0 ( 3 ! -gt 10 )
echo
b=`expr $a \< 10`
echo "If a < 10, b = 1 (true)"
echo "b = $b"
# 1 ( 3 -lt 10 )
echo
# Note escaping of operators.
b=`expr $a \<= 3`
echo "If a <= 3, b = 1 (true)"
echo "b = $b"
# 1 ( 3 -le 3 )
# There is also a "\>=" operator (greater than or equal to).

echo
echo

# String Operators
# ------ --------echo "String Operators"
echo
a=1234zipper43231
echo "The string being operated upon is \"$a\"."
# length: length of string
b=`expr length $a`
echo "Length of \"$a\" is $b."
# index: position of first character in substring
#
that matches a character in string
b=`expr index $a 23`
echo "Numerical position of first \"2\" in \"$a\" is \"$b\"."
# substr: extract substring, starting position & length specified
b=`expr substr $a 2 6`
echo "Substring of \"$a\", starting at position 2,\
and 6 chars long is \"$b\"."

# The default behavior of the 'match' operations is to
#+ search for the specified match at the BEGINNING of the string.
#
#
Using Regular Expressions ...
b=`expr match "$a" '[0-9]*'`
# Numerical count.
echo Number of digits at the beginning of \"$a\" is $b.
b=`expr match "$a" '\([0-9]*\)'`
# Note that escaped parentheses

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#
==
==
#+ trigger substring match.
echo "The digits at the beginning of \"$a\" are \"$b\"."
echo
exit 0

The : (null) operator can substitute for match. For example, b=`expr $a : [0-9]*` is
the exact equivalent of b=`expr match $a [0-9]*` in the above listing.
#!/bin/bash
echo
echo "String operations using \"expr \$string : \" construct"
echo "==================================================="
echo
a=1234zipper5FLIPPER43231
echo "The string being operated upon is \"`expr "$a" : '\(.*\)'`\"."
#
Escaped parentheses grouping operator.
== ==
#
#+
#+
#

***************************
Escaped parentheses
match a substring
***************************

# If no escaped parentheses...
#+ then 'expr' converts the string operand to an integer.
echo "Length of \"$a\" is `expr "$a" : '.*'`."

# Length of string

echo "Number of digits at the beginning of \"$a\" is `expr "$a" : '[0-9]*'`."
# ------------------------------------------------------------------------- #
echo
echo "The digits at the beginning of \"$a\" are `expr "$a" : '\([0-9]*\)'`."
#
==
==
echo "The first 7 characters of \"$a\" are `expr "$a" : '\(.......\)'`."
#
=====
==
==
# Again, escaped parentheses force a substring match.
#
echo "The last 7 characters of \"$a\" are `expr "$a" : '.*\(.......\)'`."
#
====
end of string operator ^^
# (actually means skip over one or more of any characters until specified
#+ substring)
echo
exit 0

The above script illustrates how expr uses the escaped parentheses -- \( ... \) -- grouping operator in tandem
with regular expression parsing to match a substring. Here is a another example, this time from "real life."
# Strip the whitespace from the beginning and end.
LRFDATE=`expr "$LRFDATE" : '[[:space:]]*\(.*\)[[:space:]]*$'`

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# From Peter Knowles' "booklistgen.sh" script
#+ for converting files to Sony Librie/PRS-50X format.
# (http://booklistgensh.peterknowles.com)

Perl, sed, and awk have far superior string parsing facilities. A short sed or awk "subroutine" within a script
(see Section 36.2) is an attractive alternative to expr.
See Section 10.1 for more on using expr in string operations.

16.3. Time / Date Commands
Time/date and timing
date
Simply invoked, date prints the date and time to stdout. Where this command gets interesting is in
its formatting and parsing options.

Example 16-10. Using date
#!/bin/bash
# Exercising the 'date' command
echo "The number of days since the year's beginning is `date +%j`."
# Needs a leading '+' to invoke formatting.
# %j gives day of year.
echo "The number of seconds elapsed since 01/01/1970 is `date +%s`."
# %s yields number of seconds since "UNIX epoch" began,
#+ but how is this useful?
prefix=temp
suffix=$(date +%s) # The "+%s" option to 'date' is GNU-specific.
filename=$prefix.$suffix
echo "Temporary filename = $filename"
# It's great for creating "unique and random" temp filenames,
#+ even better than using $$.
# Read the 'date' man page for more formatting options.
exit 0

The -u option gives the UTC (Universal Coordinated Time).
bash$ date
Fri Mar 29 21:07:39 MST 2002

bash$ date -u
Sat Mar 30 04:07:42 UTC 2002

This option facilitates calculating the time between different dates.

Example 16-11. Date calculations
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#!/bin/bash
# date-calc.sh
# Author: Nathan Coulter
# Used in ABS Guide with permission (thanks!).
MPHR=60
HPD=24

# Minutes per hour.
# Hours per day.

diff () {
printf '%s' $(( $(date -u -d"$TARGET" +%s) $(date -u -d"$CURRENT" +%s)))
#
%d = day of month.
}

CURRENT=$(date -u -d '2007-09-01 17:30:24' '+%F %T.%N %Z')
TARGET=$(date -u -d'2007-12-25 12:30:00' '+%F %T.%N %Z')
# %F = full date, %T = %H:%M:%S, %N = nanoseconds, %Z = time zone.
printf '\nIn 2007, %s ' \
"$(date -d"$CURRENT +
$(( $(diff) /$MPHR /$MPHR /$HPD / 2 )) days" '+%d %B')"
#
%B = name of month
^ halfway
printf 'was halfway between %s ' "$(date -d"$CURRENT" '+%d %B')"
printf 'and %s\n' "$(date -d"$TARGET" '+%d %B')"
printf '\nOn %s at %s, there were\n' \
$(date -u -d"$CURRENT" +%F) $(date -u -d"$CURRENT" +%T)
DAYS=$(( $(diff) / $MPHR / $MPHR / $HPD ))
CURRENT=$(date -d"$CURRENT +$DAYS days" '+%F %T.%N %Z')
HOURS=$(( $(diff) / $MPHR / $MPHR ))
CURRENT=$(date -d"$CURRENT +$HOURS hours" '+%F %T.%N %Z')
MINUTES=$(( $(diff) / $MPHR ))
CURRENT=$(date -d"$CURRENT +$MINUTES minutes" '+%F %T.%N %Z')
printf '%s days, %s hours, ' "$DAYS" "$HOURS"
printf '%s minutes, and %s seconds ' "$MINUTES" "$(diff)"
printf 'until Christmas Dinner!\n\n'
#
#
#
#+

Exercise:
-------Rewrite the diff () function to accept passed parameters,
rather than using global variables.

The date command has quite a number of output options. For example %N gives the nanosecond
portion of the current time. One interesting use for this is to generate random integers.
date +%N | sed -e 's/000$//' -e 's/^0//'
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Strip off leading and trailing zeroes, if present.
# Length of generated integer depends on
#+ how many zeroes stripped off.
# 115281032
# 63408725
# 394504284

There are many more options (try man date).
date +%j
# Echoes day of the year (days elapsed since January 1).

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date +%k%M
# Echoes hour and minute in 24-hour format, as a single digit string.

# The 'TZ' parameter permits overriding the default time zone.
date
# Mon Mar 28 21:42:16 MST 2005
TZ=EST date
# Mon Mar 28 23:42:16 EST 2005
# Thanks, Frank Kannemann and Pete Sjoberg, for the tip.

SixDaysAgo=$(date --date='6 days ago')
OneMonthAgo=$(date --date='1 month ago')
OneYearAgo=$(date --date='1 year ago')

# Four weeks back (not a month!)

See also Example 3-4 and Example A-43.
zdump
Time zone dump: echoes the time in a specified time zone.
bash$ zdump EST
EST Tue Sep 18 22:09:22 2001 EST

time
Outputs verbose timing statistics for executing a command.
time ls -l / gives something like this:
real
user
sys

0m0.067s
0m0.004s
0m0.005s

See also the very similar times command in the previous section.
As of version 2.0 of Bash, time became a shell reserved word, with slightly altered
behavior in a pipeline.
touch
Utility for updating access/modification times of a file to current system time or other specified time,
but also useful for creating a new file. The command touch zzz will create a new file of zero
length, named zzz, assuming that zzz did not previously exist. Time-stamping empty files in this
way is useful for storing date information, for example in keeping track of modification times on a
project.
The touch command is equivalent to : >> newfile or >> newfile (for
ordinary files).
Before doing a cp -u (copy/update), use touch to update the time stamp of files you
don't wish overwritten.
As an example, if the directory /home/bozo/tax_audit contains the files
spreadsheet-051606.data, spreadsheet-051706.data, and
spreadsheet-051806.data, then doing a touch spreadsheet*.data will protect
these files from being overwritten by files with the same names during a cp -u
/home/bozo/financial_info/spreadsheet*data /home/bozo/tax_audit.
at

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The at job control command executes a given set of commands at a specified time. Superficially, it
resembles cron, however, at is chiefly useful for one-time execution of a command set.
at 2pm January 15 prompts for a set of commands to execute at that time. These commands
should be shell-script compatible, since, for all practical purposes, the user is typing in an executable
shell script a line at a time. Input terminates with a Ctl-D.
Using either the -f option or input redirection (<), at reads a command list from a file. This file is an
executable shell script, though it should, of course, be non-interactive. Particularly clever is including
the run-parts command in the file to execute a different set of scripts.
bash$ at 2:30 am Friday < at-jobs.list
job 2 at 2000-10-27 02:30

batch
The batch job control command is similar to at, but it runs a command list when the system load
drops below .8. Like at, it can read commands from a file with the -f option.

The concept of batch processing dates back to the era of mainframe computers. It means running a
set of commands without user intervention.
cal
Prints a neatly formatted monthly calendar to stdout. Will do current year or a large range of past
and future years.
sleep
This is the shell equivalent of a wait loop. It pauses for a specified number of seconds, doing nothing.
It can be useful for timing or in processes running in the background, checking for a specific event
every so often (polling), as in Example 32-6.
sleep 3

# Pauses 3 seconds.

The sleep command defaults to seconds, but minute, hours, or days may also be
specified.
sleep 3 h

# Pauses 3 hours!

The watch command may be a better choice than sleep for running commands at
timed intervals.
usleep
Microsleep (the u may be read as the Greek mu, or micro- prefix). This is the same as sleep, above,
but "sleeps" in microsecond intervals. It can be used for fine-grained timing, or for polling an ongoing
process at very frequent intervals.
usleep 30

# Pauses 30 microseconds.

This command is part of the Red Hat initscripts / rc-scripts package.
The usleep command does not provide particularly accurate timing, and is therefore
unsuitable for critical timing loops.
hwclock, clock
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The hwclock command accesses or adjusts the machine's hardware clock. Some options require root
privileges. The /etc/rc.d/rc.sysinit startup file uses hwclock to set the system time from
the hardware clock at bootup.
The clock command is a synonym for hwclock.

16.4. Text Processing Commands
Commands affecting text and text files
sort
File sort utility, often used as a filter in a pipe. This command sorts a text stream or file forwards or
backwards, or according to various keys or character positions. Using the -m option, it merges
presorted input files. The info page lists its many capabilities and options. See Example 11-9,
Example 11-10, and Example A-8.
tsort
Topological sort, reading in pairs of whitespace-separated strings and sorting according to input
patterns. The original purpose of tsort was to sort a list of dependencies for an obsolete version of the
ld linker in an "ancient" version of UNIX.
The results of a tsort will usually differ markedly from those of the standard sort command, above.
uniq
This filter removes duplicate lines from a sorted file. It is often seen in a pipe coupled with sort.
cat list-1 list-2 list-3 | sort | uniq > final.list
# Concatenates the list files,
# sorts them,
# removes duplicate lines,
# and finally writes the result to an output file.

The useful -c option prefixes each line of the input file with its number of occurrences.
bash$ cat testfile
This line occurs only once.
This line occurs twice.
This line occurs twice.
This line occurs three times.
This line occurs three times.
This line occurs three times.

bash$ uniq -c testfile
1 This line occurs only once.
2 This line occurs twice.
3 This line occurs three times.

bash$ sort testfile | uniq -c | sort -nr
3 This line occurs three times.
2 This line occurs twice.
1 This line occurs only once.

The sort INPUTFILE | uniq -c | sort -nr command string produces a frequency of
occurrence listing on the INPUTFILE file (the -nr options to sort cause a reverse numerical sort).
This template finds use in analysis of log files and dictionary lists, and wherever the lexical structure
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of a document needs to be examined.

Example 16-12. Word Frequency Analysis
#!/bin/bash
# wf.sh: Crude word frequency analysis on a text file.
# This is a more efficient version of the "wf2.sh" script.

# Check for input file on command-line.
ARGS=1
E_BADARGS=85
E_NOFILE=86
if [ $# -ne "$ARGS" ] # Correct number of arguments passed to script?
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ]
# Check if file exists.
then
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi

########################################################
# main ()
sed -e 's/\.//g' -e 's/\,//g' -e 's/ /\
/g' "$1" | tr 'A-Z' 'a-z' | sort | uniq -c | sort -nr
#
=========================
#
Frequency of occurrence
#
#+
#+
#+

Filter out periods and commas, and
change space between words to linefeed,
then shift characters to lowercase, and
finally prefix occurrence count and sort numerically.

# Arun Giridhar suggests modifying the above to:
# . . . | sort | uniq -c | sort +1 [-f] | sort +0 -nr
# This adds a secondary sort key, so instances of
#+ equal occurrence are sorted alphabetically.
# As he explains it:
# "This is effectively a radix sort, first on the
#+ least significant column
#+ (word or string, optionally case-insensitive)
#+ and last on the most significant column (frequency)."
#
# As Frank Wang explains, the above is equivalent to
#+
. . . | sort | uniq -c | sort +0 -nr
#+ and the following also works:
#+
. . . | sort | uniq -c | sort -k1nr -k
########################################################
exit 0
# Exercises:
# ---------

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# 1)
#+
# 2)
#+

Add 'sed' commands to filter out other punctuation,
such as semicolons.
Modify the script to also filter out multiple spaces and
other whitespace.

bash$ cat testfile
This line occurs only once.
This line occurs twice.
This line occurs twice.
This line occurs three times.
This line occurs three times.
This line occurs three times.

bash$ ./wf.sh testfile
6 this
6 occurs
6 line
3 times
3 three
2 twice
1 only
1 once

expand, unexpand
The expand filter converts tabs to spaces. It is often used in a pipe.
The unexpand filter converts spaces to tabs. This reverses the effect of expand.
cut
A tool for extracting fields from files. It is similar to the print $N command set in awk, but more
limited. It may be simpler to use cut in a script than awk. Particularly important are the -d (delimiter)
and -f (field specifier) options.
Using cut to obtain a listing of the mounted filesystems:
cut -d ' ' -f1,2 /etc/mtab

Using cut to list the OS and kernel version:
uname -a | cut -d" " -f1,3,11,12

Using cut to extract message headers from an e-mail folder:
bash$ grep '^Subject:' read-messages | cut -c10-80
Re: Linux suitable for mission-critical apps?
MAKE MILLIONS WORKING AT HOME!!!
Spam complaint
Re: Spam complaint

Using cut to parse a file:
# List all the users in /etc/passwd.
FILENAME=/etc/passwd
for user in $(cut -d: -f1 $FILENAME)
do
echo $user
done

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# Thanks, Oleg Philon for suggesting this.

cut -d ' ' -f2,3 filename is equivalent to awk -F'[ ]' '{ print $2, $3 }'
filename
It is even possible to specify a linefeed as a delimiter. The trick is to actually embed a
linefeed (RETURN) in the command sequence.
bash$ cut -d'
' -f3,7,19 testfile
This is line 3 of testfile.
This is line 7 of testfile.
This is line 19 of testfile.

Thank you, Jaka Kranjc, for pointing this out.
See also Example 16-48.
paste
Tool for merging together different files into a single, multi-column file. In combination with cut,
useful for creating system log files.
join
Consider this a special-purpose cousin of paste. This powerful utility allows merging two files in a
meaningful fashion, which essentially creates a simple version of a relational database.
The join command operates on exactly two files, but pastes together only those lines with a common
tagged field (usually a numerical label), and writes the result to stdout. The files to be joined
should be sorted according to the tagged field for the matchups to work properly.
File: 1.data
100 Shoes
200 Laces
300 Socks
File: 2.data
100 $40.00
200 $1.00
300 $2.00
bash$ join 1.data 2.data
File: 1.data 2.data
100 Shoes $40.00
200 Laces $1.00
300 Socks $2.00

The tagged field appears only once in the output.
head
lists the beginning of a file to stdout. The default is 10 lines, but a different number can be
specified. The command has a number of interesting options.
Example 16-13. Which files are scripts?

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#!/bin/bash
# script-detector.sh: Detects scripts within a directory.
TESTCHARS=2
SHABANG='#!'

# Test first 2 characters.
# Scripts begin with a "sha-bang."

for file in * # Traverse all the files in current directory.
do
if [[ `head -c$TESTCHARS "$file"` = "$SHABANG" ]]
#
head -c2
#!
# The '-c' option to "head" outputs a specified
#+ number of characters, rather than lines (the default).
then
echo "File \"$file\" is a script."
else
echo "File \"$file\" is *not* a script."
fi
done
exit 0
#
#
#
#+
#+
#
#
#+
#

Exercises:
--------1) Modify this script to take as an optional argument
the directory to scan for scripts
(rather than just the current working directory).
2) As it stands, this script gives "false positives" for
Perl, awk, and other scripting language scripts.
Correct this.

Example 16-14. Generating 10-digit random numbers
#!/bin/bash
# rnd.sh: Outputs a 10-digit random number
# Script by Stephane Chazelas.
head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'

# =================================================================== #
# Analysis
# -------# head:
# -c4 option takes first 4 bytes.
# od:
# -N4 option limits output to 4 bytes.
# -tu4 option selects unsigned decimal format for output.
# sed:
# -n option, in combination with "p" flag to the "s" command,
# outputs only matched lines.

# The author of this script explains the action of 'sed', as follows.

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# head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
# ----------------------------------> |
# Assume output up to "sed" --------> |
# is 0000000 1198195154\n
#
#
#+
#
#
#
#
#+
#
#
#+

sed begins reading characters: 0000000 1198195154\n.
Here it finds a newline character,
so it is ready to process the first line (0000000 1198195154).
It looks at its s. The first and only one is
range
1

action
s/.* //p

The line number is in the range, so it executes the action:
tries to substitute the longest string ending with a space in the line
("0000000 ") with nothing (//), and if it succeeds, prints the result
("p" is a flag to the "s" command here, this is different
from the "p" command).

# sed is now ready to continue reading its input. (Note that before
#+ continuing, if -n option had not been passed, sed would have printed
#+ the line once again).
#
#+
#
#+
#

Now, sed reads the remainder of the characters, and finds the
end of the file.
It is now ready to process its 2nd line (which is also numbered '$' as
it's the last one).
It sees it is not matched by any , so its job is done.

# In few word this sed commmand means:
# "On the first line only, remove any character up to the right-most space,
#+ then print it."
# A better way to do this would have been:
#
sed -e 's/.* //;q'
# Here, two s (could have been written
#
sed -e 's/.* //' -e q):
#
#
#

range
nothing (matches line)
nothing (matches line)

action
s/.* //
q (quit)

# Here, sed only reads its first line of input.
# It performs both actions, and prints the line (substituted) before
#+ quitting (because of the "q" action) since the "-n" option is not passed.
# =================================================================== #
# An even simpler altenative to the above one-line script would be:
#
head -c4 /dev/urandom| od -An -tu4
exit

See also Example 16-39.
tail
lists the (tail) end of a file to stdout. The default is 10 lines, but this can be changed with the -n
option. Commonly used to keep track of changes to a system logfile, using the -f option, which
outputs lines appended to the file.

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Example 16-15. Using tail to monitor the system log
#!/bin/bash
filename=sys.log
cat /dev/null > $filename; echo "Creating / cleaning out file."
# Creates file if it does not already exist,
#+ and truncates it to zero length if it does.
# : > filename
and
> filename also work.
tail /var/log/messages > $filename
# /var/log/messages must have world read permission for this to work.
echo "$filename contains tail end of system log."
exit 0

To list a specific line of a text file, pipe the output of head to tail -n 1. For example
head -n 8 database.txt | tail -n 1 lists the 8th line of the file
database.txt.
To set a variable to a given block of a text file:
var=$(head -n $m $filename | tail -n $n)
# filename = name of file
# m = from beginning of file, number of lines to end of block
# n = number of lines to set variable to (trim from end of block)

Newer implementations of tail deprecate the older tail -$LINES filename usage. The
standard tail -n $LINES filename is correct.
See also Example 16-5, Example 16-39 and Example 32-6.
grep
A multi-purpose file search tool that uses Regular Expressions. It was originally a command/filter in
the venerable ed line editor: g/re/p -- global - regular expression - print.
grep pattern [file...]
Search the target file(s) for occurrences of pattern, where pattern may be literal text or a
Regular Expression.
bash$ grep '[rst]ystem.$' osinfo.txt
The GPL governs the distribution of the Linux operating system.

If no target file(s) specified, grep works as a filter on stdout, as in a pipe.
bash$ ps ax | grep clock
765 tty1
S
0:00 xclock
901 pts/1
S
0:00 grep clock

The -i option causes a case-insensitive search.
The -w option matches only whole words.
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The -l option lists only the files in which matches were found, but not the matching lines.
The -r (recursive) option searches files in the current working directory and all subdirectories below
it.
The -n option lists the matching lines, together with line numbers.
bash$ grep -n Linux osinfo.txt
2:This is a file containing information about Linux.
6:The GPL governs the distribution of the Linux operating system.

The -v (or --invert-match) option filters out matches.
grep pattern1 *.txt | grep -v pattern2
# Matches all lines in "*.txt" files containing "pattern1",
# but ***not*** "pattern2".

The -c (--count) option gives a numerical count of matches, rather than actually listing the
matches.
grep -c txt *.sgml

# (number of occurrences of "txt" in "*.sgml" files)

#
grep -cz .
#
^ dot
# means count (-c) zero-separated (-z) items matching "."
# that is, non-empty ones (containing at least 1 character).
#
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz .
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$'
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^'
#
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$'
# By default, newline chars (\n) separate items to match.

# 3
# 5
# 5
# 9

# Note that the -z option is GNU "grep" specific.

# Thanks, S.C.

The --color (or --colour) option marks the matching string in color (on the console or in an
xterm window). Since grep prints out each entire line containing the matching pattern, this lets you
see exactly what is being matched. See also the -o option, which shows only the matching portion of
the line(s).

Example 16-16. Printing out the From lines in stored e-mail messages
#!/bin/bash
# from.sh
# Emulates the useful "from" utility in Solaris, BSD, etc.
# Echoes the "From" header line in all messages
#+ in your e-mail directory.

MAILDIR=~/mail/*
# No quoting of variable. Why?
# Maybe check if exists $MAILDIR . . . if [ -d $MAILDIR ] . . .

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GREP_OPTS="-H -A 5 --color"
TARGETSTR="^From"

# Show file, plus extra context lines
#+ and display "From" in color.
# "From" at beginning of line.

for file in $MAILDIR
# No quoting of variable.
do
grep $GREP_OPTS "$TARGETSTR" "$file"
#
^^^^^^^^^^
# Again, do not quote this variable.
echo
done
exit $?
# You might wish to pipe the output of this script to 'more'
#+ or redirect it to a file . . .

When invoked with more than one target file given, grep specifies which file contains matches.
bash$ grep Linux osinfo.txt misc.txt
osinfo.txt:This is a file containing information about Linux.
osinfo.txt:The GPL governs the distribution of the Linux operating system.
misc.txt:The Linux operating system is steadily gaining in popularity.

To force grep to show the filename when searching only one target file, simply give
/dev/null as the second file.
bash$ grep Linux osinfo.txt /dev/null
osinfo.txt:This is a file containing information about Linux.
osinfo.txt:The GPL governs the distribution of the Linux operating system.

If there is a successful match, grep returns an exit status of 0, which makes it useful in a condition test
in a script, especially in combination with the -q option to suppress output.
SUCCESS=0
word=Linux
filename=data.file

# if grep lookup succeeds

grep -q "$word" "$filename"

# The "-q" option
#+ causes nothing to echo to stdout.

if [ $? -eq $SUCCESS ]
# if grep -q "$word" "$filename"
can replace lines 5 - 7.
then
echo "$word found in $filename"
else
echo "$word not found in $filename"
fi

Example 32-6 demonstrates how to use grep to search for a word pattern in a system logfile.

Example 16-17. Emulating grep in a script
#!/bin/bash
# grp.sh: Rudimentary reimplementation of grep.
E_BADARGS=85
if [ -z "$1" ]
then

# Check for argument to script.

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echo "Usage: `basename $0` pattern"
exit $E_BADARGS
fi
echo
for file in *
# Traverse all files in $PWD.
do
output=$(sed -n /"$1"/p $file) # Command substitution.
if [ ! -z "$output" ]
# What happens if "$output" is not quoted?
then
echo -n "$file: "
echo "$output"
fi
# sed -ne "/$1/s|^|${file}: |p" is equivalent to above.
echo
done
echo
exit 0
#
#
#
#

Exercises:
--------1) Add newlines to output, if more than one match in any given file.
2) Add features.

How can grep search for two (or more) separate patterns? What if you want grep to display all lines
in a file or files that contain both "pattern1" and "pattern2"?
One method is to pipe the result of grep pattern1 to grep pattern2.
For example, given the following file:
# Filename: tstfile
This
This
This
This
Here

is a sample file.
is an ordinary text file.
file does not contain any unusual text.
file is not unusual.
is some text.

Now, let's search this file for lines containing both "file" and "text" . . .
bash$ grep file tstfile
# Filename: tstfile
This is a sample file.
This is an ordinary text file.
This file does not contain any unusual text.
This file is not unusual.
bash$ grep file tstfile | grep text
This is an ordinary text file.
This file does not contain any unusual text.

Now, for an interesting recreational use of grep . . .

Example 16-18. Crossword puzzle solver

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#!/bin/bash
# cw-solver.sh
# This is actually a wrapper around a one-liner (line 46).
# Crossword puzzle and anagramming word game solver.
# You know *some* of the letters in the word you're looking for,
#+ so you need a list of all valid words
#+ with the known letters in given positions.
# For example: w...i....n
#
1???5????10
# w in position 1, 3 unknowns, i in the 5th, 4 unknowns, n at the end.
# (See comments at end of script.)

E_NOPATT=71
DICT=/usr/share/dict/word.lst
#
^^^^^^^^
Looks for word list here.
# ASCII word list, one word per line.
# If you happen to need an appropriate list,
#+ download the author's "yawl" word list package.
# http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
# or
# http://bash.webofcrafts.net/yawl-0.3.2.tar.gz

if [ -z "$1" ]
# If no word pattern specified
then
#+ as a command-line argument . . .
echo
#+ . . . then . . .
echo "Usage:" #+ Usage message.
echo
echo ""$0" \"pattern,\""
echo "where \"pattern\" is in the form"
echo "xxx..x.x..."
echo
echo "The x's represent known letters,"
echo "and the periods are unknown letters (blanks)."
echo "Letters and periods can be in any position."
echo "For example, try:
sh cw-solver.sh w...i....n"
echo
exit $E_NOPATT
fi
echo
# ===============================================
# This is where all the work gets done.
grep ^"$1"$ "$DICT"
# Yes, only one line!
#
|
|
# ^ is start-of-word regex anchor.
# $ is end-of-word regex anchor.
# From _Stupid Grep Tricks_, vol. 1,
#+ a book the ABS Guide author may yet get around
#+ to writing . . . one of these days . . .
# ===============================================
echo

exit $? # Script terminates here.
# If there are too many words generated,
#+ redirect the output to a file.
$ sh cw-solver.sh w...i....n

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wellington
workingman
workingmen

egrep -- extended grep -- is the same as grep -E. This uses a somewhat different, extended set of
Regular Expressions, which can make the search a bit more flexible. It also allows the boolean | (or)
operator.
bash $ egrep 'matches|Matches' file.txt
Line 1 matches.
Line 3 Matches.
Line 4 contains matches, but also Matches

fgrep -- fast grep -- is the same as grep -F. It does a literal string search (no Regular Expressions),
which generally speeds things up a bit.
On some Linux distros, egrep and fgrep are symbolic links to, or aliases for
grep, but invoked with the -E and -F options, respectively.
Example 16-19. Looking up definitions in Webster's 1913 Dictionary
#!/bin/bash
# dict-lookup.sh
#
#
#+
#+
#
#
#+
#
#

This script looks up definitions in the 1913 Webster's Dictionary.
This Public Domain dictionary is available for download
from various sites, including
Project Gutenberg (http://www.gutenberg.org/etext/247).
Convert it from DOS to UNIX format (with only LF at end of line)
before using it with this script.
Store the file in plain, uncompressed ASCII text.
Set DEFAULT_DICTFILE variable below to path/filename.

E_BADARGS=85
MAXCONTEXTLINES=50
# Maximum number of lines to show.
DEFAULT_DICTFILE="/usr/share/dict/webster1913-dict.txt"
# Default dictionary file pathname.
# Change this as necessary.
# Note:
# ---# This particular edition of the 1913 Webster's
#+ begins each entry with an uppercase letter
#+ (lowercase for the remaining characters).
# Only the *very first line* of an entry begins this way,
#+ and that's why the search algorithm below works.

if [[ -z $(echo "$1" | sed -n '/^[A-Z]/p') ]]
# Must at least specify word to look up, and
#+ it must start with an uppercase letter.
then
echo "Usage: `basename $0` Word-to-define [dictionary-file]"
echo
echo "Note: Word to look up must start with capital letter,"
echo "with the rest of the word in lowercase."
echo "--------------------------------------------"

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echo "Examples: Abandon, Dictionary, Marking, etc."
exit $E_BADARGS
fi

if [ -z "$2" ]

# May specify different dictionary
#+ as an argument to this script.

then
dictfile=$DEFAULT_DICTFILE
else
dictfile="$2"
fi
# --------------------------------------------------------Definition=$(fgrep -A $MAXCONTEXTLINES "$1 \\" "$dictfile")
#
Definitions in form "Word \..."
#
# And, yes, "fgrep" is fast enough
#+ to search even a very large text file.

# Now, snip out just the definition block.
echo "$Definition" |
sed -n '1,/^[A-Z]/p' |
# Print from first line of output
#+ to the first line of the next entry.
sed '$d' | sed '$d'
# Delete last two lines of output
#+ (blank line and first line of next entry).
# --------------------------------------------------------exit $?
#
#
#
#
#
#
#
#
#
#
#
#
#

Exercises:
--------1) Modify the script to accept any type of alphabetic input
+ (uppercase, lowercase, mixed case), and convert it
+ to an acceptable format for processing.
2) Convert the script to a GUI application,
+ using something like 'gdialog' or 'zenity' . . .
The script will then no longer take its argument(s)
+ from the command-line.
3) Modify the script to parse one of the other available
+ Public Domain Dictionaries, such as the U.S. Census Bureau Gazetteer.

See also Example A-41 for an example of speedy fgrep lookup on a large text file.
agrep (approximate grep) extends the capabilities of grep to approximate matching. The search string
may differ by a specified number of characters from the resulting matches. This utility is not part of
the core Linux distribution.

To search compressed files, use zgrep, zegrep, or zfgrep. These also work on
non-compressed files, though slower than plain grep, egrep, fgrep. They are handy
for searching through a mixed set of files, some compressed, some not.
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To search bzipped files, use bzgrep.
look
The command look works like grep, but does a lookup on a "dictionary," a sorted word list. By
default, look searches for a match in /usr/dict/words, but a different dictionary file may be
specified.

Example 16-20. Checking words in a list for validity
#!/bin/bash
# lookup: Does a dictionary lookup on each word in a data file.
file=words.data

# Data file from which to read words to test.

echo
while [ "$word" != end ] # Last word in data file.
do
# ^^^
read word
# From data file, because of redirection at end of loop.
look $word > /dev/null # Don't want to display lines in dictionary file.
lookup=$?
# Exit status of 'look' command.
if [ "$lookup" -eq 0 ]
then
echo "\"$word\" is valid."
else
echo "\"$word\" is invalid."
fi
done <"$file"

# Redirects stdin to $file, so "reads" come from there.

echo
exit 0
# ---------------------------------------------------------------# Code below line will not execute because of "exit" command above.

# Stephane Chazelas proposes the following, more concise alternative:
while read word && [[ $word != end ]]
do if look "$word" > /dev/null
then echo "\"$word\" is valid."
else echo "\"$word\" is invalid."
fi
done <"$file"
exit 0

sed, awk
Scripting languages especially suited for parsing text files and command output. May be embedded
singly or in combination in pipes and shell scripts.
sed
Non-interactive "stream editor", permits using many ex commands in batch mode. It finds many uses
in shell scripts.
awk
Programmable file extractor and formatter, good for manipulating and/or extracting fields (columns)
in structured text files. Its syntax is similar to C.
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wc
wc gives a "word count" on a file or I/O stream:
bash $ wc /usr/share/doc/sed-4.1.2/README
13 70 447 README
[13 lines 70 words 447 characters]

wc -w gives only the word count.
wc -l gives only the line count.
wc -c gives only the byte count.
wc -m gives only the character count.
wc -L gives only the length of the longest line.
Using wc to count how many .txt files are in current working directory:
$ ls *.txt | wc -l
# Will work as long as none of the "*.txt" files
#+ have a linefeed embedded in their name.
#
#
#

Alternative ways of doing this are:
find . -maxdepth 1 -name \*.txt -print0 | grep -cz .
(shopt -s nullglob; set -- *.txt; echo $#)

#

Thanks, S.C.

Using wc to total up the size of all the files whose names begin with letters in the range d - h
bash$ wc [d-h]* | grep total | awk '{print $3}'
71832

Using wc to count the instances of the word "Linux" in the main source file for this book.
bash$ grep Linux abs-book.sgml | wc -l
50

See also Example 16-39 and Example 20-8.
Certain commands include some of the functionality of wc as options.
... | grep foo | wc -l
# This frequently used construct can be more concisely rendered.
... | grep -c foo
# Just use the "-c" (or "--count") option of grep.
# Thanks, S.C.

tr
character translation filter.
Must use quoting and/or brackets, as appropriate. Quotes prevent the shell from
reinterpreting the special characters in tr command sequences. Brackets should be
quoted to prevent expansion by the shell.
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Either tr "A-Z" "*"  $NEWFILENAME
# Delete CR's and write to new file.
echo "Original DOS text file is \"$1\"."
echo "Converted UNIX text file is \"$NEWFILENAME\"."
exit 0
# Exercise:
# -------# Change the above script to convert from UNIX to DOS.

Example 16-24. rot13: ultra-weak encryption.
#!/bin/bash
# rot13.sh: Classic rot13 algorithm,
#
encryption that might fool a 3-year old.
# Usage: ./rot13.sh filename
# or
./rot13.sh > "$BOOKLIST"
}
# From Peter Knowles' "booklistgen.sh" script
#+ for converting files to Sony Librie/PRS-50X format.
# (http://booklistgensh.peterknowles.com)

recode
Consider this a fancier version of iconv, above. This very versatile utility for converting a file to a
different encoding scheme. Note that recode is not part of the standard Linux installation.
TeX, gs
TeX and Postscript are text markup languages used for preparing copy for printing or formatted
video display.
TeX is Donald Knuth's elaborate typsetting system. It is often convenient to write a shell script
encapsulating all the options and arguments passed to one of these markup languages.

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Ghostscript (gs) is a GPL-ed Postscript interpreter.
texexec
Utility for processing TeX and pdf files. Found in /usr/bin on many Linux distros, it is actually a
shell wrapper that calls Perl to invoke Tex.
texexec --pdfarrange --result=Concatenated.pdf *pdf
#
#+
#
#

Concatenates all the pdf files in the current working directory
into the merged file, Concatenated.pdf . . .
(The --pdfarrange option repaginates a pdf file. See also --pdfcombine.)
The above command-line could be parameterized and put into a shell script.

enscript
Utility for converting plain text file to PostScript
For example, enscript filename.txt -p filename.ps produces the PostScript output file
filename.ps.
groff, tbl, eqn
Yet another text markup and display formatting language is groff. This is the enhanced GNU version
of the venerable UNIX roff/troff display and typesetting package. Manpages use groff.
The tbl table processing utility is considered part of groff, as its function is to convert table markup
into groff commands.
The eqn equation processing utility is likewise part of groff, and its function is to convert equation
markup into groff commands.

Example 16-29. manview: Viewing formatted manpages
#!/bin/bash
# manview.sh: Formats the source of a man page for viewing.
# This script is useful when writing man page source.
# It lets you look at the intermediate results on the fly
#+ while working on it.
E_WRONGARGS=85
if [ -z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_WRONGARGS
fi
# --------------------------groff -Tascii -man $1 | less
# From the man page for groff.
# --------------------------# If the man page includes tables and/or equations,
#+ then the above code will barf.
# The following line can handle such cases.
#
#
gtbl < "$1" | geqn -Tlatin1 | groff -Tlatin1 -mtty-char -man
#
#
Thanks, S.C.

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exit $?

# See also the "maned.sh" script.

See also Example A-39.
lex, yacc
The lex lexical analyzer produces programs for pattern matching. This has been replaced by the
nonproprietary flex on Linux systems.

The yacc utility creates a parser based on a set of specifications. This has been replaced by the
nonproprietary bison on Linux systems.

16.5. File and Archiving Commands
Archiving
tar
The standard UNIX archiving utility. [73] Originally a Tape ARchiving program, it has developed into
a general purpose package that can handle all manner of archiving with all types of destination
devices, ranging from tape drives to regular files to even stdout (see Example 3-4). GNU tar has
been patched to accept various compression filters, for example: tar czvf archive_name.tar.gz *,
which recursively archives and gzips all files in a directory tree except dotfiles in the current working
directory ($PWD). [74]
Some useful tar options:
1. -c create (a new archive)
2. -x extract (files from existing archive)
3. --delete delete (files from existing archive)
This option will not work on magnetic tape devices.
4. -r append (files to existing archive)
5. -A append (tar files to existing archive)
6. -t list (contents of existing archive)
7. -u update archive
8. -d compare archive with specified filesystem
9. --after-date only process files with a date stamp after specified date
10. -z gzip the archive
(compress or uncompress, depending on whether combined with the -c or -x) option
11. -j bzip2 the archive
It may be difficult to recover data from a corrupted gzipped tar archive. When
archiving important files, make multiple backups.
shar
Shell archiving utility. The text files in a shell archive are concatenated without compression, and the
resultant archive is essentially a shell script, complete with #!/bin/sh header, containing all the
necessary unarchiving commands, as well as the files themselves. Shar archives still show up in
Usenet newsgroups, but otherwise shar has been replaced by tar/gzip. The unshar command
unpacks shar archives.
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The mailshar command is a Bash script that uses shar to concatenate multiple files into a single one
for e-mailing. This script supports compression and uuencoding.
ar
Creation and manipulation utility for archives, mainly used for binary object file libraries.
rpm
The Red Hat Package Manager, or rpm utility provides a wrapper for source or binary archives. It
includes commands for installing and checking the integrity of packages, among other things.
A simple rpm -i package_name.rpm usually suffices to install a package, though there are many
more options available.
rpm -qf identifies which package a file originates from.
bash$ rpm -qf /bin/ls
coreutils-5.2.1-31

rpm -qa gives a complete list of all installed rpm packages on a given system. An
rpm -qa package_name lists only the package(s) corresponding to
package_name.
bash$ rpm -qa
redhat-logos-1.1.3-1
glibc-2.2.4-13
cracklib-2.7-12
dosfstools-2.7-1
gdbm-1.8.0-10
ksymoops-2.4.1-1
mktemp-1.5-11
perl-5.6.0-17
reiserfs-utils-3.x.0j-2
...

bash$ rpm -qa docbook-utils
docbook-utils-0.6.9-2

bash$ rpm -qa docbook | grep docbook
docbook-dtd31-sgml-1.0-10
docbook-style-dsssl-1.64-3
docbook-dtd30-sgml-1.0-10
docbook-dtd40-sgml-1.0-11
docbook-utils-pdf-0.6.9-2
docbook-dtd41-sgml-1.0-10
docbook-utils-0.6.9-2

cpio
This specialized archiving copy command (copy input and output) is rarely seen any more, having
been supplanted by tar/gzip. It still has its uses, such as moving a directory tree. With an appropriate
block size (for copying) specified, it can be appreciably faster than tar.

Example 16-30. Using cpio to move a directory tree

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#!/bin/bash
# Copying a directory tree using cpio.
# Advantages of using 'cpio':
#
Speed of copying. It's faster than 'tar' with pipes.
#
Well suited for copying special files (named pipes, etc.)
#+ that 'cp' may choke on.
ARGS=2
E_BADARGS=65
if [ $# -ne "$ARGS" ]
then
echo "Usage: `basename $0` source destination"
exit $E_BADARGS
fi
source="$1"
destination="$2"
###################################################################
find "$source" -depth | cpio -admvp "$destination"
#
^^^^^
^^^^^
# Read the 'find' and 'cpio' info pages to decipher these options.
# The above works only relative to $PWD (current directory) . . .
#+ full pathnames are specified.
###################################################################

# Exercise:
# -------# Add code to check the exit status ($?) of the 'find | cpio' pipe
#+ and output appropriate error messages if anything went wrong.
exit $?

rpm2cpio
This command extracts a cpio archive from an rpm one.

Example 16-31. Unpacking an rpm archive
#!/bin/bash
# de-rpm.sh: Unpack an 'rpm' archive
: ${1?"Usage: `basename $0` target-file"}
# Must specify 'rpm' archive name as an argument.

TEMPFILE=$$.cpio

#
#

Tempfile with "unique" name.
$$ is process ID of script.

rpm2cpio < $1 > $TEMPFILE

#
#+
#
#

Converts rpm archive into
cpio archive.
Unpacks cpio archive.
Deletes cpio archive.

cpio --make-directories -F $TEMPFILE -i
rm -f $TEMPFILE
exit 0

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# Exercise:
# Add check for whether 1) "target-file" exists and
#+
2) it is an rpm archive.
# Hint:
Parse output of 'file' command.

pax
The pax portable archive exchange toolkit facilitates periodic file backups and is designed to be
cross-compatible between various flavors of UNIX. It was designed to replace tar and cpio.
pax -wf daily_backup.pax ~/linux-server/files
# Creates a tar archive of all files in the target directory.
# Note that the options to pax must be in the correct order -#+ pax -fw
has an entirely different effect.
pax -f daily_backup.pax
# Lists the files in the archive.
pax -rf daily_backup.pax ~/bsd-server/files
# Restores the backed-up files from the Linux machine
#+ onto a BSD one.

Note that pax handles many of the standard archiving and compression commands.
Compression
gzip
The standard GNU/UNIX compression utility, replacing the inferior and proprietary compress. The
corresponding decompression command is gunzip, which is the equivalent of gzip -d.
The -c option sends the output of gzip to stdout. This is useful when piping to
other commands.
The zcat filter decompresses a gzipped file to stdout, as possible input to a pipe or redirection. This
is, in effect, a cat command that works on compressed files (including files processed with the older
compress utility). The zcat command is equivalent to gzip -dc.
On some commercial UNIX systems, zcat is a synonym for uncompress -c, and will
not work on gzipped files.
See also Example 7-7.
bzip2
An alternate compression utility, usually more efficient (but slower) than gzip, especially on large
files. The corresponding decompression command is bunzip2.
Similar to the zcat command, bzcat decompresses a bzipped2-ed file to stdout.
Newer versions of tar have been patched with bzip2 support.
compress, uncompress
This is an older, proprietary compression utility found in commercial UNIX distributions. The more
efficient gzip has largely replaced it. Linux distributions generally include a compress workalike for
compatibility, although gunzip can unarchive files treated with compress.
The znew command transforms compressed files into gzipped ones.
sq

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Yet another compression (squeeze) utility, a filter that works only on sorted ASCII word lists. It uses
the standard invocation syntax for a filter, sq < input-file > output-file. Fast, but not nearly as
efficient as gzip. The corresponding uncompression filter is unsq, invoked like sq.
The output of sq may be piped to gzip for further compression.
zip, unzip
Cross-platform file archiving and compression utility compatible with DOS pkzip.exe. "Zipped"
archives seem to be a more common medium of file exchange on the Internet than "tarballs."
unarc, unarj, unrar
These Linux utilities permit unpacking archives compressed with the DOS arc.exe, arj.exe, and
rar.exe programs.
lzma, unlzma, lzcat
Highly efficient Lempel-Ziv-Markov compression. The syntax of lzma is similar to that of gzip. The
7-zip Website has more information.
File Information
file
A utility for identifying file types. The command file file-name will return a file specification
for file-name, such as ascii text or data. It references the magic numbers found in
/usr/share/magic, /etc/magic, or /usr/lib/magic, depending on the Linux/UNIX
distribution.
The -f option causes file to run in batch mode, to read from a designated file a list of filenames to
analyze. The -z option, when used on a compressed target file, forces an attempt to analyze the
uncompressed file type.
bash$ file test.tar.gz
test.tar.gz: gzip compressed data, deflated,
last modified: Sun Sep 16 13:34:51 2001, os: Unix
bash file -z test.tar.gz
test.tar.gz: GNU tar archive (gzip compressed data, deflated,
last modified: Sun Sep 16 13:34:51 2001, os: Unix)

# Find sh and Bash scripts in a given directory:
DIRECTORY=/usr/local/bin
KEYWORD=Bourne
# Bourne and Bourne-Again shell scripts
file $DIRECTORY/* | fgrep $KEYWORD
# Output:
#
#
#
#
#

/usr/local/bin/burn-cd:
/usr/local/bin/burnit:
/usr/local/bin/cassette.sh:
/usr/local/bin/copy-cd:
. . .

Bourne-Again
Bourne-Again
Bourne shell
Bourne-Again

shell script text executable
shell script text executable
script text executable
shell script text executable

Example 16-32. Stripping comments from C program files

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#!/bin/bash
# strip-comment.sh: Strips out the comments (/* COMMENT */) in a C program.
E_NOARGS=0
E_ARGERROR=66
E_WRONG_FILE_TYPE=67
if [ $# -eq "$E_NOARGS" ]
then
echo "Usage: `basename $0` C-program-file" >&2 # Error message to stderr.
exit $E_ARGERROR
fi
# Test for correct file type.
type=`file $1 | awk '{ print $2, $3, $4, $5 }'`
# "file $1" echoes file type . . .
# Then awk removes the first field, the filename . . .
# Then the result is fed into the variable "type."
correct_type="ASCII C program text"
if [ "$type" != "$correct_type" ]
then
echo
echo "This script works on C program files only."
echo
exit $E_WRONG_FILE_TYPE
fi

# Rather cryptic sed script:
#-------sed '
/^\/\*/d
/.*\*\//d
' $1
#-------# Easy to understand if you take several hours to learn sed fundamentals.

# Need to add one more line to the sed script to deal with
#+ case where line of code has a comment following it on same line.
# This is left as a non-trivial exercise.
# Also, the above code deletes non-comment lines with a "*/" . . .
#+ not a desirable result.
exit 0

# ---------------------------------------------------------------# Code below this line will not execute because of 'exit 0' above.
# Stephane Chazelas suggests the following alternative:
usage() {
echo "Usage: `basename $0` C-program-file" >&2
exit 1
}
WEIRD=`echo -n -e '\377'`
[[ $# -eq 1 ]] || usage
case `file "$1"` in

# or WEIRD=$'\377'

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*"C program text"*) sed -e "s%/\*%${WEIRD}%g;s%\*/%${WEIRD}%g" "$1" \
| tr '\377\n' '\n\377' \
| sed -ne 'p;n' \
| tr -d '\n' | tr '\377' '\n';;
*) usage;;
esac
#
#
#
#
#
#
#+
#+

This is still fooled by things like:
printf("/*");
or
/* /* buggy embedded comment */
To handle all special cases (comments in strings, comments in string
where there is a \", \\" ...),
the only way is to write a C parser (using lex or yacc perhaps?).

exit 0

which
which command gives the full path to "command." This is useful for finding out whether a particular
command or utility is installed on the system.
$bash which rm
/usr/bin/rm

For an interesting use of this command, see Example 36-14.
whereis
Similar to which, above, whereis command gives the full path to "command," but also to its
manpage.
$bash whereis rm
rm: /bin/rm /usr/share/man/man1/rm.1.bz2

whatis
whatis command looks up "command" in the whatis database. This is useful for identifying system
commands and important configuration files. Consider it a simplified man command.
$bash whatis whatis
whatis

(1)

- search the whatis database for complete words

Example 16-33. Exploring /usr/X11R6/bin
#!/bin/bash
# What are all those mysterious binaries in /usr/X11R6/bin?
DIRECTORY="/usr/X11R6/bin"
# Try also "/bin", "/usr/bin", "/usr/local/bin", etc.
for file in $DIRECTORY/*
do
whatis `basename $file`
done

# Echoes info about the binary.

exit 0

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#
#+
#
#
#
#

Note: For this to work, you must create a "whatis" database
with /usr/sbin/makewhatis.
You may wish to redirect output of this script, like so:
./what.sh >>whatis.db
or view it a page at a time on stdout,
./what.sh | less

See also Example 11-3.
vdir
Show a detailed directory listing. The effect is similar to ls -lb.
This is one of the GNU fileutils.
bash$ vdir
total 10
-rw-r--r--rw-r--r--rw-r--r--

1 bozo
1 bozo
1 bozo

bozo
bozo
bozo

4034 Jul 18 22:04 data1.xrolo
4602 May 25 13:58 data1.xrolo.bak
877 Dec 17 2000 employment.xrolo

bash ls -l
total 10
-rw-r--r--rw-r--r--rw-r--r--

1 bozo
1 bozo
1 bozo

bozo
bozo
bozo

4034 Jul 18 22:04 data1.xrolo
4602 May 25 13:58 data1.xrolo.bak
877 Dec 17 2000 employment.xrolo

locate, slocate
The locate command searches for files using a database stored for just that purpose. The slocate
command is the secure version of locate (which may be aliased to slocate).
$bash locate hickson
/usr/lib/xephem/catalogs/hickson.edb

getfacl, setfacl
These commands retrieve or set the file access control list -- the owner, group, and file permissions.
bash$ getfacl *
# file: test1.txt
# owner: bozo
# group: bozgrp
user::rwgroup::rwother::r-# file: test2.txt
# owner: bozo
# group: bozgrp
user::rwgroup::rwother::r--

bash$ setfacl -m u:bozo:rw yearly_budget.csv
bash$ getfacl yearly_budget.csv
# file: yearly_budget.csv
# owner: accountant
# group: budgetgrp
user::rwuser:bozo:rw-

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user:accountant:rwgroup::rwmask::rwother::r--

readlink
Disclose the file that a symbolic link points to.
bash$ readlink /usr/bin/awk
../../bin/gawk

strings
Use the strings command to find printable strings in a binary or data file. It will list sequences of
printable characters found in the target file. This might be handy for a quick 'n dirty examination of a
core dump or for looking at an unknown graphic image file (strings image-file | more
might show something like JFIF, which would identify the file as a jpeg graphic). In a script, you
would probably parse the output of strings with grep or sed. See Example 11-7 and Example 11-9.

Example 16-34. An "improved" strings command
#!/bin/bash
# wstrings.sh: "word-strings" (enhanced "strings" command)
#
# This script filters the output of "strings" by checking it
#+ against a standard word list file.
# This effectively eliminates gibberish and noise,
#+ and outputs only recognized words.
# ===========================================================
#
Standard Check for Script Argument(s)
ARGS=1
E_BADARGS=85
E_NOFILE=86
if [ $# -ne $ARGS ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
if [ ! -f "$1" ]
# Check if file exists.
then
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
# ===========================================================

MINSTRLEN=3
# Minimum string length.
WORDFILE=/usr/share/dict/linux.words # Dictionary file.
# May specify a different word list file
#+ of one-word-per-line format.
# For example, the "yawl" word-list package,
# http://bash.webofcrafts.net/yawl-0.3.2.tar.gz

wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`

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# Translate output of 'strings' command with multiple passes of 'tr'.
# "tr A-Z a-z" converts to lowercase.
# "tr '[:space:]'" converts whitespace characters to Z's.
# "tr -cs '[:alpha:]' Z" converts non-alphabetic characters to Z's,
#+ and squeezes multiple consecutive Z's.
# "tr -s '\173-\377' Z" converts all characters past 'z' to Z's
#+ and squeezes multiple consecutive Z's,
#+ which gets rid of all the weird characters that the previous
#+ translation failed to deal with.
# Finally, "tr Z ' '" converts all those Z's to whitespace,
#+ which will be seen as word separators in the loop below.
#
#
#+
#

********************************************************************
Note the technique of feeding the output of 'tr' back to itself,
but with different arguments and/or options on each successive pass.
********************************************************************

for word in $wlist

#
#
#
#

Important:
$wlist must not be quoted here.
"$wlist" does not work.
Why not?

do

#

strlen=${#word}
if [ "$strlen" -lt "$MINSTRLEN" ]
then
continue
fi

# String length.
# Skip over short strings.

grep -Fw $word "$WORDFILE"
^^^

# Match whole words only.
# "Fixed strings" and
#+ "whole words" options.

done
exit $?

Comparison
diff, patch
diff: flexible file comparison utility. It compares the target files line-by-line sequentially. In some
applications, such as comparing word dictionaries, it may be helpful to filter the files through sort and
uniq before piping them to diff. diff file-1 file-2 outputs the lines in the files that differ,
with carets showing which file each particular line belongs to.
The --side-by-side option to diff outputs each compared file, line by line, in separate columns,
with non-matching lines marked. The -c and -u options likewise make the output of the command
easier to interpret.
There are available various fancy frontends for diff, such as sdiff, wdiff, xdiff, and mgdiff.
The diff command returns an exit status of 0 if the compared files are identical, and 1
if they differ. This permits use of diff in a test construct within a shell script (see
below).
A common use for diff is generating difference files to be used with patch The -e option outputs
files suitable for ed or ex scripts.

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patch: flexible versioning utility. Given a difference file generated by diff, patch can upgrade a
previous version of a package to a newer version. It is much more convenient to distribute a relatively
small "diff" file than the entire body of a newly revised package. Kernel "patches" have become the
preferred method of distributing the frequent releases of the Linux kernel.
patch -p1  /dev/null # /dev/null buries the output of the "cmp" command.
#
cmp -s $1 $2 has same result ("-s" silent flag to "cmp")
#
Thank you Anders Gustavsson for pointing this out.
#
# Also works with 'diff', i.e.,
diff $1 $2 &> /dev/null
if [ $? -eq 0 ]
# Test exit status of "cmp" command.
then
echo "File \"$1\" is identical to file \"$2\"."
else
echo "File \"$1\" differs from file \"$2\"."
fi
exit 0

Use zcmp on gzipped files.
comm
Versatile file comparison utility. The files must be sorted for this to be useful.
comm -options first-file second-file
comm file-1 file-2 outputs three columns:
◊ column 1 = lines unique to file-1
◊ column 2 = lines unique to file-2
◊ column 3 = lines common to both.
The options allow suppressing output of one or more columns.
◊ -1 suppresses column 1
◊ -2 suppresses column 2
◊ -3 suppresses column 3
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◊ -12 suppresses both columns 1 and 2, etc.
This command is useful for comparing "dictionaries" or word lists -- sorted text files with one word
per line.
Utilities
basename
Strips the path information from a file name, printing only the file name. The construction
basename $0 lets the script know its name, that is, the name it was invoked by. This can be used
for "usage" messages if, for example a script is called with missing arguments:
echo "Usage: `basename $0` arg1 arg2 ... argn"

dirname
Strips the basename from a filename, printing only the path information.
basename and dirname can operate on any arbitrary string. The argument does not
need to refer to an existing file, or even be a filename for that matter (see Example
A-7).
Example 16-36. basename and dirname
#!/bin/bash
a=/home/bozo/daily-journal.txt
echo
echo
echo
echo
echo

"Basename of /home/bozo/daily-journal.txt = `basename $a`"
"Dirname of /home/bozo/daily-journal.txt = `dirname $a`"
"My own home is `basename ~/`."
"The home of my home is `dirname ~/`."

# `basename ~` also works.
# `dirname ~` also works.

exit 0

split, csplit
These are utilities for splitting a file into smaller chunks. Their usual use is for splitting up large files
in order to back them up on floppies or preparatory to e-mailing or uploading them.
The csplit command splits a file according to context, the split occuring where patterns are matched.

Example 16-37. A script that copies itself in sections
#!/bin/bash
# splitcopy.sh
# A script that splits itself into chunks,
#+ then reassembles the chunks into an exact copy
#+ of the original script.
CHUNKSIZE=4
OUTPREFIX=xx

# Size of first chunk of split files.
# csplit prefixes, by default,
#+ files with "xx" ...

csplit "$0" "$CHUNKSIZE"
# Some comment lines for padding . . .

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#
#
#
#
#
#

Line
Line
Line
Line
Line
Line

15
16
17
18
19
20

cat "$OUTPREFIX"* > "$0.copy"
rm "$OUTPREFIX"*

# Concatenate the chunks.
# Get rid of the chunks.

exit $?

Encoding and Encryption
sum, cksum, md5sum, sha1sum
These are utilities for generating checksums. A checksum is a number [75] mathematically calculated
from the contents of a file, for the purpose of checking its integrity. A script might refer to a list of
checksums for security purposes, such as ensuring that the contents of key system files have not been
altered or corrupted. For security applications, use the md5sum (message digest 5 checksum)
command, or better yet, the newer sha1sum (Secure Hash Algorithm). [76]
bash$ cksum /boot/vmlinuz
1670054224 804083 /boot/vmlinuz
bash$ echo -n "Top Secret" | cksum
3391003827 10

bash$ md5sum /boot/vmlinuz
0f43eccea8f09e0a0b2b5cf1dcf333ba

/boot/vmlinuz

bash$ echo -n "Top Secret" | md5sum
8babc97a6f62a4649716f4df8d61728f -

The cksum command shows the size, in bytes, of its target, whether file or stdout.
The md5sum and sha1sum commands display a dash when they receive their input
from stdout.
Example 16-38. Checking file integrity
#!/bin/bash
# file-integrity.sh: Checking whether files in a given directory
#
have been tampered with.
E_DIR_NOMATCH=70
E_BAD_DBFILE=71
dbfile=File_record.md5
# Filename for storing records (database file).

set_up_database ()
{
echo ""$directory"" > "$dbfile"

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# Write directory name to first line of file.
md5sum "$directory"/* >> "$dbfile"
# Append md5 checksums and filenames.
}
check_database ()
{
local n=0
local filename
local checksum
# ------------------------------------------- #
# This file check should be unnecessary,
#+ but better safe than sorry.
if [ ! -r "$dbfile" ]
then
echo "Unable to read checksum database file!"
exit $E_BAD_DBFILE
fi
# ------------------------------------------- #
while read record[n]
do
directory_checked="${record[0]}"
if [ "$directory_checked" != "$directory" ]
then
echo "Directories do not match up!"
# Tried to use file for a different directory.
exit $E_DIR_NOMATCH
fi
if [ "$n" -gt 0 ]
# Not directory name.
then
filename[n]=$( echo ${record[$n]} | awk '{ print $2 }' )
# md5sum writes records backwards,
#+ checksum first, then filename.
checksum[n]=$( md5sum "${filename[n]}" )

if [ "${record[n]}" = "${checksum[n]}" ]
then
echo "${filename[n]} unchanged."
elif [ "`basename ${filename[n]}`" != "$dbfile" ]
# Skip over checksum database file,
#+ as it will change with each invocation of script.
# --# This unfortunately means that when running
#+ this script on $PWD, tampering with the
#+ checksum database file will not be detected.
# Exercise: Fix this.
then
echo "${filename[n]} : CHECKSUM ERROR!"
# File has been changed since last checked.
fi
fi

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let "n+=1"
done <"$dbfile"

# Read from checksum database file.

}
# =================================================== #
# main ()
if [ -z "$1" ]
then
directory="$PWD"
else
directory="$1"
fi

# If not specified,
#+ use current working directory.

clear
# Clear screen.
echo " Running file integrity check on $directory"
echo
# ------------------------------------------------------------------ #
if [ ! -r "$dbfile" ] # Need to create database file?
then
echo "Setting up database file, \""$directory"/"$dbfile"\"."; echo
set_up_database
fi
# ------------------------------------------------------------------ #
check_database

# Do the actual work.

echo
# You may wish to redirect the stdout of this script to a file,
#+ especially if the directory checked has many files in it.
exit 0
# For a much more thorough file integrity check,
#+ consider the "Tripwire" package,
#+ http://sourceforge.net/projects/tripwire/.

Also see Example A-19, Example 36-14, and Example 10-2 for creative uses of the md5sum
command.
There have been reports that the 128-bit md5sum can be cracked, so the more secure
160-bit sha1sum is a welcome new addition to the checksum toolkit.
bash$ md5sum testfile
e181e2c8720c60522c4c4c981108e367

testfile

bash$ sha1sum testfile
5d7425a9c08a66c3177f1e31286fa40986ffc996

testfile

Security consultants have demonstrated that even sha1sum can be compromised. Fortunately, newer
Linux distros include longer bit-length sha224sum, sha256sum, sha384sum, and sha512sum
commands.
uuencode
This utility encodes binary files (images, sound files, compressed files, etc.) into ASCII characters,
making them suitable for transmission in the body of an e-mail message or in a newsgroup posting.
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This is especially useful where MIME (multimedia) encoding is not available.
uudecode
This reverses the encoding, decoding uuencoded files back into the original binaries.

Example 16-39. Uudecoding encoded files
#!/bin/bash
# Uudecodes all uuencoded files in current working directory.
lines=35

# Allow 35 lines for the header (very generous).

for File in *
# Test all the files in $PWD.
do
search1=`head -n $lines $File | grep begin | wc -w`
search2=`tail -n $lines $File | grep end | wc -w`
# Uuencoded files have a "begin" near the beginning,
#+ and an "end" near the end.
if [ "$search1" -gt 0 ]
then
if [ "$search2" -gt 0 ]
then
echo "uudecoding - $File -"
uudecode $File
fi
fi
done
# Note that running this script upon itself fools it
#+ into thinking it is a uuencoded file,
#+ because it contains both "begin" and "end".
#
#
#
#+

Exercise:
-------Modify this script to check each file for a newsgroup header,
and skip to next if not found.

exit 0

The fold -s command may be useful (possibly in a pipe) to process long uudecoded
text messages downloaded from Usenet newsgroups.
mimencode, mmencode
The mimencode and mmencode commands process multimedia-encoded e-mail attachments.
Although mail user agents (such as pine or kmail) normally handle this automatically, these particular
utilities permit manipulating such attachments manually from the command-line or in batch
processing mode by means of a shell script.
crypt
At one time, this was the standard UNIX file encryption utility. [77] Politically-motivated government
regulations prohibiting the export of encryption software resulted in the disappearance of crypt from
much of the UNIX world, and it is still missing from most Linux distributions. Fortunately,
programmers have come up with a number of decent alternatives to it, among them the author's very
own cruft (see Example A-4).
openssl
This is an Open Source implementation of Secure Sockets Layer encryption.
# To encrypt a file:

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openssl aes-128-ecb -salt -in file.txt -out file.encrypted \
-pass pass:my_password
#
^^^^^^^^^^^
User-selected password.
#
aes-128-ecb
is the encryption method chosen.
# To decrypt an openssl-encrypted file:
openssl aes-128-ecb -d -salt -in file.encrypted -out file.txt \
-pass pass:my_password
#
^^^^^^^^^^^
User-selected password.

Piping openssl to/from tar makes it possible to encrypt an entire directory tree.
# To encrypt a directory:
sourcedir="/home/bozo/testfiles"
encrfile="encr-dir.tar.gz"
password=my_secret_password
tar czvf - "$sourcedir" |
openssl des3 -salt -out "$encrfile" -pass pass:"$password"
#
^^^^
Uses des3 encryption.
# Writes encrypted file "encr-dir.tar.gz" in current working directory.
# To decrypt the resulting tarball:
openssl des3 -d -salt -in "$encrfile" -pass pass:"$password" |
tar -xzv
# Decrypts and unpacks into current working directory.

Of course, openssl has many other uses, such as obtaining signed certificates for Web sites. See the
info page.
shred
Securely erase a file by overwriting it multiple times with random bit patterns before deleting it. This
command has the same effect as Example 16-60, but does it in a more thorough and elegant manner.
This is one of the GNU fileutils.
Advanced forensic technology may still be able to recover the contents of a file, even
after application of shred.
Miscellaneous
mktemp
Create a temporary file [78] with a "unique" filename. When invoked from the command-line without
additional arguments, it creates a zero-length file in the /tmp directory.
bash$ mktemp
/tmp/tmp.zzsvql3154

PREFIX=filename
tempfile=`mktemp $PREFIX.XXXXXX`
#
^^^^^^ Need at least 6 placeholders
#+
in the filename template.
#
If no filename template supplied,
#+ "tmp.XXXXXXXXXX" is the default.
echo "tempfile name = $tempfile"
# tempfile name = filename.QA2ZpY

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#
#
#+
#
#+

or something similar...
Creates a file of that name in the current working directory
with 600 file permissions.
A "umask 177" is therefore unnecessary,
but it's good programming practice anyhow.

make
Utility for building and compiling binary packages. This can also be used for any set of operations
triggered by incremental changes in source files.
The make command checks a Makefile, a list of file dependencies and operations to be carried out.
The make utility is, in effect, a powerful scripting language similar in many ways to Bash, but with
the capability of recognizing dependencies. For in-depth coverage of this useful tool set, see the GNU
software documentation site.
install
Special purpose file copying command, similar to cp, but capable of setting permissions and attributes
of the copied files. This command seems tailormade for installing software packages, and as such it
shows up frequently in Makefiles (in the make install : section). It could likewise prove
useful in installation scripts.
dos2unix
This utility, written by Benjamin Lin and collaborators, converts DOS-formatted text files (lines
terminated by CR-LF) to UNIX format (lines terminated by LF only), and vice-versa.
ptx
The ptx [targetfile] command outputs a permuted index (cross-reference list) of the targetfile. This
may be further filtered and formatted in a pipe, if necessary.
more, less
Pagers that display a text file or stream to stdout, one screenful at a time. These may be used to
filter the output of stdout . . . or of a script.
An interesting application of more is to "test drive" a command sequence, to forestall potentially
unpleasant consequences.
ls /home/bozo | awk '{print "rm -rf " $1}' | more
#
^^^^
# Testing the effect of the following (disastrous) command-line:
#
ls /home/bozo | awk '{print "rm -rf " $1}' | sh
#
Hand off to the shell to execute . . .
^^

The less pager has the interesting property of doing a formatted display of man page source. See
Example A-39.

16.6. Communications Commands
Certain of the following commands find use in network data transfer and analysis, as well as in chasing
spammers.
Information and Statistics
host
Searches for information about an Internet host by name or IP address, using DNS.
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bash$ host surfacemail.com
surfacemail.com. has address 202.92.42.236

ipcalc
Displays IP information for a host. With the -h option, ipcalc does a reverse DNS lookup, finding the
name of the host (server) from the IP address.
bash$ ipcalc -h 202.92.42.236
HOSTNAME=surfacemail.com

nslookup
Do an Internet "name server lookup" on a host by IP address. This is essentially equivalent to ipcalc
-h or dig -x . The command may be run either interactively or noninteractively, i.e., from within a
script.
The nslookup command has allegedly been "deprecated," but it is still useful.
bash$ nslookup -sil 66.97.104.180
nslookup kuhleersparnis.ch
Server:
135.116.137.2
Address:
135.116.137.2#53
Non-authoritative answer:
Name:
kuhleersparnis.ch

dig
Domain Information Groper. Similar to nslookup, dig does an Internet name server lookup on a host.
May be run from the command-line or from within a script.
Some interesting options to dig are +time=N for setting a query timeout to N seconds, +nofail for
continuing to query servers until a reply is received, and -x for doing a reverse address lookup.
Compare the output of dig -x with ipcalc -h and nslookup.
bash$ dig -x 81.9.6.2
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0
;; QUESTION SECTION:
;2.6.9.81.in-addr.arpa.
;; AUTHORITY SECTION:
6.9.81.in-addr.arpa.
3600
2002031705 900 600 86400 3600
;;
;;
;;
;;

IN

PTR

IN

SOA

ns.eltel.net. noc.eltel.net.

Query time: 537 msec
SERVER: 135.116.137.2#53(135.116.137.2)
WHEN: Wed Jun 26 08:35:24 2002
MSG SIZE rcvd: 91

Example 16-40. Finding out where to report a spammer

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#!/bin/bash
# spam-lookup.sh: Look up abuse contact to report a spammer.
# Thanks, Michael Zick.
# Check for command-line arg.
ARGCOUNT=1
E_WRONGARGS=65
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` domain-name"
exit $E_WRONGARGS
fi

dig +short $1.contacts.abuse.net -c in -t txt
# Also try:
#
dig +nssearch $1
#
Tries to find "authoritative name servers" and display SOA records.
# The following also works:
#
whois -h whois.abuse.net $1
#
^^ ^^^^^^^^^^^^^^^ Specify host.
#
Can even lookup multiple spammers with this, i.e."
#
whois -h whois.abuse.net $spamdomain1 $spamdomain2 . . .

#
#
#
#+
#+
#

Exercise:
-------Expand the functionality of this script
so that it automatically e-mails a notification
to the responsible ISP's contact address(es).
Hint: use the "mail" command.

exit $?
# spam-lookup.sh chinatietong.com
#
A known spam domain.
# "crnet_mgr@chinatietong.com"
# "crnet_tec@chinatietong.com"
# "postmaster@chinatietong.com"

# For a more elaborate version of this script,
#+ see the SpamViz home page, http://www.spamviz.net/index.html.

Example 16-41. Analyzing a spam domain
#! /bin/bash
# is-spammer.sh: Identifying spam domains
# $Id: is-spammer, v 1.4 2004/09/01 19:37:52 mszick Exp $
# Above line is RCS ID info.
#
# This is a simplified version of the "is_spammer.bash
#+ script in the Contributed Scripts appendix.
# is-spammer 
# Uses an external program: 'dig'

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# Tested with version: 9.2.4rc5
# Uses functions.
# Uses IFS to parse strings by assignment into arrays.
# And even does something useful: checks e-mail blacklists.
# Use the domain.name(s) from the text body:
# http://www.good_stuff.spammer.biz/just_ignore_everything_else
#
^^^^^^^^^^^
# Or the domain.name(s) from any e-mail address:
# Really_Good_Offer@spammer.biz
#
# as the only argument to this script.
#(PS: have your Inet connection running)
#
# So, to invoke this script in the above two instances:
#
is-spammer.sh spammer.biz

# Whitespace == :Space:Tab:Line Feed:Carriage Return:
WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'
# No Whitespace == Line Feed:Carriage Return
No_WSP=$'\x0A'$'\x0D'
# Field separator for dotted decimal ip addresses
ADR_IFS=${No_WSP}'.'
# Get the dns text resource record.
# get_txt  
get_txt() {
# Parse $1 by assignment at the dots.
local -a dns
IFS=$ADR_IFS
dns=( $1 )
IFS=$WSP_IFS
if [ "${dns[0]}" == '127' ]
then
# See if there is a reason.
echo $(dig +short $2 -t txt)
fi
}
# Get the
# chk_adr
chk_adr()
local
local
local

dns address resource record.
 
{
reply
server
reason

server=${1}${2}
reply=$( dig +short ${server} )
# If reply might be an error code . . .
if [ ${#reply} -gt 6 ]
then
reason=$(get_txt ${reply} ${server} )
reason=${reason:-${reply}}
fi
echo ${reason:-' not blacklisted.'}
}

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# Need to get the IP address from the name.
echo 'Get address of: '$1
ip_adr=$(dig +short $1)
dns_reply=${ip_adr:-' no answer '}
echo ' Found address: '${dns_reply}
# A valid reply is at least 4 digits plus 3 dots.
if [ ${#ip_adr} -gt 6 ]
then
echo
declare query
# Parse by assignment at the dots.
declare -a dns
IFS=$ADR_IFS
dns=( ${ip_adr} )
IFS=$WSP_IFS
# Reorder octets into dns query order.
rev_dns="${dns[3]}"'.'"${dns[2]}"'.'"${dns[1]}"'.'"${dns[0]}"'.'
# See: http://www.spamhaus.org (Conservative, well maintained)
echo -n 'spamhaus.org says: '
echo $(chk_adr ${rev_dns} 'sbl-xbl.spamhaus.org')
# See: http://ordb.org (Open mail relays)
echo -n '
ordb.org says: '
echo $(chk_adr ${rev_dns} 'relays.ordb.org')
# See: http://www.spamcop.net/ (You can report spammers here)
echo -n ' spamcop.net says: '
echo $(chk_adr ${rev_dns} 'bl.spamcop.net')
# # # other blacklist operations # # #
# See: http://cbl.abuseat.org.
echo -n ' abuseat.org says: '
echo $(chk_adr ${rev_dns} 'cbl.abuseat.org')
# See: http://dsbl.org/usage (Various mail relays)
echo
echo 'Distributed Server Listings'
echo -n '
list.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'list.dsbl.org')
echo -n '
multihop.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'multihop.dsbl.org')
echo -n 'unconfirmed.dsbl.org says: '
echo $(chk_adr ${rev_dns} 'unconfirmed.dsbl.org')
else
echo
echo 'Could not use that address.'
fi
exit 0
# Exercises:
# --------

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# 1) Check arguments to script,
#
and exit with appropriate error message if necessary.
# 2) Check if on-line at invocation of script,
#
and exit with appropriate error message if necessary.
# 3) Substitute generic variables for "hard-coded" BHL domains.
# 4) Set a time-out for the script using the "+time=" option
to the 'dig' command.

For a much more elaborate version of the above script, see Example A-28.
traceroute
Trace the route taken by packets sent to a remote host. This command works within a LAN, WAN, or
over the Internet. The remote host may be specified by an IP address. The output of this command
may be filtered by grep or sed in a pipe.
bash$ traceroute 81.9.6.2
traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets
1 tc43.xjbnnbrb.com (136.30.178.8) 191.303 ms 179.400 ms 179.767 ms
2 or0.xjbnnbrb.com (136.30.178.1) 179.536 ms 179.534 ms 169.685 ms
3 192.168.11.101 (192.168.11.101) 189.471 ms 189.556 ms *
...

ping
Broadcast an ICMP ECHO_REQUEST packet to another machine, either on a local or remote
network. This is a diagnostic tool for testing network connections, and it should be used with caution.
bash$ ping localhost
PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.
64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec
64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec
--- localhost.localdomain ping statistics --2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms

A successful ping returns an exit status of 0. This can be tested for in a script.

HNAME=nastyspammer.com
# HNAME=$HOST
# Debug: test for localhost.
count=2 # Send only two pings.
if [[ `ping -c $count "$HNAME"` ]]
then
echo ""$HNAME" still up and broadcasting spam your way."
else
echo ""$HNAME" seems to be down. Pity."
fi

whois
Perform a DNS (Domain Name System) lookup. The -h option permits specifying which particular
whois server to query. See Example 4-6 and Example 16-40.
finger
Retrieve information about users on a network. Optionally, this command can display a user's
~/.plan, ~/.project, and ~/.forward files, if present.

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bash$ finger
Login Name
bozo
Bozo Bozeman
bozo
Bozo Bozeman
bozo
Bozo Bozeman

Tty
tty1
ttyp0
ttyp1

Idle
8

Login Time
Office
Jun 25 16:59
Jun 25 16:59
Jun 25 17:07

Office Phone
(:0)
(:0.0)
(:0.0)

bash$ finger bozo
Login: bozo
Name: Bozo Bozeman
Directory: /home/bozo
Shell: /bin/bash
Office: 2355 Clown St., 543-1234
On since Fri Aug 31 20:13 (MST) on tty1
1 hour 38 minutes idle
On since Fri Aug 31 20:13 (MST) on pts/0
12 seconds idle
On since Fri Aug 31 20:13 (MST) on pts/1
On since Fri Aug 31 20:31 (MST) on pts/2
1 hour 16 minutes idle
Mail last read Tue Jul 3 10:08 2007 (MST)
No Plan.

Out of security considerations, many networks disable finger and its associated daemon. [79]
chfn
Change information disclosed by the finger command.
vrfy
Verify an Internet e-mail address.
This command seems to be missing from newer Linux distros.
Remote Host Access
sx, rx
The sx and rx command set serves to transfer files to and from a remote host using the xmodem
protocol. These are generally part of a communications package, such as minicom.
sz, rz
The sz and rz command set serves to transfer files to and from a remote host using the zmodem
protocol. Zmodem has certain advantages over xmodem, such as faster transmission rate and
resumption of interrupted file transfers. Like sx and rx, these are generally part of a communications
package.
ftp
Utility and protocol for uploading / downloading files to or from a remote host. An ftp session can be
automated in a script (see Example 19-6 and Example A-4).
uucp, uux, cu
uucp: UNIX to UNIX copy. This is a communications package for transferring files between UNIX
servers. A shell script is an effective way to handle a uucp command sequence.
Since the advent of the Internet and e-mail, uucp seems to have faded into obscurity, but it still exists
and remains perfectly workable in situations where an Internet connection is not available or
appropriate. The advantage of uucp is that it is fault-tolerant, so even if there is a service interruption
the copy operation will resume where it left off when the connection is restored.
--uux: UNIX to UNIX execute. Execute a command on a remote system. This command is part of the
uucp package.

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--cu: Call Up a remote system and connect as a simple terminal. It is a sort of dumbed-down version of
telnet. This command is part of the uucp package.
telnet
Utility and protocol for connecting to a remote host.
The telnet protocol contains security holes and should therefore probably be avoided.
Its use within a shell script is not recommended.
wget
The wget utility noninteractively retrieves or downloads files from a Web or ftp site. It works well in
a script.
wget -p http://www.xyz23.com/file01.html
# The -p or --page-requisite option causes wget to fetch all files
#+ required to display the specified page.
wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -O $SAVEFILE
# The -r option recursively follows and retrieves all links
#+ on the specified site.
wget -c ftp://ftp.xyz25.net/bozofiles/filename.tar.bz2
# The -c option lets wget resume an interrupted download.
# This works with ftp servers and many HTTP sites.

Example 16-42. Getting a stock quote
#!/bin/bash
# quote-fetch.sh: Download a stock quote.

E_NOPARAMS=86
if [ -z "$1" ] # Must specify a stock (symbol) to fetch.
then echo "Usage: `basename $0` stock-symbol"
exit $E_NOPARAMS
fi
stock_symbol=$1
file_suffix=.html
# Fetches an HTML file, so name it appropriately.
URL='http://finance.yahoo.com/q?s='
# Yahoo finance board, with stock query suffix.
# ----------------------------------------------------------wget -O ${stock_symbol}${file_suffix} "${URL}${stock_symbol}"
# -----------------------------------------------------------

#
#
#
#
#
#

To look up stuff on http://search.yahoo.com:
----------------------------------------------------------URL="http://search.yahoo.com/search?fr=ush-news&p=${query}"
wget -O "$savefilename" "${URL}"
----------------------------------------------------------Saves a list of relevant URLs.

exit $?

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# Exercises:
# --------#
# 1) Add a test to ensure the user running the script is on-line.
#
(Hint: parse the output of 'ps -ax' for "ppp" or "connect."
#
# 2) Modify this script to fetch the local weather report,
#+
taking the user's zip code as an argument.

See also Example A-30 and Example A-31.
lynx
The lynx Web and file browser can be used inside a script (with the -dump option) to retrieve a file
from a Web or ftp site noninteractively.
lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE

With the -traversal option, lynx starts at the HTTP URL specified as an argument, then "crawls"
through all links located on that particular server. Used together with the -crawl option, outputs
page text to a log file.
rlogin
Remote login, initates a session on a remote host. This command has security issues, so use ssh
instead.
rsh
Remote shell, executes command(s) on a remote host. This has security issues, so use ssh
instead.
rcp
Remote copy, copies files between two different networked machines.
rsync
Remote synchronize, updates (synchronizes) files between two different networked machines.
bash$ rsync -a ~/sourcedir/*txt /node1/subdirectory/

Example 16-43. Updating FC4
#!/bin/bash
# fc4upd.sh
# Script author: Frank Wang.
# Slight stylistic modifications by ABS Guide author.
# Used in ABS Guide with permission.

#
#
#
#+

Download Fedora Core 4 update from mirror site using rsync.
Should also work for newer Fedora Cores -- 5, 6, . . .
Only download latest package if multiple versions exist,
to save space.

URL=rsync://distro.ibiblio.org/fedora-linux-core/updates/
# URL=rsync://ftp.kddilabs.jp/fedora/core/updates/
# URL=rsync://rsync.planetmirror.com/fedora-linux-core/updates/
DEST=${1:-/var/www/html/fedora/updates/}
LOG=/tmp/repo-update-$(/bin/date +%Y-%m-%d).txt
PID_FILE=/var/run/${0##*/}.pid
E_RETURN=85

# Something unexpected happened.

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#
#
#
#

General rsync options
-r: recursive download
-t: reserve time
-v: verbose

OPTS="-rtv --delete-excluded --delete-after --partial"
# rsync include pattern
# Leading slash causes absolute path name match.
INCLUDE=(
"/4/i386/kde-i18n-Chinese*"
#
^
^
# Quoting is necessary to prevent globbing.
)

# rsync exclude pattern
# Temporarily comment out unwanted pkgs using "#" . . .
EXCLUDE=(
/1
/2
/3
/testing
/4/SRPMS
/4/ppc
/4/x86_64
/4/i386/debug
"/4/i386/kde-i18n-*"
"/4/i386/openoffice.org-langpack-*"
"/4/i386/*i586.rpm"
"/4/i386/GFS-*"
"/4/i386/cman-*"
"/4/i386/dlm-*"
"/4/i386/gnbd-*"
"/4/i386/kernel-smp*"
# "/4/i386/kernel-xen*"
# "/4/i386/xen-*"
)

init () {
# Let pipe command return possible rsync error, e.g., stalled network.
set -o pipefail
# Newly introduced in Bash, version 3.
TMP=${TMPDIR:-/tmp}/${0##*/}.$$
trap "{
rm -f $TMP 2>/dev/null
}" EXIT

# Store refined download list.

# Clear temporary file on exit.

}

check_pid () {
# Check if process exists.
if [ -s "$PID_FILE" ]; then
echo "PID file exists. Checking ..."
PID=$(/bin/egrep -o "^[[:digit:]]+" $PID_FILE)
if /bin/ps --pid $PID &>/dev/null; then
echo "Process $PID found. ${0##*/} seems to be running!"
/usr/bin/logger -t ${0##*/} \
"Process $PID found. ${0##*/} seems to be running!"

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exit $E_RETURN
fi
echo "Process $PID not found. Start new process . . ."
fi
}

# Set overall file update range starting from root or $URL,
#+ according to above patterns.
set_range () {
include=
exclude=
for p in "${INCLUDE[@]}"; do
include="$include --include \"$p\""
done
for p in "${EXCLUDE[@]}"; do
exclude="$exclude --exclude \"$p\""
done
}

# Retrieve and refine rsync update list.
get_list () {
echo $$ > $PID_FILE || {
echo "Can't write to pid file $PID_FILE"
exit $E_RETURN
}
echo -n "Retrieving and refining update list . . ."
# Retrieve list -- 'eval' is needed to run rsync as a single command.
# $3 and $4 is the date and time of file creation.
# $5 is the full package name.
previous=
pre_file=
pre_date=0
eval /bin/nice /usr/bin/rsync \
-r $include $exclude $URL | \
egrep '^dr.x|^-r' | \
awk '{print $3, $4, $5}' | \
sort -k3 | \
{ while read line; do
# Get seconds since epoch, to filter out obsolete pkgs.
cur_date=$(date -d "$(echo $line | awk '{print $1, $2}')" +%s)
# echo $cur_date
# Get file name.
cur_file=$(echo $line | awk '{print $3}')
# echo $cur_file
# Get rpm pkg name from file name, if possible.
if [[ $cur_file == *rpm ]]; then
pkg_name=$(echo $cur_file | sed -r -e \
's/(^([^_-]+[_-])+)[[:digit:]]+\..*[_-].*$/\1/')
else
pkg_name=
fi
# echo $pkg_name
if [ -z "$pkg_name" ]; then
echo $cur_file >> $TMP

# If not a rpm file,
#+ then append to download list.

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elif [ "$pkg_name" != "$previous" ]; then
# A new pkg found.
echo $pre_file >> $TMP
# Output latest file.
previous=$pkg_name
# Save current.
pre_date=$cur_date
pre_file=$cur_file
elif [ "$cur_date" -gt "$pre_date" ]; then
# If same pkg, but newer,
pre_date=$cur_date
#+ then update latest pointer.
pre_file=$cur_file
fi
done
echo $pre_file >> $TMP
# TMP contains ALL
#+ of refined list now.
# echo "subshell=$BASH_SUBSHELL"
}

# Bracket required here to let final "echo $pre_file >> $TMP"
# Remained in the same subshell ( 1 ) with the entire loop.

RET=$?

# Get return code of the pipe command.

[ "$RET" -ne 0 ] && {
echo "List retrieving failed with code $RET"
exit $E_RETURN
}
echo "done"; echo
}
# Real rsync download part.
get_file () {
echo "Downloading..."
/bin/nice /usr/bin/rsync \
$OPTS \
--filter "merge,+/ $TMP" \
--exclude '*' \
$URL $DEST
\
| /usr/bin/tee $LOG
RET=$?
#
#
#
#+

--filter merge,+/ is crucial for the intention.
+ modifier means include and / means absolute path.
Then sorted list in $TMP will contain ascending dir name and
prevent the following --exclude '*' from "shortcutting the circuit."

echo "Done"
rm -f $PID_FILE 2>/dev/null
return $RET
}
# ------# Main
init
check_pid
set_range
get_list
get_file
RET=$?
# -------

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if [ "$RET" -eq 0 ]; then
/usr/bin/logger -t ${0##*/} "Fedora update mirrored successfully."
else
/usr/bin/logger -t ${0##*/} \
"Fedora update mirrored with failure code: $RET"
fi
exit $RET

See also Example A-32.
Using rcp, rsync, and similar utilities with security implications in a shell script may
not be advisable. Consider, instead, using ssh, scp, or an expect script.
ssh
Secure shell, logs onto a remote host and executes commands there. This secure replacement for
telnet, rlogin, rcp, and rsh uses identity authentication and encryption. See its manpage for details.

Example 16-44. Using ssh
#!/bin/bash
# remote.bash: Using ssh.
# This example by Michael Zick.
# Used with permission.

#
#
#
#
#
#
#
#+

Presumptions:
-----------fd-2 isn't being captured ( '2>/dev/null' ).
ssh/sshd presumes stderr ('2') will display to user.

#
#
#
#
#
#
#
#

sshd is running on your machine.
For any 'standard' distribution, it probably is,
and without any funky ssh-keygen having been done.
Try ssh to your machine from the command-line:
$ ssh $HOSTNAME
Without extra set-up you'll be asked for your password.
enter password
when done, $ exit
Did that work? If so, you're ready for more fun.

# Try ssh to your machine as 'root':
#
#
$ ssh -l root $HOSTNAME
#
When asked for password, enter root's, not yours.
#
Last login: Tue Aug 10 20:25:49 2004 from localhost.localdomain
#
Enter 'exit' when done.
#
#
#+
#
#+

The above gives you an interactive shell.
It is possible for sshd to be set up in a 'single command' mode,
but that is beyond the scope of this example.
The only thing to note is that the following will work in
'single command' mode.

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# A basic, write stdout (local) command.
ls -l
# Now the same basic command on a remote machine.
# Pass a different 'USERNAME' 'HOSTNAME' if desired:
USER=${USERNAME:-$(whoami)}
HOST=${HOSTNAME:-$(hostname)}
# Now excute the above command-line on the remote host,
#+ with all transmissions encrypted.
ssh -l ${USER} ${HOST} " ls -l "
#
#+
#
#+

The expected result is a listing of your username's home
directory on the remote machine.
To see any difference, run this script from somewhere
other than your home directory.

# In other words, the Bash command is passed as a quoted line
#+ to the remote shell, which executes it on the remote machine.
# In this case, sshd does ' bash -c "ls -l" '
on your behalf.
# For information on topics such as not having to enter a
#+ password/passphrase for every command-line, see
#+
man ssh
#+
man ssh-keygen
#+
man sshd_config.
exit 0

Within a loop, ssh may cause unexpected behavior. According to a Usenet post in the
comp.unix shell archives, ssh inherits the loop's stdin. To remedy this, pass ssh
either the -n or -f option.
Thanks, Jason Bechtel, for pointing this out.
scp
Secure copy, similar in function to rcp, copies files between two different networked machines,
but does so using authentication, and with a security level similar to ssh.
Local Network
write
This is a utility for terminal-to-terminal communication. It allows sending lines from your terminal
(console or xterm) to that of another user. The mesg command may, of course, be used to disable
write access to a terminal
Since write is interactive, it would not normally find use in a script.
netconfig
A command-line utility for configuring a network adapter (using DHCP). This command is native to
Red Hat centric Linux distros.
Mail
mail
Send or read e-mail messages.
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This stripped-down command-line mail client works fine as a command embedded in a script.

Example 16-45. A script that mails itself
#!/bin/sh
# self-mailer.sh: Self-mailing script
adr=${1:-`whoami`}
# Default to current user, if not specified.
# Typing 'self-mailer.sh wiseguy@superdupergenius.com'
#+ sends this script to that addressee.
# Just 'self-mailer.sh' (no argument) sends the script
#+ to the person invoking it, for example, bozo@localhost.localdomain.
#
# For more on the ${parameter:-default} construct,
#+ see the "Parameter Substitution" section
#+ of the "Variables Revisited" chapter.
# ============================================================================
cat $0 | mail -s "Script \"`basename $0`\" has mailed itself to you." "$adr"
# ============================================================================
# -------------------------------------------# Greetings from the self-mailing script.
# A mischievous person has run this script,
#+ which has caused it to mail itself to you.
# Apparently, some people have nothing better
#+ to do with their time.
# -------------------------------------------echo "At `date`, script \"`basename $0`\" mailed to "$adr"."
exit 0
# Note that the "mailx" command (in "send" mode) may be substituted
#+ for "mail" ... but with somewhat different options.

mailto
Similar to the mail command, mailto sends e-mail messages from the command-line or in a script.
However, mailto also permits sending MIME (multimedia) messages.
mailstats
Show mail statistics. This command may be invoked only by root.
root# mailstats
Statistics from Tue Jan 1 20:32:08 2008
M
msgsfr bytes_from
msgsto
bytes_to msgsrej msgsdis msgsqur
4
1682
24118K
0
0K
0
0
0
9
212
640K
1894
25131K
0
0
0
=====================================================================
T
1894
24758K
1894
25131K
0
0
0
C
414
0

Mailer
esmtp
local

vacation
This utility automatically replies to e-mails that the intended recipient is on vacation and temporarily
unavailable. It runs on a network, in conjunction with sendmail, and is not applicable to a dial-up
POPmail account.

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16.7. Terminal Control Commands
Command affecting the console or terminal
tput
Initialize terminal and/or fetch information about it from terminfo data. Various options permit certain
terminal operations: tput clear is the equivalent of clear; tput reset is the equivalent of reset.
bash$ tput longname
xterm terminal emulator (X Window System)

Issuing a tput cup X Y moves the cursor to the (X,Y) coordinates in the current terminal. A clear to
erase the terminal screen would normally precede this.
Some interesting options to tput are:
◊ bold, for high-intensity text
◊ smul, to underline text in the terminal
◊ smso, to render text in reverse
◊ sgr0, to reset the terminal parameters (to normal), without clearing the screen
Example scripts using tput:
1. Example 36-13
2. Example 36-11
3. Example A-44
4. Example A-42
5. Example 27-2
Note that stty offers a more powerful command set for controlling a terminal.
infocmp
This command prints out extensive information about the current terminal. It references the terminfo
database.
bash$ infocmp
#
Reconstructed via infocmp from file:
/usr/share/terminfo/r/rxvt
rxvt|rxvt terminal emulator (X Window System),
am, bce, eo, km, mir, msgr, xenl, xon,
colors#8, cols#80, it#8, lines#24, pairs#64,
acsc=``aaffggjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~,
bel=^G, blink=\E[5m, bold=\E[1m,
civis=\E[?25l,
clear=\E[H\E[2J, cnorm=\E[?25h, cr=^M,
...

reset
Reset terminal parameters and clear text screen. As with clear, the cursor and prompt reappear in the
upper lefthand corner of the terminal.
clear
The clear command simply clears the text screen at the console or in an xterm. The prompt and cursor
reappear at the upper lefthand corner of the screen or xterm window. This command may be used
either at the command line or in a script. See Example 11-25.
resize
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Echoes commands necessary to set $TERM and $TERMCAP to duplicate the size (dimensions) of the
current terminal.
bash$ resize
set noglob;
setenv COLUMNS '80';
setenv LINES '24';
unset noglob;

script
This utility records (saves to a file) all the user keystrokes at the command-line in a console or an
xterm window. This, in effect, creates a record of a session.

16.8. Math Commands
"Doing the numbers"
factor
Decompose an integer into prime factors.
bash$ factor 27417
27417: 3 13 19 37

Example 16-46. Generating prime numbers
#!/bin/bash
# primes2.sh
# Generating prime numbers the quick-and-easy way,
#+ without resorting to fancy algorithms.
CEILING=10000
PRIME=0
E_NOTPRIME=

# 1 to 10000

is_prime ()
{
local factors
factors=( $(factor $1) )

# Load output of `factor` into array.

if [ -z "${factors[2]}" ]
# Third element of "factors" array:
#+ ${factors[2]} is 2nd factor of argument.
# If it is blank, then there is no 2nd factor,
#+ and the argument is therefore prime.
then
return $PRIME
# 0
else
return $E_NOTPRIME
# null
fi
}
echo
for n in $(seq $CEILING)
do
if is_prime $n

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then
printf %5d $n
fi
#
^ Five positions per number suffices.
done
#
For a higher $CEILING, adjust upward, as necessary.
echo
exit

bc
Bash can't handle floating point calculations, and it lacks operators for certain important mathematical
functions. Fortunately, bc gallops to the rescue.
Not just a versatile, arbitrary precision calculation utility, bc offers many of the facilities of a
programming language. It has a syntax vaguely resembling C.
Since it is a fairly well-behaved UNIX utility, and may therefore be used in a pipe, bc comes in handy
in scripts.

Here is a simple template for using bc to calculate a script variable. This uses command substitution.
variable=$(echo "OPTIONS; OPERATIONS" | bc)

Example 16-47. Monthly Payment on a Mortgage
#!/bin/bash
# monthlypmt.sh: Calculates monthly payment on a mortgage.

#
#+
#+
#+
#+
#

This is a modification of code in the
"mcalc" (mortgage calculator) package,
by Jeff Schmidt
and
Mendel Cooper (yours truly, the ABS Guide author).
http://www.ibiblio.org/pub/Linux/apps/financial/mcalc-1.6.tar.gz

echo
echo "Given the principal, interest rate, and term of a mortgage,"
echo "calculate the monthly payment."
bottom=1.0
echo
echo
read
echo
read
echo
read

-n "Enter principal (no commas) "
principal
-n "Enter interest rate (percent) "
interest_r
-n "Enter term (months) "
term

# If 12%, enter "12", not ".12".

interest_r=$(echo "scale=9; $interest_r/100.0" | bc) # Convert to decimal.
#
^^^^^^^^^^^^^^^^^ Divide by 100.
# "scale" determines how many decimal places.
interest_rate=$(echo "scale=9; $interest_r/12 + 1.0" | bc)

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top=$(echo "scale=9; $principal*$interest_rate^$term" | bc)
#
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#
Standard formula for figuring interest.
echo; echo "Please be patient. This may take a while."
let "months = $term - 1"
# ====================================================================
for ((x=$months; x > 0; x--))
do
bot=$(echo "scale=9; $interest_rate^$x" | bc)
bottom=$(echo "scale=9; $bottom+$bot" | bc)
# bottom = $(($bottom + $bot"))
done
# ====================================================================
# -------------------------------------------------------------------# Rick Boivie pointed out a more efficient implementation
#+ of the above loop, which decreases computation time by 2/3.
# for ((x=1; x <= $months; x++))
# do
#
bottom=$(echo "scale=9; $bottom * $interest_rate + 1" | bc)
# done

# And then he came up with an even more efficient alternative,
#+ one that cuts down the run time by about 95%!
# bottom=`{
#
echo "scale=9; bottom=$bottom; interest_rate=$interest_rate"
#
for ((x=1; x <= $months; x++))
#
do
#
echo 'bottom = bottom * interest_rate + 1'
#
done
#
echo 'bottom'
#
} | bc`
# Embeds a 'for loop' within command substitution.
# -------------------------------------------------------------------------# On the other hand, Frank Wang suggests:
# bottom=$(echo "scale=9; ($interest_rate^$term-1)/($interest_rate-1)" | bc)
# Because . . .
# The algorithm behind the loop
#+ is actually a sum of geometric proportion series.
# The sum formula is e0(1-q^n)/(1-q),
#+ where e0 is the first element and q=e(n+1)/e(n)
#+ and n is the number of elements.
# --------------------------------------------------------------------------

# let "payment = $top/$bottom"
payment=$(echo "scale=2; $top/$bottom" | bc)
# Use two decimal places for dollars and cents.
echo
echo "monthly payment = \$$payment"
echo

# Echo a dollar sign in front of amount.

exit 0

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# Exercises:
#
1) Filter
#
2) Filter
#
3) If you
#+
expand

input to permit commas in principal amount.
input to permit interest to be entered as percent or decimal.
are really ambitious,
this script to print complete amortization tables.

Example 16-48. Base Conversion
#!/bin/bash
###########################################################################
# Shellscript: base.sh - print number to different bases (Bourne Shell)
# Author
: Heiner Steven (heiner.steven@odn.de)
# Date
: 07-03-95
# Category
: Desktop
# $Id: base.sh,v 1.2 2000/02/06 19:55:35 heiner Exp $
# ==> Above line is RCS ID info.
###########################################################################
# Description
#
# Changes
# 21-03-95 stv fixed error occuring with 0xb as input (0.2)
###########################################################################
# ==> Used in ABS Guide with the script author's permission.
# ==> Comments added by ABS Guide author.
NOARGS=85
PN=`basename "$0"`
VER=`echo '$Revision: 1.2 $' | cut -d' ' -f2`

# Program name
# ==> VER=1.2

Usage () {
echo "$PN - print number to different bases, $VER (stv '95)
usage: $PN [number ...]
If no number is given, the numbers are read from standard input.
A number may be
binary (base 2)
starting with 0b (i.e. 0b1100)
octal (base 8)
starting with 0 (i.e. 014)
hexadecimal (base 16)
starting with 0x (i.e. 0xc)
decimal
otherwise (i.e. 12)" >&2
exit $NOARGS
}
# ==> Prints usage message.
Msg () {
for i
# ==> in [list] missing. Why?
do echo "$PN: $i" >&2
done
}
Fatal () { Msg "$@"; exit 66; }
PrintBases () {
# Determine base of the number
for i
# ==> in [list] missing...
do
# ==> so operates on command-line arg(s).
case "$i" in
0b*)
ibase=2;;
# binary
0x*|[a-f]*|[A-F]*) ibase=16;;
# hexadecimal

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0*)
ibase=8;;
# octal
[1-9]*)
ibase=10;;
# decimal
*)
Msg "illegal number $i - ignored"
continue;;
esac
# Remove prefix, convert hex digits to uppercase (bc needs this).
number=`echo "$i" | sed -e 's:^0[bBxX]::' | tr '[a-f]' '[A-F]'`
# ==> Uses ":" as sed separator, rather than "/".
# Convert number to decimal
dec=`echo "ibase=$ibase; $number" | bc`
case "$dec" in
[0-9]*)
;;
*)
continue;;
esac

# ==> 'bc' is calculator utility.
# number ok
# error: ignore

# Print all conversions in one line.
# ==> 'here document' feeds command list to 'bc'.
echo `bc < Is a "while loop" really necessary here,
# ==>+ since all the cases either break out of the loop
# ==>+ or terminate the script.
# ==> (Above comment by Paulo Marcel Coelho Aragao.)
do
case "$1" in
--)
shift; break;;
-h)
Usage;;
# ==> Help message.
-*)
Usage;;
*)
break;;
# First number
esac
# ==> Error checking for illegal input might be appropriate.
shift
done
if [ $# -gt 0 ]
then
PrintBases "$@"
else
while read line
do
PrintBases $line
done
fi

# Read from stdin.

exit

An alternate method of invoking bc involves using a here document embedded within a command
substitution block. This is especially appropriate when a script needs to pass a list of options and
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commands to bc.
variable=`bc << LIMIT_STRING
options
statements
operations
LIMIT_STRING
`
...or...

variable=$(bc << LIMIT_STRING
options
statements
operations
LIMIT_STRING
)

Example 16-49. Invoking bc using a here document
#!/bin/bash
# Invoking 'bc' using command substitution
# in combination with a 'here document'.

var1=`bc << EOF
18.33 * 19.78
EOF
`
echo $var1

# 362.56

# $( ... ) notation also works.
v1=23.53
v2=17.881
v3=83.501
v4=171.63
var2=$(bc << EOF
scale = 4
a = ( $v1 + $v2 )
b = ( $v3 * $v4 )
a * b + 15.35
EOF
)
echo $var2
# 593487.8452

var3=$(bc -l << EOF
scale = 9
s ( 1.7 )
EOF
)
# Returns the sine of 1.7 radians.
# The "-l" option calls the 'bc' math library.
echo $var3
# .991664810

# Now, try it in a function...
hypotenuse ()
# Calculate hypotenuse of a right triangle.

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{
# c = sqrt( a^2 + b^2 )
hyp=$(bc -l << EOF
scale = 9
sqrt ( $1 * $1 + $2 * $2 )
EOF
)
# Can't directly return floating point values from a Bash function.
# But, can echo-and-capture:
echo "$hyp"
}
hyp=$(hypotenuse 3.68 7.31)
echo "hypotenuse = $hyp"
# 8.184039344

exit 0

Example 16-50. Calculating PI
#!/bin/bash
# cannon.sh: Approximating PI by firing cannonballs.
# Author: Mendel Cooper
# License: Public Domain
# Version 2.2, reldate 13oct08.
# This is a very simple instance of a "Monte Carlo" simulation:
#+ a mathematical model of a real-life event,
#+ using pseudorandom numbers to emulate random chance.
#
#
#+
#
#
#
#
#+
#
#+
#
#+
#
#+
#
#
#+
#+
#
#
#+
#+
#
#
#
#
#+
#

Consider a perfectly square plot of land, 10000 units on a side.
This land has a perfectly circular lake in its center,
with a diameter of 10000 units.
The plot is actually mostly water, except for land in the four corners.
(Think of it as a square with an inscribed circle.)
We will fire iron cannonballs from an old-style cannon
at the square.
All the shots impact somewhere on the square,
either in the lake or on the dry corners.
Since the lake takes up most of the area,
most of the shots will SPLASH! into the water.
Just a few shots will THUD! into solid ground
in the four corners of the square.
If we take enough random, unaimed shots at the square,
Then the ratio of SPLASHES to total shots will approximate
the value of PI/4.
The simplified explanation is that the cannon is actually
shooting only at the upper right-hand quadrant of the square,
i.e., Quadrant I of the Cartesian coordinate plane.

Theoretically, the more shots taken, the better the fit.
However, a shell script, as opposed to a compiled language
with floating-point math built in, requires some compromises.
This decreases the accuracy of the simulation.

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DIMENSION=10000

# Length of each side of the plot.
# Also sets ceiling for random integers generated.

MAXSHOTS=1000

# Fire this many shots.
# 10000 or more would be better, but would take too long.
# Scaling factor.

PMULTIPLIER=4.0

declare -r M_PI=3.141592654
# Actual 9-place value of PI, for comparison purposes.
get_random ()
{
SEED=$(head -n 1 /dev/urandom | od -N 1 | awk '{
RANDOM=$SEED
#
#+
let "rnum = $RANDOM % $DIMENSION"
#
echo $rnum
}

print $2 }')
From "seeding-random.sh"
example script.
Range less than 10000.

distance=
# Declare global variable.
hypotenuse ()
# Calculate hypotenuse of a right triangle.
{
# From "alt-bc.sh" example.
distance=$(bc -l << EOF
scale = 0
sqrt ( $1 * $1 + $2 * $2 )
EOF
)
# Setting "scale" to zero rounds down result to integer value,
#+ a necessary compromise in this script.
# It decreases the accuracy of this simulation.
}

# ==========================================================
# main() {
# "Main" code block, mimicking a C-language main() function.
# Initialize variables.
shots=0
splashes=0
thuds=0
Pi=0
error=0
while [ "$shots" -lt
do

"$MAXSHOTS" ]

xCoord=$(get_random)
yCoord=$(get_random)
hypotenuse $xCoord $yCoord

# Main loop.

# Get random X and Y coords.
# Hypotenuse of
#+ right-triangle = distance.

((shots++))
printf
printf
printf
printf

"#%4d
" $shots
"Xc = %4d " $xCoord
"Yc = %4d " $yCoord
"Distance = %5d " $distance

#
#+
#+
#+

Distance from
center of lake
-- the "origin" -coordinate (0,0).

if [ "$distance" -le "$DIMENSION" ]

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then
echo -n "SPLASH!
((splashes++))
else
echo -n "THUD!
((thuds++))
fi

"

"

Pi=$(echo "scale=9; $PMULTIPLIER*$splashes/$shots" | bc)
# Multiply ratio by 4.0.
echo -n "PI ~ $Pi"
echo
done
echo
echo "After $shots shots, PI looks like approximately
$Pi"
# Tends to run a bit high,
#+ possibly due to round-off error and imperfect randomness of $RANDOM.
# But still usually within plus-or-minus 5% . . .
#+ a pretty fair rough approximation.
error=$(echo "scale=9; $Pi - $M_PI" | bc)
pct_error=$(echo "scale=2; 100.0 * $error / $M_PI" | bc)
echo -n "Deviation from mathematical value of PI =
$error"
echo " ($pct_error% error)"
echo
# End of "main" code block.
# }
# ==========================================================
exit 0
#
#+
#
#
#
#
#+

One might well wonder whether a shell script is appropriate for
an application as complex and computation-intensive as a simulation.
There
1) As
2) To
it

are at least two justifications.
a proof of concept: to show it can be done.
prototype and test the algorithms before rewriting
in a compiled high-level language.

See also Example A-37.
dc
The dc (desk calculator) utility is stack-oriented and uses RPN (Reverse Polish Notation). Like bc, it
has much of the power of a programming language.
Similar to the procedure with bc, echo a command-string to dc.
echo "[Printing a string ... ]P" | dc
# The P command prints the string between the preceding brackets.
# And now for some simple arithmetic.
echo "7 8 * p" | dc
# 56
# Pushes 7, then 8 onto the stack,
#+ multiplies ("*" operator), then prints the result ("p" operator).

Most persons avoid dc, because of its non-intuitive input and rather cryptic operators. Yet, it has its
uses.

Example 16-51. Converting a decimal number to hexadecimal
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#!/bin/bash
# hexconvert.sh: Convert a decimal number to hexadecimal.
E_NOARGS=85 # Command-line arg missing.
BASE=16
# Hexadecimal.
if [ -z "$1" ]
then
# Need a command-line argument.
echo "Usage: $0 number"
exit $E_NOARGS
fi
# Exercise: add argument validity checking.

hexcvt ()
{
if [ -z "$1" ]
then
echo 0
return
# "Return" 0 if no arg passed to function.
fi
echo ""$1" "$BASE" o p" | dc
#
o
sets radix (numerical base) of output.
#
p prints the top of stack.
# For other options: 'man dc' ...
return
}
hexcvt "$1"
exit

Studying the info page for dc is a painful path to understanding its intricacies. There seems to be a
small, select group of dc wizards who delight in showing off their mastery of this powerful, but
arcane utility.
bash$ echo "16i[q]sa[ln0=aln100%Pln100/snlbx]sbA0D68736142snlbxq" | dc
Bash

dc
#
#
#
#
#
#
#
#
#

<<< 10k5v1+2/p # 1.6180339887
^^^
Feed operations to dc using a Here String.
^^^
Pushes 10 and sets that as the precision (10k).
^^
Pushes 5 and takes its square root
(5v, v = square root).
^^
Pushes 1 and adds it to the running total (1+).
^^ Pushes 2 and divides the running total by that (2/).
^ Pops and prints the result (p)
The result is 1.6180339887 ...
... which happens to be the Pythagorean Golden Ratio, to 10 places.

Example 16-52. Factoring
#!/bin/bash
# factr.sh: Factor a number
MIN=2
# Will not work for number smaller than this.
E_NOARGS=85
E_TOOSMALL=86

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if [ -z $1 ]
then
echo "Usage: $0 number"
exit $E_NOARGS
fi
if [ "$1" -lt "$MIN" ]
then
echo "Number to factor must be $MIN or greater."
exit $E_TOOSMALL
fi
# Exercise: Add type checking (to reject non-integer arg).
echo "Factors of $1:"
# ------------------------------------------------------echo "$1[p]s2[lip/dli%0=1dvsr]s12sid2%0=13sidvsr[dli%0=\
1lrli2+dsi!>.]ds.xd1<2" | dc
# ------------------------------------------------------# Above code written by Michel Charpentier 
# (as a one-liner, here broken into two lines for display purposes).
# Used in ABS Guide with permission (thanks!).
exit
#
#
#
#
#

$ sh factr.sh 270138
2
3
11
4093

awk
Yet another way of doing floating point math in a script is using awk's built-in math functions in a
shell wrapper.

Example 16-53. Calculating the hypotenuse of a triangle
#!/bin/bash
# hypotenuse.sh: Returns the "hypotenuse" of a right triangle.
#
(square root of sum of squares of the "legs")
ARGS=2
E_BADARGS=85

# Script needs sides of triangle passed.
# Wrong number of arguments.

if [ $# -ne "$ARGS" ] # Test number of arguments to script.
then
echo "Usage: `basename $0` side_1 side_2"
exit $E_BADARGS
fi

AWKSCRIPT=' { printf( "%3.7f\n", sqrt($1*$1 + $2*$2) ) } '
#
command(s) / parameters passed to awk

# Now, pipe the parameters to awk.
echo -n "Hypotenuse of $1 and $2 = "
echo $1 $2 | awk "$AWKSCRIPT"
#
^^^^^^^^^^^^
# An echo-and-pipe is an easy way of passing shell parameters to awk.

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exit
# Exercise: Rewrite this script using 'bc' rather than awk.
#
Which method is more intuitive?

16.9. Miscellaneous Commands
Command that fit in no special category
jot, seq
These utilities emit a sequence of integers, with a user-selectable increment.
The default separator character between each integer is a newline, but this can be changed with the -s
option.
bash$ seq 5
1
2
3
4
5

bash$ seq -s : 5
1:2:3:4:5

Both jot and seq come in handy in a for loop.

Example 16-54. Using seq to generate loop arguments
#!/bin/bash
# Using "seq"
echo
for a in `seq 80` # or
for a in $( seq 80 )
# Same as
for a in 1 2 3 4 5 ... 80
(saves much typing!).
# May also use 'jot' (if present on system).
do
echo -n "$a "
done
# 1 2 3 4 5 ... 80
# Example of using the output of a command to generate
# the [list] in a "for" loop.
echo; echo

COUNT=80

# Yes, 'seq' also accepts a replaceable parameter.

for a in `seq $COUNT` # or
do
echo -n "$a "
done
# 1 2 3 4 5 ... 80

for a in $( seq $COUNT )

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echo; echo
BEGIN=75
END=80
for a in `seq $BEGIN $END`
# Giving "seq" two arguments starts the count at the first one,
#+ and continues until it reaches the second.
do
echo -n "$a "
done
# 75 76 77 78 79 80
echo; echo
BEGIN=45
INTERVAL=5
END=80
for a in `seq $BEGIN $INTERVAL $END`
# Giving "seq" three arguments starts the count at the first one,
#+ uses the second for a step interval,
#+ and continues until it reaches the third.
do
echo -n "$a "
done
# 45 50 55 60 65 70 75 80
echo; echo
exit 0

A simpler example:
# Create a set of 10 files,
#+ named file.1, file.2 . . . file.10.
COUNT=10
PREFIX=file
for filename in `seq $COUNT`
do
touch $PREFIX.$filename
# Or, can do other operations,
#+ such as rm, grep, etc.
done

Example 16-55. Letter Count"
#!/bin/bash
# letter-count.sh: Counting letter occurrences in a text file.
# Written by Stefano Palmeri.
# Used in ABS Guide with permission.
# Slightly modified by document author.
MINARGS=2
E_BADARGS=65
FILE=$1

# Script requires at least two arguments.

let LETTERS=$#-1

# How many letters specified (as command-line args).
# (Subtract 1 from number of command-line args.)

show_help(){

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echo
echo Usage: `basename $0` file letters
echo Note: `basename $0` arguments are case sensitive.
echo Example: `basename $0` foobar.txt G n U L i N U x.
echo
}
# Checks number of arguments.
if [ $# -lt $MINARGS ]; then
echo
echo "Not enough arguments."
echo
show_help
exit $E_BADARGS
fi

# Checks if file exists.
if [ ! -f $FILE ]; then
echo "File \"$FILE\" does not exist."
exit $E_BADARGS
fi

# Counts letter occurrences .
for n in `seq $LETTERS`; do
shift
if [[ `echo -n "$1" | wc -c` -eq 1 ]]; then
#
echo "$1" -\> `cat $FILE | tr -cd "$1" | wc -c` #
else
echo "$1 is not a single char."
fi
done

Checks arg.
Counting.

exit $?
# This script has exactly the same functionality as letter-count2.sh,
#+ but executes faster.
# Why?

Somewhat more capable than seq, jot is a classic UNIX utility that is not normally
included in a standard Linux distro. However, the source rpm is available for
download from the MIT repository.

Unlike seq, jot can generate a sequence of random numbers, using the -r option.
bash$ jot -r 3 999
1069
1272
1428

getopt
The getopt command parses command-line options preceded by a dash. This external command
corresponds to the getopts Bash builtin. Using getopt permits handling long options by means of the
-l flag, and this also allows parameter reshuffling.

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Example 16-56. Using getopt to parse command-line options
#!/bin/bash
# Using getopt
# Try the following when invoking this script:
#
sh ex33a.sh -a
#
sh ex33a.sh -abc
#
sh ex33a.sh -a -b -c
#
sh ex33a.sh -d
#
sh ex33a.sh -dXYZ
#
sh ex33a.sh -d XYZ
#
sh ex33a.sh -abcd
#
sh ex33a.sh -abcdZ
#
sh ex33a.sh -z
#
sh ex33a.sh a
# Explain the results of each of the above.
E_OPTERR=65
if [ "$#" -eq 0 ]
then
# Script needs at least one command-line argument.
echo "Usage $0 -[options a,b,c]"
exit $E_OPTERR
fi
set -- `getopt "abcd:" "$@"`
# Sets positional parameters to command-line arguments.
# What happens if you use "$*" instead of "$@"?
while [ ! -z "$1" ]
do
case "$1" in
-a) echo "Option
-b) echo "Option
-c) echo "Option
-d) echo "Option
*) break;;
esac

\"a\"";;
\"b\"";;
\"c\"";;
\"d\" $2";;

shift
done
#
#

It is usually better to use the 'getopts' builtin in a script.
See "ex33.sh."

exit 0

As Peggy Russell points out:
It is often necessary to include an eval to correctly process whitespace and quotes.
args=$(getopt -o a:bc:d -- "$@")
eval set -- "$args"

See Example 10-5 for a simplified emulation of getopt.
run-parts
The run-parts command [80] executes all the scripts in a target directory, sequentially in
ASCII-sorted filename order. Of course, the scripts need to have execute permission.

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The cron daemon invokes run-parts to run the scripts in the /etc/cron.* directories.
yes
In its default behavior the yes command feeds a continuous string of the character y followed by a
line feed to stdout. A control-C terminates the run. A different output string may be specified, as
in yes different string, which would continually output different string to
stdout.
One might well ask the purpose of this. From the command-line or in a script, the output of yes can be
redirected or piped into a program expecting user input. In effect, this becomes a sort of poor man's
version of expect.
yes | fsck /dev/hda1 runs fsck non-interactively (careful!).
yes | rm -r dirname has same effect as rm -rf dirname (careful!).
Caution advised when piping yes to a potentially dangerous system command, such as
fsck or fdisk. It might have unintended consequences.
The yes command parses variables, or more accurately, it echoes parsed variables. For
example:
bash$ yes $BASH_VERSION
3.1.17(1)-release
3.1.17(1)-release
3.1.17(1)-release
3.1.17(1)-release
3.1.17(1)-release
. . .

This particular "feature" may be used to create a very large ASCII file on the fly:
bash$ yes $PATH > huge_file.txt
Ctl-C

Hit Ctl-C very quickly, or you just might get more than you bargained for. . . .
The yes command may be emulated in a very simple script function.
yes ()
{ # Trivial emulation of "yes" ...
local DEFAULT_TEXT="y"
while [ true ]
# Endless loop.
do
if [ -z "$1" ]
then
echo "$DEFAULT_TEXT"
else
# If argument ...
echo "$1"
# ... expand and echo it.
fi
done
# The only things missing are the
}
#+ --help and --version options.

banner
Prints arguments as a large vertical banner to stdout, using an ASCII character (default '#'). This
may be redirected to a printer for hardcopy.

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Note that banner has been dropped from many Linux distros.
printenv
Show all the environmental variables set for a particular user.
bash$ printenv | grep HOME
HOME=/home/bozo

lp
The lp and lpr commands send file(s) to the print queue, to be printed as hard copy. [81] These
commands trace the origin of their names to the line printers of another era.
bash$ lp file1.txt or bash lp  to file
|
==========================|====================
command ---> command ---> |tee ---> command ---> ---> output of pipe
===============================================

cat listfile* | sort | tee check.file | uniq > result.file
#
^^^^^^^^^^^^^^
^^^^
# The file "check.file" contains the concatenated sorted "listfiles,"
#+ before the duplicate lines are removed by 'uniq.'

mkfifo
This obscure command creates a named pipe, a temporary first-in-first-out buffer for transferring data
between processes. [82] Typically, one process writes to the FIFO, and the other reads from it. See
Example A-14.
#!/bin/bash
# This short script by Omair Eshkenazi.

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# Used in ABS Guide with permission (thanks!).
mkfifo pipe1
mkfifo pipe2

# Yes, pipes can be given names.
# Hence the designation "named pipe."

(cut -d' ' -f1 | tr "a-z" "A-Z") >pipe2  $filename.uppercase
#
lcase
# For lower case conversion

Some basic options to dd are:
◊ if=INFILE
INFILE is the source file.
◊ of=OUTFILE
OUTFILE is the target file, the file that will have the data written to it.
◊ bs=BLOCKSIZE
This is the size of each block of data being read and written, usually a power of 2.
◊ skip=BLOCKS
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How many blocks of data to skip in INFILE before starting to copy. This is useful when the
INFILE has "garbage" or garbled data in its header or when it is desirable to copy only a
portion of the INFILE.
◊ seek=BLOCKS
How many blocks of data to skip in OUTFILE before starting to copy, leaving blank data at
beginning of OUTFILE.
◊ count=BLOCKS
Copy only this many blocks of data, rather than the entire INFILE.
◊ conv=CONVERSION
Type of conversion to be applied to INFILE data before copying operation.
A dd --help lists all the options this powerful utility takes.

Example 16-57. A script that copies itself
#!/bin/bash
# self-copy.sh
# This script copies itself.
file_subscript=copy
dd if=$0 of=$0.$file_subscript 2>/dev/null
# Suppress messages from dd:
^^^^^^^^^^^
exit $?
# A program whose only output is its own source code
#+ is called a "quine" per Willard Quine.
# Does this script qualify as a quine?

Example 16-58. Exercising dd
#!/bin/bash
# exercising-dd.sh
# Script by Stephane Chazelas.
# Somewhat modified by ABS Guide author.
infile=$0
outfile=log.txt
n=8
p=11

# This script.
# Output file left behind.

dd if=$infile of=$outfile bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null
# Extracts characters n to p (8 to 11) from this script ("bash").
# ---------------------------------------------------------------echo -n "hello vertical world" | dd cbs=1 conv=unblock 2> /dev/null
# Echoes "hello vertical world" vertically downward.
# Why? A newline follows each character dd emits.
exit $?

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To demonstrate just how versatile dd is, let's use it to capture keystrokes.

Example 16-59. Capturing Keystrokes
#!/bin/bash
# dd-keypress.sh: Capture keystrokes without needing to press ENTER.

keypresses=4

# Number of keypresses to capture.

old_tty_setting=$(stty -g)

# Save old terminal settings.

echo "Press $keypresses keys."
stty -icanon -echo

# Disable canonical mode.
# Disable local echo.
keys=$(dd bs=1 count=$keypresses 2> /dev/null)
# 'dd' uses stdin, if "if" (input file) not specified.
stty "$old_tty_setting"

# Restore old terminal settings.

echo "You pressed the \"$keys\" keys."
# Thanks, Stephane Chazelas, for showing the way.
exit 0

The dd command can do random access on a data stream.
echo -n . | dd bs=1 seek=4 of=file conv=notrunc
# The "conv=notrunc" option means that the output file
#+ will not be truncated.
# Thanks, S.C.

The dd command can copy raw data and disk images to and from devices, such as floppies and tape
drives (Example A-5). A common use is creating boot floppies.
dd if=kernel-image of=/dev/fd0H1440
Similarly, dd can copy the entire contents of a floppy, even one formatted with a "foreign" OS, to the
hard drive as an image file.
dd if=/dev/fd0 of=/home/bozo/projects/floppy.img

Other applications of dd include initializing temporary swap files (Example 31-2) and ramdisks
(Example 31-3). It can even do a low-level copy of an entire hard drive partition, although this is not
necessarily recommended.
People (with presumably nothing better to do with their time) are constantly thinking of interesting
applications of dd.

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Example 16-60. Securely deleting a file
#!/bin/bash
# blot-out.sh: Erase "all" traces of a file.
#
#+
#
#+

This script overwrites a target file alternately
with random bytes, then zeros before finally deleting it.
After that, even examining the raw disk sectors by conventional methods
will not reveal the original file data.

PASSES=7

BLOCKSIZE=1

#
#
#+
#
#+
#

E_BADARGS=70
E_NOT_FOUND=71
E_CHANGED_MIND=72

Number of file-shredding passes.
Increasing this slows script execution,
especially on large target files.
I/O with /dev/urandom requires unit block size,
otherwise you get weird results.
Various error exit codes.

if [ -z "$1" ]
# No filename specified.
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
file=$1
if [ ! -e "$file" ]
then
echo "File \"$file\" not found."
exit $E_NOT_FOUND
fi
echo; echo -n "Are you absolutely sure you want to blot out \"$file\" (y/n)? "
read answer
case "$answer" in
[nN]) echo "Changed your mind, huh?"
exit $E_CHANGED_MIND
;;
*)
echo "Blotting out file \"$file\".";;
esac

flength=$(ls -l "$file" | awk '{print $5}')
pass_count=1
chmod u+w "$file"

# Field 5 is file length.

# Allow overwriting/deleting the file.

echo
while [ "$pass_count" -le "$PASSES" ]
do
echo "Pass #$pass_count"
sync
# Flush buffers.
dd if=/dev/urandom of=$file bs=$BLOCKSIZE count=$flength
# Fill with random bytes.
sync
# Flush buffers again.
dd if=/dev/zero of=$file bs=$BLOCKSIZE count=$flength
# Fill with zeros.
sync
# Flush buffers yet again.
let "pass_count += 1"
echo

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done

rm -f $file
sync

# Finally, delete scrambled and shredded file.
# Flush buffers a final time.

echo "File \"$file\" blotted out and deleted."; echo

exit 0
#
#+
#
#+

This is a fairly secure, if inefficient and slow method
of thoroughly "shredding" a file.
The "shred" command, part of the GNU "fileutils" package,
does the same thing, although more efficiently.

#
#
#+
#+

The file cannot not be "undeleted" or retrieved by normal methods.
However . . .
this simple method would *not* likely withstand
sophisticated forensic analysis.

#
#

This script may not play well with a journaled file system.
Exercise (difficult): Fix it so it does.

# Tom Vier's "wipe" file-deletion package does a much more thorough job
#+ of file shredding than this simple script.
#
http://www.ibiblio.org/pub/Linux/utils/file/wipe-2.0.0.tar.bz2
# For an in-depth analysis on the topic of file deletion and security,
#+ see Peter Gutmann's paper,
#+
"Secure Deletion of Data From Magnetic and Solid-State Memory".
#
http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html

See also the dd thread entry in the bibliography.
od
The od, or octal dump filter converts input (or files) to octal (base-8) or other bases. This is useful for
viewing or processing binary data files or otherwise unreadable system device files, such as
/dev/urandom, and as a filter for binary data.
head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
# Sample output: 1324725719, 3918166450, 2989231420, etc.
# From rnd.sh example script, by Stéphane Chazelas

See also Example 9-16 and Example A-36.
hexdump
Performs a hexadecimal, octal, decimal, or ASCII dump of a binary file. This command is the rough
equivalent of od, above, but not nearly as useful. May be used to view the contents of a binary file, in
combination with dd and less.
dd if=/bin/ls | hexdump -C | less
# The -C option nicely formats the output in tabular form.

objdump
Displays information about an object file or binary executable in either hexadecimal form or as a
disassembled listing (with the -d option).
bash$ objdump -d /bin/ls
/bin/ls:
file format elf32-i386

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Disassembly of section .init:
080490bc <.init>:
80490bc:
55
80490bd:
89 e5
. . .

push
mov

%ebp
%esp,%ebp

mcookie
This command generates a "magic cookie," a 128-bit (32-character) pseudorandom hexadecimal
number, normally used as an authorization "signature" by the X server. This also available for use in a
script as a "quick 'n dirty" random number.
random000=$(mcookie)

Of course, a script could use md5sum for the same purpose.
# Generate md5 checksum on the script itself.
random001=`md5sum $0 | awk '{print $1}'`
# Uses 'awk' to strip off the filename.

The mcookie command gives yet another way to generate a "unique" filename.

Example 16-61. Filename generator
#!/bin/bash
# tempfile-name.sh:

temp filename generator

BASE_STR=`mcookie`
POS=11
LEN=5

# 32-character magic cookie.
# Arbitrary position in magic cookie string.
# Get $LEN consecutive characters.

prefix=temp

#
#
#+
#+

This is, after all, a "temp" file.
For more "uniqueness," generate the
filename prefix using the same method
as the suffix, below.

suffix=${BASE_STR:POS:LEN}
# Extract a 5-character string,
#+ starting at position 11.
temp_filename=$prefix.$suffix
# Construct the filename.
echo "Temp filename = "$temp_filename""
# sh tempfile-name.sh
# Temp filename = temp.e19ea
# Compare this method of generating "unique" filenames
#+ with the 'date' method in ex51.sh.
exit 0

units
This utility converts between different units of measure. While normally invoked in interactive mode,
units may find use in a script.

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Example 16-62. Converting meters to miles
#!/bin/bash
# unit-conversion.sh

convert_units () # Takes as arguments the units to convert.
{
cf=$(units "$1" "$2" | sed --silent -e '1p' | awk '{print $2}')
# Strip off everything except the actual conversion factor.
echo "$cf"
}
Unit1=miles
Unit2=meters
cfactor=`convert_units $Unit1 $Unit2`
quantity=3.73
result=$(echo $quantity*$cfactor | bc)
echo "There are $result $Unit2 in $quantity $Unit1."
# What happens if you pass incompatible units,
#+ such as "acres" and "miles" to the function?
exit 0

m4
A hidden treasure, m4 is a powerful macro [84] processing filter, virtually a complete language.
Although originally written as a pre-processor for RatFor, m4 turned out to be useful as a stand-alone
utility. In fact, m4 combines some of the functionality of eval, tr, and awk, in addition to its extensive
macro expansion facilities.
The April, 2002 issue of Linux Journal has a very nice article on m4 and its uses.

Example 16-63. Using m4
#!/bin/bash
# m4.sh: Using the m4 macro processor
# Strings
string=abcdA01
echo "len($string)" | m4
echo "substr($string,4)" | m4
echo "regexp($string,[0-1][0-1],\&Z)" | m4

#
7
# A01
# 01Z

# Arithmetic
var=99
echo "incr($var)" | m4
echo "eval($var / 3)" | m4

#
#

100
33

exit

xmessage
This X-based variant of echo pops up a message/query window on the desktop.
xmessage Left click to continue -button okay

zenity
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The zenity utility is adept at displaying GTK+ dialog widgets and very suitable for scripting purposes.
doexec
The doexec command enables passing an arbitrary list of arguments to a binary executable. In
particular, passing argv[0] (which corresponds to $0 in a script) lets the executable be invoked by
various names, and it can then carry out different sets of actions, according to the name by which it
was called. What this amounts to is roundabout way of passing options to an executable.
For example, the /usr/local/bin directory might contain a binary called "aaa". Invoking doexec
/usr/local/bin/aaa list would list all those files in the current working directory beginning with an "a",
while invoking (the same executable with) doexec /usr/local/bin/aaa delete would delete those files.
The various behaviors of the executable must be defined within the code of the
executable itself, analogous to something like the following in a shell script:
case `basename $0` in
"name1" ) do_something;;
"name2" ) do_something_else;;
"name3" ) do_yet_another_thing;;
*
) bail_out;;
esac

dialog
The dialog family of tools provide a method of calling interactive "dialog" boxes from a script. The
more elaborate variations of dialog -- gdialog, Xdialog, and kdialog -- actually invoke X-Windows
widgets.
sox
The sox, or "sound exchange" command plays and performs transformations on sound files. In fact,
the /usr/bin/play executable (now deprecated) is nothing but a shell wrapper for sox.
For example, sox soundfile.wav soundfile.au changes a WAV sound file into a (Sun audio format)
AU sound file.
Shell scripts are ideally suited for batch-processing sox operations on sound files. For examples, see
the Linux Radio Timeshift HOWTO and the MP3do Project.

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The startup and shutdown scripts in /etc/rc.d illustrate the uses (and usefulness) of many of these
comands. These are usually invoked by root and used for system maintenance or emergency filesystem
repairs. Use with caution, as some of these commands may damage your system if misused.
Users and Groups
users
Show all logged on users. This is the approximate equivalent of who -q.
groups
Lists the current user and the groups she belongs to. This corresponds to the $GROUPS internal
variable, but gives the group names, rather than the numbers.
bash$ groups
bozita cdrom cdwriter audio xgrp
bash$ echo $GROUPS
501

chown, chgrp
The chown command changes the ownership of a file or files. This command is a useful method that
root can use to shift file ownership from one user to another. An ordinary user may not change the
ownership of files, not even her own files. [85]
root# chown bozo *.txt

The chgrp command changes the group ownership of a file or files. You must be owner of the
file(s) as well as a member of the destination group (or root) to use this operation.
chgrp --recursive dunderheads *.data
# The "dunderheads" group will now own all the "*.data" files
#+ all the way down the $PWD directory tree (that's what "recursive" means).

useradd, userdel
The useradd administrative command adds a user account to the system and creates a home directory
for that particular user, if so specified. The corresponding userdel command removes a user account
from the system [86] and deletes associated files.
The adduser command is a synonym for useradd and is usually a symbolic link to it.
usermod
Modify a user account. Changes may be made to the password, group membership, expiration date,
and other attributes of a given user's account. With this command, a user's password may be locked,
which has the effect of disabling the account.
groupmod
Modify a given group. The group name and/or ID number may be changed using this command.
id
The id command lists the real and effective user IDs and the group IDs of the user associated with the
current process. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables.
bash$ id
uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio)

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bash$ echo $UID
501

The id command shows the effective IDs only when they differ from the real ones.
Also see Example 9-5.
lid
The lid (list ID) command shows the group(s) that a given user belongs to, or alternately, the users
belonging to a given group. May be invoked only by root.
root# lid bozo
bozo(gid=500)

root# lid daemon
bin(gid=1)
daemon(gid=2)
adm(gid=4)
lp(gid=7)

who
Show all users logged on to the system.
bash$ who
bozo tty1
bozo pts/0
bozo pts/1
bozo pts/2

Apr 27 17:45
Apr 27 17:46
Apr 27 17:47
Apr 27 17:49

The -m gives detailed information about only the current user. Passing any two arguments to who is
the equivalent of who -m, as in who am i or who The Man.
bash$ who -m
localhost.localdomain!bozo

pts/2

Apr 27 17:49

whoami is similar to who -m, but only lists the user name.
bash$ whoami
bozo

w
Show all logged on users and the processes belonging to them. This is an extended version of who.
The output of w may be piped to grep to find a specific user and/or process.
bash$ w | grep startx
bozo tty1
-

4:22pm

6:41

4.47s

0.45s

startx

logname
Show current user's login name (as found in /var/run/utmp). This is a near-equivalent to
whoami, above.
bash$ logname
bozo
bash$ whoami
bozo

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However . . .
bash$ su
Password: ......
bash# whoami
root
bash# logname
bozo

While logname prints the name of the logged in user, whoami gives the name of the
user attached to the current process. As we have just seen, sometimes these are not the
same.
su
Runs a program or script as a substitute user. su rjones starts a shell as user rjones. A naked su
defaults to root. See Example A-14.
sudo
Runs a command as root (or another user). This may be used in a script, thus permitting a regular
user to run the script.
#!/bin/bash
# Some commands.
sudo cp /root/secretfile /home/bozo/secret
# Some more commands.

The file /etc/sudoers holds the names of users permitted to invoke sudo.
passwd
Sets, changes, or manages a user's password.
The passwd command can be used in a script, but probably should not be.

Example 17-1. Setting a new password
#!/bin/bash
# setnew-password.sh: For demonstration purposes only.
#
Not a good idea to actually run this script.
# This script must be run as root.
ROOT_UID=0
E_WRONG_USER=65

# Root has $UID 0.
# Not root?

E_NOSUCHUSER=70
SUCCESS=0

if [ "$UID" -ne "$ROOT_UID" ]
then
echo; echo "Only root can run this script."; echo
exit $E_WRONG_USER
else
echo
echo "You should know better than to run this script, root."
echo "Even root users get the blues... "
echo
fi

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username=bozo
NEWPASSWORD=security_violation
# Check if bozo lives here.
grep -q "$username" /etc/passwd
if [ $? -ne $SUCCESS ]
then
echo "User $username does not exist."
echo "No password changed."
exit $E_NOSUCHUSER
fi
echo "$NEWPASSWORD" | passwd --stdin "$username"
# The '--stdin' option to 'passwd' permits
#+ getting a new password from stdin (or a pipe).
echo; echo "User $username's password changed!"
# Using the 'passwd' command in a script is dangerous.
exit 0

The passwd command's -l, -u, and -d options permit locking, unlocking, and deleting a user's
password. Only root may use these options.
ac
Show users' logged in time, as read from /var/log/wtmp. This is one of the GNU accounting
utilities.
bash$ ac
total

68.08

last
List last logged in users, as read from /var/log/wtmp. This command can also show remote
logins.
For example, to show the last few times the system rebooted:
bash$ last reboot
reboot
system boot 2.6.9-1.667
reboot
system boot 2.6.9-1.667
reboot
system boot 2.6.9-1.667
reboot
system boot 2.6.9-1.667
. . .
wtmp begins Tue Feb

Fri Feb 4 18:18
Fri Feb 4 15:20
Fri Feb 4 12:56
Thu Feb 3 21:08

(00:02)
(01:27)
(00:49)
(02:17)

1 12:50:09 2005

newgrp
Change user's group ID without logging out. This permits access to the new group's files. Since users
may be members of multiple groups simultaneously, this command finds only limited use.
Kurt Glaesemann points out that the newgrp command could prove helpful in setting
the default group permissions for files a user writes. However, the chgrp command
might be more convenient for this purpose.
Terminals
tty
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Echoes the name (filename) of the current user's terminal. Note that each separate xterm window
counts as a different terminal.
bash$ tty
/dev/pts/1

stty
Shows and/or changes terminal settings. This complex command, used in a script, can control
terminal behavior and the way output displays. See the info page, and study it carefully.

Example 17-2. Setting an erase character
#!/bin/bash
# erase.sh: Using "stty" to set an erase character when reading input.
echo -n "What is your name? "
read name

# Try to backspace
#+ to erase characters of input.
# Problems?

echo "Your name is $name."
stty
echo
read
echo

erase '#'
-n "What is your name? "
name
"Your name is $name."

#

Set "hashmark" (#) as erase character.

#

Use # to erase last character typed.

exit 0
# Even after the script exits, the new key value remains set.
# Exercise: How would you reset the erase character to the default value?

Example 17-3. secret password: Turning off terminal echoing
#!/bin/bash
# secret-pw.sh: secret password
echo
echo
read
echo
echo
echo

-n "Enter password "
passwd
"password is $passwd"
-n "If someone had been looking over your shoulder, "
"your password would have been compromised."

echo && echo

# Two line-feeds in an "and list."

stty -echo

# Turns off screen echo.

echo -n "Enter password again "
read passwd
echo
echo "password is $passwd"
echo
stty echo

# Restores screen echo.

exit 0
# Do an 'info stty' for more on this useful-but-tricky command.

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A creative use of stty is detecting a user keypress (without hitting ENTER).

Example 17-4. Keypress detection
#!/bin/bash
# keypress.sh: Detect a user keypress ("hot keys").
echo
old_tty_settings=$(stty -g)
stty -icanon
Keypress=$(head -c1)

# Save old settings (why?).
# or $(dd bs=1 count=1 2> /dev/null)
# on non-GNU systems

echo
echo "Key pressed was \""$Keypress"\"."
echo
stty "$old_tty_settings"

# Restore old settings.

# Thanks, Stephane Chazelas.
exit 0

Also see Example 9-3 and Example A-43.

terminals and modes
Normally, a terminal works in the canonical mode. When a user hits a key, the resulting character does
not immediately go to the program actually running in this terminal. A buffer local to the terminal stores
keystrokes. When the user hits the ENTER key, this sends all the stored keystrokes to the program
running. There is even a basic line editor inside the terminal.
bash$ stty -a
speed 9600 baud; rows 36; columns 96; line = 0;
intr = ^C; quit = ^\; erase = ^H; kill = ^U; eof = ^D; eol = ; eol2 = ;
start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O;
...
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt

Using canonical mode, it is possible to redefine the special keys for the local terminal line editor.
bash$ cat > filexxx
whaIfoo barhello world

bash$ cat filexxx
hello world
bash$ wc -c < filexxx
12

The process controlling the terminal receives only 12 characters (11 alphabetic ones, plus a newline),
although the user hit 26 keys.

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In non-canonical ("raw") mode, every key hit (including special editing keys such as ctl-H) sends a
character immediately to the controlling process.
The Bash prompt disables both icanon and echo, since it replaces the basic terminal line editor with its
own more elaborate one. For example, when you hit ctl-A at the Bash prompt, there's no ^A echoed by
the terminal, but Bash gets a \1 character, interprets it, and moves the cursor to the begining of the line.
Stéphane Chazelas
setterm
Set certain terminal attributes. This command writes to its terminal's stdout a string that changes
the behavior of that terminal.
bash$ setterm -cursor off
bash$

The setterm command can be used within a script to change the appearance of text written to
stdout, although there are certainly better tools available for this purpose.
setterm -bold on
echo bold hello
setterm -bold off
echo normal hello

tset
Show or initialize terminal settings. This is a less capable version of stty.
bash$ tset -r
Terminal type is xterm-xfree86.
Kill is control-U (^U).
Interrupt is control-C (^C).

setserial
Set or display serial port parameters. This command must be run by root and is usually found in a
system setup script.
# From /etc/pcmcia/serial script:
IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'`
setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ

getty, agetty
The initialization process for a terminal uses getty or agetty to set it up for login by a user. These
commands are not used within user shell scripts. Their scripting counterpart is stty.
mesg
Enables or disables write access to the current user's terminal. Disabling access would prevent another
user on the network to write to the terminal.
It can be quite annoying to have a message about ordering pizza suddenly appear in
the middle of the text file you are editing. On a multi-user network, you might
therefore wish to disable write access to your terminal when you need to avoid
interruptions.
wall

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This is an acronym for "write all," i.e., sending a message to all users at every terminal logged into the
network. It is primarily a system administrator's tool, useful, for example, when warning everyone
that the system will shortly go down due to a problem (see Example 19-1).
bash$ wall System going down for maintenance in 5 minutes!
Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001...
System going down for maintenance in 5 minutes!

If write access to a particular terminal has been disabled with mesg, then wall cannot
send a message to that terminal.
Information and Statistics
uname
Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the -a option, gives
verbose system info (see Example 16-5). The -s option shows only the OS type.
bash$ uname
Linux
bash$ uname -s
Linux

bash$ uname -a
Linux iron.bozo 2.6.15-1.2054_FC5 #1 Tue Mar 14 15:48:33 EST 2006
i686 i686 i386 GNU/Linux

arch
Show system architecture. Equivalent to uname -m. See Example 11-26.
bash$ arch
i686
bash$ uname -m
i686

lastcomm
Gives information about previous commands, as stored in the /var/account/pacct file.
Command name and user name can be specified by options. This is one of the GNU accounting
utilities.
lastlog
List the last login time of all system users. This references the /var/log/lastlog file.
bash$ lastlog
root
tty1
bin
daemon
...
bozo
tty1

bash$ lastlog | grep root
root
tty1

Chapter 17. System and Administrative Commands

Fri Dec 7 18:43:21 -0700 2001
**Never logged in**
**Never logged in**
Sat Dec

Fri Dec

8 21:14:29 -0700 2001

7 18:43:21 -0700 2001

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This command will fail if the user invoking it does not have read permission for the
/var/log/lastlog file.
lsof
List open files. This command outputs a detailed table of all currently open files and gives
information about their owner, size, the processes associated with them, and more. Of course, lsof
may be piped to grep and/or awk to parse and analyze its results.
bash$ lsof
COMMAND
PID
init
1
init
1
init
1
cardmgr
213
...

USER
root
root
root
root

FD
mem
mem
mem
mem

TYPE
REG
REG
REG
REG

DEVICE
3,5
3,5
3,5
3,5

SIZE
30748
73120
931668
36956

NODE NAME
30303 /sbin/init
8069 /lib/ld-2.1.3.so
8075 /lib/libc-2.1.3.so
30357 /sbin/cardmgr

The lsof command is a useful, if complex administrative tool. If you are unable to dismount a
filesystem and get an error message that it is still in use, then running lsof helps determine which files
are still open on that filesystem. The -i option lists open network socket files, and this can help trace
intrusion or hack attempts.
bash$ lsof -an -i tcp
COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME
firefox 2330 bozo 32u IPv4
9956
TCP 66.0.118.137:57596->67.112.7.104:http ...
firefox 2330 bozo 38u IPv4 10535
TCP 66.0.118.137:57708->216.79.48.24:http ...

See Example 30-2 for an effective use of lsof.
strace
System trace: diagnostic and debugging tool for tracing system calls and signals. This command and
ltrace, following, are useful for diagnosing why a given program or package fails to run . . . perhaps
due to missing libraries or related causes.
bash$ strace df
execve("/bin/df", ["df"], [/* 45 vars */]) = 0
uname({sys="Linux", node="bozo.localdomain", ...}) = 0
brk(0)
= 0x804f5e4
...

This is the Linux equivalent of the Solaris truss command.
ltrace
Library trace: diagnostic and debugging tool that traces library calls invoked by a given command.
bash$ ltrace df
__libc_start_main(0x804a910, 1, 0xbfb589a4, 0x804fb70, 0x804fb68 :
setlocale(6, "")
= "en_US.UTF-8"
bindtextdomain("coreutils", "/usr/share/locale") = "/usr/share/locale"
textdomain("coreutils")
= "coreutils"
__cxa_atexit(0x804b650, 0, 0, 0x8052bf0, 0xbfb58908) = 0
getenv("DF_BLOCK_SIZE")
= NULL
...

nc
The nc (netcat) utility is a complete toolkit for connecting to and listening to TCP and UDP ports. It is
useful as a diagnostic and testing tool and as a component in simple script-based HTTP clients and
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servers.
bash$ nc localhost.localdomain 25
220 localhost.localdomain ESMTP Sendmail 8.13.1/8.13.1;
Thu, 31 Mar 2005 15:41:35 -0700

A real-life usage example.

Example 17-5. Checking a remote server for identd
#! /bin/sh
## Duplicate DaveG's ident-scan thingie using netcat. Oooh, he'll be p*ssed.
## Args: target port [port port port ...]
## Hose stdout _and_ stderr together.
##
## Advantages: runs slower than ident-scan, giving remote inetd less cause
##+ for alarm, and only hits the few known daemon ports you specify.
## Disadvantages: requires numeric-only port args, the output sleazitude,
##+ and won't work for r-services when coming from high source ports.
# Script author: Hobbit 
# Used in ABS Guide with permission.
# --------------------------------------------------E_BADARGS=65
# Need at least two args.
TWO_WINKS=2
# How long to sleep.
THREE_WINKS=3
IDPORT=113
# Authentication "tap ident" port.
RAND1=999
RAND2=31337
TIMEOUT0=9
TIMEOUT1=8
TIMEOUT2=4
# --------------------------------------------------case "${2}" in
"" ) echo "Need HOST and at least one PORT." ; exit $E_BADARGS ;;
esac
# Ping 'em once and see if they *are* running identd.
nc -z -w $TIMEOUT0 "$1" $IDPORT || \
{ echo "Oops, $1 isn't running identd." ; exit 0 ; }
# -z scans for listening daemons.
#
-w $TIMEOUT = How long to try to connect.
# Generate a randomish base port.
RP=`expr $$ % $RAND1 + $RAND2`
TRG="$1"
shift
while test "$1" ; do
nc -v -w $TIMEOUT1 -p ${RP} "$TRG" ${1} < /dev/null > /dev/null &
PROC=$!
sleep $THREE_WINKS
echo "${1},${RP}" | nc -w $TIMEOUT2 -r "$TRG" $IDPORT 2>&1
sleep $TWO_WINKS
# Does this look like a lamer script or what . . . ?
# ABS Guide author comments: "Ain't really all that bad . . .
#+
kinda clever, actually."

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kill -HUP $PROC
RP=`expr ${RP} + 1`
shift
done
exit $?
#
#

Notes:
-----

# Try commenting out line 30 and running this script
#+ with "localhost.localdomain 25" as arguments.
# For more of Hobbit's 'nc' example scripts,
#+ look in the documentation:
#+ the /usr/share/doc/nc-X.XX/scripts directory.

And, of course, there's Dr. Andrew Tridgell's notorious one-line script in the BitKeeper Affair:
echo clone | nc thunk.org 5000 > e2fsprogs.dat

free
Shows memory and cache usage in tabular form. The output of this command lends itself to parsing,
using grep, awk or Perl. The procinfo command shows all the information that free does, and much
more.
bash$ free
total
Mem:
30504
-/+ buffers/cache:
Swap:
68540

used
28624
10640
3128

free
1880
19864
65412

shared
15820

buffers
1608

cached
16376

To show unused RAM memory:
bash$ free | grep Mem | awk '{ print $4 }'
1880

procinfo
Extract and list information and statistics from the /proc pseudo-filesystem. This gives a very
extensive and detailed listing.
bash$ procinfo | grep Bootup
Bootup: Wed Mar 21 15:15:50 2001

Load average: 0.04 0.21 0.34 3/47 6829

lsdev
List devices, that is, show installed hardware.
bash$ lsdev
Device
DMA
IRQ I/O Ports
-----------------------------------------------cascade
4
2
dma
0080-008f
dma1
0000-001f
dma2
00c0-00df
fpu
00f0-00ff
ide0
14 01f0-01f7 03f6-03f6
...

du
Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified.
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bash$ du -ach
1.0k
./wi.sh
1.0k
./tst.sh
1.0k
./random.file
6.0k
.
6.0k
total

df
Shows filesystem usage in tabular form.
bash$ df
Filesystem
/dev/hda5
/dev/hda8
/dev/hda7

1k-blocks
273262
222525
1408796

Used Available Use% Mounted on
92607
166547 36% /
123951
87085 59% /home
1075744
261488 80% /usr

dmesg
Lists all system bootup messages to stdout. Handy for debugging and ascertaining which device
drivers were installed and which system interrupts in use. The output of dmesg may, of course, be
parsed with grep, sed, or awk from within a script.
bash$ dmesg | grep hda
Kernel command line: ro root=/dev/hda2
hda: IBM-DLGA-23080, ATA DISK drive
hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63
hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4

stat
Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files.
bash$ stat test.cru
File: "test.cru"
Size: 49970
Allocated Blocks: 100
Filetype: Regular File
Mode: (0664/-rw-rw-r--)
Uid: ( 501/ bozo) Gid: ( 501/ bozo)
Device: 3,8
Inode: 18185
Links: 1
Access: Sat Jun 2 16:40:24 2001
Modify: Sat Jun 2 16:40:24 2001
Change: Sat Jun 2 16:40:24 2001

If the target file does not exist, stat returns an error message.
bash$ stat nonexistent-file
nonexistent-file: No such file or directory

In a script, you can use stat to extract information about files (and filesystems) and set variables
accordingly.
#!/bin/bash
# fileinfo2.sh
# Per suggestion of Joël Bourquard and . . .
# http://www.linuxquestions.org/questions/showthread.php?t=410766

FILENAME=testfile.txt
file_name=$(stat -c%n "$FILENAME")
# Same as "$FILENAME" of course.
file_owner=$(stat -c%U "$FILENAME")
file_size=$(stat -c%s "$FILENAME")
# Certainly easier than using "ls -l $FILENAME"

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#+ and then parsing with sed.
file_inode=$(stat -c%i "$FILENAME")
file_type=$(stat -c%F "$FILENAME")
file_access_rights=$(stat -c%A "$FILENAME")
echo
echo
echo
echo
echo
echo

"File
"File
"File
"File
"File
"File

name:
owner:
size:
inode:
type:
access rights:

$file_name"
$file_owner"
$file_size"
$file_inode"
$file_type"
$file_access_rights"

exit 0
sh fileinfo2.sh
File
File
File
File
File
File

name:
owner:
size:
inode:
type:
access rights:

testfile.txt
bozo
418
1730378
regular file
-rw-rw-r--

vmstat
Display virtual memory statistics.
bash$ vmstat
procs
r b w
swpd
0 0 0
0

free
11040

buff
2636

memory
cache
38952

si
0

swap
so
0

bi
33

io system
bo
in
7 271

cs
88

us
8

cpu
sy id
3 89

uptime
Shows how long the system has been running, along with associated statistics.
bash$ uptime
10:28pm up 1:57,

3 users,

load average: 0.17, 0.34, 0.27

A load average of 1 or less indicates that the system handles processes immediately. A
load average greater than 1 means that processes are being queued. When the load
average gets above 3 (on a single-core processor), then system performance is
significantly degraded.
hostname
Lists the system's host name. This command sets the host name in an /etc/rc.d setup script
(/etc/rc.d/rc.sysinit or similar). It is equivalent to uname -n, and a counterpart to the
$HOSTNAME internal variable.
bash$ hostname
localhost.localdomain
bash$ echo $HOSTNAME
localhost.localdomain

Similar to the hostname command are the domainname, dnsdomainname, nisdomainname, and
ypdomainname commands. Use these to display or set the system DNS or NIS/YP domain name.
Various options to hostname also perform these functions.
hostid
Echo a 32-bit hexadecimal numerical identifier for the host machine.

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bash$ hostid
7f0100

This command allegedly fetches a "unique" serial number for a particular system.
Certain product registration procedures use this number to brand a particular user
license. Unfortunately, hostid only returns the machine network address in
hexadecimal, with pairs of bytes transposed.
The network address of a typical non-networked Linux machine, is found in
/etc/hosts.
bash$ cat /etc/hosts
127.0.0.1

localhost.localdomain localhost

As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0, which
translates in hex to 007f0100, the exact equivalent of what hostid returns, above.
There exist only a few million other Linux machines with this identical hostid.
sar
Invoking sar (System Activity Reporter) gives a very detailed rundown on system statistics. The
Santa Cruz Operation ("Old" SCO) released sar as Open Source in June, 1999.
This command is not part of the base Linux distribution, but may be obtained as part of the sysstat
utilities package, written by Sebastien Godard.
bash$ sar
Linux 2.4.9 (brooks.seringas.fr)
10:30:00
10:40:00
10:50:00
11:00:00
Average:

CPU
all
all
all
all

%user
2.21
3.36
1.12
2.23

14:32:30

LINUX RESTART

15:00:00
15:10:00
15:20:00
15:30:00
Average:

CPU
all
all
all
all

%user
8.59
4.07
0.79
6.33

09/26/03
%nice
10.90
0.00
0.00
3.63

%system
65.48
72.36
80.77
72.87

%iowait
0.00
0.00
0.00
0.00

%idle
21.41
24.28
18.11
21.27

%nice
2.40
1.00
2.94
1.70

%system
17.47
11.95
7.56
14.71

%iowait
0.00
0.00
0.00
0.00

%idle
71.54
82.98
88.71
77.26

readelf
Show information and statistics about a designated elf binary. This is part of the binutils package.
bash$ readelf -h /bin/bash
ELF Header:
Magic:
7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
Class:
ELF32
Data:
2's complement, little endian
Version:
1 (current)
OS/ABI:
UNIX - System V
ABI Version:
0
Type:
EXEC (Executable file)
. . .

size

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The size [/path/to/binary] command gives the segment sizes of a binary executable or archive file.
This is mainly of use to programmers.
bash$ size /bin/bash
text
data
bss
495971
22496
17392

dec
535859

hex filename
82d33 /bin/bash

System Logs
logger
Appends a user-generated message to the system log (/var/log/messages). You do not have to
be root to invoke logger.
logger Experiencing instability in network connection at 23:10, 05/21.
# Now, do a 'tail /var/log/messages'.

By embedding a logger command in a script, it is possible to write debugging information to
/var/log/messages.
logger -t $0 -i Logging at line "$LINENO".
# The "-t" option specifies the tag for the logger entry.
# The "-i" option records the process ID.
# tail /var/log/message
# ...
# Jul 7 20:48:58 localhost ./test.sh[1712]: Logging at line 3.

logrotate
This utility manages the system log files, rotating, compressing, deleting, and/or e-mailing them, as
appropriate. This keeps the /var/log from getting cluttered with old log files. Usually cron runs
logrotate on a daily basis.
Adding an appropriate entry to /etc/logrotate.conf makes it possible to manage personal log
files, as well as system-wide ones.
Stefano Falsetto has created rottlog, which he considers to be an improved version of
logrotate.
Job Control
ps
Process Statistics: lists currently executing processes by owner and PID (process ID). This is usually
invoked with ax or aux options, and may be piped to grep or sed to search for a specific process (see
Example 15-14 and Example 29-3).
bash$
295 ?

ps ax | grep sendmail
S
0:00 sendmail: accepting connections on port 25

To display system processes in graphical "tree" format: ps afjx or ps ax --forest.
pgrep, pkill
Combining the ps command with grep or kill.
bash$ ps a | grep mingetty
2212 tty2
Ss+
0:00 /sbin/mingetty tty2
2213 tty3
Ss+
0:00 /sbin/mingetty tty3

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2214
2215
2216
4849

tty4
tty5
tty6
pts/2

Ss+
Ss+
Ss+
S+

0:00
0:00
0:00
0:00

/sbin/mingetty tty4
/sbin/mingetty tty5
/sbin/mingetty tty6
grep mingetty

bash$ pgrep mingetty
2212 mingetty
2213 mingetty
2214 mingetty
2215 mingetty
2216 mingetty

Compare the action of pkill with killall.
pstree
Lists currently executing processes in "tree" format. The -p option shows the PIDs, as well as the
process names.
top
Continuously updated display of most cpu-intensive processes. The -b option displays in text mode,
so that the output may be parsed or accessed from a script.
bash$ top -b
8:30pm up 3 min, 3 users, load average: 0.49, 0.32, 0.13
45 processes: 44 sleeping, 1 running, 0 zombie, 0 stopped
CPU states: 13.6% user, 7.3% system, 0.0% nice, 78.9% idle
Mem:
78396K av,
65468K used,
12928K free,
0K shrd,
Swap: 157208K av,
0K used, 157208K free
PID
848
1
2
...

USER
bozo
root
root

PRI
17
8
9

NI
0
0
0

SIZE
996
512
0

RSS SHARE STAT %CPU %MEM
996
800 R
5.6 1.2
512
444 S
0.0 0.6
0
0 SW
0.0 0.0

TIME
0:00
0:04
0:00

2352K buff
37244K cached

COMMAND
top
init
keventd

nice
Run a background job with an altered priority. Priorities run from 19 (lowest) to -20 (highest). Only
root may set the negative (higher) priorities. Related commands are renice and snice, which change
the priority of a running process or processes, and skill, which sends a kill signal to a process or
processes.
nohup
Keeps a command running even after user logs off. The command will run as a foreground process
unless followed by &. If you use nohup within a script, consider coupling it with a wait to avoid
creating an orphan or zombie process.
pidof
Identifies process ID (PID) of a running job. Since job control commands, such as kill and renice act
on the PID of a process (not its name), it is sometimes necessary to identify that PID. The pidof
command is the approximate counterpart to the $PPID internal variable.
bash$ pidof xclock
880

Example 17-6. pidof helps kill a process

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#!/bin/bash
# kill-process.sh
NOPROCESS=2
process=xxxyyyzzz # Use nonexistent process.
# For demo purposes only...
# ... don't want to actually kill any actual process with this script.
#
# If, for example, you wanted to use this script to logoff the Internet,
#
process=pppd
t=`pidof $process`
# Find pid (process id) of $process.
# The pid is needed by 'kill' (can't 'kill' by program name).
if [ -z "$t" ]
# If process not present, 'pidof' returns null.
then
echo "Process $process was not running."
echo "Nothing killed."
exit $NOPROCESS
fi
kill $t

# May need 'kill -9' for stubborn process.

# Need a check here to see if process allowed itself to be killed.
# Perhaps another " t=`pidof $process` " or ...

# This entire script could be replaced by
#
kill $(pidof -x process_name)
# or
#
killall process_name
# but it would not be as instructive.
exit 0

fuser
Identifies the processes (by PID) that are accessing a given file, set of files, or directory. May also be
invoked with the -k option, which kills those processes. This has interesting implications for system
security, especially in scripts preventing unauthorized users from accessing system services.
bash$ fuser -u /usr/bin/vim
/usr/bin/vim:
3207e(bozo)

bash$ fuser -u /dev/null
/dev/null:
3009(bozo)

3010(bozo)

3197(bozo)

3199(bozo)

One important application for fuser is when physically inserting or removing storage media, such as
CD ROM disks or USB flash drives. Sometimes trying a umount fails with a device is busy error
message. This means that some user(s) and/or process(es) are accessing the device. An fuser -um
/dev/device_name will clear up the mystery, so you can kill any relevant processes.
bash$ umount /mnt/usbdrive
umount: /mnt/usbdrive: device is busy

bash$ fuser -um /dev/usbdrive

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/mnt/usbdrive:

1772c(bozo)

bash$ kill -9 1772
bash$ umount /mnt/usbdrive

The fuser command, invoked with the -n option identifies the processes accessing a port. This is
especially useful in combination with nmap.
root# nmap localhost.localdomain
PORT
STATE SERVICE
25/tcp
open smtp

root# fuser -un tcp 25
25/tcp:
2095(root)
root# ps ax | grep 2095 | grep -v grep
2095 ?
Ss
0:00 sendmail: accepting connections

cron
Administrative program scheduler, performing such duties as cleaning up and deleting system log
files and updating the slocate database. This is the superuser version of at (although each user may
have their own crontab file which can be changed with the crontab command). It runs as a daemon
and executes scheduled entries from /etc/crontab.
Some flavors of Linux run crond, Matthew Dillon's version of cron.
Process Control and Booting
init
The init command is the parent of all processes. Called in the final step of a bootup, init determines
the runlevel of the system from /etc/inittab. Invoked by its alias telinit, and by root only.
telinit
Symlinked to init, this is a means of changing the system runlevel, usually done for system
maintenance or emergency filesystem repairs. Invoked only by root. This command can be dangerous
-- be certain you understand it well before using!
runlevel
Shows the current and last runlevel, that is, whether the system is halted (runlevel 0), in single-user
mode (1), in multi-user mode (2 or 3), in X Windows (5), or rebooting (6). This command accesses
the /var/run/utmp file.
halt, shutdown, reboot
Command set to shut the system down, usually just prior to a power down.
On some Linux distros, the halt command has 755 permissions, so it can be invoked
by a non-root user. A careless halt in a terminal or a script may shut down the system!
service
Starts or stops a system service. The startup scripts in /etc/init.d and /etc/rc.d use this
command to start services at bootup.

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root# /sbin/service iptables stop
Flushing firewall rules:
Setting chains to policy ACCEPT: filter
Unloading iptables modules:

[
[
[

OK ]
OK ]
OK ]

Network
nmap
Network mapper and port scanner. This command scans a server to locate open ports and the services
associated with those ports. It can also report information about packet filters and firewalls. This is an
important security tool for locking down a network against hacking attempts.
#!/bin/bash
SERVER=$HOST
PORT_NUMBER=25

# localhost.localdomain (127.0.0.1).
# SMTP port.

nmap $SERVER | grep -w "$PORT_NUMBER" # Is that particular port open?
#
grep -w matches whole words only,
#+
so this wouldn't match port 1025, for example.
exit 0
# 25/tcp

open

smtp

ifconfig
Network interface configuration and tuning utility.
bash$ ifconfig -a
lo
Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:10 errors:0 dropped:0 overruns:0 frame:0
TX packets:10 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:700 (700.0 b) TX bytes:700 (700.0 b)

The ifconfig command is most often used at bootup to set up the interfaces, or to shut them down
when rebooting.
# Code snippets from /etc/rc.d/init.d/network
# ...
# Check that networking is up.
[ ${NETWORKING} = "no" ] && exit 0
[ -x /sbin/ifconfig ] || exit 0
# ...
for i in $interfaces ; do
if ifconfig $i 2>/dev/null | grep -q "UP" >/dev/null 2>&1 ; then
action "Shutting down interface $i: " ./ifdown $i boot
fi
# The GNU-specific "-q" option to "grep" means "quiet", i.e.,
#+ producing no output.
# Redirecting output to /dev/null is therefore not strictly necessary.

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# ...
echo "Currently active devices:"
echo `/sbin/ifconfig | grep ^[a-z] | awk '{print $1}'`
#
^^^^^ should be quoted to prevent globbing.
# The following also work.
#
echo $(/sbin/ifconfig | awk '/^[a-z]/ { print $1 })'
#
echo $(/sbin/ifconfig | sed -e 's/ .*//')
# Thanks, S.C., for additional comments.

See also Example 32-6.
netstat
Show current network statistics and information, such as routing tables and active connections. This
utility accesses information in /proc/net (Chapter 29). See Example 29-4.
netstat -r is equivalent to route.
bash$ netstat
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address
Foreign Address
State
Active UNIX domain sockets (w/o servers)
Proto RefCnt Flags
Type
State
I-Node Path
unix 11
[ ]
DGRAM
906
/dev/log
unix 3
[ ]
STREAM
CONNECTED
4514
/tmp/.X11-unix/X0
unix 3
[ ]
STREAM
CONNECTED
4513
. . .

A netstat -lptu shows sockets that are listening to ports, and the associated processes.
This can be useful for determining whether a computer has been hacked or
compromised.
iwconfig
This is the command set for configuring a wireless network. It is the wireless equivalent of ifconfig,
above.
ip
General purpose utility for setting up, changing, and analyzing IP (Internet Protocol) networks and
attached devices. This command is part of the iproute2 package.
bash$ ip link show
1: lo:  mtu 16436 qdisc noqueue
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0:  mtu 1500 qdisc pfifo_fast qlen 1000
link/ether 00:d0:59:ce:af:da brd ff:ff:ff:ff:ff:ff
3: sit0:  mtu 1480 qdisc noop
link/sit 0.0.0.0 brd 0.0.0.0

bash$ ip route list
169.254.0.0/16 dev lo

scope link

Or, in a script:

#!/bin/bash
# Script by Juan Nicolas Ruiz
# Used with his kind permission.
# Setting up (and stopping) a GRE tunnel.

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# --- start-tunnel.sh --LOCAL_IP="192.168.1.17"
REMOTE_IP="10.0.5.33"
OTHER_IFACE="192.168.0.100"
REMOTE_NET="192.168.3.0/24"
/sbin/ip tunnel add netb mode gre remote $REMOTE_IP \
local $LOCAL_IP ttl 255
/sbin/ip addr add $OTHER_IFACE dev netb
/sbin/ip link set netb up
/sbin/ip route add $REMOTE_NET dev netb
exit 0

#############################################

# --- stop-tunnel.sh --REMOTE_NET="192.168.3.0/24"
/sbin/ip route del $REMOTE_NET dev netb
/sbin/ip link set netb down
/sbin/ip tunnel del netb
exit 0

route
Show info about or make changes to the kernel routing table.
bash$ route
Destination
Gateway
Genmask
Flags
pm3-67.bozosisp *
255.255.255.255 UH
127.0.0.0
*
255.0.0.0
U
default
pm3-67.bozosisp 0.0.0.0
UG

MSS Window
40 0
40 0
40 0

irtt Iface
0 ppp0
0 lo
0 ppp0

iptables
The iptables command set is a packet filtering tool used mainly for such security purposes as setting
up network firewalls. This is a complex tool, and a detailed explanation of its use is beyond the scope
of this document. Oskar Andreasson's tutorial is a reasonable starting point.
See also shutting down iptables and Example 30-2.
chkconfig
Check network and system configuration. This command lists and manages the network and system
services started at bootup in the /etc/rc?.d directory.
Originally a port from IRIX to Red Hat Linux, chkconfig may not be part of the core installation of
some Linux flavors.
bash$ chkconfig --list
atd
0:off
rwhod
0:off
...

1:off
1:off

2:off
2:off

3:on
3:off

4:on
4:off

5:on
5:off

6:off
6:off

tcpdump
Network packet "sniffer." This is a tool for analyzing and troubleshooting traffic on a network by
dumping packet headers that match specified criteria.

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Dump ip packet traffic between hosts bozoville and caduceus:
bash$ tcpdump ip host bozoville and caduceus

Of course, the output of tcpdump can be parsed with certain of the previously discussed text
processing utilities.
Filesystem
mount
Mount a filesystem, usually on an external device, such as a floppy or CDROM. The file
/etc/fstab provides a handy listing of available filesystems, partitions, and devices, including
options, that may be automatically or manually mounted. The file /etc/mtab shows the currently
mounted filesystems and partitions (including the virtual ones, such as /proc).
mount -a mounts all filesystems and partitions listed in /etc/fstab, except those with a noauto
option. At bootup, a startup script in /etc/rc.d (rc.sysinit or something similar) invokes this
to get everything mounted.
mount -t iso9660 /dev/cdrom /mnt/cdrom
# Mounts CD ROM. ISO 9660 is a standard CD ROM filesystem.
mount /mnt/cdrom
# Shortcut, if /mnt/cdrom listed in /etc/fstab

The versatile mount command can even mount an ordinary file on a block device, and the file will act
as if it were a filesystem. Mount accomplishes that by associating the file with a loopback device. One
application of this is to mount and examine an ISO9660 filesystem image before burning it onto a
CDR. [87]

Example 17-7. Checking a CD image
# As root...
mkdir /mnt/cdtest

# Prepare a mount point, if not already there.

mount -r -t iso9660 -o loop cd-image.iso /mnt/cdtest
# Mount the image.
#
"-o loop" option equivalent to "losetup /dev/loop0"
cd /mnt/cdtest
# Now, check the image.
ls -alR
# List the files in the directory tree there.
# And so forth.

umount
Unmount a currently mounted filesystem. Before physically removing a previously mounted floppy or
CDROM disk, the device must be umounted, else filesystem corruption may result.
umount /mnt/cdrom
# You may now press the eject button and safely remove the disk.

The automount utility, if properly installed, can mount and unmount floppies or
CDROM disks as they are accessed or removed. On "multispindle" laptops with
swappable floppy and optical drives, this can cause problems, however.
gnome-mount

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The newer Linux distros have deprecated mount and umount. The successor, for command-line
mounting of removable storage devices, is gnome-mount. It can take the -d option to mount a device
file by its listing in /dev.
For example, to mount a USB flash drive:
bash$ gnome-mount -d /dev/sda1
gnome-mount 0.4

bash$ df
. . .
/dev/sda1

63584

12034

51550

19% /media/disk

sync
Forces an immediate write of all updated data from buffers to hard drive (synchronize drive with
buffers). While not strictly necessary, a sync assures the sys admin or user that the data just changed
will survive a sudden power failure. In the olden days, a sync; sync (twice, just to make
absolutely sure) was a useful precautionary measure before a system reboot.
At times, you may wish to force an immediate buffer flush, as when securely deleting a file (see
Example 16-60) or when the lights begin to flicker.
losetup
Sets up and configures loopback devices.

Example 17-8. Creating a filesystem in a file
SIZE=1000000

# 1 meg

head -c $SIZE < /dev/zero > file
losetup /dev/loop0 file
mke2fs /dev/loop0
mount -o loop /dev/loop0 /mnt

#
#
#
#

Set up file of designated size.
Set it up as loopback device.
Create filesystem.
Mount it.

# Thanks, S.C.

mkswap
Creates a swap partition or file. The swap area must subsequently be enabled with swapon.
swapon, swapoff
Enable / disable swap partitition or file. These commands usually take effect at bootup and shutdown.
mke2fs
Create a Linux ext2 filesystem. This command must be invoked as root.

Example 17-9. Adding a new hard drive
#!/bin/bash
#
#
#
#

Adding a second hard drive to system.
Software configuration. Assumes hardware already mounted.
From an article by the author of the ABS Guide.
In issue #38 of _Linux Gazette_, http://www.linuxgazette.com.

ROOT_UID=0
E_NOTROOT=67

# This script must be run as root.
# Non-root exit error.

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if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
# Use with extreme caution!
# If something goes wrong, you may wipe out your current filesystem.

NEWDISK=/dev/hdb
MOUNTPOINT=/mnt/newdisk

fdisk $NEWDISK
mke2fs -cv $NEWDISK1
# Note:
^
mkdir $MOUNTPOINT
chmod 777 $MOUNTPOINT

#
#
#
#

# Assumes /dev/hdb vacant. Check!
# Or choose another mount point.

# Check for bad blocks (verbose output).
/dev/hdb1, *not* /dev/hdb!
# Makes new drive accessible to all users.

Now, test ...
mount -t ext2 /dev/hdb1 /mnt/newdisk
Try creating a directory.
If it works, umount it, and proceed.

# Final step:
# Add the following line to /etc/fstab.
# /dev/hdb1 /mnt/newdisk ext2 defaults

1 1

exit

See also Example 17-8 and Example 31-3.
tune2fs
Tune ext2 filesystem. May be used to change filesystem parameters, such as maximum mount count.
This must be invoked as root.
This is an extremely dangerous command. Use it at your own risk, as you may
inadvertently destroy your filesystem.
dumpe2fs
Dump (list to stdout) very verbose filesystem info. This must be invoked as root.
root# dumpe2fs /dev/hda7 |
dumpe2fs 1.19, 13-Jul-2000
Mount count:
Maximum mount count:

grep 'ount count'
for EXT2 FS 0.5b, 95/08/09
6
20

hdparm
List or change hard disk parameters. This command must be invoked as root, and it may be dangerous
if misused.
fdisk
Create or change a partition table on a storage device, usually a hard drive. This command must be
invoked as root.
Use this command with extreme caution. If something goes wrong, you may destroy
an existing filesystem.
fsck, e2fsck, debugfs
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Filesystem check, repair, and debug command set.
fsck: a front end for checking a UNIX filesystem (may invoke other utilities). The actual filesystem
type generally defaults to ext2.
e2fsck: ext2 filesystem checker.
debugfs: ext2 filesystem debugger. One of the uses of this versatile, but dangerous command is to
(attempt to) recover deleted files. For advanced users only!
All of these should be invoked as root, and they can damage or destroy a filesystem if
misused.
badblocks
Checks for bad blocks (physical media flaws) on a storage device. This command finds use when
formatting a newly installed hard drive or testing the integrity of backup media. [88] As an example,
badblocks /dev/fd0 tests a floppy disk.
The badblocks command may be invoked destructively (overwrite all data) or in non-destructive
read-only mode. If root user owns the device to be tested, as is generally the case, then root must
invoke this command.
lsusb, usbmodules
The lsusb command lists all USB (Universal Serial Bus) buses and the devices hooked up to them.
The usbmodules command outputs information about the driver modules for connected USB devices.
bash$ lsusb
Bus 001 Device 001: ID 0000:0000
Device Descriptor:
bLength
18
bDescriptorType
1
bcdUSB
1.00
bDeviceClass
9 Hub
bDeviceSubClass
0
bDeviceProtocol
0
bMaxPacketSize0
8
idVendor
0x0000
idProduct
0x0000
. . .

lspci
Lists pci busses present.
bash$ lspci
00:00.0 Host
(Brookdale)
00:01.0 PCI
(Brookdale)
00:1d.0 USB
00:1d.1 USB
00:1d.2 USB
00:1e.0 PCI

bridge: Intel Corporation 82845 845
Chipset Host Bridge (rev 04)
bridge: Intel Corporation 82845 845
Chipset AGP Bridge (rev 04)
Controller: Intel Corporation 82801CA/CAM USB (Hub #1)
Controller: Intel Corporation 82801CA/CAM USB (Hub #2)
Controller: Intel Corporation 82801CA/CAM USB (Hub #3)
bridge: Intel Corporation 82801 Mobile PCI Bridge (rev

(rev 02)
(rev 02)
(rev 02)
42)

. . .

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mkbootdisk
Creates a boot floppy which can be used to bring up the system if, for example, the MBR (master boot
record) becomes corrupted. Of special interest is the --iso option, which uses mkisofs to create a
bootable ISO9660 filesystem image suitable for burning a bootable CDR.
The mkbootdisk command is actually a Bash script, written by Erik Troan, in the /sbin directory.
mkisofs
Creates an ISO9660 filesystem suitable for a CDR image.
chroot
CHange ROOT directory. Normally commands are fetched from $PATH, relative to /, the default
root directory. This changes the root directory to a different one (and also changes the working
directory to there). This is useful for security purposes, for instance when the system administrator
wishes to restrict certain users, such as those telnetting in, to a secured portion of the filesystem (this
is sometimes referred to as confining a guest user to a "chroot jail"). Note that after a chroot, the
execution path for system binaries is no longer valid.
A chroot /opt would cause references to /usr/bin to be translated to /opt/usr/bin.
Likewise, chroot /aaa/bbb /bin/ls would redirect future instances of ls to /aaa/bbb as
the base directory, rather than / as is normally the case. An alias XX 'chroot /aaa/bbb ls' in a user's
~/.bashrc effectively restricts which portion of the filesystem she may run command "XX" on.
The chroot command is also handy when running from an emergency boot floppy (chroot to
/dev/fd0), or as an option to lilo when recovering from a system crash. Other uses include
installation from a different filesystem (an rpm option) or running a readonly filesystem from a CD
ROM. Invoke only as root, and use with care.
It might be necessary to copy certain system files to a chrooted directory, since the
normal $PATH can no longer be relied upon.
lockfile
This utility is part of the procmail package (www.procmail.org). It creates a lock file, a semaphore
that controls access to a file, device, or resource.

Definition: A semaphore is a flag or signal. (The usage originated in railroading, where a
colored flag, lantern, or striped movable arm semaphore indicated whether a particular track was in
use and therefore unavailable for another train.) A UNIX process can check the appropriate
semaphore to determine whether a particular resource is available/accessible.
The lock file serves as a flag that this particular file, device, or resource is in use by a process (and is
therefore "busy"). The presence of a lock file permits only restricted access (or no access) to other
processes.
lockfile /home/bozo/lockfiles/$0.lock
# Creates a write-protected lockfile prefixed with the name of the script.
lockfile /home/bozo/lockfiles/${0##*/}.lock
# A safer version of the above, as pointed out by E. Choroba.

Lock files are used in such applications as protecting system mail folders from simultaneously being
changed by multiple users, indicating that a modem port is being accessed, and showing that an
instance of Firefox is using its cache. Scripts may check for the existence of a lock file created by a
certain process to check if that process is running. Note that if a script attempts to create a lock file
that already exists, the script will likely hang.
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Normally, applications create and check for lock files in the /var/lock directory. [89] A script can
test for the presence of a lock file by something like the following.
appname=xyzip
# Application "xyzip" created lock file "/var/lock/xyzip.lock".
if [ -e "/var/lock/$appname.lock" ]
then
#+ Prevent other programs & scripts
# from accessing files/resources used by xyzip.
...

flock
Much less useful than the lockfile command is flock. It sets an "advisory" lock on a file and then
executes a command while the lock is on. This is to prevent any other process from setting a lock on
that file until completion of the specified command.
flock $0 cat $0 > lockfile__$0
# Set a lock on the script the above line appears in,
#+ while listing the script to stdout.

Unlike lockfile, flock does not automatically create a lock file.
mknod
Creates block or character device files (may be necessary when installing new hardware on the
system). The MAKEDEV utility has virtually all of the functionality of mknod, and is easier to use.
MAKEDEV
Utility for creating device files. It must be run as root, and in the /dev directory. It is a sort of
advanced version of mknod.
tmpwatch
Automatically deletes files which have not been accessed within a specified period of time. Usually
invoked by cron to remove stale log files.
Backup
dump, restore
The dump command is an elaborate filesystem backup utility, generally used on larger installations
and networks. [90] It reads raw disk partitions and writes a backup file in a binary format. Files to be
backed up may be saved to a variety of storage media, including disks and tape drives. The restore
command restores backups made with dump.
fdformat
Perform a low-level format on a floppy disk (/dev/fd0*).
System Resources
ulimit
Sets an upper limit on use of system resources. Usually invoked with the -f option, which sets a limit
on file size (ulimit -f 1000 limits files to 1 meg maximum). [91] The -t option limits the coredump
size (ulimit -c 0 eliminates coredumps). Normally, the value of ulimit would be set in
/etc/profile and/or ~/.bash_profile (see Appendix G).
Judicious use of ulimit can protect a system against the dreaded fork bomb.
#!/bin/bash
# This script is for illustrative purposes only.

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# Run it at your own peril -- it WILL freeze your system.
while true
do
$0 &

#

Endless loop.

done

#
#+
#+
#

This script invokes itself . . .
forks an infinite number of times . . .
until the system freezes up because all resources exhausted.
This is the notorious "sorcerer's appentice" scenario.

exit 0

#

Will not exit here, because this script will never terminate.

A ulimit -Hu XX (where XX is the user process limit) in /etc/profile would abort this
script when it exceeded the preset limit.
quota
Display user or group disk quotas.
setquota
Set user or group disk quotas from the command-line.
umask
User file creation permissions mask. Limit the default file attributes for a particular user. All files
created by that user take on the attributes specified by umask. The (octal) value passed to umask
defines the file permissions disabled. For example, umask 022 ensures that new files will have at
most 755 permissions (777 NAND 022). [92] Of course, the user may later change the attributes of
particular files with chmod. The usual practice is to set the value of umask in /etc/profile
and/or ~/.bash_profile (see Appendix G).

Example 17-10. Using umask to hide an output file from prying eyes
#!/bin/bash
# rot13a.sh: Same as "rot13.sh" script, but writes output to "secure" file.
# Usage: ./rot13a.sh filename
# or
./rot13a.sh  $OUTFILE
#
^^ Input from stdin or a file.
^^^^^^^^^^ Output redirected to file.
exit 0

rdev
Get info about or make changes to root device, swap space, or video mode. The functionality of rdev
has generally been taken over by lilo, but rdev remains useful for setting up a ram disk. This is a
dangerous command, if misused.
Modules
lsmod
List installed kernel modules.
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bash$ lsmod
Module
autofs
opl3
serial_cs
sb
uart401
sound
soundlow
soundcore
ds
i82365
pcmcia_core

Size Used by
9456
2 (autoclean)
11376
0
5456
0 (unused)
34752
0
6384
0 [sb]
58368
0 [opl3 sb uart401]
464
0 [sound]
2800
6 [sb sound]
6448
2 [serial_cs]
22928
2
45984
0 [serial_cs ds i82365]

Doing a cat /proc/modules gives the same information.
insmod
Force installation of a kernel module (use modprobe instead, when possible). Must be invoked as
root.
rmmod
Force unloading of a kernel module. Must be invoked as root.
modprobe
Module loader that is normally invoked automatically in a startup script. Must be invoked as root.
depmod
Creates module dependency file. Usually invoked from a startup script.
modinfo
Output information about a loadable module.
bash$ modinfo hid
filename:
/lib/modules/2.4.20-6/kernel/drivers/usb/hid.o
description: "USB HID support drivers"
author:
"Andreas Gal, Vojtech Pavlik "
license:
"GPL"

Miscellaneous
env
Runs a program or script with certain environmental variables set or changed (without changing the
overall system environment). The [varname=xxx] permits changing the environmental variable
varname for the duration of the script. With no options specified, this command lists all the
environmental variable settings. [93]
The first line of a script (the "sha-bang" line) may use env when the path to the shell
or interpreter is unknown.
#! /usr/bin/env perl
print "This Perl script will run,\n";
print "even when I don't know where to find Perl.\n";
# Good for portable cross-platform scripts,
# where the Perl binaries may not be in the expected place.
# Thanks, S.C.

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Or even ...
#!/bin/env bash
# Queries the $PATH enviromental variable for the location of bash.
# Therefore ...
# This script will run where Bash is not in its usual place, in /bin.
...

ldd
Show shared lib dependencies for an executable file.
bash$ ldd /bin/ls
libc.so.6 => /lib/libc.so.6 (0x4000c000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)

watch
Run a command repeatedly, at specified time intervals.
The default is two-second intervals, but this may be changed with the -n option.
watch -n 5 tail /var/log/messages
# Shows tail end of system log, /var/log/messages, every five seconds.

Unfortunately, piping the output of watch command to grep does not work.
strip
Remove the debugging symbolic references from an executable binary. This decreases its size, but
makes debugging it impossible.
This command often occurs in a Makefile, but rarely in a shell script.
nm
List symbols in an unstripped compiled binary.
rdist
Remote distribution client: synchronizes, clones, or backs up a file system on a remote server.

17.1. Analyzing a System Script
Using our knowledge of administrative commands, let us examine a system script. One of the shortest and
simplest to understand scripts is "killall," [94] used to suspend running processes at system shutdown.

Example 17-11. killall, from /etc/rc.d/init.d
#!/bin/sh
# --> Comments added by the author of this document marked by "# -->".
# --> This is part of the 'rc' script package
# --> by Miquel van Smoorenburg, .
# --> This particular script seems to be Red Hat / FC specific
# --> (may not be present in other distributions).
# Bring down all unneeded services that are still running
#+ (there shouldn't be any, so this is just a sanity check)

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for i in /var/lock/subsys/*; do
# --> Standard for/in loop, but since "do" is on same line,
# --> it is necessary to add ";".
# Check if the script is there.
[ ! -f $i ] && continue
# --> This is a clever use of an "and list", equivalent to:
# --> if [ ! -f "$i" ]; then continue
# Get the subsystem name.
subsys=${i#/var/lock/subsys/}
# --> Match variable name, which, in this case, is the file name.
# --> This is the exact equivalent of subsys=`basename $i`.
#
#
#
#

-->
-->+
-->+
-->

It gets it from the lock file name
(if there is a lock file,
that's proof the process has been running).
See the "lockfile" entry, above.

# Bring the subsystem down.
if [ -f /etc/rc.d/init.d/$subsys.init ]; then
/etc/rc.d/init.d/$subsys.init stop
else
/etc/rc.d/init.d/$subsys stop
# --> Suspend running jobs and daemons.
# --> Note that "stop" is a positional parameter,
# -->+ not a shell builtin.
fi
done

That wasn't so bad. Aside from a little fancy footwork with variable matching, there is no new material there.
Exercise 1. In /etc/rc.d/init.d, analyze the halt script. It is a bit longer than killall, but similar in
concept. Make a copy of this script somewhere in your home directory and experiment with it (do not run it as
root). Do a simulated run with the -vn flags (sh -vn scriptname). Add extensive comments. Change
the commands to echos.
Exercise 2. Look at some of the more complex scripts in /etc/rc.d/init.d. Try to understand at least
portions of them. Follow the above procedure to analyze them. For some additional insight, you might also
examine the file sysvinitfiles in /usr/share/doc/initscripts-?.??, which is part of the
"initscripts" documentation.

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Part 5. Advanced Topics
At this point, we are ready to delve into certain of the difficult and unusual aspects of scripting. Along the
way, we will attempt to "push the envelope" in various ways and examine boundary conditions (what happens
when we move into uncharted territory?).
Table of Contents
18. Regular Expressions
18.1. A Brief Introduction to Regular Expressions
18.2. Globbing
19. Here Documents
19.1. Here Strings
20. I/O Redirection
20.1. Using exec
20.2. Redirecting Code Blocks
20.3. Applications
21. Subshells
22. Restricted Shells
23. Process Substitution
24. Functions
24.1. Complex Functions and Function Complexities
24.2. Local Variables
24.3. Recursion Without Local Variables
25. Aliases
26. List Constructs
27. Arrays
28. Indirect References
29. /dev and /proc
29.1. /dev
29.2. /proc
30. Network Programming
31. Of Zeros and Nulls
32. Debugging
33. Options
34. Gotchas
35. Scripting With Style
35.1. Unofficial Shell Scripting Stylesheet
36. Miscellany
36.1. Interactive and non-interactive shells and scripts
36.2. Shell Wrappers
36.3. Tests and Comparisons: Alternatives
36.4. Recursion: a script calling itself
36.5. "Colorizing" Scripts
36.6. Optimizations
36.7. Assorted Tips
36.8. Security Issues
36.9. Portability Issues
36.10. Shell Scripting Under Windows
37. Bash, versions 2, 3, and 4
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37.1. Bash, version 2
37.2. Bash, version 3
37.3. Bash, version 4

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Chapter 18. Regular Expressions
. . . the intellectual activity associated with
software development is largely one of gaining
insight.
--Stowe Boyd
To fully utilize the power of shell scripting, you need to master Regular Expressions. Certain commands and
utilities commonly used in scripts, such as grep, expr, sed and awk, interpret and use REs. As of version 3,
Bash has acquired its own RE-match operator: =~.

18.1. A Brief Introduction to Regular Expressions
An expression is a string of characters. Those characters having an interpretation above and beyond their
literal meaning are called metacharacters. A quote symbol, for example, may denote speech by a person,
ditto, or a meta-meaning [95] for the symbols that follow. Regular Expressions are sets of characters and/or
metacharacters that match (or specify) patterns.
A Regular Expression contains one or more of the following:
• A character set. These are the characters retaining their literal meaning. The simplest type of Regular
Expression consists only of a character set, with no metacharacters.
•
An anchor. These designate (anchor) the position in the line of text that the RE is to match. For
example, ^, and $ are anchors.
• Modifiers. These expand or narrow (modify) the range of text the RE is to match. Modifiers include
the asterisk, brackets, and the backslash.
The main uses for Regular Expressions (REs) are text searches and string manipulation. An RE matches a
single character or a set of characters -- a string or a part of a string.
• The asterisk -- * -- matches any number of repeats of the character string or RE preceding it,
including zero instances.
"1133*" matches 11 + one or more 3's: 113, 1133, 1133333, and so forth.
• The dot -- . -- matches any one character, except a newline. [96]
"13." matches 13 + at least one of any character (including a space):
1133, 11333, but not 13 (additional character missing).
See Example 16-18 for a demonstration of dot single-character matching.
• The caret -- ^ -- matches the beginning of a line, but sometimes, depending on context, negates the
meaning of a set of characters in an RE.
•
The dollar sign -- $ -- at the end of an RE matches the end of a line.
"XXX$" matches XXX at the end of a line.

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"^$" matches blank lines.
•
Brackets -- [...] -- enclose a set of characters to match in a single RE.
"[xyz]" matches any one of the characters x, y, or z.
"[c-n]" matches any one of the characters in the range c to n.
"[B-Pk-y]" matches any one of the characters in the ranges B to P and k to y.
"[a-z0-9]" matches any single lowercase letter or any digit.
"[^b-d]" matches any character except those in the range b to d. This is an instance of ^ negating or
inverting the meaning of the following RE (taking on a role similar to ! in a different context).
Combined sequences of bracketed characters match common word patterns. "[Yy][Ee][Ss]" matches
yes, Yes, YES, yEs, and so forth. "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" matches any
Social Security number.
•
The backslash -- \ -- escapes a special character, which means that character gets interpreted literally
(and is therefore no longer special).
A "\$" reverts back to its literal meaning of "$", rather than its RE meaning of end-of-line. Likewise a
"\\" has the literal meaning of "\".
•
Escaped "angle brackets" -- \<...\> -- mark word boundaries.
The angle brackets must be escaped, since otherwise they have only their literal character meaning.
"\" matches the word "the," but not the words "them," "there," "other," etc.
bash$ cat textfile
This is line 1, of which there is only one instance.
This is the only instance of line 2.
This is line 3, another line.
This is line 4.

bash$ grep 'the' textfile
This is line 1, of which there is only one instance.
This is the only instance of line 2.
This is line 3, another line.

bash$ grep '\' textfile
This is the only instance of line 2.

The only way to be certain that a particular RE works is to test it.
TEST FILE: tstfile

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# No match.

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Run

grep "1133*"

This
This
This
This
This
This
This
This

line
line
line
line
line
line
line
line

bash$
Run
This
This
This
This

contains
contains
contains
contains
contains
contains
contains
contains

grep
grep
line
line
line
line

on this file.

the number
the number
the number
the number
the number
the number
the number
no numbers

113.
13.
133.
1133.
113312.
1112.
113312312.
at all.

"1133*" tstfile
"1133*" on this file.
contains the number 113.
contains the number 1133.
contains the number 113312.
contains the number 113312312.

#
#
#
#
#
#
#
#
#
#
#

Match.
No match.
No match.
Match.
No match.
No match.
Match.
Match.
No match.
Match.
No match.

# Match.
# Match.
# Match.
# Match.
# Match.

• Extended REs. Additional metacharacters added to the basic set. Used in egrep, awk, and Perl.
•
The question mark -- ? -- matches zero or one of the previous RE. It is generally used for matching
single characters.
•
The plus -- + -- matches one or more of the previous RE. It serves a role similar to the *, but does not
match zero occurrences.
# GNU versions of sed and awk can use "+",
# but it needs to be escaped.
echo a111b | sed -ne '/a1\+b/p'
echo a111b | grep 'a1\+b'
echo a111b | gawk '/a1+b/'
# All of above are equivalent.
# Thanks, S.C.

• Escaped "curly brackets" -- \{ \} -- indicate the number of occurrences of a preceding RE to match.
It is necessary to escape the curly brackets since they have only their literal character meaning
otherwise. This usage is technically not part of the basic RE set.
"[0-9]\{5\}" matches exactly five digits (characters in the range of 0 to 9).
Curly brackets are not available as an RE in the "classic" (non-POSIX compliant)
version of awk. However, the GNU extended version of awk, gawk, has the
--re-interval option that permits them (without being escaped).
bash$ echo 2222 | gawk --re-interval '/2{3}/'
2222

Perl and some egrep versions do not require escaping the curly brackets.
•
Parentheses -- ( ) -- enclose a group of REs. They are useful with the following "|" operator and in
substring extraction using expr.
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• The -- | -- "or" RE operator matches any of a set of alternate characters.
bash$ egrep 're(a|e)d' misc.txt
People who read seem to be better informed than those who do not.
The clarinet produces sound by the vibration of its reed.

Some versions of sed, ed, and ex support escaped versions of the extended Regular Expressions
described above, as do the GNU utilities.
• POSIX Character Classes. [:class:]
This is an alternate method of specifying a range of characters to match.
• [:alnum:] matches alphabetic or numeric characters. This is equivalent to A-Za-z0-9.
• [:alpha:] matches alphabetic characters. This is equivalent to A-Za-z.
• [:blank:] matches a space or a tab.
• [:cntrl:] matches control characters.
• [:digit:] matches (decimal) digits. This is equivalent to 0-9.
• [:graph:] (graphic printable characters). Matches characters in the range of ASCII 33 - 126. This
is the same as [:print:], below, but excluding the space character.
• [:lower:] matches lowercase alphabetic characters. This is equivalent to a-z.
• [:print:] (printable characters). Matches characters in the range of ASCII 32 - 126. This is the
same as [:graph:], above, but adding the space character.
• [:space:] matches whitespace characters (space and horizontal tab).
• [:upper:] matches uppercase alphabetic characters. This is equivalent to A-Z.
• [:xdigit:] matches hexadecimal digits. This is equivalent to 0-9A-Fa-f.
POSIX character classes generally require quoting or double brackets ([[ ]]).
bash$ grep [[:digit:]] test.file
abc=723

# ...
if [[ $arow =~ [[:digit:]] ]]
# Numerical input?
then
# POSIX char class
if [[ $acol =~ [[:alpha:]] ]] # Number followed by a letter? Illegal!
# ...
# From ktour.sh example script.

These character classes may even be used with globbing, to a limited extent.
bash$ ls -l ?[[:digit:]][[:digit:]]?
-rw-rw-r-1 bozo bozo
0 Aug 21 14:47 a33b

POSIX character classes are used in Example 16-21 and Example 16-22.
Sed, awk, and Perl, used as filters in scripts, take REs as arguments when "sifting" or transforming files or I/O
streams. See Example A-12 and Example A-16 for illustrations of this.
The standard reference on this complex topic is Friedl's Mastering Regular Expressions. Sed & Awk, by
Dougherty and Robbins, also gives a very lucid treatment of REs. See the Bibliography for more information
on these books.
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18.2. Globbing
Bash itself cannot recognize Regular Expressions. Inside scripts, it is commands and utilities -- such as sed
and awk -- that interpret RE's.
Bash does carry out filename expansion [97] -- a process known as globbing -- but this does not use the
standard RE set. Instead, globbing recognizes and expands wild cards. Globbing interprets the standard wild
card characters [98] -- * and ?, character lists in square brackets, and certain other special characters (such as ^
for negating the sense of a match). There are important limitations on wild card characters in globbing,
however. Strings containing * will not match filenames that start with a dot, as, for example, .bashrc. [99]
Likewise, the ? has a different meaning in globbing than as part of an RE.
bash$ ls -l
total 2
-rw-rw-r--rw-rw-r--rw-rw-r--rw-rw-r--rw-rw-r--

1
1
1
1
1

bozo
bozo
bozo
bozo
bozo

bash$ ls -l t?.sh
-rw-rw-r-1 bozo

bozo
bozo
bozo
bozo
bozo

bozo

0
0
0
466
758

Aug 6
Aug 6
Aug 6
Aug 6
Jul 30

466 Aug

18:42
18:42
18:42
17:48
09:02

a.1
b.1
c.1
t2.sh
test1.txt

6 17:48 t2.sh

bash$ ls -l [ab]*
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo

0 Aug 6 18:42 a.1
0 Aug 6 18:42 b.1

bash$ ls -l [a-c]*
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo

0 Aug 6 18:42 a.1
0 Aug 6 18:42 b.1
0 Aug 6 18:42 c.1

bash$ ls -l [^ab]*
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo

0 Aug 6 18:42 c.1
466 Aug 6 17:48 t2.sh
758 Jul 30 09:02 test1.txt

bash$ ls -l {b*,c*,*est*}
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo
-rw-rw-r-1 bozo bozo

0 Aug 6 18:42 b.1
0 Aug 6 18:42 c.1
758 Jul 30 09:02 test1.txt

Bash performs filename expansion on unquoted command-line arguments. The echo command demonstrates
this.
bash$ echo *
a.1 b.1 c.1 t2.sh test1.txt
bash$ echo t*
t2.sh test1.txt
bash$ echo t?.sh
t2.sh

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It is possible to modify the way Bash interprets special characters in globbing. A set -f command
disables globbing, and the nocaseglob and nullglob options to shopt change globbing behavior.
See also Example 11-4.

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Chapter 19. Here Documents
Here and now, boys.
--Aldous Huxley, Island
A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to
an interactive program or a command, such as ftp, cat, or the ex text editor.
COMMAND <.
# Bram Moolenaar points out that this may not work with 'vim'
#+ because of possible problems with terminal interaction.
exit

The above script could just as effectively have been implemented with ex, rather than vi. Here documents
containing a list of ex commands are common enough to form their own category, known as ex scripts.
#!/bin/bash
# Replace all instances of "Smith" with "Jones"
#+ in files with a ".txt" filename suffix.
ORIGINAL=Smith
REPLACEMENT=Jones
for word in $(fgrep -l $ORIGINAL *.txt)
do
# ------------------------------------ex $word < $Newfile < $OUTFILE
# ----------------------------------------------------------# Quoting the 'limit string' prevents variable expansion
#+ within the body of the above 'here document.'
# This permits outputting literal strings in the output file.
if [ -f "$OUTFILE" ]
then
chmod 755 $OUTFILE
# Make the generated file executable.
else
echo "Problem in creating file: \"$OUTFILE\""
fi
# This method can also be used for generating
#+ C programs, Perl programs, Python programs, Makefiles,
#+ and the like.
exit 0

It is possible to set a variable from the output of a here document. This is actually a devious form of command
substitution.
variable=$(cat < EOF
lsof
1213 bozo
0r
REG
3,5
0 30386 /tmp/t1213-0-sh (deleted)

Some utilities will not work inside a here document.
The closing limit string, on the final line of a here document, must start in the first character position.
There can be no leading whitespace. Trailing whitespace after the limit string likewise causes
unexpected behavior. The whitespace prevents the limit string from being recognized. [100]

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#!/bin/bash
echo "----------------------------------------------------------------------"
cat <&2
done <<< $(echo ${ArrayVar[*]})
# element0 element1 element2 A B C D

Example 19-13. Prepending a line to a file
#!/bin/bash
# prepend.sh: Add text at beginning of file.
#
# Example contributed by Kenny Stauffer,
#+ and slightly modified by document author.

E_NOSUCHFILE=85

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read -p "File: " file
# -p arg to 'read' displays prompt.
if [ ! -e "$file" ]
then
# Bail out if no such file.
echo "File $file not found."
exit $E_NOSUCHFILE
fi
read -p "Title: " title
cat - $file <<<$title > $file.new
echo "Modified file is $file.new"
exit

# Ends script execution.

from 'man bash':
Here Strings
A variant of here documents, the format is:
<<
# Redirect stdout to a file.
# Creates the file if not present, otherwise overwrites it.
ls -lR > dir-tree.list
# Creates a file containing a listing of the directory tree.
: > filename
# The > truncates file "filename" to zero length.
# If file not present, creates zero-length file (same effect as 'touch').
# The : serves as a dummy placeholder, producing no output.
> filename
# The > truncates file "filename" to zero length.
# If file not present, creates zero-length file (same effect as 'touch').
# (Same result as ": >", above, but this does not work with some shells.)
COMMAND_OUTPUT >>
# Redirect stdout to a file.
# Creates the file if not present, otherwise appends to it.

# Single-line redirection commands (affect only the line they are on):
# -------------------------------------------------------------------1>filename
# Redirect stdout to file "filename."
1>>filename
# Redirect and append stdout to file "filename."
2>filename
# Redirect stderr to file "filename."
2>>filename
# Redirect and append stderr to file "filename."
&>filename
# Redirect both stdout and stderr to file "filename."
# This operator is now functional, as of Bash 4, final release.
M>N
# "M" is a file descriptor, which defaults to 1, if not explicitly set.
# "N" is a filename.
# File descriptor "M" is redirect to file "N."
M>&N
# "M" is a file descriptor, which defaults to 1, if not set.

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# "N" is another file descriptor.
#==============================================================================
# Redirecting stdout, one line at a time.
LOGFILE=script.log
echo "This statement is sent to the log file, \"$LOGFILE\"." 1>$LOGFILE
echo "This statement is appended to \"$LOGFILE\"." 1>>$LOGFILE
echo "This statement is also appended to \"$LOGFILE\"." 1>>$LOGFILE
echo "This statement is echoed to stdout, and will not appear in \"$LOGFILE\"."
# These redirection commands automatically "reset" after each line.

# Redirecting stderr, one line at a time.
ERRORFILE=script.errors
bad_command1 2>$ERRORFILE
bad_command2 2>>$ERRORFILE
bad_command3

# Error message sent to $ERRORFILE.
# Error message appended to $ERRORFILE.
# Error message echoed to stderr,
#+ and does not appear in $ERRORFILE.
# These redirection commands also automatically "reset" after each line.
#=======================================================================
2>&1
# Redirects stderr to stdout.
# Error messages get sent to same place as standard output.
>>filename 2>&1
bad_command >>filename 2>&1
# Appends both stdout and stderr to the file "filename" ...
2>&1 | [command(s)]
bad_command 2>&1 | awk '{print $5}'
# found
# Sends stderr through a pipe.
# |& was added to Bash 4 as an abbreviation for 2>&.
i>&j
# Redirects file descriptor i to j.
# All output of file pointed to by i gets sent to file pointed to by j.
>&j
# Redirects, by default, file descriptor 1 (stdout) to j.
# All stdout gets sent to file pointed to by j.
0< FILENAME
< FILENAME
# Accept input from a file.
# Companion command to ">", and often used in combination with it.
#
# grep search-word filename
# Open file "filename" for reading and writing,
#+ and assign file descriptor "j" to it.
# If "filename" does not exist, create it.
# If file descriptor "j" is not specified, default to fd 0, stdin.
#
# An application of this is writing at a specified place in a file.
echo 1234567890 > File
# Write string to "File".
exec 3<> File
# Open "File" and assign fd 3 to it.

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read -n 4 <&3
#
echo -n . >&3
#
exec 3>&#
cat File
#
# Random access, by golly.

Read only 4 characters.
Write a decimal point there.
Close fd 3.
==> 1234.67890

|
# Pipe.
# General purpose process and command chaining tool.
# Similar to ">", but more general in effect.
# Useful for chaining commands, scripts, files, and programs together.
cat *.txt | sort | uniq > result-file
# Sorts the output of all the .txt files and deletes duplicate lines,
# finally saves results to "result-file".

Multiple instances of input and output redirection and/or pipes can be combined in a single command line.
command < input-file > output-file
# Or the equivalent:
< input-file command > output-file

# Although this is non-standard.

command1 | command2 | command3 > output-file

See Example 16-31 and Example A-14.
Multiple output streams may be redirected to one file.
ls
#
#
#+

-yz >> command.log 2>&1
Capture result of illegal options "yz" in file "command.log."
Because stderr is redirected to the file,
any error messages will also be there.

#
ls
#
#
#+

Note, however, that the following does *not* give the same result.
-yz 2>&1 >> command.log
Outputs an error message, but does not write to file.
More precisely, the command output (in this case, null)
writes to the file, but the error message goes only to stdout.

# If redirecting both stdout and stderr,
#+ the order of the commands makes a difference.

Closing File Descriptors
n<&Close input file descriptor n.
0<&-, <&Close stdin.
n>&Close output file descriptor n.
1>&-, >&Close stdout.

Child processes inherit open file descriptors. This is why pipes work. To prevent an fd from being inherited,
close it.

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# Redirecting only stderr to a pipe.
exec 3>&1
ls -l 2>&1 >&3 3>&- | grep bad 3>&#
^^^^
^^^^
exec 3>&-

# Save current "value" of stdout.
# Close fd 3 for 'grep' (but not 'ls').
# Now close it for the remainder of the script.

# Thanks, S.C.

For a more detailed introduction to I/O redirection see Appendix E.

20.1. Using exec
An exec filename command redirects stdout to a designated file. This sends all command
output that would normally go to stdout to that file.
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exec N > filename affects the entire script or current shell. Redirection in the PID of the script or shell
from that point on has changed. However . . .
N > filename affects only the newly-forked process, not the entire script or shell.
Thank you, Ahmed Darwish, for pointing this out.
Example 20-2. Redirecting stdout using exec
#!/bin/bash
# reassign-stdout.sh
LOGFILE=logfile.txt
exec 6>&1

# Link file descriptor #6 with stdout.
# Saves stdout.

exec > $LOGFILE

# stdout replaced with file "logfile.txt".

# ----------------------------------------------------------- #
# All output from commands in this block sent to file $LOGFILE.
echo -n "Logfile: "
date
echo "-------------------------------------"
echo
echo "Output of \"ls -al\" command"
echo
ls -al
echo; echo
echo "Output of \"df\" command"
echo
df
# ----------------------------------------------------------- #
exec 1>&6 6>&-

# Restore stdout and close file descriptor #6.

echo
echo "== stdout now restored to default == "
echo
ls -al
echo
exit 0

Example 20-3. Redirecting both stdin and stdout in the same script with exec
#!/bin/bash
# upperconv.sh
# Converts a specified input file to uppercase.
E_FILE_ACCESS=70
E_WRONG_ARGS=71
if [ ! -r "$1" ]
# Is specified input file readable?
then
echo "Can't read from input file!"

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echo "Usage: $0 input-file output-file"
exit $E_FILE_ACCESS
fi
# Will exit with same error
#+ even if input file ($1) not specified (why?).
if [ -z "$2" ]
then
echo "Need to specify output file."
echo "Usage: $0 input-file output-file"
exit $E_WRONG_ARGS
fi

exec 4<&0
exec < $1

# Will read from input file.

exec 7>&1
exec > $2

# Will write to output file.
# Assumes output file writable (add check?).

# ----------------------------------------------cat - | tr a-z A-Z
# Uppercase conversion.
#
^^^^^
# Reads from stdin.
#
^^^^^^^^^^
# Writes to stdout.
# However, both stdin and stdout were redirected.
# Note that the 'cat' can be omitted.
# ----------------------------------------------exec 1>&7 7>&exec 0<&4 4<&-

# Restore stout.
# Restore stdin.

# After restoration, the following line prints to stdout as expected.
echo "File \"$1\" written to \"$2\" as uppercase conversion."
exit 0

I/O redirection is a clever way of avoiding the dreaded inaccessible variables within a subshell problem.

Example 20-4. Avoiding a subshell
#!/bin/bash
# avoid-subshell.sh
# Suggested by Matthew Walker.
Lines=0
echo
cat myfile.txt | while read line;
do {
echo $line
(( Lines++ ));

# Incremented values of this variable
#+ inaccessible outside loop.
# Subshell problem.

}
done
echo "Number of lines read = $Lines"

# 0
# Wrong!

echo "------------------------"

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exec 3<> myfile.txt
while read line <&3
do {
echo "$line"
(( Lines++ ));

# Incremented values of this variable
#+ accessible outside loop.
# No subshell, no problem.

}
done
exec 3>&echo "Number of lines read = $Lines"

# 8

echo
exit 0
# Lines below not seen by script.
$ cat myfile.txt
Line
Line
Line
Line
Line
Line
Line
Line

1.
2.
3.
4.
5.
6.
7.
8.

20.2. Redirecting Code Blocks
Blocks of code, such as while, until, and for loops, even if/then test blocks can also incorporate redirection of
stdin. Even a function may use this form of redirection (see Example 24-11). The < operator at the end of
the code block accomplishes this.

Example 20-5. Redirected while loop
#!/bin/bash
# redir2.sh
if [ -z "$1" ]
then
Filename=names.data
# Default, if no filename specified.
else
Filename=$1
fi
#+ Filename=${1:-names.data}
# can replace the above test (parameter substitution).
count=0
echo
while [ "$name" != Smith ]
do

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Advanced Bash-Scripting Guide
read name
echo $name
let "count += 1"
done <"$Filename"
#
^^^^^^^^^^^^

# Reads from $Filename, rather than stdin.

# Redirects stdin to file $Filename.

echo; echo "$count names read"; echo
exit 0
#
#+
#
#
#+
#

Note that in some older shell scripting languages,
the redirected loop would run as a subshell.
Therefore, $count would return 0, the initialized value outside the loop.
Bash and ksh avoid starting a subshell *whenever possible*,
so that this script, for example, runs correctly.
(Thanks to Heiner Steven for pointing this out.)

# However . . .
# Bash *can* sometimes start a subshell in a PIPED "while-read" loop,
#+ as distinct from a REDIRECTED "while" loop.
abc=hi
echo -e "1\n2\n3" | while read l
do abc="$l"
echo $abc
done
echo $abc
# Thanks, Bruno de Oliveira Schneider, for demonstrating this
#+ with the above snippet of code.
# And, thanks, Brian Onn, for correcting an annotation error.

Example 20-6. Alternate form of redirected while loop
#!/bin/bash
# This is an alternate form of the preceding script.
# Suggested by Heiner Steven
#+ as a workaround in those situations when a redirect loop
#+ runs as a subshell, and therefore variables inside the loop
# +do not keep their values upon loop termination.

if [ -z "$1" ]
then
Filename=names.data
else
Filename=$1
fi

exec 3<&0
exec 0<"$Filename"

# Default, if no filename specified.

# Save stdin to file descriptor 3.
# Redirect standard input.

count=0
echo

while [ "$name" != Smith ]
do

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read name
echo $name
let "count += 1"
done

# Reads from redirected stdin ($Filename).

# Loop reads from file $Filename
#+ because of line 20.

# The original version of this script terminated the "while" loop with
#+
done <"$Filename"
# Exercise:
# Why is this unnecessary?

exec 0<&3
exec 3<&-

# Restore old stdin.
# Close temporary fd 3.

echo; echo "$count names read"; echo
exit 0

Example 20-7. Redirected until loop
#!/bin/bash
# Same as previous example, but with "until" loop.
if [ -z "$1" ]
then
Filename=names.data
else
Filename=$1
fi
# while [ "$name" != Smith ]
until [ "$name" = Smith ]
do
read name
echo $name
done <"$Filename"
#
^^^^^^^^^^^^

# Default, if no filename specified.

# Change

!=

to =.

# Reads from $Filename, rather than stdin.
# Redirects stdin to file $Filename.

# Same results as with "while" loop in previous example.
exit 0

Example 20-8. Redirected for loop
#!/bin/bash
if [ -z "$1" ]
then
Filename=names.data
else
Filename=$1
fi

# Default, if no filename specified.

line_count=`wc $Filename | awk '{ print $1 }'`
#
Number of lines in target file.
#
# Very contrived and kludgy, nevertheless shows that
#+ it's possible to redirect stdin within a "for" loop...
#+ if you're clever enough.

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#
# More concise is

line_count=$(wc -l < "$Filename")

for name in `seq $line_count`
# while [ "$name" != Smith ]
do
read name
echo $name
if [ "$name" = Smith ]
then
break
fi
done <"$Filename"
#
^^^^^^^^^^^^

# Recall that "seq" prints sequence of numbers.
-more complicated than a "while" loop
-# Reads from $Filename, rather than stdin.
# Need all this extra baggage here.

# Redirects stdin to file $Filename.

exit 0

We can modify the previous example to also redirect the output of the loop.

Example 20-9. Redirected for loop (both stdin and stdout redirected)
#!/bin/bash
if [ -z "$1" ]
then
Filename=names.data
else
Filename=$1
fi

# Default, if no filename specified.

Savefile=$Filename.new
FinalName=Jonah

# Filename to save results in.
# Name to terminate "read" on.

line_count=`wc $Filename | awk '{ print $1 }'`

for name in `seq $line_count`
do
read name
echo "$name"
if [ "$name" = "$FinalName" ]
then
break
fi
done < "$Filename" > "$Savefile"
#
^^^^^^^^^^^^^^^^^^^^^^^^^^^

# Number of lines in target file.

# Redirects stdin to file $Filename,
and saves it to backup file.

exit 0

Example 20-10. Redirected if/then test
#!/bin/bash
if [ -z "$1" ]
then
Filename=names.data
else
Filename=$1
fi

# Default, if no filename specified.

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TRUE=1
if [ "$TRUE" ]
then
read name
echo $name
fi <"$Filename"
# ^^^^^^^^^^^^

# if true

and

if :

also work.

# Reads only first line of file.
# An "if/then" test has no way of iterating unless embedded in a loop.
exit 0

Example 20-11. Data file names.data for above examples
Aristotle
Belisarius
Capablanca
Euler
Goethe
Hegel
Jonah
Laplace
Maroczy
Purcell
Schmidt
Semmelweiss
Smith
Turing
Venn
Warshawski
Znosko-Borowski
# This is a data file for
#+ "redir2.sh", "redir3.sh", "redir4.sh", "redir4a.sh", "redir5.sh".

Redirecting the stdout of a code block has the effect of saving its output to a file. See Example 3-2.
Here documents are a special case of redirected code blocks. That being the case, it should be possible to feed
the output of a here document into the stdin for a while loop.
# This example by Albert Siersema
# Used with permission (thanks!).
function doesOutput()
# Could be an external command too, of course.
# Here we show you can use a function as well.
{
ls -al *.jpg | awk '{print $5,$9}'
}

nr=0
totalSize=0

# We want the while loop to be able to manipulate these and
#+ to be able to see the changes after the 'while' finished.

while read fileSize fileName ; do
echo "$fileName is $fileSize bytes"
let nr++
totalSize=$((totalSize+fileSize))
# Or: "let totalSize+=fileSize"

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done<&7
# This *appends* the date to the file.
#
^^^^^^^ command substitution
# See below.
}

case $LOG_LEVEL in
1) exec 3>&2
2) exec 3>&2
3) exec 3>&2
*) exec 3> /dev/null
esac

4> /dev/null
4>&2
4>&2
4> /dev/null

5> /dev/null;;
5> /dev/null;;
5>&2;;
5> /dev/null;;

FD_LOGVARS=6

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if [[ $LOG_VARS ]]
then exec 6>> /var/log/vars.log
else exec 6> /dev/null
fi

# Bury output.

FD_LOGEVENTS=7
if [[ $LOG_EVENTS ]]
then
# exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log)
# Above line fails in versions of Bash more recent than 2.04. Why?
exec 7>> /var/log/event.log
# Append to "event.log".
log
# Write time and date.
else exec 7> /dev/null
# Bury output.
fi
echo "DEBUG3: beginning" >&${FD_DEBUG3}
ls -l >&5 2>&4

# command1 >&5 2>&4

echo "Done"

# command2

echo "sending mail" >&${FD_LOGEVENTS}
# Writes "sending mail" to file descriptor #7.

exit 0

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Chapter 21. Subshells
Running a shell script launches a new process, a subshell.

Definition: A subshell is a child process launched by a shell (or shell script).
A subshell is a separate instance of the command processor -- the shell that gives you the prompt at the
console or in an xterm window. Just as your commands are interpreted at the command-line prompt, similarly
does a script batch-process a list of commands. Each shell script running is, in effect, a subprocess (child
process) of the parent shell.
A shell script can itself launch subprocesses. These subshells let the script do parallel processing, in effect
executing multiple subtasks simultaneously.
#!/bin/bash
# subshell-test.sh
(
# Inside parentheses, and therefore a subshell . . .
while [ 1 ]
# Endless loop.
do
echo "Subshell running . . ."
done
)
# Script will run forever,
#+ or at least until terminated by a Ctl-C.
exit $?

# End of script (but will never get here).

Now, run the script:
sh subshell-test.sh
And, while the script is running, from a different xterm:
ps -ef | grep subshell-test.sh
UID
500
500

PID
2698
2699

PPID C STIME TTY
2502 0 14:26 pts/4
2698 21 14:26 pts/4

TIME
CMD
00:00:00 sh subshell-test.sh
00:00:24 sh subshell-test.sh

^^^^
Analysis:
PID 2698, the script, launched PID 2699, the subshell.
Note: The "UID ..." line would be filtered out by the "grep" command,
but is shown here for illustrative purposes.

In general, an external command in a script forks off a subprocess, [104] whereas a Bash builtin does not. For
this reason, builtins execute more quickly and use fewer system resources than their external command
equivalents.
Command List within Parentheses
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Advanced Bash-Scripting Guide
( command1; command2; command3; ... )
A command list embedded between parentheses runs as a subshell.
Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible to the
parent process, to the shell that launched the subshell. These are, in effect, variables local to the child process.

Example 21-1. Variable scope in a subshell
#!/bin/bash
# subshell.sh
echo
echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
# Bash, version 3, adds the new
$BASH_SUBSHELL variable.
echo; echo
outer_variable=Outer
global_variable=
# Define global variable for "storage" of
#+ value of subshell variable.
(
echo "We are inside the subshell."
echo "Subshell level INSIDE subshell = $BASH_SUBSHELL"
inner_variable=Inner
echo "From inside subshell, \"inner_variable\" = $inner_variable"
echo "From inside subshell, \"outer\" = $outer_variable"
global_variable="$inner_variable"

# Will this allow "exporting"
#+ a subshell variable?

)
echo; echo
echo "We are outside the subshell."
echo "Subshell level OUTSIDE subshell = $BASH_SUBSHELL"
echo
if [ -z "$inner_variable" ]
then
echo "inner_variable undefined in main body of shell"
else
echo "inner_variable defined in main body of shell"
fi
echo "From main body of shell, \"inner_variable\" = $inner_variable"
# $inner_variable will show as blank (uninitialized)
#+ because variables defined in a subshell are "local variables".
# Is there a remedy for this?
echo "global_variable = "$global_variable"" # Why doesn't this work?
echo
# =======================================================================
# Additionally ...

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echo "-----------------"; echo
var=41

# Global variable.

( let "var+=1"; echo "\$var INSIDE subshell = $var" )

# 42

echo "\$var OUTSIDE subshell = $var"
# 41
# Variable operations inside a subshell, even to a GLOBAL variable
#+ do not affect the value of the variable outside the subshell!

exit 0
#
#
#
#+
#+

Question:
-------Once having exited a subshell,
is there any way to reenter that very same subshell
to modify or access the subshell variables?

See also $BASHPID and Example 34-2.

Definition: The scope of a variable is the context in which it has meaning, in which it has a value that
can be referenced. For example, the scope of a local variable lies only within the function, block of code, or
subshell within which it is defined, while the scope of a global variable is the entire script in which it
appears.
While the $BASH_SUBSHELL internal variable indicates the nesting level of a subshell, the $SHLVL
variable shows no change within a subshell.
echo " \$BASH_SUBSHELL outside subshell
= $BASH_SUBSHELL"
# 0
( echo " \$BASH_SUBSHELL inside subshell
= $BASH_SUBSHELL" )
# 1
( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2
# ^ ^
*** nested ***
^ ^
echo
echo " \$SHLVL outside subshell = $SHLVL"
( echo " \$SHLVL inside subshell = $SHLVL" )

# 3
# 3 (No change!)

Directory changes made in a subshell do not carry over to the parent shell.

Example 21-2. List User Profiles
#!/bin/bash
# allprofs.sh: Print all user profiles.
# This script written by Heiner Steven, and modified by the document author.
FILE=.bashrc

# File containing user profile,
#+ was ".profile" in original script.

for home in `awk -F: '{print $6}' /etc/passwd`
do
[ -d "$home" ] || continue
# If no home directory, go to next.
[ -r "$home" ] || continue
# If not readable, go to next.
(cd $home; [ -e $FILE ] && less $FILE)

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done
# When script terminates, there is no need to 'cd' back to original directory,
#+ because 'cd $home' takes place in a subshell.
exit 0

A subshell may be used to set up a "dedicated environment" for a command group.
COMMAND1
COMMAND2
COMMAND3
(
IFS=:
PATH=/bin
unset TERMINFO
set -C
shift 5
COMMAND4
COMMAND5
exit 3 # Only exits the subshell!
)
# The parent shell has not been affected, and the environment is preserved.
COMMAND6
COMMAND7

As seen here, the exit command only terminates the subshell in which it is running, not the parent shell or
script.
One application of such a "dedicated environment" is testing whether a variable is defined.
if (set -u; : $variable) 2> /dev/null
then
echo "Variable is set."
fi
# Variable has been set in current script,
#+ or is an an internal Bash variable,
#+ or is present in environment (has been exported).
#
#
#
#

Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]
or
[[ ${variable-x} != x$variable ]]
or
[[ ${variable+x} = x ]]
or
[[ ${variable-x} != x ]]

Another application is checking for a lock file:
if (set -C; : > lock_file) 2> /dev/null
then
:
# lock_file didn't exist: no user running the script
else
echo "Another user is already running that script."
exit 65
fi
# Code snippet by Stéphane Chazelas,
#+ with modifications by Paulo Marcel Coelho Aragao.

+
Processes may execute in parallel within different subshells. This permits breaking a complex task into
subcomponents processed concurrently.

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Example 21-3. Running parallel processes in subshells
(cat list1 list2 list3 | sort | uniq > list123) &
(cat list4 list5 list6 | sort | uniq > list456) &
# Merges and sorts both sets of lists simultaneously.
# Running in background ensures parallel execution.
#
# Same effect as
#
cat list1 list2 list3 | sort | uniq > list123 &
#
cat list4 list5 list6 | sort | uniq > list456 &
wait

# Don't execute the next command until subshells finish.

diff list123 list456

Redirecting I/O to a subshell uses the "|" pipe operator, as in ls -al | (command).
A code block between curly brackets does not launch a subshell.
{ command1; command2; command3; . . . commandN; }
var1=23
echo "$var1"

# 23

{ var1=76; }
echo "$var1"

# 76

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385

Chapter 22. Restricted Shells
Disabled commands in restricted shells
. Running a script or portion of a script in restricted mode disables certain commands that would
otherwise be available. This is a security measure intended to limit the privileges of the script user and
to minimize possible damage from running the script.
The following commands and actions are disabled:
• Using cd to change the working directory.
• Changing the values of the $PATH, $SHELL, $BASH_ENV, or $ENV environmental variables.
• Reading or changing the $SHELLOPTS, shell environmental options.
• Output redirection.
• Invoking commands containing one or more /'s.
• Invoking exec to substitute a different process for the shell.
• Various other commands that would enable monkeying with or attempting to subvert the script for an
unintended purpose.
• Getting out of restricted mode within the script.

Example 22-1. Running a script in restricted mode
#!/bin/bash
# Starting the script with "#!/bin/bash -r"
#+ runs entire script in restricted mode.
echo
echo "Changing directory."
cd /usr/local
echo "Now in `pwd`"
echo "Coming back home."
cd
echo "Now in `pwd`"
echo
# Everything up to here in normal, unrestricted mode.
set -r
# set --restricted
has same effect.
echo "==> Now in restricted mode. <=="
echo
echo
echo "Attempting directory change in restricted mode."
cd ..
echo "Still in `pwd`"
echo
echo
echo "\$SHELL = $SHELL"
echo "Attempting to change shell in restricted mode."

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SHELL="/bin/ash"
echo
echo "\$SHELL= $SHELL"
echo
echo
echo "Attempting to redirect output in restricted mode."
ls -l /usr/bin > bin.files
ls -l bin.files
# Try to list attempted file creation effort.
echo
exit 0

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387

Chapter 23. Process Substitution
Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need
to pipe the stdout of multiple commands? This is where process substitution comes in.
Process substitution feeds the output of a process (or processes) into the stdin of another process.
Template
Command list enclosed within parentheses
>(command_list)
<(command_list)
Process substitution uses /dev/fd/ files to send the results of the process(es) within
parentheses to another process. [105]
There is no space between the the "<" or ">" and the parentheses. Space there would
give an error message.
bash$ echo >(true)
/dev/fd/63
bash$ echo <(true)
/dev/fd/63
bash$ echo >(true) <(true)
/dev/fd/63 /dev/fd/62

bash$ wc <(cat /usr/share/dict/linux.words)
483523 483523 4992010 /dev/fd/63
bash$ grep script /usr/share/dict/linux.words | wc
262
262
3601
bash$ wc <(grep script /usr/share/dict/linux.words)
262
262
3601 /dev/fd/63

Bash creates a pipe with two file descriptors, --fIn and fOut--. The stdin of true connects to
fOut (dup2(fOut, 0)), then Bash passes a /dev/fd/fIn argument to echo. On systems lacking
/dev/fd/ files, Bash may use temporary files. (Thanks, S.C.)
Process substitution can compare the output of two different commands, or even the output of different
options to the same command.
bash$ comm <(ls -l) <(ls -al)
total 12
-rw-rw-r-1 bozo bozo
78 Mar 10 12:58 File0
-rw-rw-r-1 bozo bozo
42 Mar 10 12:58 File2
-rw-rw-r-1 bozo bozo
103 Mar 10 12:58 t2.sh
total 20
drwxrwxrwx
2 bozo bozo
4096 Mar 10 18:10 .

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drwx------rw-rw-r--rw-rw-r--rw-rw-r--

72
1
1
1

bozo
bozo
bozo
bozo

bozo
bozo
bozo
bozo

4096
78
42
103

Mar
Mar
Mar
Mar

10
10
10
10

17:58
12:58
12:58
12:58

..
File0
File2
t2.sh

Process substitution can compare the contents of two directories -- to see which filenames are in one, but not
the other.
diff <(ls $first_directory) <(ls $second_directory)

Some other usages and uses of process substitution:

read -a list < <( od -Ad -w24 -t u2 /dev/urandom )
# Read a list of random numbers from /dev/urandom,
#+ process with "od"
#+ and feed into stdin of "read" . . .
#
#

From "insertion-sort.bash" example script.
Courtesy of JuanJo Ciarlante.

PORT=6881

# bittorrent

# Scan the port to make sure nothing nefarious is going on.
netcat -l $PORT | tee>(md5sum ->mydata-orig.md5) |
gzip | tee>(md5sum - | sed 's/-$/mydata.lz2/'>mydata-gz.md5)>mydata.gz
# Check the decompression:
gzip -d(bzip2 -c > file.tar.bz2) $directory_name
# Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2".
#
# Because of the /dev/fd/ system feature,
# the pipe between both commands does not need to be named.
#
# This can be emulated.
#
bzip2 -c < pipe > file.tar.bz2&
tar cf pipe $directory_name
rm pipe
#
or
exec 3>&1
tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&exec 3>&-

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# Thanks, Stéphane Chazelas

Here is a method of circumventing the problem of an echo piped to a while-read loop running in a subshell.

Example 23-1. Code block redirection without forking
#!/bin/bash
# wr-ps.bash: while-read loop with process substitution.
# This example contributed by Tomas Pospisek.
# (Heavily edited by the ABS Guide author.)
echo
echo "random input" | while read i
do
global=3D": Not available outside the loop."
# ... because it runs in a subshell.
done
echo "\$global (from outside the subprocess) = $global"
# $global (from outside the subprocess) =
echo; echo "--"; echo
while read i
do
echo $i
global=3D": Available outside the loop."
# ... because it does *not* run in a subshell.
done < <( echo "random input" )
#
^ ^
echo "\$global (using process substitution) = $global"
# Random input
# $global (using process substitution) = 3D: Available outside the loop.

echo; echo "##########"; echo

# And likewise . . .
declare -a inloop
index=0
cat $0 | while read line
do
inloop[$index]="$line"
((index++))
# It runs in a subshell, so ...
done
echo "OUTPUT = "
echo ${inloop[*]}
# ... nothing echoes.

echo; echo "--"; echo

declare -a outloop

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index=0
while read line
do
outloop[$index]="$line"
((index++))
# It does *not* run in a subshell, so ...
done < <( cat $0 )
echo "OUTPUT = "
echo ${outloop[*]}
# ... the entire script echoes.
exit $?

This is a similar example.

Example 23-2. Redirecting the output of process substitution into a loop.
#!/bin/bash
# psub.bash
# As inspired by Diego Molina (thanks!).
declare -a array0
while read
do
array0[${#array0[@]}]="$REPLY"
done < <( sed -e 's/bash/CRASH-BANG!/' $0 | grep bin | awk '{print $1}' )
# Sets the default 'read' variable, $REPLY, by process substitution,
#+ then copies it into an array.
echo "${array0[@]}"
exit $?
# ====================================== #
bash psub.bash
#!/bin/CRASH-BANG! done #!/bin/CRASH-BANG!

A reader sent in the following interesting example of process substitution.
# Script fragment taken from SuSE distribution:
# --------------------------------------------------------------#
while read des what mask iface; do
# Some commands ...
done < <(route -n)
#
^ ^ First < is redirection, second is process substitution.
# To test it, let's make it do something.
while read des what mask iface; do
echo $des $what $mask $iface
done < <(route -n)
#
#
#
#
#
#

Output:
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
--------------------------------------------------------------#
As Stéphane Chazelas points out,

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#+ an easier-to-understand equivalent is:
route -n |
while read des what mask iface; do
# Variables set from output of pipe.
echo $des $what $mask $iface
done # This yields the same output as above.
# However, as Ulrich Gayer points out . . .
#+ this simplified equivalent uses a subshell for the while loop,
#+ and therefore the variables disappear when the pipe terminates.
# --------------------------------------------------------------#
# However, Filip Moritz comments that there is a subtle difference
#+ between the above two examples, as the following shows.
(
route -n | while read x; do ((y++)); done
echo $y # $y is still unset
while read x; do ((y++)); done < <(route -n)
echo $y # $y has the number of lines of output of route -n
)
More generally spoken
(
: | x=x
# seems to start a subshell like
: | ( x=x )
# while
x=x < <(:)
# does not
)
# This is useful, when parsing csv and the like.
# That is, in effect, what the original SuSE code fragment does.

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392

Chapter 24. Functions
Like "real" programming languages, Bash has functions, though in a somewhat limited implementation. A
function is a subroutine, a code block that implements a set of operations, a "black box" that performs a
specified task. Wherever there is repetitive code, when a task repeats with only slight variations in procedure,
then consider using a function.
function function_name {
command...
}
or
function_name () {
command...
}
This second form will cheer the hearts of C programmers (and is more portable).
As in C, the function's opening bracket may optionally appear on the second line.
function_name ()
{
command...
}
A function may be "compacted" into a single line.
fun () { echo "This is a function"; echo; }
#
^
^

In this case, however, a semicolon must follow the final command in the function.
fun () { echo "This is a function"; echo } # Error!
#
^
fun2 () { echo "Even a single-command function? Yes!"; }
#
^

Functions are called, triggered, simply by invoking their names. A function call is equivalent to a command.

Example 24-1. Simple functions
#!/bin/bash
JUST_A_SECOND=1
funky ()
{ # This is about as simple as functions get.
echo "This is a funky function."
echo "Now exiting funky function."
} # Function declaration must precede call.

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fun ()
{ # A somewhat more complex function.
i=0
REPEATS=30
echo
echo "And now the fun really begins."
echo
sleep $JUST_A_SECOND
# Hey, wait a second!
while [ $i -lt $REPEATS ]
do
echo "----------FUNCTIONS---------->"
echo "<------------ARE-------------"
echo "<------------FUN------------>"
echo
let "i+=1"
done
}
# Now, call the functions.
funky
fun
exit 0

The function definition must precede the first call to it. There is no method of "declaring" the function, as, for
example, in C.
f1
# Will give an error message, since function "f1" not yet defined.
declare -f f1
f1

# This doesn't help either.
# Still an error message.

# However...

f1 ()
{
echo "Calling function \"f2\" from within function \"f1\"."
f2
}
f2 ()
{
echo "Function \"f2\"."
}
f1

# Function "f2" is not actually called until this point,
#+ although it is referenced before its definition.
# This is permissible.
# Thanks, S.C.

Functions may not be empty!

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#!/bin/bash
# empty-function.sh
empty ()
{
}
exit 0

# Will not exit here!

# $ sh empty-function.sh
# empty-function.sh: line 6: syntax error near unexpected token `}'
# empty-function.sh: line 6: `}'
# $ echo $?
# 2

# Note that a function containing only comments is empty.
func ()
{
# Comment 1.
# Comment 2.
# This is still an empty function.
# Thank you, Mark Bova, for pointing this out.
}
# Results in same error message as above.

# However ...
not_quite_empty ()
{
illegal_command
} # A script containing this function will *not* bomb
#+ as long as the function is not called.

# Thank you, Thiemo Kellner, for pointing this out.

It is even possible to nest a function within another function, although this is not very useful.
f1 ()
{
f2 () # nested
{
echo "Function \"f2\", inside \"f1\"."
}
}
f2

#
#

Gives an error message.
Even a preceding "declare -f f2" wouldn't help.

echo
f1
f2

# Does nothing, since calling "f1" does not automatically call "f2".
# Now, it's all right to call "f2",
#+ since its definition has been made visible by calling "f1".
# Thanks, S.C.

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Function declarations can appear in unlikely places, even where a command would otherwise go.
ls -l | foo() { echo "foo"; }

# Permissible, but useless.

if [ "$USER" = bozo ]
then
bozo_greet ()
# Function definition embedded in an if/then construct.
{
echo "Hello, Bozo."
}
fi
bozo_greet

# Works only for Bozo, and other users get an error.

# Something like this might be useful in some contexts.
NO_EXIT=1
# Will enable function definition below.
[[ $NO_EXIT -eq 1 ]] && exit() { true; }
# Function definition in an "and-list".
# If $NO_EXIT is 1, declares "exit ()".
# This disables the "exit" builtin by aliasing it to "true".
exit

# Invokes "exit ()" function, not "exit" builtin.

# Or, similarly:
filename=file1
[ -f "$filename" ] &&
foo () { rm -f "$filename"; echo "File "$filename" deleted."; } ||
foo () { echo "File "$filename" not found."; touch bar; }
foo
# Thanks, S.C. and Christopher Head

Function names can take strange forms.
_(){ for i in {1..10}; do echo -n "$FUNCNAME"; done; echo; }
# ^^^
No space between function name and parentheses.
#
This doesn't always work. Why not?
# Now, let's invoke the function.
_
# __________
#
^^^^^^^^^^
10 underscores (10 x function name)!
# A "naked" underscore is an acceptable function name.

# In fact, a colon is likewise an acceptable function name.
:(){ echo ":"; }; :
# Of what use is this?
# It's a devious way to obfuscate the code in a script.

See also Example A-55

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What happens when different versions of the same function appear in a script?
#
#
#
#

As Yan Chen points out,
when a function is defined multiple times,
the final version is what is invoked.
This is not, however, particularly useful.

func ()
{
echo "First version of func ()."
}
func ()
{
echo "Second version of func ()."
}
func

# Second version of func ().

exit $?
# It is even possible to use functions to override
#+ or preempt system commands.
# Of course, this is *not* advisable.

24.1. Complex Functions and Function Complexities
Functions may process arguments passed to them and return an exit status to the script for further processing.
function_name $arg1 $arg2

The function refers to the passed arguments by position (as if they were positional parameters), that is, $1,
$2, and so forth.

Example 24-2. Function Taking Parameters
#!/bin/bash
# Functions and parameters
DEFAULT=default
func2 () {
if [ -z "$1" ]
then
echo "-Parameter #1 is zero length.-"
else
echo "-Param #1 is \"$1\".-"
fi
variable=${1-$DEFAULT}
echo "variable = $variable"

# Default param value.

# Is parameter #1 zero length?
# Or no parameter passed.

#
#+
#
#
#+

What does
parameter substitution show?
--------------------------It distinguishes between
no param and a null param.

if [ "$2" ]

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then
echo "-Parameter #2 is \"$2\".-"
fi
return 0
}
echo
echo "Nothing passed."
func2
echo

# Called with no params

echo "Zero-length parameter passed."
func2 ""
# Called with zero-length param
echo
echo "Null parameter passed."
func2 "$uninitialized_param"
echo

# Called with uninitialized param

echo "One parameter passed."
func2 first
# Called with one param
echo
echo "Two parameters passed."
func2 first second
# Called with two params
echo
echo "\"\" \"second\" passed."
func2 "" second
# Called with zero-length first parameter
echo
# and ASCII string as a second one.
exit 0

The shift command works on arguments passed to functions (see Example 36-16).
But, what about command-line arguments passed to the script? Does a function see them? Well, let's clear up
the confusion.

Example 24-3. Functions and command-line args passed to the script
#!/bin/bash
# func-cmdlinearg.sh
# Call this script with a command-line argument,
#+ something like $0 arg1.

func ()
{
echo "$1"
}
echo "First call to function: no arg passed."
echo "See if command-line arg is seen."
func
# No! Command-line arg not seen.

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echo "============================================================"
echo
echo "Second call to function: command-line arg passed explicitly."
func $1
# Now it's seen!
exit 0

In contrast to certain other programming languages, shell scripts normally pass only value parameters to
functions. Variable names (which are actually pointers), if passed as parameters to functions, will be treated
as string literals. Functions interpret their arguments literally.

Indirect variable references (see Example 37-2) provide a clumsy sort of mechanism for passing variable
pointers to functions.

Example 24-4. Passing an indirect reference to a function
#!/bin/bash
# ind-func.sh: Passing an indirect reference to a function.
echo_var ()
{
echo "$1"
}
message=Hello
Hello=Goodbye
echo_var "$message"
# Hello
# Now, let's pass an indirect reference to the function.
echo_var "${!message}"
# Goodbye
echo "-------------"
# What happens if we change the contents of "hello" variable?
Hello="Hello, again!"
echo_var "$message"
# Hello
echo_var "${!message}"
# Hello, again!
exit 0

The next logical question is whether parameters can be dereferenced after being passed to a function.

Example 24-5. Dereferencing a parameter passed to a function
#!/bin/bash
# dereference.sh
# Dereferencing parameter passed to a function.
# Script by Bruce W. Clare.
dereference ()
{
y=\$"$1"
echo $y

# Name of variable (not value!).
# $Junk

x=`eval "expr \"$y\" "`

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Advanced Bash-Scripting Guide
echo $1=$x
eval "$1=\"Some Different Text \""

# Assign new value.

}
Junk="Some Text"
echo $Junk "before"

# Some Text before

dereference Junk
echo $Junk "after"

# Some Different Text after

exit 0

Example 24-6. Again, dereferencing a parameter passed to a function
#!/bin/bash
# ref-params.sh: Dereferencing a parameter passed to a function.
#
(Complex Example)
ITERATIONS=3
icount=1

# How many times to get input.

my_read () {
# Called with my_read varname,
#+ outputs the previous value between brackets as the default value,
#+ then asks for a new value.
local local_var
echo -n "Enter a value "
eval 'echo -n "[$'$1'] "'
# eval echo -n "[\$$1] "

# Previous value.
# Easier to understand,
#+ but loses trailing space in user prompt.

read local_var
[ -n "$local_var" ] && eval $1=\$local_var
# "And-list": if "local_var" then set "$1" to its value.
}
echo
while [ "$icount" -le "$ITERATIONS" ]
do
my_read var
echo "Entry #$icount = $var"
let "icount += 1"
echo
done

# Thanks to Stephane Chazelas for providing this instructive example.
exit 0

Exit and Return
exit status
Functions return a value, called an exit status. This is analogous to the exit status returned by a
command. The exit status may be explicitly specified by a return statement, otherwise it is the exit
status of the last command in the function (0 if successful, and a non-zero error code if not). This exit
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status may be used in the script by referencing it as $?. This mechanism effectively permits script
functions to have a "return value" similar to C functions.
return
Terminates a function. A return command [106] optionally takes an integer argument, which is
returned to the calling script as the "exit status" of the function, and this exit status is assigned to the
variable $?.

Example 24-7. Maximum of two numbers
#!/bin/bash
# max.sh: Maximum of two integers.
E_PARAM_ERR=250
# If less than 2 params passed to function.
EQUAL=251
# Return value if both params equal.
# Error values out of range of any
#+ params that might be fed to the function.
max2 ()
# Returns larger of two numbers.
{
# Note: numbers compared must be less than 250.
if [ -z "$2" ]
then
return $E_PARAM_ERR
fi
if [ "$1" -eq "$2" ]
then
return $EQUAL
else
if [ "$1" -gt "$2" ]
then
return $1
else
return $2
fi
fi
}
max2 33 34
return_val=$?
if [ "$return_val" -eq $E_PARAM_ERR ]
then
echo "Need to pass two parameters to the function."
elif [ "$return_val" -eq $EQUAL ]
then
echo "The two numbers are equal."
else
echo "The larger of the two numbers is $return_val."
fi

exit 0
#
#
#
#+

Exercise (easy):
--------------Convert this to an interactive script,
that is, have the script ask for input (two numbers).

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For a function to return a string or array, use a dedicated variable.
count_lines_in_etc_passwd()
{
[[ -r /etc/passwd ]] && REPLY=$(echo $(wc -l < /etc/passwd))
# If /etc/passwd is readable, set REPLY to line count.
# Returns both a parameter value and status information.
# The 'echo' seems unnecessary, but . . .
#+ it removes excess whitespace from the output.
}
if count_lines_in_etc_passwd
then
echo "There are $REPLY lines in /etc/passwd."
else
echo "Cannot count lines in /etc/passwd."
fi
# Thanks, S.C.

Example 24-8. Converting numbers to Roman numerals
#!/bin/bash
# Arabic number to Roman numeral conversion
# Range: 0 - 200
# It's crude, but it works.
# Extending the range and otherwise improving the script is left as an exercise.
# Usage: roman number-to-convert
LIMIT=200
E_ARG_ERR=65
E_OUT_OF_RANGE=66
if [ -z "$1" ]
then
echo "Usage: `basename $0` number-to-convert"
exit $E_ARG_ERR
fi
num=$1
if [ "$num" -gt $LIMIT ]
then
echo "Out of range!"
exit $E_OUT_OF_RANGE
fi
to_roman ()
# Must declare function before first call to it.
{
number=$1
factor=$2
rchar=$3
let "remainder = number - factor"
while [ "$remainder" -ge 0 ]
do
echo -n $rchar
let "number -= factor"
let "remainder = number - factor"

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done
return $number
# Exercises:
# --------# 1) Explain how this function works.
#
Hint: division by successive subtraction.
# 2) Extend to range of the function.
#
Hint: use "echo" and command-substitution capture.
}

to_roman $num 100 C
num=$?
to_roman $num 90 LXXXX
num=$?
to_roman $num 50 L
num=$?
to_roman $num 40 XL
num=$?
to_roman $num 10 X
num=$?
to_roman $num 9 IX
num=$?
to_roman $num 5 V
num=$?
to_roman $num 4 IV
num=$?
to_roman $num 1 I
# Successive calls to conversion function!
# Is this really necessary??? Can it be simplified?
echo
exit

See also Example 11-28.
The largest positive integer a function can return is 255. The return command is closely
tied to the concept of exit status, which accounts for this particular limitation. Fortunately,
there are various workarounds for those situations requiring a large integer return value
from a function.

Example 24-9. Testing large return values in a function
#!/bin/bash
# return-test.sh
# The largest positive value a function can return is 255.
return_test ()
{
return $1
}

# Returns whatever passed to it.

return_test 27
echo $?

# o.k.
# Returns 27.

return_test 255

# Still o.k.

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echo $?

# Returns 255.

return_test 257
echo $?

# Error!
# Returns 1 (return code for miscellaneous error).

# ======================================================
return_test -151896
# Do large negative numbers work?
echo $?
# Will this return -151896?
# No! It returns 168.
# Version of Bash before 2.05b permitted
#+ large negative integer return values.
# Newer versions of Bash plug this loophole.
# This may break older scripts.
# Caution!
# ======================================================
exit 0

A workaround for obtaining large integer "return values" is to simply assign the "return
value" to a global variable.
Return_Val=

# Global variable to hold oversize return value of function.

alt_return_test ()
{
fvar=$1
Return_Val=$fvar
return
# Returns 0 (success).
}
alt_return_test 1
echo $?
echo "return value = $Return_Val"

# 0
# 1

alt_return_test 256
echo "return value = $Return_Val"

# 256

alt_return_test 257
echo "return value = $Return_Val"

# 257

alt_return_test 25701
echo "return value = $Return_Val"

#25701

A more elegant method is to have the function echo its "return value to stdout," and then
capture it by command substitution. See the discussion of this in Section 36.7.

Example 24-10. Comparing two large integers
#!/bin/bash
# max2.sh: Maximum of two LARGE integers.
# This is the previous "max.sh" example,
#+ modified to permit comparing large integers.
EQUAL=0
E_PARAM_ERR=-99999
#
^^^^^^

# Return value if both params equal.
# Not enough params passed to function.
Out of range of any params that might be passed.

max2 ()

# "Returns" larger of two numbers.

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{
if [ -z "$2" ]
then
echo $E_PARAM_ERR
return
fi
if [ "$1" -eq "$2" ]
then
echo $EQUAL
return
else
if [ "$1" -gt "$2" ]
then
retval=$1
else
retval=$2
fi
fi
echo $retval

# Echoes (to stdout), rather than returning value.
# Why?

}

return_val=$(max2 33001 33997)
#
^^^^
Function name
#
^^^^^ ^^^^^ Params passed
# This is actually a form of command substitution:
#+ treating a function as if it were a command,
#+ and assigning the stdout of the function to the variable "return_val."

# ========================= OUTPUT ========================
if [ "$return_val" -eq "$E_PARAM_ERR" ]
then
echo "Error in parameters passed to comparison function!"
elif [ "$return_val" -eq "$EQUAL" ]
then
echo "The two numbers are equal."
else
echo "The larger of the two numbers is $return_val."
fi
# =========================================================
exit 0
#
#
#
#+
#
#

Exercises:
--------1) Find a more elegant way of testing
the parameters passed to the function.
2) Simplify the if/then structure at "OUTPUT."
3) Rewrite the script to take input from command-line parameters.

Here is another example of capturing a function "return value." Understanding it requires
some knowledge of awk.
month_length ()
{
monthD="31 28 31
echo "$monthD" |
#

Chapter 24. Functions

# Takes month number as an argument.
# Returns number of days in month.
30 31 30 31 31 30 31 30 31" # Declare as local?
awk '{ print $'"${1}"' }'
# Tricky.
^^^^^^^^^

405

Advanced Bash-Scripting Guide
# Parameter passed to function ($1 -- month number), then to awk.
# Awk sees this as "print $1 . . . print $12" (depending on month number)
# Template for passing a parameter to embedded awk script:
#
$'"${script_parameter}"'
# Needs error checking for correct parameter range (1-12)
#+ and for February in leap year.
}
# ---------------------------------------------# Usage example:
month=4
# April, for example (4th month).
days_in=$(month_length $month)
echo $days_in # 30
# ----------------------------------------------

See also Example A-7 and Example A-37.
Exercise: Using what we have just learned, extend the previous Roman numerals
example to accept arbitrarily large input.
Redirection
Redirecting the stdin of a function
A function is essentially a code block, which means its stdin can be redirected (as in Example 3-1).

Example 24-11. Real name from username
#!/bin/bash
# realname.sh
#
# From username, gets "real name" from /etc/passwd.

ARGCOUNT=1
E_WRONGARGS=85

# Expect one arg.

file=/etc/passwd
pattern=$1
if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` USERNAME"
exit $E_WRONGARGS
fi
file_excerpt ()
# Scan file for pattern,
{
#+ then print relevant portion of line.
while read line # "while" does not necessarily need [ condition ]
do
echo "$line" | grep $1 | awk -F":" '{ print $5 }'
# Have awk use ":" delimiter.
done
} <$file # Redirect into function's stdin.
file_excerpt $pattern
# Yes, this entire script could be reduced to
#
grep PATTERN /etc/passwd | awk -F":" '{ print $5 }'

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# or
#
awk -F: '/PATTERN/ {print $5}'
# or
#
awk -F: '($1 == "username") { print $5 }' # real name from username
# However, it might not be as instructive.
exit 0

There is an alternate, and perhaps less confusing method of redirecting a function's stdin. This
involves redirecting the stdin to an embedded bracketed code block within the function.
# Instead of:
Function ()
{
...
} < file
# Try this:
Function ()
{
{
...
} < file
}
# Similarly,
Function () # This works.
{
{
echo $*
} | tr a b
}
Function ()
{
echo $*
} | tr a b

# This doesn't work.

# A nested code block is mandatory here.

# Thanks, S.C.

Emmanuel Rouat's sample bashrc file contains some instructive examples of
functions.

24.2. Local Variables
What makes a variable local?
local variables
A variable declared as local is one that is visible only within the block of code in which it appears. It
has local scope. In a function, a local variable has meaning only within that function block. [107]

Example 24-12. Local variable visibility
#!/bin/bash

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# Global and local variables inside a function.
func ()
{
local loc_var=23
# Declared as local variable.
echo
# Uses the 'local' builtin.
echo "\"loc_var\" in function = $loc_var"
global_var=999
# Not declared as local.
# Defaults to global.
echo "\"global_var\" in function = $global_var"
}
func
# Now, to see if local variable "loc_var" exists outside function.
echo
echo "\"loc_var\" outside function = $loc_var"
# $loc_var outside function =
# No, $loc_var not visible globally.
echo "\"global_var\" outside function = $global_var"
# $global_var outside function = 999
# $global_var is visible globally.
echo
exit 0
# In contrast to C, a Bash variable declared inside a function
#+ is local *only* if declared as such.

Before a function is called, all variables declared within the function are invisible outside the
body of the function, not just those explicitly declared as local.
#!/bin/bash
func ()
{
global_var=37
}

# Visible only within the function block
#+ before the function has been called.
# END OF FUNCTION

echo "global_var = $global_var"

func
echo "global_var = $global_var"

# global_var =
# Function "func" has not yet been called,
#+ so $global_var is not visible here.

# global_var = 37
# Has been set by function call.

As Evgeniy Ivanov points out, when declaring and setting a local variable in a single
command, apparently the order of operations is to first set the variable, and only
afterwards restrict it to local scope. This is reflected in the return value.
#!/bin/bash
echo "==OUTSIDE Function (global)=="
t=$(exit 1)
echo $?
# 1
# As expected.

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echo
function0 ()
{
echo "==INSIDE Function=="
echo "Global"
t0=$(exit 1)
echo $?
# 1
# As expected.
echo
echo "Local declared & assigned in same command."
local t1=$(exit 1)
echo $?
# 0
# Unexpected!
# Apparently, the variable assignment takes place before
#+ the local declaration.
#+ The return value is for the latter.
echo
echo "Local declared, then assigned (separate commands)."
local t2
t2=$(exit 1)
echo $?
# 1
# As expected.
}
function0

24.2.1. Local variables and recursion.

Recursion is an interesting and sometimes useful form of self-reference. Herbert Mayer defines it as ". . .
expressing an algorithm by using a simpler version of that same algorithm . . ."
Consider a definition defined in terms of itself, [108] an expression implicit in its own expression, [109] a
snake swallowing its own tail, [110] or . . . a function that calls itself. [111]

Example 24-13. Demonstration of a simple recursive function
#!/bin/bash
# recursion-demo.sh
# Demonstration of recursion.
RECURSIONS=9
r_count=0

# How many times to recurse.
# Must be global. Why?

recurse ()
{
var="$1"

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while [ "$var" -ge 0 ]
do
echo "Recursion count = "$r_count" +-+ \$var = "$var""
(( var-- )); (( r_count++ ))
recurse "$var" # Function calls itself (recurses)
done
#+ until what condition is met?
}
recurse $RECURSIONS
exit $?

Example 24-14. Another simple demonstration
#!/bin/bash
# recursion-def.sh
# A script that defines "recursion" in a rather graphic way.
RECURSIONS=10
r_count=0
sp=" "
define_recursion ()
{
((r_count++))
sp="$sp"" "
echo -n "$sp"
echo "\"The act of recurring ... \""

# Per 1913 Webster's dictionary.

while [ $r_count -le $RECURSIONS ]
do
define_recursion
done
}
echo
echo "Recursion: "
define_recursion
echo
exit $?

Local variables are a useful tool for writing recursive code, but this practice generally involves a great deal of
computational overhead and is definitely not recommended in a shell script. [112]

Example 24-15. Recursion, using a local variable
#!/bin/bash
#
#

factorial
---------

# Does bash permit recursion?
# Well, yes, but...
# It's so slow that you gotta have rocks in your head to try it.

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MAX_ARG=5
E_WRONG_ARGS=85
E_RANGE_ERR=86

if [ -z "$1" ]
then
echo "Usage: `basename $0` number"
exit $E_WRONG_ARGS
fi
if [ "$1" -gt $MAX_ARG ]
then
echo "Out of range ($MAX_ARG is maximum)."
# Let's get real now.
# If you want greater range than this,
#+ rewrite it in a Real Programming Language.
exit $E_RANGE_ERR
fi
fact ()
{
local number=$1
# Variable "number" must be declared as local,
#+ otherwise this doesn't work.
if [ "$number" -eq 0 ]
then
factorial=1
# Factorial of 0 = 1.
else
let "decrnum = number - 1"
fact $decrnum # Recursive function call (the function calls itself).
let "factorial = $number * $?"
fi
return $factorial
}
fact $1
echo "Factorial of $1 is $?."
exit 0

Also see Example A-15 for an example of recursion in a script. Be aware that recursion is resource-intensive
and executes slowly, and is therefore generally not appropriate in a script.

24.3. Recursion Without Local Variables
A function may recursively call itself even without use of local variables.

Example 24-16. The Fibonacci Sequence
#!/bin/bash
# fibo.sh : Fibonacci sequence (recursive)
# Author: M. Cooper
# License: GPL3

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#
#
#
#
#
#

--------------------------------Fibo(0) = 0
Fibo(1) = 1
else
Fibo(j) = Fibo(j-1) + Fibo(j-2)
---------------------------------

MAXTERM=15
MINIDX=2

# Number of terms (+1) to generate.
# If idx is less than 2, then Fibo(idx) = idx.

Fibonacci ()
{
idx=$1
# Doesn't need to be local. Why not?
if [ "$idx" -lt "$MINIDX" ]
then
echo "$idx" # First two terms are 0 1 ... see above.
else
(( --idx )) # j-1
term1=$( Fibonacci $idx )
# Fibo(j-1)
(( --idx )) # j-2
term2=$( Fibonacci $idx )

#

Fibo(j-2)

echo $(( term1 + term2 ))
fi
# An ugly, ugly kludge.
# The more elegant implementation of recursive fibo in C
#+ is a straightforward translation of the algorithm in lines 7 - 10.
}
for i in $(seq 0 $MAXTERM)
do # Calculate $MAXTERM+1 terms.
FIBO=$(Fibonacci $i)
echo -n "$FIBO "
done
# 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610
# Takes a while, doesn't it? Recursion in a script is slow.
echo
exit 0

Example 24-17. The Towers of Hanoi
#! /bin/bash
#
# The Towers Of Hanoi
# Bash script
# Copyright (C) 2000 Amit Singh. All Rights Reserved.
# http://hanoi.kernelthread.com
#
# Tested under Bash version 2.05b.0(13)-release.
# Also works under Bash version 3.x.
#
# Used in "Advanced Bash Scripting Guide"
#+ with permission of script author.
# Slightly modified and commented by ABS author.
#=================================================================#

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# The Tower of Hanoi is a mathematical puzzle attributed to
#+ Edouard Lucas, a nineteenth-century French mathematician.
#
# There are three vertical posts set in a base.
# The first post has a set of annular rings stacked on it.
# These rings are disks with a hole drilled out of the center,
#+ so they can slip over the posts and rest flat.
# The rings have different diameters, and they stack in ascending
#+ order, according to size.
# The smallest ring is on top, and the largest on the bottom.
#
# The task is to transfer the stack of rings
#+ to one of the other posts.
# You can move only one ring at a time to another post.
# You are permitted to move rings back to the original post.
# You may place a smaller ring atop a larger one,
#+ but *not* vice versa.
# Again, it is forbidden to place a larger ring atop a smaller one.
#
# For a small number of rings, only a few moves are required.
#+ For each additional ring,
#+ the required number of moves approximately doubles,
#+ and the "strategy" becomes increasingly complicated.
#
# For more information, see http://hanoi.kernelthread.com
#+ or pp. 186-92 of _The Armchair Universe_ by A.K. Dewdney.
#
#
#
...
...
...
#
| |
| |
| |
#
_|_|_
| |
| |
#
|_____|
| |
| |
#
|_______|
| |
| |
#
|_________|
| |
| |
#
|___________|
| |
| |
#
|
|
| |
| |
# .--------------------------------------------------------------.
# |**************************************************************|
#
#1
#2
#3
#
#=================================================================#

E_NOPARAM=66 # No parameter passed to script.
E_BADPARAM=67 # Illegal number of disks passed to script.
Moves=
# Global variable holding number of moves.
# Modification to original script.
dohanoi() {
# Recursive function.
case $1 in
0)
;;
*)
dohanoi "$(($1-1))" $2 $4 $3
echo move $2 "-->" $3
((Moves++))
# Modification to original script.
dohanoi "$(($1-1))" $4 $3 $2
;;
esac
}
case $# in

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1) case $(($1>0)) in
# Must have at least one disk.
1) # Nested case statement.
dohanoi $1 1 3 2
echo "Total moves = $Moves"
# 2^n - 1, where n = # of disks.
exit 0;
;;
*)
echo "$0: illegal value for number of disks";
exit $E_BADPARAM;
;;
esac
;;
*)
echo "usage: $0 N"
echo "
Where \"N\" is the number of disks."
exit $E_NOPARAM;
;;
esac
#
#
#
#
#
#

Exercises:
--------1) Would commands beyond this point ever be executed?
Why not? (Easy)
2) Explain the workings of the workings of the "dohanoi" function.
(Difficult -- see the Dewdney reference, above.)

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414

Chapter 25. Aliases
A Bash alias is essentially nothing more than a keyboard shortcut, an abbreviation, a means of avoiding
typing a long command sequence. If, for example, we include alias lm="ls -l | more" in the ~/.bashrc
file, then each lm [113] typed at the command-line will automatically be replaced by a ls -l | more. This can
save a great deal of typing at the command-line and avoid having to remember complex combinations of
commands and options. Setting alias rm="rm -i" (interactive mode delete) may save a good deal of grief,
since it can prevent inadvertently deleting important files.
In a script, aliases have very limited usefulness. It would be nice if aliases could assume some of the
functionality of the C preprocessor, such as macro expansion, but unfortunately Bash does not expand
arguments within the alias body. [114] Moreover, a script fails to expand an alias itself within "compound
constructs," such as if/then statements, loops, and functions. An added limitation is that an alias will not
expand recursively. Almost invariably, whatever we would like an alias to do could be accomplished much
more effectively with a function.

Example 25-1. Aliases within a script
#!/bin/bash
# alias.sh
shopt -s expand_aliases
# Must set this option, else script will not expand aliases.

# First, some fun.
alias Jesse_James='echo "\"Alias Jesse James\" was a 1959 comedy starring Bob Hope."'
Jesse_James
echo; echo; echo;
alias ll="ls -l"
# May use either single (') or double (") quotes to define an alias.
echo "Trying aliased \"ll\":"
ll /usr/X11R6/bin/mk*
#* Alias works.
echo
directory=/usr/X11R6/bin/
prefix=mk* # See if wild card causes problems.
echo "Variables \"directory\" + \"prefix\" = $directory$prefix"
echo
alias lll="ls -l $directory$prefix"
echo "Trying aliased \"lll\":"
lll
# Long listing of all files in /usr/X11R6/bin stating with mk.
# An alias can handle concatenated variables -- including wild card -- o.k.

TRUE=1

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echo
if [ TRUE ]
then
alias rr="ls -l"
echo "Trying aliased \"rr\" within if/then statement:"
rr /usr/X11R6/bin/mk*
#* Error message results!
# Aliases not expanded within compound statements.
echo "However, previously expanded alias still recognized:"
ll /usr/X11R6/bin/mk*
fi
echo
count=0
while [ $count -lt 3 ]
do
alias rrr="ls -l"
echo "Trying aliased \"rrr\" within \"while\" loop:"
rrr /usr/X11R6/bin/mk*
#* Alias will not expand here either.
# alias.sh: line 57: rrr: command not found
let count+=1
done
echo; echo
alias xyz='cat $0'

# Script lists itself.
# Note strong quotes.

xyz
# This seems to work,
#+ although the Bash documentation suggests that it shouldn't.
#
# However, as Steve Jacobson points out,
#+ the "$0" parameter expands immediately upon declaration of the alias.
exit 0

The unalias command removes a previously set alias.

Example 25-2. unalias: Setting and unsetting an alias
#!/bin/bash
# unalias.sh
shopt -s expand_aliases

# Enables alias expansion.

alias llm='ls -al | more'
llm
echo
unalias llm
# Unset alias.
llm
# Error message results, since 'llm' no longer recognized.
exit 0
bash$ ./unalias.sh
total 6
drwxrwxr-x
2 bozo

Chapter 25. Aliases

bozo

3072 Feb

6 14:04 .

416

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drwxr-xr-x
-rwxr-xr-x

40 bozo
1 bozo

bozo
bozo

2048 Feb
199 Feb

6 14:04 ..
6 14:04 unalias.sh

./unalias.sh: llm: command not found

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417

Chapter 26. List Constructs
The and list and or list constructs provide a means of processing a number of commands consecutively. These
can effectively replace complex nested if/then or even case statements.
Chaining together commands
and list
command-1 && command-2 && command-3 && ... command-n

Each command executes in turn, provided that the previous command has given a return value of
true (zero). At the first false (non-zero) return, the command chain terminates (the first command
returning false is the last one to execute).
Example 26-1. Using an and list to test for command-line arguments
#!/bin/bash
# and list
if [ ! -z "$1" ] && echo "Argument #1 = $1" && [ ! -z "$2" ] && \
#
^^
^^
^^
echo "Argument #2 = $2"
then
echo "At least 2 arguments passed to script."
# All the chained commands return true.
else
echo "Fewer than 2 arguments passed to script."
# At least one of the chained commands returns false.
fi
# Note that "if [ ! -z $1 ]" works, but its alleged equivalent,
#
"if [ -n $1 ]" does not.
#
However, quoting fixes this.
# if "[ -n "$1" ]" works.
#
^ ^
Careful!
# It is always best to QUOTE the variables being tested.

# This
if [ !
then
echo
fi
if [ !
then
echo
echo
else
echo
fi
# It's

accomplishes the same thing, using "pure" if/then statements.
-z "$1" ]
"Argument #1 = $1"
-z "$2" ]
"Argument #2 = $2"
"At least 2 arguments passed to script."
"Fewer than 2 arguments passed to script."
longer and more ponderous than using an "and list".

exit $?

Example 26-2. Another command-line arg test using an and list
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#!/bin/bash
ARGS=1
E_BADARGS=85

# Number of arguments expected.
# Exit value if incorrect number of args passed.

test $# -ne $ARGS && \
#
^^^^^^^^^^^^ condition #1
echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS
#
^^
# If condition #1 tests true (wrong number of args passed to script),
#+ then the rest of the line executes, and script terminates.
# Line below executes only if the above test fails.
echo "Correct number of arguments passed to this script."
exit 0
# To check exit value, do a "echo $?" after script termination.

Of course, an and list can also set variables to a default value.
arg1=$@ && [ -z "$arg1" ] && arg1=DEFAULT
# Set $arg1 to command-line arguments, if any.
# But . . . set to DEFAULT if not specified on command-line.

or list
command-1 || command-2 || command-3 || ... command-n

Each command executes in turn for as long as the previous command returns false. At the first true
return, the command chain terminates (the first command returning true is the last one to execute).
This is obviously the inverse of the "and list".
Example 26-3. Using or lists in combination with an and list
#!/bin/bash
#
#

delete.sh, a not-so-cunning file deletion utility.
Usage: delete filename

E_BADARGS=85
if [ -z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS # No arg? Bail out.
else
file=$1
# Set filename.
fi

[ ! -f "$file" ] && echo "File \"$file\" not found. \
Cowardly refusing to delete a nonexistent file."
# AND LIST, to give error message if file not present.
# Note echo message continuing on to a second line after an escape.
[ ! -f "$file" ] || (rm -f $file; echo "File \"$file\" deleted.")
# OR LIST, to delete file if present.
# Note logic inversion above.

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# AND LIST executes on true, OR LIST on false.
exit $?

If the first command in an or list returns true, it will execute.
# ==>
#+==>
#+==>
# ==>

The following snippets from the /etc/rc.d/init.d/single
script by Miquel van Smoorenburg
illustrate use of "and" and "or" lists.
"Arrowed" comments added by document author.

[ -x /usr/bin/clear ] && /usr/bin/clear
# ==> If /usr/bin/clear exists, then invoke it.
# ==> Checking for the existence of a command before calling it
#+==> avoids error messages and other awkward consequences.
# ==> . . .
# If they want to run something in single user mode, might as well run it...
for i in /etc/rc1.d/S[0-9][0-9]* ; do
# Check if the script is there.
[ -x "$i" ] || continue
# ==> If corresponding file in $PWD *not* found,
#+==> then "continue" by jumping to the top of the loop.
# Reject backup files and files generated by rpm.
case "$1" in
*.rpmsave|*.rpmorig|*.rpmnew|*~|*.orig)
continue;;
esac
[ "$i" = "/etc/rc1.d/S00single" ] && continue
# ==> Set script name, but don't execute it yet.
$i start
done
# ==> . . .

The exit status of an and list or an or list is the exit status of the last command executed.
Clever combinations of and and or lists are possible, but the logic may easily become convoluted and require
close attention to operator precedence rules, and possibly extensive debugging.
false && true || echo false
# Same result as
( false && true ) || echo false
# But NOT
false && ( true || echo false )

# false

# false
# (nothing echoed)

# Note left-to-right grouping and evaluation of statements,
#+ since the logic operators "&&" and "||" have equal precedence.
#

It's usually best to avoid such complexities.

#

Thanks, S.C.

See Example A-7 and Example 7-4 for illustrations of using and / or list constructs to test variables.

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Chapter 27. Arrays
Newer versions of Bash support one-dimensional arrays. Array elements may be initialized with the
variable[xx] notation. Alternatively, a script may introduce the entire array by an explicit declare -a
variable statement. To dereference (retrieve the contents of) an array element, use curly bracket notation,
that is, ${element[xx]}.

Example 27-1. Simple array usage
#!/bin/bash

area[11]=23
area[13]=37
area[51]=UFOs
#

Array members need not be consecutive or contiguous.

#
#
#
#+

Some members of the array can be left uninitialized.
Gaps in the array are okay.
In fact, arrays with sparse data ("sparse arrays")
are useful in spreadsheet-processing software.

echo -n "area[11] = "
echo ${area[11]}
#

{curly brackets} needed.

echo -n "area[13] = "
echo ${area[13]}
echo "Contents of area[51] are ${area[51]}."
# Contents of uninitialized array variable print blank (null variable).
echo -n "area[43] = "
echo ${area[43]}
echo "(area[43] unassigned)"
echo
# Sum of two array variables assigned to third
area[5]=`expr ${area[11]} + ${area[13]}`
echo "area[5] = area[11] + area[13]"
echo -n "area[5] = "
echo ${area[5]}
area[6]=`expr ${area[11]} + ${area[51]}`
echo "area[6] = area[11] + area[51]"
echo -n "area[6] = "
echo ${area[6]}
# This fails because adding an integer to a string is not permitted.
echo; echo; echo
# ----------------------------------------------------------------# Another array, "area2".

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# Another way of assigning array variables...
# array_name=( XXX YYY ZZZ ... )
area2=( zero one two three four )
echo -n "area2[0] = "
echo ${area2[0]}
# Aha, zero-based indexing (first element of array is [0], not [1]).
echo -n "area2[1] = "
echo ${area2[1]}
# [1] is second element of array.
# ----------------------------------------------------------------echo; echo; echo
#
#
#
#

----------------------------------------------Yet another array, "area3".
Yet another way of assigning array variables...
array_name=([xx]=XXX [yy]=YYY ...)

area3=([17]=seventeen [24]=twenty-four)
echo -n "area3[17] = "
echo ${area3[17]}
echo -n "area3[24] = "
echo ${area3[24]}
# ----------------------------------------------exit 0

As we have seen, a convenient way of initializing an entire array is the array=( element1 element2
... elementN ) notation.
base64_charset=( {A..Z} {a..z} {0..9} + / = )
# Using extended brace expansion
#+ to initialize the elements of the array.
# Excerpted from vladz's "base64.sh" script
#+ in the "Contributed Scripts" appendix.

Bash permits array operations on variables, even if the variables are not explicitly declared as arrays.
string=abcABC123ABCabc
echo ${string[@]}
echo ${string[*]}
echo ${string[0]}
echo ${string[1]}
echo ${#string[@]}

#
#
#
#
#
#
#
#

abcABC123ABCabc
abcABC123ABCabc
abcABC123ABCabc
No output!
Why?
1
One element in the array.
The string itself.

# Thank you, Michael Zick, for pointing this out.

Once again this demonstrates that Bash variables are untyped.
Example 27-2. Formatting a poem

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#!/bin/bash
# poem.sh: Pretty-prints one of the ABS Guide author's favorite poems.
# Lines of the poem (single stanza).
Line[1]="I do not know which to prefer,"
Line[2]="The beauty of inflections"
Line[3]="Or the beauty of innuendoes,"
Line[4]="The blackbird whistling"
Line[5]="Or just after."
# Note that quoting permits embedding whitespace.
# Attribution.
Attrib[1]=" Wallace Stevens"
Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\""
# This poem is in the Public Domain (copyright expired).
echo
tput bold

# Bold print.

for index in 1 2 3 4 5
# Five lines.
do
printf "
%s\n" "${Line[index]}"
done
for index in 1 2
do
printf "
done
tput sgr0

# Two attribution lines.
%s\n" "${Attrib[index]}"

# Reset terminal.
# See 'tput' docs.

echo
exit 0
# Exercise:
# -------# Modify this script to pretty-print a poem from a text data file.

Array variables have a syntax all their own, and even standard Bash commands and operators have special
options adapted for array use.

Example 27-3. Various array operations
#!/bin/bash
# array-ops.sh: More fun with arrays.

array=( zero one two three four five )
# Element 0
1
2
3
4
5
echo ${array[0]}
echo ${array:0}

echo ${array:1}

Chapter 27. Arrays

#
#
#
#+
#
#

zero
zero
Parameter expansion of first element,
starting at position # 0 (1st character).
ero
Parameter expansion of first element,

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Advanced Bash-Scripting Guide
#+ starting at position # 1 (2nd character).
echo "--------------"
echo ${#array[0]}

#
#
#
#
#

4
Length of first element of array.
4
Length of first element of array.
(Alternate notation)

echo ${#array[1]}

#
#
#

3
Length of second element of array.
Arrays in Bash have zero-based indexing.

echo ${#array[*]}

#
#
#
#

6
Number of elements in array.
6
Number of elements in array.

echo ${#array}

echo ${#array[@]}

echo "--------------"
array2=( [0]="first element" [1]="second element" [3]="fourth element" )
#
^
^
^
^
^
^
^
^
^
# Quoting permits embedding whitespace within individual array elements.
echo ${array2[0]}
echo ${array2[1]}
echo ${array2[2]}
echo ${array2[3]}
echo ${#array2[0]}
echo ${#array2[*]}

#
#
#
#
#
#
#

first element
second element
Skipped in initialization, and therefore null.
fourth element
13
(length of first element)
3
(number of elements in array)

exit

Many of the standard string operations work on arrays.

Example 27-4. String operations on arrays
#!/bin/bash
# array-strops.sh: String operations on arrays.
# Script by Michael Zick.
# Used in ABS Guide with permission.
# Fixups: 05 May 08, 04 Aug 08.
# In general, any string operation using the ${name ... } notation
#+ can be applied to all string elements in an array,
#+ with the ${name[@] ... } or ${name[*] ...} notation.

arrayZ=( one two three four five five )
echo
# Trailing Substring Extraction
echo ${arrayZ[@]:0}
# one two three four five five
#
^
All elements.

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echo ${arrayZ[@]:1}
#
^

# two three four five five
All elements following element[0].

echo ${arrayZ[@]:1:2}
#
^

# two three
Only the two elements after element[0].

echo "---------"

# Substring Removal
# Removes shortest match from front of string(s).
echo ${arrayZ[@]#f*r}
#
^

# one two three five five
# Applied to all elements of the array.
# Matches "four" and removes it.

# Longest match from front of string(s)
echo ${arrayZ[@]##t*e} # one two four five five
#
^^
# Applied to all elements of the array.
# Matches "three" and removes it.
# Shortest match from back of string(s)
echo ${arrayZ[@]%h*e}
# one two t four five five
#
^
# Applied to all elements of the array.
# Matches "hree" and removes it.
# Longest match from back
echo ${arrayZ[@]%%t*e} #
#
^^
#
#

of string(s)
one two four five five
Applied to all elements of the array.
Matches "three" and removes it.

echo "----------------------"

# Substring Replacement
# Replace first occurrence of substring with replacement.
echo ${arrayZ[@]/fiv/XYZ}
# one two three four XYZe XYZe
#
^
# Applied to all elements of the array.
# Replace all occurrences of substring.
echo ${arrayZ[@]//iv/YY}
# one two three four fYYe fYYe
# Applied to all elements of the array.
# Delete all occurrences of substring.
# Not specifing a replacement defaults to 'delete' ...
echo ${arrayZ[@]//fi/}
# one two three four ve ve
#
^^
# Applied to all elements of the array.
# Replace front-end occurrences of substring.
echo ${arrayZ[@]/#fi/XY}
# one two three four XYve XYve
#
^
# Applied to all elements of the array.
# Replace back-end occurrences of substring.
echo ${arrayZ[@]/%ve/ZZ}
# one two three four fiZZ fiZZ
#
^
# Applied to all elements of the array.
echo ${arrayZ[@]/%o/XX}
#
^

# one twXX three four five five
# Why?

echo "-----------------------------"

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replacement() {
echo -n "!!!"
}
echo ${arrayZ[@]/%e/$(replacement)}
#
^ ^^^^^^^^^^^^^^
# on!!! two thre!!! four fiv!!! fiv!!!
# The stdout of replacement() is the replacement string.
# Q.E.D: The replacement action is, in effect, an 'assignment.'
echo "------------------------------------"
# Accessing the "for-each":
echo ${arrayZ[@]//*/$(replacement optional_arguments)}
#
^^ ^^^^^^^^^^^^^
# !!! !!! !!! !!! !!! !!!
# Now, if Bash would only pass the matched string
#+ to the function being called . . .
echo
exit 0
# Before reaching for a Big Hammer -- Perl, Python, or all the rest -# recall:
#
$( ... ) is command substitution.
#
A function runs as a sub-process.
#
A function writes its output (if echo-ed) to stdout.
#
Assignment, in conjunction with "echo" and command substitution,
#+
can read a function's stdout.
#
The name[@] notation specifies (the equivalent of) a "for-each"
#+
operation.
# Bash is more powerful than you think!

Command substitution can construct the individual elements of an array.

Example 27-5. Loading the contents of a script into an array
#!/bin/bash
# script-array.sh: Loads this script into an array.
# Inspired by an e-mail from Chris Martin (thanks!).
script_contents=( $(cat "$0") )

# Stores contents of this script ($0)
#+ in an array.

for element in $(seq 0 $((${#script_contents[@]} - 1)))
do
# ${#script_contents[@]}
#+ gives number of elements in the array.
#
# Question:
# Why is seq 0 necessary?
# Try changing it to seq 1.
echo -n "${script_contents[$element]}"
# List each field of this script on a single line.
# echo -n "${script_contents[element]}" also works because of ${ ... }.
echo -n " -- "
# Use " -- " as a field separator.
done

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echo
exit 0
# Exercise:
# -------# Modify this script so it lists itself
#+ in its original format,
#+ complete with whitespace, line breaks, etc.

In an array context, some Bash builtins have a slightly altered meaning. For example, unset deletes array
elements, or even an entire array.

Example 27-6. Some special properties of arrays
#!/bin/bash
declare -a colors
# All subsequent commands in this script will treat
#+ the variable "colors" as an array.
echo "Enter your favorite colors (separated from each other by a space)."
read -a colors
# Enter at least 3 colors to demonstrate features below.
# Special option to 'read' command,
#+ allowing assignment of elements in an array.
echo
element_count=${#colors[@]}
# Special syntax to extract number of elements in array.
#
element_count=${#colors[*]} works also.
#
# The "@" variable allows word splitting within quotes
#+ (extracts variables separated by whitespace).
#
# This corresponds to the behavior of "$@" and "$*"
#+ in positional parameters.
index=0
while [ "$index" -lt "$element_count" ]
do
# List all the elements in the array.
echo ${colors[$index]}
#
${colors[index]} also works because it's within ${ ... } brackets.
let "index = $index + 1"
# Or:
#
((index++))
done
# Each array element listed on a separate line.
# If this is not desired, use echo -n "${colors[$index]} "
#
# Doing it with a "for" loop instead:
#
for i in "${colors[@]}"
#
do
#
echo "$i"
#
done
# (Thanks, S.C.)

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echo
# Again, list all the elements in the array, but using a more elegant method.
echo ${colors[@]}
# echo ${colors[*]} also works.
echo
# The "unset" command deletes elements of an array, or entire array.
unset colors[1]
# Remove 2nd element of array.
# Same effect as
colors[1]=
echo ${colors[@]}
# List array again, missing 2nd element.
unset colors

# Delete entire array.
# unset colors[*] and
#+ unset colors[@] also work.

echo; echo -n "Colors gone."
echo ${colors[@]}
# List array again, now empty.
exit 0

As seen in the previous example, either ${array_name[@]} or ${array_name[*]} refers to all the elements
of the array. Similarly, to get a count of the number of elements in an array, use either ${#array_name[@]}
or ${#array_name[*]}. ${#array_name} is the length (number of characters) of ${array_name[0]}, the first
element of the array.

Example 27-7. Of empty arrays and empty elements
#!/bin/bash
# empty-array.sh
# Thanks to Stephane Chazelas for the original example,
#+ and to Michael Zick and Omair Eshkenazi, for extending it.
# And to Nathan Coulter for clarifications and corrections.

# An empty array is not the same as an array with empty elements.
array0=( first second third )
array1=( '' )
# "array1" consists of one empty element.
array2=( )
# No elements . . . "array2" is empty.
array3=(
)
# What about this array?

echo
ListArray()
{
echo
echo "Elements in array0: ${array0[@]}"
echo "Elements in array1: ${array1[@]}"
echo "Elements in array2: ${array2[@]}"
echo "Elements in array3: ${array3[@]}"
echo
echo "Length of first element in array0 = ${#array0}"
echo "Length of first element in array1 = ${#array1}"
echo "Length of first element in array2 = ${#array2}"
echo "Length of first element in array3 = ${#array3}"
echo
echo "Number of elements in array0 = ${#array0[*]}" # 3

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echo "Number of elements in array1 = ${#array1[*]}"
echo "Number of elements in array2 = ${#array2[*]}"
echo "Number of elements in array3 = ${#array3[*]}"
}

# 1
# 0
# 0

(Surprise!)

# ===================================================================
ListArray
# Try extending those arrays.
# Adding
array0=(
array1=(
array2=(
array3=(

an element to an array.
"${array0[@]}" "new1" )
"${array1[@]}" "new1" )
"${array2[@]}" "new1" )
"${array3[@]}" "new1" )

ListArray
# or
array0[${#array0[*]}]="new2"
array1[${#array1[*]}]="new2"
array2[${#array2[*]}]="new2"
array3[${#array3[*]}]="new2"
ListArray
# When extended as above, arrays are 'stacks' ...
# Above is the 'push' ...
# The stack 'height' is:
height=${#array2[@]}
echo
echo "Stack height for array2 = $height"
# The 'pop' is:
unset array2[${#array2[@]}-1]
# Arrays are zero-based,
height=${#array2[@]}
#+ which means first element has index 0.
echo
echo "POP"
echo "New stack height for array2 = $height"
ListArray
# List only 2nd and 3rd elements of array0.
from=1
# Zero-based numbering.
to=2
array3=( ${array0[@]:1:2} )
echo
echo "Elements in array3: ${array3[@]}"
# Works like a string (array of characters).
# Try some other "string" forms.
# Replacement:
array4=( ${array0[@]/second/2nd} )
echo
echo "Elements in array4: ${array4[@]}"
# Replace all matching wildcarded string.
array5=( ${array0[@]//new?/old} )
echo
echo "Elements in array5: ${array5[@]}"

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# Just when you are getting the feel for this . . .
array6=( ${array0[@]#*new} )
echo # This one might surprise you.
echo "Elements in array6: ${array6[@]}"
array7=( ${array0[@]#new1} )
echo # After array6 this should not be a surprise.
echo "Elements in array7: ${array7[@]}"
# Which looks a lot like . . .
array8=( ${array0[@]/new1/} )
echo
echo "Elements in array8: ${array8[@]}"
#

So what can one say about this?

#
#+
#
#
#+
#

The string operations are performed on
each of the elements in var[@] in succession.
Therefore : Bash supports string vector operations.
If the result is a zero length string,
that element disappears in the resulting assignment.
However, if the expansion is in quotes, the null elements remain.

# Michael Zick:
Question, are those strings hard or soft quotes?
# Nathan Coulter: There is no such thing as "soft quotes."
#!
What's really happening is that
#!+
the pattern matching happens after
#!+
all the other expansions of [word]
#!+
in cases like ${parameter#word}.

zap='new*'
array9=( ${array0[@]/$zap/} )
echo
echo "Number of elements in array9: ${#array9[@]}"
array9=( "${array0[@]/$zap/}" )
echo "Elements in array9: ${array9[@]}"
# This time the null elements remain.
echo "Number of elements in array9: ${#array9[@]}"

# Just when you thought you were still in Kansas . . .
array10=( ${array0[@]#$zap} )
echo
echo "Elements in array10: ${array10[@]}"
# But, the asterisk in zap won't be interpreted if quoted.
array10=( ${array0[@]#"$zap"} )
echo
echo "Elements in array10: ${array10[@]}"
# Well, maybe we _are_ still in Kansas . . .
# (Revisions to above code block by Nathan Coulter.)

#
#

Compare array7 with array10.
Compare array8 with array9.

#
#
#
#+
#

Reiterating: No such thing as soft quotes!
Nathan Coulter explains:
Pattern matching of 'word' in ${parameter#word} is done after
parameter expansion and *before* quote removal.
In the normal case, pattern matching is done *after* quote removal.

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exit

The relationship of ${array_name[@]} and ${array_name[*]} is analogous to that between $@ and $*. This
powerful array notation has a number of uses.

# Copying an array.
array2=( "${array1[@]}" )
# or
array2="${array1[@]}"
#
# However, this fails with "sparse" arrays,
#+ arrays with holes (missing elements) in them,
#+ as Jochen DeSmet points out.
# -----------------------------------------array1[0]=0
# array1[1] not assigned
array1[2]=2
array2=( "${array1[@]}" )
# Copy it?
echo ${array2[0]}
# 0
echo ${array2[2]}
# (null), should be 2
# ------------------------------------------

# Adding an element to an array.
array=( "${array[@]}" "new element" )
# or
array[${#array[*]}]="new element"
# Thanks, S.C.

The array=( element1 element2 ... elementN ) initialization operation, with the help of command
substitution, makes it possible to load the contents of a text file into an array.
#!/bin/bash
filename=sample_file
#
#
#
#

cat sample_file
1 a b c
2 d e fg

declare -a array1
array1=( `cat "$filename"`)
# Loads contents
#
List file to stdout
#+ of $filename into array1.
#
# array1=( `cat "$filename" | tr '\n' ' '`)
#
change linefeeds in file to spaces.
# Not necessary because Bash does word splitting,
#+ changing linefeeds to spaces.
echo ${array1[@]}
#
#

Chapter 27. Arrays

# List the array.
1 a b c 2 d e fg

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# Each whitespace-separated "word" in the file
#+ has been assigned to an element of the array.
element_count=${#array1[*]}
echo $element_count

# 8

Clever scripting makes it possible to add array operations.

Example 27-8. Initializing arrays
#! /bin/bash
# array-assign.bash
# Array operations are Bash-specific,
#+ hence the ".bash" in the script name.
#
#
#
#
#

Copyright (c) Michael S. Zick, 2003, All rights reserved.
License: Unrestricted reuse in any form, for any purpose.
Version: $ID$
Clarification and additional comments by William Park.

# Based on an example provided by Stephane Chazelas
#+ which appeared in an earlier version of the
#+ Advanced Bash Scripting Guide.
# Output format of the 'times' command:
# User CPU  System CPU
# User CPU of dead children  System CPU of dead children
#
#+
#
#+
#
#+

Bash has two versions of assigning all elements of an array
to a new array variable.
Both drop 'null reference' elements
in Bash versions 2.04 and later.
An additional array assignment that maintains the relationship of
[subscript]=value for arrays may be added to newer versions.

# Constructs a large array using an internal command,
#+ but anything creating an array of several thousand elements
#+ will do just fine.
declare -a bigOne=( /dev/* ) # All the files in /dev . . .
echo
echo 'Conditions: Unquoted, default IFS, All-Elements-Of'
echo "Number of elements in array is ${#bigOne[@]}"
# set -vx

echo
echo '- - testing: =( ${array[@]} ) - -'
times
declare -a bigTwo=( ${bigOne[@]} )
# Note parens:
^
^
times

echo

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echo '- - testing: =${array[@]} - -'
times
declare -a bigThree=${bigOne[@]}
# No parentheses this time.
times
#
#+
#
#
#+
#+
#
#
#
#
#
#

Comparing the numbers shows that the second form, pointed out
by Stephane Chazelas, is faster.
As William Park explains:
The bigTwo array assigned element by element (because of parentheses),
whereas bigThree assigned as a single string.
So, in essence, you have:
bigTwo=( [0]="..." [1]="..." [2]="..." ... )
bigThree=( [0]="... ... ..." )
Verify this by:

echo ${bigTwo[0]}
echo ${bigThree[0]}

# I will continue to use the first form in my example descriptions
#+ because I think it is a better illustration of what is happening.
# The reusable portions of my examples will actual contain
#+ the second form where appropriate because of the speedup.
# MSZ: Sorry about that earlier oversight folks.

#
#
#
#+
#+
#
#+
#

Note:
---The "declare -a" statements in lines 32
are not strictly necessary, since it is
in the Array=( ... ) assignment form.
However, eliminating these declarations
the execution of the following sections
Try it, and see.

and 44
implicit
slows down
of the script.

exit 0

Adding a superfluous declare -a statement to an array declaration may speed up execution of subsequent
operations on the array.

Example 27-9. Copying and concatenating arrays
#! /bin/bash
# CopyArray.sh
#
# This script written by Michael Zick.
# Used here with permission.
# How-To "Pass by Name & Return by Name"
#+ or "Building your own assignment statement".

CpArray_Mac() {
# Assignment Command Statement Builder

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echo
echo
echo
echo
echo

-n
-n
-n
-n
-n

'eval '
"$2"
'=( ${'
"$1"
'[@]} )'

# Destination name
# Source name

# That could all be a single command.
# Matter of style only.
}
declare -f CopyArray
CopyArray=CpArray_Mac

# Function "Pointer"
# Statement Builder

Hype()
{
# Hype the array named $1.
# (Splice it together with array containing "Really Rocks".)
# Return in array named $2.
local -a TMP
local -a hype=( Really Rocks )
$($CopyArray $1 TMP)
TMP=( ${TMP[@]} ${hype[@]} )
$($CopyArray TMP $2)
}
declare -a before=( Advanced Bash Scripting )
declare -a after
echo "Array Before = ${before[@]}"
Hype before after
echo "Array After = ${after[@]}"
# Too much hype?
echo "What ${after[@]:3:2}?"
declare -a modest=( ${after[@]:2:1} ${after[@]:3:2} )
#
---- substring extraction ---echo "Array Modest = ${modest[@]}"
# What happened to 'before' ?
echo "Array Before = ${before[@]}"
exit 0

Example 27-10. More on concatenating arrays
#! /bin/bash
# array-append.bash
# Copyright (c) Michael S. Zick, 2003, All rights reserved.
# License: Unrestricted reuse in any form, for any purpose.
# Version: $ID$
#

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434

Advanced Bash-Scripting Guide
# Slightly modified in formatting by M.C.

# Array operations are Bash-specific.
# Legacy UNIX /bin/sh lacks equivalents.

# Pipe the output of this script to 'more'
#+ so it doesn't scroll off the terminal.
# Or, redirect output to a file.

declare -a array1=( zero1 one1 two1 )
# Subscript packed.
declare -a array2=( [0]=zero2 [2]=two2 [3]=three2 )
# Subscript sparse -- [1] is not defined.
echo
echo '- Confirm that the array is really subscript sparse. -'
echo "Number of elements: 4"
# Hard-coded for illustration.
for (( i = 0 ; i < 4 ; i++ ))
do
echo "Element [$i]: ${array2[$i]}"
done
# See also the more general code example in basics-reviewed.bash.

declare -a dest
# Combine (append) two arrays into a third array.
echo
echo 'Conditions: Unquoted, default IFS, All-Elements-Of operator'
echo '- Undefined elements not present, subscripts not maintained. -'
# # The undefined elements do not exist; they are not being dropped.
dest=( ${array1[@]} ${array2[@]} )
# dest=${array1[@]}${array2[@]}

# Strange results, possibly a bug.

# Now, list the result.
echo
echo '- - Testing Array Append - -'
cnt=${#dest[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${dest[$i]}"
done
# Assign an array to a single array element (twice).
dest[0]=${array1[@]}
dest[1]=${array2[@]}
# List the result.
echo
echo '- - Testing modified array - -'
cnt=${#dest[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${dest[$i]}"

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done
# Examine the modified second element.
echo
echo '- - Reassign and list second element - -'
declare -a subArray=${dest[1]}
cnt=${#subArray[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${subArray[$i]}"
done
#
#+
#+
#+

The assignment of an entire array to a
of another array using the '=${ ... }'
has converted the array being assigned
with the elements separated by a space

single element
array assignment
into a string,
(the first character of IFS).

# If the original elements didn't contain whitespace . . .
# If the original array isn't subscript sparse . . .
# Then we could get the original array structure back again.
# Restore from the modified second element.
echo
echo '- - Listing restored element - -'
declare -a subArray=( ${dest[1]} )
cnt=${#subArray[@]}
echo "Number of elements: $cnt"
for (( i = 0 ; i < cnt ; i++ ))
do
echo "Element [$i]: ${subArray[$i]}"
done
echo '- - Do not depend on this behavior. - -'
echo '- - This behavior is subject to change - -'
echo '- - in versions of Bash newer than version 2.05b - -'
# MSZ: Sorry about any earlier confusion folks.
exit 0

-Arrays permit deploying old familiar algorithms as shell scripts. Whether this is necessarily a good idea is left
for the reader to decide.

Example 27-11. The Bubble Sort
#!/bin/bash
# bubble.sh: Bubble sort, of sorts.
# Recall the algorithm for a bubble sort. In this particular version...
# With each successive pass through the array to be sorted,
#+ compare two adjacent elements, and swap them if out of order.
# At the end of the first pass, the "heaviest" element has sunk to bottom.

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#
#
#
#

At the end of the second pass, the next "heaviest" one has sunk next to bottom.
And so forth.
This means that each successive pass needs to traverse less of the array.
You will therefore notice a speeding up in the printing of the later passes.

exchange()
{
# Swaps two members of the array.
local temp=${Countries[$1]} # Temporary storage
#+ for element getting swapped out.
Countries[$1]=${Countries[$2]}
Countries[$2]=$temp
return
}
declare -a Countries

# Declare array,
#+ optional here since it's initialized below.

# Is it permissable to split an array variable over multiple lines
#+ using an escape (\)?
# Yes.
Countries=(Netherlands Ukraine Zaire Turkey Russia Yemen Syria \
Brazil Argentina Nicaragua Japan Mexico Venezuela Greece England \
Israel Peru Canada Oman Denmark Wales France Kenya \
Xanadu Qatar Liechtenstein Hungary)
# "Xanadu" is the mythical place where, according to Coleridge,
#+ Kubla Khan did a pleasure dome decree.

clear

# Clear the screen to start with.

echo "0: ${Countries[*]}"

# List entire array at pass 0.

number_of_elements=${#Countries[@]}
let "comparisons = $number_of_elements - 1"
count=1 # Pass number.
while [ "$comparisons" -gt 0 ]
do
index=0

# Beginning of outer loop

# Reset index to start of array after each pass.

while [ "$index" -lt "$comparisons" ] # Beginning of inner loop
do
if [ ${Countries[$index]} \> ${Countries[`expr $index + 1`]} ]
# If out of order...
# Recalling that \> is ASCII comparison operator
#+ within single brackets.
# if [[ ${Countries[$index]} > ${Countries[`expr $index + 1`]} ]]
#+ also works.
then
exchange $index `expr $index + 1` # Swap.
fi
let "index += 1" # Or,
index+=1
on Bash, ver. 3.1 or newer.
done # End of inner loop

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# ---------------------------------------------------------------------# Paulo Marcel Coelho Aragao suggests for-loops as a simpler altenative.
#
# for (( last = $number_of_elements - 1 ; last > 0 ; last-- ))
##
Fix by C.Y. Hunt
^
(Thanks!)
# do
#
for (( i = 0 ; i < last ; i++ ))
#
do
#
[[ "${Countries[$i]}" > "${Countries[$((i+1))]}" ]] \
#
&& exchange $i $((i+1))
#
done
# done
# ----------------------------------------------------------------------

let "comparisons -= 1" # Since "heaviest" element bubbles to bottom,
#+ we need do one less comparison each pass.
echo
echo "$count: ${Countries[@]}"
echo
let "count += 1"
done

# Print resultant array at end of each pass.
# Increment pass count.
# End of outer loop
# All done.

exit 0

--

Is it possible to nest arrays within arrays?
#!/bin/bash
# "Nested" array.
# Michael Zick provided this example,
#+ with corrections and clarifications by William Park.
AnArray=( $(ls --inode --ignore-backups --almost-all \
--directory --full-time --color=none --time=status \
--sort=time -l ${PWD} ) ) # Commands and options.
# Spaces are significant . . . and don't quote anything in the above.
SubArray=( ${AnArray[@]:11:1} ${AnArray[@]:6:5} )
# This array has six elements:
#+
SubArray=( [0]=${AnArray[11]} [1]=${AnArray[6]} [2]=${AnArray[7]}
#
[3]=${AnArray[8]} [4]=${AnArray[9]} [5]=${AnArray[10]} )
#
# Arrays in Bash are (circularly) linked lists
#+ of type string (char *).
# So, this isn't actually a nested array,
#+ but it's functionally similar.
echo "Current directory and date of last status change:"
echo "${SubArray[@]}"
exit 0

--

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Embedded arrays in combination with indirect references create some fascinating possibilities

Example 27-12. Embedded arrays and indirect references
#!/bin/bash
# embedded-arrays.sh
# Embedded arrays and indirect references.
# This script by Dennis Leeuw.
# Used with permission.
# Modified by document author.

ARRAY1=(
VAR1_1=value11
VAR1_2=value12
VAR1_3=value13
)
ARRAY2=(
VARIABLE="test"
STRING="VAR1=value1 VAR2=value2 VAR3=value3"
ARRAY21=${ARRAY1[*]}
)
# Embed ARRAY1 within this second array.
function print () {
OLD_IFS="$IFS"
IFS=$'\n'

# To print each array element
#+ on a separate line.
TEST1="ARRAY2[*]"
local ${!TEST1} # See what happens if you delete this line.
# Indirect reference.
# This makes the components of $TEST1
#+ accessible to this function.

# Let's see what we've got so far.
echo
echo "\$TEST1 = $TEST1"
# Just the name of the variable.
echo; echo
echo "{\$TEST1} = ${!TEST1}" # Contents of the variable.
# That's what an indirect
#+ reference does.
echo
echo "-------------------------------------------"; echo
echo

# Print variable
echo "Variable VARIABLE: $VARIABLE"
# Print a string element
IFS="$OLD_IFS"
TEST2="STRING[*]"
local ${!TEST2}
# Indirect reference (as above).
echo "String element VAR2: $VAR2 from STRING"
# Print an array element
TEST2="ARRAY21[*]"

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local ${!TEST2}
# Indirect reference (as above).
echo "Array element VAR1_1: $VAR1_1 from ARRAY21"
}
print
echo
exit 0
#
As the author of the script notes,
#+ "you can easily expand it to create named-hashes in bash."
#
(Difficult) exercise for the reader: implement this.

--

Arrays enable implementing a shell script version of the Sieve of Eratosthenes. Of course, a resource-intensive
application of this nature should really be written in a compiled language, such as C. It runs excruciatingly
slowly as a script.

Example 27-13. The Sieve of Eratosthenes
#!/bin/bash
# sieve.sh (ex68.sh)
# Sieve of Eratosthenes
# Ancient algorithm for finding prime numbers.
# This runs a couple of orders of magnitude slower
#+ than the equivalent program written in C.
LOWER_LIMIT=1
# Starting with 1.
UPPER_LIMIT=1000
# Up to 1000.
# (You may set this higher . . . if you have time on your hands.)
PRIME=1
NON_PRIME=0
let SPLIT=UPPER_LIMIT/2
# Optimization:
# Need to test numbers only halfway to upper limit. Why?

declare -a Primes
# Primes[] is an array.

initialize ()
{
# Initialize the array.
i=$LOWER_LIMIT
until [ "$i" -gt "$UPPER_LIMIT" ]
do
Primes[i]=$PRIME
let "i += 1"
done
# Assume all array members guilty (prime)
#+ until proven innocent.
}

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440

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print_primes ()
{
# Print out the members of the Primes[] array tagged as prime.
i=$LOWER_LIMIT
until [ "$i" -gt "$UPPER_LIMIT" ]
do
if [ "${Primes[i]}" -eq "$PRIME" ]
then
printf "%8d" $i
# 8 spaces per number gives nice, even columns.
fi
let "i += 1"
done
}
sift () # Sift out the non-primes.
{
let i=$LOWER_LIMIT+1
# Let's start with 2.
until [ "$i" -gt "$UPPER_LIMIT" ]
do
if [ "${Primes[i]}" -eq "$PRIME" ]
# Don't bother sieving numbers already sieved (tagged as non-prime).
then
t=$i
while [ "$t" -le "$UPPER_LIMIT" ]
do
let "t += $i "
Primes[t]=$NON_PRIME
# Tag as non-prime all multiples.
done
fi
let "i += 1"
done

}

# ==============================================
# main ()
# Invoke the functions sequentially.
initialize
sift
print_primes
# This is what they call structured programming.
# ==============================================

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echo
exit 0

# -------------------------------------------------------- #
# Code below line will not execute, because of 'exit.'
# This improved version of the Sieve, by Stephane Chazelas,
#+ executes somewhat faster.
# Must invoke with command-line argument (limit of primes).
UPPER_LIMIT=$1
let SPLIT=UPPER_LIMIT/2

# From command-line.
# Halfway to max number.

Primes=( '' $(seq $UPPER_LIMIT) )
i=1
until (( ( i += 1 ) > SPLIT )) # Need check only halfway.
do
if [[ -n $Primes[i] ]]
then
t=$i
until (( ( t += i ) > UPPER_LIMIT ))
do
Primes[t]=
done
fi
done
echo ${Primes[*]}
exit $?

Example 27-14. The Sieve of Eratosthenes, Optimized
#!/bin/bash
# Optimized Sieve of Eratosthenes
# Script by Jared Martin, with very minor changes by ABS Guide author.
# Used in ABS Guide with permission (thanks!).
# Based on script in Advanced Bash Scripting Guide.
# http://tldp.org/LDP/abs/html/arrays.html#PRIMES0 (ex68.sh).
# http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf (reference)
# Check results against http://primes.utm.edu/lists/small/1000.txt
# Necessary but not sufficient would be, e.g.,
#
(($(sieve 7919 | wc -w) == 1000)) && echo "7919 is the 1000th prime"
UPPER_LIMIT=${1:?"Need an upper limit of primes to search."}
Primes=( '' $(seq ${UPPER_LIMIT}) )
typeset -i i t
Primes[i=1]='' # 1 is not a prime.
until (( ( i += 1 ) > (${UPPER_LIMIT}/i) )) # Need check only ith-way.
do
# Why?
if ((${Primes[t=i*(i-1), i]}))
# Obscure, but instructive, use of arithmetic expansion in subscript.

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then
until (( ( t += i ) > ${UPPER_LIMIT} ))
do Primes[t]=; done
fi
done
# echo ${Primes[*]}
echo
# Change to original script for pretty-printing (80-col. display).
printf "%8d" ${Primes[*]}
echo; echo
exit $?

Compare these array-based prime number generators with alternatives that do not use arrays, Example A-15,
and Example 16-46.
-Arrays lend themselves, to some extent, to emulating data structures for which Bash has no native support.

Example 27-15. Emulating a push-down stack
#!/bin/bash
# stack.sh: push-down stack simulation
# Similar to the CPU stack, a push-down stack stores data items
#+ sequentially, but releases them in reverse order, last-in first-out.

BP=100

#
#

Base Pointer of stack array.
Begin at element 100.

SP=$BP

#
#

Stack Pointer.
Initialize it to "base" (bottom) of stack.

Data=

# Contents of stack location.
# Must use global variable,
#+ because of limitation on function return range.

# 100
# 99
# 98
# ...
#

Base pointer
First data item
Second data item
More data
Last data item

<-- Base Pointer

<-- Stack pointer

declare -a stack

push()
{
if [ -z "$1" ]
then
return
fi

# Push item on stack.

let "SP -= 1"

# Bump stack pointer.

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443

Advanced Bash-Scripting Guide
stack[$SP]=$1
return
}
pop()
{
Data=

# Pop item off stack.

if [ "$SP" -eq "$BP" ]
then
return
fi

# Stack empty?

# Empty out data item.

Data=${stack[$SP]}
let "SP += 1"
return
}

# This also keeps SP from getting past 100,
#+ i.e., prevents a runaway stack.

# Bump stack pointer.

status_report()
# Find out what's happening.
{
echo "-------------------------------------"
echo "REPORT"
echo "Stack Pointer = $SP"
echo "Just popped \""$Data"\" off the stack."
echo "-------------------------------------"
echo
}

# =======================================================
# Now, for some fun.
echo
# See if you can pop anything off empty stack.
pop
status_report
echo
push garbage
pop
status_report

# Garbage in, garbage out.

value1=23;
value2=skidoo;
value3=LAST;

push $value1
push $value2
push $value3

pop
status_report
pop
status_report
pop
status_report

# LAST
# skidoo
# 23
# Last-in, first-out!

# Notice how the stack pointer decrements with each push,
#+ and increments with each pop.
echo

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exit 0
# =======================================================

# Exercises:
# --------# 1) Modify the "push()" function to permit pushing
#
+ multiple element on the stack with a single function call.
# 2) Modify the "pop()" function to permit popping
#
+ multiple element from the stack with a single function call.
# 3)
#
#
+
#
+

Add error checking to the critical functions.
That is, return an error code, depending on
successful or unsuccessful completion of the operation,
and take appropriate action.

# 4) Using this script as a starting point,
#
+ write a stack-based 4-function calculator.

-Fancy manipulation of array "subscripts" may require intermediate variables. For projects involving this,
again consider using a more powerful programming language, such as Perl or C.

Example 27-16. Complex array application: Exploring a weird mathematical series
#!/bin/bash
# Douglas Hofstadter's notorious "Q-series":
# Q(1) = Q(2) = 1
# Q(n) = Q(n - Q(n-1)) + Q(n - Q(n-2)), for n>2
#
#+
#
#

This is a "chaotic" integer series with strange
and unpredictable behavior.
The first 20 terms of the series are:
1 1 2 3 3 4 5 5 6 6 6 8 8 8 10 9 10 11 11 12

# See Hofstadter's book, _Goedel, Escher, Bach: An Eternal Golden Braid_,
#+ p. 137, ff.

LIMIT=100
LINEWIDTH=20

# Number of terms to calculate.
# Number of terms printed per line.

Q[1]=1
Q[2]=1

# First two terms of series are 1.

echo
echo "Q-series [$LIMIT terms]:"
echo -n "${Q[1]} "
# Output first two terms.
echo -n "${Q[2]} "
for ((n=3; n <= $LIMIT; n++)) # C-like loop expression.
do
# Q[n] = Q[n - Q[n-1]] + Q[n - Q[n-2]] for n>2
#
Need to break the expression into intermediate terms,
#+
since Bash doesn't handle complex array arithmetic very well.

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let "n1 = $n - 1"
let "n2 = $n - 2"

# n-1
# n-2

t0=`expr $n - ${Q[n1]}`
t1=`expr $n - ${Q[n2]}`

# n - Q[n-1]
# n - Q[n-2]

T0=${Q[t0]}
T1=${Q[t1]}

# Q[n - Q[n-1]]
# Q[n - Q[n-2]]

Q[n]=`expr $T0 + $T1`
echo -n "${Q[n]} "

# Q[n - Q[n-1]] + Q[n - Q[n-2]]

if [ `expr $n % $LINEWIDTH` -eq 0 ]
# Format output.
then
#
^ modulo
echo # Break lines into neat chunks.
fi
done
echo
exit 0
#
#
#
#+

This is an iterative implementation of the Q-series.
The more intuitive recursive implementation is left as an exercise.
Warning: calculating this series recursively takes a VERY long time
via a script. C/C++ would be orders of magnitude faster.

--

Bash supports only one-dimensional arrays, though a little trickery permits simulating multi-dimensional
ones.

Example 27-17. Simulating a two-dimensional array, then tilting it
#!/bin/bash
# twodim.sh: Simulating a two-dimensional array.
# A one-dimensional array consists of a single row.
# A two-dimensional array stores rows sequentially.
Rows=5
Columns=5
# 5 X 5 Array.
declare -a alpha

# char alpha [Rows] [Columns];
# Unnecessary declaration. Why?

load_alpha ()
{
local rc=0
local index
for i in A B C D E F G H I J K L M N O P Q R S T U V W X Y
do
# Use different symbols if you like.
local row=`expr $rc / $Columns`
local column=`expr $rc % $Rows`
let "index = $row * $Rows + $column"

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alpha[$index]=$i
# alpha[$row][$column]
let "rc += 1"
done
# Simpler would be
#+
declare -a alpha=( A B C D E F G H I J K L M N O P Q R S T U V W X Y )
#+ but this somehow lacks the "flavor" of a two-dimensional array.
}
print_alpha ()
{
local row=0
local index
echo
while [ "$row" -lt "$Rows" ]
do

# Print out in "row major" order:
#+ columns vary,
#+ while row (outer loop) remains the same.

local column=0
echo -n "

"

#

Lines up "square" array with rotated one.

while [ "$column" -lt "$Columns" ]
do
let "index = $row * $Rows + $column"
echo -n "${alpha[index]} " # alpha[$row][$column]
let "column += 1"
done
let "row += 1"
echo
done
# The simpler equivalent is
#
echo ${alpha[*]} | xargs -n $Columns
echo
}
filter ()
{
echo -n "

# Filter out negative array indices.

"

# Provides the tilt.
# Explain how.

if [[ "$1" -ge 0 && "$1" -lt "$Rows" && "$2" -ge 0 && "$2" -lt "$Columns" ]]
then
let "index = $1 * $Rows + $2"
# Now, print it rotated.
echo -n " ${alpha[index]}"
#
alpha[$row][$column]
fi
}

rotate ()

#

Rotate the array 45 degrees --

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447

Advanced Bash-Scripting Guide
{
#+ "balance" it on its lower lefthand corner.
local row
local column
for (( row = Rows; row > -Rows; row-- ))
do
# Step through the array backwards. Why?
for (( column = 0; column < Columns; column++ ))
do
if [ "$row"
then
let "t1 =
let "t2 =
else
let "t1 =
let "t2 =
fi

-ge 0 ]
$column - $row"
$column"
$column"
$column + $row"

filter $t1 $t2

# Filter out negative array indices.
# What happens if you don't do this?

done
echo; echo
done
#
#+
#+
#
#+

Array rotation inspired by examples (pp. 143-146) in
"Advanced C Programming on the IBM PC," by Herbert Mayer
(see bibliography).
This just goes to show that much of what can be done in C
can also be done in shell scripting.

}

#--------------- Now, let the show begin. ------------#
load_alpha
# Load the array.
print_alpha
# Print it out.
rotate
# Rotate it 45 degrees counterclockwise.
#-----------------------------------------------------#
exit 0
# This is a rather contrived, not to mention inelegant simulation.
#
#
#
#
#
#
#
#
#
#
#

Exercises:
--------1) Rewrite the array loading and printing functions
in a more intuitive and less kludgy fashion.
2)

Figure out how the array rotation functions work.
Hint: think about the implications of backwards-indexing an array.

3)

Rewrite this script to handle a non-square array,
such as a 6 X 4 one.
Try to minimize "distortion" when the array is rotated.

A two-dimensional array is essentially equivalent to a one-dimensional one, but with additional addressing
modes for referencing and manipulating the individual elements by row and column position.
For an even more elaborate example of simulating a two-dimensional array, see Example A-10.
Chapter 27. Arrays

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Advanced Bash-Scripting Guide
-For more interesting scripts using arrays, see:
• Example 12-3
• Example 16-46
• Example A-22
• Example A-44
• Example A-41
• Example A-42

Chapter 27. Arrays

449

Chapter 28. Indirect References
We have seen that referencing a variable, $var, fetches its value. But, what about the value of a value? What
about $$var?
The actual notation is \$$var, usually preceded by an eval (and sometimes an echo). This is called an
indirect reference.

Example 28-1. Indirect Variable References
#!/bin/bash
# ind-ref.sh: Indirect variable referencing.
# Accessing the contents of the contents of a variable.
# First, let's fool around a little.
var=23
echo "\$var
= $var"
# $var
= 23
# So far, everything as expected. But ...
echo "\$\$var = $$var"
# $$var = 4570var
# Not useful ...
# \$\$ expanded to PID of the script
# -- refer to the entry on the $$ variable -#+ and "var" is echoed as plain text.
# (Thank you, Jakob Bohm, for pointing this out.)
echo "\\\$\$var = \$$var"
# \$$var = $23
# As expected. The first $ is escaped and pasted on to
#+ the value of var ($var = 23 ).
# Meaningful, but still not useful.
# Now, let's start over and do it the right way.
# ============================================== #

a=letter_of_alphabet
letter_of_alphabet=z

# Variable "a" holds the name of another variable.

echo
# Direct reference.
echo "a = $a"

# a = letter_of_alphabet

# Indirect reference.
eval a=\$$a
# ^^^
Forcing an eval(uation), and ...
#
^
Escaping the first $ ...
# -----------------------------------------------------------------------# The 'eval' forces an update of $a, sets it to the updated value of \$$a.
# So, we see why 'eval' so often shows up in indirect reference notation.
# -----------------------------------------------------------------------echo "Now a = $a"
# Now a = z

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echo

# Now, let's try changing the second-order reference.
t=table_cell_3
table_cell_3=24
echo "\"table_cell_3\" = $table_cell_3"
# "table_cell_3" = 24
echo -n "dereferenced \"t\" = "; eval echo \$$t
# dereferenced "t" = 24
# In this simple case, the following also works (why?).
#
eval t=\$$t; echo "\"t\" = $t"
echo
t=table_cell_3
NEW_VAL=387
table_cell_3=$NEW_VAL
echo "Changing value of \"table_cell_3\" to $NEW_VAL."
echo "\"table_cell_3\" now $table_cell_3"
echo -n "dereferenced \"t\" now "; eval echo \$$t
# "eval" takes the two arguments "echo" and "\$$t" (set equal to $table_cell_3)

echo
# (Thanks, Stephane Chazelas, for clearing up the above behavior.)

#
A more straightforward method is the ${!t} notation, discussed in the
#+ "Bash, version 2" section.
#
See also ex78.sh.
exit 0

Indirect referencing in Bash is a multi-step process. First, take the name of a variable: varname. Then,
reference it: $varname. Then, reference the reference: $$varname. Then, escape the first $:
\$$varname. Finally, force a reevaluation of the expression and assign it: eval newvar=\$$varname.
Of what practical use is indirect referencing of variables? It gives Bash a little of the functionality of pointers
in C, for instance, in table lookup. And, it also has some other very interesting applications. . . .
Nils Radtke shows how to build "dynamic" variable names and evaluate their contents. This can be useful
when sourcing configuration files.
#!/bin/bash

# --------------------------------------------# This could be "sourced" from a separate file.
isdnMyProviderRemoteNet=172.16.0.100
isdnYourProviderRemoteNet=10.0.0.10
isdnOnlineService="MyProvider"
# ---------------------------------------------

remoteNet=$(eval "echo \$$(echo isdn${isdnOnlineService}RemoteNet)")
remoteNet=$(eval "echo \$$(echo isdnMyProviderRemoteNet)")
remoteNet=$(eval "echo \$isdnMyProviderRemoteNet")

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remoteNet=$(eval "echo $isdnMyProviderRemoteNet")
echo "$remoteNet"

# 172.16.0.100

# ================================================================
#

And, it gets even better.

# Consider the following snippet given a variable named getSparc,
#+ but no such variable getIa64:
chkMirrorArchs () {
arch="$1";
if [ "$(eval "echo \${$(echo get$(echo -ne $arch |
sed 's/^\(.\).*/\1/g' | tr 'a-z' 'A-Z'; echo $arch |
sed 's/^.\(.*\)/\1/g')):-false}")" = true ]
then
return 0;
else
return 1;
fi;
}
getSparc="true"
unset getIa64
chkMirrorArchs sparc
echo $?
# 0
# True
chkMirrorArchs Ia64
echo $?
# 1
# False
#
#
#
#
#

Notes:
----Even the to-be-substituted variable name part is built explicitly.
The parameters to the chkMirrorArchs calls are all lower case.
The variable name is composed of two parts: "get" and "Sparc" . . .

Example 28-2. Passing an indirect reference to awk
#!/bin/bash
# Another version of the "column totaler" script
#+ that adds up a specified column (of numbers) in the target file.
# This one uses indirect references.
ARGS=2
E_WRONGARGS=85
if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2

# Name of file to operate on.
# Which column to total up.

#===== Same as original script, up to this point =====#

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# A multi-line awk script is invoked by
#
awk "
#
...
#
...
#
...
#
"

# Begin awk script.
# ------------------------------------------------awk "
{ total += \$${column_number} # Indirect reference
}
END {
print total
}
" "$filename"
# Note that awk doesn't need an eval preceding \$$.
# ------------------------------------------------# End awk script.
# Indirect variable reference avoids the hassles
#+ of referencing a shell variable within the embedded awk script.
# Thanks, Stephane Chazelas.

exit $?

This method of indirect referencing is a bit tricky. If the second order variable changes its value,
then the first order variable must be properly dereferenced (as in the above example).
Fortunately, the ${!variable} notation introduced with version 2 of Bash (see Example
37-2 and Example A-22) makes indirect referencing more intuitive.
Bash does not support pointer arithmetic, and this severely limits the usefulness of indirect referencing. In
fact, indirect referencing in a scripting language is, at best, something of an afterthought.

Chapter 28. Indirect References

453

Chapter 29. /dev and /proc
A Linux or UNIX filesystem typically has the /dev and /proc special-purpose directories.

29.1. /dev
The /dev directory contains entries for the physical devices that may or may not be present in the hardware.
[115] Appropriately enough, these are called device files. As an example, the hard drive partitions containing
the mounted filesystem(s) have entries in /dev, as df shows.
bash$ df
Filesystem
Mounted on
/dev/hda6
/dev/hda1
/dev/hda8
/dev/hda5

1k-blocks
495876
50755
367013
1714416

Used Available Use%
222748
3887
13262
1123624

247527
44248
334803
503704

48%
9%
4%
70%

/
/boot
/home
/usr

Among other things, the /dev directory contains loopback devices, such as /dev/loop0. A loopback
device is a gimmick that allows an ordinary file to be accessed as if it were a block device. [116] This permits
mounting an entire filesystem within a single large file. See Example 17-8 and Example 17-7.
A few of the pseudo-devices in /dev have other specialized uses, such as /dev/null, /dev/zero,
/dev/urandom, /dev/sda1 (hard drive partition), /dev/udp (User Datagram Packet port), and
/dev/tcp.
For instance:
To manually mount a USB flash drive, append the following line to /etc/fstab. [117]
/dev/sda1

/mnt/flashdrive

auto

noauto,user,noatime

0 0

(See also Example A-23.)
Checking whether a disk is in the CD-burner (soft-linked to /dev/hdc):
head -1 /dev/hdc

#
#

head: cannot open '/dev/hdc' for reading: No medium found
(No disc in the drive.)

# head: error reading '/dev/hdc': Input/output error
# (There is a disk in the drive, but it can't be read;
#+ possibly it's an unrecorded CDR blank.)
#
#
#+
#
#+

Stream of characters and assorted gibberish
(There is a pre-recorded disk in the drive,
and this is raw output -- a stream of ASCII and binary data.)
Here we see the wisdom of using 'head' to limit the output
to manageable proportions, rather than 'cat' or something similar.

#

Now, it's just a matter of checking/parsing the output and taking

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Advanced Bash-Scripting Guide
#+ appropriate action.

When executing a command on a /dev/tcp/$host/$port pseudo-device file, Bash opens a TCP
connection to the associated socket.

A socket is a communications node associated with a specific I/O port. (This is analogous to a hardware
socket, or receptacle, for a connecting cable.) It permits data transfer between hardware devices on the same
machine, between machines on the same network, between machines across different networks, and, of
course, between machines at different locations on the Internet.
The following examples assume an active Internet connection.
Getting the time from nist.gov:
bash$ cat /dev/tcp/www.net.cn/80
bash$ echo -e "GET / HTTP/1.0\n" >&5
bash$ cat <&5

[Thanks, Mark and Mihai Maties.]

Example 29-1. Using /dev/tcp for troubleshooting
#!/bin/bash
# dev-tcp.sh: /dev/tcp redirection to check Internet connection.
# Script by Troy Engel.
# Used with permission.
TCP_HOST=www.dns-diy.com
TCP_PORT=80

# A known spam-friendly ISP.
# Port 80 is http.

# Try to connect. (Somewhat similar to a 'ping' . . .)
echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT}
MYEXIT=$?

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Advanced Bash-Scripting Guide
: <From the bash reference:
/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer
port number or service name, Bash attempts to open a TCP connection to the
corresponding socket.
EXPLANATION

if [ "X$MYEXIT" = "X0" ]; then
echo "Connection successful. Exit code: $MYEXIT"
else
echo "Connection unsuccessful. Exit code: $MYEXIT"
fi
exit $MYEXIT

Example 29-2. Playing music
#!/bin/bash
# music.sh
# MUSIC WITHOUT EXTERNAL FILES
# Author: Antonio Macchi
# Used in ABS Guide with permission

# /dev/dsp default = 8000 frames per second, 8 bits per frame (1 byte),
#+ 1 channel (mono)
duration=2000
volume=$'\xc0'
mute=$'\x80'

# If 8000 bytes = 1 second, then 2000 = 1/4 second.
# Max volume = \xff (or \x00).
# No volume = \x80 (the middle).

function mknote () # $1=Note Hz in bytes (e.g. A = 440Hz ::
{
#+ 8000 fps / 440 = 16 :: A = 16 bytes per second)
for t in `seq 0 $duration`
do
test $(( $t % $1 )) = 0 && echo -n $volume || echo -n $mute
done
}
e=`mknote 49`
g=`mknote 41`
a=`mknote 36`
b=`mknote 32`
c=`mknote 30`
cis=`mknote 29`
d=`mknote 27`
e2=`mknote 24`
n=`mknote 32767`
# European notation.

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Advanced Bash-Scripting Guide
echo -n "$g$e2$d$c$d$c$a$g$n$g$e$n$g$e2$d$c$c$b$c$cis$n$cis$d \
$n$g$e2$d$c$d$c$a$g$n$g$e$n$g$a$d$c$b$a$b$c" > /dev/dsp
# dsp = Digital Signal Processor
exit

# A "bonny" example of a shell script!

29.2. /proc
The /proc directory is actually a pseudo-filesystem. The files in /proc mirror currently running system and
kernel processes and contain information and statistics about them.
bash$ cat /proc/devices
Character devices:
1 mem
2 pty
3 ttyp
4 ttyS
5 cua
7 vcs
10 misc
14 sound
29 fb
36 netlink
128 ptm
136 pts
162 raw
254 pcmcia
Block devices:
1 ramdisk
2 fd
3 ide0
9 md

bash$ cat /proc/interrupts
CPU0
0:
84505
XT-PIC
1:
3375
XT-PIC
2:
0
XT-PIC
5:
1
XT-PIC
8:
1
XT-PIC
12:
4231
XT-PIC
14:
109373
XT-PIC
NMI:
0
ERR:
0

bash$ cat /proc/partitions
major minor #blocks name
3
3
3
3
...

0
1
2
4

3007872
52416
1
165280

timer
keyboard
cascade
soundblaster
rtc
PS/2 Mouse
ide0

rio rmerge rsect ruse wio wmerge wsect wuse running use aveq

hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030
hda1 27 395 844 960 4 2 14 180 0 800 1140
hda2 0 0 0 0 0 0 0 0 0 0 0
hda4 10 0 20 210 0 0 0 0 0 210 210

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bash$ cat /proc/loadavg
0.13 0.42 0.27 2/44 1119

bash$ cat /proc/apm
1.16 1.2 0x03 0x01 0xff 0x80 -1% -1 ?

bash$ cat /proc/acpi/battery/BAT0/info
present:
yes
design capacity:
43200 mWh
last full capacity:
36640 mWh
battery technology:
rechargeable
design voltage:
10800 mV
design capacity warning: 1832 mWh
design capacity low:
200 mWh
capacity granularity 1: 1 mWh
capacity granularity 2: 1 mWh
model number:
IBM-02K6897
serial number:
1133
battery type:
LION
OEM info:
Panasonic

bash$ fgrep Mem /proc/meminfo
MemTotal:
515216 kB
MemFree:
266248 kB

Shell scripts may extract data from certain of the files in /proc. [118]
FS=iso

# ISO filesystem support in kernel?

grep $FS /proc/filesystems

# iso9660

kernel_version=$( awk '{ print $3 }' /proc/version )
CPU=$( awk '/model name/ {print $5}' < /proc/cpuinfo )
if [ "$CPU" = "Pentium(R)" ]
then
run_some_commands
...
else
run_other_commands
...
fi

cpu_speed=$( fgrep "cpu MHz" /proc/cpuinfo | awk '{print $4}' )
# Current operating speed (in MHz) of the cpu on your machine.
# On a laptop this may vary, depending on use of battery
#+ or AC power.

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#!/bin/bash
# get-commandline.sh
# Get the command-line parameters of a process.
OPTION=cmdline
# Identify PID.
pid=$( echo $(pidof "$1") | awk '{ print $1 }' )
# Get only first
^^^^^^^^^^^^^^^^^^ of multiple instances.
echo
echo "Process ID of (first instance of) "$1" = $pid"
echo -n "Command-line arguments: "
cat /proc/"$pid"/"$OPTION" | xargs -0 echo
#
Formats output:
^^^^^^^^^^^^^^^
#
(Thanks, Han Holl, for the fixup!)
echo; echo

# For example:
# sh get-commandline.sh xterm

+
devfile="/proc/bus/usb/devices"
text="Spd"
USB1="Spd=12"
USB2="Spd=480"

bus_speed=$(fgrep -m 1 "$text" $devfile | awk '{print $9}')
#
^^^^ Stop after first match.
if [ "$bus_speed" = "$USB1" ]
then
echo "USB 1.1 port found."
# Do something appropriate for USB 1.1.
fi

It is even possible to control certain peripherals with commands sent to the /proc directory.
root# echo on > /proc/acpi/ibm/light

This turns on the Thinklight in certain models of IBM/Lenovo Thinkpads. (May not work on all Linux
distros.)
Of course, caution is advised when writing to /proc.
The /proc directory contains subdirectories with unusual numerical names. Every one of these names maps
to the process ID of a currently running process. Within each of these subdirectories, there are a number of
files that hold useful information about the corresponding process. The stat and status files keep running
statistics on the process, the cmdline file holds the command-line arguments the process was invoked with,
and the exe file is a symbolic link to the complete path name of the invoking process. There are a few more
such files, but these seem to be the most interesting from a scripting standpoint.

Example 29-3. Finding the process associated with a PID
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Advanced Bash-Scripting Guide
#!/bin/bash
# pid-identifier.sh:
# Gives complete path name to process associated with pid.
ARGNO=1 # Number of arguments the script expects.
E_WRONGARGS=65
E_BADPID=66
E_NOSUCHPROCESS=67
E_NOPERMISSION=68
PROCFILE=exe
if [ $# -ne $ARGNO ]
then
echo "Usage: `basename $0` PID-number" >&2
exit $E_WRONGARGS
fi

# Error message >stderr.

pidno=$( ps ax | grep $1 | awk '{ print $1 }' | grep $1 )
# Checks for pid in "ps" listing, field #1.
# Then makes sure it is the actual process, not the process invoked by this script.
# The last "grep $1" filters out this possibility.
#
#
pidno=$( ps ax | awk '{ print $1 }' | grep $1 )
#
also works, as Teemu Huovila, points out.
if [ -z "$pidno" ] # If, after all the filtering, the result is a zero-length string,
then
#+ no running process corresponds to the pid given.
echo "No such process running."
exit $E_NOSUCHPROCESS
fi
# Alternatively:
#
if ! ps $1 > /dev/null 2>&1
#
then
# no running process corresponds to the pid given.
#
echo "No such process running."
#
exit $E_NOSUCHPROCESS
#
fi
# To simplify the entire process, use "pidof".

if [ !
then
echo
echo
exit
fi

-r "/proc/$1/$PROCFILE" ]

# Check for read permission.

"Process $1 running, but..."
"Can't get read permission on /proc/$1/$PROCFILE."
$E_NOPERMISSION # Ordinary user can't access some files in /proc.

# The last two tests may be replaced by:
#
if ! kill -0 $1 > /dev/null 2>&1 # '0' is not a signal, but
# this will test whether it is possible
# to send a signal to the process.
#
then echo "PID doesn't exist or you're not its owner" >&2
#
exit $E_BADPID
#
fi

exe_file=$( ls -l /proc/$1 | grep "exe" | awk '{ print $11 }' )
# Or
exe_file=$( ls -l /proc/$1/exe | awk '{print $11}' )
#
# /proc/pid-number/exe is a symbolic link

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Advanced Bash-Scripting Guide
#+ to the complete path name of the invoking process.
if [ -e "$exe_file" ] # If /proc/pid-number/exe exists,
then
#+ then the corresponding process exists.
echo "Process #$1 invoked by $exe_file."
else
echo "No such process running."
fi

# This elaborate script can *almost* be replaced by
#
ps ax | grep $1 | awk '{ print $5 }'
# However, this will not work...
#+ because the fifth field of 'ps' is argv[0] of the process,
#+ not the executable file path.
#
# However, either of the following would work.
#
find /proc/$1/exe -printf '%l\n'
#
lsof -aFn -p $1 -d txt | sed -ne 's/^n//p'
# Additional commentary by Stephane Chazelas.
exit 0

Example 29-4. On-line connect status
#!/bin/bash
# connect-stat.sh
# Note that this script may need modification
#+ to work with a wireless connection.
PROCNAME=pppd
PROCFILENAME=status
NOTCONNECTED=85
INTERVAL=2

# ppp daemon
# Where to look.
# Update every 2 seconds.

pidno=$( ps ax | grep -v "ps ax" | grep -v grep | grep $PROCNAME |
awk '{ print $1 }' )
# Finding the process number of 'pppd', the 'ppp daemon'.
# Have to filter out the process lines generated by the search itself.
#
# However, as Oleg Philon points out,
#+ this could have been considerably simplified by using "pidof".
# pidno=$( pidof $PROCNAME )
#
# Moral of the story:
#+ When a command sequence gets too complex, look for a shortcut.

if [ -z "$pidno" ]
# If no pid, then process is not running.
then
echo "Not connected."
# exit $NOTCONNECTED
else
echo "Connected."; echo
fi
while [ true ]
do

# Endless loop, script can be improved here.

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if [ ! -e "/proc/$pidno/$PROCFILENAME" ]
# While process running, then "status" file exists.
then
echo "Disconnected."
#
exit $NOTCONNECTED
fi
netstat -s | grep "packets received" # Get some connect statistics.
netstat -s | grep "packets delivered"

sleep $INTERVAL
echo; echo
done
exit 0
# As it stands, this script must be terminated with a Control-C.
#
#
#
#
#

Exercises:
--------Improve the script so it exits on a "q" keystroke.
Make the script more user-friendly in other ways.
Fix the script to work with wireless/DSL connections.

In general, it is dangerous to write to the files in /proc, as this can corrupt the filesystem or crash the
machine.

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Chapter 30. Network Programming
The Net's a cross between an elephant and a
white elephant sale: it never forgets, and it's
always crap.
--Nemo
A Linux system has quite a number of tools for accessing, manipulating, and troubleshooting network
connections. We can incorporate some of these tools into scripts -- scripts that expand our knowledge of
networking, useful scripts that can facilitate the administration of a network.
Here is a simple CGI script that demonstrates connecting to a remote server.

Example 30-1. Print the server environment
#!/bin/bash
# test-cgi.sh
# by Michael Zick
# Used with permission
#
#
#
#

May have to change the location for your site.
(At the ISP's servers, Bash may not be in the usual place.)
Other places: /usr/bin or /usr/local/bin
Might even try it without any path in sha-bang.

# Disable filename globbing.
set -f
# Header tells browser what to expect.
echo Content-type: text/plain
echo
echo CGI/1.0 test script report:
echo
echo environment settings:
set
echo
echo whereis bash?
whereis bash
echo

echo who are we?
echo ${BASH_VERSINFO[*]}
echo
echo argc is $#. argv is "$*".
echo
# CGI/1.0 expected environment variables.
echo SERVER_SOFTWARE = $SERVER_SOFTWARE
echo SERVER_NAME = $SERVER_NAME
echo GATEWAY_INTERFACE = $GATEWAY_INTERFACE

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Advanced Bash-Scripting Guide
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo

SERVER_PROTOCOL = $SERVER_PROTOCOL
SERVER_PORT = $SERVER_PORT
REQUEST_METHOD = $REQUEST_METHOD
HTTP_ACCEPT = "$HTTP_ACCEPT"
PATH_INFO = "$PATH_INFO"
PATH_TRANSLATED = "$PATH_TRANSLATED"
SCRIPT_NAME = "$SCRIPT_NAME"
QUERY_STRING = "$QUERY_STRING"
REMOTE_HOST = $REMOTE_HOST
REMOTE_ADDR = $REMOTE_ADDR
REMOTE_USER = $REMOTE_USER
AUTH_TYPE = $AUTH_TYPE
CONTENT_TYPE = $CONTENT_TYPE
CONTENT_LENGTH = $CONTENT_LENGTH

exit 0
# Here document to give short instructions.
:<<-'_test_CGI_'
1) Drop this in your http://domain.name/cgi-bin directory.
2) Then, open http://domain.name/cgi-bin/test-cgi.sh.
_test_CGI_

For security purposes, it may be helpful to identify the IP addresses a computer is accessing.

Example 30-2. IP addresses
#!/bin/bash
# ip-addresses.sh
# List the IP addresses your computer is connected to.
#
#
#
#
#
#+

Inspired by Greg Bledsoe's ddos.sh script,
Linux Journal, 09 March 2011.
URL:
http://www.linuxjournal.com/content/back-dead-simple-bash-complex-ddos
Greg licensed his script under the GPL2,
and as a derivative, this script is likewise GPL2.

connection_type=TCP
# Also try UDP.
field=2
# Which field of the output we're interested in.
no_match=LISTEN
# Filter out records containing this. Why?
lsof_args=-ni
# -i lists Internet-associated files.
# -n preserves numerical IP addresses.
# What happens without the -n option? Try it.
router="[0-9][0-9][0-9][0-9][0-9]->"
#
Delete the router info.
lsof "$lsof_args" | grep $connection_type | grep -v "$no_match" |
awk '{print $9}' | cut -d : -f $field | sort | uniq |
sed s/"^$router"//
#
#
#
#
#
#

Bledsoe's script assigns the output of a filtered IP list,
(similar to lines 19-22, above) to a variable.
He checks for multiple connections to a single IP address,
then uses:
iptables -I INPUT -s $ip -p tcp -j REJECT --reject-with tcp-reset

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#
#

... within a 60-second delay loop to bounce packets from DDOS attacks.

#
#
#
#+

Exercise:
-------Use the 'iptables' command to extend this script
to reject connection attempts from well-known spammer IP domains.

More examples of network programming:
1. Getting the time from nist.gov
2. Downloading a URL
3. A GRE tunnel
4. Checking if an Internet server is up
5. Example 16-41
6. Example A-28
7. Example A-29
8. Example 29-1
See also the networking commands in the System and Administrative Commands chapter and the
communications commands in the External Filters, Programs and Commands chapter.

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Chapter 31. Of Zeros and Nulls
Faultily faultless, icily regular, splendidly null
Dead perfection; no more.
--Alfred Lord Tennyson
/dev/zero ... /dev/null
Uses of /dev/null
Think of /dev/null as a black hole. It is essentially the equivalent of a write-only file. Everything
written to it disappears. Attempts to read or output from it result in nothing. All the same,
/dev/null can be quite useful from both the command-line and in scripts.
Suppressing stdout.
cat $filename >/dev/null
# Contents of the file will not list to stdout.

Suppressing stderr (from Example 16-3).
rm $badname 2>/dev/null
#
So error messages [stderr] deep-sixed.

Suppressing output from both stdout and stderr.
cat $filename 2>/dev/null >/dev/null
# If "$filename" does not exist, there will be no error message output.
# If "$filename" does exist, the contents of the file will not list to stdout.
# Therefore, no output at all will result from the above line of code.
#
# This can be useful in situations where the return code from a command
#+ needs to be tested, but no output is desired.
#
# cat $filename &>/dev/null
#
also works, as Baris Cicek points out.

Deleting contents of a file, but preserving the file itself, with all attendant permissions (from Example
2-1 and Example 2-3):
cat /dev/null > /var/log/messages
# : > /var/log/messages
has same effect, but does not spawn a new process.
cat /dev/null > /var/log/wtmp

Automatically emptying the contents of a logfile (especially good for dealing with those nasty
"cookies" sent by commercial Web sites):

Example 31-1. Hiding the cookie jar
# Obsolete Netscape browser.
# Same principle applies to newer browsers.
if [ -f ~/.netscape/cookies ]
then

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rm -f ~/.netscape/cookies
fi
ln -s /dev/null ~/.netscape/cookies
# All cookies now get sent to a black hole, rather than saved to disk.

Uses of /dev/zero
Like /dev/null, /dev/zero is a pseudo-device file, but it actually produces a stream of nulls
(binary zeros, not the ASCII kind). Output written to /dev/zero disappears, and it is fairly difficult
to actually read the nulls emitted there, though it can be done with od or a hex editor. The chief use of
/dev/zero is creating an initialized dummy file of predetermined length intended as a temporary
swap file.

Example 31-2. Setting up a swapfile using /dev/zero
#!/bin/bash
# Creating a swap file.
# A swap file provides a temporary storage cache
#+ which helps speed up certain filesystem operations.
ROOT_UID=0
E_WRONG_USER=85

# Root has $UID 0.
# Not root?

FILE=/swap
BLOCKSIZE=1024
MINBLOCKS=40
SUCCESS=0

# This script must be run as root.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo; echo "You must be root to run this script."; echo
exit $E_WRONG_USER
fi

blocks=${1:-$MINBLOCKS}
#
#
#
#
#
#
#
#
#

# Set to default of 40 blocks,
#+ if nothing specified on command-line.
This is the equivalent of the command block below.
-------------------------------------------------if [ -n "$1" ]
then
blocks=$1
else
blocks=$MINBLOCKS
fi
--------------------------------------------------

if [ "$blocks" -lt $MINBLOCKS ]
then
blocks=$MINBLOCKS
fi

# Must be at least 40 blocks long.

######################################################################
echo "Creating swap file of size $blocks blocks (KB)."
dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file.

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mkswap $FILE $blocks
# Designate it a swap file.
swapon $FILE
# Activate swap file.
retcode=$?
# Everything worked?
# Note that if one or more of these commands fails,
#+ then it could cause nasty problems.
######################################################################
# Exercise:
# Rewrite the above block of code so that if it does not execute
#+ successfully, then:
#
1) an error message is echoed to stderr,
#
2) all temporary files are cleaned up, and
#
3) the script exits in an orderly fashion with an
#+
appropriate error code.
echo "Swap file created and activated."
exit $retcode

Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose,
such as mounting a filesystem on a loopback device (see Example 17-8) or "securely" deleting a file
(see Example 16-60).

Example 31-3. Creating a ramdisk
#!/bin/bash
# ramdisk.sh
#
#+
#
#
#+
#
#
#
#+

A "ramdisk" is a segment of system RAM memory
which acts as if it were a filesystem.
Its advantage is very fast access (read/write time).
Disadvantages: volatility, loss of data on reboot or powerdown.
less RAM available to system.
Of what use is a ramdisk?
Keeping a large dataset, such as a table or dictionary on ramdisk,
speeds up data lookup, since memory access is much faster than disk access.

E_NON_ROOT_USER=70
ROOTUSER_NAME=root
MOUNTPT=/mnt/ramdisk
SIZE=2000
BLOCKSIZE=1024
DEVICE=/dev/ram0

# Must run as root.

# 2K blocks (change as appropriate)
# 1K (1024 byte) block size
# First ram device

username=`id -nu`
if [ "$username" != "$ROOTUSER_NAME" ]
then
echo "Must be root to run \"`basename $0`\"."
exit $E_NON_ROOT_USER
fi
if [ ! -d "$MOUNTPT" ]
then
mkdir $MOUNTPT
fi

# Test whether mount point already there,
#+ so no error if this script is run
#+ multiple times.

##############################################################################

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dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE

# Zero out RAM device.
# Why is this necessary?
mke2fs $DEVICE
# Create an ext2 filesystem on it.
mount $DEVICE $MOUNTPT
# Mount it.
chmod 777 $MOUNTPT
# Enables ordinary user to access ramdisk.
# However, must be root to unmount it.
##############################################################################
# Need to test whether above commands succeed. Could cause problems otherwise.
# Exercise: modify this script to make it safer.
echo "\"$MOUNTPT\" now available for use."
# The ramdisk is now accessible for storing files, even by an ordinary user.
# Caution, the ramdisk is volatile, and its contents will disappear
#+ on reboot or power loss.
# Copy anything you want saved to a regular directory.
# After reboot, run this script to again set up ramdisk.
# Remounting /mnt/ramdisk without the other steps will not work.
# Suitably modified, this script can by invoked in /etc/rc.d/rc.local,
#+ to set up ramdisk automatically at bootup.
# That may be appropriate on, for example, a database server.
exit 0

In addition to all the above, /dev/zero is needed by ELF (Executable and Linking Format)
UNIX/Linux binaries.

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Chapter 32. Debugging
Debugging is twice as hard as writing the code in
the first place. Therefore, if you write the code as
cleverly as possible, you are, by definition, not
smart enough to debug it.
--Brian Kernighan
The Bash shell contains no built-in debugger, and only bare-bones debugging-specific commands and
constructs. Syntax errors or outright typos in the script generate cryptic error messages that are often of no
help in debugging a non-functional script.

Example 32-1. A buggy script
#!/bin/bash
# ex74.sh
# This is a buggy script.
# Where, oh where is the error?
a=37
if [$a -gt 27 ]
then
echo $a
fi
exit 0

Output from script:
./ex74.sh: [37: command not found

What's wrong with the above script? Hint: after the if.
Example 32-2. Missing keyword
#!/bin/bash
# missing-keyword.sh: What error message will this generate?
for a in 1 2 3
do
echo "$a"
# done
# Required keyword 'done' commented out in line 7.
exit 0

Output from script:
missing-keyword.sh: line 10: syntax error: unexpected end of file

Note that the error message does not necessarily reference the line in which the error occurs, but the line
where the Bash interpreter finally becomes aware of the error.
Error messages may disregard comment lines in a script when reporting the line number of a syntax error.

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What if the script executes, but does not work as expected? This is the all too familiar logic error.

Example 32-3. test24: another buggy script
#!/bin/bash
#
#+
#
#

This script is supposed to delete all filenames in current directory
containing embedded spaces.
It doesn't work.
Why not?

badname=`ls | grep ' '`
# Try this:
# echo "$badname"
rm "$badname"
exit 0

Try to find out what's wrong with Example 32-3 by uncommenting the echo "$badname" line. Echo
statements are useful for seeing whether what you expect is actually what you get.
In this particular case, rm "$badname" will not give the desired results because $badname should not be
quoted. Placing it in quotes ensures that rm has only one argument (it will match only one filename). A partial
fix is to remove to quotes from $badname and to reset $IFS to contain only a newline, IFS=$'\n'.
However, there are simpler ways of going about it.
# Correct methods of deleting filenames containing spaces.
rm *\ *
rm *" "*
rm *' '*
# Thank you. S.C.

Summarizing the symptoms of a buggy script,
1. It bombs with a "syntax error" message, or
2. It runs, but does not work as expected (logic error).
3. It runs, works as expected, but has nasty side effects (logic bomb).

Tools for debugging non-working scripts include
1. Inserting echo statements at critical points in the script to trace the variables, and otherwise give a
snapshot of what is going on.
Even better is an echo that echoes only when debug is on.
### debecho (debug-echo), by Stefano Falsetto ###
### Will echo passed parameters only if DEBUG is set to a value. ###
debecho () {
if [ ! -z "$DEBUG" ]; then
echo "$1" >&2
#
^^^ to stderr
fi

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}
DEBUG=on
Whatever=whatnot
debecho $Whatever

# whatnot

DEBUG=
Whatever=notwhat
debecho $Whatever

# (Will not echo.)

2. Using the tee filter to check processes or data flows at critical points.
3. Setting option flags -n -v -x
sh -n scriptname checks for syntax errors without actually running the script. This is the
equivalent of inserting set -n or set -o noexec into the script. Note that certain types of
syntax errors can slip past this check.
sh -v scriptname echoes each command before executing it. This is the equivalent of inserting
set -v or set -o verbose in the script.
The -n and -v flags work well together. sh -nv scriptname gives a verbose syntax check.
sh -x scriptname echoes the result each command, but in an abbreviated manner. This is the
equivalent of inserting set -x or set -o xtrace in the script.

Inserting set -u or set -o nounset in the script runs it, but gives an unbound variable error
message and aborts the script.
set -u

# Or

set -o nounset

# Setting a variable to null will not trigger the error/abort.
# unset_var=
echo $unset_var

# Unset (and undeclared) variable.

echo "Should not echo!"
# sh t2.sh
# t2.sh: line 6: unset_var: unbound variable

4. Using an "assert" function to test a variable or condition at critical points in a script. (This is an idea
borrowed from C.)
Example 32-4. Testing a condition with an assert
#!/bin/bash
# assert.sh
#######################################################################
assert ()
# If condition false,
{
#+ exit from script
#+ with appropriate error message.
E_PARAM_ERR=98
E_ASSERT_FAILED=99

if [ -z "$2" ]

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#

Not enough parameters passed

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Advanced Bash-Scripting Guide
then
return $E_PARAM_ERR
fi

#+ to assert() function.
# No damage done.

lineno=$2
if [ ! $1 ]
then
echo "Assertion failed: \"$1\""
echo "File \"$0\", line $lineno"
# Give name of file and line number.
exit $E_ASSERT_FAILED
# else
#
return
#
and continue executing the script.
fi
} # Insert a similar assert() function into a script you need to debug.
#######################################################################

a=5
b=4
condition="$a -lt $b"

# Error message and exit from script.
# Try setting "condition" to something else
#+ and see what happens.

assert "$condition" $LINENO
# The remainder of the script executes only if the "assert" does not fail.

# Some commands.
# Some more commands . . .
echo "This statement echoes only if the \"assert\" does not fail."
# . . .
# More commands . . .
exit $?

5. Using the $LINENO variable and the caller builtin.
6. Trapping at exit.
The exit command in a script triggers a signal 0, terminating the process, that is, the script itself. [119]
It is often useful to trap the exit, forcing a "printout" of variables, for example. The trap must be the
first command in the script.
Trapping signals
trap
Specifies an action on receipt of a signal; also useful for debugging.

A signal is a message sent to a process, either by the kernel or another process, telling it to take
some specified action (usually to terminate). For example, hitting a Control-C sends a user interrupt,
an INT signal, to a running program.
A simple instance:
trap '' 2
# Ignore interrupt 2 (Control-C), with no action specified.

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trap 'echo "Control-C disabled."' 2
# Message when Control-C pressed.

Example 32-5. Trapping at exit
#!/bin/bash
# Hunting variables with a trap.
trap 'echo Variable Listing --- a = $a b = $b' EXIT
# EXIT is the name of the signal generated upon exit from a script.
#
# The command specified by the "trap" doesn't execute until
#+ the appropriate signal is sent.
echo "This prints before the \"trap\" --"
echo "even though the script sees the \"trap\" first."
echo
a=39
b=36
exit 0
# Note that commenting out the 'exit' command makes no difference,
#+ since the script exits in any case after running out of commands.

Example 32-6. Cleaning up after Control-C
#!/bin/bash
# logon.sh: A quick 'n dirty script to check whether you are on-line yet.
umask 177

# Make sure temp files are not world readable.

TRUE=1
LOGFILE=/var/log/messages
# Note that $LOGFILE must be readable
#+ (as root, chmod 644 /var/log/messages).
TEMPFILE=temp.$$
# Create a "unique" temp file name, using process id of the script.
#
Using 'mktemp' is an alternative.
#
For example:
#
TEMPFILE=`mktemp temp.XXXXXX`
KEYWORD=address
# At logon, the line "remote IP address xxx.xxx.xxx.xxx"
#
appended to /var/log/messages.
ONLINE=22
USER_INTERRUPT=13
CHECK_LINES=100
# How many lines in log file to check.
trap 'rm -f $TEMPFILE; exit $USER_INTERRUPT' TERM INT
# Cleans up the temp file if script interrupted by control-c.
echo
while [ $TRUE ]
do

#Endless loop.

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tail -n $CHECK_LINES $LOGFILE> $TEMPFILE
# Saves last 100 lines of system log file as temp file.
# Necessary, since newer kernels generate many log messages at log on.
search=`grep $KEYWORD $TEMPFILE`
# Checks for presence of the "IP address" phrase,
#+ indicating a successful logon.
if [ ! -z "$search" ] #
then
echo "On-line"
rm -f $TEMPFILE
#
exit $ONLINE
else
echo -n "."
#
#+
fi

Quotes necessary because of possible spaces.

Clean up temp file.

The -n option to echo suppresses newline,
so you get continuous rows of dots.

sleep 1
done

# Note: if you change the KEYWORD variable to "Exit",
#+ this script can be used while on-line
#+ to check for an unexpected logoff.
# Exercise: Change the script, per the above note,
#
and prettify it.
exit 0

# Nick Drage suggests an alternate method:
while true
do ifconfig ppp0 | grep UP 1> /dev/null && echo "connected" && exit 0
echo -n "."
# Prints dots (.....) until connected.
sleep 2
done
# Problem: Hitting Control-C to terminate this process may be insufficient.
#+
(Dots may keep on echoing.)
# Exercise: Fix this.

# Stephane Chazelas has yet another alternative:
CHECK_INTERVAL=1
while ! tail -n 1 "$LOGFILE" | grep -q "$KEYWORD"
do echo -n .
sleep $CHECK_INTERVAL
done
echo "On-line"
# Exercise: Discuss the relative strengths and weaknesses
#
of each of these various approaches.

Example 32-7. A Simple Implementation of a Progress Bar
#! /bin/bash

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# progress-bar2.sh
# Author: Graham Ewart (with reformatting by ABS Guide author).
# Used in ABS Guide with permission (thanks!).
# Invoke this script with bash. It doesn't work with sh.
interval=1
long_interval=10
{
trap "exit" SIGUSR1
sleep $interval; sleep $interval
while true
do
echo -n '.'
# Use dots.
sleep $interval
done; } &
# Start a progress bar as a background process.
pid=$!
trap "echo !; kill -USR1 $pid; wait $pid"

EXIT

# To handle ^C.

echo -n 'Long-running process '
sleep $long_interval
echo ' Finished!'
kill -USR1 $pid
wait $pid
trap EXIT

# Stop the progress bar.

exit $?

The DEBUG argument to trap causes a specified action to execute after every command in a script. This
permits tracing variables, for example.
Example 32-8. Tracing a variable
#!/bin/bash
trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG
# Echoes the value of $variable after every command.
variable=29
echo "

Just initialized \$variable to $variable."

let "variable *= 3"
echo " Just multiplied \$variable by 3."
exit
#
#+
#+
#+

The "trap 'command1 . . . command2 . . .' DEBUG" construct is
more appropriate in the context of a complex script,
where inserting multiple "echo $variable" statements might be
awkward and time-consuming.

# Thanks, Stephane Chazelas for the pointer.

Output of script:

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VARIABLE-TRACE> $variable = ""
VARIABLE-TRACE> $variable = "29"
Just initialized $variable to 29.
VARIABLE-TRACE> $variable = "29"
VARIABLE-TRACE> $variable = "87"
Just multiplied $variable by 3.
VARIABLE-TRACE> $variable = "87"

Of course, the trap command has other uses aside from debugging, such as disabling certain keystrokes
within a script (see Example A-43).

Example 32-9. Running multiple processes (on an SMP box)
#!/bin/bash
# parent.sh
# Running multiple processes on an SMP box.
# Author: Tedman Eng
# This is the first of two scripts,
#+ both of which must be present in the current working directory.

LIMIT=$1
# Total number of process to start
NUMPROC=4
# Number of concurrent threads (forks?)
PROCID=1
# Starting Process ID
echo "My PID is $$"
function start_thread() {
if [ $PROCID -le $LIMIT ] ; then
./child.sh $PROCID&
let "PROCID++"
else
echo "Limit reached."
wait
exit
fi
}
while [ "$NUMPROC" -gt 0 ]; do
start_thread;
let "NUMPROC--"
done

while true
do
trap "start_thread" SIGRTMIN
done
exit 0

# ======== Second script follows ========

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#!/bin/bash
# child.sh
# Running multiple processes on an SMP box.
# This script is called by parent.sh.
# Author: Tedman Eng
temp=$RANDOM
index=$1
shift
let "temp %= 5"
let "temp += 4"
echo "Starting $index Time:$temp" "$@"
sleep ${temp}
echo "Ending $index"
kill -s SIGRTMIN $PPID
exit 0

# ======================= SCRIPT AUTHOR'S NOTES ======================= #
# It's not completely bug free.
# I ran it with limit = 500 and after the first few hundred iterations,
#+ one of the concurrent threads disappeared!
# Not sure if this is collisions from trap signals or something else.
# Once the trap is received, there's a brief moment while executing the
#+ trap handler but before the next trap is set. During this time, it may
#+ be possible to miss a trap signal, thus miss spawning a child process.
# No doubt someone may spot the bug and will be writing
#+ . . . in the future.

# ===================================================================== #

# ----------------------------------------------------------------------#

#################################################################
# The following is the original script written by Vernia Damiano.
# Unfortunately, it doesn't work properly.
#################################################################
#!/bin/bash
# Must call script with at least one integer parameter
#+ (number of concurrent processes).
# All other parameters are passed through to the processes started.

INDICE=8
TEMPO=5
E_BADARGS=65

# Total number of process to start
# Maximum sleep time per process
# No arg(s) passed to script.

if [ $# -eq 0 ] # Check for at least one argument passed to script.
then
echo "Usage: `basename $0` number_of_processes [passed params]"
exit $E_BADARGS
fi

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NUMPROC=$1
shift
PARAMETRI=( "$@" )

# Number of concurrent process
# Parameters of each process

function avvia() {
local temp
local index
temp=$RANDOM
index=$1
shift
let "temp %= $TEMPO"
let "temp += 1"
echo "Starting $index Time:$temp" "$@"
sleep ${temp}
echo "Ending $index"
kill -s SIGRTMIN $$
}
function parti() {
if [ $INDICE -gt 0 ] ; then
avvia $INDICE "${PARAMETRI[@]}" &
let "INDICE--"
else
trap : SIGRTMIN
fi
}
trap parti SIGRTMIN
while [ "$NUMPROC" -gt 0 ]; do
parti;
let "NUMPROC--"
done
wait
trap - SIGRTMIN
exit $?
: <|)
List double-quoted strings prefixed by $, but do not execute commands in
script
Export all defined variables
Notify when jobs running in background terminate (not of much use in a
script)
Read commands from ...
Informs user of any open jobs upon shell exit. Introduced in version 4 of
Bash, and still "experimental." Usage: shopt -s checkjobs (Caution: may
hang!)
Abort script at first error, when a command exits with non-zero status
(except in until or while loops, if-tests, list constructs)
Filename expansion (globbing) disabled
Enables the ** globbing operator (version 4+ of Bash). Usage: shopt -s
globstar
Script runs in interactive mode
Read commands in script, but do not execute them (syntax check)
Invoke the Option-Name option
Change the behavior of Bash, or invoked script, to conform to POSIX
standard.
Causes a pipeline to return the exit status of the last command in the pipe
that returned a non-zero return value.
Script runs as "suid" (caution!)
Script runs in restricted mode (see Chapter 22).
Read commands from stdin
Exit after first command
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-u

nounset

-v
-x
--

verbose
xtrace
(none)
(none)

Chapter 33. Options

Attempt to use undefined variable outputs error message, and forces an
exit
Print each command to stdout before executing it
Similar to -v, but expands commands
End of options flag. All other arguments are positional parameters.
Unset positional parameters. If arguments given (-- arg1 arg2),
positional parameters set to arguments.

483

Chapter 34. Gotchas
Turandot: Gli enigmi sono tre, la morte una!
Caleph: No, no! Gli enigmi sono tre, una la vita!
--Puccini
Here are some (non-recommended!) scripting practices that will bring excitement into an otherwise dull life.
•
Assigning reserved words or characters to variable names.
case=value0
# Causes problems.
23skidoo=value1
# Also problems.
# Variable names starting with a digit are reserved by the shell.
# Try _23skidoo=value1. Starting variables with an underscore is okay.
# However . . .
_=25
echo $_
# But . . .

using just an underscore will not work.
# $_ is a special variable set to last arg of last command.
_ is a valid function name!

xyz((!*=value2
# Causes severe problems.
# As of version 3 of Bash, periods are not allowed within variable names.

• Using a hyphen or other reserved characters in a variable name (or function name).
var-1=23
# Use 'var_1' instead.
function-whatever ()
# Error
# Use 'function_whatever ()' instead.

# As of version 3 of Bash, periods are not allowed within function names.
function.whatever ()
# Error
# Use 'functionWhatever ()' instead.

• Using the same name for a variable and a function. This can make a script difficult to understand.
do_something ()
{
echo "This function does something with \"$1\"."
}
do_something=do_something
do_something do_something
# All this is legal, but highly confusing.

• Using whitespace inappropriately. In contrast to other programming languages, Bash can be quite
finicky about whitespace.
var1 = 23
# 'var1=23' is correct.
# On line above, Bash attempts to execute command "var1"
# with the arguments "=" and "23".

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let c = $a - $b

# Instead:

let c=$a-$b

or

let "c = $a - $b"

if [ $a -le 5]
#
^^

# if [ $a -le 5 ]
is correct.
if [ "$a" -le 5 ]
is even better.
# [[ $a -le 5 ]] also works.

•
Not terminating with a semicolon the final command in a code block within curly brackets.
{ ls -l; df; echo "Done." }
# bash: syntax error: unexpected end of file
{ ls -l; df; echo "Done."; }
#
^

### Final command needs semicolon.

•
Assuming uninitialized variables (variables before a value is assigned to them) are "zeroed out". An
uninitialized variable has a value of null, not zero.

#!/bin/bash
echo "uninitialized_var = $uninitialized_var"
# uninitialized_var =
# However . . .
# if $BASH_VERSION ≥ 4.2; then
if [[ ! -v uninitialized_var ]]
then
uninitialized_var=0
# Initialize it to zero!
fi

•
Mixing up = and -eq in a test. Remember, = is for comparing literal variables and -eq for integers.
if [ "$a" = 273 ]
if [ "$a" -eq 273 ]

# Is $a an integer or string?
# If $a is an integer.

# Sometimes you can interchange -eq and = without adverse consequences.
# However . . .

a=273.0

# Not an integer.

if [ "$a" = 273 ]
then
echo "Comparison works."
else
echo "Comparison does not work."
fi
# Comparison does not work.
# Same with

a=" 273"

and a="0273".

# Likewise, problems trying to use "-eq" with non-integer values.
if [ "$a" -eq 273.0 ]
then
echo "a = $a"
fi # Aborts with an error message.
# test.sh: [: 273.0: integer expression expected

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• Misusing string comparison operators.

Example 34-1. Numerical and string comparison are not equivalent
#!/bin/bash
# bad-op.sh: Trying to use a string comparison on integers.
echo
number=1
# The following while-loop has two errors:
#+ one blatant, and the other subtle.
while [ "$number" < 5 ]
# Wrong! Should be: while [ "$number" -lt 5 ]
do
echo -n "$number "
let "number += 1"
done
# Attempt to run this bombs with the error message:
#+ bad-op.sh: line 10: 5: No such file or directory
# Within single brackets, "<" must be escaped,
#+ and even then, it's still wrong for comparing integers.
echo "---------------------"
while [ "$number" \< 5 ]
do
echo -n "$number "
let "number += 1"
done

#
#
#
#+
#+

1 2 3 4
It *seems* to work, but . . .
it actually does an ASCII comparison,
rather than a numerical one.

echo; echo "---------------------"
# This can cause problems. For example:
lesser=5
greater=105
if [ "$greater" \< "$lesser" ]
then
echo "$greater is less than $lesser"
fi
# 105 is less than 5
# In fact, "105" actually is less than "5"
#+ in a string comparison (ASCII sort order).
echo
exit 0

•
Attempting to use let to set string variables.
let "a = hello, you"
echo "$a"
# 0

•
Sometimes variables within "test" brackets ([ ]) need to be quoted (double quotes). Failure to do so
may cause unexpected behavior. See Example 7-6, Example 20-5, and Example 9-6.

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• Quoting a variable containing whitespace prevents splitting. Sometimes this produces unintended
consequences.
•
Commands issued from a script may fail to execute because the script owner lacks execute permission
for them. If a user cannot invoke a command from the command-line, then putting it into a script will
likewise fail. Try changing the attributes of the command in question, perhaps even setting the suid bit
(as root, of course).
•
Attempting to use - as a redirection operator (which it is not) will usually result in an unpleasant
surprise.
command1 2> - | command2
# Trying to redirect error output of command1 into a pipe . . .
# . . . will not work.
command1 2>& - | command2

# Also futile.

Thanks, S.C.

•
Using Bash version 2+ functionality may cause a bailout with error messages. Older Linux machines
may have version 1.XX of Bash as the default installation.
#!/bin/bash
minimum_version=2
# Since Chet Ramey is constantly adding features to Bash,
# you may set $minimum_version to 2.XX, 3.XX, or whatever is appropriate.
E_BAD_VERSION=80
if [ "$BASH_VERSION" \< "$minimum_version" ]
then
echo "This script works only with Bash, version $minimum or greater."
echo "Upgrade strongly recommended."
exit $E_BAD_VERSION
fi
...

• Using Bash-specific functionality in a Bourne shell script (#!/bin/sh) on a non-Linux machine
may cause unexpected behavior. A Linux system usually aliases sh to bash, but this does not
necessarily hold true for a generic UNIX machine.
•
Using undocumented features in Bash turns out to be a dangerous practice. In previous releases of this
book there were several scripts that depended on the "feature" that, although the maximum value of
an exit or return value was 255, that limit did not apply to negative integers. Unfortunately, in version
2.05b and later, that loophole disappeared. See Example 24-9.
•
In certain contexts, a misleading exit status may be returned. This may occur when setting a local
variable within a function or when assigning an arithmetic value to a variable.
• The exit status of an arithmetic expression is not equivalent to an error code.
var=1 && ((--var)) && echo $var
#
^^^^^^^^^ Here the and-list terminates with exit status 1.
#
$var doesn't echo!
echo $?
# 1

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• A script with DOS-type newlines (\r\n) will fail to execute, since #!/bin/bash\r\n is not
recognized, not the same as the expected #!/bin/bash\n. The fix is to convert the script to
UNIX-style newlines.
#!/bin/bash
echo "Here"
unix2dos $0
chmod 755 $0

# Script changes itself to DOS format.
# Change back to execute permission.
# The 'unix2dos' command removes execute permission.

./$0

# Script tries to run itself again.
# But it won't work as a DOS file.

echo "There"
exit 0

•
A shell script headed by #!/bin/sh will not run in full Bash-compatibility mode. Some
Bash-specific functions might be disabled. Scripts that need complete access to all the Bash-specific
extensions should start with #!/bin/bash.
• Putting whitespace in front of the terminating limit string of a here document will cause unexpected
behavior in a script.
• Putting more than one echo statement in a function whose output is captured.
add2 ()
{
echo "Whatever ... "
# Delete this line!
let "retval = $1 + $2"
echo $retval
}
num1=12
num2=43
echo "Sum of $num1 and $num2 = $(add2 $num1 $num2)"
#
#

Sum of 12 and 43 = Whatever ...
55

#

The "echoes" concatenate.

This will not work.
•
A script may not export variables back to its parent process, the shell, or to the environment. Just as
we learned in biology, a child process can inherit from a parent, but not vice versa.
WHATEVER=/home/bozo
export WHATEVER
exit 0
bash$ echo $WHATEVER
bash$

Sure enough, back at the command prompt, $WHATEVER remains unset.
•
Setting and manipulating variables in a subshell, then attempting to use those same variables outside
the scope of the subshell will result an unpleasant surprise.

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Example 34-2. Subshell Pitfalls
#!/bin/bash
# Pitfalls of variables in a subshell.
outer_variable=outer
echo
echo "outer_variable = $outer_variable"
echo
(
# Begin subshell
echo "outer_variable inside subshell = $outer_variable"
inner_variable=inner # Set
echo "inner_variable inside subshell = $inner_variable"
outer_variable=inner # Will value change globally?
echo "outer_variable inside subshell = $outer_variable"
# Will 'exporting' make a difference?
#
export inner_variable
#
export outer_variable
# Try it and see.
# End subshell
)
echo
echo "inner_variable outside subshell = $inner_variable"
echo "outer_variable outside subshell = $outer_variable"
echo

# Unset.
# Unchanged.

exit 0
# What happens if you uncomment lines 19 and 20?
# Does it make a difference?

•
Piping echo output to a read may produce unexpected results. In this scenario, the read acts as if it
were running in a subshell. Instead, use the set command (as in Example 15-18).

Example 34-3. Piping the output of echo to a read
#!/bin/bash
# badread.sh:
# Attempting to use 'echo and 'read'
#+ to assign variables non-interactively.
#

shopt -s lastpipe

a=aaa
b=bbb
c=ccc
echo "one two three" | read a b c
# Try to reassign a, b, and c.
echo
echo "a = $a"
echo "b = $b"

Chapter 34. Gotchas

# a = aaa
# b = bbb

489

Advanced Bash-Scripting Guide
echo "c = $c" # c = ccc
# Reassignment failed.
###
##
#
##+
###

However . . .
Uncommenting line 6:
shopt -s lastpipe
fixes the problem!
This is a new feature in Bash, version 4.2.

# -----------------------------# Try the following alternative.
var=`echo "one two three"`
set -- $var
a=$1; b=$2; c=$3
echo "-------"
echo "a = $a"
echo "b = $b"
echo "c = $c"
# Reassignment

# a = one
# b = two
# c = three
succeeded.

# -----------------------------#
#

Note also that an echo to a 'read' works within a subshell.
However, the value of the variable changes *only* within the subshell.

a=aaa
b=bbb
c=ccc

# Starting all over again.

echo; echo
echo "one two three" | ( read a b c;
echo "Inside subshell: "; echo "a = $a"; echo "b = $b"; echo "c = $c" )
# a = one
# b = two
# c = three
echo "-----------------"
echo "Outside subshell: "
echo "a = $a" # a = aaa
echo "b = $b" # b = bbb
echo "c = $c" # c = ccc
echo
exit 0

In fact, as Anthony Richardson points out, piping to any loop can cause a similar problem.
# Loop piping troubles.
# This example by Anthony Richardson,
#+ with addendum by Wilbert Berendsen.

foundone=false
find $HOME -type f -atime +30 -size 100k |
while true
do
read f
echo "$f is over 100KB and has not been accessed in over 30 days"
echo "Consider moving the file to archives."

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foundone=true
# -----------------------------------echo "Subshell level = $BASH_SUBSHELL"
# Subshell level = 1
# Yes, we're inside a subshell.
# -----------------------------------done
# foundone will always be false here since it is
#+ set to true inside a subshell
if [ $foundone = false ]
then
echo "No files need archiving."
fi
# =====================Now, here is the correct way:=================
foundone=false
for f in $(find $HOME -type f -atime +30 -size 100k) # No pipe here.
do
echo "$f is over 100KB and has not been accessed in over 30 days"
echo "Consider moving the file to archives."
foundone=true
done
if [ $foundone = false ]
then
echo "No files need archiving."
fi
# ==================And here is another alternative==================
# Places the part of the script that reads the variables
#+ within a code block, so they share the same subshell.
# Thank you, W.B.
find $HOME -type f -atime +30 -size 100k | {
foundone=false
while read f
do
echo "$f is over 100KB and has not been accessed in over 30 days"
echo "Consider moving the file to archives."
foundone=true
done
if ! $foundone
then
echo "No files need archiving."
fi
}

A lookalike problem occurs when trying to write the stdout of a tail -f piped to grep.
tail -f /var/log/messages | grep "$ERROR_MSG" >> error.log
# The "error.log" file will not have anything written to it.
# As Samuli Kaipiainen points out, this results from grep
#+ buffering its output.
# The fix is to add the "--line-buffered" parameter to grep.

•
Using "suid" commands within scripts is risky, as it may compromise system security. [120]
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• Using shell scripts for CGI programming may be problematic. Shell script variables are not
"typesafe," and this can cause undesirable behavior as far as CGI is concerned. Moreover, it is
difficult to "cracker-proof" shell scripts.
• Bash does not handle the double slash (//) string correctly.
•
Bash scripts written for Linux or BSD systems may need fixups to run on a commercial UNIX (or
Apple OSX) machine. Such scripts often employ the GNU set of commands and filters, which have
greater functionality than their generic UNIX counterparts. This is particularly true of such text
processing utilites as tr.
Danger is near thee -Beware, beware, beware, beware.
Many brave hearts are asleep in the deep.
So beware -Beware.
--A.J. Lamb and H.W. Petrie

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Chapter 35. Scripting With Style
Get into the habit of writing shell scripts in a structured and systematic manner. Even on-the-fly and "written
on the back of an envelope" scripts will benefit if you take a few minutes to plan and organize your thoughts
before sitting down and coding.
Herewith are a few stylistic guidelines. This is not (necessarily) intended as an Official Shell Scripting
Stylesheet.

35.1. Unofficial Shell Scripting Stylesheet
• Comment your code. This makes it easier for others to understand (and appreciate), and easier for you
to maintain.
PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"
# It made perfect sense when you wrote it last year,
#+ but now it's a complete mystery.
# (From Antek Sawicki's "pw.sh" script.)

Add descriptive headers to your scripts and functions.
#!/bin/bash
#************************************************#
#
xyz.sh
#
#
written by Bozo Bozeman
#
#
July 05, 2001
#
#
#
#
Clean up project files.
#
#************************************************#
E_BADDIR=85
projectdir=/home/bozo/projects

# No such directory.
# Directory to clean up.

# --------------------------------------------------------# cleanup_pfiles ()
# Removes all files in designated directory.
# Parameter: $target_directory
# Returns: 0 on success, $E_BADDIR if something went wrong.
# --------------------------------------------------------cleanup_pfiles ()
{
if [ ! -d "$1" ] # Test if target directory exists.
then
echo "$1 is not a directory."
return $E_BADDIR
fi

#
#
#
#
#
#

rm -f "$1"/*
return 0
# Success.
}
cleanup_pfiles $projectdir
exit $?

• Avoid using "magic numbers," [121] that is, "hard-wired" literal constants. Use meaningful variable
names instead. This makes the script easier to understand and permits making changes and updates
without breaking the application.
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if [ -f /var/log/messages ]
then
...
fi
# A year later, you decide to change the script to check /var/log/syslog.
# It is now necessary to manually change the script, instance by instance,
#+ and hope nothing breaks.
# A better way:
LOGFILE=/var/log/messages
if [ -f "$LOGFILE" ]
then
...
fi

# Only line that needs to be changed.

• Choose descriptive names for variables and functions.
fl=`ls -al $dirname`
file_listing=`ls -al $dirname`

# Cryptic.
# Better.

MAXVAL=10
# All caps used for a script constant.
while [ "$index" -le "$MAXVAL" ]
...

E_NOTFOUND=95

# Uppercase for an errorcode,
#+ and name prefixed with E_.

if [ ! -e "$filename" ]
then
echo "File $filename not found."
exit $E_NOTFOUND
fi

MAIL_DIRECTORY=/var/spool/mail/bozo
export MAIL_DIRECTORY

# Uppercase for an environmental
#+ variable.

GetAnswer ()
{
prompt=$1
echo -n $prompt
read answer
return $answer
}

# Mixed case works well for a
#+ function name, especially
#+ when it improves legibility.

GetAnswer "What is your favorite number? "
favorite_number=$?
echo $favorite_number

_uservariable=23
# Permissible, but not recommended.
# It's better for user-defined variables not to start with an underscore.
# Leave that for system variables.

• Use exit codes in a systematic and meaningful way.
E_WRONG_ARGS=95
...
...
exit $E_WRONG_ARGS

See also Appendix D.

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Ender suggests using the exit codes in /usr/include/sysexits.h in shell scripts, though these
are primarily intended for C and C++ programming.
• Use standardized parameter flags for script invocation. Ender proposes the following set of flags.
-a
-b
-c
-d
-e
-h
-l
-m
-n
-r
-s
-u
-v
-V

All: Return all information (including hidden file info).
Brief: Short version, usually for other scripts.
Copy, concatenate, etc.
Daily: Use information from the whole day, and not merely
information for a specific instance/user.
Extended/Elaborate: (often does not include hidden file info).
Help: Verbose usage w/descs, aux info, discussion, help.
See also -V.
Log output of script.
Manual: Launch man-page for base command.
Numbers: Numerical data only.
Recursive: All files in a directory (and/or all sub-dirs).
Setup & File Maintenance: Config files for this script.
Usage: List of invocation flags for the script.
Verbose: Human readable output, more or less formatted.
Version / License / Copy(right|left) / Contribs (email too).

See also Section F.1.
• Break complex scripts into simpler modules. Use functions where appropriate. See Example 37-4.
• Don't use a complex construct where a simpler one will do.
COMMAND
if [ $? -eq 0 ]
...
# Redundant and non-intuitive.
if COMMAND
...
# More concise (if perhaps not quite as legible).

... reading the UNIX source code to the Bourne
shell (/bin/sh). I was shocked at how much simple
algorithms could be made cryptic, and therefore
useless, by a poor choice of code style. I asked
myself, "Could someone be proud of this code?"
--Landon Noll

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Chapter 36. Miscellany
Nobody really knows what the Bourne shell's
grammar is. Even examination of the source code
is little help.
--Tom Duff

36.1. Interactive and non-interactive shells and scripts
An interactive shell reads commands from user input on a tty. Among other things, such a shell reads startup
files on activation, displays a prompt, and enables job control by default. The user can interact with the shell.
A shell running a script is always a non-interactive shell. All the same, the script can still access its tty. It is
even possible to emulate an interactive shell in a script.
#!/bin/bash
MY_PROMPT='$ '
while :
do
echo -n "$MY_PROMPT"
read line
eval "$line"
done
exit 0
# This example script, and much of the above explanation supplied by
# Stéphane Chazelas (thanks again).

Let us consider an interactive script to be one that requires input from the user, usually with read statements
(see Example 15-3). "Real life" is actually a bit messier than that. For now, assume an interactive script is
bound to a tty, a script that a user has invoked from the console or an xterm.
Init and startup scripts are necessarily non-interactive, since they must run without human intervention. Many
administrative and system maintenance scripts are likewise non-interactive. Unvarying repetitive tasks cry out
for automation by non-interactive scripts.
Non-interactive scripts can run in the background, but interactive ones hang, waiting for input that never
comes. Handle that difficulty by having an expect script or embedded here document feed input to an
interactive script running as a background job. In the simplest case, redirect a file to supply input to a read
statement (read variable > $LOGFILE
# Now, do it.
exec $OPERATION "$@"
# It's necessary to do the logging before the operation.
# Why?

Example 36-4. A shell wrapper around an awk script
#!/bin/bash
# pr-ascii.sh: Prints a table of ASCII characters.
START=33
END=127

# Range of printable ASCII characters (decimal).
# Will not work for unprintable characters (> 127).

echo " Decimal
echo " -------

Hex
---

Character"
---------"

# Header.

for ((i=START; i<=END; i++))
do
echo $i | awk '{printf(" %3d
%2x
%c\n", $1, $1, $1)}'
# The Bash printf builtin will not work in this context:
#
printf "%c" "$i"

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done
exit 0

#
#
#
#
#
#
#
#
#
#
#
#
#

Decimal
------33
34
35
36

Hex
--21
22
23
24

Character
--------!
"
#
$

7a
7b
7c
7d

z
{
|
}

. . .
122
123
124
125

# Redirect the output of this script to a file
#+ or pipe it to "more": sh pr-asc.sh | more

Example 36-5. A shell wrapper around another awk script
#!/bin/bash
# Adds up a specified column (of numbers) in the target file.
# Floating-point (decimal) numbers okay, because awk can handle them.
ARGS=2
E_WRONGARGS=85
if [ $# -ne "$ARGS" ] # Check for proper number of command-line args.
then
echo "Usage: `basename $0` filename column-number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
#
#
#+
#
#
#
#

Passing shell variables to the awk part of the script is a bit tricky.
One method is to strong-quote the Bash-script variable
within the awk script.
$'$BASH_SCRIPT_VAR'
^
^
This is done in the embedded awk script below.
See the awk documentation for more details.

# A multi-line awk script is here invoked by
#
awk '
#
...
#
...
#
...
#
'

# Begin awk script.
# ----------------------------awk '

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{ total += $'"${column_number}"'
}
END {
print total
}
' "$filename"
# ----------------------------# End awk script.

#
#+
#
#
#
#
#
#
#
#

It may not be safe to pass shell variables to an embedded awk script,
so Stephane Chazelas proposes the following alternative:
--------------------------------------awk -v column_number="$column_number" '
{ total += $column_number
}
END {
print total
}' "$filename"
---------------------------------------

exit 0

For those scripts needing a single do-it-all tool, a Swiss army knife, there is Perl. Perl combines the
capabilities of sed and awk, and throws in a large subset of C, to boot. It is modular and contains support for
everything ranging from object-oriented programming up to and including the kitchen sink. Short Perl scripts
lend themselves to embedding within shell scripts, and there may be some substance to the claim that Perl can
totally replace shell scripting (though the author of the ABS Guide remains skeptical).

Example 36-6. Perl embedded in a Bash script
#!/bin/bash
# Shell commands may precede the Perl script.
echo "This precedes the embedded Perl script within \"$0\"."
echo "==============================================================="
perl -e 'print "This is an embedded Perl script.\n";'
# Like sed, Perl also uses the "-e" option.
echo "==============================================================="
echo "However, the script may also contain shell and system commands."
exit

It is even possible to combine a Bash script and Perl script within the same file. Depending on how the script
is invoked, either the Bash part or the Perl part will execute.

Example 36-7. Bash and Perl scripts combined
#!/bin/bash
# bashandperl.sh

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echo "Greetings from the Bash part of the script, $0."
# More Bash commands may follow here.
exit
# End of Bash part of the script.
# =======================================================
#!/usr/bin/perl
# This part of the script must be invoked with
#
perl -x bashandperl.sh
print "Greetings from the Perl part of the script, $0.\n";
#
Perl doesn't seem to like "echo" ...
# More Perl commands may follow here.
# End of Perl part of the script.
bash$ bash bashandperl.sh
Greetings from the Bash part of the script.

bash$ perl -x bashandperl.sh
Greetings from the Perl part of the script.

One interesting example of a complex shell wrapper is Martin Matusiak's undvd script, which provides an
easy-to-use command-line interface to the complex mencoder utility. Another example is Itzchak Rehberg's
Ext3Undel, a set of scripts to recover deleted file on an ext3 filesystem.

36.3. Tests and Comparisons: Alternatives
For tests, the [[ ]] construct may be more appropriate than [ ]. Likewise, arithmetic comparisons might
benefit from the (( )) construct.
a=8
# All of the comparisons below are equivalent.
test "$a" -lt 16 && echo "yes, $a < 16"
/bin/test "$a" -lt 16 && echo "yes, $a < 16"
[ "$a" -lt 16 ] && echo "yes, $a < 16"
[[ $a -lt 16 ]] && echo "yes, $a < 16"
(( a < 16 )) && echo "yes, $a < 16"

# "and list"

# Quoting variables within
# [[ ]] and (( )) not necessary.

city="New York"
# Again, all of the comparisons below are equivalent.
test "$city" \< Paris && echo "Yes, Paris is greater than $city"
# Greater ASCII order.
/bin/test "$city" \< Paris && echo "Yes, Paris is greater than $city"
[ "$city" \< Paris ] && echo "Yes, Paris is greater than $city"
[[ $city < Paris ]] && echo "Yes, Paris is greater than $city"
# Need not quote $city.
# Thank you, S.C.

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36.4. Recursion: a script calling itself
Can a script recursively call itself? Indeed.

Example 36-8. A (useless) script that recursively calls itself
#!/bin/bash
# recurse.sh
#
#
#

Can a script recursively call itself?
Yes, but is this of any practical use?
(See the following.)

RANGE=10
MAXVAL=9
i=$RANDOM
let "i %= $RANGE"

# Generate a random number between 0 and $RANGE - 1.

if [ "$i" -lt "$MAXVAL" ]
then
echo "i = $i"
./$0
# Script recursively spawns a new instance of itself.
fi
# Each child script does the same, until
#+ a generated $i equals $MAXVAL.
#
#

Using a "while" loop instead of an "if/then" test causes problems.
Explain why.

exit 0
#
#
#
#
#

Note:
---This script must have execute permission for it to work properly.
This is the case even if it is invoked by an "sh" command.
Explain why.

Example 36-9. A (useful) script that recursively calls itself
#!/bin/bash
# pb.sh: phone book
# Written by Rick Boivie, and used with permission.
# Modifications by ABS Guide author.
MINARGS=1
# Script needs at least one argument.
DATAFILE=./phonebook
# A data file in current working directory
#+ named "phonebook" must exist.
PROGNAME=$0
E_NOARGS=70
# No arguments error.
if [ $# -lt $MINARGS ]; then
echo "Usage: "$PROGNAME" data-to-look-up"
exit $E_NOARGS
fi

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if [ $# -eq $MINARGS ]; then
grep $1 "$DATAFILE"
# 'grep' prints an error message if $DATAFILE not present.
else
( shift; "$PROGNAME" $* ) | grep $1
# Script recursively calls itself.
fi
exit 0

# Script exits here.
# Therefore, it's o.k. to put
#+ non-hashmarked comments and data after this point.

# -----------------------------------------------------------------------Sample "phonebook" datafile:
John Doe
1555 Main St., Baltimore, MD 21228
(410) 222-3333
Mary Moe
9899 Jones Blvd., Warren, NH 03787
(603) 898-3232
Richard Roe
856 E. 7th St., New York, NY 10009
(212) 333-4567
Sam Roe
956 E. 8th St., New York, NY 10009
(212) 444-5678
Zoe Zenobia
4481 N. Baker St., San Francisco, SF 94338 (415) 501-1631
# -----------------------------------------------------------------------$bash pb.sh Roe
Richard Roe
856 E. 7th St., New York, NY 10009
Sam Roe
956 E. 8th St., New York, NY 10009

(212) 333-4567
(212) 444-5678

$bash pb.sh Roe Sam
Sam Roe
956 E. 8th St., New York, NY 10009

(212) 444-5678

# When more than one argument is passed to this script,
#+ it prints *only* the line(s) containing all the arguments.

Example 36-10. Another (useful) script that recursively calls itself
#!/bin/bash
# usrmnt.sh, written by Anthony Richardson
# Used with permission.
# usage:
usrmnt.sh
# description: mount device, invoking user must be listed in the
#
MNTUSERS group in the /etc/sudoers file.
# ---------------------------------------------------------# This is a usermount script that reruns itself using sudo.
# A user with the proper permissions only has to type
#

usermount /dev/fd0 /mnt/floppy

# instead of
#

sudo usermount /dev/fd0 /mnt/floppy

# I use this same technique for all of my
#+ sudo scripts, because I find it convenient.
# ---------------------------------------------------------# If SUDO_COMMAND variable is not set we are not being run through
#+ sudo, so rerun ourselves. Pass the user's real and group id . . .
if [ -z "$SUDO_COMMAND" ]

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then
mntusr=$(id -u) grpusr=$(id -g) sudo $0 $*
exit 0
fi
# We will only get here if we are being run by sudo.
/bin/mount $* -o uid=$mntusr,gid=$grpusr
exit 0
# Additional notes (from the author of this script):
# ------------------------------------------------# 1) Linux allows the "users" option in the /etc/fstab
#
file so that any user can mount removable media.
#
But, on a server, I like to allow only a few
#
individuals access to removable media.
#
I find using sudo gives me more control.
# 2) I also find sudo to be more convenient than
#
accomplishing this task through groups.
# 3) This method gives anyone with proper permissions
#
root access to the mount command, so be careful
#
about who you allow access.
#
You can get finer control over which access can be mounted
#
by using this same technique in separate mntfloppy, mntcdrom,
#
and mntsamba scripts.

Too many levels of recursion can exhaust the script's stack space, causing a segfault.

36.5. "Colorizing" Scripts
The ANSI [123] escape sequences set screen attributes, such as bold text, and color of foreground and
background. DOS batch files commonly used ANSI escape codes for color output, and so can Bash scripts.

Example 36-11. A "colorized" address database
#!/bin/bash
# ex30a.sh: "Colorized" version of ex30.sh.
#
Crude address database

clear

# Clear the screen.

echo -n "
"
echo -e '\E[37;44m'"\033[1mContact List\033[0m"
# White on blue background
echo; echo
echo -e "\033[1mChoose one of the following persons:\033[0m"
# Bold
tput sgr0
# Reset attributes.
echo "(Enter only the first letter of name.)"
echo
echo -en '\E[47;34m'"\033[1mE\033[0m"
# Blue
tput sgr0
# Reset colors to "normal."

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echo
echo
tput
echo
echo
tput
echo
echo
tput
echo
echo

"vans, Roland"
-en '\E[47;35m'"\033[1mJ\033[0m"
sgr0
"ones, Mildred"
-en '\E[47;32m'"\033[1mS\033[0m"
sgr0
"mith, Julie"
-en '\E[47;31m'"\033[1mZ\033[0m"
sgr0
"ane, Morris"

# "[E]vans, Roland"
# Magenta

# Green

# Red

read person
case "$person" in
# Note variable is quoted.
"E" | "e" )
# Accept upper or lowercase input.
echo
echo "Roland Evans"
echo "4321 Flash Dr."
echo "Hardscrabble, CO 80753"
echo "(303) 734-9874"
echo "(303) 734-9892 fax"
echo "revans@zzy.net"
echo "Business partner & old friend"
;;
"J" | "j" )
echo
echo "Mildred Jones"
echo "249 E. 7th St., Apt. 19"
echo "New York, NY 10009"
echo "(212) 533-2814"
echo "(212) 533-9972 fax"
echo "milliej@loisaida.com"
echo "Girlfriend"
echo "Birthday: Feb. 11"
;;
# Add info for Smith & Zane later.
* )
# Default option.
# Empty input (hitting RETURN) fits here, too.
echo
echo "Not yet in database."
;;
esac
tput sgr0

# Reset colors to "normal."

echo
exit 0

Example 36-12. Drawing a box
#!/bin/bash

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# Draw-box.sh: Drawing a box using ASCII characters.
# Script by Stefano Palmeri, with minor editing by document author.
# Minor edits suggested by Jim Angstadt.
# Used in the ABS Guide with permission.

######################################################################
### draw_box function doc ###
# The "draw_box" function lets the user
#+ draw a box in a terminal.
#
# Usage: draw_box ROW COLUMN HEIGHT WIDTH [COLOR]
# ROW and COLUMN represent the position
#+ of the upper left angle of the box you're going to draw.
# ROW and COLUMN must be greater than 0
#+ and less than current terminal dimension.
# HEIGHT is the number of rows of the box, and must be > 0.
# HEIGHT + ROW must be <= than current terminal height.
# WIDTH is the number of columns of the box and must be > 0.
# WIDTH + COLUMN must be <= than current terminal width.
#
# E.g.: If your terminal dimension is 20x80,
# draw_box 2 3 10 45 is good
# draw_box 2 3 19 45 has bad HEIGHT value (19+2 > 20)
# draw_box 2 3 18 78 has bad WIDTH value (78+3 > 80)
#
# COLOR is the color of the box frame.
# This is the 5th argument and is optional.
# 0=black 1=red 2=green 3=tan 4=blue 5=purple 6=cyan 7=white.
# If you pass the function bad arguments,
#+ it will just exit with code 65,
#+ and no messages will be printed on stderr.
#
# Clear the terminal before you start to draw a box.
# The clear command is not contained within the function.
# This allows the user to draw multiple boxes, even overlapping ones.
### end of draw_box function doc ###
######################################################################
draw_box(){
#=============#
HORZ="-"
VERT="|"
CORNER_CHAR="+"
MINARGS=4
E_BADARGS=65
#=============#

if [ $# -lt "$MINARGS" ]; then
exit $E_BADARGS
fi

# If args are less than 4, exit.

# Looking for non digit chars in arguments.
# Probably it could be done better (exercise for the reader?).
if echo $@ | tr -d [:blank:] | tr -d [:digit:] | grep . &> /dev/null; then
exit $E_BADARGS

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fi
BOX_HEIGHT=`expr $3 - 1`
BOX_WIDTH=`expr $4 - 1`
T_ROWS=`tput lines`
T_COLS=`tput cols`

#
#+
#
#+

-1 correction needed because angle char "+"
is a part of both box height and width.
Define current terminal dimension
in rows and columns.

if [ $1 -lt 1 ] || [ $1 -gt $T_ROWS ]; then
# Start checking if arguments
exit $E_BADARGS
#+ are correct.
fi
if [ $2 -lt 1 ] || [ $2 -gt $T_COLS ]; then
exit $E_BADARGS
fi
if [ `expr $1 + $BOX_HEIGHT + 1` -gt $T_ROWS ]; then
exit $E_BADARGS
fi
if [ `expr $2 + $BOX_WIDTH + 1` -gt $T_COLS ]; then
exit $E_BADARGS
fi
if [ $3 -lt 1 ] || [ $4 -lt 1 ]; then
exit $E_BADARGS
fi
# End checking arguments.
plot_char(){
echo -e "\E[${1};${2}H"$3
}

# Function within a function.

echo -ne "\E[3${5}m"

# Set box frame color, if defined.

# start drawing the box
count=1
for (( r=$1; count<=$BOX_HEIGHT; r++)); do
plot_char $r $2 $VERT
let count=count+1
done

# Draw vertical lines using
#+ plot_char function.

count=1
c=`expr $2 + $BOX_WIDTH`
for (( r=$1; count<=$BOX_HEIGHT; r++)); do
plot_char $r $c $VERT
let count=count+1
done
count=1
for (( c=$2; count<=$BOX_WIDTH; c++)); do
plot_char $1 $c $HORZ
let count=count+1
done

# Draw horizontal lines using
#+ plot_char function.

count=1
r=`expr $1 + $BOX_HEIGHT`
for (( c=$2; count<=$BOX_WIDTH; c++)); do
plot_char $r $c $HORZ
let count=count+1
done
plot_char
plot_char
plot_char
plot_char

$1 $2 $CORNER_CHAR
# Draw box angles.
$1 `expr $2 + $BOX_WIDTH` $CORNER_CHAR
`expr $1 + $BOX_HEIGHT` $2 $CORNER_CHAR
`expr $1 + $BOX_HEIGHT` `expr $2 + $BOX_WIDTH` $CORNER_CHAR

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echo -ne "\E[0m"

#

Restore old colors.

P_ROWS=`expr $T_ROWS - 1`

#

Put the prompt at bottom of the terminal.

echo -e "\E[${P_ROWS};1H"
}

# Now, let's try drawing a box.
clear
# Clear the terminal.
R=2
# Row
C=3
# Column
H=10
# Height
W=45
# Width
col=1
# Color (red)
draw_box $R $C $H $W $col
# Draw the box.
exit 0
# Exercise:
# -------# Add the option of printing text within the drawn box.

The simplest, and perhaps most useful ANSI escape sequence is bold text, \033[1m ... \033[0m. The \033
represents an escape, the "[1" turns on the bold attribute, while the "[0" switches it off. The "m" terminates
each term of the escape sequence.
bash$ echo -e "\033[1mThis is bold text.\033[0m"

A similar escape sequence switches on the underline attribute (on an rxvt and an aterm).
bash$ echo -e "\033[4mThis is underlined text.\033[0m"

With an echo, the -e option enables the escape sequences.
Other escape sequences change the text and/or background color.
bash$ echo -e '\E[34;47mThis prints in blue.'; tput sgr0

bash$ echo -e '\E[33;44m'"yellow text on blue background"; tput sgr0

bash$ echo -e '\E[1;33;44m'"BOLD yellow text on blue background"; tput sgr0

It's usually advisable to set the bold attribute for light-colored foreground text.
The tput sgr0 restores the terminal settings to normal. Omitting this lets all subsequent output from that
particular terminal remain blue.
Since tput sgr0 fails to restore terminal settings under certain circumstances, echo -ne \E[0m may be a
better choice.

Use the following template for writing colored text on a colored background.
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echo -e '\E[COLOR1;COLOR2mSome text goes here.'
The "\E[" begins the escape sequence. The semicolon-separated numbers "COLOR1" and "COLOR2"
specify a foreground and a background color, according to the table below. (The order of the numbers does
not matter, since the foreground and background numbers fall in non-overlapping ranges.) The "m"
terminates the escape sequence, and the text begins immediately after that.
Note also that single quotes enclose the remainder of the command sequence following the echo -e.
The numbers in the following table work for an rxvt terminal. Results may vary for other terminal emulators.

Table 36-1. Numbers representing colors in Escape Sequences
Color
black
red
green
yellow
blue
magenta
cyan
white

Foreground
30
31
32
33
34
35
36
37

Background
40
41
42
43
44
45
46
47

Example 36-13. Echoing colored text
#!/bin/bash
# color-echo.sh: Echoing text messages in color.
# Modify this script for your own purposes.
# It's easier than hand-coding color.
black='\E[30;47m'
red='\E[31;47m'
green='\E[32;47m'
yellow='\E[33;47m'
blue='\E[34;47m'
magenta='\E[35;47m'
cyan='\E[36;47m'
white='\E[37;47m'

alias Reset="tput sgr0"

# Reset text attributes to normal
#+ without clearing screen.

cecho ()

# Color-echo.
# Argument $1 = message
# Argument $2 = color

{
local default_msg="No message passed."
# Doesn't really need to be a local variable.

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message=${1:-$default_msg}
color=${2:-$black}
echo -e "$color"
echo "$message"
Reset

# Defaults to default message.
# Defaults to black, if not specified.

# Reset to normal.

return
}

# Now, let's try it out.
# ---------------------------------------------------cecho "Feeling blue..." $blue
cecho "Magenta looks more like purple." $magenta
cecho "Green with envy." $green
cecho "Seeing red?" $red
cecho "Cyan, more familiarly known as aqua." $cyan
cecho "No color passed (defaults to black)."
# Missing $color argument.
cecho "\"Empty\" color passed (defaults to black)." ""
# Empty $color argument.
cecho
# Missing $message and $color arguments.
cecho "" ""
# Empty $message and $color arguments.
# ---------------------------------------------------echo
exit 0
#
#
#
#

Exercises:
--------1) Add the "bold" attribute to the 'cecho ()' function.
2) Add options for colored backgrounds.

Example 36-14. A "horserace" game
#!/bin/bash
# horserace.sh: Very simple horserace simulation.
# Author: Stefano Palmeri
# Used with permission.
################################################################
# Goals of the script:
# playing with escape sequences and terminal colors.
#
# Exercise:
# Edit the script to make it run less randomly,
#+ set up a fake betting shop . . .
# Um . . . um . . . it's starting to remind me of a movie . . .
#
# The script gives each horse a random handicap.
# The odds are calculated upon horse handicap
#+ and are expressed in European(?) style.
# E.g., odds=3.75 means that if you bet $1 and win,
#+ you receive $3.75.
#
# The script has been tested with a GNU/Linux OS,

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#+ using xterm and rxvt, and konsole.
# On a machine with an AMD 900 MHz processor,
#+ the average race time is 75 seconds.
# On faster computers the race time would be lower.
# So, if you want more suspense, reset the USLEEP_ARG variable.
#
# Script by Stefano Palmeri.
################################################################
E_RUNERR=65
# Check if md5sum and bc are installed.
if ! which bc &> /dev/null; then
echo bc is not installed.
echo "Can\'t run . . . "
exit $E_RUNERR
fi
if ! which md5sum &> /dev/null; then
echo md5sum is not installed.
echo "Can\'t run . . . "
exit $E_RUNERR
fi
# Set the following variable to slow down script execution.
# It will be passed as the argument for usleep (man usleep)
#+ and is expressed in microseconds (500000 = half a second).
USLEEP_ARG=0
# Clean up the temp directory, restore terminal cursor and
#+ terminal colors -- if script interrupted by Ctl-C.
trap 'echo -en "\E[?25h"; echo -en "\E[0m"; stty echo;\
tput cup 20 0; rm -fr $HORSE_RACE_TMP_DIR' TERM EXIT
# See the chapter on debugging for an explanation of 'trap.'
# Set a unique (paranoid) name for the temp directory the script needs.
HORSE_RACE_TMP_DIR=$HOME/.horserace-`date +%s`-`head -c10 /dev/urandom \
| md5sum | head -c30`
# Create the temp directory and move right in.
mkdir $HORSE_RACE_TMP_DIR
cd $HORSE_RACE_TMP_DIR

# This function moves the cursor to line $1 column $2 and then prints $3.
# E.g.: "move_and_echo 5 10 linux" is equivalent to
#+ "tput cup 4 9; echo linux", but with one command instead of two.
# Note: "tput cup" defines 0 0 the upper left angle of the terminal,
#+ echo defines 1 1 the upper left angle of the terminal.
move_and_echo() {
echo -ne "\E[${1};${2}H""$3"
}
# Function to generate a pseudo-random number between 1 and 9.
random_1_9 ()
{
head -c10 /dev/urandom | md5sum | tr -d [a-z] | tr -d 0 | cut -c1
}
# Two functions that simulate "movement," when drawing the horses.
draw_horse_one() {
echo -n " "//$MOVE_HORSE//
}

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draw_horse_two(){
echo -n " "\\\\$MOVE_HORSE\\\\
}

# Define current terminal dimension.
N_COLS=`tput cols`
N_LINES=`tput lines`
# Need at least a 20-LINES X 80-COLUMNS terminal. Check it.
if [ $N_COLS -lt 80 ] || [ $N_LINES -lt 20 ]; then
echo "`basename $0` needs a 80-cols X 20-lines terminal."
echo "Your terminal is ${N_COLS}-cols X ${N_LINES}-lines."
exit $E_RUNERR
fi

# Start drawing the race field.
# Need a string of 80 chars. See below.
BLANK80=`seq -s "" 100 | head -c80`
clear
# Set foreground and background colors to white.
echo -ne '\E[37;47m'
# Move the cursor on the upper left angle of the terminal.
tput cup 0 0
# Draw six white lines.
for n in `seq 5`; do
echo $BLANK80
# Use the 80 chars string to colorize the terminal.
done
# Sets foreground color to black.
echo -ne '\E[30m'
move_and_echo
move_and_echo
move_and_echo
move_and_echo
move_and_echo
move_and_echo
move_and_echo
move_and_echo
move_and_echo
move_and_echo

3
3
1
1
2
2
4
4
5
5

1 "START 1"
75 FINISH
5 "|"
80 "|"
5 "|"
80 "|"
5 "| 2"
80 "|"
5 "V 3"
80 "V"

# Set foreground color to red.
echo -ne '\E[31m'
# Some ASCII art.
move_and_echo 1 8 "..@@@..@@@@@...@@@@@.@...@..@@@@..."
move_and_echo 2 8 ".@...@...@.......@...@...@.@......."
move_and_echo 3 8 ".@@@@@...@.......@...@@@@@.@@@@...."
move_and_echo 4 8 ".@...@...@.......@...@...@.@......."
move_and_echo 5 8 ".@...@...@.......@...@...@..@@@@..."
move_and_echo 1 43 "@@@@...@@@...@@@@..@@@@..@@@@."
move_and_echo 2 43 "@...@.@...@.@.....@.....@....."
move_and_echo 3 43 "@@@@..@@@@@.@.....@@@@...@@@.."
move_and_echo 4 43 "@..@..@...@.@.....@.........@."

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move_and_echo 5 43 "@...@.@...@..@@@@..@@@@.@@@@.."

# Set foreground and background colors to green.
echo -ne '\E[32;42m'
# Draw eleven green lines.
tput cup 5 0
for n in `seq 11`; do
echo $BLANK80
done
# Set foreground color to black.
echo -ne '\E[30m'
tput cup 5 0
# Draw the fences.
echo "++++++++++++++++++++++++++++++++++++++\
++++++++++++++++++++++++++++++++++++++++++"
tput cup 15 0
echo "++++++++++++++++++++++++++++++++++++++\
++++++++++++++++++++++++++++++++++++++++++"
# Set foreground and background colors to white.
echo -ne '\E[37;47m'
# Draw three white lines.
for n in `seq 3`; do
echo $BLANK80
done
# Set foreground color to black.
echo -ne '\E[30m'
# Create 9 files to stores handicaps.
for n in `seq 10 7 68`; do
touch $n
done
# Set the first type of "horse" the script will draw.
HORSE_TYPE=2
# Create position-file and odds-file for every "horse".
#+ In these files, store the current position of the horse,
#+ the type and the odds.
for HN in `seq 9`; do
touch horse_${HN}_position
touch odds_${HN}
echo \-1 > horse_${HN}_position
echo $HORSE_TYPE >> horse_${HN}_position
# Define a random handicap for horse.
HANDICAP=`random_1_9`
# Check if the random_1_9 function returned a good value.
while ! echo $HANDICAP | grep [1-9] &> /dev/null; do
HANDICAP=`random_1_9`
done
# Define last handicap position for horse.
LHP=`expr $HANDICAP \* 7 + 3`
for FILE in `seq 10 7 $LHP`; do
echo $HN >> $FILE
done

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# Calculate odds.
case $HANDICAP in
1) ODDS=`echo $HANDICAP \* 0.25 + 1.25 | bc`
echo $ODDS > odds_${HN}
;;
2 | 3) ODDS=`echo $HANDICAP \* 0.40 + 1.25 | bc`
echo $ODDS > odds_${HN}
;;
4 | 5 | 6) ODDS=`echo $HANDICAP \* 0.55 + 1.25 | bc`
echo $ODDS > odds_${HN}
;;
7 | 8) ODDS=`echo $HANDICAP \* 0.75 + 1.25 | bc`
echo $ODDS > odds_${HN}
;;
9) ODDS=`echo $HANDICAP \* 0.90 + 1.25 | bc`
echo $ODDS > odds_${HN}
esac

done

# Print odds.
print_odds() {
tput cup 6 0
echo -ne '\E[30;42m'
for HN in `seq 9`; do
echo "#$HN odds->" `cat odds_${HN}`
done
}
# Draw the horses at starting line.
draw_horses() {
tput cup 6 0
echo -ne '\E[30;42m'
for HN in `seq 9`; do
echo /\\$HN/\\"
done
}

"

print_odds
echo -ne '\E[47m'
# Wait for a enter key press to start the race.
# The escape sequence '\E[?25l' disables the cursor.
tput cup 17 0
echo -e '\E[?25l'Press [enter] key to start the race...
read -s
# Disable normal echoing in the terminal.
# This avoids key presses that might "contaminate" the screen
#+ during the race.
stty -echo
# -------------------------------------------------------# Start the race.
draw_horses
echo -ne '\E[37;47m'
move_and_echo 18 1 $BLANK80
echo -ne '\E[30m'

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move_and_echo 18 1 Starting...
sleep 1
# Set the column of the finish line.
WINNING_POS=74
# Define the time the race started.
START_TIME=`date +%s`
# COL variable needed by following "while" construct.
COL=0
while [ $COL -lt $WINNING_POS ]; do
MOVE_HORSE=0
# Check if the random_1_9 function has returned a good value.
while ! echo $MOVE_HORSE | grep [1-9] &> /dev/null; do
MOVE_HORSE=`random_1_9`
done
# Define old type and position of the "randomized horse".
HORSE_TYPE=`cat horse_${MOVE_HORSE}_position | tail -n 1`
COL=$(expr `cat horse_${MOVE_HORSE}_position | head -n 1`)
ADD_POS=1
# Check if the current position is an handicap position.
if seq 10 7 68 | grep -w $COL &> /dev/null; then
if grep -w $MOVE_HORSE $COL &> /dev/null; then
ADD_POS=0
grep -v -w $MOVE_HORSE $COL > ${COL}_new
rm -f $COL
mv -f ${COL}_new $COL
else ADD_POS=1
fi
else ADD_POS=1
fi
COL=`expr $COL + $ADD_POS`
echo $COL > horse_${MOVE_HORSE}_position # Store new position.
# Choose the type of horse to draw.
case $HORSE_TYPE in
1) HORSE_TYPE=2; DRAW_HORSE=draw_horse_two
;;
2) HORSE_TYPE=1; DRAW_HORSE=draw_horse_one
esac
echo $HORSE_TYPE >> horse_${MOVE_HORSE}_position
# Store current type.
# Set foreground color to black and background to green.
echo -ne '\E[30;42m'
# Move the cursor to new horse position.
tput cup `expr $MOVE_HORSE + 5` \
`cat horse_${MOVE_HORSE}_position | head -n 1`
# Draw the horse.
$DRAW_HORSE
usleep $USLEEP_ARG
# When all horses have gone beyond field line 15, reprint odds.
touch fieldline15

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if [ $COL = 15 ]; then
echo $MOVE_HORSE >> fieldline15
fi
if [ `wc -l fieldline15 | cut -f1 -d " "` = 9 ]; then
print_odds
: > fieldline15
fi
# Define the leading horse.
HIGHEST_POS=`cat *position | sort -n | tail -1`
# Set background color to white.
echo -ne '\E[47m'
tput cup 17 0
echo -n Current leader: `grep -w $HIGHEST_POS *position | cut -c7`\
"
"
done
# Define the time the race finished.
FINISH_TIME=`date +%s`
# Set background color to green and enable blinking text.
echo -ne '\E[30;42m'
echo -en '\E[5m'
# Make the winning horse blink.
tput cup `expr $MOVE_HORSE + 5` \
`cat horse_${MOVE_HORSE}_position | head -n 1`
$DRAW_HORSE
# Disable blinking text.
echo -en '\E[25m'
# Set foreground and background color to white.
echo -ne '\E[37;47m'
move_and_echo 18 1 $BLANK80
# Set foreground color to black.
echo -ne '\E[30m'
# Make winner blink.
tput cup 17 0
echo -e "\E[5mWINNER: $MOVE_HORSE\E[25m"" Odds: `cat odds_${MOVE_HORSE}`"\
" Race time: `expr $FINISH_TIME - $START_TIME` secs"
# Restore cursor and old colors.
echo -en "\E[?25h"
echo -en "\E[0m"
# Restore echoing.
stty echo
# Remove race temp directory.
rm -rf $HORSE_RACE_TMP_DIR
tput cup 19 0
exit 0

See also Example A-21, Example A-44, Example A-52, and Example A-40.

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There is, however, a major problem with all this. ANSI escape sequences are emphatically non-portable.
What works fine on some terminal emulators (or the console) may work differently, or not at all, on
others. A "colorized" script that looks stunning on the script author's machine may produce unreadable
output on someone else's. This somewhat compromises the usefulness of colorizing scripts, and possibly
relegates this technique to the status of a gimmick. Colorized scripts are probably inappropriate in a
commercial setting, i.e., your supervisor might disapprove.
Alister's ansi-color utility (based on Moshe Jacobson's color utility considerably simplifies using ANSI
escape sequences. It substitutes a clean and logical syntax for the clumsy constructs just discussed.
Henry/teikedvl has likewise created a utility (http://scriptechocolor.sourceforge.net/) to simplify creation of
colorized scripts.

36.6. Optimizations
Most shell scripts are quick 'n dirty solutions to non-complex problems. As such, optimizing them for speed is
not much of an issue. Consider the case, though, where a script carries out an important task, does it well, but
runs too slowly. Rewriting it in a compiled language may not be a palatable option. The simplest fix would be
to rewrite the parts of the script that slow it down. Is it possible to apply principles of code optimization even
to a lowly shell script?
Check the loops in the script. Time consumed by repetitive operations adds up quickly. If at all possible,
remove time-consuming operations from within loops.
Use builtin commands in preference to system commands. Builtins execute faster and usually do not launch a
subshell when invoked.

Avoid unnecessary commands, particularly in a pipe.
cat "$file" | grep "$word"
grep "$word" "$file"
# The above command-lines have an identical effect,
#+ but the second runs faster since it launches one fewer subprocess.

The cat command seems especially prone to overuse in scripts.
Use the time and times tools to profile computation-intensive commands. Consider rewriting time-critical
code sections in C, or even in assembler.
Try to minimize file I/O. Bash is not particularly efficient at handling files, so consider using more
appropriate tools for this within the script, such as awk or Perl.
Write your scripts in a modular and coherent form, [124] so they can be reorganized and tightened up as
necessary. Some of the optimization techniques applicable to high-level languages may work for scripts, but
others, such as loop unrolling, are mostly irrelevant. Above all, use common sense.
For an excellent demonstration of how optimization can dramatically reduce the execution time of a script, see
Example 16-47.

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36.7. Assorted Tips
36.7.1. Ideas for more powerful scripts
•
You have a problem that you want to solve by writing a Bash script. Unfortunately, you don't know
quite where to start. One method is to plunge right in and code those parts of the script that come
easily, and write the hard parts as pseudo-code.
#!/bin/bash
ARGCOUNT=1
E_WRONGARGS=65

# Need name as argument.

if [ number-of-arguments is-not-equal-to "$ARGCOUNT" ]
#
^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^
# Can't figure out how to code this . . .
#+ . . . so write it in pseudo-code.
then
echo "Usage: name-of-script name"
#
^^^^^^^^^^^^^^
More pseudo-code.
exit $E_WRONGARGS
fi
. . .
exit 0

# Later on, substitute working code for the pseudo-code.
# Line 6 becomes:
if [ $# -ne "$ARGCOUNT" ]
# Line 12 becomes:
echo "Usage: `basename $0` name"

For an example of using pseudo-code, see the Square Root exercise.
•
To keep a record of which user scripts have run during a particular session or over a number of
sessions, add the following lines to each script you want to keep track of. This will keep a continuing
file record of the script names and invocation times.
# Append (>>) following to end of each script tracked.
whoami>> $SAVE_FILE
echo $0>> $SAVE_FILE
date>> $SAVE_FILE
echo>> $SAVE_FILE

#
#
#
#

User invoking the script.
Script name.
Date and time.
Blank line as separator.

# Of course, SAVE_FILE defined and exported as environmental variable in ~/.bashrc
#+ (something like ~/.scripts-run)

•
The >> operator appends lines to a file. What if you wish to prepend a line to an existing file, that is,
to paste it in at the beginning?

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file=data.txt
title="***This is the title line of data text file***"
echo $title | cat - $file >$file.new
# "cat -" concatenates stdout to $file.
# End result is
#+ to write a new file with $title appended at *beginning*.

This is a simplified variant of the Example 19-13 script given earlier. And, of course, sed can also do
this.
•
A shell script may act as an embedded command inside another shell script, a Tcl or wish script, or
even a Makefile. It can be invoked as an external shell command in a C program using the
system() call, i.e., system("script_name");.
•
Setting a variable to the contents of an embedded sed or awk script increases the readability of the
surrounding shell wrapper. See Example A-1 and Example 15-20.
•
Put together files containing your favorite and most useful definitions and functions. As necessary,
"include" one or more of these "library files" in scripts with either the dot (.) or source command.
# SCRIPT LIBRARY
# ------ ------# Note:
# No "#!" here.
# No "live code" either.

# Useful variable definitions
ROOT_UID=0
E_NOTROOT=101
MAXRETVAL=255
SUCCESS=0
FAILURE=-1

# Root has $UID 0.
# Not root user error.
# Maximum (positive) return value of a function.

# Functions
Usage ()
{
if [ -z "$1" ]
then
msg=filename
else
msg=$@
fi

# "Usage:" message.
# No arg passed.

echo "Usage: `basename $0` "$msg""
}

Check_if_root ()
# Check if root running script.
{
# From "ex39.sh" example.
if [ "$UID" -ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT

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fi
}

CreateTempfileName () # Creates a "unique" temp filename.
{
# From "ex51.sh" example.
prefix=temp
suffix=`eval date +%s`
Tempfilename=$prefix.$suffix
}

isalpha2 ()
# Tests whether *entire string* is alphabetic.
{
# From "isalpha.sh" example.
[ $# -eq 1 ] || return $FAILURE
case $1 in
*[!a-zA-Z]*|"") return $FAILURE;;
*) return $SUCCESS;;
esac
# Thanks, S.C.
}

abs ()
{
E_ARGERR=-999999

# Absolute value.
# Caution: Max return value = 255.

if [ -z "$1" ]
then
return $E_ARGERR
fi

# Need arg passed.

if [ "$1" -ge 0 ]
then
absval=$1
else
let "absval = (( 0 - $1 ))"
fi

#
#
#
#
#

# Obvious error value returned.

If non-negative,
stays as-is.
Otherwise,
change sign.

return $absval
}

tolower ()
{
if [ -z "$1" ]
then
echo "(null)"
return
fi

# Converts string(s) passed as argument(s)
#+ to lowercase.
#
#+
#+
#+

If no argument(s) passed,
send error message
(C-style void-pointer error message)
and return from function.

echo "$@" | tr A-Z a-z
# Translate all passed arguments ($@).
return
# Use command substitution to set a variable to function output.
# For example:
#
oldvar="A seT of miXed-caSe LEtTerS"
#
newvar=`tolower "$oldvar"`
#
echo "$newvar"
# a set of mixed-case letters

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#
# Exercise: Rewrite this function to change lowercase passed argument(s)
#
to uppercase ... toupper() [easy].
}

•
Use special-purpose comment headers to increase clarity and legibility in scripts.
## Caution.
rm -rf *.zzy

#+
#
#+
#+

## The "-rf" options to "rm" are very dangerous,
##+ especially with wild cards.

Line continuation.
This is line 1
of a multi-line comment,
and this is the final line.

#* Note.
#o List item.
#> Another point of view.
while [ "$var1" != "end" ]

#> while test "$var1" != "end"

•
Dotan Barak contributes template code for a progress bar in a script.

Example 36-15. A Progress Bar
#!/bin/bash
# progress-bar.sh
# Author: Dotan Barak (very minor revisions by ABS Guide author).
# Used in ABS Guide with permission (thanks!).

BAR_WIDTH=50
BAR_CHAR_START="["
BAR_CHAR_END="]"
BAR_CHAR_EMPTY="."
BAR_CHAR_FULL="="
BRACKET_CHARS=2
LIMIT=100
print_progress_bar()
{
# Calculate how many characters will be full.
let "full_limit = ((($1 - $BRACKET_CHARS) * $2) / $LIMIT)"
# Calculate how many characters will be empty.
let "empty_limit = ($1 - $BRACKET_CHARS) - ${full_limit}"
# Prepare the bar.
bar_line="${BAR_CHAR_START}"
for ((j=0; j/dev/null
# An integer is either equal to 0 or not equal to 0.

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# 2>/dev/null suppresses error message.
if [ $? -ne "$SUCCESS" ]
then
echo "Usage: `basename $0` integer-input"
exit $E_BADINPUT
fi
let "sum = $1 + 25"
echo "Sum = $sum"

# Would give error if $1 not integer.

# Any variable, not just a command-line parameter, can be tested this way.
exit 0

• The 0 - 255 range for function return values is a severe limitation. Global variables and other
workarounds are often problematic. An alternative method for a function to communicate a value
back to the main body of the script is to have the function write to stdout (usually with echo) the
"return value," and assign this to a variable. This is actually a variant of command substitution.
Example 36-16. Return value trickery
#!/bin/bash
# multiplication.sh
multiply ()
{

# Multiplies params passed.
# Will accept a variable number of args.

local product=1
until [ -z "$1" ]
do
let "product *= $1"
shift
done

# Until uses up arguments passed...

echo $product

# Will not echo to stdout,
#+ since this will be assigned to a variable.

}

mult1=15383; mult2=25211
val1=`multiply $mult1 $mult2`
# Assigns stdout (echo) of function to the variable val1.
echo "$mult1 X $mult2 = $val1"
# 387820813
mult1=25; mult2=5; mult3=20
val2=`multiply $mult1 $mult2 $mult3`
echo "$mult1 X $mult2 X $mult3 = $val2"

# 2500

mult1=188; mult2=37; mult3=25; mult4=47
val3=`multiply $mult1 $mult2 $mult3 $mult4`
echo "$mult1 X $mult2 X $mult3 X $mult4 = $val3" # 8173300
exit 0

The same technique also works for alphanumeric strings. This means that a function can "return" a
non-numeric value.
capitalize_ichar ()
{
string0="$@"

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# Capitalizes initial character
#+ of argument string(s) passed.
# Accepts multiple arguments.

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Advanced Bash-Scripting Guide
firstchar=${string0:0:1}
string1=${string0:1}

# First character.
# Rest of string(s).

FirstChar=`echo "$firstchar" | tr a-z A-Z`
# Capitalize first character.
echo "$FirstChar$string1"

# Output to stdout.

}
newstring=`capitalize_ichar "every sentence should start with a capital letter."`
echo "$newstring"
# Every sentence should start with a capital letter.

It is even possible for a function to "return" multiple values with this method.

Example 36-17. Even more return value trickery
#!/bin/bash
# sum-product.sh
# A function may "return" more than one value.
sum_and_product ()
# Calculates both sum and product of passed args.
{
echo $(( $1 + $2 )) $(( $1 * $2 ))
# Echoes to stdout each calculated value, separated by space.
}
echo
echo "Enter first number "
read first
echo
echo "Enter second number "
read second
echo
retval=`sum_and_product $first $second`
sum=`echo "$retval" | awk '{print $1}'`
product=`echo "$retval" | awk '{print $2}'`

# Assigns output of function.
# Assigns first field.
# Assigns second field.

echo "$first + $second = $sum"
echo "$first * $second = $product"
echo
exit 0

There can be only one echo statement in the function for this to work. If you alter the
previous example:
sum_and_product ()
{
echo "This is the sum_and_product function." # This messes things up!
echo $(( $1 + $2 )) $(( $1 * $2 ))
}
...
retval=`sum_and_product $first $second`
# Assigns output of function.
# Now, this will not work correctly.

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Next in our bag of tricks are techniques for passing an array to a function, then "returning" an array
back to the main body of the script.
Passing an array involves loading the space-separated elements of the array into a variable with
command substitution. Getting an array back as the "return value" from a function uses the previously
mentioned strategem of echoing the array in the function, then invoking command substitution and
the ( ... ) operator to assign it to an array.

Example 36-18. Passing and returning arrays
#!/bin/bash
# array-function.sh: Passing an array to a function and ...
#
"returning" an array from a function

Pass_Array ()
{
local passed_array
# Local variable!
passed_array=( `echo "$1"` )
echo "${passed_array[@]}"
# List all the elements of the new array
#+ declared and set within the function.
}

original_array=( element1 element2 element3 element4 element5 )
echo
echo "original_array = ${original_array[@]}"
#
List all elements of original array.

# This is the trick that permits passing an array to a function.
# **********************************
argument=`echo ${original_array[@]}`
# **********************************
# Pack a variable
#+ with all the space-separated elements of the original array.
#
# Attempting to just pass the array itself will not work.

# This is the trick that allows grabbing an array as a "return value".
# *****************************************
returned_array=( `Pass_Array "$argument"` )
# *****************************************
# Assign 'echoed' output of function to array variable.
echo "returned_array = ${returned_array[@]}"
echo "============================================================="
# Now, try it again,
#+ attempting to access (list) the array from outside the function.
Pass_Array "$argument"
# The function itself lists the array, but ...
#+ accessing the array from outside the function is forbidden.
echo "Passed array (within function) = ${passed_array[@]}"

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• # NULL VALUE since the array is a variable local to the function.
echo
############################################
# And here is an even more explicit example:
ret_array ()
{
for element in {11..20}
do
echo "$element "
# Echo individual elements
done
#+ of what will be assembled into an array.
}
arr=( $(ret_array) )
echo
echo
echo
echo

#

Assemble into array.

"Capturing array \"arr\" from function ret_array () ..."
"Third element of array \"arr\" is ${arr[2]}."
# 13 (zero-indexed)
-n "Entire array is: "
${arr[@]}
# 11 12 13 14 15 16 17 18 19 20

echo
exit 0

For a more elaborate example of passing arrays to functions, see Example A-10.
•
Using the double-parentheses construct, it is possible to use C-style syntax for setting and
incrementing/decrementing variables and in for and while loops. See Example 11-12 and Example
11-17.
•
Setting the path and umask at the beginning of a script makes it more portable -- more likely to run on
a "foreign" machine whose user may have bollixed up the $PATH and umask.
#!/bin/bash
PATH=/bin:/usr/bin:/usr/local/bin ; export PATH
umask 022
# Files that the script creates will have 755 permission.
# Thanks to Ian D. Allen, for this tip.

•
A useful scripting technique is to repeatedly feed the output of a filter (by piping) back to the same
filter, but with a different set of arguments and/or options. Especially suitable for this are tr and grep.
# From "wstrings.sh" example.
wlist=`strings "$1" | tr A-Z a-z | tr '[:space:]' Z | \
tr -cs '[:alpha:]' Z | tr -s '\173-\377' Z | tr Z ' '`

Example 36-19. Fun with anagrams
#!/bin/bash
# agram.sh: Playing games with anagrams.
# Find anagrams of...
LETTERSET=etaoinshrdlu
FILTER='.......'
# How many letters minimum?
#
1234567

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anagram "$LETTERSET" |
grep "$FILTER" |
grep '^is' |
grep -v 's$' |
grep -v 'ed$'
# Possible to add many
#
#+
#
#

# Find all anagrams of the letterset...
# With at least 7 letters,
# starting with 'is'
# no plurals
# no past tense verbs
combinations of conditions and filters.

Uses "anagram" utility
that is part of the author's "yawl" word list package.
http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
http://bash.webofcrafts.net/yawl-0.3.2.tar.gz

exit 0

# End of code.

bash$ sh agram.sh
islander
isolate
isolead
isotheral

#
#
#
#
#+

Exercises:
--------Modify this script to take the LETTERSET as a command-line parameter.
Parameterize the filters in lines 11 - 13 (as with $FILTER),
so that they can be specified by passing arguments to a function.

# For a slightly different approach to anagramming,
#+ see the agram2.sh script.

See also Example 29-4, Example 16-25, and Example A-9.
•
Use "anonymous here documents" to comment out blocks of code, to save having to individually
comment out each line with a #. See Example 19-11.
•
Running a script on a machine that relies on a command that might not be installed is dangerous. Use
whatis to avoid potential problems with this.
CMD=command1
PlanB=command2

# First choice.
# Fallback option.

command_test=$(whatis "$CMD" | grep 'nothing appropriate')
# If 'command1' not found on system , 'whatis' will return
#+ "command1: nothing appropriate."
#
# A safer alternative is:
#
command_test=$(whereis "$CMD" | grep \/)
# But then the sense of the following test would have to be reversed,
#+ since the $command_test variable holds content only if
#+ the $CMD exists on the system.
#
(Thanks, bojster.)

if [[ -z "$command_test" ]]
then
$CMD option1 option2
else
$PlanB

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# Check whether command present.
# Run command1 with options.
# Otherwise,
#+ run command2.

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fi

•
An if-grep test may not return expected results in an error case, when text is output to stderr, rather
that stdout.
if ls -l nonexistent_filename | grep -q 'No such file or directory'
then echo "File \"nonexistent_filename\" does not exist."
fi

Redirecting stderr to stdout fixes this.
if ls -l nonexistent_filename 2>&1 | grep -q 'No such file or directory'
#
^^^^
then echo "File \"nonexistent_filename\" does not exist."
fi
# Thanks, Chris Martin, for pointing this out.

• If you absolutely must access a subshell variable outside the subshell, here's a way to do it.
TMPFILE=tmpfile
(
# Inside the subshell ...
inner_variable=Inner
echo $inner_variable
echo $inner_variable >>$TMPFILE
)

# Create a temp file to store the variable.

# Append to temp file.

# Outside the subshell ...
echo; echo "-----"; echo
echo $inner_variable
echo "-----"; echo
# Now ...
read inner_variable <$TMPFILE
rm -f "$TMPFILE"
echo "$inner_variable"

# Null, as expected.

# Read back shell variable.
# Get rid of temp file.
# It's an ugly kludge, but it works.

•
The run-parts command is handy for running a set of command scripts in a particular sequence,
especially in combination with cron or at.
•
For doing multiple revisions on a complex script, use the rcs Revision Control System package.
Among other benefits of this is automatically updated ID header tags. The co command in rcs does a
parameter replacement of certain reserved key words, for example, replacing # $Id$ in a script with
something like:
# $Id: hello-world.sh,v 1.1 2004/10/16 02:43:05 bozo Exp $

36.7.2. Widgets
It would be nice to be able to invoke X-Windows widgets from a shell script. There happen to exist several
packages that purport to do so, namely Xscript, Xmenu, and widtools. The first two of these no longer seem to
be maintained. Fortunately, it is still possible to obtain widtools here.

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The widtools (widget tools) package requires the XForms library to be installed. Additionally, the
Makefile needs some judicious editing before the package will build on a typical Linux system. Finally,
three of the six widgets offered do not work (and, in fact, segfault).
The dialog family of tools offers a method of calling "dialog" widgets from a shell script. The original dialog
utility works in a text console, but its successors, gdialog, Xdialog, and kdialog use X-Windows-based widget
sets.

Example 36-20. Widgets invoked from a shell script
#!/bin/bash
# dialog.sh: Using 'gdialog' widgets.
# Must have 'gdialog' installed on your system to run this script.
# Or, you can replace all instance of 'gdialog' below with 'kdialog' ...
# Version 1.1 (corrected 04/05/05)
# This script was inspired by the following article.
#
"Scripting for X Productivity," by Marco Fioretti,
#
LINUX JOURNAL, Issue 113, September 2003, pp. 86-9.
# Thank you, all you good people at LJ.

# Input error in dialog box.
E_INPUT=65
# Dimensions of display, input widgets.
HEIGHT=50
WIDTH=60
# Output file name (constructed out of script name).
OUTFILE=$0.output
# Display this script in a text widget.
gdialog --title "Displaying: $0" --textbox $0 $HEIGHT $WIDTH

# Now, we'll try saving input in a file.
echo -n "VARIABLE=" > $OUTFILE
gdialog --title "User Input" --inputbox "Enter variable, please:" \
$HEIGHT $WIDTH 2>> $OUTFILE

if [ "$?" -eq 0 ]
# It's good practice to check exit status.
then
echo "Executed \"dialog box\" without errors."
else
echo "Error(s) in \"dialog box\" execution."
# Or, clicked on "Cancel", instead of "OK" button.
rm $OUTFILE
exit $E_INPUT
fi

# Now, we'll retrieve and display the saved variable.
. $OUTFILE
# 'Source' the saved file.

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echo "The variable input in the \"input box\" was: "$VARIABLE""

rm $OUTFILE

# Clean up by removing the temp file.
# Some applications may need to retain this file.

exit $?
# Exercise: Rewrite this script using the 'zenity' widget set.

The xmessage command is a simple method of popping up a message/query window. For example:
xmessage Fatal error in script! -button exit

The latest entry in the widget sweepstakes is zenity. This utility pops up GTK+ dialog widgets-and-windows,
and it works very nicely within a script.
get_info ()
{
zenity --entry

# Pops up query window . . .
#+ and prints user entry to stdout.
#

Also try the --calendar and --scale options.

#

Capture stdout in $answer variable.

}
answer=$( get_info )

echo "User entered: "$answer""

For other methods of scripting with widgets, try Tk or wish (Tcl derivatives), PerlTk (Perl with Tk
extensions), tksh (ksh with Tk extensions), XForms4Perl (Perl with XForms extensions), Gtk-Perl (Perl with
Gtk extensions), or PyQt (Python with Qt extensions).

36.8. Security Issues
36.8.1. Infected Shell Scripts
A brief warning about script security is indicated. A shell script may contain a worm, trojan, or even a virus.
For that reason, never run as root a script (or permit it to be inserted into the system startup scripts in
/etc/rc.d) unless you have obtained said script from a trusted source or you have carefully analyzed it to
make certain it does nothing harmful.
Various researchers at Bell Labs and other sites, including M. Douglas McIlroy, Tom Duff, and Fred Cohen
have investigated the implications of shell script viruses. They conclude that it is all too easy for even a
novice, a "script kiddie," to write one. [125]
Here is yet another reason to learn scripting. Being able to look at and understand scripts may protect your
system from being compromised by a rogue script.

36.8.2. Hiding Shell Script Source
For security purposes, it may be necessary to render a script unreadable. If only there were a utility to create a
stripped binary executable from a script. Francisco Rosales' shc -- generic shell script compiler does exactly
that.
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Unfortunately, according to an article in the October, 2005 Linux Journal, the binary can, in at least some
cases, be decrypted to recover the original script source. Still, this could be a useful method of keeping scripts
secure from all but the most skilled hackers.

36.8.3. Writing Secure Shell Scripts
Dan Stromberg suggests the following guidelines for writing (relatively) secure shell scripts.
• Don't put secret data in environment variables.
• Don't pass secret data in an external command's arguments (pass them in via a pipe or redirection
instead).
• Set your $PATH carefully. Don't just trust whatever path you inherit from the caller if your script is
running as root. In fact, whenever you use an environment variable inherited from the caller, think
about what could happen if the caller put something misleading in the variable, e.g., if the caller set
$HOME to /etc.

36.9. Portability Issues
It is easier to port a shell than a shell script.
--Larry Wall
This book deals specifically with Bash scripting on a GNU/Linux system. All the same, users of sh and ksh
will find much of value here.
As it happens, many of the various shells and scripting languages seem to be converging toward the POSIX
1003.2 standard. Invoking Bash with the --posix option or inserting a set -o posix at the head of a script
causes Bash to conform very closely to this standard. Another alternative is to use a #!/bin/sh sha-bang header
in the script, rather than #!/bin/bash. [126] Note that /bin/sh is a link to /bin/bash in Linux and certain
other flavors of UNIX, and a script invoked this way disables extended Bash functionality.
Most Bash scripts will run as-is under ksh, and vice-versa, since Chet Ramey has been busily porting ksh
features to the latest versions of Bash.
On a commercial UNIX machine, scripts using GNU-specific features of standard commands may not work.
This has become less of a problem in the last few years, as the GNU utilities have pretty much displaced their
proprietary counterparts even on "big-iron" UNIX. Caldera's release of the source to many of the original
UNIX utilities has accelerated the trend.

Bash has certain features that the traditional Bourne shell lacks. Among these are:
• Certain extended invocation options
• Command substitution using $( ) notation
• Brace expansion
• Certain array operations, and associative arrays
• The double brackets extended test construct
• The double-parentheses arithmetic-evaluation construct
• Certain string manipulation operations
• Process substitution
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• A Regular Expression matching operator
• Bash-specific builtins
• Coprocesses
See the Bash F.A.Q. for a complete listing.

36.9.1. A Test Suite
Let us illustrate some of the incompatibilities between Bash and the classic Bourne shell. Download and
install the "Heirloom Bourne Shell" and run the following script, first using Bash, then the classic sh.

Example 36-21. Test Suite
#!/bin/bash
# test-suite.sh
# A partial Bash compatibility test suite.
# Run this on your version of Bash, or some other shell.
default_option=FAIL
echo
echo -n "Testing
sleep 1; echo -n
sleep 1; echo -n
sleep 1; echo ".
echo

# Tests below will fail unless . . .

"
". "
". "
"

# Double brackets
String="Double brackets supported?"
echo -n "Double brackets test: "
if [[ "$String" = "Double brackets supported?" ]]
then
echo "PASS"
else
echo "FAIL"
fi

# Double brackets and regex matching
String="Regex matching supported?"
echo -n "Regex matching: "
if [[ "$String" =~ R.....matching* ]]
then
echo "PASS"
else
echo "FAIL"
fi

# Arrays
test_arr=$default_option
# FAIL
Array=( If supports arrays will print PASS )
test_arr=${Array[5]}
echo "Array test: $test_arr"

# Command Substitution

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csub_test ()
{
echo "PASS"
}
test_csub=$default_option
# FAIL
test_csub=$(csub_test)
echo "Command substitution test: $test_csub"
echo
# Completing this script is an exercise for the reader.
# Add to the above similar tests for double parentheses,
#+ brace expansion, process substitution, etc.
exit $?

36.10. Shell Scripting Under Windows
Even users running that other OS can run UNIX-like shell scripts, and therefore benefit from many of the
lessons of this book. The Cygwin package from Cygnus and the MKS utilities from Mortice Kern Associates
add shell scripting capabilities to Windows.
In 2006, Microsoft released the Windows Powershell®, which contains limited Bash-like command-line
scripting capabilities.

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Chapter 37. Bash, versions 2, 3, and 4
37.1. Bash, version 2
The current version of Bash, the one you have running on your machine, is most likely version 2.xx.yy,
3.xx.yy, or 4.xx.yy.
bash$ echo $BASH_VERSION
3.2.25(1)-release

The version 2 update of the classic Bash scripting language added array variables, string and parameter
expansion, and a better method of indirect variable references, among other features.

Example 37-1. String expansion
#!/bin/bash
# String expansion.
# Introduced with version 2 of Bash.
# Strings of the form $'xxx'
#+ have the standard escaped characters interpreted.
echo $'Ringing bell 3 times \a \a \a'
# May only ring once with certain terminals.
# Or ...
# May not ring at all, depending on terminal settings.
echo $'Three form feeds \f \f \f'
echo $'10 newlines \n\n\n\n\n\n\n\n\n\n'
echo $'\102\141\163\150'
#
B
a
s
h
# Octal equivalent of characters.
exit

Example 37-2. Indirect variable references - the new way
#!/bin/bash
# Indirect variable referencing.
# This has a few of the attributes of references in C++.

a=letter_of_alphabet
letter_of_alphabet=z
echo "a = $a"

# Direct reference.

echo "Now a = ${!a}"
# Indirect reference.
# The ${!variable} notation is more intuitive than the old
#+ eval var1=\$$var2
echo

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t=table_cell_3
table_cell_3=24
echo "t = ${!t}"
table_cell_3=387
echo "Value of t changed to ${!t}"
# No 'eval' necessary.
#
#+
#
#+

# t = 24
# 387

This is useful for referencing members of an array or table,
or for simulating a multi-dimensional array.
An indexing option (analogous to pointer arithmetic)
would have been nice. Sigh.

exit 0
# See also, ind-ref.sh example.

Example 37-3. Simple database application, using indirect variable referencing
#!/bin/bash
# resistor-inventory.sh
# Simple database / table-lookup application.
# ============================================================== #
# Data
B1723_value=470
B1723_powerdissip=.25
B1723_colorcode="yellow-violet-brown"
B1723_loc=173
B1723_inventory=78

#
#
#
#
#

Ohms
Watts
Color bands
Where they are
How many

B1724_value=1000
B1724_powerdissip=.25
B1724_colorcode="brown-black-red"
B1724_loc=24N
B1724_inventory=243
B1725_value=10000
B1725_powerdissip=.125
B1725_colorcode="brown-black-orange"
B1725_loc=24N
B1725_inventory=89
# ============================================================== #

echo
PS3='Enter catalog number: '
echo
select catalog_number in "B1723" "B1724" "B1725"
do
Inv=${catalog_number}_inventory
Val=${catalog_number}_value
Pdissip=${catalog_number}_powerdissip
Loc=${catalog_number}_loc
Ccode=${catalog_number}_colorcode

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echo
echo "Catalog number $catalog_number:"
# Now, retrieve value, using indirect referencing.
echo "There are ${!Inv} of [${!Val} ohm / ${!Pdissip} watt]\
resistors in stock." #
^
^
# As of Bash 4.2, you can replace "ohm" with \u2126 (using echo -e).
echo "These are located in bin # ${!Loc}."
echo "Their color code is \"${!Ccode}\"."
break
done
echo; echo
# Exercises:
# --------# 1) Rewrite this script to read its data from an external file.
# 2) Rewrite this script to use arrays,
#+
rather than indirect variable referencing.
#
Which method is more straightforward and intuitive?
#
Which method is easier to code?

# Notes:
# ----# Shell scripts are inappropriate for anything except the most simple
#+ database applications, and even then it involves workarounds and kludges.
# Much better is to use a language with native support for data structures,
#+ such as C++ or Java (or even Perl).
exit 0

Example 37-4. Using arrays and other miscellaneous trickery to deal four random hands from a deck of
cards
#!/bin/bash
# cards.sh
# Deals four random hands from a deck of cards.
UNPICKED=0
PICKED=1
DUPE_CARD=99
LOWER_LIMIT=0
UPPER_LIMIT=51
CARDS_IN_SUIT=13
CARDS=52
declare -a Deck
declare -a Suits
declare -a Cards
# It would have been easier to implement and more intuitive
#+ with a single, 3-dimensional array.
# Perhaps a future version of Bash will support multidimensional arrays.

initialize_Deck ()
{
i=$LOWER_LIMIT

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until [ "$i" -gt $UPPER_LIMIT ]
do
Deck[i]=$UNPICKED
# Set each card of "Deck" as unpicked.
let "i += 1"
done
echo
}
initialize_Suits ()
{
Suits[0]=C #Clubs
Suits[1]=D #Diamonds
Suits[2]=H #Hearts
Suits[3]=S #Spades
}
initialize_Cards ()
{
Cards=(2 3 4 5 6 7 8 9 10 J Q K A)
# Alternate method of initializing an array.
}
pick_a_card ()
{
card_number=$RANDOM
let "card_number %= $CARDS" # Restrict range to 0 - 51, i.e., 52 cards.
if [ "${Deck[card_number]}" -eq $UNPICKED ]
then
Deck[card_number]=$PICKED
return $card_number
else
return $DUPE_CARD
fi
}
parse_card ()
{
number=$1
let "suit_number = number / CARDS_IN_SUIT"
suit=${Suits[suit_number]}
echo -n "$suit-"
let "card_no = number % CARDS_IN_SUIT"
Card=${Cards[card_no]}
printf %-4s $Card
# Print cards in neat columns.
}
seed_random () # Seed random number generator.
{
# What happens if you don't do this?
seed=`eval date +%s`
let "seed %= 32766"
RANDOM=$seed
# What are some other methods
#+ of seeding the random number generator?
}
deal_cards ()
{
echo
cards_picked=0
while [ "$cards_picked" -le $UPPER_LIMIT ]

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do
pick_a_card
t=$?
if [ "$t" -ne $DUPE_CARD ]
then
parse_card $t
u=$cards_picked+1
# Change back to 1-based indexing (temporarily). Why?
let "u %= $CARDS_IN_SUIT"
if [ "$u" -eq 0 ]
# Nested if/then condition test.
then
echo
echo
fi
# Each hand set apart with a blank line.
let "cards_picked += 1"
fi
done
echo
return 0
}

# Structured programming:
# Entire program logic modularized in functions.
#===============
seed_random
initialize_Deck
initialize_Suits
initialize_Cards
deal_cards
#===============
exit

# Exercise 1:
# Add comments to thoroughly document this script.
# Exercise 2:
# Add a routine (function) to print out each hand sorted in suits.
# You may add other bells and whistles if you like.
# Exercise 3:
# Simplify and streamline the logic of the script.

37.2. Bash, version 3
On July 27, 2004, Chet Ramey released version 3 of Bash. This update fixed quite a number of bugs and
added new features.
Some of the more important added features:
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• A new, more generalized {a..z} brace expansion operator.
#!/bin/bash
for i in {1..10}
# Simpler and more straightforward than
#+ for i in $(seq 10)
do
echo -n "$i "
done
echo
# 1 2 3 4 5 6 7 8 9 10

# Or just . . .
echo {a..z}
echo {e..m}
echo {z..a}
echo {25..30}
echo {3..-2}
echo {X..d}

echo {]..a}

#
#
#
#
#
#
#
#
#+
#
#

a b c d e f g h i j k l m n o p q r s t u v w x y z
e f g h i j k l m
z y x w v u t s r q p o n m l k j i h g f e d c b a
Works backwards, too.
25 26 27 28 29 30
3 2 1 0 -1 -2
X Y Z [ ] ^ _ ` a b c d
Shows (some of) the ASCII characters between Z and a,
but don't rely on this type of behavior because . . .
{]..a}
Why?

# You can tack on prefixes and suffixes.
echo "Number #"{1..4}, "..."
# Number #1, Number #2, Number #3, Number #4, ...

# You can concatenate brace-expansion sets.
echo {1..3}{x..z}" +" "..."
# 1x + 1y + 1z + 2x + 2y + 2z + 3x + 3y + 3z + ...
# Generates an algebraic expression.
# This could be used to find permutations.
# You can nest brace-expansion sets.
echo {{a..c},{1..3}}
# a b c 1 2 3
# The "comma operator" splices together strings.

# Unfortunately, brace expansion does not lend itself to parameterization.
var1=1
var2=5
echo {$var1..$var2}
# {1..5}

• The ${!array[@]} operator, which expands to all the indices of a given array.
#!/bin/bash
Array=(element-zero element-one element-two element-three)
echo ${Array[0]}

# element-zero
# First element of array.

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echo ${!Array[@]}

# 0 1 2 3
# All the indices of Array.

for i in ${!Array[@]}
do
echo ${Array[i]} # element-zero
# element-one
# element-two
# element-three
#
# All the elements in Array.
done

•
The =~ Regular Expression matching operator within a double brackets test expression. (Perl has a
similar operator.)
#!/bin/bash
variable="This is a fine mess."
echo "$variable"
# Regex matching with =~ operator within [[ double brackets ]].
if [[ "$variable" =~ T.........fin*es* ]]
# NOTE: As of version 3.2 of Bash, expression to match no longer quoted.
then
echo "match found"
# match found
fi

Or, more usefully:
#!/bin/bash
input=$1

if [[ "$input" =~ "[0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9][0-9][0-9]" ]]
#
^ NOTE: Quoting not necessary, as of version 3.2 of Bash.
# NNN-NN-NNNN (where each N is a digit).
then
echo "Social Security number."
# Process SSN.
else
echo "Not a Social Security number!"
# Or, ask for corrected input.
fi

For additional examples of using the =~ operator, see Example A-29, Example 19-14, Example A-35,
and Example A-24.
•
The new set -o pipefail option is useful for debugging pipes. If this option is set, then the exit
status of a pipe is the exit status of the last command in the pipe to fail (return a non-zero value),
rather than the actual final command in the pipe.
See Example 16-43.

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The update to version 3 of Bash breaks a few scripts that worked under earlier versions. Test critical
legacy scripts to make sure they still work!
As it happens, a couple of the scripts in the Advanced Bash Scripting Guide had to be fixed up (see
Example 9-4, for instance).

37.2.1. Bash, version 3.1
The version 3.1 update of Bash introduces a number of bugfixes and a few minor changes.
• The += operator is now permitted in in places where previously only the = assignment operator was
recognized.

a=1
echo $a

# 1

a+=5
echo $a

# Won't work under versions of Bash earlier than 3.1.
# 15

a+=Hello
echo $a

# 15Hello

Here, += functions as a string concatenation operator. Note that its behavior in this particular context
is different than within a let construct.
a=1
echo $a

# 1

let a+=5
echo $a

# Integer arithmetic, rather than string concatenation.
# 6

let a+=Hello
echo $a

# Doesn't "add" anything to a.
# 6

Jeffrey Haemer points out that this concatenation operator can be quite useful. In this instance, we
append a directory to the $PATH.
bash$ echo $PATH
/usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games

bash$ PATH+=:/opt/bin
bash$ echo $PATH
/usr/bin:/bin:/usr/local/bin:/usr/X11R6/bin/:/usr/games:/opt/bin

37.2.2. Bash, version 3.2
This is pretty much a bugfix update.
• In global parameter substitutions, the pattern no longer anchors at the start of the string.
• The --wordexp option disables process substitution.
• The =~ Regular Expression match operator no longer requires quoting of the pattern within [[ ... ]].
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In fact, quoting in this context is not advisable as it may cause regex evaluation to fail.
Chet Ramey states in the Bash FAQ that quoting explicitly disables regex evaluation.
See also the Ubuntu Bug List and Wikinerds on Bash syntax.
Setting shopt -s compat31 in a script causes reversion to the original behavior.

37.3. Bash, version 4
Chet Ramey announced Version 4 of Bash on the 20th of February, 2009. This release has a number of
significant new features, as well as some important bugfixes.
Among the new goodies:
• Associative arrays. [127]
An associative array can be thought of as a set of two linked arrays -- one holding the data, and the
other the keys that index the individual elements of the data array.
Example 37-5. A simple address database
#!/bin/bash4
# fetch_address.sh
declare -A address
#
-A option declares associative array.
address[Charles]="414 W. 10th Ave., Baltimore, MD 21236"
address[John]="202 E. 3rd St., New York, NY 10009"
address[Wilma]="1854 Vermont Ave, Los Angeles, CA 90023"

echo "Charles's address is ${address[Charles]}."
# Charles's address is 414 W. 10th Ave., Baltimore, MD 21236.
echo "Wilma's address is ${address[Wilma]}."
# Wilma's address is 1854 Vermont Ave, Los Angeles, CA 90023.
echo "John's address is ${address[John]}."
# John's address is 202 E. 3rd St., New York, NY 10009.
echo
echo "${!address[*]}"
# Charles John Wilma

# The array indices ...

Example 37-6. A somewhat more elaborate address database
#!/bin/bash4
# fetch_address-2.sh
# A more elaborate version of fetch_address.sh.
SUCCESS=0
E_DB=99

# Error code for missing entry.

declare -A address

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#

-A option declares associative array.

store_address ()
{
address[$1]="$2"
return $?
}

fetch_address ()
{
if [[ -z "${address[$1]}" ]]
then
echo "$1's address is not in database."
return $E_DB
fi
echo "$1's address is ${address[$1]}."
return $?
}

store_address "Lucas Fayne" "414 W. 13th Ave., Baltimore, MD 21236"
store_address "Arvid Boyce" "202 E. 3rd St., New York, NY 10009"
store_address "Velma Winston" "1854 Vermont Ave, Los Angeles, CA 90023"
# Exercise:
# Rewrite the above store_address calls to read data from a file,
#+ then assign field 1 to name, field 2 to address in the array.
# Each line in the file would have a format corresponding to the above.
# Use a while-read loop to read from file, sed or awk to parse the fields.
fetch_address "Lucas Fayne"
# Lucas Fayne's address is 414 W. 13th Ave., Baltimore, MD 21236.
fetch_address "Velma Winston"
# Velma Winston's address is 1854 Vermont Ave, Los Angeles, CA 90023.
fetch_address "Arvid Boyce"
# Arvid Boyce's address is 202 E. 3rd St., New York, NY 10009.
fetch_address "Bozo Bozeman"
# Bozo Bozeman's address is not in database.
exit $?

# In this case, exit code = 99, since that is function return.

See Example A-53 for an interesting usage of an associative array.
Elements of the index array may include embedded space characters, or even leading
and/or trailing space characters. However, index array elements containing only
whitespace are not permitted.
address[

]="Blank"

# Error!

• Enhancements to the case construct: the ;;& and ;& terminators.
Example 37-7. Testing characters
#!/bin/bash4
test_char ()
{
case "$1" in
[[:print:]] )

echo "$1 is a printable character.";;&

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# The ;;& terminator continues to the next pattern test.
|
[[:alnum:]] ) echo "$1 is an alpha/numeric character.";;& # v
[[:alpha:]] ) echo "$1 is an alphabetic character.";;&
# v
[[:lower:]] ) echo "$1 is a lowercase alphabetic character.";;&
[[:digit:]] ) echo "$1 is an numeric character.";&
# |
# The ;& terminator executes the next statement ...
# |
%%%@@@@@
) echo "********************************";;
# v
#
^^^^^^^^ ... even with a dummy pattern.
esac
}
echo
test_char 3
# 3 is a printable character.
# 3 is an alpha/numeric character.
# 3 is an numeric character.
# ********************************
echo
test_char m
# m is a printable character.
# m is an alpha/numeric character.
# m is an alphabetic character.
# m is a lowercase alphabetic character.
echo
test_char /
# / is a printable character.
echo
# The ;;& terminator can save complex if/then conditions.
# The ;& is somewhat less useful.

• The new coproc builtin enables two parallel processes to communicate and interact. As Chet Ramey
states in the Bash FAQ [128] , ver. 4.01:
There is a new 'coproc' reserved word that
an asynchronous command run with two pipes
shell. Coprocs can be named. The input and
and the PID of the coprocess are available
variables with coproc-specific names.

specifies a coprocess:
connected to the creat
output file descriptor
to the calling shell i

George Dimitriu explains,
"... coproc ... is a feature used in Bash process substitution,
which now is made publicly available."
This means it can be explicitly invoked in a script, rather than
just being a behind-the-scenes mechanism used by Bash.
See http://linux010.blogspot.com/2008/12/bash-process-substitutio

Coprocesses use file descriptors. File descriptors enable processes and pipes to communicate.
#!/bin/bash4
# A coprocess communicates with a while-read loop.

coproc { cat mx_data.txt; sleep 2; }

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#
^^^^^^^
# Try running this without "sleep 2" and see what happens.
while read -u ${COPROC[0]} line
# ${COPROC[0]} is the
do
#+ file descriptor of the coprocess.
echo "$line" | sed -e 's/line/NOT-ORIGINAL-TEXT/'
done
kill $COPROC_PID

# No longer need the coprocess,
#+ so kill its PID.

But, be careful!
#!/bin/bash4
echo; echo
a=aaa
b=bbb
c=ccc
coproc echo "one two three"
while read -u ${COPROC[0]} a b c; # Note that this loop
do
#+ runs in a subshell.
echo "Inside while-read loop: ";
echo "a = $a"; echo "b = $b"; echo "c = $c"
echo "coproc file descriptor: ${COPROC[0]}"
done
#
#
#
#

a = one
b = two
c = three
So far, so good, but ...

echo "-----------------"
echo "Outside while-read loop: "
echo "a = $a" # a =
echo "b = $b" # b =
echo "c = $c" # c =
echo "coproc file descriptor: ${COPROC[0]}"
echo
# The coproc is still running, but ...
#+ it still doesn't enable the parent process
#+ to "inherit" variables from the child process, the while-read loop.
#

Compare this to the "badread.sh" script.

The coprocess is asynchronous, and this might cause a problem. It may terminate
before another process has finished communicating with it.
#!/bin/bash4
coproc cpname { for i in {0..10}; do echo "index = $i"; done; }
#
^^^^^^ This is a *named* coprocess.
read -u ${cpname[0]}
echo $REPLY
# index = 0
echo ${COPROC[0]}
#+ No output ... the coprocess timed out
# after the first loop iteration.

# However, George Dimitriu has a partial fix.

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coproc cpname { for i in {0..10}; do echo "index = $i"; done; sleep 1;
echo hi > myo; cat - >> myo; }
#
^^^^^ This is a *named* coprocess.
echo "I am main"$'\04' >&${cpname[1]}
myfd=${cpname[0]}
echo myfd=$myfd
### while read -u $myfd
### do
###
echo $REPLY;
### done
echo $cpname_PID
# Run this with and without the commented-out while-loop, and it is
#+ apparent that each process, the executing shell and the coprocess,
#+ waits for the other to finish writing in its own write-enabled pipe.

• The new mapfile builtin makes it possible to load an array with the contents of a text file without
using a loop or command substitution.
#!/bin/bash4
mapfile Arr1 < $0
# Same result as
echo "${Arr1[@]}"

Arr1=( $(cat $0) )
# Copies this entire script out to stdout.

echo "--"; echo
# But, not the same as
read -a
!!!
read -a Arr2 < $0
echo "${Arr2[@]}" # Reads only first line of script into the array.
exit

• The read builtin got a minor facelift. The -t timeout option now accepts (decimal) fractional values
[129] and the -i option permits preloading the edit buffer. [130] Unfortunately, these enhancements
are still a work in progress and not (yet) usable in scripts.
• Parameter substitution gets case-modification operators.
#!/bin/bash4
var=veryMixedUpVariable
echo ${var}
#
echo ${var^}
#
#
*
echo ${var^^}
#
#
**
echo ${var,}
#
#
*
echo ${var,,}
#
#
**

veryMixedUpVariable
VeryMixedUpVariable
First char --> uppercase.
VERYMIXEDUPVARIABLE
All chars --> uppercase.
veryMixedUpVariable
First char --> lowercase.
verymixedupvariable
All chars --> lowercase.

•
The declare builtin now accepts the -l lowercase and -c capitalize options.
#!/bin/bash4
declare -l var1

# Will change to lowercase

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var1=MixedCaseVARIABLE
echo "$var1"
# Same effect as
declare -c var2
var2=originally_lowercase
echo "$var2"
# NOT the same effect as

# mixedcasevariable
echo $var1 | tr A-Z a-z
# Changes only initial char to uppercase.
# Originally_lowercase
echo $var2 | tr a-z A-Z

• Brace expansion has more options.
Increment/decrement, specified in the final term within braces.
#!/bin/bash4
echo {40..60..2}
# 40 42 44 46 48 50 52 54 56 58 60
# All the even numbers, between 40 and 60.
echo {60..40..2}
# 60 58 56 54 52 50 48 46 44 42 40
# All the even numbers, between 40 and 60, counting backwards.
# In effect, a decrement.
echo {60..40..-2}
# The same output. The minus sign is not necessary.
# But, what about letters and symbols?
echo {X..d}
# X Y Z [ ] ^ _ ` a b c d
# Does not echo the \ which escapes a space.

Zero-padding, specified in the first term within braces, prefixes each term in the output with the same
number of zeroes.
bash4$ echo {010..15}
010 011 012 013 014 015

bash4$ echo {000..10}
000 001 002 003 004 005 006 007 008 009 010

•
Substring extraction on positional parameters now starts with $0 as the zero-index. (This corrects an
inconsistency in the treatment of positional parameters.)
#!/bin/bash
# show-params.bash
# Requires version 4+ of Bash.
# Invoke this scripts with at least one positional parameter.
E_BADPARAMS=99
if [ -z "$1" ]
then
echo "Usage $0 param1 ..."
exit $E_BADPARAMS
fi
echo ${@:0}

Chapter 37. Bash, versions 2, 3, and 4

548

Advanced Bash-Scripting Guide
# bash3 show-params.bash4 one two three
# one two three
# bash4 show-params.bash4 one two three
# show-params.bash4 one two three
# $0

$1

$2

$3

• The new ** globbing operator matches filenames and directories recursively.
#!/bin/bash4
# filelist.bash4
shopt -s globstar

# Must enable globstar, otherwise ** doesn't work.
# The globstar shell option is new to version 4 of Bash.

echo "Using *"; echo
for filename in *
do
echo "$filename"
done
# Lists only files in current directory ($PWD).
echo; echo "--------------"; echo
echo "Using **"
for filename in **
do
echo "$filename"
done
# Lists complete file tree, recursively.
exit
Using *
allmyfiles
filelist.bash4
-------------Using **
allmyfiles
allmyfiles/file.index.txt
allmyfiles/my_music
allmyfiles/my_music/me-singing-60s-folksongs.ogg
allmyfiles/my_music/me-singing-opera.ogg
allmyfiles/my_music/piano-lesson.1.ogg
allmyfiles/my_pictures
allmyfiles/my_pictures/at-beach-with-Jade.png
allmyfiles/my_pictures/picnic-with-Melissa.png
filelist.bash4

• The new $BASHPID internal variable.
•
There is a new builtin error-handling function named command_not_found_handle.
#!/bin/bash4
command_not_found_handle ()
{ # Accepts implicit parameters.
echo "The following command is not valid: \""$1\"""
echo "With the following argument(s): \""$2\"" \""$3\"""

Chapter 37. Bash, versions 2, 3, and 4

# $4, $5 ...

549

Advanced Bash-Scripting Guide
} # $1, $2, etc. are not explicitly passed to the function.
bad_command arg1 arg2
# The following command is not valid: "bad_command"
# With the following argument(s): "arg1" "arg2"

Editorial comment
Associative arrays? Coprocesses? Whatever happened to the lean and mean Bash we have come to know
and love? Could it be suffering from (horrors!) "feature creep"? Or perhaps even Korn shell envy?
Note to Chet Ramey: Please add only essential features in future Bash releases -- perhaps for-each loops and
support for multi-dimensional arrays. [131] Most Bash users won't need, won't use, and likely won't greatly
appreciate complex "features" like built-in debuggers, Perl interfaces, and bolt-on rocket boosters.

37.3.1. Bash, version 4.1
Version 4.1 of Bash, released in May, 2010, was primarily a bugfix update.
• The printf command now accepts a -v option for setting array indices.
• Within double brackets, the > and < string comparison operators now conform to the locale. Since the
locale setting may affect the sorting order of string expressions, this has side-effects on comparison
tests within [[ ... ]] expressions.
• The read builtin now takes a -N option (read -N chars), which causes the read to terminate after
chars characters.
Example 37-8. Reading N characters
#!/bin/bash
# Requires Bash version -ge 4.1 ...
num_chars=61
read -N $num_chars var < $0
echo "$var"
exit

# Read first 61 characters of script!

####### Output of Script #######
#!/bin/bash
# Requires Bash version -ge 4.1 ...
num_chars=61

• Here documents embedded in $( ... ) command substitution constructs may terminate with a
simple ).
Example 37-9. Using a here document to set a variable
#!/bin/bash
# here-commsub.sh
# Requires Bash version -ge 4.1 ...

Chapter 37. Bash, versions 2, 3, and 4

550

Advanced Bash-Scripting Guide
multi_line_var=$( cat <?
Update: upgraded to a T60 Thinkpad running Mandriva 2010. No longer starving .

38.4.2. Software and Printware
i. Bram Moolenaar's powerful SGML-aware vim text editor.
ii. OpenJade, a DSSSL rendering engine for converting SGML documents into other formats.
iii. Norman Walsh's DSSSL stylesheets.
iv. DocBook, The Definitive Guide, by Norman Walsh and Leonard Muellner (O'Reilly, ISBN
1-56592-580-7). This is still the standard reference for anyone attempting to write a document in
Docbook SGML format.

38.5. Credits
Community participation made this project possible. The author gratefully acknowledges that writing this
book would have been unthinkable without help and feedback from all you people out there.
Philippe Martin translated the first version (0.1) of this document into DocBook/SGML. While not on the job
at a small French company as a software developer, he enjoys working on GNU/Linux documentation and
software, reading literature, playing music, and, for his peace of mind, making merry with friends. You may
run across him somewhere in France or in the Basque Country, or you can email him at feloy@free.fr.
Philippe Martin also pointed out that positional parameters past $9 are possible using {bracket} notation. (See
Example 4-5).
Stéphane Chazelas sent a long list of corrections, additions, and example scripts. More than a contributor, he
had, in effect, for a while taken on the role of co-editor for this document. Merci beaucoup!
Chapter 38. Endnotes

556

Advanced Bash-Scripting Guide
Paulo Marcel Coelho Aragao offered many corrections, both major and minor, and contributed quite a number
of helpful suggestions.
I would like to especially thank Patrick Callahan, Mike Novak, and Pal Domokos for catching bugs, pointing
out ambiguities, and for suggesting clarifications and changes in the preliminary version (0.1) of this
document. Their lively discussion of shell scripting and general documentation issues inspired me to try to
make this document more readable.
I'm grateful to Jim Van Zandt for pointing out errors and omissions in version 0.2 of this document. He also
contributed an instructive example script.
Many thanks to Jordi Sanfeliu for giving permission to use his fine tree script (Example A-16), and to Rick
Boivie for revising it.
Likewise, thanks to Michel Charpentier for permission to use his dc factoring script (Example 16-52).
Kudos to Noah Friedman for permission to use his string function script (Example A-18).
Emmanuel Rouat suggested corrections and additions on command substitution and aliases. He also
contributed a very nice sample .bashrc file (Appendix L).
Heiner Steven kindly gave permission to use his base conversion script, Example 16-48. He also made a
number of corrections and many helpful suggestions. Special thanks.
Rick Boivie contributed the delightfully recursive pb.sh script (Example 36-9), revised the tree.sh script
(Example A-16), and suggested performance improvements for the monthlypmt.sh script (Example 16-47).
Florian Wisser enlightened me on some of the fine points of testing strings (see Example 7-6), and on other
matters.
Oleg Philon sent suggestions concerning cut and pidof.
Michael Zick extended the empty array example to demonstrate some surprising array properties. He also
contributed the isspammer scripts (Example 16-41 and Example A-28).
Marc-Jano Knopp sent corrections and clarifications on DOS batch files.
Hyun Jin Cha found several typos in the document in the process of doing a Korean translation. Thanks for
pointing these out.
Andreas Abraham sent in a long list of typographical errors and other corrections. Special thanks!
Others contributing scripts, making helpful suggestions, and pointing out errors were Gabor Kiss, Leopold
Toetsch, Peter Tillier, Marcus Berglof, Tony Richardson, Nick Drage (script ideas!), Rich Bartell, Jess
Thrysoee, Adam Lazur, Bram Moolenaar, Baris Cicek, Greg Keraunen, Keith Matthews, Sandro Magi, Albert
Reiner, Dim Segebart, Rory Winston, Lee Bigelow, Wayne Pollock, "jipe," "bojster," "nyal," "Hobbit,"
"Ender," "Little Monster" (Alexis), "Mark," "Patsie," "vladz," Peggy Russell, Emilio Conti, Ian. D. Allen,
Hans-Joerg Diers, Arun Giridhar, Dennis Leeuw, Dan Jacobson, Aurelio Marinho Jargas, Edward Scholtz,
Jean Helou, Chris Martin, Lee Maschmeyer, Bruno Haible, Wilbert Berendsen, Sebastien Godard, Bjön
Eriksson, John MacDonald, John Lange, Joshua Tschida, Troy Engel, Manfred Schwarb, Amit Singh, Bill
Gradwohl, E. Choroba, David Lombard, Jason Parker, Steve Parker, Bruce W. Clare, William Park, Vernia
Chapter 38. Endnotes

557

Advanced Bash-Scripting Guide
Damiano, Mihai Maties, Mark Alexander, Jeremy Impson, Ken Fuchs, Jared Martin, Frank Wang, Sylvain
Fourmanoit, Matthew Sage, Matthew Walker, Kenny Stauffer, Filip Moritz, Andrzej Stefanski, Daniel Albers,
Jeffrey Haemer, Stefano Palmeri, Nils Radtke, Sigurd Solaas, Serghey Rodin, Jeroen Domburg, Alfredo
Pironti, Phil Braham, Bruno de Oliveira Schneider, Stefano Falsetto, Chris Morgan, Walter Dnes, Linc
Fessenden, Michael Iatrou, Pharis Monalo, Jesse Gough, Fabian Kreutz, Mark Norman, Harald Koenig, Dan
Stromberg, Peter Knowles, Francisco Lobo, Mariusz Gniazdowski, Sebastian Arming, Chetankumar
Phulpagare, Benno Schulenberg, Tedman Eng, Jochen DeSmet, Juan Nicolas Ruiz, Oliver Beckstein, Achmed
Darwish, Dotan Barak, Richard Neill, Albert Siersema, Omair Eshkenazi, Geoff Lee, Graham Ewart, JuanJo
Ciarlante, Cliff Bamford, Nathan Coulter, Ramses Rodriguez Martinez, Evgeniy Ivanov, Craig Barnes,
George Dimitriu, Kevin LeBlanc, Antonio Macchi, Tomas Pospisek, Andreas Kühne, Pádraig Brady, and
David Lawyer (himself an author of four HOWTOs).
My gratitude to Chet Ramey and Brian Fox for writing Bash, and building into it elegant and powerful
scripting capabilities rivaling those of ksh.
Very special thanks to the hard-working volunteers at the Linux Documentation Project. The LDP hosts a
repository of Linux knowledge and lore, and has, to a great extent, enabled the publication of this book.
Thanks and appreciation to IBM, Red Hat, Google, the Free Software Foundation, and all the good people
fighting the good fight to keep Open Source software free and open.
Belated thanks to my fourth grade teacher, Miss Spencer, for emotional support and for convincing me that
maybe, just maybe I wasn't a total loss.
Thanks most of all to my wife, Anita, for her encouragement, inspiration, and emotional support.

38.6. Disclaimer
(This is a variant of the standard LDP disclaimer.)
No liability for the contents of this document can be accepted. Use the concepts, examples and information at
your own risk. There may be errors, omissions, and inaccuracies that could cause you to lose data, harm your
system, or induce involuntary electrocution, so proceed with appropriate caution. The author takes no
responsibility for any damages, incidental or otherwise.
As it happens, it is highly unlikely that either you or your system will suffer ill effects, aside from
uncontrollable hiccups. In fact, the raison d'etre of this book is to enable its readers to analyze shell scripts
and determine whether they have unanticipated consequences.

Chapter 38. Endnotes

558

Bibliography
Those who do not understand UNIX are
condemned to reinvent it, poorly.
--Henry Spencer

Edited by Peter Denning, Computers Under Attack: Intruders, Worms, and Viruses, ACM Press, 1990,
0-201-53067-8.
This compendium contains a couple of articles on shell script viruses.
*

Ken Burtch, Linux Shell Scripting with Bash, 1st edition, Sams Publishing (Pearson), 2004, 0672326426.
Covers much of the same material as the ABS Guide, though in a different style.
*

Dale Dougherty and Arnold Robbins, Sed and Awk, 2nd edition, O'Reilly and Associates, 1997,
1-156592-225-5.
Unfolding the full power of shell scripting requires at least a passing familiarity with sed and awk. This is the
standard tutorial. It includes an excellent introduction to Regular Expressions. Recommended.
*

Jeffrey Friedl, Mastering Regular Expressions, O'Reilly and Associates, 2002, 0-596-00289-0.
Still the best all-around reference on Regular Expressions.
*

Aeleen Frisch, Essential System Administration, 3rd edition, O'Reilly and Associates, 2002, 0-596-00343-9.
This excellent manual provides a decent introduction to shell scripting from a sys admin point of view. It
includes comprehensive explanations of the startup and initialization scripts in a UNIX system.
*

Stephen Kochan and Patrick Wood, Unix Shell Programming, Hayden, 1990, 067248448X.
Bibliography

559

Advanced Bash-Scripting Guide
Still considered a standard reference, though somewhat dated, and a bit "wooden" stylistically speaking. [138]
In fact, this book was the ABS Guide author's first exposure to UNIX shell scripting, lo these many years ago.
For more information, see the Kochan-Wood website.
*

Neil Matthew and Richard Stones, Beginning Linux Programming, Wrox Press, 1996, 1874416680.
Surprisingly good in-depth coverage of various programming languages available for Linux, including a fairly
strong chapter on shell scripting.
*

Herbert Mayer, Advanced C Programming on the IBM PC, Windcrest Books, 1989, 0830693637.
Excellent coverage of algorithms and general programming practices. Highly recommended, but unfortunately
out of print.
*

David Medinets, Unix Shell Programming Tools, McGraw-Hill, 1999, 0070397333.
Pretty good treatment of shell scripting, with examples, and a short intro to Tcl and Perl.
*

Cameron Newham and Bill Rosenblatt, Learning the Bash Shell, 2nd edition, O'Reilly and Associates, 1998,
1-56592-347-2.
This is a valiant effort at a decent shell primer, but sadly deficient in its coverage of writing scripts and
lacking sufficient examples.
*

Anatole Olczak, Bourne Shell Quick Reference Guide, ASP, Inc., 1991, 093573922X.
A very handy pocket reference, despite lacking coverage of Bash-specific features.
*

Jerry Peek, Tim O'Reilly, and Mike Loukides, Unix Power Tools, 3rd edition, O'Reilly and Associates,
Random House, 2002, 0-596-00330-7.

Bibliography

560

Advanced Bash-Scripting Guide
Contains a couple of sections of very informative in-depth articles on shell programming, but falls short of
being a self-teaching manual. It reproduces much of the Regular Expressions tutorial from the Dougherty and
Robbins book, above. The comprehensive coverage of UNIX commands makes this book worthy of a place
on your bookshelf.
*

Clifford Pickover, Computers, Pattern, Chaos, and Beauty, St. Martin's Press, 1990, 0-312-04123-3.
A treasure trove of ideas and recipes for computer-based exploration of mathematical oddities.
*

George Polya, How To Solve It, Princeton University Press, 1973, 0-691-02356-5.
The classic tutorial on problem-solving methods (i.e., algorithms), with special emphasis on how to teach
them.
*

Chet Ramey and Brian Fox, The GNU Bash Reference Manual, Network Theory Ltd, 2003, 0-9541617-7-7.
This manual is the definitive reference for GNU Bash. The authors of this manual, Chet Ramey and Brian
Fox, are the original developers of GNU Bash. For each copy sold, the publisher donates $1 to the Free
Software Foundation.
*

Arnold Robbins, Bash Reference Card, SSC, 1998, 1-58731-010-5.
Excellent Bash pocket reference (don't leave home without it, especially if you're a sysadmin). A bargain at
$4.95, but unfortunately no longer available for free download.
*

Arnold Robbins, Effective Awk Programming, Free Software Foundation / O'Reilly and Associates, 2000,
1-882114-26-4.
The absolute best awk tutorial and reference. The free electronic version of this book is part of the awk
documentation, and printed copies of the latest version are available from O'Reilly and Associates.
This book has served as an inspiration for the author of the ABS Guide.
*

Bibliography

561

Advanced Bash-Scripting Guide
Bill Rosenblatt, Learning the Korn Shell, O'Reilly and Associates, 1993, 1-56592-054-6.
This well-written book contains some excellent pointers on shell scripting in general.
*

Paul Sheer, LINUX: Rute User's Tutorial and Exposition, 1st edition, , 2002, 0-13-033351-4.
Very detailed and readable introduction to Linux system administration.
The book is available in print, or on-line.
*

Ellen Siever and the staff of O'Reilly and Associates, Linux in a Nutshell, 2nd edition, O'Reilly and
Associates, 1999, 1-56592-585-8.
The all-around best Linux command reference. It even has a Bash section.
*

Dave Taylor, Wicked Cool Shell Scripts: 101 Scripts for Linux, Mac OS X, and Unix Systems, 1st edition, No
Starch Press, 2004, 1-59327-012-7.
Just what the title promises . . .
*

The UNIX CD Bookshelf, 3rd edition, O'Reilly and Associates, 2003, 0-596-00392-7.
An array of seven UNIX books on CD ROM, including UNIX Power Tools, Sed and Awk, and Learning the
Korn Shell. A complete set of all the UNIX references and tutorials you would ever need at about $130. Buy
this one, even if it means going into debt and not paying the rent.
Update: Seems to have somehow fallen out of print. Ah, well. You can still buy the dead-tree editions of these
books.
*

The O'Reilly books on Perl. (Actually, any O'Reilly books.)

***
Other Resources

Bibliography

562

Advanced Bash-Scripting Guide
Fioretti, Marco, "Scripting for X Productivity," Linux Journal, Issue 113, September, 2003, pp. 86-9.

Ben Okopnik's well-written introductory Bash scripting articles in issues 53, 54, 55, 57, and 59 of the Linux
Gazette, and his explanation of "The Deep, Dark Secrets of Bash" in issue 56.

Chet Ramey's Bash - The GNU Shell, a two-part series published in issues 3 and 4 of the Linux Journal,
July-August 1994.

Mike G's Bash-Programming-Intro HOWTO.

Richard's Unix Scripting Universe.

Chet Ramey's Bash FAQ.

Greg's WIKI: Bash FAQ.

Example shell scripts at Lucc's Shell Scripts .

Example shell scripts at SHELLdorado .

Example shell scripts at Noah Friedman's script site.

Examples from the The Bash Scripting Cookbook, by Albing, Vossen, and Newham.

Example shell scripts at zazzybob.

Steve Parker's Shell Programming Stuff. In fact, all of his shell scripting books are highly recommended. See
also Steve's Arcade Games written in a shell script.

An excellent collection of Bash scripting tips, tricks, and resources at the Bash Hackers Wiki.

Giles Orr's Bash-Prompt HOWTO.

The Pixelbeat command-line reference.

Bibliography

563

Advanced Bash-Scripting Guide
Very nice sed, awk, and regular expression tutorials at The UNIX Grymoire.

The GNU sed and gawk manuals. As you recall, gawk is the enhanced GNU version of awk.

Many interesting sed scripts at the seder's grab bag.

Tips and tricks at Linux Reviews.

Trent Fisher's groff tutorial.

"Shelltris" and "shellitaire" at Shell Script Games.

Mark Komarinski's Printing-Usage HOWTO.

The Linux USB subsystem (helpful in writing scripts affecting USB peripherals).

There is some nice material on I/O redirection in chapter 10 of the textutils documentation at the University
of Alberta site.

Rick Hohensee has written the osimpa i386 assembler entirely as Bash scripts.

dgatwood has a very nice shell script games site, featuring a Tetris® clone and solitaire.

Aurelio Marinho Jargas has written a Regular expression wizard. He has also written an informative book on
Regular Expressions, in Portuguese.

Ben Tomkins has created the Bash Navigator directory management tool.

William Park has been working on a project to incorporate certain Awk and Python features into Bash. Among
these is a gdbm interface. He has released bashdiff on Freshmeat.net. He has an article in the November, 2004
issue of the Linux Gazette on adding string functions to Bash, with a followup article in the December issue,
and yet another in the January, 2005 issue.

Peter Knowles has written an elaborate Bash script that generates a book list on the Sony Librie e-book
reader. This useful tool facilitates loading non-DRM user content on the Librie (and the newer PRS-xxx-series
devices).

Bibliography

564

Advanced Bash-Scripting Guide
Tim Waugh's xmlto is an elaborate Bash script for converting Docbook XML documents to other formats.

Philip Patterson's logforbash logging/debugging script.

Of historical interest are Colin Needham's original International Movie Database (IMDB) reader polling
scripts, which nicely illustrate the use of awk for string parsing. Unfortunately, the URL link is broken.
---

Fritz Mehner has written a bash-support plugin for the vim text editor. He has also also come up with his own
stylesheet for Bash. Compare it with the ABS Guide Unofficial Stylesheet.
---

Penguin Pete has quite a number of shell scripting tips and hints on his superb site. Highly recommended.

The excellent Bash Reference Manual, by Chet Ramey and Brian Fox, distributed as part of the bash-2-doc
package (available as an rpm). See especially the instructive example scripts in this package.

John Lion's classic, A Commentary on the Sixth Edition UNIX Operating System.

The comp.os.unix.shell newsgroup.

The dd thread on Linux Questions.

The comp.os.unix.shell FAQ.

Assorted comp.os.unix FAQs.

The Wikipedia article covering dc.

The manpages for bash and bash2, date, expect, expr, find, grep, gzip, ln, patch, tar, tr, bc, xargs. The
texinfo documentation on bash, dd, m4, gawk, and sed.

Bibliography

565

Appendix A. Contributed Scripts
These scripts, while not fitting into the text of this document, do illustrate some interesting shell programming
techniques. They are useful, too. Have fun analyzing and running them.

Example A-1. mailformat: Formatting an e-mail message
#!/bin/bash
# mail-format.sh (ver. 1.1): Format e-mail messages.
# Gets rid of carets, tabs, and also folds excessively long lines.
# =================================================================
#
Standard Check for Script Argument(s)
ARGS=1
E_BADARGS=65
E_NOFILE=66
if [ $# -ne $ARGS ] # Correct number of arguments passed to script?
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
if [ -f "$1" ]
# Check if file exists.
then
file_name=$1
else
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
# =================================================================
MAXWIDTH=70

# Width to fold excessively long lines to.

# --------------------------------# A variable can hold a sed script.
sedscript='s/^>//
s/^ *>//
s/^ *//
s/
*//'
# --------------------------------# Delete carets and tabs at beginning of lines,
#+ then fold lines to $MAXWIDTH characters.
sed "$sedscript" $1 | fold -s --width=$MAXWIDTH
# -s option to "fold"
#+ breaks lines at whitespace, if possible.

#
#+
#
#
#+

This script was inspired by an article in a well-known trade journal
extolling a 164K MS Windows utility with similar functionality.
An nice set of text processing utilities and an efficient
scripting language provide an alternative to bloated executables.

exit

Appendix A. Contributed Scripts

566

Advanced Bash-Scripting Guide
Example A-2. rn: A simple-minded file renaming utility
This script is a modification of Example 16-22.
#! /bin/bash
# rn.sh
# Very simpleminded filename "rename" utility (based on "lowercase.sh").
#
# The "ren" utility, by Vladimir Lanin (lanin@csd2.nyu.edu),
#+ does a much better job of this.

ARGS=2
E_BADARGS=85
ONE=1

# For getting singular/plural right (see below).

if [ $# -ne "$ARGS" ]
then
echo "Usage: `basename $0` old-pattern new-pattern"
# As in "rn gif jpg", which renames all gif files in working directory to jpg.
exit $E_BADARGS
fi
number=0

# Keeps track of how many files actually renamed.

for filename in *$1*
#Traverse all matching files in directory.
do
if [ -f "$filename" ] # If finds match...
then
fname=`basename $filename`
# Strip off path.
n=`echo $fname | sed -e "s/$1/$2/"`
# Substitute new for old in filename.
mv $fname $n
# Rename.
let "number += 1"
fi
done
if [ "$number" -eq "$ONE" ]
then
echo "$number file renamed."
else
echo "$number files renamed."
fi

# For correct grammar.

exit $?

#
#
#
#

Exercises:
--------What types of files will this not work on?
How can this be fixed?

Example A-3. blank-rename: Renames filenames containing blanks
This is an even simpler-minded version of previous script.
#! /bin/bash
# blank-rename.sh
#

Appendix A. Contributed Scripts

567

Advanced Bash-Scripting Guide
# Substitutes underscores for blanks in all the filenames in a directory.
ONE=1
number=0
FOUND=0

# For getting singular/plural right (see below).
# Keeps track of how many files actually renamed.
# Successful return value.

for filename in *
#Traverse all files in directory.
do
echo "$filename" | grep -q " "
# Check whether filename
if [ $? -eq $FOUND ]
#+ contains space(s).
then
fname=$filename
# Yes, this filename needs work.
n=`echo $fname | sed -e "s/ /_/g"`
# Substitute underscore for blank.
mv "$fname" "$n"
# Do the actual renaming.
let "number += 1"
fi
done
if [ "$number" -eq "$ONE" ]
then
echo "$number file renamed."
else
echo "$number files renamed."
fi

# For correct grammar.

exit 0

Example A-4. encryptedpw: Uploading to an ftp site, using a locally encrypted password
#!/bin/bash
# Example "ex72.sh" modified to use encrypted password.
# Note that this is still rather insecure,
#+ since the decrypted password is sent in the clear.
# Use something like "ssh" if this is a concern.
E_BADARGS=85
if [ -z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
Username=bozo
# Change to suit.
pword=/home/bozo/secret/password_encrypted.file
# File containing encrypted password.
Filename=`basename $1`

# Strips pathname out of file name.

Server="XXX"
Directory="YYY"

# Change above to actual server name & directory.

Password=`cruft <$pword`
# Decrypt password.
# Uses the author's own "cruft" file encryption package,
#+ based on the classic "onetime pad" algorithm,
#+ and obtainable from:
#+ Primary-site:
ftp://ibiblio.org/pub/Linux/utils/file
#+
cruft-0.2.tar.gz [16k]

Appendix A. Contributed Scripts

568

Advanced Bash-Scripting Guide

ftp -n $Server <32000), try increasing MAX_ITERATIONS.
h=${1:-$$}

# Seed.
# Use $PID as seed,
#+ if not specified as command-line arg.

echo
echo "C($h) -*- $MAX_ITERATIONS Iterations"
echo
for ((i=1; i<=MAX_ITERATIONS; i++))
do
# echo -n "$h
"
#
^^^
#
tab
# printf does it better ...
COLWIDTH=%7d
printf $COLWIDTH $h
let "remainder = h % 2"
if [ "$remainder" -eq 0 ]
then
let "h /= 2"
else

Appendix A. Contributed Scripts

# Even?
# Divide by 2.

570

Advanced Bash-Scripting Guide
let "h = h*3 + 1"
fi

# Multiply by 3 and add 1.

COLUMNS=10
# Output 10 values per line.
let "line_break = i % $COLUMNS"
if [ "$line_break" -eq 0 ]
then
echo
fi
done
echo
# For more information on this strange mathematical function,
#+ see _Computers, Pattern, Chaos, and Beauty_, by Pickover, p. 185 ff.,
#+ as listed in the bibliography.
exit 0

Example A-7. days-between: Days between two dates
#!/bin/bash
# days-between.sh:
Number of days between two dates.
# Usage: ./days-between.sh [M]M/[D]D/YYYY [M]M/[D]D/YYYY
#
# Note: Script modified to account for changes in Bash, v. 2.05b +,
#+
that closed the loophole permitting large negative
#+
integer return values.
ARGS=2
E_PARAM_ERR=85

# Two command-line parameters expected.
# Param error.

REFYR=1600
CENTURY=100
DIY=365
ADJ_DIY=367
MIY=12
DIM=31
LEAPCYCLE=4

# Reference year.

MAXRETVAL=255

# Largest permissible
#+ positive return value from a function.

diff=
value=
day=
month=
year=

# Declare global variable for date difference.
# Declare global variable for absolute value.
# Declare globals for day, month, year.

# Adjusted for leap year + fraction.

Param_Error ()
# Command-line parameters wrong.
{
echo "Usage: `basename $0` [M]M/[D]D/YYYY [M]M/[D]D/YYYY"
echo "
(date must be after 1/3/1600)"
exit $E_PARAM_ERR
}

Appendix A. Contributed Scripts

571

Advanced Bash-Scripting Guide
Parse_Date ()
{
month=${1%%/**}
dm=${1%/**}
day=${dm#*/}
let "year = `basename $1`"
}

# Parse date from command-line params.

# Day and month.
# Not a filename, but works just the same.

check_date ()
# Checks for invalid date(s) passed.
{
[ "$day" -gt "$DIM" ] || [ "$month" -gt "$MIY" ] ||
[ "$year" -lt "$REFYR" ] && Param_Error
# Exit script on bad value(s).
# Uses or-list / and-list.
#
# Exercise: Implement more rigorous date checking.
}

strip_leading_zero () # Better to strip
{
#+ from day and/or
return ${1#0}
#+ since otherwise
}
#+ as octal values

day_index ()
{

possible leading zero(s)
month
Bash will interpret them
(POSIX.2, sect 2.9.2.1).

# Gauss' Formula:
# Days from March 1, 1600 to date passed as param.
#
^^^^^^^^^^^^^

day=$1
month=$2
year=$3
let "month = $month - 2"
if [ "$month" -le 0 ]
then
let "month += 12"
let "year -= 1"
fi
let "year -= $REFYR"
let "indexyr = $year / $CENTURY"

let "Days = $DIY*$year + $year/$LEAPCYCLE - $indexyr \
+ $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day - $DIM"
# For an in-depth explanation of this algorithm, see
#+
http://weblogs.asp.net/pgreborio/archive/2005/01/06/347968.aspx

echo $Days
}

calculate_difference ()
{
let "diff = $1 - $2"
}

# Difference between two day indices.

abs ()

#

Appendix A. Contributed Scripts

# Global variable.

Absolute value

572

Advanced Bash-Scripting Guide
{
if [ "$1" -lt 0 ]
then
let "value = 0 - $1"
else
let "value = $1"
fi

#
#
#+
#+
#+
#+

Uses global "value" variable.
If negative
then
change sign,
else
leave it alone.

}

if [ $# -ne "$ARGS" ]
then
Param_Error
fi
Parse_Date $1
check_date $day $month $year
strip_leading_zero $day
day=$?
strip_leading_zero $month
month=$?

# Require two command-line params.

#

See if valid date.

# Remove any leading zeroes
#+ on day and/or month.

let "date1 = `day_index $day $month $year`"

Parse_Date $2
check_date $day $month $year
strip_leading_zero $day
day=$?
strip_leading_zero $month
month=$?
date2=$(day_index $day $month $year) # Command substitution.

calculate_difference $date1 $date2
abs $diff
diff=$value

# Make sure it's positive.

echo $diff
exit 0
#
#
#
#+

Exercise:
-------If given only one command-line parameter, have the script
use today's date as the second.

# Compare this script with
#+ the implementation of Gauss' Formula in a C program at
#+
http://buschencrew.hypermart.net/software/datedif

Example A-8. Making a dictionary
#!/bin/bash
# makedict.sh

[make dictionary]

Appendix A. Contributed Scripts

573

Advanced Bash-Scripting Guide
# Modification of /usr/sbin/mkdict (/usr/sbin/cracklib-forman) script.
# Original script copyright 1993, by Alec Muffett.
#
# This modified script included in this document in a manner
#+ consistent with the "LICENSE" document of the "Crack" package
#+ that the original script is a part of.
#
#+
#
#+

This script processes text files to produce a sorted list
of words found in the files.
This may be useful for compiling dictionaries
and for other lexicographic purposes.

E_BADARGS=65
if [ ! -r "$1" ]
# Need at least one
then
#+ valid file argument.
echo "Usage: $0 files-to-process"
exit $E_BADARGS
fi

# SORT="sort"

# No longer necessary to define
#+ options to sort. Changed from
#+ original script.

cat $* |
tr A-Z a-z |
tr ' ' '\012' |
#
tr -cd '\012[a-z][0-9]' |

# Contents of specified files to stdout.
# Convert to lowercase.
# New: change spaces to newlines.
# Get rid of everything
#+ non-alphanumeric (in orig. script).
# Rather than deleting non-alpha
#+ chars, change them to newlines.
# $SORT options unnecessary now.
# Remove duplicates.
# Delete lines starting with hashmark.
# Delete blank lines.

tr -c '\012a-z'
sort
uniq
grep
grep

'\012' |

|
|
-v '^#' |
-v '^$'

exit 0

Example A-9. Soundex conversion
#!/bin/bash
# soundex.sh: Calculate "soundex" code for names
# =======================================================
#
Soundex script
#
by
#
Mendel Cooper
#
thegrendel.abs@gmail.com
#
reldate: 23 January, 2002
#
#
Placed in the Public Domain.
#
# A slightly different version of this script appeared in
#+ Ed Schaefer's July, 2002 "Shell Corner" column
#+ in "Unix Review" on-line,
#+ http://www.unixreview.com/documents/uni1026336632258/
# =======================================================

Appendix A. Contributed Scripts

574

Advanced Bash-Scripting Guide

ARGCOUNT=1
E_WRONGARGS=90

# Need name as argument.

if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` name"
exit $E_WRONGARGS
fi

assign_value ()
{
val1=bfpv
val2=cgjkqsxz
val3=dt
val4=l
val5=mn
val6=r

# Assigns numerical value
#+ to letters of name.
# 'b,f,p,v' = 1
# 'c,g,j,k,q,s,x,z' = 2
# etc.

# Exceptionally clever use of 'tr' follows.
# Try to figure out what is going on here.
value=$( echo "$1" \
| tr -d wh \
| tr $val1 1 | tr $val2 2 | tr $val3 3 \
| tr $val4 4 | tr $val5 5 | tr $val6 6 \
| tr -s 123456 \
| tr -d aeiouy )
#
#
#
#
#
#

Assign
Remove
Ignore
Ignore

letter values.
duplicate numbers, except when separated by vowels.
vowels, except as separators, so delete them last.
'w' and 'h', even as separators, so delete them first.

The above command substitution lays more pipe than a plumber .

}

input_name="$1"
echo
echo "Name = $input_name"

# Change all characters of name input to lowercase.
# -----------------------------------------------name=$( echo $input_name | tr A-Z a-z )
# -----------------------------------------------# Just in case argument to script is mixed case.

# Prefix of soundex code: first letter of name.
# --------------------------------------------

char_pos=0
# Initialize character position.
prefix0=${name:$char_pos:1}
prefix=`echo $prefix0 | tr a-z A-Z`
# Uppercase 1st letter of soundex.

Appendix A. Contributed Scripts

575

Advanced Bash-Scripting Guide
let "char_pos += 1"
name1=${name:$char_pos}

# Bump character position to 2nd letter of name.

# ++++++++++++++++++++++++++ Exception Patch ++++++++++++++++++++++++++++++
# Now, we run both the input name and the name shifted one char
#+ to the right through the value-assigning function.
# If we get the same value out, that means that the first two characters
#+ of the name have the same value assigned, and that one should cancel.
# However, we also need to test whether the first letter of the name
#+ is a vowel or 'w' or 'h', because otherwise this would bollix things up.
char1=`echo $prefix | tr A-Z a-z`
assign_value $name
s1=$value
assign_value $name1
s2=$value
assign_value $char1
s3=$value
s3=9$s3

# First letter of name, lowercased.

#
#+
#+
#+
#+

If first letter of name is a vowel
or 'w' or 'h',
then its "value" will be null (unset).
Therefore, set it to 9, an otherwise
unused value, which can be tested for.

if [[ "$s1" -ne "$s2" || "$s3" -eq 9 ]]
then
suffix=$s2
else
suffix=${s2:$char_pos}
fi
# ++++++++++++++++++++++ end Exception Patch ++++++++++++++++++++++++++++++

padding=000

# Use at most 3 zeroes to pad.

soun=$prefix$suffix$padding

# Pad with zeroes.

MAXLEN=4
soundex=${soun:0:$MAXLEN}

# Truncate to maximum of 4 chars.

echo "Soundex = $soundex"
echo
#
#+
#
#+
#
#
#
#
#
#
#

The soundex code is a method of indexing and classifying names
by grouping together the ones that sound alike.
The soundex code for a given name is the first letter of the name,
followed by a calculated three-number code.
Similar sounding names should have almost the same soundex codes.
Examples:
Smith and Smythe both have a "S-530" soundex.
Harrison = H-625
Hargison = H-622
Harriman = H-655
This works out fairly well in practice, but there are numerous anomalies.

Appendix A. Contributed Scripts

576

Advanced Bash-Scripting Guide
#
#
#
#
#
#
#+
#+

The U.S. Census and certain other governmental agencies use soundex,
as do genealogical researchers.
For more information,
see the "National Archives and Records Administration home page",
http://www.nara.gov/genealogy/soundex/soundex.html

# Exercise:
# -------# Simplify the "Exception Patch" section of this script.
exit 0

Example A-10. Game of Life
#!/bin/bash
# life.sh: "Life in the Slow Lane"
# Author: Mendel Cooper
# License: GPL3
# Version 0.2:
Patched by Daniel Albers
#+
to allow non-square grids as input.
# Version 0.2.1: Added 2-second delay between generations.
# ##################################################################### #
# This is the Bash script version of John Conway's "Game of Life".
#
# "Life" is a simple implementation of cellular automata.
#
# --------------------------------------------------------------------- #
# On a rectangular grid, let each "cell" be either "living" or "dead." #
# Designate a living cell with a dot, and a dead one with a blank space.#
#
Begin with an arbitrarily drawn dot-and-blank grid,
#
#+
and let this be the starting generation: generation 0.
#
# Determine each successive generation by the following rules:
#
#
1) Each cell has 8 neighbors, the adjoining cells
#
#+
left, right, top, bottom, and the 4 diagonals.
#
#
#
#
123
#
#
4*5
The * is the cell under consideration. #
#
678
#
#
#
# 2) A living cell with either 2 or 3 living neighbors remains alive.
#
SURVIVE=2
#
# 3) A dead cell with 3 living neighbors comes alive, a "birth."
#
BIRTH=3
#
# 4) All other cases result in a dead cell for the next generation.
#
# ##################################################################### #

startfile=gen0

# Read the starting generation from the file "gen0" ...
# Default, if no other file specified when invoking script.
#
# Specify another "generation 0" file.

if [ -n "$1" ]
then
startfile="$1"
fi

Appendix A. Contributed Scripts

577

Advanced Bash-Scripting Guide
############################################
# Abort script if "startfile" not specified
#+ and
#+ default file "gen0" not present.
E_NOSTARTFILE=86
if [ ! -e "$startfile" ]
then
echo "Startfile \""$startfile"\" missing!"
exit $E_NOSTARTFILE
fi
############################################

ALIVE1=.
DEAD1=_
# Represent living and dead cells in the start-up file.
# -----------------------------------------------------#
# This script uses a 10 x 10 grid (may be increased,
#+ but a large grid will slow down execution).
ROWS=10
COLS=10
# Change above two variables to match desired grid size.
# -----------------------------------------------------#
GENERATIONS=10

# How many generations to cycle through.
# Adjust this upwards
#+ if you have time on your hands.

NONE_ALIVE=85

# Exit status on premature bailout,
#+ if no cells left alive.
# Pause between generations.

DELAY=2
TRUE=0
FALSE=1
ALIVE=0
DEAD=1
avar=
generation=0

# Global; holds current generation.
# Initialize generation count.

# =================================================================
let "cells = $ROWS * $COLS"

# How many cells.

# Arrays containing "cells."
declare -a initial
declare -a current
display ()
{
alive=0

declare -a arr
arr=( `echo "$1"` )

# How many cells alive at any given time.
# Initially zero.

# Convert passed arg to array.

element_count=${#arr[*]}
local i

Appendix A. Contributed Scripts

578

Advanced Bash-Scripting Guide
local rowcheck
for ((i=0; i<$element_count; i++))
do
# Insert newline at end of each row.
let "rowcheck = $i % COLS"
if [ "$rowcheck" -eq 0 ]
then
echo
# Newline.
echo -n "
"
# Indent.
fi
cell=${arr[i]}
if [ "$cell" = . ]
then
let "alive += 1"
fi
echo -n "$cell" | sed -e 's/_/ /g'
# Print out array, changing underscores to spaces.
done
return
}
IsValid ()
{
if [ -z "$1" -o -z "$2" ]
then
return $FALSE
fi
local
local
local
local
local

row
lower_limit=0
upper_limit
left
right

# Test if cell coordinate valid.

# Mandatory arguments missing?

# Disallow negative coordinate.

let "upper_limit = $ROWS * $COLS - 1" # Total number of cells.

if [ "$1" -lt "$lower_limit" -o "$1" -gt "$upper_limit" ]
then
return $FALSE
# Out of array bounds.
fi
row=$2
let "left = $row * $COLS"
let "right = $left + $COLS - 1"

# Left limit.
# Right limit.

if [ "$1" -lt "$left" -o "$1" -gt "$right" ]
then
return $FALSE
# Beyond row boundary.
fi
return $TRUE

# Valid coordinate.

}

Appendix A. Contributed Scripts

579

Advanced Bash-Scripting Guide

IsAlive ()
{
GetCount "$1" $2
local nhbd=$?

#
#
#+
#

Test whether cell is alive.
Takes array, cell number, and
state of cell as arguments.
Get alive cell count in neighborhood.

if [ "$nhbd" -eq "$BIRTH" ]
then
return $ALIVE
fi

# Alive in any case.

if [ "$3" = "." -a "$nhbd" -eq "$SURVIVE" ]
then
# Alive only if previously alive.
return $ALIVE
fi
return $DEAD

# Defaults to dead.

}

GetCount ()

#
#
#
#

Count live cells in passed cell's neighborhood.
Two arguments needed:
$1) variable holding array
$2) cell number

{
local
local
local
local
local
local
local
local
local
local
local
local
local

cell_number=$2
array
top
center
bottom
r
row
i
t_top
t_cen
t_bot
count=0
ROW_NHBD=3

array=( `echo "$1"` )
let
let
let
let

"top = $cell_number - $COLS - 1"
# Set up cell neighborhood.
"center = $cell_number - 1"
"bottom = $cell_number + $COLS - 1"
"r = $cell_number / $COLS"

for ((i=0; i<$ROW_NHBD; i++))
do
let "t_top = $top + $i"
let "t_cen = $center + $i"
let "t_bot = $bottom + $i"

# Traverse from left to right.

let "row = $r"
#
IsValid $t_cen $row
#
if [ $? -eq "$TRUE" ]
then
if [ ${array[$t_cen]} = "$ALIVE1" ] #
then
#

Appendix A. Contributed Scripts

Count center row.
Valid cell position?

Is it alive?
If yes, then ...

580

Advanced Bash-Scripting Guide
let "count += 1"
fi

# Increment count.

fi
let "row = $r - 1"
# Count top row.
IsValid $t_top $row
if [ $? -eq "$TRUE" ]
then
if [ ${array[$t_top]} = "$ALIVE1" ] # Redundancy here.
then
# Can it be optimized?
let "count += 1"
fi
fi
let "row = $r + 1"
# Count bottom row.
IsValid $t_bot $row
if [ $? -eq "$TRUE" ]
then
if [ ${array[$t_bot]} = "$ALIVE1" ]
then
let "count += 1"
fi
fi
done

if [ ${array[$cell_number]} = "$ALIVE1" ]
then
let "count -= 1"
# Make sure value of tested cell itself
fi
#+ is not counted.

return $count
}
next_gen ()
{

# Update generation array.

local array
local i=0
array=( `echo "$1"` )

# Convert passed arg to array.

while [ "$i" -lt "$cells" ]
do
IsAlive "$1" $i ${array[$i]}
if [ $? -eq "$ALIVE" ]
then
array[$i]=.
else
array[$i]="_"
fi
let "i += 1"
done

#
###

# Is the cell alive?
# If alive, then
#+ represent the cell as a period.
# Otherwise underscore
#+ (will later be converted to space).

let "generation += 1"
# Increment generation count.
Why was the above line commented out?

Appendix A. Contributed Scripts

581

Advanced Bash-Scripting Guide
# Set variable to pass as parameter to "display" function.
avar=`echo ${array[@]}`
# Convert array back to string variable.
display "$avar"
# Display it.
echo; echo
echo "Generation $generation - $alive alive"
if [ "$alive" -eq 0 ]
then
echo
echo "Premature exit: no more cells alive!"
exit $NONE_ALIVE
# No point in continuing
fi
#+ if no live cells.
}

# =========================================================
# main ()
# {
# Load initial array with contents of startup file.
initial=( `cat "$startfile" | sed -e '/#/d' | tr -d '\n' |\
# Delete lines containing '#' comment character.
sed -e 's/\./\. /g' -e 's/_/_ /g'` )
# Remove linefeeds and insert space between elements.
clear

# Clear screen.

echo #
Title
setterm -reverse on
echo "======================="
setterm -reverse off
echo "
$GENERATIONS generations"
echo "
of"
echo "\"Life in the Slow Lane\""
setterm -reverse on
echo "======================="
setterm -reverse off
sleep $DELAY

# Display "splash screen" for 2 seconds.

# -------- Display first generation. -------Gen0=`echo ${initial[@]}`
display "$Gen0"
# Display only.
echo; echo
echo "Generation $generation - $alive alive"
sleep $DELAY
# -------------------------------------------

let "generation += 1"
echo

# Bump generation count.

# ------- Display second generation. ------Cur=`echo ${initial[@]}`
next_gen "$Cur"
# Update & display.
sleep $DELAY
# -----------------------------------------let "generation += 1"

# Increment generation count.

Appendix A. Contributed Scripts

582

Advanced Bash-Scripting Guide
# ------ Main loop for displaying subsequent generations -----while [ "$generation" -le "$GENERATIONS" ]
do
Cur="$avar"
next_gen "$Cur"
let "generation += 1"
sleep $DELAY
done
# ==============================================================
echo
# }
exit 0

# CEOF:EOF

# The grid in this script has a "boundary problem."
# The the top, bottom, and sides border on a void of dead cells.
# Exercise: Change the script to have the grid wrap around,
# +
so that the left and right sides will "touch,"
# +
as will the top and bottom.
#
# Exercise: Create a new "gen0" file to seed this script.
#
Use a 12 x 16 grid, instead of the original 10 x 10 one.
#
Make the necessary changes to the script,
#+
so it will run with the altered file.
#
# Exercise: Modify this script so that it can determine the grid size
#+
from the "gen0" file, and set any variables necessary
#+
for the script to run.
#
This would make unnecessary any changes to variables
#+
in the script for an altered grid size.
#
# Exercise: Optimize this script.
#
It has redundant code.

Example A-11. Data file for Game of Life
# gen0
#
# This is an example "generation 0" start-up file for "life.sh".
# -------------------------------------------------------------# The "gen0" file is a 10 x 10 grid using a period (.) for live cells,
#+ and an underscore (_) for dead ones. We cannot simply use spaces
#+ for dead cells in this file because of a peculiarity in Bash arrays.
# [Exercise for the reader: explain this.]
#
# Lines beginning with a '#' are comments, and the script ignores them.
__.__..___
___._.____
____.___..
_._______.
____._____
..__...___
____._____
___...____
__.._..___
_..___..__

+++
Appendix A. Contributed Scripts

583

Advanced Bash-Scripting Guide
The following script is by Mark Moraes of the University of Toronto. See the file Moraes-COPYRIGHT for
permissions and restrictions. This file is included in the combined HTML/source tarball of the ABS Guide.

Example A-12. behead: Removing mail and news message headers
#! /bin/sh
# Strips off the header from a mail/News message i.e. till the first
# empty line.
# Author: Mark Moraes, University of Toronto
# ==> These comments added by author of this document.
if [ $# -eq 0 ]; then
# ==> If no command-line args present, then works on file redirected to stdin.
sed -e '1,/^$/d' -e '/^[
]*$/d'
# --> Delete empty lines and all lines until
# --> first one beginning with white space.
else
# ==> If command-line args present, then work on files named.
for i do
sed -e '1,/^$/d' -e '/^[
]*$/d' $i
# --> Ditto, as above.
done
fi
exit
#
#
#
#

==> Exercise: Add error checking and other options.
==>
==> Note that the small sed script repeats, except for the arg passed.
==> Does it make sense to embed it in a function? Why or why not?

/*
* Copyright University of Toronto 1988, 1989.
* Written by Mark Moraes
*
* Permission is granted to anyone to use this software for any purpose on
* any computer system, and to alter it and redistribute it freely, subject
* to the following restrictions:
*
* 1. The author and the University of Toronto are not responsible
*
for the consequences of use of this software, no matter how awful,
*
even if they arise from flaws in it.
*
* 2. The origin of this software must not be misrepresented, either by
*
explicit claim or by omission. Since few users ever read sources,
*
credits must appear in the documentation.
*
* 3. Altered versions must be plainly marked as such, and must not be
*
misrepresented as being the original software. Since few users
*
ever read sources, credits must appear in the documentation.
*
* 4. This notice may not be removed or altered.
*/

+
Antek Sawicki contributed the following script, which makes very clever use of the parameter substitution
operators discussed in Section 10.2.
Appendix A. Contributed Scripts

584

Advanced Bash-Scripting Guide
Example A-13. password: Generating random 8-character passwords
#!/bin/bash
# May need to be invoked with #!/bin/bash2 on older machines.
#
# Random password generator for Bash 2.x +
#+ by Antek Sawicki ,
#+ who generously gave usage permission to the ABS Guide author.
#
# ==> Comments added by document author ==>

MATRIX="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
# ==> Password will consist of alphanumeric characters.
LENGTH="8"
# ==> May change 'LENGTH' for longer password.

while [ "${n:=1}" -le "$LENGTH" ]
# ==> Recall that := is "default substitution" operator.
# ==> So, if 'n' has not been initialized, set it to 1.
do
PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"
# ==> Very clever, but tricky.
# ==> Starting from the innermost nesting...
# ==> ${#MATRIX} returns length of array MATRIX.
# ==> $RANDOM%${#MATRIX} returns random number between 1
# ==> and [length of MATRIX] - 1.
#
#
#
#

==>
==>
==>
==>

${MATRIX:$(($RANDOM%${#MATRIX})):1}
returns expansion of MATRIX at random position, by length 1.
See {var:pos:len} parameter substitution in Chapter 9.
and the associated examples.

# ==> PASS=... simply pastes this result onto previous PASS (concatenation).
# ==> To visualize this more clearly, uncomment the following line
#
echo "$PASS"
# ==> to see PASS being built up,
# ==> one character at a time, each iteration of the loop.
let n+=1
# ==> Increment 'n' for next pass.
done
echo "$PASS"

# ==> Or, redirect to a file, as desired.

exit 0

+
James R. Van Zandt contributed this script which uses named pipes and, in his words, "really exercises
quoting and escaping."

Example A-14. fifo: Making daily backups, using named pipes
#!/bin/bash
# ==> Script by James R. Van Zandt, and used here with his permission.

Appendix A. Contributed Scripts

585

Advanced Bash-Scripting Guide
# ==> Comments added by author of this document.

HERE=`uname -n`
# ==> hostname
THERE=bilbo
echo "starting remote backup to $THERE at `date +%r`"
# ==> `date +%r` returns time in 12-hour format, i.e. "08:08:34 PM".
# make sure /pipe really is a pipe and not a plain file
rm -rf /pipe
mkfifo /pipe
# ==> Create a "named pipe", named "/pipe" ...
# ==> 'su xyz' runs commands as user "xyz".
# ==> 'ssh' invokes secure shell (remote login client).
su xyz -c "ssh $THERE \"cat > /home/xyz/backup/${HERE}-daily.tar.gz\" < /pipe"&
cd /
tar -czf - bin boot dev etc home info lib man root sbin share usr var > /pipe
# ==> Uses named pipe, /pipe, to communicate between processes:
# ==> 'tar/gzip' writes to /pipe and 'ssh' reads from /pipe.
# ==> The end result is this backs up the main directories, from / on down.
# ==> What are the advantages of a "named pipe" in this situation,
# ==>+ as opposed to an "anonymous pipe", with |?
# ==> Will an anonymous pipe even work here?
# ==>
# ==>

Is it necessary to delete the pipe before exiting the script?
How could that be done?

exit 0

+

Stéphane Chazelas used the following script to demonstrate generating prime numbers without arrays.

Example A-15. Generating prime numbers using the modulo operator
#!/bin/bash
# primes.sh: Generate prime numbers, without using arrays.
# Script contributed by Stephane Chazelas.
# This does *not* use the classic "Sieve of Eratosthenes" algorithm,
#+ but instead the more intuitive method of testing each candidate number
#+ for factors (divisors), using the "%" modulo operator.

LIMIT=1000
Primes()
{
(( n = $1 + 1 ))
shift
# echo "_n=$n i=$i_"

# Primes, 2 ... 1000.

# Bump to next integer.
# Next parameter in list.

if (( n == LIMIT ))
then echo $*

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586

Advanced Bash-Scripting Guide
return
fi
for i; do
#
echo "-n=$n i=$i-"
(( i * i > n )) && break
(( n % i )) && continue
Primes $n $@
return
done
Primes $n $@ $n

# "i" set to "@", previous values of $n.
# Optimization.
# Sift out non-primes using modulo operator.
# Recursion inside loop.

#
#
#+
#

Recursion outside loop.
Successively accumulate
positional parameters.
"$@" is the accumulating list of primes.

}
Primes 1
exit $?
# Pipe output of the script to 'fmt' for prettier printing.
#

Uncomment lines 16 and 24 to help figure out what is going on.

# Compare the speed of this algorithm for generating primes
#+ with the Sieve of Eratosthenes (ex68.sh).

#

Exercise: Rewrite this script without recursion.

+
Rick Boivie's revision of Jordi Sanfeliu's tree script.

Example A-16. tree: Displaying a directory tree
#!/bin/bash
# tree.sh
#
#
#
#+
#
#+
#

Written by Rick Boivie.
Used with permission.
This is a revised and simplified version of a script
by Jordi Sanfeliu (the original author), and patched by Ian Kjos.
This script replaces the earlier version used in
previous releases of the Advanced Bash Scripting Guide.
Copyright (c) 2002, by Jordi Sanfeliu, Rick Boivie, and Ian Kjos.

# ==> Comments added by the author of this document.

search () {
for dir in `echo *`
# ==> `echo *` lists all the files in current working directory,
#+ ==> without line breaks.
# ==> Similar effect to for dir in *
# ==> but "dir in `echo *`" will not handle filenames with blanks.
do
if [ -d "$dir" ] ; then # ==> If it is a directory (-d)...
zz=0
# ==> Temp variable, keeping track of

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587

Advanced Bash-Scripting Guide
while [ $zz != $1 ]
do
echo -n "| "

zz=`expr $zz + 1`
done

#
directory level.
# Keep track of inner nested loop.
# ==> Display vertical connector symbol,
# ==> with 2 spaces & no line feed
#
in order to indent.
# ==> Increment zz.

if [ -L "$dir" ] ; then # ==> If directory is a symbolic link...
echo "+---$dir" `ls -l $dir | sed 's/^.*'$dir' //'`
# ==> Display horiz. connector and list directory name, but...
# ==> delete date/time part of long listing.
else
echo "+---$dir"
# ==> Display horizontal connector symbol...
# ==> and print directory name.
numdirs=`expr $numdirs + 1` # ==> Increment directory count.
if cd "$dir" ; then
# ==> If can move to subdirectory...
search `expr $1 + 1`
# with recursion ;-)
# ==> Function calls itself.
cd ..
fi
fi
fi
done
}
if [ $# != 0 ] ; then
cd $1
# Move to indicated directory.
#else
# stay in current directory
fi
echo "Initial directory = `pwd`"
numdirs=0
search 0
echo "Total directories = $numdirs"
exit 0

Patsie's version of a directory tree script.

Example A-17. tree2: Alternate directory tree script
#!/bin/bash
# tree2.sh
# Lightly modified/reformatted by ABS Guide author.
# Included in ABS Guide with permission of script author (thanks!).
##
##
##
##
##
##
##
##

Recursive file/dirsize checking script, by Patsie
This script builds a list of files/directories and their size (du -akx)
and processes this list to a human readable tree shape
The 'du -akx' is only as good as the permissions the owner has.
So preferably run as root* to get the best results, or use only on
directories for which you have read permissions. Anything you can't
read is not in the list.

#* ABS Guide author advises caution when running scripts as root!

Appendix A. Contributed Scripts

588

Advanced Bash-Scripting Guide
##########
TOP=5
MAXRECURS=5
E_BL=80
E_DIR=81

##########

THIS IS CONFIGURABLE
#
#
#
#

##########

Top 5 biggest (sub)directories.
Max 5 subdirectories/recursions deep.
Blank line already returned.
Directory not specified.

DON'T CHANGE ANYTHING BELOW THIS LINE

PID=$$
SELF=`basename $0`
TMP="/tmp/${SELF}.${PID}.tmp"

##########

# Our own process ID.
# Our own program name.
# Temporary 'du' result.

# Convert number to dotted thousand.
function dot { echo "
$*" |
sed -e :a -e 's/\(.*[0-9]\)\([0-9]\{3\}\)/\1,\2/;ta' |
tail -c 12; }
# Usage: tree 
function tree {
recurs="$1"
#
prefix="$2"
#
minsize="$3"
#
dirname="$4"
#

  
How deep nested are we?
What do we display before file/dirname?
What is the minumum file/dirsize?
Which directory are we checking?

# Get ($TOP) biggest subdirs/subfiles from TMP file.
LIST=`egrep "[[:space:]]${dirname}/[^/]*$" "$TMP" |
awk '{if($1>'$minsize') print;}' | sort -nr | head -$TOP`
[ -z "$LIST" ] && return
# Empty list, then go back.
cnt=0
num=`echo "$LIST" | wc -l`

# How many entries in the list.

## Main loop
echo "$LIST" | while read size name; do
((cnt+=1))
# Count entry number.
bname=`basename "$name"`
# We only need a basename of the entry.
[ -d "$name" ] && bname="$bname/"
# If it's a directory, append a slash.
echo "`dot $size`$prefix +-$bname"
# Display the result.
# Call ourself recursively if it's a directory
#+ and we're not nested too deep ($MAXRECURS).
# The recursion goes up: $((recurs+1))
# The prefix gets a space if it's the last entry,
#+ or a pipe if there are more entries.
# The minimum file/dirsize becomes
#+ a tenth of his parent: $((size/10)).
# Last argument is the full directory name to check.
if [ -d "$name" -a $recurs -lt $MAXRECURS ]; then
[ $cnt -lt $num ] \
|| (tree $((recurs+1)) "$prefix " $((size/10)) "$name") \
&& (tree $((recurs+1)) "$prefix |" $((size/10)) "$name")
fi
done
[ $? -eq 0 ] && echo "
$prefix"
# Every time we jump back add a 'blank' line.
return $E_BL
# We return 80 to tell we added a blank line already.

Appendix A. Contributed Scripts

589

Advanced Bash-Scripting Guide
}
###
###
###

main program

###
###
###

rootdir="$@"
[ -d "$rootdir" ] ||
{ echo "$SELF: Usage: $SELF " >&2; exit $E_DIR; }
# We should be called with a directory name.
echo "Building inventory list, please wait ..."
# Show "please wait" message.
du -akx "$rootdir" 1>"$TMP" 2>/dev/null
# Build a temporary list of all files/dirs and their size.
size=`tail -1 "$TMP" | awk '{print $1}'`
# What is our rootdirectory's size?
echo "`dot $size` $rootdir"
# Display rootdirectory's entry.
tree 0 "" 0 "$rootdir"
# Display the tree below our rootdirectory.
rm "$TMP" 2>/dev/null
# Clean up TMP file.
exit $?

Noah Friedman permitted use of his string function script. It essentially reproduces some of the C-library
string manipulation functions.

Example A-18. string functions: C-style string functions
#!/bin/bash
#
#
#
#
#
#

string.bash --- bash emulation of string(3) library routines
Author: Noah Friedman 
==>
Used with his kind permission in this document.
Created: 1992-07-01
Last modified: 1993-09-29
Public domain

# Conversion to bash v2 syntax done by Chet Ramey
# Commentary:
# Code:
#:docstring strcat:
# Usage: strcat s1 s2
#
# Strcat appends the value of variable s2 to variable s1.
#
# Example:
#
a="foo"
#
b="bar"
#
strcat a b
#
echo $a
#
=> foobar
#
#:end docstring:

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590

Advanced Bash-Scripting Guide
###;;;autoload
==> Autoloading of function commented out.
function strcat ()
{
local s1_val s2_val
s1_val=${!1}
# indirect variable expansion
s2_val=${!2}
eval "$1"=\'"${s1_val}${s2_val}"\'
# ==> eval $1='${s1_val}${s2_val}' avoids problems,
# ==> if one of the variables contains a single quote.
}
#:docstring strncat:
# Usage: strncat s1 s2 $n
#
# Line strcat, but strncat appends a maximum of n characters from the value
# of variable s2. It copies fewer if the value of variabl s2 is shorter
# than n characters. Echoes result on stdout.
#
# Example:
#
a=foo
#
b=barbaz
#
strncat a b 3
#
echo $a
#
=> foobar
#
#:end docstring:
###;;;autoload
function strncat ()
{
local s1="$1"
local s2="$2"
local -i n="$3"
local s1_val s2_val
s1_val=${!s1}
s2_val=${!s2}
if [ ${#s2_val} -gt ${n} ]; then
s2_val=${s2_val:0:$n}
fi

# ==> indirect variable expansion

# ==> substring extraction

eval "$s1"=\'"${s1_val}${s2_val}"\'
# ==> eval $1='${s1_val}${s2_val}' avoids problems,
# ==> if one of the variables contains a single quote.
}
#:docstring strcmp:
# Usage: strcmp $s1 $s2
#
# Strcmp compares its arguments and returns an integer less than, equal to,
# or greater than zero, depending on whether string s1 is lexicographically
# less than, equal to, or greater than string s2.
#:end docstring:
###;;;autoload
function strcmp ()
{
[ "$1" = "$2" ] && return 0
[ "${1}" '<' "${2}" ] > /dev/null && return -1

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591

Advanced Bash-Scripting Guide
return 1
}
#:docstring strncmp:
# Usage: strncmp $s1 $s2 $n
#
# Like strcmp, but makes the comparison by examining a maximum of n
# characters (n less than or equal to zero yields equality).
#:end docstring:
###;;;autoload
function strncmp ()
{
if [ -z "${3}" -o "${3}" -le "0" ]; then
return 0
fi
if [ ${3} -ge ${#1} -a ${3} -ge ${#2} ]; then
strcmp "$1" "$2"
return $?
else
s1=${1:0:$3}
s2=${2:0:$3}
strcmp $s1 $s2
return $?
fi
}
#:docstring strlen:
# Usage: strlen s
#
# Strlen returns the number of characters in string literal s.
#:end docstring:
###;;;autoload
function strlen ()
{
eval echo "\${#${1}}"
# ==> Returns the length of the value of the variable
# ==> whose name is passed as an argument.
}
#:docstring strspn:
# Usage: strspn $s1 $s2
#
# Strspn returns the length of the maximum initial segment of string s1,
# which consists entirely of characters from string s2.
#:end docstring:
###;;;autoload
function strspn ()
{
# Unsetting IFS allows whitespace to be handled as normal chars.
local IFS=
local result="${1%%[!${2}]*}"
echo ${#result}
}
#:docstring strcspn:
# Usage: strcspn $s1 $s2

Appendix A. Contributed Scripts

592

Advanced Bash-Scripting Guide
#
# Strcspn returns the length of the maximum initial segment of string s1,
# which consists entirely of characters not from string s2.
#:end docstring:
###;;;autoload
function strcspn ()
{
# Unsetting IFS allows whitspace to be handled as normal chars.
local IFS=
local result="${1%%[${2}]*}"
echo ${#result}
}
#:docstring strstr:
# Usage: strstr s1 s2
#
# Strstr echoes a substring starting at the first occurrence of string s2 in
# string s1, or nothing if s2 does not occur in the string. If s2 points to
# a string of zero length, strstr echoes s1.
#:end docstring:
###;;;autoload
function strstr ()
{
# if s2 points to a string of zero length, strstr echoes s1
[ ${#2} -eq 0 ] && { echo "$1" ; return 0; }
# strstr echoes nothing if s2 does not occur in s1
case "$1" in
*$2*) ;;
*) return 1;;
esac
# use the pattern matching code to strip off the match and everything
# following it
first=${1/$2*/}
# then strip off the first unmatched portion of the string
echo "${1##$first}"
}
#:docstring strtok:
# Usage: strtok s1 s2
#
# Strtok considers the string s1 to consist of a sequence of zero or more
# text tokens separated by spans of one or more characters from the
# separator string s2. The first call (with a non-empty string s1
# specified) echoes a string consisting of the first token on stdout. The
# function keeps track of its position in the string s1 between separate
# calls, so that subsequent calls made with the first argument an empty
# string will work through the string immediately following that token. In
# this way subsequent calls will work through the string s1 until no tokens
# remain. The separator string s2 may be different from call to call.
# When no token remains in s1, an empty value is echoed on stdout.
#:end docstring:
###;;;autoload
function strtok ()
{
:

Appendix A. Contributed Scripts

593

Advanced Bash-Scripting Guide
}
#:docstring strtrunc:
# Usage: strtrunc $n $s1 {$s2} {$...}
#
# Used by many functions like strncmp to truncate arguments for comparison.
# Echoes the first n characters of each string s1 s2 ... on stdout.
#:end docstring:
###;;;autoload
function strtrunc ()
{
n=$1 ; shift
for z; do
echo "${z:0:$n}"
done
}
# provide string
# string.bash ends here

# ========================================================================== #
# ==> Everything below here added by the document author.
# ==> Suggested use of this script is to delete everything below here,
# ==> and "source" this file into your own scripts.
# strcat
string0=one
string1=two
echo
echo "Testing \"strcat\" function:"
echo "Original \"string0\" = $string0"
echo "\"string1\" = $string1"
strcat string0 string1
echo "New \"string0\" = $string0"
echo
# strlen
echo
echo "Testing \"strlen\" function:"
str=123456789
echo "\"str\" = $str"
echo -n "Length of \"str\" = "
strlen str
echo

# Exercise:
# -------# Add code to test all the other string functions above.

exit 0

Michael Zick's complex array example uses the md5sum check sum command to encode directory
information.

Appendix A. Contributed Scripts

594

Advanced Bash-Scripting Guide
Example A-19. Directory information
#! /bin/bash
# directory-info.sh
# Parses and lists directory information.
# NOTE: Change lines 273 and 353 per "README" file.
# Michael Zick is the author of this script.
# Used here with his permission.
#
#
#
#
#
#
#
#

Controls
If overridden by command arguments, they must be in the order:
Arg1: "Descriptor Directory"
Arg2: "Exclude Paths"
Arg3: "Exclude Directories"
Environment Settings override Defaults.
Command arguments override Environment Settings.

# Default location for content addressed file descriptors.
MD5UCFS=${1:-${MD5UCFS:-'/tmpfs/ucfs'}}
# Directory paths never to list or enter
declare -a \
EXCLUDE_PATHS=${2:-${EXCLUDE_PATHS:-'(/proc /dev /devfs /tmpfs)'}}
# Directories never to list or enter
declare -a \
EXCLUDE_DIRS=${3:-${EXCLUDE_DIRS:-'(ucfs lost+found tmp wtmp)'}}
# Files never to list or enter
declare -a \
EXCLUDE_FILES=${3:-${EXCLUDE_FILES:-'(core "Name with Spaces")'}}

#
:
#
#
#
#
#
#
#

Here document used as a comment block.
< /dev/null
/proc/982/fd/1 -> /home/mszick/.xsession-errors
/proc/982/fd/13 -> /tmp/tmpfZVVOCs (deleted)
/proc/982/fd/7 -> /tmp/kde-mszick/ksycoca
/proc/982/fd/8 -> socket:[11586]
/proc/982/fd/9 -> pipe:[11588]
If that isn't enough to keep your parser guessing,
either or both of the path components may be relative:
../Built-Shared -> Built-Static
../linux-2.4.20.tar.bz2 -> ../../../SRCS/linux-2.4.20.tar.bz2
The first character of the 11 (10?) character permissions field:
's' Socket
'd' Directory
'b' Block device
'c' Character device
'l' Symbolic link
NOTE: Hard links not marked - test for identical inode numbers
on identical filesystems.
All information about hard linked files are shared, except
for the names and the name's location in the directory system.
NOTE: A "Hard link" is known as a "File Alias" on some systems.
'-' An undistingushed file
Followed by three groups of letters for: User, Group, Others
Character 1: '-' Not readable; 'r' Readable
Character 2: '-' Not writable; 'w' Writable
Character 3, User and Group: Combined execute and special
'-' Not Executable, Not Special
'x' Executable, Not Special
's' Executable, Special
'S' Not Executable, Special
Character 3, Others: Combined execute and sticky (tacky?)
'-' Not Executable, Not Tacky
'x' Executable, Not Tacky
't' Executable, Tacky
'T' Not Executable, Tacky
Followed by an access indicator
Haven't tested this one, it may be the eleventh character
or it may generate another field
' ' No alternate access
'+' Alternate access
LSfieldsDoc

ListDirectory()
{
local -a T
local -i of=0
#
OLD_IFS=$IFS

# Default return in variable
# Using BASH default ' \t\n'

case "$#" in
3)
case "$1" in
-of)
of=1 ; shift ;;
* )
return 1 ;;
esac ;;
2)
: ;;
# Poor man's "continue"

Appendix A. Contributed Scripts

596

Advanced Bash-Scripting Guide
*)
esac

return 1 ;;

# NOTE: the (ls) command is NOT quoted (")
T=( $(ls --inode --ignore-backups --almost-all --directory \
--full-time --color=none --time=status --sort=none \
--format=long $1) )
case $of in
# Assign T back to the array whose name was passed as $2
0) eval $2=\( \"\$\{T\[@\]\}\" \) ;;
# Write T into filename passed as $2
1) echo "${T[@]}" > "$2" ;;
esac
return 0
}
# # # # # Is that string a legal number? # # # # #
#
#
IsNumber "Var"
# # # # # There has to be a better way, sigh...
IsNumber()
{
local -i int
if [ $# -eq 0 ]
then
return 1
else
(let int=$1)
return $?
fi
}

2>/dev/null
# Exit status of the let thread

# # # # # Index Filesystem Directory Information # # # # #
#
#
IndexList "Field-Array-Name" "Index-Array-Name"
# or
#
IndexList -if Field-Array-Filename Index-Array-Name
#
IndexList -of Field-Array-Name Index-Array-Filename
#
IndexList -if -of Field-Array-Filename Index-Array-Filename
# # # # #
: <= Lcnt ))
do
if IsNumber ${LIST[$Lidx]}
then
local -i inode name
local ft
inode=Lidx
local m=${LIST[$Lidx+2]}
# Hard Links field
ft=${LIST[$Lidx+1]:0:1}
# Fast-Stat
case $ft in
b)
((Lidx+=12)) ;;
# Block device
c)
((Lidx+=12)) ;;
# Character device
*)
((Lidx+=11)) ;;
# Anything else
esac
name=Lidx
case $ft in
-)
((Lidx+=1)) ;;
# The easy one
b)
((Lidx+=1)) ;;
# Block device
c)
((Lidx+=1)) ;;
# Character device
d)
((Lidx+=1)) ;;
# The other easy one
l)
((Lidx+=3)) ;;
# At LEAST two more fields
# A little more elegance here would handle pipes,
#+ sockets, deleted files - later.
*)
until IsNumber ${LIST[$Lidx]} || ((Lidx >= Lcnt))
do
((Lidx+=1))
done
;;
# Not required

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598

Advanced Bash-Scripting Guide
esac
INDEX[${#INDEX[*]}]=$inode
INDEX[${#INDEX[*]}]=$name
INDEX[0]=${INDEX[0]}+1
# One more "line" found
# echo "Line: ${INDEX[0]} Type: $ft Links: $m Inode: \
# ${LIST[$inode]} Name: ${LIST[$name]}"
else
((Lidx+=1))
fi
done
case "$of" in
0) eval $2=\( \"\$\{INDEX\[@\]\}\" \) ;;
1) echo "${INDEX[@]}" > "$2" ;;
esac
return 0
# What could go wrong?
}
# # # # # Content Identify File # # # # #
#
#
DigestFile Input-Array-Name Digest-Array-Name
# or
#
DigestFile -if Input-FileName Digest-Array-Name
# # # # #
# Here document used as a comment block.
: < realname
stat -t linkname returns the linkname (link) information
stat -lt linkname returns the realname information
stat -tf and stat -ltf fields
[0]
name
[1]
ID-0?
# Maybe someday, but Linux stat structure
[2]
ID-0?
# does not have either LABEL nor UUID
# fields, currently information must come
# from file-system specific utilities
These will be munged into:
[1]
UUID if possible
[2]
Volume Label if possible
Note: 'mount -l' does return the label and could return the UUID
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]

Maximum length of filenames
Filesystem type
Total blocks in the filesystem
Free blocks
Free blocks for non-root user(s)
Block size of the filesystem
Total inodes
Free inodes

-*-*- Per:
Return code: 0
Size of array: 11
Contents of array
Element 0: /home/mszick
Element 1: 0
Element 2: 0
Element 3: 255
Element 4: ef53
Element 5: 2581445
Element 6: 2277180

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601

Advanced Bash-Scripting Guide
Element
Element
Element
Element

7: 2146050
8: 4096
9: 1311552
10: 1276425

StatFieldsDoc

#
#

LocateFile [-l] FileName Location-Array-Name
LocateFile [-l] -of FileName Location-Array-FileName

LocateFile()
{
local -a LOC LOC1 LOC2
local lk="" of=0
case "$#" in
0) return 1 ;;
1) return 1 ;;
2) : ;;
*) while (( "$#" > 2 ))
do
case "$1" in
-l) lk=-1 ;;
-of) of=1 ;;
*) return 1 ;;
esac
shift
done ;;
esac
# More Sanscrit-2.0.5
# LOC1=( $(stat -t $lk $1) )
# LOC2=( $(stat -tf $lk $1) )
# Uncomment above two lines if system has "stat" command installed.
LOC=( ${LOC1[@]:0:1} ${LOC1[@]:3:11}
${LOC2[@]:1:2} ${LOC2[@]:4:1} )
case "$of" in
0) eval $2=\( \"\$\{LOC\[@\]\}\" \) ;;
1) echo "${LOC[@]}" > "$2" ;;
esac
return 0
# Which yields (if you are lucky, and have "stat" installed)
# -*-*- Location Discriptor -*-*#
Return code: 0
#
Size of array: 15
#
Contents of array
#
Element 0: /home/mszick
20th Century name
#
Element 1: 41e8
Type and Permissions
#
Element 2: 500
User
#
Element 3: 500
Group
#
Element 4: 303
Device
#
Element 5: 32385
inode
#
Element 6: 22
Link count
#
Element 7: 0
Device Major
#
Element 8: 0
Device Minor
#
Element 9: 1051224608
Last Access
#
Element 10: 1051214068
Last Modify
#
Element 11: 1051214068
Last Status
#
Element 12: 0
UUID (to be)
#
Element 13: 0
Volume Label (to be)

Appendix A. Contributed Scripts

602

Advanced Bash-Scripting Guide
#
}

Element 14: ef53

Filesystem type

# And then there was some test code
ListArray() # ListArray Name
{
local -a Ta
eval Ta=\( \"\$\{$1\[@\]\}\" \)
echo
echo "-*-*- List of Array -*-*-"
echo "Size of array $1: ${#Ta[*]}"
echo "Contents of array $1:"
for (( i=0 ; i<${#Ta[*]} ; i++ ))
do
echo -e "\tElement $i: ${Ta[$i]}"
done
return 0
}
declare -a CUR_DIR
# For small arrays
ListDirectory "${PWD}" CUR_DIR
ListArray CUR_DIR
declare -a DIR_DIG
DigestFile CUR_DIR DIR_DIG
echo "The new \"name\" (checksum) for ${CUR_DIR[9]} is ${DIR_DIG[0]}"
declare -a DIR_ENT
# BIG_DIR # For really big arrays - use a temporary file in ramdisk
# BIG-DIR # ListDirectory -of "${CUR_DIR[11]}/*" "/tmpfs/junk2"
ListDirectory "${CUR_DIR[11]}/*" DIR_ENT
declare -a DIR_IDX
# BIG-DIR # IndexList -if "/tmpfs/junk2" DIR_IDX
IndexList DIR_ENT DIR_IDX
declare -a IDX_DIG
# BIG-DIR # DIR_ENT=( $(cat /tmpfs/junk2) )
# BIG-DIR # DigestFile -if /tmpfs/junk2 IDX_DIG
DigestFile DIR_ENT IDX_DIG
# Small (should) be able to parallize IndexList & DigestFile
# Large (should) be able to parallize IndexList & DigestFile & the assignment
echo "The \"name\" (checksum) for the contents of ${PWD} is ${IDX_DIG[0]}"
declare -a FILE_LOC
LocateFile ${PWD} FILE_LOC
ListArray FILE_LOC
exit 0

Stéphane Chazelas demonstrates object-oriented programming in a Bash script.
Mariusz Gniazdowski contributed a hash library for use in scripts.

Example A-20. Library of hash functions
Appendix A. Contributed Scripts

603

Advanced Bash-Scripting Guide
#
#
#
#

Hash:
Hash function library
Author: Mariusz Gniazdowski 
Date: 2005-04-07

# Functions making emulating hashes in Bash a little less painful.

#
#
#
#
#+
#
#+
#
#
#
#
#
#
#

Limitations:
* Only global variables are supported.
* Each hash instance generates one global variable per value.
* Variable names collisions are possible
if you define variable like __hash__hashname_key
* Keys must use chars that can be part of a Bash variable name
(no dashes, periods, etc.).
* The hash is created as a variable:
... hashname_keyname
So if somone will create hashes like:
myhash_ + mykey = myhash__mykey
myhash + _mykey = myhash__mykey
Then there will be a collision.
(This should not pose a major problem.)

Hash_config_varname_prefix=__hash__

# Emulates: hash[key]=value
#
# Params:
# 1 - hash
# 2 - key
# 3 - value
function hash_set {
eval "${Hash_config_varname_prefix}${1}_${2}=\"${3}\""
}

# Emulates: value=hash[key]
#
# Params:
# 1 - hash
# 2 - key
# 3 - value (name of global variable to set)
function hash_get_into {
eval "$3=\"\$${Hash_config_varname_prefix}${1}_${2}\""
}

# Emulates: echo hash[key]
#
# Params:
# 1 - hash
# 2 - key
# 3 - echo params (like -n, for example)
function hash_echo {
eval "echo $3 \"\$${Hash_config_varname_prefix}${1}_${2}\""
}

# Emulates:
#

hash1[key1]=hash2[key2]

Appendix A. Contributed Scripts

604

Advanced Bash-Scripting Guide
# Params:
# 1 - hash1
# 2 - key1
# 3 - hash2
# 4 - key2
function hash_copy {
eval "${Hash_config_varname_prefix}${1}_${2}\
=\"\$${Hash_config_varname_prefix}${3}_${4}\""
}

# Emulates: hash[keyN-1]=hash[key2]=...hash[key1]
#
# Copies first key to rest of keys.
#
# Params:
# 1 - hash1
# 2 - key1
# 3 - key2
# . . .
# N - keyN
function hash_dup {
local hashName="$1" keyName="$2"
shift 2
until [ ${#} -le 0 ]; do
eval "${Hash_config_varname_prefix}${hashName}_${1}\
=\"\$${Hash_config_varname_prefix}${hashName}_${keyName}\""
shift;
done;
}

# Emulates: unset hash[key]
#
# Params:
# 1 - hash
# 2 - key
function hash_unset {
eval "unset ${Hash_config_varname_prefix}${1}_${2}"
}

# Emulates something similar to: ref=&hash[key]
#
# The reference is name of the variable in which value is held.
#
# Params:
# 1 - hash
# 2 - key
# 3 - ref - Name of global variable to set.
function hash_get_ref_into {
eval "$3=\"${Hash_config_varname_prefix}${1}_${2}\""
}

#
#
#
#
#
#
#

Emulates something similar to:

echo &hash[key]

That reference is name of variable in which value is held.
Params:
1 - hash
2 - key

Appendix A. Contributed Scripts

605

Advanced Bash-Scripting Guide
# 3 - echo params (like -n for example)
function hash_echo_ref {
eval "echo $3 \"${Hash_config_varname_prefix}${1}_${2}\""
}

# Emulates something similar to: $$hash[key](param1, param2, ...)
#
# Params:
# 1 - hash
# 2 - key
# 3,4, ... - Function parameters
function hash_call {
local hash key
hash=$1
key=$2
shift 2
eval "eval \"\$${Hash_config_varname_prefix}${hash}_${key} \\\"\\\$@\\\"\""
}

# Emulates something similar to: isset(hash[key]) or hash[key]==NULL
#
# Params:
# 1 - hash
# 2 - key
# Returns:
# 0 - there is such key
# 1 - there is no such key
function hash_is_set {
eval "if [[ \"\${${Hash_config_varname_prefix}${1}_${2}-a}\" = \"a\" &&
\"\${${Hash_config_varname_prefix}${1}_${2}-b}\" = \"b\" ]]
then return 1; else return 0; fi"
}

# Emulates something similar to:
#
foreach($hash as $key => $value) { fun($key,$value); }
#
# It is possible to write different variations of this function.
# Here we use a function call to make it as "generic" as possible.
#
# Params:
# 1 - hash
# 2 - function name
function hash_foreach {
local keyname oldIFS="$IFS"
IFS=' '
for i in $(eval "echo \${!${Hash_config_varname_prefix}${1}_*}"); do
keyname=$(eval "echo \${i##${Hash_config_varname_prefix}${1}_}")
eval "$2 $keyname \"\$$i\""
done
IFS="$oldIFS"
}
#
#

NOTE: In lines 103 and 116, ampersand changed.
But, it doesn't matter, because these are comment lines anyhow.

Here is an example script using the foregoing hash library.

Appendix A. Contributed Scripts

606

Advanced Bash-Scripting Guide
Example A-21. Colorizing text using hash functions
#!/bin/bash
# hash-example.sh: Colorizing text.
# Author: Mariusz Gniazdowski 
. Hash.lib

# Load the library of functions.

hash_set
hash_set
hash_set
hash_set
hash_set
hash_set
hash_set
hash_set
hash_set
hash_set
hash_set
hash_set

red
blue
light_blue
light_red
cyan
light_green
light_gray
green
yellow
light_purple
purple
reset_color

colors
colors
colors
colors
colors
colors
colors
colors
colors
colors
colors
colors

"\033[0;31m"
"\033[0;34m"
"\033[1;34m"
"\033[1;31m"
"\033[0;36m"
"\033[1;32m"
"\033[0;37m"
"\033[0;32m"
"\033[1;33m"
"\033[1;35m"
"\033[0;35m"
"\033[0;00m"

# $1 - keyname
# $2 - value
try_colors() {
echo -en "$2"
echo "This line is $1."
}
hash_foreach colors try_colors
hash_echo colors reset_color -en
echo -e '\nLet us overwrite some colors with yellow.\n'
# It's hard to read yellow text on some terminals.
hash_dup colors yellow
red light_green blue green light_gray cyan
hash_foreach colors try_colors
hash_echo colors reset_color -en
echo -e '\nLet us delete them and try colors once more . . .\n'
for i in red light_green blue green light_gray cyan; do
hash_unset colors $i
done
hash_foreach colors try_colors
hash_echo colors reset_color -en
hash_set other txt "Other examples . . ."
hash_echo other txt
hash_get_into other txt text
echo $text
hash_set other my_fun try_colors
hash_call other my_fun
purple "`hash_echo colors purple`"
hash_echo colors reset_color -en
echo; echo "Back to normal?"; echo
exit $?
#
#
#

On some terminals, the "light" colors print in bold,
and end up looking darker than the normal ones.
Why is this?

Appendix A. Contributed Scripts

607

Advanced Bash-Scripting Guide
An example illustrating the mechanics of hashing, but from a different point of view.

Example A-22. More on hash functions
#!/bin/bash
# $Id: ha.sh,v 1.2 2005/04/21 23:24:26 oliver Exp $
# Copyright 2005 Oliver Beckstein
# Released under the GNU Public License
# Author of script granted permission for inclusion in ABS Guide.
# (Thank you!)
#---------------------------------------------------------------# pseudo hash based on indirect parameter expansion
# API: access through functions:
#
# create the hash:
#
#
newhash Lovers
#
# add entries (note single quotes for spaces)
#
#
addhash Lovers Tristan Isolde
#
addhash Lovers 'Romeo Montague' 'Juliet Capulet'
#
# access value by key
#
#
gethash Lovers Tristan
----> Isolde
#
# show all keys
#
#
keyshash Lovers
----> 'Tristan' 'Romeo Montague'
#
#
# Convention: instead of perls' foo{bar} = boing' syntax,
# use
#
'_foo_bar=boing' (two underscores, no spaces)
#
# 1) store key
in _NAME_keys[]
# 2) store value in _NAME_values[] using the same integer index
# The integer index for the last entry is _NAME_ptr
#
# NOTE: No error or sanity checks, just bare bones.

function _inihash () {
# private function
# call at the beginning of each procedure
# defines: _keys _values _ptr
#
# Usage: _inihash NAME
local name=$1
_keys=_${name}_keys
_values=_${name}_values
_ptr=_${name}_ptr
}
function newhash () {
# Usage: newhash NAME
#
NAME should not contain spaces or dots.
#
Actually: it must be a legal name for a Bash variable.

Appendix A. Contributed Scripts

608

Advanced Bash-Scripting Guide
# We rely on Bash automatically recognising arrays.
local name=$1
local _keys _values _ptr
_inihash ${name}
eval ${_ptr}=0
}

function addhash () {
# Usage: addhash NAME KEY 'VALUE with spaces'
#
arguments with spaces need to be quoted with single quotes ''
local name=$1 k="$2" v="$3"
local _keys _values _ptr
_inihash ${name}
#echo "DEBUG(addhash): ${_ptr}=${!_ptr}"
eval let ${_ptr}=${_ptr}+1
eval "$_keys[${!_ptr}]=\"${k}\""
eval "$_values[${!_ptr}]=\"${v}\""
}
function gethash () {
# Usage: gethash NAME KEY
#
Returns boing
#
ERR=0 if entry found, 1 otherwise
# That's not a proper hash -#+ we simply linearly search through the keys.
local name=$1 key="$2"
local _keys _values _ptr
local k v i found h
_inihash ${name}
# _ptr holds the highest index in the hash
found=0
for i in $(seq 1 ${!_ptr}); do
h="\${${_keys}[${i}]}" # Safer to do it in two steps,
eval k=${h}
#+ especially when quoting for spaces.
if [ "${k}" = "${key}" ]; then found=1; break; fi
done;
[ ${found} = 0 ] && return 1;
# else: i is the index that matches the key
h="\${${_values}[${i}]}"
eval echo "${h}"
return 0;
}
function keyshash () {
# Usage: keyshash NAME
# Returns list of all keys defined for hash name.
local name=$1 key="$2"
local _keys _values _ptr
local k i h
_inihash ${name}
# _ptr holds the highest index in the hash
for i in $(seq 1 ${!_ptr}); do
h="\${${_keys}[${i}]}"
# Safer to do it in two steps,
eval k=${h}
#+ especially when quoting for spaces.
echo -n "'${k}' "

Appendix A. Contributed Scripts

609

Advanced Bash-Scripting Guide
done;
}

# ----------------------------------------------------------------------# Now, let's test it.
# (Per comments at the beginning of the script.)
newhash Lovers
addhash Lovers Tristan Isolde
addhash Lovers 'Romeo Montague' 'Juliet Capulet'
# Output results.
echo
gethash Lovers Tristan
echo
keyshash Lovers
echo; echo

# Isolde
# 'Tristan' 'Romeo Montague'

exit 0
# Exercise:
# -------# Add error checks to the functions.

Now for a script that installs and mounts those cute USB keychain solid-state "hard drives."

Example A-23. Mounting USB keychain storage devices
#!/bin/bash
# ==> usb.sh
# ==> Script for mounting and installing pen/keychain USB storage devices.
# ==> Runs as root at system startup (see below).
# ==>
# ==> Newer Linux distros (2004 or later) autodetect
# ==> and install USB pen drives, and therefore don't need this script.
# ==> But, it's still instructive.
#
#
#
#
#+
#
#
#
#
#
#+
#
#+
#
#
#
#
#+
#+
#
#

This code is free software covered by GNU GPL license version 2 or above.
Please refer to http://www.gnu.org/ for the full license text.
Some code lifted from usb-mount by Michael Hamilton's usb-mount (LGPL)
see http://users.actrix.co.nz/michael/usbmount.html
INSTALL
------Put this in /etc/hotplug/usb/diskonkey.
Then look in /etc/hotplug/usb.distmap, and copy all usb-storage entries
into /etc/hotplug/usb.usermap, substituting "usb-storage" for "diskonkey".
Otherwise this code is only run during the kernel module invocation/removal
(at least in my tests), which defeats the purpose.
TODO
---Handle more than one diskonkey device at one time (e.g. /dev/diskonkey1
and /mnt/diskonkey1), etc. The biggest problem here is the handling in
devlabel, which I haven't yet tried.
AUTHOR and SUPPORT

Appendix A. Contributed Scripts

610

Advanced Bash-Scripting Guide
# -----------------# Konstantin Riabitsev, .
# Send any problem reports to my email address at the moment.
#
# ==> Comments added by ABS Guide author.

SYMLINKDEV=/dev/diskonkey
MOUNTPOINT=/mnt/diskonkey
DEVLABEL=/sbin/devlabel
DEVLABELCONFIG=/etc/sysconfig/devlabel
IAM=$0
##
# Functions lifted near-verbatim from usb-mount code.
#
function allAttachedScsiUsb {
find /proc/scsi/ -path '/proc/scsi/usb-storage*' -type f |
xargs grep -l 'Attached: Yes'
}
function scsiDevFromScsiUsb {
echo $1 | awk -F"[-/]" '{ n=$(NF-1);
print "/dev/sd" substr("abcdefghijklmnopqrstuvwxyz", n+1, 1) }'
}
if [ "${ACTION}" = "add" ] && [ -f "${DEVICE}" ]; then
##
# lifted from usbcam code.
#
if [ -f /var/run/console.lock ]; then
CONSOLEOWNER=`cat /var/run/console.lock`
elif [ -f /var/lock/console.lock ]; then
CONSOLEOWNER=`cat /var/lock/console.lock`
else
CONSOLEOWNER=
fi
for procEntry in $(allAttachedScsiUsb); do
scsiDev=$(scsiDevFromScsiUsb $procEntry)
# Some bug with usb-storage?
# Partitions are not in /proc/partitions until they are accessed
#+ somehow.
/sbin/fdisk -l $scsiDev >/dev/null
##
# Most devices have partitioning info, so the data would be on
#+ /dev/sd?1. However, some stupider ones don't have any partitioning
#+ and use the entire device for data storage. This tries to
#+ guess semi-intelligently if we have a /dev/sd?1 and if not, then
#+ it uses the entire device and hopes for the better.
#
if grep -q `basename $scsiDev`1 /proc/partitions; then
part="$scsiDev""1"
else
part=$scsiDev
fi
##
# Change ownership of the partition to the console user so they can
#+ mount it.
#
if [ ! -z "$CONSOLEOWNER" ]; then
chown $CONSOLEOWNER:disk $part
fi

Appendix A. Contributed Scripts

611

Advanced Bash-Scripting Guide
##
# This checks if we already have this UUID defined with devlabel.
# If not, it then adds the device to the list.
#
prodid=`$DEVLABEL printid -d $part`
if ! grep -q $prodid $DEVLABELCONFIG; then
# cross our fingers and hope it works
$DEVLABEL add -d $part -s $SYMLINKDEV 2>/dev/null
fi
##
# Check if the mount point exists and create if it doesn't.
#
if [ ! -e $MOUNTPOINT ]; then
mkdir -p $MOUNTPOINT
fi
##
# Take care of /etc/fstab so mounting is easy.
#
if ! grep -q "^$SYMLINKDEV" /etc/fstab; then
# Add an fstab entry
echo -e \
"$SYMLINKDEV\t\t$MOUNTPOINT\t\tauto\tnoauto,owner,kudzu 0 0" \
>> /etc/fstab
fi
done
if [ ! -z "$REMOVER" ]; then
##
# Make sure this script is triggered on device removal.
#
mkdir -p `dirname $REMOVER`
ln -s $IAM $REMOVER
fi
elif [ "${ACTION}" = "remove" ]; then
##
# If the device is mounted, unmount it cleanly.
#
if grep -q "$MOUNTPOINT" /etc/mtab; then
# unmount cleanly
umount -l $MOUNTPOINT
fi
##
# Remove it from /etc/fstab if it's there.
#
if grep -q "^$SYMLINKDEV" /etc/fstab; then
grep -v "^$SYMLINKDEV" /etc/fstab > /etc/.fstab.new
mv -f /etc/.fstab.new /etc/fstab
fi
fi
exit 0

Converting a text file to HTML format.

Example A-24. Converting to HTML
#!/bin/bash
# tohtml.sh [v. 0.2, reldate: 06/26/08, still buggy]
# Convert a text file to HTML format.
# Author: Mendel Cooper
# License: GPL3

Appendix A. Contributed Scripts

612

Advanced Bash-Scripting Guide
# Usage: sh tohtml.sh < textfile > htmlfile
# Script can easily be modified to accept source and target filenames.
#
# 1)
# 2)
#
#
# 3)
#+
#+

Assumptions:
Paragraphs in (target) text file are separated by a blank line.
Jpeg images (*.jpg) are located in "images" subdirectory.
In the target file, the image names are enclosed in square brackets,
for example, [image01.jpg].
Emphasized (italic) phrases begin with a space+underscore
or the first character on the line is an underscore,
and end with an underscore+space or underscore+end-of-line.

# Settings
FNTSIZE=2
# Small-medium font size
IMGDIR="images" # Image directory
# Headers
HDR01=''
HDR02=''
HDR03=''
HDR10=''
HDR11=''
HDR11a=''
HDR12a=''
HDR121=''
HDR13=''
# Change background color to suit.
HDR14a=''
# Footers
FTR10=''
FTR11=''
# Tags
BOLD=""
CENTER=""
END_CENTER=""
LF="
"

write_headers ()
{
echo "$HDR01"
echo
echo "$HDR02"
echo "$HDR03"
echo
echo
echo "$HDR10"
echo "$HDR11"
echo "$HDR121"
echo "$HDR11a"
echo "$HDR13"
echo
echo -n "$HDR14a"
echo -n "$FNTSIZE"
echo "$HDR14b"
echo
echo "$BOLD"
}

# Everything in bold (more easily readable).

process_text ()

Appendix A. Contributed Scripts

613

Advanced Bash-Scripting Guide
{
while read line
do
{
if [ ! "$line" ]
then
echo
echo "$LF"
echo "$LF"
echo
continue
else

# Read one line at a time.

# Blank line?
# Then new paragraph must follow.
# Insert two 
 tags.

# Skip the underscore test.
# Otherwise . . .

if [[ "$line" =~ "\[*jpg\]" ]] # Is a graphic?
then
# Strip away brackets.
temp=$( echo "$line" | sed -e 's/\[//' -e 's/\]//' )
line=""$CENTER"  "$END_CENTER" "
# Add image tag.
# And, center it.
fi
fi

echo "$line" | grep -q _
if [ "$?" -eq 0 ]
# If line contains underscore ...
then
# ===================================================
# Convert underscored phrase to italics.
temp=$( echo "$line" |
sed -e 's/ _/ /' -e 's/_/<\/i> /' |
sed -e 's/^_//' -e 's/_/<\/i>/' )
# Process only underscores prefixed by space,
#+ or at beginning or end of line.
# Do not convert underscores embedded within a word!
line="$temp"
# Slows script execution. Can be optimized?
# ===================================================
fi

echo
echo "$line"
echo
} # End while
done
}
# End process_text ()

write_footers ()
{
echo "$FTR10"
echo "$FTR11"
}

# Termination tags.

# main () {
# =========
write_headers
process_text
write_footers

Appendix A. Contributed Scripts

614

Advanced Bash-Scripting Guide
# =========
#
}
exit $?
#
#
#
#
#+

Exercises:
--------1) Fixup: Check for closing underscore before a comma or period.
2) Add a test for the presence of a closing underscore
in phrases to be italicized.

Here is something to warm the hearts of webmasters and mistresses: a script that saves weblogs.

Example A-25. Preserving weblogs
#!/bin/bash
# archiveweblogs.sh v1.0
# Troy Engel 
# Slightly modified by document author.
# Used with permission.
#
# This script will preserve the normally rotated and
#+ thrown away weblogs from a default RedHat/Apache installation.
# It will save the files with a date/time stamp in the filename,
#+ bzipped, to a given directory.
#
# Run this from crontab nightly at an off hour,
#+ as bzip2 can suck up some serious CPU on huge logs:
# 0 2 * * * /opt/sbin/archiveweblogs.sh

PROBLEM=66
# Set this to your backup dir.
BKP_DIR=/opt/backups/weblogs
# Default Apache/RedHat stuff
LOG_DAYS="4 3 2 1"
LOG_DIR=/var/log/httpd
LOG_FILES="access_log error_log"
# Default RedHat program locations
LS=/bin/ls
MV=/bin/mv
ID=/usr/bin/id
CUT=/bin/cut
COL=/usr/bin/column
BZ2=/usr/bin/bzip2
# Are we root?
USER=`$ID -u`
if [ "X$USER" != "X0" ]; then
echo "PANIC: Only root can run this script!"
exit $PROBLEM
fi
# Backup dir exists/writable?
if [ ! -x $BKP_DIR ]; then
echo "PANIC: $BKP_DIR doesn't exist or isn't writable!"
exit $PROBLEM

Appendix A. Contributed Scripts

615

Advanced Bash-Scripting Guide
fi
# Move, rename and bzip2 the logs
for logday in $LOG_DAYS; do
for logfile in $LOG_FILES; do
MYFILE="$LOG_DIR/$logfile.$logday"
if [ -w $MYFILE ]; then
DTS=`$LS -lgo --time-style=+%Y%m%d $MYFILE | $COL -t | $CUT -d ' ' -f7`
$MV $MYFILE $BKP_DIR/$logfile.$DTS
$BZ2 $BKP_DIR/$logfile.$DTS
else
# Only spew an error if the file exits (ergo non-writable).
if [ -f $MYFILE ]; then
echo "ERROR: $MYFILE not writable. Skipping."
fi
fi
done
done
exit 0

How to keep the shell from expanding and reinterpreting text strings.

Example A-26. Protecting literal strings
#! /bin/bash
# protect_literal.sh
# set -vx
:<<-'_Protect_Literal_String_Doc'
Copyright (c) Michael S. Zick, 2003; All Rights Reserved
License: Unrestricted reuse in any form, for any purpose.
Warranty: None
Revision: $ID$
Documentation redirected to the Bash no-operation.
Bash will '/dev/null' this block when the script is first read.
(Uncomment the above set command to see this action.)
Remove the first (Sha-Bang) line when sourcing this as a library
procedure. Also comment out the example use code in the two
places where shown.

Usage:
_protect_literal_str 'Whatever string meets your ${fancy}'
Just echos the argument to standard out, hard quotes
restored.
$(_protect_literal_str 'Whatever string meets your ${fancy}')
as the right-hand-side of an assignment statement.
Does:
As the right-hand-side of an assignment, preserves the
hard quotes protecting the contents of the literal during
assignment.
Notes:
The strange names (_*) are used to avoid trampling on

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616

Advanced Bash-Scripting Guide
the user's chosen names when this is sourced as a
library.
_Protect_Literal_String_Doc
# The 'for illustration' function form
_protect_literal_str() {
# Pick an un-used, non-printing character as local IFS.
# Not required, but shows that we are ignoring it.
local IFS=$'\x1B'
# \ESC character
# Enclose the All-Elements-Of in hard quotes during assignment.
local tmp=$'\x27'$@$'\x27'
#
local tmp=$'\''$@$'\''
# Even uglier.
local len=${#tmp}
echo $tmp is $len long.

# Info only.
# Output AND information.

}
# This is the short-named version.
_pls() {
local IFS=$'x1B'
echo $'\x27'$@$'\x27'
}

# \ESC character (not required)
# Hard quoted parameter glob

# :<<-'_Protect_Literal_String_Test'
# # # Remove the above "# " to disable this code. # # #
# See how that looks
echo
echo "- - Test One _protect_literal_str
_protect_literal_str
echo
#
#
#
#

when printed.
-"
'Hello $user'
'Hello "${username}"'

#
#
#+
#

Which yields:
- - Test One - 'Hello $user' is 13 long.
'Hello "${username}"' is 21 long.
Looks as expected, but why all of the trouble?
The difference is hidden inside the Bash internal order
of operations.
Which shows when you use it on the RHS of an assignment.

# Declare an array for test values.
declare -a arrayZ
# Assign elements with various types of quotes and escapes.
arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "\'Pass: ${pw}\'" )
# Now list that array and see what is there.
echo "- - Test Two - -"
for (( i=0 ; i<${#arrayZ[*]} ; i++ ))
do
echo Element $i: ${arrayZ[$i]} is: ${#arrayZ[$i]} long.
done
echo
# Which yields:

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Advanced Bash-Scripting Guide
#
#
#
#
#

- - Test Two - Element 0: zero is: 4 long.
Element 1: 'Hello ${Me}' is: 13 long.
Element 2: Hello ${You} is: 12 long.
Element 3: \'Pass: \' is: 10 long.

#
#
#
#

Our marker element
Our "$(_pls '...' )"
Quotes are missing
${pw} expanded to nothing

# Now make an assignment with that result.
declare -a array2=( ${arrayZ[@]} )
# And print what happened.
echo "- - Test Three - -"
for (( i=0 ; i<${#array2[*]} ; i++ ))
do
echo Element $i: ${array2[$i]} is: ${#array2[$i]} long.
done
echo
#
#
#
#
#
#
#

Which yields:
- - Test Three - Element 0: zero is: 4 long.
Element 1: Hello ${Me} is: 11 long.
Element 2: Hello is: 5 long.
Element 3: 'Pass: is: 6 long.
Element 4: ' is: 1 long.

#
#
#
#
#

Our marker element.
Intended result.
${You} expanded to nothing.
Split on the whitespace.
The end quote is here now.

#
#
#
#+

Our Element 1 has had its leading and trailing hard quotes stripped.
Although not shown, leading and trailing whitespace is also stripped.
Now that the string contents are set, Bash will always, internally,
hard quote the contents as required during its operations.

#
#
#
#
#
#
#+
#
#
#+
#

Why?
Considering our "$(_pls 'Hello ${Me}')" construction:
" ... " -> Expansion required, strip the quotes.
$( ... ) -> Replace with the result of..., strip this.
_pls ' ... ' -> called with literal arguments, strip the quotes.
The result returned includes hard quotes; BUT the above processing
has already been done, so they become part of the value assigned.
Similarly, during further usage of the string variable, the ${Me}
is part of the contents (result) and survives any operations
(Until explicitly told to evaluate the string).

# Hint: See what happens when the hard quotes ($'\x27') are replaced
#+ with soft quotes ($'\x22') in the above procedures.
# Interesting also is to remove the addition of any quoting.
# _Protect_Literal_String_Test
# # # Remove the above "# " to disable this code. # # #
exit 0

But, what if you want the shell to expand and reinterpret strings?

Example A-27. Unprotecting literal strings
#! /bin/bash
# unprotect_literal.sh
# set -vx
:<<-'_UnProtect_Literal_String_Doc'

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Copyright (c) Michael S. Zick, 2003; All Rights Reserved
License: Unrestricted reuse in any form, for any purpose.
Warranty: None
Revision: $ID$
Documentation redirected to the Bash no-operation. Bash will
'/dev/null' this block when the script is first read.
(Uncomment the above set command to see this action.)
Remove the first (Sha-Bang) line when sourcing this as a library
procedure. Also comment out the example use code in the two
places where shown.

Usage:
Complement of the "$(_pls 'Literal String')" function.
(See the protect_literal.sh example.)
StringVar=$(_upls ProtectedSringVariable)
Does:
When used on the right-hand-side of an assignment statement;
makes the substitions embedded in the protected string.
Notes:
The strange names (_*) are used to avoid trampling on
the user's chosen names when this is sourced as a
library.

_UnProtect_Literal_String_Doc
_upls() {
local IFS=$'x1B'
eval echo $@
}

# \ESC character (not required)
# Substitution on the glob.

# :<<-'_UnProtect_Literal_String_Test'
# # # Remove the above "# " to disable this code. # # #

_pls() {
local IFS=$'x1B'
echo $'\x27'$@$'\x27'
}

# \ESC character (not required)
# Hard quoted parameter glob

# Declare an array for test values.
declare -a arrayZ
# Assign elements with various types of quotes and escapes.
arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "\'Pass: ${pw}\'" )
# Now make an assignment with that result.
declare -a array2=( ${arrayZ[@]} )
#
#
#
#
#
#

Which yielded:
- - Test Three - Element 0: zero is: 4 long
Element 1: Hello ${Me} is: 11 long
Element 2: Hello is: 5 long
Element 3: 'Pass: is: 6 long

Appendix A. Contributed Scripts

#
#
#
#

Our marker element.
Intended result.
${You} expanded to nothing.
Split on the whitespace.

619

Advanced Bash-Scripting Guide
# Element 4: ' is: 1 long

# The end quote is here now.

# set -vx
#
#
#+
#

Initialize 'Me' to something for the embedded ${Me} substitution.
This needs to be done ONLY just prior to evaluating the
protected string.
(This is why it was protected to begin with.)

Me="to the array guy."
# Set a string variable destination to the result.
newVar=$(_upls ${array2[1]})
# Show what the contents are.
echo $newVar
# Do we really need a function to do this?
newerVar=$(eval echo ${array2[1]})
echo $newerVar
#
#+
#
#+

I guess not, but the _upls function gives us a place to hang
the documentation on.
This helps when we forget what a # construction like:
$(eval echo ... ) means.

# What if Me isn't set when the protected string is evaluated?
unset Me
newestVar=$(_upls ${array2[1]})
echo $newestVar
# Just gone, no hints, no runs, no errors.
#
#
#+
#+
#
#
#+

Why in the world?
Setting the contents of a string variable containing character
sequences that have a meaning in Bash is a general problem in
script programming.
This problem is now solved in eight lines of code
(and four pages of description).

# Where is all this going?
# Dynamic content Web pages as an array of Bash strings.
# Content set per request by a Bash 'eval' command
#+ on the stored page template.
# Not intended to replace PHP, just an interesting thing to do.
###
# Don't have a webserver application?
# No problem, check the example directory of the Bash source;
#+ there is a Bash script for that also.
# _UnProtect_Literal_String_Test
# # # Remove the above "# " to disable this code. # # #
exit 0

This interesting script helps hunt down spammers.

Example A-28. Spammer Identification
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#!/bin/bash
# $Id: is_spammer.bash,v 1.12.2.11 2004/10/01 21:42:33 mszick Exp $
# Above line is RCS info.
#
#
#
#
#

The latest version of this script is available from http://www.morethan.org.
Spammer-identification
by Michael S. Zick
Used in the ABS Guide with permission.

#######################################################
# Documentation
# See also "Quickstart" at end of script.
#######################################################
:<<-'__is_spammer_Doc_'
Copyright (c) Michael S. Zick, 2004
License: Unrestricted reuse in any form, for any purpose.
Warranty: None -{Its a script; the user is on their own.}Impatient?
Application code: goto "# # # Hunt the Spammer' program code # # #"
Example output: ":<<-'_is_spammer_outputs_'"
How to use: Enter script name without arguments.
Or goto "Quickstart" at end of script.
Provides
Given a domain name or IP(v4) address as input:
Does an exhaustive set of queries to find the associated
network resources (short of recursing into TLDs).
Checks the IP(v4) addresses found against Blacklist
nameservers.
If found to be a blacklisted IP(v4) address,
reports the blacklist text records.
(Usually hyper-links to the specific report.)
Requires
A working Internet connection.
(Exercise: Add check and/or abort if not on-line when running script.)
Bash with arrays (2.05b+).
The external program 'dig' -a utility program provided with the 'bind' set of programs.
Specifically, the version which is part of Bind series 9.x
See: http://www.isc.org
All usages of 'dig' are limited to wrapper functions,
which may be rewritten as required.
See: dig_wrappers.bash for details.
("Additional documentation" -- below)
Usage
Script requires a single argument, which may be:
1) A domain name;
2) An IP(v4) address;

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Advanced Bash-Scripting Guide
3) A filename, with one name or address per line.
Script accepts an optional second argument, which may be:
1) A Blacklist server name;
2) A filename, with one Blacklist server name per line.
If the second argument is not provided, the script uses
a built-in set of (free) Blacklist servers.
See also, the Quickstart at the end of this script (after 'exit').
Return Codes
0 - All OK
1 - Script failure
2 - Something is Blacklisted
Optional environment variables
SPAMMER_TRACE
If set to a writable file,
script will log an execution flow trace.
SPAMMER_DATA
If set to a writable file, script will dump its
discovered data in the form of GraphViz file.
See: http://www.research.att.com/sw/tools/graphviz
SPAMMER_LIMIT
Limits the depth of resource tracing.
Default is 2 levels.
A setting of 0 (zero) means 'unlimited' . . .
Caution: script might recurse the whole Internet!
A limit of 1 or 2 is most useful when processing
a file of domain names and addresses.
A higher limit can be useful when hunting spam gangs.

Additional documentation
Download the archived set of scripts
explaining and illustrating the function contained within this script.
http://bash.webofcrafts.net/mszick_clf.tar.bz2

Study notes
This script uses a large number of functions.
Nearly all general functions have their own example script.
Each of the example scripts have tutorial level comments.
Scripting project
Add support for IP(v6) addresses.
IP(v6) addresses are recognized but not processed.
Advanced project
Add the reverse lookup detail to the discovered information.
Report the delegation chain and abuse contacts.
Modify the GraphViz file output to include the
newly discovered information.

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Advanced Bash-Scripting Guide
__is_spammer_Doc_
#######################################################

#### Special IFS settings used for string parsing. ####
# Whitespace == :Space:Tab:Line Feed:Carriage Return:
WSP_IFS=$'\x20'$'\x09'$'\x0A'$'\x0D'
# No Whitespace == Line Feed:Carriage Return
NO_WSP=$'\x0A'$'\x0D'
# Field separator for dotted decimal IP addresses
ADR_IFS=${NO_WSP}'.'
# Array to dotted string conversions
DOT_IFS='.'${WSP_IFS}
#
#
#
#

# # Pending operations stack machine # # #
This set of functions described in func_stack.bash.
(See "Additional documentation" above.)
# #

# Global stack of pending operations.
declare -f -a _pending_
# Global sentinel for stack runners
declare -i _p_ctrl_
# Global holder for currently executing function
declare -f _pend_current_
# # # Debug version only - remove for regular use # # #
#
# The function stored in _pend_hook_ is called
# immediately before each pending function is
# evaluated. Stack clean, _pend_current_ set.
#
# This thingy demonstrated in pend_hook.bash.
declare -f _pend_hook_
# # #
# The do nothing function
pend_dummy() { : ; }
# Clear and initialize the function stack.
pend_init() {
unset _pending_[@]
pend_func pend_stop_mark
_pend_hook_='pend_dummy' # Debug only.
}
# Discard the top function on the stack.
pend_pop() {
if [ ${#_pending_[@]} -gt 0 ]
then
local -i _top_
_top_=${#_pending_[@]}-1
unset _pending_[$_top_]
fi
}

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Advanced Bash-Scripting Guide
# pend_func function_name [$(printf '%q\n' arguments)]
pend_func() {
local IFS=${NO_WSP}
set -f
_pending_[${#_pending_[@]}]=$@
set +f
}
# The function which stops the release:
pend_stop_mark() {
_p_ctrl_=0
}
pend_mark() {
pend_func pend_stop_mark
}
# Execute functions until 'pend_stop_mark' . . .
pend_release() {
local -i _top_
# Declare _top_ as integer.
_p_ctrl_=${#_pending_[@]}
while [ ${_p_ctrl_} -gt 0 ]
do
_top_=${#_pending_[@]}-1
_pend_current_=${_pending_[$_top_]}
unset _pending_[$_top_]
$_pend_hook_
# Debug only.
eval $_pend_current_
done
}
# Drop functions until 'pend_stop_mark' . . .
pend_drop() {
local -i _top_
local _pd_ctrl_=${#_pending_[@]}
while [ ${_pd_ctrl_} -gt 0 ]
do
_top_=$_pd_ctrl_-1
if [ "${_pending_[$_top_]}" == 'pend_stop_mark' ]
then
unset _pending_[$_top_]
break
else
unset _pending_[$_top_]
_pd_ctrl_=$_top_
fi
done
if [ ${#_pending_[@]} -eq 0 ]
then
pend_func pend_stop_mark
fi
}
#### Array editors ####
# This function described in edit_exact.bash.
# (See "Additional documentation," above.)
# edit_exact  
edit_exact() {
[ $# -eq 2 ] ||
[ $# -eq 3 ] || return 1

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Advanced Bash-Scripting Guide
local -a _ee_Excludes
local -a _ee_Target
local _ee_x
local _ee_t
local IFS=${NO_WSP}
set -f
eval _ee_Excludes=\( \$\{$1\[@\]\} \)
eval _ee_Target=\( \$\{$2\[@\]\} \)
local _ee_len=${#_ee_Target[@]}
# Original length.
local _ee_cnt=${#_ee_Excludes[@]}
# Exclude list length.
[ ${_ee_len} -ne 0 ] || return 0
# Can't edit zero length.
[ ${_ee_cnt} -ne 0 ] || return 0
# Can't edit zero length.
for (( x = 0; x < ${_ee_cnt} ; x++ ))
do
_ee_x=${_ee_Excludes[$x]}
for (( n = 0 ; n < ${_ee_len} ; n++ ))
do
_ee_t=${_ee_Target[$n]}
if [ x"${_ee_t}" == x"${_ee_x}" ]
then
unset _ee_Target[$n]
# Discard match.
[ $# -eq 2 ] && break
# If 2 arguments, then done.
fi
done
done
eval $2=\( \$\{_ee_Target\[@\]\} \)
set +f
return 0
}
# This function described in edit_by_glob.bash.
# edit_by_glob  
edit_by_glob() {
[ $# -eq 2 ] ||
[ $# -eq 3 ] || return 1
local -a _ebg_Excludes
local -a _ebg_Target
local _ebg_x
local _ebg_t
local IFS=${NO_WSP}
set -f
eval _ebg_Excludes=\( \$\{$1\[@\]\} \)
eval _ebg_Target=\( \$\{$2\[@\]\} \)
local _ebg_len=${#_ebg_Target[@]}
local _ebg_cnt=${#_ebg_Excludes[@]}
[ ${_ebg_len} -ne 0 ] || return 0
[ ${_ebg_cnt} -ne 0 ] || return 0
for (( x = 0; x < ${_ebg_cnt} ; x++ ))
do
_ebg_x=${_ebg_Excludes[$x]}
for (( n = 0 ; n < ${_ebg_len} ; n++ ))
do
[ $# -eq 3 ] && _ebg_x=${_ebg_x}'*' # Do prefix edit
if [ ${_ebg_Target[$n]:=} ]
#+ if defined & set.
then
_ebg_t=${_ebg_Target[$n]/#${_ebg_x}/}
[ ${#_ebg_t} -eq 0 ] && unset _ebg_Target[$n]
fi
done
done
eval $2=\( \$\{_ebg_Target\[@\]\} \)
set +f

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Advanced Bash-Scripting Guide
return 0
}
# This function described in unique_lines.bash.
# unique_lines  
unique_lines() {
[ $# -eq 2 ] || return 1
local -a _ul_in
local -a _ul_out
local -i _ul_cnt
local -i _ul_pos
local _ul_tmp
local IFS=${NO_WSP}
set -f
eval _ul_in=\( \$\{$1\[@\]\} \)
_ul_cnt=${#_ul_in[@]}
for (( _ul_pos = 0 ; _ul_pos < ${_ul_cnt} ; _ul_pos++ ))
do
if [ ${_ul_in[${_ul_pos}]:=} ]
# If defined & not empty
then
_ul_tmp=${_ul_in[${_ul_pos}]}
_ul_out[${#_ul_out[@]}]=${_ul_tmp}
for (( zap = _ul_pos ; zap < ${_ul_cnt} ; zap++ ))
do
[ ${_ul_in[${zap}]:=} ] &&
[ 'x'${_ul_in[${zap}]} == 'x'${_ul_tmp} ] &&
unset _ul_in[${zap}]
done
fi
done
eval $2=\( \$\{_ul_out\[@\]\} \)
set +f
return 0
}
# This function described in char_convert.bash.
# to_lower 
to_lower() {
[ $# -eq 1 ] || return 1
local _tl_out
_tl_out=${1//A/a}
_tl_out=${_tl_out//B/b}
_tl_out=${_tl_out//C/c}
_tl_out=${_tl_out//D/d}
_tl_out=${_tl_out//E/e}
_tl_out=${_tl_out//F/f}
_tl_out=${_tl_out//G/g}
_tl_out=${_tl_out//H/h}
_tl_out=${_tl_out//I/i}
_tl_out=${_tl_out//J/j}
_tl_out=${_tl_out//K/k}
_tl_out=${_tl_out//L/l}
_tl_out=${_tl_out//M/m}
_tl_out=${_tl_out//N/n}
_tl_out=${_tl_out//O/o}
_tl_out=${_tl_out//P/p}
_tl_out=${_tl_out//Q/q}
_tl_out=${_tl_out//R/r}
_tl_out=${_tl_out//S/s}
_tl_out=${_tl_out//T/t}
_tl_out=${_tl_out//U/u}
_tl_out=${_tl_out//V/v}

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Advanced Bash-Scripting Guide
_tl_out=${_tl_out//W/w}
_tl_out=${_tl_out//X/x}
_tl_out=${_tl_out//Y/y}
_tl_out=${_tl_out//Z/z}
echo ${_tl_out}
return 0
}
#### Application helper functions ####
# Not everybody uses dots as separators (APNIC, for example).
# This function described in to_dot.bash
# to_dot 
to_dot() {
[ $# -eq 1 ] || return 1
echo ${1//[#|@|%]/.}
return 0
}
# This function described in is_number.bash.
# is_number 
is_number() {
[ "$#" -eq 1 ]
|| return 1 # is blank?
[ x"$1" == 'x0' ] && return 0 # is zero?
local -i tst
let tst=$1 2>/dev/null
# else is numeric!
return $?
}
# This function described in is_address.bash.
# is_address 
is_address() {
[ $# -eq 1 ] || return 1
# Blank ==> false
local -a _ia_input
local IFS=${ADR_IFS}
_ia_input=( $1 )
if [ ${#_ia_input[@]} -eq 4 ] &&
is_number ${_ia_input[0]}
&&
is_number ${_ia_input[1]}
&&
is_number ${_ia_input[2]}
&&
is_number ${_ia_input[3]}
&&
[ ${_ia_input[0]} -lt 256 ] &&
[ ${_ia_input[1]} -lt 256 ] &&
[ ${_ia_input[2]} -lt 256 ] &&
[ ${_ia_input[3]} -lt 256 ]
then
return 0
else
return 1
fi
}
# This function described in split_ip.bash.
# split_ip 
#+  []
split_ip() {
[ $# -eq 3 ] ||
# Either three
[ $# -eq 2 ] || return 1
#+ or two arguments
local -a _si_input
local IFS=${ADR_IFS}
_si_input=( $1 )
IFS=${WSP_IFS}

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Advanced Bash-Scripting Guide
eval $2=\(\ \$\{_si_input\[@\]\}\ \)
if [ $# -eq 3 ]
then
# Build query order array.
local -a _dns_ip
_dns_ip[0]=${_si_input[3]}
_dns_ip[1]=${_si_input[2]}
_dns_ip[2]=${_si_input[1]}
_dns_ip[3]=${_si_input[0]}
eval $3=\(\ \$\{_dns_ip\[@\]\}\ \)
fi
return 0
}
# This function described in dot_array.bash.
# dot_array 
dot_array() {
[ $# -eq 1 ] || return 1
# Single argument required.
local -a _da_input
eval _da_input=\(\ \$\{$1\[@\]\}\ \)
local IFS=${DOT_IFS}
local _da_output=${_da_input[@]}
IFS=${WSP_IFS}
echo ${_da_output}
return 0
}
# This function described in file_to_array.bash
# file_to_array  
file_to_array() {
[ $# -eq 2 ] || return 1 # Two arguments required.
local IFS=${NO_WSP}
local -a _fta_tmp_
_fta_tmp_=( $(cat $1) )
eval $2=\( \$\{_fta_tmp_\[@\]\} \)
return 0
}
# Columnized print of an array of multi-field strings.
# col_print   <
#+ tab_stop [tab_stops]>
col_print() {
[ $# -gt 2 ] || return 0
local -a _cp_inp
local -a _cp_spc
local -a _cp_line
local _cp_min
local _cp_mcnt
local _cp_pos
local _cp_cnt
local _cp_tab
local -i _cp
local -i _cpf
local _cp_fld
# WARNING: FOLLOWING LINE NOT BLANK -- IT IS QUOTED SPACES.
local _cp_max='
set -f
local IFS=${NO_WSP}
eval _cp_inp=\(\ \$\{$1\[@\]\}\ \)
[ ${#_cp_inp[@]} -gt 0 ] || return 0 # Empty is easy.
_cp_mcnt=$2
_cp_min=${_cp_max:1:${_cp_mcnt}}

Appendix A. Contributed Scripts

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628

Advanced Bash-Scripting Guide
shift
shift
_cp_cnt=$#
for (( _cp = 0 ; _cp < _cp_cnt ; _cp++ ))
do
_cp_spc[${#_cp_spc[@]}]="${_cp_max:2:$1}" #"
shift
done
_cp_cnt=${#_cp_inp[@]}
for (( _cp = 0 ; _cp < _cp_cnt ; _cp++ ))
do
_cp_pos=1
IFS=${NO_WSP}$'\x20'
_cp_line=( ${_cp_inp[${_cp}]} )
IFS=${NO_WSP}
for (( _cpf = 0 ; _cpf < ${#_cp_line[@]} ; _cpf++ ))
do
_cp_tab=${_cp_spc[${_cpf}]:${_cp_pos}}
if [ ${#_cp_tab} -lt ${_cp_mcnt} ]
then
_cp_tab="${_cp_min}"
fi
echo -n "${_cp_tab}"
(( _cp_pos = ${_cp_pos} + ${#_cp_tab} ))
_cp_fld="${_cp_line[${_cpf}]}"
echo -n ${_cp_fld}
(( _cp_pos = ${_cp_pos} + ${#_cp_fld} ))
done
echo
done
set +f
return 0
}
# # # # 'Hunt the Spammer' data flow # # # #
# Application return code
declare -i _hs_RC
# Original input, from which IP addresses are removed
# After which, domain names to check
declare -a uc_name
# Original input IP addresses are moved here
# After which, IP addresses to check
declare -a uc_address
# Names against which address expansion run
# Ready for name detail lookup
declare -a chk_name
# Addresses against which name expansion run
# Ready for address detail lookup
declare -a chk_address
# Recursion is depth-first-by-name.
# The expand_input_address maintains this list
#+ to prohibit looking up addresses twice during
#+ domain name recursion.
declare -a been_there_addr
been_there_addr=( '127.0.0.1' ) # Whitelist localhost

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Advanced Bash-Scripting Guide
# Names which we have checked (or given up on)
declare -a known_name
# Addresses which we have checked (or given up on)
declare -a known_address
# List
# Each
#+ with
declare

of zero or more Blacklist servers to check.
'known_address' will be checked against each server,
negative replies and failures suppressed.
-a list_server

# Indirection limit - set to zero == no limit
indirect=${SPAMMER_LIMIT:=2}
# # # # 'Hunt the Spammer' information output data # # # #
# Any domain name may have multiple IP addresses.
# Any IP address may have multiple domain names.
# Therefore, track unique address-name pairs.
declare -a known_pair
declare -a reverse_pair
# In addition to the data flow variables; known_address
#+ known_name and list_server, the following are output to the
#+ external graphics interface file.
# Authority chain, parent -> SOA fields.
declare -a auth_chain
# Reference chain, parent name -> child name
declare -a ref_chain
# DNS chain - domain name -> address
declare -a name_address
# Name and service pairs - domain name -> service
declare -a name_srvc
# Name and resource pairs - domain name -> Resource Record
declare -a name_resource
# Parent and Child pairs - parent name -> child name
# This MAY NOT be the same as the ref_chain followed!
declare -a parent_child
# Address and Blacklist hit pairs - address->server
declare -a address_hits
# Dump interface file data
declare -f _dot_dump
_dot_dump=pend_dummy
# Initially a no-op
# Data dump is enabled by setting the environment variable SPAMMER_DATA
#+ to the name of a writable file.
declare _dot_file
# Helper function for the dump-to-dot-file function
# dump_to_dot  
dump_to_dot() {
local -a _dda_tmp
local -i _dda_cnt
local _dda_form='
'${2}'%04u %s\n'

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Advanced Bash-Scripting Guide
local IFS=${NO_WSP}
eval _dda_tmp=\(\ \$\{$1\[@\]\}\ \)
_dda_cnt=${#_dda_tmp[@]}
if [ ${_dda_cnt} -gt 0 ]
then
for (( _dda = 0 ; _dda < _dda_cnt ; _dda++ ))
do
printf "${_dda_form}" \
"${_dda}" "${_dda_tmp[${_dda}]}" >>${_dot_file}
done
fi
}
# Which will also set _dot_dump to this function . . .
dump_dot() {
local -i _dd_cnt
echo '# Data vintage: '$(date -R) >${_dot_file}
echo '# ABS Guide: is_spammer.bash; v2, 2004-msz' >>${_dot_file}
echo >>${_dot_file}
echo 'digraph G {' >>${_dot_file}
if [ ${#known_name[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known domain name nodes' >>${_dot_file}
_dd_cnt=${#known_name[@]}
for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))
do
printf '
N%04u [label="%s"] ;\n' \
"${_dd}" "${known_name[${_dd}]}" >>${_dot_file}
done
fi
if [ ${#known_address[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known address nodes' >>${_dot_file}
_dd_cnt=${#known_address[@]}
for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))
do
printf '
A%04u [label="%s"] ;\n' \
"${_dd}" "${known_address[${_dd}]}" >>${_dot_file}
done
fi
echo
echo
echo
echo
echo

>>${_dot_file}
'/*'
>>${_dot_file}
' * Known relationships :: User conversion to' >>${_dot_file}
' * graphic form by hand or program required.' >>${_dot_file}
' *'
>>${_dot_file}

if [ ${#auth_chain[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Authority ref. edges followed & field source.' >>${_dot_file}
dump_to_dot auth_chain AC
fi
if [ ${#ref_chain[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Name ref. edges followed and field source.' >>${_dot_file}

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Advanced Bash-Scripting Guide
dump_to_dot ref_chain RC
fi
if [ ${#name_address[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known name->address edges' >>${_dot_file}
dump_to_dot name_address NA
fi
if [ ${#name_srvc[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known name->service edges' >>${_dot_file}
dump_to_dot name_srvc NS
fi
if [ ${#name_resource[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known name->resource edges' >>${_dot_file}
dump_to_dot name_resource NR
fi
if [ ${#parent_child[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known parent->child edges' >>${_dot_file}
dump_to_dot parent_child PC
fi
if [ ${#list_server[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known Blacklist nodes' >>${_dot_file}
_dd_cnt=${#list_server[@]}
for (( _dd = 0 ; _dd < _dd_cnt ; _dd++ ))
do
printf '
LS%04u [label="%s"] ;\n' \
"${_dd}" "${list_server[${_dd}]}" >>${_dot_file}
done
fi
unique_lines address_hits address_hits
if [ ${#address_hits[@]} -gt 0 ]
then
echo >>${_dot_file}
echo '# Known address->Blacklist_hit edges' >>${_dot_file}
echo '# CAUTION: dig warnings can trigger false hits.' >>${_dot_file}
dump_to_dot address_hits AH
fi
echo
>>${_dot_file}
echo ' *'
>>${_dot_file}
echo ' * That is a lot of relationships. Happy graphing.' >>${_dot_file}
echo ' */'
>>${_dot_file}
echo '}'
>>${_dot_file}
return 0
}
# # # # 'Hunt the Spammer' execution flow # # # #
#

Execution trace is enabled by setting the

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632

Advanced Bash-Scripting Guide
#+ environment variable SPAMMER_TRACE to the name of a writable file.
declare -a _trace_log
declare _log_file
# Function to fill the trace log
trace_logger() {
_trace_log[${#_trace_log[@]}]=${_pend_current_}
}
# Dump trace log to file function variable.
declare -f _log_dump
_log_dump=pend_dummy
# Initially a no-op.
# Dump the trace log to a file.
dump_log() {
local -i _dl_cnt
_dl_cnt=${#_trace_log[@]}
for (( _dl = 0 ; _dl < _dl_cnt ; _dl++ ))
do
echo ${_trace_log[${_dl}]} >> ${_log_file}
done
_dl_cnt=${#_pending_[@]}
if [ ${_dl_cnt} -gt 0 ]
then
_dl_cnt=${_dl_cnt}-1
echo '# # # Operations stack not empty # # #' >> ${_log_file}
for (( _dl = ${_dl_cnt} ; _dl >= 0 ; _dl-- ))
do
echo ${_pending_[${_dl}]} >> ${_log_file}
done
fi
}
# # # Utility program 'dig' wrappers # # #
#
# These wrappers are derived from the
#+ examples shown in dig_wrappers.bash.
#
# The major difference is these return
#+ their results as a list in an array.
#
# See dig_wrappers.bash for details and
#+ use that script to develop any changes.
#
# # #
# Short form answer: 'dig' parses answer.
# Forward lookup :: Name -> Address
# short_fwd  
short_fwd() {
local -a _sf_reply
local -i _sf_rc
local -i _sf_cnt
IFS=${NO_WSP}
echo -n '.'
# echo 'sfwd: '${1}
_sf_reply=( $(dig +short ${1} -c in -t a 2>/dev/null) )
_sf_rc=$?
if [ ${_sf_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='## Lookup error '${_sf_rc}' on '${1}' ##'

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Advanced Bash-Scripting Guide
# [ ${_sf_rc} -ne 9 ] && pend_drop
return ${_sf_rc}
else
# Some versions of 'dig' return warnings on stdout.
_sf_cnt=${#_sf_reply[@]}
for (( _sf = 0 ; _sf < ${_sf_cnt} ; _sf++ ))
do
[ 'x'${_sf_reply[${_sf}]:0:2} == 'x;;' ] &&
unset _sf_reply[${_sf}]
done
eval $2=\( \$\{_sf_reply\[@\]\} \)
fi
return 0
}
# Reverse lookup :: Address -> Name
# short_rev  
short_rev() {
local -a _sr_reply
local -i _sr_rc
local -i _sr_cnt
IFS=${NO_WSP}
echo -n '.'
# echo 'srev: '${1}
_sr_reply=( $(dig +short -x ${1} 2>/dev/null) )
_sr_rc=$?
if [ ${_sr_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='## Lookup error '${_sr_rc}' on '${1}' ##'
# [ ${_sr_rc} -ne 9 ] && pend_drop
return ${_sr_rc}
else
# Some versions of 'dig' return warnings on stdout.
_sr_cnt=${#_sr_reply[@]}
for (( _sr = 0 ; _sr < ${_sr_cnt} ; _sr++ ))
do
[ 'x'${_sr_reply[${_sr}]:0:2} == 'x;;' ] &&
unset _sr_reply[${_sr}]
done
eval $2=\( \$\{_sr_reply\[@\]\} \)
fi
return 0
}
# Special format lookup used to query blacklist servers.
# short_text  
short_text() {
local -a _st_reply
local -i _st_rc
local -i _st_cnt
IFS=${NO_WSP}
# echo 'stxt: '${1}
_st_reply=( $(dig +short ${1} -c in -t txt 2>/dev/null) )
_st_rc=$?
if [ ${_st_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='##Text lookup error '${_st_rc}' on '${1}'##'
# [ ${_st_rc} -ne 9 ] && pend_drop
return ${_st_rc}
else
# Some versions of 'dig' return warnings on stdout.
_st_cnt=${#_st_reply[@]}

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Advanced Bash-Scripting Guide
for (( _st = 0 ; _st < ${#_st_cnt} ; _st++ ))
do
[ 'x'${_st_reply[${_st}]:0:2} == 'x;;' ] &&
unset _st_reply[${_st}]
done
eval $2=\( \$\{_st_reply\[@\]\} \)
fi
return 0
}
# The long forms, a.k.a., the parse it yourself versions
#
#
#
#
#

RFC 2782
Service lookups
dig +noall +nofail +answer _ldap._tcp.openldap.org -t srv
_._.
_ldap._tcp.openldap.org. 3600
IN
SRV
0 0 389 ldap.openldap.org.
domain TTL Class SRV Priority Weight Port Target

# Forward lookup :: Name -> poor man's zone transfer
# long_fwd  
long_fwd() {
local -a _lf_reply
local -i _lf_rc
local -i _lf_cnt
IFS=${NO_WSP}
echo -n ':'
# echo 'lfwd: '${1}
_lf_reply=( $(
dig +noall +nofail +answer +authority +additional \
${1} -t soa ${1} -t mx ${1} -t any 2>/dev/null) )
_lf_rc=$?
if [ ${_lf_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='# Zone lookup err '${_lf_rc}' on '${1}' #'
# [ ${_lf_rc} -ne 9 ] && pend_drop
return ${_lf_rc}
else
# Some versions of 'dig' return warnings on stdout.
_lf_cnt=${#_lf_reply[@]}
for (( _lf = 0 ; _lf < ${_lf_cnt} ; _lf++ ))
do
[ 'x'${_lf_reply[${_lf}]:0:2} == 'x;;' ] &&
unset _lf_reply[${_lf}]
done
eval $2=\( \$\{_lf_reply\[@\]\} \)
fi
return 0
}
# The reverse lookup domain name corresponding to the IPv6 address:
#
4321:0:1:2:3:4:567:89ab
# would be (nibble, I.E: Hexdigit) reversed:
# b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.IP6.ARPA.
# Reverse lookup :: Address -> poor man's delegation chain
# long_rev  
long_rev() {
local -a _lr_reply
local -i _lr_rc
local -i _lr_cnt
local _lr_dns
_lr_dns=${1}'.in-addr.arpa.'
IFS=${NO_WSP}

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Advanced Bash-Scripting Guide
echo -n ':'
# echo 'lrev: '${1}
_lr_reply=( $(
dig +noall +nofail +answer +authority +additional \
${_lr_dns} -t soa ${_lr_dns} -t any 2>/dev/null) )
_lr_rc=$?
if [ ${_lr_rc} -ne 0 ]
then
_trace_log[${#_trace_log[@]}]='# Deleg lkp error '${_lr_rc}' on '${1}' #'
# [ ${_lr_rc} -ne 9 ] && pend_drop
return ${_lr_rc}
else
# Some versions of 'dig' return warnings on stdout.
_lr_cnt=${#_lr_reply[@]}
for (( _lr = 0 ; _lr < ${_lr_cnt} ; _lr++ ))
do
[ 'x'${_lr_reply[${_lr}]:0:2} == 'x;;' ] &&
unset _lr_reply[${_lr}]
done
eval $2=\( \$\{_lr_reply\[@\]\} \)
fi
return 0
}
# # # Application specific functions # # #
# Mung a possible name; suppresses root and TLDs.
# name_fixup 
name_fixup(){
local -a _nf_tmp
local -i _nf_end
local _nf_str
local IFS
_nf_str=$(to_lower ${1})
_nf_str=$(to_dot ${_nf_str})
_nf_end=${#_nf_str}-1
[ ${_nf_str:${_nf_end}} != '.' ] &&
_nf_str=${_nf_str}'.'
IFS=${ADR_IFS}
_nf_tmp=( ${_nf_str} )
IFS=${WSP_IFS}
_nf_end=${#_nf_tmp[@]}
case ${_nf_end} in
0) # No dots, only dots.
echo
return 1
;;
1) # Only a TLD.
echo
return 1
;;
2) # Maybe okay.
echo ${_nf_str}
return 0
# Needs a lookup table?
if [ ${#_nf_tmp[1]} -eq 2 ]
then # Country coded TLD.
echo
return 1
else
echo ${_nf_str}
return 0

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Advanced Bash-Scripting Guide
fi
;;
esac
echo ${_nf_str}
return 0
}
# Grope and mung original input(s).
split_input() {
[ ${#uc_name[@]} -gt 0 ] || return 0
local -i _si_cnt
local -i _si_len
local _si_str
unique_lines uc_name uc_name
_si_cnt=${#uc_name[@]}
for (( _si = 0 ; _si < _si_cnt ; _si++ ))
do
_si_str=${uc_name[$_si]}
if is_address ${_si_str}
then
uc_address[${#uc_address[@]}]=${_si_str}
unset uc_name[$_si]
else
if ! uc_name[$_si]=$(name_fixup ${_si_str})
then
unset ucname[$_si]
fi
fi
done
uc_name=( ${uc_name[@]} )
_si_cnt=${#uc_name[@]}
_trace_log[${#_trace_log[@]}]='#Input '${_si_cnt}' unchkd name input(s).#'
_si_cnt=${#uc_address[@]}
_trace_log[${#_trace_log[@]}]='#Input '${_si_cnt}' unchkd addr input(s).#'
return 0
}
# # # Discovery functions -- recursively interlocked by external data # # #
# # # The leading 'if list is empty; return 0' in each is required. # # #
# Recursion limiter
# limit_chk() 
limit_chk() {
local -i _lc_lmt
# Check indirection limit.
if [ ${indirect} -eq 0 ] || [ $# -eq 0 ]
then
# The 'do-forever' choice
echo 1
# Any value will do.
return 0
# OK to continue.
else
# Limiting is in effect.
if [ ${indirect} -lt ${1} ]
then
echo ${1}
# Whatever.
return 1
# Stop here.
else
_lc_lmt=${1}+1
# Bump the given limit.
echo ${_lc_lmt}
# Echo it.
return 0
# OK to continue.
fi
fi

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Advanced Bash-Scripting Guide
}
# For each name in uc_name:
#
Move name to chk_name.
#
Add addresses to uc_address.
#
Pend expand_input_address.
#
Repeat until nothing new found.
# expand_input_name 
expand_input_name() {
[ ${#uc_name[@]} -gt 0 ] || return 0
local -a _ein_addr
local -a _ein_new
local -i _ucn_cnt
local -i _ein_cnt
local _ein_tst
_ucn_cnt=${#uc_name[@]}
if ! _ein_cnt=$(limit_chk ${1})
then
return 0
fi
for (( _ein = 0 ; _ein < _ucn_cnt ; _ein++ ))
do
if short_fwd ${uc_name[${_ein}]} _ein_new
then
for (( _ein_cnt = 0 ; _ein_cnt < ${#_ein_new[@]}; _ein_cnt++ ))
do
_ein_tst=${_ein_new[${_ein_cnt}]}
if is_address ${_ein_tst}
then
_ein_addr[${#_ein_addr[@]}]=${_ein_tst}
fi
done
fi
done
unique_lines _ein_addr _ein_addr
# Scrub duplicates.
edit_exact chk_address _ein_addr
# Scrub pending detail.
edit_exact known_address _ein_addr
# Scrub already detailed.
if [ ${#_ein_addr[@]} -gt 0 ]
# Anything new?
then
uc_address=( ${uc_address[@]} ${_ein_addr[@]} )
pend_func expand_input_address ${1}
_trace_log[${#_trace_log[@]}]='#Add '${#_ein_addr[@]}' unchkd addr inp.#'
fi
edit_exact chk_name uc_name
# Scrub pending detail.
edit_exact known_name uc_name
# Scrub already detailed.
if [ ${#uc_name[@]} -gt 0 ]
then
chk_name=( ${chk_name[@]} ${uc_name[@]} )
pend_func detail_each_name ${1}
fi
unset uc_name[@]
return 0
}
# For each address in uc_address:
#
Move address to chk_address.
#
Add names to uc_name.
#
Pend expand_input_name.
#
Repeat until nothing new found.
# expand_input_address 

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638

Advanced Bash-Scripting Guide
expand_input_address() {
[ ${#uc_address[@]} -gt 0 ] || return 0
local -a _eia_addr
local -a _eia_name
local -a _eia_new
local -i _uca_cnt
local -i _eia_cnt
local _eia_tst
unique_lines uc_address _eia_addr
unset uc_address[@]
edit_exact been_there_addr _eia_addr
_uca_cnt=${#_eia_addr[@]}
[ ${_uca_cnt} -gt 0 ] &&
been_there_addr=( ${been_there_addr[@]} ${_eia_addr[@]} )
for (( _eia = 0 ; _eia < _uca_cnt ; _eia++ ))
do
if short_rev ${_eia_addr[${_eia}]} _eia_new
then
for (( _eia_cnt = 0 ; _eia_cnt < ${#_eia_new[@]} ; _eia_cnt++ ))
do
_eia_tst=${_eia_new[${_eia_cnt}]}
if _eia_tst=$(name_fixup ${_eia_tst})
then
_eia_name[${#_eia_name[@]}]=${_eia_tst}
fi
done
fi
done
unique_lines _eia_name _eia_name
# Scrub duplicates.
edit_exact chk_name _eia_name
# Scrub pending detail.
edit_exact known_name _eia_name
# Scrub already detailed.
if [ ${#_eia_name[@]} -gt 0 ]
# Anything new?
then
uc_name=( ${uc_name[@]} ${_eia_name[@]} )
pend_func expand_input_name ${1}
_trace_log[${#_trace_log[@]}]='#Add '${#_eia_name[@]}' unchkd name inp.#'
fi
edit_exact chk_address _eia_addr
# Scrub pending detail.
edit_exact known_address _eia_addr
# Scrub already detailed.
if [ ${#_eia_addr[@]} -gt 0 ]
# Anything new?
then
chk_address=( ${chk_address[@]} ${_eia_addr[@]} )
pend_func detail_each_address ${1}
fi
return 0
}
# The parse-it-yourself zone reply.
# The input is the chk_name list.
# detail_each_name 
detail_each_name() {
[ ${#chk_name[@]} -gt 0 ] || return 0
local -a _den_chk
# Names to check
local -a _den_name
# Names found here
local -a _den_address
# Addresses found here
local -a _den_pair
# Pairs found here
local -a _den_rev
# Reverse pairs found here
local -a _den_tmp
# Line being parsed
local -a _den_auth
# SOA contact being parsed
local -a _den_new
# The zone reply
local -a _den_pc
# Parent-Child gets big fast

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Advanced Bash-Scripting Guide
local
local
local
local
local
local
local
local
local
local
local
local
local

-a _den_ref
-a _den_nr
-a _den_na
-a _den_ns
-a _den_achn
-i _den_cnt
-i _den_lmt
_den_who
_den_rec
_den_cont
_den_str
_den_str2
IFS=${WSP_IFS}

#
#
#
#
#
#
#
#
#
#
#
#

So does reference chain
Name-Resource can be big
Name-Address
Name-Service
Chain of Authority
Count of names to detail
Indirection limit
Named being processed
Record type being processed
Contact domain
Fixed up name string
Fixed up reverse

# Local, unique copy of names to check
unique_lines chk_name _den_chk
unset chk_name[@]
# Done with globals.
# Less any names already known
edit_exact known_name _den_chk
_den_cnt=${#_den_chk[@]}
# If anything left, add to known_name.
[ ${_den_cnt} -gt 0 ] &&
known_name=( ${known_name[@]} ${_den_chk[@]} )
# for the list of (previously) unknown names . . .
for (( _den = 0 ; _den < _den_cnt ; _den++ ))
do
_den_who=${_den_chk[${_den}]}
if long_fwd ${_den_who} _den_new
then
unique_lines _den_new _den_new
if [ ${#_den_new[@]} -eq 0 ]
then
_den_pair[${#_den_pair[@]}]='0.0.0.0 '${_den_who}
fi
# Parse each line in the reply.
for (( _line = 0 ; _line < ${#_den_new[@]} ; _line++ ))
do
IFS=${NO_WSP}$'\x09'$'\x20'
_den_tmp=( ${_den_new[${_line}]} )
IFS=${WSP_IFS}
# If usable record and not a warning message . . .
if [ ${#_den_tmp[@]} -gt 4 ] && [ 'x'${_den_tmp[0]} != 'x;;' ]
then
_den_rec=${_den_tmp[3]}
_den_nr[${#_den_nr[@]}]=${_den_who}' '${_den_rec}
# Begin at RFC1033 (+++)
case ${_den_rec} in
# []

[] SOA  
SOA) # Start Of Authority
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_str}' SOA'
# SOA origin -- domain name of master zone record
if _den_str2=$(name_fixup ${_den_tmp[4]})
then
_den_name[${#_den_name[@]}]=${_den_str2}

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_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_str2}' SOA.O'
fi
# Responsible party e-mail address (possibly bogus).
# Possibility of first.last@domain.name ignored.
set -f
if _den_str2=$(name_fixup ${_den_tmp[5]})
then
IFS=${ADR_IFS}
_den_auth=( ${_den_str2} )
IFS=${WSP_IFS}
if [ ${#_den_auth[@]} -gt 2 ]
then
_den_cont=${_den_auth[1]}
for (( _auth = 2 ; _auth < ${#_den_auth[@]} ; _auth++ ))
do
_den_cont=${_den_cont}'.'${_den_auth[${_auth}]}
done
_den_name[${#_den_name[@]}]=${_den_cont}'.'
_den_achn[${#_den_achn[@]}]=${_den_who}' '${_den_cont}'. SOA.C'
fi
fi
set +f
fi
;;

A) # IP(v4) Address Record
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' '${_den_str}
_den_na[${#_den_na[@]}]=${_den_str}' '${_den_tmp[4]}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' A'
else
_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' unknown.domain'
_den_na[${#_den_na[@]}]='unknown.domain '${_den_tmp[4]}
_den_ref[${#_den_ref[@]}]=${_den_who}' unknown.domain A'
fi
_den_address[${#_den_address[@]}]=${_den_tmp[4]}
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_tmp[4]}
;;
NS) # Name Server Record
# Domain name being serviced (may be other than current)
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' NS'
# Domain name of service provider
if _den_str2=$(name_fixup ${_den_tmp[4]})
then
_den_name[${#_den_name[@]}]=${_den_str2}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str2}' NSH'
_den_ns[${#_den_ns[@]}]=${_den_str2}' NS'
_den_pc[${#_den_pc[@]}]=${_den_str}' '${_den_str2}
fi
fi
;;
MX) # Mail Server Record
# Domain name being serviced (wildcards not handled here)

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if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' MX'
fi
# Domain name of service provider
if _den_str=$(name_fixup ${_den_tmp[5]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' MXH'
_den_ns[${#_den_ns[@]}]=${_den_str}' MX'
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}
fi
;;
PTR) # Reverse address record
# Special name
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' PTR'
# Host name (not a CNAME)
if _den_str2=$(name_fixup ${_den_tmp[4]})
then
_den_rev[${#_den_rev[@]}]=${_den_str}' '${_den_str2}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str2}' PTRH'
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}
fi
fi
;;
AAAA) # IP(v6) Address Record
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' '${_den_str}
_den_na[${#_den_na[@]}]=${_den_str}' '${_den_tmp[4]}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' AAAA'
else
_den_pair[${#_den_pair[@]}]=${_den_tmp[4]}' unknown.domain'
_den_na[${#_den_na[@]}]='unknown.domain '${_den_tmp[4]}
_den_ref[${#_den_ref[@]}]=${_den_who}' unknown.domain'
fi
# No processing for IPv6 addresses
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_tmp[4]}
;;
CNAME) # Alias name record
# Nickname
if _den_str=$(name_fixup ${_den_tmp[0]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' CNAME'
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}
fi
# Hostname
if _den_str=$(name_fixup ${_den_tmp[4]})
then
_den_name[${#_den_name[@]}]=${_den_str}
_den_ref[${#_den_ref[@]}]=${_den_who}' '${_den_str}' CHOST'
_den_pc[${#_den_pc[@]}]=${_den_who}' '${_den_str}
fi
;;

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#
#

TXT)
;;
esac
fi
done
else # Lookup error == 'A' record 'unknown address'
_den_pair[${#_den_pair[@]}]='0.0.0.0 '${_den_who}
fi
done
# Control dot array growth.
unique_lines _den_achn _den_achn
# Works best, all the same.
edit_exact auth_chain _den_achn
# Works best, unique items.
if [ ${#_den_achn[@]} -gt 0 ]
then
IFS=${NO_WSP}
auth_chain=( ${auth_chain[@]} ${_den_achn[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_ref _den_ref
# Works best, all the same.
edit_exact ref_chain _den_ref
# Works best, unique items.
if [ ${#_den_ref[@]} -gt 0 ]
then
IFS=${NO_WSP}
ref_chain=( ${ref_chain[@]} ${_den_ref[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_na _den_na
edit_exact name_address _den_na
if [ ${#_den_na[@]} -gt 0 ]
then
IFS=${NO_WSP}
name_address=( ${name_address[@]} ${_den_na[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_ns _den_ns
edit_exact name_srvc _den_ns
if [ ${#_den_ns[@]} -gt 0 ]
then
IFS=${NO_WSP}
name_srvc=( ${name_srvc[@]} ${_den_ns[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_nr _den_nr
edit_exact name_resource _den_nr
if [ ${#_den_nr[@]} -gt 0 ]
then
IFS=${NO_WSP}
name_resource=( ${name_resource[@]} ${_den_nr[@]} )
IFS=${WSP_IFS}
fi
unique_lines _den_pc _den_pc
edit_exact parent_child _den_pc
if [ ${#_den_pc[@]} -gt 0 ]
then
IFS=${NO_WSP}
parent_child=( ${parent_child[@]} ${_den_pc[@]} )

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IFS=${WSP_IFS}
fi
# Update list known_pair (Address and Name).
unique_lines _den_pair _den_pair
edit_exact known_pair _den_pair
if [ ${#_den_pair[@]} -gt 0 ] # Anything new?
then
IFS=${NO_WSP}
known_pair=( ${known_pair[@]} ${_den_pair[@]} )
IFS=${WSP_IFS}
fi
# Update list of reverse pairs.
unique_lines _den_rev _den_rev
edit_exact reverse_pair _den_rev
if [ ${#_den_rev[@]} -gt 0 ]
# Anything new?
then
IFS=${NO_WSP}
reverse_pair=( ${reverse_pair[@]} ${_den_rev[@]} )
IFS=${WSP_IFS}
fi
# Check indirection limit -- give up if reached.
if ! _den_lmt=$(limit_chk ${1})
then
return 0
fi
# Execution engine is LIFO. Order of pend operations is important.
# Did we define any new addresses?
unique_lines _den_address _den_address
# Scrub duplicates.
edit_exact known_address _den_address
# Scrub already processed.
edit_exact un_address _den_address
# Scrub already waiting.
if [ ${#_den_address[@]} -gt 0 ]
# Anything new?
then
uc_address=( ${uc_address[@]} ${_den_address[@]} )
pend_func expand_input_address ${_den_lmt}
_trace_log[${#_trace_log[@]}]='# Add '${#_den_address[@]}' unchkd addr. #'
fi
# Did we find any new names?
unique_lines _den_name _den_name
# Scrub duplicates.
edit_exact known_name _den_name
# Scrub already processed.
edit_exact uc_name _den_name
# Scrub already waiting.
if [ ${#_den_name[@]} -gt 0 ]
# Anything new?
then
uc_name=( ${uc_name[@]} ${_den_name[@]} )
pend_func expand_input_name ${_den_lmt}
_trace_log[${#_trace_log[@]}]='#Added '${#_den_name[@]}' unchkd name#'
fi
return 0
}
# The parse-it-yourself delegation reply
# Input is the chk_address list.
# detail_each_address 
detail_each_address() {
[ ${#chk_address[@]} -gt 0 ] || return 0
unique_lines chk_address chk_address
edit_exact known_address chk_address
if [ ${#chk_address[@]} -gt 0 ]

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then
known_address=( ${known_address[@]} ${chk_address[@]} )
unset chk_address[@]
fi
return 0
}
# # # Application specific output functions # # #
# Pretty print the known pairs.
report_pairs() {
echo
echo 'Known network pairs.'
col_print known_pair 2 5 30
if [ ${#auth_chain[@]} -gt 0 ]
then
echo
echo 'Known chain of authority.'
col_print auth_chain 2 5 30 55
fi
if [ ${#reverse_pair[@]} -gt 0 ]
then
echo
echo 'Known reverse pairs.'
col_print reverse_pair 2 5 55
fi
return 0
}
# Check an address against the list of blacklist servers.
# A good place to capture for GraphViz: address->status(server(reports))
# check_lists 
check_lists() {
[ $# -eq 1 ] || return 1
local -a _cl_fwd_addr
local -a _cl_rev_addr
local -a _cl_reply
local -i _cl_rc
local -i _ls_cnt
local _cl_dns_addr
local _cl_lkup
split_ip ${1} _cl_fwd_addr _cl_rev_addr
_cl_dns_addr=$(dot_array _cl_rev_addr)'.'
_ls_cnt=${#list_server[@]}
echo '
Checking address '${1}
for (( _cl = 0 ; _cl < _ls_cnt ; _cl++ ))
do
_cl_lkup=${_cl_dns_addr}${list_server[${_cl}]}
if short_text ${_cl_lkup} _cl_reply
then
if [ ${#_cl_reply[@]} -gt 0 ]
then
echo '
Records from '${list_server[${_cl}]}
address_hits[${#address_hits[@]}]=${1}' '${list_server[${_cl}]}
_hs_RC=2
for (( _clr = 0 ; _clr < ${#_cl_reply[@]} ; _clr++ ))
do
echo '
'${_cl_reply[${_clr}]}
done

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fi
fi
done
return 0
}
# # # The usual application glue # # #
# Who did it?
credits() {
echo
echo 'Advanced Bash Scripting Guide: is_spammer.bash, v2, 2004-msz'
}
# How to use it?
# (See also, "Quickstart" at end of script.)
usage() {
cat <<-'_usage_statement_'
The script is_spammer.bash requires either one or two arguments.
arg 1)
a)
b)
c)

May be one of:
A domain name
An IPv4 address
The name of a file with any mix of names
and addresses, one per line.

arg 2) May be one of:
a) A Blacklist server domain name
b) The name of a file with Blacklist server
domain names, one per line.
c) If not present, a default list of (free)
Blacklist servers is used.
d) If a filename of an empty, readable, file
is given,
Blacklist server lookup is disabled.
All script output is written to stdout.
Return codes: 0 -> All OK, 1 -> Script failure,
2 -> Something is Blacklisted.
Requires the external program 'dig' from the 'bind-9'
set of DNS programs. See: http://www.isc.org
The domain name lookup depth limit defaults to 2 levels.
Set the environment variable SPAMMER_LIMIT to change.
SPAMMER_LIMIT=0 means 'unlimited'
Limit may also be set on the command-line.
If arg#1 is an integer, the limit is set to that value
and then the above argument rules are applied.
Setting the environment variable 'SPAMMER_DATA' to a filename
will cause the script to write a GraphViz graphic file.
For the development version;
Setting the environment variable 'SPAMMER_TRACE' to a filename
will cause the execution engine to log a function call trace.
_usage_statement_
}

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# The default list of Blacklist servers:
# Many choices, see: http://www.spews.org/lists.html
declare -a default_servers
# See: http://www.spamhaus.org (Conservative, well maintained)
default_servers[0]='sbl-xbl.spamhaus.org'
# See: http://ordb.org (Open mail relays)
default_servers[1]='relays.ordb.org'
# See: http://www.spamcop.net/ (You can report spammers here)
default_servers[2]='bl.spamcop.net'
# See: http://www.spews.org (An 'early detect' system)
default_servers[3]='l2.spews.dnsbl.sorbs.net'
# See: http://www.dnsbl.us.sorbs.net/using.shtml
default_servers[4]='dnsbl.sorbs.net'
# See: http://dsbl.org/usage (Various mail relay lists)
default_servers[5]='list.dsbl.org'
default_servers[6]='multihop.dsbl.org'
default_servers[7]='unconfirmed.dsbl.org'
# User input argument #1
setup_input() {
if [ -e ${1} ] && [ -r ${1} ] # Name of readable file
then
file_to_array ${1} uc_name
echo 'Using filename >'${1}'< as input.'
else
if is_address ${1}
# IP address?
then
uc_address=( ${1} )
echo 'Starting with address >'${1}'<'
else
# Must be a name.
uc_name=( ${1} )
echo 'Starting with domain name >'${1}'<'
fi
fi
return 0
}
# User input argument #2
setup_servers() {
if [ -e ${1} ] && [ -r ${1} ] # Name of a readable file
then
file_to_array ${1} list_server
echo 'Using filename >'${1}'< as blacklist server list.'
else
list_server=( ${1} )
echo 'Using blacklist server >'${1}'<'
fi
return 0
}
# User environment variable SPAMMER_TRACE
live_log_die() {
if [ ${SPAMMER_TRACE:=} ]
# Wants trace log?
then
if [ ! -e ${SPAMMER_TRACE} ]
then
if ! touch ${SPAMMER_TRACE} 2>/dev/null
then
pend_func echo $(printf '%q\n' \
'Unable to create log file >'${SPAMMER_TRACE}'<')
pend_release

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exit 1
fi
_log_file=${SPAMMER_TRACE}
_pend_hook_=trace_logger
_log_dump=dump_log
else
if [ ! -w ${SPAMMER_TRACE} ]
then
pend_func echo $(printf '%q\n' \
'Unable to write log file >'${SPAMMER_TRACE}'<')
pend_release
exit 1
fi
_log_file=${SPAMMER_TRACE}
echo '' > ${_log_file}
_pend_hook_=trace_logger
_log_dump=dump_log
fi
fi
return 0
}
# User environment variable SPAMMER_DATA
data_capture() {
if [ ${SPAMMER_DATA:=} ]
# Wants a data dump?
then
if [ ! -e ${SPAMMER_DATA} ]
then
if ! touch ${SPAMMER_DATA} 2>/dev/null
then
pend_func echo $(printf '%q]n' \
'Unable to create data output file >'${SPAMMER_DATA}'<')
pend_release
exit 1
fi
_dot_file=${SPAMMER_DATA}
_dot_dump=dump_dot
else
if [ ! -w ${SPAMMER_DATA} ]
then
pend_func echo $(printf '%q\n' \
'Unable to write data output file >'${SPAMMER_DATA}'<')
pend_release
exit 1
fi
_dot_file=${SPAMMER_DATA}
_dot_dump=dump_dot
fi
fi
return 0
}
# Grope user specified arguments.
do_user_args() {
if [ $# -gt 0 ] && is_number $1
then
indirect=$1
shift
fi
case $# in
1)

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if ! setup_input $1
# Needs error checking.
then
pend_release
$_log_dump
exit 1
fi
list_server=( ${default_servers[@]} )
_list_cnt=${#list_server[@]}
echo 'Using default blacklist server list.'
echo 'Search depth limit: '${indirect}
;;
2)
if ! setup_input $1
# Needs error checking.
then
pend_release
$_log_dump
exit 1
fi
if ! setup_servers $2 # Needs error checking.
then
pend_release
$_log_dump
exit 1
fi
echo 'Search depth limit: '${indirect}
;;
*)
pend_func usage
pend_release
$_log_dump
exit 1
;;
esac
return 0
}
# A general purpose debug tool.
# list_array 
list_array() {
[ $# -eq 1 ] || return 1 # One argument required.
local -a _la_lines
set -f
local IFS=${NO_WSP}
eval _la_lines=\(\ \$\{$1\[@\]\}\ \)
echo
echo "Element count "${#_la_lines[@]}" array "${1}
local _ln_cnt=${#_la_lines[@]}
for (( _i = 0; _i < ${_ln_cnt}; _i++ ))
do
echo 'Element '$_i' >'${_la_lines[$_i]}'<'
done
set +f
return 0
}
# # # 'Hunt the Spammer' program code # # #
pend_init
# Ready stack engine.
pend_func credits
# Last thing to print.
# # # Deal with user # # #

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live_log_die
data_capture
echo
do_user_args $@

# Setup debug trace log.
# Setup data capture file.

# # # Haven't exited yet - There is some hope # # #
# Discovery group - Execution engine is LIFO - pend
# in reverse order of execution.
_hs_RC=0
# Hunt the Spammer return code
pend_mark
pend_func report_pairs
# Report name-address pairs.
# The two detail_* are mutually recursive functions.
# They also pend expand_* functions as required.
# These two (the last of ???) exit the recursion.
pend_func detail_each_address
# Get all resources of addresses.
pend_func detail_each_name
# Get all resources of names.
# The two expand_* are mutually recursive functions,
#+ which pend additional detail_* functions as required.
pend_func expand_input_address 1
# Expand input names by address.
pend_func expand_input_name 1
# #xpand input addresses by name.
# Start with a unique set of names and addresses.
pend_func unique_lines uc_address uc_address
pend_func unique_lines uc_name uc_name
# Separate mixed input of names and addresses.
pend_func split_input
pend_release
# # # Pairs reported -- Unique list of IP addresses found
echo
_ip_cnt=${#known_address[@]}
if [ ${#list_server[@]} -eq 0 ]
then
echo 'Blacklist server list empty, none checked.'
else
if [ ${_ip_cnt} -eq 0 ]
then
echo 'Known address list empty, none checked.'
else
_ip_cnt=${_ip_cnt}-1
# Start at top.
echo 'Checking Blacklist servers.'
for (( _ip = _ip_cnt ; _ip >= 0 ; _ip-- ))
do
pend_func check_lists $( printf '%q\n' ${known_address[$_ip]} )
done
fi
fi
pend_release
$_dot_dump
# Graphics file dump
$_log_dump
# Execution trace
echo

##############################
# Example output from script #
##############################
:<<-'_is_spammer_outputs_'
./is_spammer.bash 0 web4.alojamentos7.com

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Starting with domain name >web4.alojamentos7.com<
Using default blacklist server list.
Search depth limit: 0
.:....::::...:::...:::.......::..::...:::.......::
Known network pairs.
66.98.208.97
web4.alojamentos7.com.
66.98.208.97
ns1.alojamentos7.com.
69.56.202.147
ns2.alojamentos.ws.
66.98.208.97
alojamentos7.com.
66.98.208.97
web.alojamentos7.com.
69.56.202.146
ns1.alojamentos.ws.
69.56.202.146
alojamentos.ws.
66.235.180.113
ns1.alojamentos.org.
66.235.181.192
ns2.alojamentos.org.
66.235.180.113
alojamentos.org.
66.235.180.113
web6.alojamentos.org.
216.234.234.30
ns1.theplanet.com.
12.96.160.115
ns2.theplanet.com.
216.185.111.52
mail1.theplanet.com.
69.56.141.4
spooling.theplanet.com.
216.185.111.40
theplanet.com.
216.185.111.40
www.theplanet.com.
216.185.111.52
mail.theplanet.com.
Checking Blacklist servers.
Checking address 66.98.208.97
Records from dnsbl.sorbs.net
"Spam Received See: http://www.dnsbl.sorbs.net/lookup.shtml?66.98.208.97"
Checking address 69.56.202.147
Checking address 69.56.202.146
Checking address 66.235.180.113
Checking address 66.235.181.192
Checking address 216.185.111.40
Checking address 216.234.234.30
Checking address 12.96.160.115
Checking address 216.185.111.52
Checking address 69.56.141.4
Advanced Bash Scripting Guide: is_spammer.bash, v2, 2004-msz
_is_spammer_outputs_
exit ${_hs_RC}
####################################################
# The script ignores everything from here on down #
#+ because of the 'exit' command, just above.
#
####################################################

Quickstart
==========
Prerequisites
Bash version 2.05b or 3.00 (bash --version)
A version of Bash which supports arrays. Array
support is included by default Bash configurations.
'dig,' version 9.x.x (dig $HOSTNAME, see first line of output)

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A version of dig which supports the +short options.
See: dig_wrappers.bash for details.

Optional Prerequisites
'named,' a local DNS caching program. Any flavor will do.
Do twice: dig $HOSTNAME
Check near bottom of output for: SERVER: 127.0.0.1#53
That means you have one running.

Optional Graphics Support
'date,' a standard *nix thing. (date -R)
dot Program to convert graphic description file to a
diagram. (dot -V)
A part of the Graph-Viz set of programs.
See: [http://www.research.att.com/sw/tools/graphviz||GraphViz]
'dotty,' a visual editor for graphic description files.
Also a part of the Graph-Viz set of programs.

Quick Start
In the same directory as the is_spammer.bash script;
Do: ./is_spammer.bash
Usage Details
1. Blacklist server choices.
(a) To use default, built-in list: Do nothing.
(b) To use your own list:
i. Create a file with a single Blacklist server
domain name per line.
ii. Provide that filename as the last argument to
the script.
(c) To use a single Blacklist server: Last argument
to the script.
(d) To disable Blacklist lookups:
i. Create an empty file (touch spammer.nul)
Your choice of filename.
ii. Provide the filename of that empty file as the
last argument to the script.
2. Search depth limit.
(a) To use the default value of 2: Do nothing.
(b) To set a different limit:

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A limit of 0 means: no limit.
i. export SPAMMER_LIMIT=1
or whatever limit you want.
ii. OR provide the desired limit as the first
argument to the script.
3. Optional execution trace log.
(a) To use the default setting of no log output: Do nothing.
(b) To write an execution trace log:
export SPAMMER_TRACE=spammer.log
or whatever filename you want.
4. Optional graphic description file.
(a) To use the default setting of no graphic file: Do nothing.
(b) To write a Graph-Viz graphic description file:
export SPAMMER_DATA=spammer.dot
or whatever filename you want.
5. Where to start the search.
(a) Starting with a single domain name:
i. Without a command-line search limit: First
argument to script.
ii. With a command-line search limit: Second
argument to script.
(b) Starting with a single IP address:
i. Without a command-line search limit: First
argument to script.
ii. With a command-line search limit: Second
argument to script.
(c) Starting with (mixed) multiple name(s) and/or address(es):
Create a file with one name or address per line.
Your choice of filename.
i. Without a command-line search limit: Filename as
first argument to script.
ii. With a command-line search limit: Filename as
second argument to script.
6. What to do with the display output.
(a) To view display output on screen: Do nothing.
(b) To save display output to a file: Redirect stdout to a filename.
(c) To discard display output: Redirect stdout to /dev/null.
7. Temporary end of decision making.
press RETURN

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wait (optionally, watch the dots and colons).
8. Optionally check the return code.
(a) Return code 0: All OK
(b) Return code 1: Script setup failure
(c) Return code 2: Something was blacklisted.
9. Where is my graph (diagram)?
The script does not directly produce a graph (diagram).
It only produces a graphic description file. You can
process the graphic descriptor file that was output
with the 'dot' program.
Until you edit that descriptor file, to describe the
relationships you want shown, all that you will get is
a bunch of labeled name and address nodes.
All of the script's discovered relationships are within
a comment block in the graphic descriptor file, each
with a descriptive heading.
The editing required to draw a line between a pair of
nodes from the information in the descriptor file may
be done with a text editor.
Given these lines somewhere in the descriptor file:
# Known domain name nodes
N0000 [label="guardproof.info."] ;
N0002 [label="third.guardproof.info."] ;

# Known address nodes
A0000 [label="61.141.32.197"] ;

/*
# Known name->address edges
NA0000 third.guardproof.info. 61.141.32.197

# Known parent->child edges
PC0000 guardproof.info. third.guardproof.info.
*/
Turn that into the following lines by substituting node
identifiers into the relationships:

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Advanced Bash-Scripting Guide
# Known domain name nodes
N0000 [label="guardproof.info."] ;
N0002 [label="third.guardproof.info."] ;

# Known address nodes
A0000 [label="61.141.32.197"] ;

# PC0000 guardproof.info. third.guardproof.info.
N0000->N0002 ;

# NA0000 third.guardproof.info. 61.141.32.197
N0002->A0000 ;

/*
# Known name->address edges
NA0000 third.guardproof.info. 61.141.32.197

# Known parent->child edges
PC0000 guardproof.info. third.guardproof.info.
*/
Process that with the 'dot' program, and you have your
first network diagram.
In addition to the conventional graphic edges, the
descriptor file includes similar format pair-data that
describes services, zone records (sub-graphs?),
blacklisted addresses, and other things which might be
interesting to include in your graph. This additional
information could be displayed as different node
shapes, colors, line sizes, etc.
The descriptor file can also be read and edited by a
Bash script (of course). You should be able to find
most of the functions required within the
"is_spammer.bash" script.
# End Quickstart.

Additional Note
========== ====

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Advanced Bash-Scripting Guide
Michael Zick points out that there is a "makeviz.bash" interactive
Web site at rediris.es. Can't give the full URL, since this is not
a publically accessible site.

Another anti-spam script.

Example A-29. Spammer Hunt
#!/bin/bash
# whx.sh: "whois" spammer lookup
# Author: Walter Dnes
# Slight revisions (first section) by ABS Guide author.
# Used in ABS Guide with permission.
# Needs version 3.x or greater of Bash to run (because of =~ operator).
# Commented by script author and ABS Guide author.

E_BADARGS=65
E_NOHOST=66
E_TIMEOUT=67
E_UNDEF=68
HOSTWAIT=10
OUTFILE=whois.txt
PORT=4321

#
#
#
#
#
#
#

Missing command-line arg.
Host not found.
Host lookup timed out.
Some other (undefined) error.
Specify up to 10 seconds for host query reply.
The actual wait may be a bit longer.
Output file.

if [ -z "$1" ]
# Check for (required) command-line arg.
then
echo "Usage: $0 domain name or IP address"
exit $E_BADARGS
fi

if [[ "$1" =~ "[a-zA-Z][a-zA-Z]$" ]] # Ends in two alpha chars?
then
# It's a domain name && must do host lookup.
IPADDR=$(host -W $HOSTWAIT $1 | awk '{print $4}')
# Doing host lookup to get IP address.
# Extract final field.
else
IPADDR="$1"
# Command-line arg was IP address.
fi
echo; echo "IP Address is: "$IPADDR""; echo
if [ -e "$OUTFILE" ]
then
rm -f "$OUTFILE"
echo "Stale output file \"$OUTFILE\" removed."; echo
fi

#
#
#
if

Sanity checks.
(This section needs more work.)
===============================
[ -z "$IPADDR" ]

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# No response.
then
echo "Host not found!"
exit $E_NOHOST
# Bail out.
fi
if [[ "$IPADDR" =~ "^[;;]" ]]
# ;; connection timed out; no servers could be reached
then
echo "Host lookup timed out!"
exit $E_TIMEOUT
# Bail out.
fi
if [[ "$IPADDR" =~ "[(NXDOMAIN)]$" ]]
# Host xxxxxxxxx.xxx not found: 3(NXDOMAIN)
then
echo "Host not found!"
exit $E_NOHOST
# Bail out.
fi
if [[ "$IPADDR" =~ "[(SERVFAIL)]$" ]]
# Host xxxxxxxxx.xxx not found: 2(SERVFAIL)
then
echo "Host not found!"
exit $E_NOHOST
# Bail out.
fi

# ======================== Main body of script ========================
AFRINICquery() {
# Define the function that queries AFRINIC. Echo a notification to the
#+ screen, and then run the actual query, redirecting output to $OUTFILE.
echo "Searching for $IPADDR in whois.afrinic.net"
whois -h whois.afrinic.net "$IPADDR" > $OUTFILE
# Check for presence of reference to an rwhois.
# Warn about non-functional rwhois.infosat.net server
#+ and attempt rwhois query.
if grep -e "^remarks: .*rwhois\.[^ ]\+" "$OUTFILE"
then
echo " " >> $OUTFILE
echo "***" >> $OUTFILE
echo "***" >> $OUTFILE
echo "Warning: rwhois.infosat.net was not working as of 2005/02/02" >> $OUTFILE
echo "
when this script was written." >> $OUTFILE
echo "***" >> $OUTFILE
echo "***" >> $OUTFILE
echo " " >> $OUTFILE
RWHOIS=`grep "^remarks: .*rwhois\.[^ ]\+" "$OUTFILE" | tail -n 1 |\
sed "s/\(^.*\)\(rwhois\..*\)\(:4.*\)/\2/"`
whois -h ${RWHOIS}:${PORT} "$IPADDR" >> $OUTFILE
fi
}
APNICquery() {
echo "Searching for $IPADDR in whois.apnic.net"
whois -h whois.apnic.net "$IPADDR" > $OUTFILE

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#
#
#+
#
#+
#
#
#+
#
#+

Just about every country has its own internet registrar.
I don't normally bother consulting them, because the regional registry
usually supplies sufficient information.
There are a few exceptions, where the regional registry simply
refers to the national registry for direct data.
These are Japan and South Korea in APNIC, and Brasil in LACNIC.
The following if statement checks $OUTFILE (whois.txt) for the presence
of "KR" (South Korea) or "JP" (Japan) in the country field.
If either is found, the query is re-run against the appropriate
national registry.
if grep -E "^country:[ ]+KR$" "$OUTFILE"
then
echo "Searching for $IPADDR in whois.krnic.net"
whois -h whois.krnic.net "$IPADDR" >> $OUTFILE
elif grep -E "^country:[ ]+JP$" "$OUTFILE"
then
echo "Searching for $IPADDR in whois.nic.ad.jp"
whois -h whois.nic.ad.jp "$IPADDR"/e >> $OUTFILE
fi

}
ARINquery() {
echo "Searching for $IPADDR in whois.arin.net"
whois -h whois.arin.net "$IPADDR" > $OUTFILE
# Several large internet providers listed by ARIN have their own
#+ internal whois service, referred to as "rwhois".
# A large block of IP addresses is listed with the provider
#+ under the ARIN registry.
# To get the IP addresses of 2nd-level ISPs or other large customers,
#+ one has to refer to the rwhois server on port 4321.
# I originally started with a bunch of "if" statements checking for
#+ the larger providers.
# This approach is unwieldy, and there's always another rwhois server
#+ that I didn't know about.
# A more elegant approach is to check $OUTFILE for a reference
#+ to a whois server, parse that server name out of the comment section,
#+ and re-run the query against the appropriate rwhois server.
# The parsing looks a bit ugly, with a long continued line inside
#+ backticks.
# But it only has to be done once, and will work as new servers are added.
#@
ABS Guide author comment: it isn't all that ugly, and is, in fact,
#@+ an instructive use of Regular Expressions.
if grep -E "^Comment: .*rwhois.[^ ]+" "$OUTFILE"
then
RWHOIS=`grep -e "^Comment:.*rwhois\.[^ ]\+" "$OUTFILE" | tail -n 1 |\
sed "s/^\(.*\)\(rwhois\.[^ ]\+\)\(.*$\)/\2/"`
echo "Searching for $IPADDR in ${RWHOIS}"
whois -h ${RWHOIS}:${PORT} "$IPADDR" >> $OUTFILE
fi
}
LACNICquery() {
echo "Searching for $IPADDR in whois.lacnic.net"
whois -h whois.lacnic.net "$IPADDR" > $OUTFILE
# The following if statement checks $OUTFILE (whois.txt) for the presence of
#+ "BR" (Brasil) in the country field.
# If it is found, the query is re-run against whois.registro.br.

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if grep -E "^country:[ ]+BR$" "$OUTFILE"
then
echo "Searching for $IPADDR in whois.registro.br"
whois -h whois.registro.br "$IPADDR" >> $OUTFILE
fi
}
RIPEquery() {
echo "Searching for $IPADDR in whois.ripe.net"
whois -h whois.ripe.net "$IPADDR" > $OUTFILE
}
#
#
#
#

Initialize a few variables.
* slash8 is the most significant octet
* slash16 consists of the two most significant octets
* octet2 is the second most significant octet

slash8=`echo $IPADDR | cut -d. -f 1`
if [ -z "$slash8" ] # Yet another sanity check.
then
echo "Undefined error!"
exit $E_UNDEF
fi
slash16=`echo $IPADDR | cut -d. -f 1-2`
#
^ Period specified as 'cut" delimiter.
if [ -z "$slash16" ]
then
echo "Undefined error!"
exit $E_UNDEF
fi
octet2=`echo $slash16 | cut -d. -f 2`
if [ -z "$octet2" ]
then
echo "Undefined error!"
exit $E_UNDEF
fi

#
#

Check for various odds and ends of reserved space.
There is no point in querying for those addresses.

if [ $slash8 == 0 ]; then
echo $IPADDR is '"This Network"' space\; Not querying
elif [ $slash8 == 10 ]; then
echo $IPADDR is RFC1918 space\; Not querying
elif [ $slash8 == 14 ]; then
echo $IPADDR is '"Public Data Network"' space\; Not querying
elif [ $slash8 == 127 ]; then
echo $IPADDR is loopback space\; Not querying
elif [ $slash16 == 169.254 ]; then
echo $IPADDR is link-local space\; Not querying
elif [ $slash8 == 172 ] && [ $octet2 -ge 16 ] && [ $octet2 -le 31 ];then
echo $IPADDR is RFC1918 space\; Not querying
elif [ $slash16 == 192.168 ]; then
echo $IPADDR is RFC1918 space\; Not querying
elif [ $slash8 -ge 224 ]; then
echo $IPADDR is either Multicast or reserved space\; Not querying
elif [ $slash8 -ge 200 ] && [ $slash8 -le 201 ]; then LACNICquery "$IPADDR"
elif [ $slash8 -ge 202 ] && [ $slash8 -le 203 ]; then APNICquery "$IPADDR"

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elif [ $slash8 -ge 210 ] && [ $slash8 -le 211 ]; then APNICquery "$IPADDR"
elif [ $slash8 -ge 218 ] && [ $slash8 -le 223 ]; then APNICquery "$IPADDR"
# If we got this far without making a decision, query ARIN.
# If a reference is found in $OUTFILE to APNIC, AFRINIC, LACNIC, or RIPE,
#+ query the appropriate whois server.
else
ARINquery "$IPADDR"
if grep "whois.afrinic.net" "$OUTFILE"; then
AFRINICquery "$IPADDR"
elif grep -E "^OrgID:[ ]+RIPE$" "$OUTFILE"; then
RIPEquery "$IPADDR"
elif grep -E "^OrgID:[ ]+APNIC$" "$OUTFILE"; then
APNICquery "$IPADDR"
elif grep -E "^OrgID:[ ]+LACNIC$" "$OUTFILE"; then
LACNICquery "$IPADDR"
fi
fi
#@
#
#
#@

--------------------------------------------------------------Try also:
wget http://logi.cc/nw/whois.php3?ACTION=doQuery&DOMAIN=$IPADDR
---------------------------------------------------------------

#
#

We've now finished the querying.
Echo a copy of the final result to the screen.

cat $OUTFILE
# Or "less $OUTFILE" . . .

exit 0
#@
#@
#@
#@+
#@+
#@

ABS Guide author comments:
Nothing fancy here, but still a very useful tool for hunting spammers.
Sure, the script can be cleaned up some, and it's still a bit buggy,
(exercise for reader), but all the same, it's a nice piece of coding
by Walter Dnes.
Thank you!

"Little Monster's" front end to wget.

Example A-30. Making wget easier to use
#!/bin/bash
# wgetter2.bash
#
#
#
#

Author: Little Monster [monster@monstruum.co.uk]
==> Used in ABS Guide with permission of script author.
==> This script still needs debugging and fixups (exercise for reader).
==> It could also use some additional editing in the comments.

# This is wgetter2 -#+ a Bash script to make wget a bit more friendly, and save typing.
#
#
#
#+

Carefully crafted by Little Monster.
More or less complete on 02/02/2005.
If you think this script can be improved,
email me at: monster@monstruum.co.uk

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# ==> and cc: to the author of the ABS Guide, please.
# This script is licenced under the GPL.
# You are free to copy, alter and re-use it,
#+ but please don't try to claim you wrote it.
# Log your changes here instead.
# =======================================================================
# changelog:
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#
#

07/02/2005.
02/02/2005.

Fixups by Little Monster.
Minor additions by Little Monster.
(See after # +++++++++++ )
29/01/2005. Minor stylistic edits and cleanups by author of ABS Guide.
Added exit error codes.
22/11/2004. Finished initial version of second version of wgetter:
wgetter2 is born.
01/12/2004. Changed 'runn' function so it can be run 2 ways -either ask for a file name or have one input on the CL.
01/12/2004. Made sensible handling of no URL's given.
01/12/2004. Made loop of main options, so you don't
have to keep calling wgetter 2 all the time.
Runs as a session instead.
01/12/2004. Added looping to 'runn' function.
Simplified and improved.
01/12/2004. Added state to recursion setting.
Enables re-use of previous value.
05/12/2004. Modified the file detection routine in the 'runn' function
so it's not fooled by empty values, and is cleaner.
01/02/2004. Added cookie finding routine from later version (which
isn't ready yet), so as not to have hard-coded paths.
=======================================================================

# Error codes for
E_USAGE=67
E_NO_OPTS=68
E_NO_URLS=69
E_NO_SAVEFILE=70
E_USER_EXIT=71

abnormal exit.
# Usage message, then quit.
# No command-line args entered.
# No URLs passed to script.
# No save filename passed to script.
# User decides to quit.

# Basic default wget command we want to use.
# This is the place to change it, if required.
# NB: if using a proxy, set http_proxy = yourproxy in .wgetrc.
# Otherwise delete --proxy=on, below.
# ====================================================================
CommandA="wget -nc -c -t 5 --progress=bar --random-wait --proxy=on -r"
# ====================================================================

# -------------------------------------------------------------------# Set some other variables and explain them.
pattern=" -A .jpg,.JPG,.jpeg,.JPEG,.gif,.GIF,.htm,.html,.shtml,.php"
# wget's option to only get certain types of file.
# comment out if not using
today=`date +%F`
# Used for a filename.
home=$HOME
# Set HOME to an internal variable.
# In case some other path is used, change it here.
depthDefault=3
# Set a sensible default recursion.
Depth=$depthDefault # Otherwise user feedback doesn't tie in properly.
RefA=""
# Set blank referring page.

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661

Advanced Bash-Scripting Guide
Flag=""

# Default to not saving anything,
#+ or whatever else might be wanted in future.
lister=""
# Used for passing a list of urls directly to wget.
Woptions=""
# Used for passing wget some options for itself.
inFile=""
# Used for the run function.
newFile=""
# Used for the run function.
savePath="$home/w-save"
Config="$home/.wgetter2rc"
# This is where some variables can be stored,
#+ if permanently changed from within the script.
Cookie_List="$home/.cookielist"
# So we know where the cookies are kept . . .
cFlag=""
# Part of the cookie file selection routine.
# Define the options available. Easy to change letters here if needed.
# These are the optional options; you don't just wait to be asked.
save=s
# Save command instead of executing it.
cook=c
# Change cookie file for this session.
help=h
# Usage guide.
list=l
# Pass wget the -i option and URL list.
runn=r
# Run saved commands as an argument to the option.
inpu=i
# Run saved commands interactively.
wopt=w
# Allow to enter options to pass directly to wget.
# --------------------------------------------------------------------

if [ -z
echo
echo
exit
fi

"$1" ]; then
# Make sure we get something for wget to eat.
"You must at least enter a URL or option!"
"-$help for usage."
$E_NO_OPTS

# +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
# added added added added added added added added added added added added
if [ ! -e "$Config" ]; then
# See if configuration file exists.
echo "Creating configuration file, $Config"
echo "# This is the configuration file for wgetter2" > "$Config"
echo "# Your customised settings will be saved in this file" >> "$Config"
else
source $Config
# Import variables we set outside the script.
fi
if [ ! -e "$Cookie_List" ]; then
# Set up a list of cookie files, if there isn't one.
echo "Hunting for cookies . . ."
find -name cookies.txt >> $Cookie_List # Create the list of cookie files.
fi # Isolate this in its own 'if' statement,
#+ in case we got interrupted while searching.
if [ -z "$cFlag" ]; then # If we haven't already done this . . .
echo
# Make a nice space after the command prompt.
echo "Looks like you haven't set up your source of cookies yet."
n=0
# Make sure the counter
#+ doesn't contain random values.
while read; do
Cookies[$n]=$REPLY # Put the cookie files we found into an array.
echo "$n) ${Cookies[$n]}" # Create a menu.
n=$(( n + 1 ))
# Increment the counter.

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Advanced Bash-Scripting Guide
done < $Cookie_List
# Feed the read statement.
echo "Enter the number of the cookie file you want to use."
echo "If you won't be using cookies, just press RETURN."
echo
echo "I won't be asking this again. Edit $Config"
echo "If you decide to change at a later date"
echo "or use the -${cook} option for per session changes."
read
if [ ! -z $REPLY ]; then
# User didn't just press return.
Cookie=" --load-cookies ${Cookies[$REPLY]}"
# Set the variable here as well as in the config file.
echo "Cookie=\" --load-cookies ${Cookies[$REPLY]}\"" >> $Config
fi
echo "cFlag=1" >> $Config # So we know not to ask again.
fi
# end added section end added section end added section end added section
# +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

# Another variable.
# This one may or may not be subject
# A bit like the small print.
CookiesON=$Cookie
# echo "cookie file is $CookiesON" #
# echo "home is ${home}"
#
#

to variation.

For debugging.
For debugging.
Got caught with this one!

wopts()
{
echo "Enter options to pass to wget."
echo "It is assumed you know what you're doing."
echo
echo "You can pass their arguments here too."
# That is to say, everything passed here is passed to wget.
read Wopts
# Read in the options to be passed to wget.
Woptions=" $Wopts"
#
^ Why the leading space?
# Assign to another variable.
# Just for fun, or something . . .
echo "passing options ${Wopts} to wget"
# Mainly for debugging.
# Is cute.
return
}

save_func()
{
echo "Settings will be saved."
if [ ! -d $savePath ]; then # See if directory exists.
mkdir $savePath
# Create the directory to save things in
#+ if it isn't already there.
fi

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Flag=S
# Tell the final bit of code what to do.
# Set a flag since stuff is done in main.
return
}

usage() # Tell them how it works.
{
echo "Welcome to wgetter. This is a front end to wget."
echo "It will always run wget with these options:"
echo "$CommandA"
echo "and the pattern to match: $pattern \
(which you can change at the top of this script)."
echo "It will also ask you for recursion depth, \
and if you want to use a referring page."
echo "Wgetter accepts the following options:"
echo ""
echo "-$help : Display this help."
echo "-$save : Save the command to a file $savePath/wget-($today) \
instead of running it."
echo "-$runn : Run saved wget commands instead of starting a new one -"
echo "Enter filename as argument to this option."
echo "-$inpu : Run saved wget commands interactively --"
echo "The script will ask you for the filename."
echo "-$cook : Change the cookies file for this session."
echo "-$list : Tell wget to use URL's from a list instead of \
from the command-line."
echo "-$wopt : Pass any other options direct to wget."
echo ""
echo "See the wget man page for additional options \
you can pass to wget."
echo ""
exit $E_USAGE

# End here. Don't process anything else.

}

list_func() # Gives the user the option to use the -i option to wget,
#+ and a list of URLs.
{
while [ 1 ]; do
echo "Enter the name of the file containing URL's (press q to change
your mind)."
read urlfile
if [ ! -e "$urlfile" ] && [ "$urlfile" != q ]; then
# Look for a file, or the quit option.
echo "That file does not exist!"
elif [ "$urlfile" = q ]; then # Check quit option.
echo "Not using a url list."
return
else
echo "using $urlfile."
echo "If you gave url's on the command-line, I'll use those first."
# Report wget standard behaviour to the user.
lister=" -i $urlfile" # This is what we want to pass to wget.
return
fi
done

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}

cookie_func() # Give the user the option to use a different cookie file.
{
while [ 1 ]; do
echo "Change the cookies file. Press return if you don't want to change
it."
read Cookies
# NB: this is not the same as Cookie, earlier.
# There is an 's' on the end.
# Bit like chocolate chips.
if [ -z "$Cookies" ]; then
# Escape clause for wusses.
return
elif [ ! -e "$Cookies" ]; then
echo "File does not exist. Try again." # Keep em going . . .
else
CookiesON=" --load-cookies $Cookies"
# File is good -- use it!
return
fi
done
}

run_func()
{
if [ -z "$OPTARG" ]; then
# Test to see if we used the in-line option or the query one.
if [ ! -d "$savePath" ]; then
# If directory doesn't exist . . .
echo "$savePath does not appear to exist."
echo "Please supply path and filename of saved wget commands:"
read newFile
until [ -f "$newFile" ]; do # Keep going till we get something.
echo "Sorry, that file does not exist. Please try again."
# Try really hard to get something.
read newFile
done

# ----------------------------------------------------------------------#
if [ -z ( grep wget ${newfile} ) ]; then
# Assume they haven't got the right file and bail out.
#
echo "Sorry, that file does not contain wget commands. Aborting."
#
exit
#
fi
#
# This is bogus code.
# It doesn't actually work.
# If anyone wants to fix it, feel free!
# -----------------------------------------------------------------------

filePath="${newFile}"
else
echo "Save path is $savePath"
echo "Please enter name of the file which you want to use."
echo "You have a choice of:"
ls $savePath
# Give them a choice.
read inFile
until [ -f "$savePath/$inFile" ]; do
# Keep going till
#+ we get something.

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if [ ! -f "${savePath}/${inFile}" ]; then # If file doesn't exist.
echo "Sorry, that file does not exist. Please choose from:"
ls $savePath
# If a mistake is made.
read inFile
fi
done
filePath="${savePath}/${inFile}" # Make one variable . . .
fi
else filePath="${savePath}/${OPTARG}"
fi

# Which can be many things . . .

if [ ! -f "$filePath" ]; then
# If a bogus file got through.
echo "You did not specify a suitable file."
echo "Run this script with the -${save} option first."
echo "Aborting."
exit $E_NO_SAVEFILE
fi
echo "Using: $filePath"
while read; do
eval $REPLY
echo "Completed: $REPLY"
done < $filePath # Feed the actual file we are using into a 'while' loop.
exit
}

# Fish out any options we are using for the script.
# This is based on the demo in "Learning The Bash Shell" (O'Reilly).
while getopts ":$save$cook$help$list$runn:$inpu$wopt" opt
do
case $opt in
$save) save_func;;
# Save some wgetter sessions for later.
$cook) cookie_func;; # Change cookie file.
$help) usage;;
# Get help.
$list) list_func;;
# Allow wget to use a list of URLs.
$runn) run_func;;
# Useful if you are calling wgetter from,
#+ for example, a cron script.
$inpu) run_func;;
# When you don't know what your files are named.
$wopt) wopts;;
# Pass options directly to wget.
\?) echo "Not a valid option."
echo "Use -${wopt} to pass options directly to wget,"
echo "or -${help} for help";;
# Catch anything else.
esac
done
shift $((OPTIND - 1))
# Do funky magic stuff with $#.

if [ -z "$1" ] && [ -z "$lister" ]; then
# We should be left with at least one URL
#+ on the command-line, unless a list is
#+ being used -- catch empty CL's.
echo "No URL's given! You must enter them on the same line as wgetter2."
echo "E.g., wgetter2 http://somesite http://anothersite."
echo "Use $help option for more information."
exit $E_NO_URLS
# Bail out, with appropriate error code.
fi
URLS=" $@"
# Use this so that URL list can be changed if we stay in the option loop.

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666

Advanced Bash-Scripting Guide
while [ 1 ]; do
# This is where we ask for the most used options.
# (Mostly unchanged from version 1 of wgetter)
if [ -z $curDepth ]; then
Current=""
else Current=" Current value is $curDepth"
fi
echo "How deep should I go? \
(integer: Default is $depthDefault.$Current)"
read Depth
# Recursion -- how far should we go?
inputB=""
# Reset this to blank on each pass of the loop.
echo "Enter the name of the referring page (default is none)."
read inputB # Need this for some sites.
echo "Do you want to have the output logged to the terminal"
echo "(y/n, default is yes)?"
read noHide # Otherwise wget will just log it to a file.
case $noHide in
# Now you see me, now you don't.
y|Y ) hide="";;
n|N ) hide=" -b";;
* ) hide="";;
esac
if [ -z ${Depth} ]; then
# User accepted either default or current depth,
#+ in which case Depth is now empty.
if [ -z ${curDepth} ]; then
# See if a depth was set on a previous iteration.
Depth="$depthDefault"
# Set the default recursion depth if nothing
#+ else to use.
else Depth="$curDepth" # Otherwise, set the one we used before.
fi
fi
Recurse=" -l $Depth"
# Set how deep we want to go.
curDepth=$Depth
# Remember setting for next time.
if [ ! -z $inputB ]; then
RefA=" --referer=$inputB"
fi

# Option to use referring page.

WGETTER="${CommandA}${pattern}${hide}${RefA}${Recurse}\
${CookiesON}${lister}${Woptions}${URLS}"
# Just string the whole lot together . . .
# NB: no embedded spaces.
# They are in the individual elements so that if any are empty,
#+ we don't get an extra space.
if [ -z "${CookiesON}" ] && [ "$cFlag" = "1" ] ; then
echo "Warning -- can't find cookie file"
# This should be changed,
#+ in case the user has opted to not use cookies.
fi
if [ "$Flag" = "S" ]; then
echo "$WGETTER" >> $savePath/wget-${today}
# Create a unique filename for today, or append to it if it exists.
echo "$inputB" >> $savePath/site-list-${today}
# Make a list, so it's easy to refer back to,
#+ since the whole command is a bit confusing to look at.
echo "Command saved to the file $savePath/wget-${today}"

Appendix A. Contributed Scripts

667

Advanced Bash-Scripting Guide
# Tell the user.
echo "Referring page URL saved to the file$ \
savePath/site-list-${today}"
# Tell the user.
Saver=" with save option"
# Stick this somewhere, so it appears in the loop if set.
else
echo "*****************"
echo "*****Getting*****"
echo "*****************"
echo ""
echo "$WGETTER"
echo ""
echo "*****************"
eval "$WGETTER"
fi
echo ""
echo "Starting over$Saver."
echo "If you want to stop, press q."
echo "Otherwise, enter some URL's:"
# Let them go again. Tell about save option being set.
read
case $REPLY in
# Need to change this to a 'trap' clause.
q|Q ) exit $E_USER_EXIT;; # Exercise for the reader?
* ) URLS=" $REPLY";;
esac
echo ""
done

exit 0

Example A-31. A podcasting script
#!/bin/bash
#
#
#
#+
#
#
#+
#

bashpodder.sh:
By Linc 10/1/2004
Find the latest script at
http://linc.homeunix.org:8080/scripts/bashpodder
Last revision 12/14/2004 - Many Contributors!
If you use this and have made improvements or have comments
drop me an email at linc dot fessenden at gmail dot com
I'd appreciate it!

# ==>

ABS Guide extra comments.

# ==> Author of this script has kindly granted permission
# ==>+ for inclusion in ABS Guide.

# ==> ################################################################
#
# ==> What is "podcasting"?
# ==> It's broadcasting "radio shows" over the Internet.
# ==> These shows can be played on iPods and other music file players.

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668

Advanced Bash-Scripting Guide
# ==> This script makes it possible.
# ==> See documentation at the script author's site, above.
# ==> ################################################################

# Make script crontab friendly:
cd $(dirname $0)
# ==> Change to directory where this script lives.
# datadir is the directory you want podcasts saved to:
datadir=$(date +%Y-%m-%d)
# ==> Will create a date-labeled directory, named: YYYY-MM-DD
# Check for and create datadir if necessary:
if test ! -d $datadir
then
mkdir $datadir
fi
# Delete any temp file:
rm -f temp.log
# Read the bp.conf file and wget any url not already
#+ in the podcast.log file:
while read podcast
do # ==> Main action follows.
file=$(wget -q $podcast -O - | tr '\r' '\n' | tr \' \" | \
sed -n 's/.*url="\([^"]*\)".*/\1/p')
for url in $file
do
echo $url >> temp.log
if ! grep "$url" podcast.log > /dev/null
then
wget -q -P $datadir "$url"
fi
done
done < bp.conf
# Move dynamically created log file to permanent log file:
cat podcast.log >> temp.log
sort temp.log | uniq > podcast.log
rm temp.log
# Create an m3u playlist:
ls $datadir | grep -v m3u > $datadir/podcast.m3u

exit 0
#################################################
For a different scripting approach to Podcasting,
see Phil Salkie's article,
"Internet Radio to Podcast with Shell Tools"
in the September, 2005 issue of LINUX JOURNAL,
http://www.linuxjournal.com/article/8171
#################################################

Example A-32. Nightly backup to a firewire HD
#!/bin/bash

Appendix A. Contributed Scripts

669

Advanced Bash-Scripting Guide
#
#
#
#
#
#
#
#+
#
#
#
#
#
#+
#
#+

nightly-backup.sh
http://www.richardneill.org/source.php#nightly-backup-rsync
Copyright (c) 2005 Richard Neill .
This is Free Software licensed under the GNU GPL.
==> Included in ABS Guide with script author's kind permission.
==> (Thanks!)
This does a backup from the host computer to a locally connected
firewire HDD using rsync and ssh.
(Script should work with USB-connected device (see lines 40-43).
It then rotates the backups.
Run it via cron every night at 5am.
This only backs up the home directory.
If ownerships (other than the user's) should be preserved,
then run the rsync process as root (and re-instate the -o).
We save every day for 7 days, then every week for 4 weeks,
then every month for 3 months.

# See: http://www.mikerubel.org/computers/rsync_snapshots/
#+ for more explanation of the theory.
# Save as: $HOME/bin/nightly-backup_firewire-hdd.sh
#
#
#

Known bugs:
---------i) Ideally, we want to exclude ~/.tmp and the browser caches.

# ii) If the user is sitting at the computer at 5am,
#+
and files are modified while the rsync is occurring,
#+
then the BACKUP_JUSTINCASE branch gets triggered.
#
To some extent, this is a
#+
feature, but it also causes a "disk-space leak".

##### BEGIN CONFIGURATION SECTION ############################################
LOCAL_USER=rjn
# User whose home directory should be backed up.
MOUNT_POINT=/backup
# Mountpoint of backup drive.
# NO trailing slash!
# This must be unique (eg using a udev symlink)
# MOUNT_POINT=/media/disk
# For USB-connected device.
SOURCE_DIR=/home/$LOCAL_USER # NO trailing slash - it DOES matter to rsync.
BACKUP_DEST_DIR=$MOUNT_POINT/backup/`hostname -s`.${LOCAL_USER}.nightly_backup
DRY_RUN=false
#If true, invoke rsync with -n, to do a dry run.
# Comment out or set to false for normal use.
VERBOSE=false
# If true, make rsync verbose.
# Comment out or set to false otherwise.
COMPRESS=false
# If true, compress.
# Good for internet, bad on LAN.
# Comment out or set to false otherwise.
### Exit Codes ###
E_VARS_NOT_SET=64
E_COMMANDLINE=65
E_MOUNT_FAIL=70
E_NOSOURCEDIR=71
E_UNMOUNTED=72
E_BACKUP=73
##### END CONFIGURATION SECTION ##############################################

Appendix A. Contributed Scripts

670

Advanced Bash-Scripting Guide
# Check
if [ -z
[ -z
[ -z
[ -z
then
echo
exit
fi

that all the important variables have been set:
"$LOCAL_USER" ] ||
"$SOURCE_DIR" ] ||
"$MOUNT_POINT" ] ||
"$BACKUP_DEST_DIR" ]
'One of the variables is not set! Edit the file: $0. BACKUP FAILED.'
$E_VARS_NOT_SET

if [ "$#" != 0 ] # If command-line param(s) . . .
then
# Here document(ation).
cat <<-ENDOFTEXT
Automatic Nightly backup run from cron.
Read the source for more details: $0
The backup directory is $BACKUP_DEST_DIR .
It will be created if necessary; initialisation is no longer required.
WARNING: Contents of $BACKUP_DEST_DIR are rotated.
Directories named 'backup.\$i' will eventually be DELETED.
We keep backups from every day for 7 days (1-8),
then every week for 4 weeks (9-12),
then every month for 3 months (13-15).
You may wish to add this to your crontab using 'crontab -e'
# Back up files: $SOURCE_DIR to $BACKUP_DEST_DIR
#+ every night at 3:15 am
15 03 * * * /home/$LOCAL_USER/bin/nightly-backup_firewire-hdd.sh
Don't forget to verify the backups are working,
especially if you don't read cron's mail!"
ENDOFTEXT
exit $E_COMMANDLINE
fi

# Parse the options.
# ==================
if [ "$DRY_RUN" == "true" ]; then
DRY_RUN="-n"
echo "WARNING:"
echo "THIS IS A 'DRY RUN'!"
echo "No data will actually be transferred!"
else
DRY_RUN=""
fi
if [ "$VERBOSE" == "true" ]; then
VERBOSE="-v"
else
VERBOSE=""
fi
if [ "$COMPRESS" == "true" ]; then
COMPRESS="-z"
else
COMPRESS=""
fi

#

Every week (actually of 8 days) and every month,

Appendix A. Contributed Scripts

671

Advanced Bash-Scripting Guide
#+ extra backups are preserved.
DAY_OF_MONTH=`date +%d`
# Day of month (01..31).
if [ $DAY_OF_MONTH = 01 ]; then
# First of month.
MONTHSTART=true
elif [ $DAY_OF_MONTH = 08 \
-o $DAY_OF_MONTH = 16 \
-o $DAY_OF_MONTH = 24 ]; then
# Day 8,16,24 (use 8, not 7 to better handle 31-day months)
WEEKSTART=true
fi

# Check that the HDD is mounted.
# At least, check that *something* is mounted here!
# We can use something unique to the device, rather than just guessing
#+ the scsi-id by having an appropriate udev rule in
#+ /etc/udev/rules.d/10-rules.local
#+ and by putting a relevant entry in /etc/fstab.
# Eg: this udev rule:
# BUS="scsi", KERNEL="sd*", SYSFS{vendor}="WDC WD16",
# SYSFS{model}="00JB-00GVA0
", NAME="%k", SYMLINK="lacie_1394d%n"
if mount | grep $MOUNT_POINT >/dev/null; then
echo "Mount point $MOUNT_POINT is indeed mounted. OK"
else
echo -n "Attempting to mount $MOUNT_POINT..."
# If it isn't mounted, try to mount it.
sudo mount $MOUNT_POINT 2>/dev/null
if mount | grep $MOUNT_POINT >/dev/null; then
UNMOUNT_LATER=TRUE
echo "OK"
# Note: Ensure that this is also unmounted
#+ if we exit prematurely with failure.
else
echo "FAILED"
echo -e "Nothing is mounted at $MOUNT_POINT. BACKUP FAILED!"
exit $E_MOUNT_FAIL
fi
fi

# Check that source dir exists and is readable.
if [ ! -r $SOURCE_DIR ] ; then
echo "$SOURCE_DIR does not exist, or cannot be read. BACKUP FAILED."
exit $E_NOSOURCEDIR
fi

#
#
#
#

Check that the backup directory structure is as it should be.
If not, create it.
Create the subdirectories.
Note that backup.0 will be created as needed by rsync.

for ((i=1;i<=15;i++)); do
if [ ! -d $BACKUP_DEST_DIR/backup.$i ]; then
if /bin/mkdir -p $BACKUP_DEST_DIR/backup.$i ; then
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ No [ ] test brackets. Why?
echo "Warning: directory $BACKUP_DEST_DIR/backup.$i is missing,"
echo "or was not initialised. (Re-)creating it."
else

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672

Advanced Bash-Scripting Guide
echo "ERROR: directory $BACKUP_DEST_DIR/backup.$i"
echo "is missing and could not be created."
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
# Before we exit, unmount the mount point if necessary.
cd
sudo umount $MOUNT_POINT &&
echo "Unmounted $MOUNT_POINT again. Giving up."
fi
exit $E_UNMOUNTED
fi
fi
done

# Set the permission to 700 for security
#+ on an otherwise permissive multi-user system.
if ! /bin/chmod 700 $BACKUP_DEST_DIR ; then
echo "ERROR: Could not set permissions on $BACKUP_DEST_DIR to 700."
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
# Before we exit, unmount the mount point if necessary.
cd ; sudo umount $MOUNT_POINT \
&& echo "Unmounted $MOUNT_POINT again. Giving up."
fi
exit $E_UNMOUNTED
fi
# Create the symlink: current -> backup.1 if required.
# A failure here is not critical.
cd $BACKUP_DEST_DIR
if [ ! -h current ] ; then
if ! /bin/ln -s backup.1 current ; then
echo "WARNING: could not create symlink current -> backup.1"
fi
fi

# Now, do the rsync.
echo "Now doing backup with rsync..."
echo "Source dir: $SOURCE_DIR"
echo -e "Backup destination dir: $BACKUP_DEST_DIR\n"

/usr/bin/rsync $DRY_RUN $VERBOSE -a -S --delete --modify-window=60 \
--link-dest=../backup.1 $SOURCE_DIR $BACKUP_DEST_DIR/backup.0/
#
#+
#
#
#
#+

Only warn, rather than exit if the rsync
since it may only be a minor problem.
E.g., if one file is not readable, rsync
This shouldn't prevent the rotation.
Not using, e.g., `date +%a` since these
are just full of links and don't consume

failed,
will fail.
directories
*that much* space.

if [ $? != 0 ]; then
BACKUP_JUSTINCASE=backup.`date +%F_%T`.justincase
echo "WARNING: the rsync process did not entirely succeed."
echo "Something might be wrong."
echo "Saving an extra copy at: $BACKUP_JUSTINCASE"
echo "WARNING: if this occurs regularly, a LOT of space will be consumed,"
echo "even though these are just hard-links!"
fi

Appendix A. Contributed Scripts

673

Advanced Bash-Scripting Guide
# Save a readme in the backup parent directory.
# Save another one in the recent subdirectory.
echo "Backup of $SOURCE_DIR on `hostname` was last run on \
`date`" > $BACKUP_DEST_DIR/README.txt
echo "This backup of $SOURCE_DIR on `hostname` was created on \
`date`" > $BACKUP_DEST_DIR/backup.0/README.txt
# If we are not in a dry run, rotate the backups.
[ -z "$DRY_RUN" ] &&
# Check how full the backup disk is.
# Warn if 90%. if 98% or more, we'll probably fail, so give up.
# (Note: df can output to more than one line.)
# We test this here, rather than before
#+ so that rsync may possibly have a chance.
DISK_FULL_PERCENT=`/bin/df $BACKUP_DEST_DIR |
tr "\n" ' ' | awk '{print $12}' | grep -oE [0-9]+ `
echo "Disk space check on backup partition \
$MOUNT_POINT $DISK_FULL_PERCENT% full."
if [ $DISK_FULL_PERCENT -gt 90 ]; then
echo "Warning: Disk is greater than 90% full."
fi
if [ $DISK_FULL_PERCENT -gt 98 ]; then
echo "Error: Disk is full! Giving up."
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
# Before we exit, unmount the mount point if necessary.
cd; sudo umount $MOUNT_POINT &&
echo "Unmounted $MOUNT_POINT again. Giving up."
fi
exit $E_UNMOUNTED
fi

# Create an extra backup.
# If this copy fails, give up.
if [ -n "$BACKUP_JUSTINCASE" ]; then
if ! /bin/cp -al $BACKUP_DEST_DIR/backup.0 \
$BACKUP_DEST_DIR/$BACKUP_JUSTINCASE
then
echo "ERROR: Failed to create extra copy \
$BACKUP_DEST_DIR/$BACKUP_JUSTINCASE"
if [ "$UNMOUNT_LATER" == "TRUE" ]; then
# Before we exit, unmount the mount point if necessary.
cd ;sudo umount $MOUNT_POINT &&
echo "Unmounted $MOUNT_POINT again. Giving up."
fi
exit $E_UNMOUNTED
fi
fi

# At start of month, rotate the oldest 8.
if [ "$MONTHSTART" == "true" ]; then
echo -e "\nStart of month. \
Removing oldest backup: $BACKUP_DEST_DIR/backup.15" &&
/bin/rm -rf $BACKUP_DEST_DIR/backup.15 &&
echo "Rotating monthly,weekly backups: \
$BACKUP_DEST_DIR/backup.[8-14] -> $BACKUP_DEST_DIR/backup.[9-15]" &&
/bin/mv $BACKUP_DEST_DIR/backup.14 $BACKUP_DEST_DIR/backup.15 &&
/bin/mv $BACKUP_DEST_DIR/backup.13 $BACKUP_DEST_DIR/backup.14 &&
/bin/mv $BACKUP_DEST_DIR/backup.12 $BACKUP_DEST_DIR/backup.13 &&

Appendix A. Contributed Scripts

674

Advanced Bash-Scripting Guide
/bin/mv
/bin/mv
/bin/mv
/bin/mv

$BACKUP_DEST_DIR/backup.11 $BACKUP_DEST_DIR/backup.12 &&
$BACKUP_DEST_DIR/backup.10 $BACKUP_DEST_DIR/backup.11 &&
$BACKUP_DEST_DIR/backup.9 $BACKUP_DEST_DIR/backup.10 &&
$BACKUP_DEST_DIR/backup.8 $BACKUP_DEST_DIR/backup.9

# At start of week, rotate the second-oldest 4.
elif [ "$WEEKSTART" == "true" ]; then
echo -e "\nStart of week. \
Removing oldest weekly backup: $BACKUP_DEST_DIR/backup.12"
/bin/rm -rf $BACKUP_DEST_DIR/backup.12 &&

&&

echo "Rotating weekly backups: \
$BACKUP_DEST_DIR/backup.[8-11] -> $BACKUP_DEST_DIR/backup.[9-12]" &&
/bin/mv $BACKUP_DEST_DIR/backup.11 $BACKUP_DEST_DIR/backup.12 &&
/bin/mv $BACKUP_DEST_DIR/backup.10 $BACKUP_DEST_DIR/backup.11 &&
/bin/mv $BACKUP_DEST_DIR/backup.9 $BACKUP_DEST_DIR/backup.10 &&
/bin/mv $BACKUP_DEST_DIR/backup.8 $BACKUP_DEST_DIR/backup.9
else
echo -e "\nRemoving oldest daily backup: $BACKUP_DEST_DIR/backup.8"
/bin/rm -rf $BACKUP_DEST_DIR/backup.8
fi

&&

&&

# Every day, rotate the newest 8.
echo "Rotating daily backups: \
$BACKUP_DEST_DIR/backup.[1-7] -> $BACKUP_DEST_DIR/backup.[2-8]"
/bin/mv $BACKUP_DEST_DIR/backup.7 $BACKUP_DEST_DIR/backup.8
/bin/mv $BACKUP_DEST_DIR/backup.6 $BACKUP_DEST_DIR/backup.7
/bin/mv $BACKUP_DEST_DIR/backup.5 $BACKUP_DEST_DIR/backup.6
/bin/mv $BACKUP_DEST_DIR/backup.4 $BACKUP_DEST_DIR/backup.5
/bin/mv $BACKUP_DEST_DIR/backup.3 $BACKUP_DEST_DIR/backup.4
/bin/mv $BACKUP_DEST_DIR/backup.2 $BACKUP_DEST_DIR/backup.3
/bin/mv $BACKUP_DEST_DIR/backup.1 $BACKUP_DEST_DIR/backup.2
/bin/mv $BACKUP_DEST_DIR/backup.0 $BACKUP_DEST_DIR/backup.1

&&
&&
&&
&&
&&
&&
&&
&&
&&

SUCCESS=true

if

[ "$UNMOUNT_LATER" == "TRUE" ]; then
# Unmount the mount point if it wasn't mounted to begin with.
cd ; sudo umount $MOUNT_POINT && echo "Unmounted $MOUNT_POINT again."

fi

if [ "$SUCCESS" == "true" ]; then
echo 'SUCCESS!'
exit 0
fi
# Should have already exited if backup worked.
echo 'BACKUP FAILED! Is this just a dry run? Is the disk full?) '
exit $E_BACKUP

Example A-33. An expanded cd command
###########################################################################
#
#
cdll
#
by Phil Braham
#

Appendix A. Contributed Scripts

675

Advanced Bash-Scripting Guide
#
############################################
#
Latest version of this script available from
#
http://freshmeat.net/projects/cd/
#
############################################
#
#
.cd_new
#
#
An enhancement of the Unix cd command
#
#
There are unlimited stack entries and special entries. The stack
#
entries keep the last cd_maxhistory
#
directories that have been used. The special entries can be
#
assigned to commonly used directories.
#
#
The special entries may be pre-assigned by setting the environment
#
variables CDSn or by using the -u or -U command.
#
#
The following is a suggestion for the .profile file:
#
#
. cdll
# Set up the cd command
#
alias cd='cd_new'
# Replace the cd command
#
cd -U
# Upload pre-assigned entries for
#
#+ the stack and special entries
#
cd -D
# Set non-default mode
#
alias @="cd_new @" # Allow @ to be used to get history
#
#
For help type:
#
#
cd -h or
#
cd -H
#
#
###########################################################################
#
#
Version 1.2.1
#
#
Written by Phil Braham - Realtime Software Pty Ltd
#
(realtime@mpx.com.au)
#
Please send any suggestions or enhancements to the author (also at
#
phil@braham.net)
#
############################################################################
cd_hm ()
{
${PRINTF} "%s" "cd [dir] [0-9] [@[s|h] [-g []] [-d] \
[-D] [-r] [dir|0-9] [-R] [|0-9]
[-s] [-S] [-u] [-U] [-f] [-F] [-h] [-H] [-v]
 Go to directory
0-n
Go to previous directory (0 is previous, 1 is last but 1 etc)
n is up to max history (default is 50)
@
List history and special entries
@h
List history entries
@s
List special entries
-g [] Go to literal name (bypass special names)
This is to allow access to dirs called '0','1','-h' etc
-d
Change default action - verbose. (See note)
-D
Change default action - silent. (See note)
-s Go to the special entry *
-S Go to the special entry 
and replace it with the current dir*
-r [] Go to directory 

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676

Advanced Bash-Scripting Guide
and then put it on special entry *
-R [] Go to directory 
and put current dir on special entry *
-a
Alternative suggested directory. See note below.
-f [] File entries to .
-u [] Update entries from .
If no filename supplied then default file
(${CDPath}${2:-"$CDFile"}) is used
-F and -U are silent versions
-v
Print version number
-h
Help
-H
Detailed help
*The special entries (0 - 9) are held until log off, replaced by another
entry or updated with the -u command
Alternative suggested directories:
If a directory is not found then CD will suggest any
possibilities. These are directories starting with the same letters
and if any are found they are listed prefixed with -a
where  is a number.
It's possible to go to the directory by entering cd -a
on the command line.
The directory for -r or -R may be a number.
For example:
$ cd -r3 4 Go to history entry 4 and put it on special entry 3
$ cd -R3 4 Put current dir on the special entry 3
and go to history entry 4
$ cd -s3
Go to special entry 3
Note that commands R,r,S and s may be
and refer to 0:
$ cd -s
Go to special entry 0
$ cd -S
Go to special entry 0
entry 0 current dir
$ cd -r 1
Go to history entry 1
$ cd -r
Go to history entry 0
"
if ${TEST} "$CD_MODE" = "PREV"
then
${PRINTF} "$cd_mnset"
else
${PRINTF} "$cd_mset"
fi

used without a number

and make special
and put it on special entry 0
and put it on special entry 0

}
cd_Hm ()
{
cd_hm
${PRINTF} "%s" "
The previous directories (0-$cd_maxhistory) are stored in the
environment variables CD[0] - CD[$cd_maxhistory]
Similarly the special directories S0 - $cd_maxspecial are in
the environment variable CDS[0] - CDS[$cd_maxspecial]
and may be accessed from the command line
The default pathname for the -f and -u commands is $CDPath
The default filename for the -f and -u commands is $CDFile
Set the following environment variables:
CDL_PROMPTLEN - Set to the length of prompt you require.

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Advanced Bash-Scripting Guide
Prompt string is set to the right characters of the
current directory.
If not set then prompt is left unchanged
CDL_PROMPT_PRE - Set to the string to prefix the prompt.
Default is:
non-root: \"\\[\\e[01;34m\\]\" (sets colour to blue).
root:
\"\\[\\e[01;31m\\]\" (sets colour to red).
CDL_PROMPT_POST
- Set to the string to suffix the prompt.
Default is:
non-root: \"\\[\\e[00m\\]$\"
(resets colour and displays $).
root:
\"\\[\\e[00m\\]#\"
(resets colour and displays #).
CDPath - Set the default path for the -f & -u options.
Default is home directory
CDFile - Set the default filename for the -f & -u options.
Default is cdfile
"
cd_version
}
cd_version ()
{
printf "Version: ${VERSION_MAJOR}.${VERSION_MINOR} Date: ${VERSION_DATE}\n"
}
#
# Truncate right.
#
# params:
#
p1 - string
#
p2 - length to truncate to
#
# returns string in tcd
#
cd_right_trunc ()
{
local tlen=${2}
local plen=${#1}
local str="${1}"
local diff
local filler="<--"
if ${TEST} ${plen} -le ${tlen}
then
tcd="${str}"
else
let diff=${plen}-${tlen}
elen=3
if ${TEST} ${diff} -le 2
then
let elen=${diff}
fi
tlen=-${tlen}
let tlen=${tlen}+${elen}
tcd=${filler:0:elen}${str:tlen}
fi
}
#
# Three versions of do history:

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#
cd_dohistory - packs history and specials side by side
#
cd_dohistoryH - Shows only hstory
#
cd_dohistoryS - Shows only specials
#
cd_dohistory ()
{
cd_getrc
${PRINTF} "History:\n"
local -i count=${cd_histcount}
while ${TEST} ${count} -ge 0
do
cd_right_trunc "${CD[count]}" ${cd_lchar}
${PRINTF} "%2d %-${cd_lchar}.${cd_lchar}s " ${count} "${tcd}"
cd_right_trunc "${CDS[count]}" ${cd_rchar}
${PRINTF} "S%d %-${cd_rchar}.${cd_rchar}s\n" ${count} "${tcd}"
count=${count}-1
done
}
cd_dohistoryH ()
{
cd_getrc
${PRINTF} "History:\n"
local -i count=${cd_maxhistory}
while ${TEST} ${count} -ge 0
do
${PRINTF} "${count} %-${cd_flchar}.${cd_flchar}s\n" ${CD[$count]}
count=${count}-1
done
}
cd_dohistoryS ()
{
cd_getrc
${PRINTF} "Specials:\n"
local -i count=${cd_maxspecial}
while ${TEST} ${count} -ge 0
do
${PRINTF} "S${count} %-${cd_flchar}.${cd_flchar}s\n" ${CDS[$count]}
count=${count}-1
done
}
cd_getrc ()
{
cd_flchar=$(stty -a | awk -F \;
'/rows/ { print $2 $3 }' | awk -F \ '{ print $4 }')
if ${TEST} ${cd_flchar} -ne 0
then
cd_lchar=${cd_flchar}/2-5
cd_rchar=${cd_flchar}/2-5
cd_flchar=${cd_flchar}-5
else
cd_flchar=${FLCHAR:=75}
# cd_flchar is used for for the @s & @h history
cd_lchar=${LCHAR:=35}
cd_rchar=${RCHAR:=35}
fi
}
cd_doselection ()

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{
local -i nm=0
cd_doflag="TRUE"
if ${TEST} "${CD_MODE}" = "PREV"
then
if ${TEST} -z "$cd_npwd"
then
cd_npwd=0
fi
fi
tm=$(echo "${cd_npwd}" | cut -b 1)
if ${TEST} "${tm}" = "-"
then
pm=$(echo "${cd_npwd}" | cut -b 2)
nm=$(echo "${cd_npwd}" | cut -d $pm -f2)
case "${pm}" in
a) cd_npwd=${cd_sugg[$nm]} ;;
s) cd_npwd="${CDS[$nm]}" ;;
S) cd_npwd="${CDS[$nm]}" ; CDS[$nm]=`pwd` ;;
r) cd_npwd="$2" ; cd_specDir=$nm ; cd_doselection "$1" "$2";;
R) cd_npwd="$2" ; CDS[$nm]=`pwd` ; cd_doselection "$1" "$2";;
esac
fi
if ${TEST} "${cd_npwd}" != "." -a "${cd_npwd}" \
!= ".." -a "${cd_npwd}" -le ${cd_maxhistory} >>/dev/null 2>&1
then
cd_npwd=${CD[$cd_npwd]}
else
case "$cd_npwd" in
@) cd_dohistory ; cd_doflag="FALSE" ;;
@h) cd_dohistoryH ; cd_doflag="FALSE" ;;
@s) cd_dohistoryS ; cd_doflag="FALSE" ;;
-h) cd_hm ; cd_doflag="FALSE" ;;
-H) cd_Hm ; cd_doflag="FALSE" ;;
-f) cd_fsave "SHOW" $2 ; cd_doflag="FALSE" ;;
-u) cd_upload "SHOW" $2 ; cd_doflag="FALSE" ;;
-F) cd_fsave "NOSHOW" $2 ; cd_doflag="FALSE" ;;
-U) cd_upload "NOSHOW" $2 ; cd_doflag="FALSE" ;;
-g) cd_npwd="$2" ;;
-d) cd_chdefm 1; cd_doflag="FALSE" ;;
-D) cd_chdefm 0; cd_doflag="FALSE" ;;
-r) cd_npwd="$2" ; cd_specDir=0 ; cd_doselection "$1" "$2";;
-R) cd_npwd="$2" ; CDS[0]=`pwd` ; cd_doselection "$1" "$2";;
-s) cd_npwd="${CDS[0]}" ;;
-S) cd_npwd="${CDS[0]}" ; CDS[0]=`pwd` ;;
-v) cd_version ; cd_doflag="FALSE";;
esac
fi
}
cd_chdefm ()
{
if ${TEST} "${CD_MODE}" = "PREV"
then
CD_MODE=""
if ${TEST} $1 -eq 1
then
${PRINTF} "${cd_mset}"
fi
else
CD_MODE="PREV"

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Advanced Bash-Scripting Guide
if ${TEST} $1 -eq 1
then
${PRINTF} "${cd_mnset}"
fi
fi
}
cd_fsave ()
{
local sfile=${CDPath}${2:-"$CDFile"}
if ${TEST} "$1" = "SHOW"
then
${PRINTF} "Saved to %s\n" $sfile
fi
${RM} -f ${sfile}
local -i count=0
while ${TEST} ${count} -le ${cd_maxhistory}
do
echo "CD[$count]=\"${CD[$count]}\"" >> ${sfile}
count=${count}+1
done
count=0
while ${TEST} ${count} -le ${cd_maxspecial}
do
echo "CDS[$count]=\"${CDS[$count]}\"" >> ${sfile}
count=${count}+1
done
}
cd_upload ()
{
local sfile=${CDPath}${2:-"$CDFile"}
if ${TEST} "${1}" = "SHOW"
then
${PRINTF} "Loading from %s\n" ${sfile}
fi
. ${sfile}
}
cd_new ()
{
local -i count
local -i choose=0
cd_npwd="${1}"
cd_specDir=-1
cd_doselection "${1}" "${2}"
if ${TEST} ${cd_doflag} = "TRUE"
then
if ${TEST} "${CD[0]}" != "`pwd`"
then
count=$cd_maxhistory
while ${TEST} $count -gt 0
do
CD[$count]=${CD[$count-1]}
count=${count}-1
done
CD[0]=`pwd`
fi
command cd "${cd_npwd}" 2>/dev/null
if ${TEST} $? -eq 1

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Advanced Bash-Scripting Guide
then
${PRINTF} "Unknown dir: %s\n" "${cd_npwd}"
local -i ftflag=0
for i in "${cd_npwd}"*
do
if ${TEST} -d "${i}"
then
if ${TEST} ${ftflag} -eq 0
then
${PRINTF} "Suggest:\n"
ftflag=1
fi
${PRINTF} "\t-a${choose} %s\n" "$i"
cd_sugg[$choose]="${i}"
choose=${choose}+1
fi
done
fi
fi
if ${TEST} ${cd_specDir} -ne -1
then
CDS[${cd_specDir}]=`pwd`
fi
if ${TEST} ! -z "${CDL_PROMPTLEN}"
then
cd_right_trunc "${PWD}" ${CDL_PROMPTLEN}
cd_rp=${CDL_PROMPT_PRE}${tcd}${CDL_PROMPT_POST}
export PS1="$(echo -ne ${cd_rp})"
fi
}
#########################################################################
#
#
#
Initialisation here
#
#
#
#########################################################################
#
VERSION_MAJOR="1"
VERSION_MINOR="2.1"
VERSION_DATE="24-MAY-2003"
#
alias cd=cd_new
#
# Set up commands
RM=/bin/rm
TEST=test
PRINTF=printf
# Use builtin printf
#########################################################################
#
#
# Change this to modify the default pre- and post prompt strings.
#
# These only come into effect if CDL_PROMPTLEN is set.
#
#
#
#########################################################################
if ${TEST} ${EUID} -eq 0
then
#
CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="$HOSTNAME@"}
CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="\\[\\e[01;31m\\]"} # Root is in red
CDL_PROMPT_POST=${CDL_PROMPT_POST:="\\[\\e[00m\\]#"}
else
CDL_PROMPT_PRE=${CDL_PROMPT_PRE:="\\[\\e[01;34m\\]"} # Users in blue

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CDL_PROMPT_POST=${CDL_PROMPT_POST:="\\[\\e[00m\\]$"}
fi
#########################################################################
#
# cd_maxhistory defines the max number of history entries allowed.
typeset -i cd_maxhistory=50
#########################################################################
#
# cd_maxspecial defines the number of special entries.
typeset -i cd_maxspecial=9
#
#
#########################################################################
#
# cd_histcount defines the number of entries displayed in
#+ the history command.
typeset -i cd_histcount=9
#
#########################################################################
export CDPath=${HOME}/
# Change these to use a different
#
#+ default path and filename
#
export CDFile=${CDFILE:=cdfile}
# for the -u and -f commands #
#
#########################################################################
#
typeset -i cd_lchar cd_rchar cd_flchar
# This is the number of chars to allow for the #
cd_flchar=${FLCHAR:=75} #+ cd_flchar is used for for the @s & @h history#
typeset -ax CD CDS
#
cd_mset="\n\tDefault mode is now set - entering cd with no parameters \
has the default action\n\tUse cd -d or -D for cd to go to \
previous directory with no parameters\n"
cd_mnset="\n\tNon-default mode is now set - entering cd with no \
parameters is the same as entering cd 0\n\tUse cd -d or \
-D to change default cd action\n"
# ==================================================================== #

: </cdll
For example if cdll is in your local home directory:
. ~/cdll
If in /usr/bin then:
. /usr/bin/cdll
If you want to use this instead of the buitin cd command then add:
alias cd='cd_new'
We would also recommend the following commands:
alias @='cd_new @'
cd -U
cd -D
If you want to use cdll's prompt facilty then add the following:
CDL_PROMPTLEN=nn
Where nn is a number described below. Initially 99 would be suitable
number.
Thus the script looks something like this:
######################################################################
# CD Setup

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Advanced Bash-Scripting Guide
######################################################################
CDL_PROMPTLEN=21
# Allow a prompt length of up to 21 characters
. /usr/bin/cdll
# Initialise cdll
alias cd='cd_new'
# Replace the built in cd command
alias @='cd_new @'
# Allow @ at the prompt to display history
cd -U
# Upload directories
cd -D
# Set default action to non-posix
######################################################################
The full meaning of these commands will become clear later.
There are a couple of caveats. If another program changes the directory
without calling cdll, then the directory won't be put on the stack and
also if the prompt facility is used then this will not be updated. Two
programs that can do this are pushd and popd. To update the prompt and
stack simply enter:
cd .
Note that if the previous entry on the stack is the current directory
then the stack is not updated.
Usage
=====
cd [dir] [0-9] [@[s|h] [-g ] [-d] [-D] [-r]
[dir|0-9] [-R] [|0-9] [-s] [-S]
[-u] [-U] [-f] [-F] [-h] [-H] [-v]

0-n

Go to directory
Goto previous directory (0 is previous,
1 is last but 1, etc.)
n is up to max history (default is 50)
@
List history and special entries (Usually available as $ @)
@h
List history entries
@s
List special entries
-g [] Go to literal name (bypass special names)
This is to allow access to dirs called '0','1','-h' etc
-d
Change default action - verbose. (See note)
-D
Change default action - silent. (See note)
-s
Go to the special entry 
-S
Go to the special entry 
and replace it with the current dir
-r [] Go to directory 
and then put it on special entry 
-R [] Go to directory 
and put current dir on special entry 
-a
Alternative suggested directory. See note below.
-f [] File entries to .
-u [] Update entries from .
If no filename supplied then default file (~/cdfile) is used
-F and -U are silent versions
-v
Print version number
-h
Help
-H
Detailed help

Examples
========
These examples assume non-default mode is set (that is, cd with no
parameters will go to the most recent stack directory), that aliases

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Advanced Bash-Scripting Guide
have been set up for cd and @ as described above and that cd's prompt
facility is active and the prompt length is 21 characters.
/home/phil$ @
# List the entries with the @
History:
# Output of the @ command
.....
# Skipped these entries for brevity
1 /home/phil/ummdev
S1 /home/phil/perl
# Most recent two history entries
0 /home/phil/perl/eg
S0 /home/phil/umm/ummdev
# and two special entries are shown
/home/phil$ cd /home/phil/utils/Cdll
# Now change directories
/home/phil/utils/Cdll$ @
# Prompt reflects the directory.
History:
# New history
.....
1 /home/phil/perl/eg
S1 /home/phil/perl
# History entry 0 has moved to 1
0 /home/phil
S0 /home/phil/umm/ummdev
# and the most recent has entered
To go to a history entry:
/home/phil/utils/Cdll$ cd 1
# Go to history entry 1.
/home/phil/perl/eg$
# Current directory is now what was 1
To go to a special entry:
/home/phil/perl/eg$ cd -s1
# Go to special entry 1
/home/phil/umm/ummdev$
# Current directory is S1
To go to a directory called, for example, 1:
/home/phil$ cd -g 1
# -g ignores the special meaning of 1
/home/phil/1$
To put current directory on the special list as S1:
cd -r1 .
# OR
cd -R1 .
# These have the same effect if the directory is
#+ . (the current directory)
To go to a directory and add it as a special
The directory for -r or -R may be a number.
For example:
$ cd -r3 4 Go to history entry 4 and put it on special entry 3
$ cd -R3 4 Put current dir on the special entry 3 and go to
history entry 4
$ cd -s3
Go to special entry 3
Note that commands R,r,S and s may be used without a number and
refer to 0:
$ cd -s
Go to special entry 0

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Advanced Bash-Scripting Guide
$ cd -S
$ cd -r 1
$ cd -r

Go to special entry 0 and make special entry 0
current dir
Go to history entry 1 and put it on special entry 0
Go to history entry 0 and put it on special entry 0

Alternative suggested directories:
If a directory
possibilities.
and if any are
where  is a
by entering cd

is not found, then CD will suggest any
These are directories starting with the same letters
found they are listed prefixed with -a
number. It's possible to go to the directory
-a on the command line.

Use cd -d or -D to change default cd action. cd -H will show
current action.
The history entries (0-n) are stored in the environment variables
CD[0] - CD[n]
Similarly the special directories S0 - 9 are in the environment
variable CDS[0] - CDS[9]
and may be accessed from the command line, for example:
ls -l ${CDS[3]}
cat ${CD[8]}/file.txt
The default pathname for the -f and -u commands is ~
The default filename for the -f and -u commands is cdfile

Configuration
=============
The following environment variables can be set:
CDL_PROMPTLEN - Set to the length of prompt you require.
Prompt string is set to the right characters of the current
directory. If not set, then prompt is left unchanged. Note
that this is the number of characters that the directory is
shortened to, not the total characters in the prompt.
CDL_PROMPT_PRE - Set to the string to prefix the prompt.
Default is:
non-root: "\\[\\e[01;34m\\]" (sets colour to blue).
root:
"\\[\\e[01;31m\\]" (sets colour to red).
CDL_PROMPT_POST
Default is:
non-root:
root:

- Set to the string to suffix the prompt.
"\\[\\e[00m\\]$"
(resets colour and displays $).
"\\[\\e[00m\\]#"
(resets colour and displays #).

Note:
CDL_PROMPT_PRE & _POST only t
CDPath - Set the
Default
CDFile - Set the
Default

default path for the -f & -u options.
is home directory
default filename for the -f & -u options.
is cdfile

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Advanced Bash-Scripting Guide
There are three variables defined in the file cdll which control the
number of entries stored or displayed. They are in the section labeled
'Initialisation here' towards the end of the file.
cd_maxhistory
cd_maxspecial
cd_histcount

- The number
Default is
- The number
Default is
- The number
displayed.

of history
50.
of special
9.
of history
Default is

entries stored.
entries allowed.
and special entries
9.

Note that cd_maxspecial should be >= cd_histcount to avoid displaying
special entries that can't be set.

Version: 1.2.1 Date: 24-MAY-2003
DOCUMENTATION

Example A-34. A soundcard setup script
#!/bin/bash
# soundcard-on.sh
#
#
#
#
#
#+
#+

Script author: Mkarcher
http://www.thinkwiki.org/wiki ...
/Script_for_configuring_the_CS4239_sound_chip_in_PnP_mode
ABS Guide author made minor changes and added comments.
Couldn't contact script author to ask for permission to use, but ...
the script was released under the FDL,
so its use here should be both legal and ethical.

#
#+
#+
#+
#
#
#
#
#+
#+

Sound-via-pnp-script for Thinkpad 600E
and possibly other computers with onboard CS4239/CS4610
that do not work with the PCI driver
and are not recognized by the PnP code of snd-cs4236.
Also for some 770-series Thinkpads, such as the 770x.
Run as root user, of course.

#

Search for sound card pnp device:

These are old and very obsolete laptop computers,
but this particular script is very instructive,
as it shows how to set up and hack device files.

for dev in /sys/bus/pnp/devices/*
do
grep CSC0100 $dev/id > /dev/null && WSSDEV=$dev
grep CSC0110 $dev/id > /dev/null && CTLDEV=$dev
done
# On 770x:
# WSSDEV = /sys/bus/pnp/devices/00:07
# CTLDEV = /sys/bus/pnp/devices/00:06
# These are symbolic links to /sys/devices/pnp0/ ...

# Activate devices:
# Thinkpad boots with devices disabled unless "fast boot" is turned off
#+ (in BIOS).

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Advanced Bash-Scripting Guide
echo activate > $WSSDEV/resources
echo activate > $CTLDEV/resources

# Parse resource settings.
{ read # Discard "state = active" (see below).
read bla port1
read bla port2
read bla port3
read bla irq
read bla dma1
read bla dma2
# The "bla's" are labels in the first field: "io," "state," etc.
# These are discarded.
# Hack: with PnPBIOS: ports are: port1: WSS, port2:
#+ OPL, port3: sb (unneeded)
#
with ACPI-PnP:ports are: port1: OPL, port2: sb, port3: WSS
# (ACPI bios seems to be wrong here, the PnP-card-code in snd-cs4236.c
#+ uses the PnPBIOS port order)
# Detect port order using the fixed OPL port as reference.
if [ ${port2%%-*} = 0x388 ]
#
^^^^ Strip out everything following hyphen in port address.
#
So, if port1 is 0x530-0x537
#+
we're left with 0x530 -- the start address of the port.
then
# PnPBIOS: usual order
port=${port1%%-*}
oplport=${port2%%-*}
else
# ACPI: mixed-up order
port=${port3%%-*}
oplport=${port1%%-*}
fi
} < $WSSDEV/resources
# To see what's going on here:
# --------------------------#
cat /sys/devices/pnp0/00:07/resources
#
#
state = active
#
io 0x530-0x537
#
io 0x388-0x38b
#
io 0x220-0x233
#
irq 5
#
dma 1
#
dma 0
#
^^^
"bla" labels in first field (discarded).

{ read # Discard first line, as above.
read bla port1
cport=${port1%%-*}
#
^^^^
# Just want _start_ address of port.
} < $CTLDEV/resources

# Load the module:
modprobe --ignore-install snd-cs4236 port=$port cport=$cport\

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Advanced Bash-Scripting Guide
fm_port=$oplport irq=$irq dma1=$dma1 dma2=$dma2 isapnp=0 index=0
# See the modprobe manpage.
exit $?

Example A-35. Locating split paragraphs in a text file
#!/bin/bash
# find-splitpara.sh
# Finds split paragraphs in a text file,
#+ and tags the line numbers.

ARGCOUNT=1
E_WRONGARGS=65

# Expect one arg.

file="$1"
lineno=1
Flag=0

# Target filename.
# Line number. Start at 1.
# Blank line flag.

if [ $# -ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` FILENAME"
exit $E_WRONGARGS
fi
file_read ()
{
while read line
do

# Scan file for pattern, then print line.

if [[ "$line" =~ ^[a-z] && $Flag -eq 1 ]]
then # Line begins with lc character, following blank line.
echo -n "$lineno::
"
echo "$line"
fi

if [[ "$line" =~ "^$" ]]
then
# If blank line,
Flag=1
#+ set flag.
else
Flag=0
fi
((lineno++))
done
} < $file

# Redirect file into function's stdin.

file_read

exit $?

# ---------------------------------------------------------------This is line one of an example paragraph, bla, bla, bla.
This is line two, and line three should follow on next line, but

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690

Advanced Bash-Scripting Guide
there is a blank line separating the two parts of the paragraph.
# ---------------------------------------------------------------Running this script on a file containing the above paragraph
yields:
4::

there is a blank line separating the two parts of the paragraph.

There will be additional output for all the other split paragraphs
in the target file.

Example A-36. Insertion sort
#!/bin/bash
# insertion-sort.bash: Insertion sort implementation in Bash
#
Heavy use of Bash array features:
#+
(string) slicing, merging, etc
# URL: http://www.lugmen.org.ar/~jjo/jjotip/insertion-sort.bash.d
#+
/insertion-sort.bash.sh
#
# Author: JuanJo Ciarlante 
# Lightly reformatted by ABS Guide author.
# License: GPLv2
# Used in ABS Guide with author's permission (thanks!).
#
# Test with:
./insertion-sort.bash -t
# Or:
bash insertion-sort.bash -t
# The following *doesn't* work:
#
sh insertion-sort.bash -t
# Why not? Hint: which Bash-specific features are disabled
#+ when running a script by 'sh script.sh'?
#
: ${DEBUG:=0} # Debug, override with: DEBUG=1 ./scriptname . . .
# Parameter substitution -- set DEBUG to 0 if not previously set.
# Global array: "list"
typeset -a list
# Load whitespace-separated numbers from stdin.
if [ "$1" = "-t" ]; then
DEBUG=1
read -a list < <( od -Ad -w24 -t u2 /dev/urandom ) # Random list.
#
^ ^ process substition
else
read -a list
fi
numelem=${#list[*]}
# Shows the list, marking the element whose index is $1
#+ by surrounding it with the two chars passed as $2.
# Whole line prefixed with $3.
showlist()
{
echo "$3"${list[@]:0:$1} ${2:0:1}${list[$1]}${2:1:1} ${list[@]:$1+1};
}
# Loop _pivot_ -- from second element to end of list.
for(( i=1; i" "*"
done

echo
echo "------"
echo $'Result:\n'${list[@]}
exit $?

Example A-37. Standard Deviation
#!/bin/bash
# sd.sh: Standard Deviation
#
#
#+
#
#+
#
#
#
#
#
#
#
#
#
#
#

The Standard Deviation indicates how consistent a set of data is.
It shows to what extent the individual data points deviate from the
arithmetic mean, i.e., how much they "bounce around" (or cluster).
It is essentially the average deviation-distance of the
data points from the mean.
=========================================================== #
To calculate the Standard Deviation:
1
2

Find the arithmetic mean (average) of all the data points.
Subtract each data point from the arithmetic mean,
and square that difference.
3 Add all of the individual difference-squares in # 2.
4 Divide the sum in # 3 by the number of data points.
This is known as the "variance."
5 The square root of # 4 gives the Standard Deviation.
=========================================================== #

count=0
SC=9
E_DATAFILE=90

# Number of data points; global.
# Scale to be used by bc. Nine decimal places.
# Data file error.

# ----------------- Set data file --------------------if [ ! -z "$1" ] # Specify filename as cmd-line arg?
then
datafile="$1" # ASCII text file,
else
#+ one (numerical) data point per line!
datafile=sample.dat
fi
# See example data file, below.
if [ ! -e "$datafile" ]
then
echo "\""$datafile"\" does not exist!"
exit $E_DATAFILE
fi
# -----------------------------------------------------

Appendix A. Contributed Scripts

692

Advanced Bash-Scripting Guide

arith_mean ()
{
local rt=0
local am=0
local ct=0

# Running total.
# Arithmetic mean.
# Number of data points.

while read value
# Read one data point at a time.
do
rt=$(echo "scale=$SC; $rt + $value" | bc)
(( ct++ ))
done
am=$(echo "scale=$SC; $rt / $ct" | bc)
echo $am; return $ct
# This function "returns" TWO values!
# Caution: This little trick will not work if $ct > 255!
# To handle a larger number of data points,
#+ simply comment out the "return $ct" above.
} <"$datafile"
# Feed in data file.
sd ()
{
mean1=$1
n=$2
sum2=0
avg2=0
sdev=0

#
#
#
#
#

Arithmetic mean (passed to function).
How many data points.
Sum of squared differences ("variance").
Average of $sum2.
Standard Deviation.

while read value
# Read one line at a time.
do
diff=$(echo "scale=$SC; $mean1 - $value" | bc)
# Difference between arith. mean and data point.
dif2=$(echo "scale=$SC; $diff * $diff" | bc) # Squared.
sum2=$(echo "scale=$SC; $sum2 + $dif2" | bc) # Sum of squares.
done
avg2=$(echo "scale=$SC; $sum2 / $n" | bc) # Avg. of sum of squares.
sdev=$(echo "scale=$SC; sqrt($avg2)" | bc) # Square root =
echo $sdev
# Standard Deviation.
} <"$datafile"

# Rewinds data file.

# ======================================================= #
mean=$(arith_mean); count=$?
# Two returns from function!
std_dev=$(sd $mean $count)
echo
echo "Number of data points in \""$datafile"\" = $count"
echo "Arithmetic mean (average) = $mean"
echo "Standard Deviation = $std_dev"
echo
# ======================================================= #
exit
# This script could stand some drastic streamlining,
#+ but not at the cost of reduced legibility, please.

Appendix A. Contributed Scripts

693

Advanced Bash-Scripting Guide
# ++++++++++++++++++++++++++++++++++++++++ #
# A sample data file (sample1.dat):
#
#
#
#
#

18.35
19.0
18.88
18.91
18.64

# $ sh sd.sh sample1.dat
#
#
#
#

Number of data points in "sample1.dat" = 5
Arithmetic mean (average) = 18.756000000
Standard Deviation = .235338054
++++++++++++++++++++++++++++++++++++++++ #

Example A-38. A pad file generator for shareware authors
#!/bin/bash
# pad.sh
#######################################################
#
PAD (xml) file creator
#+ Written by Mendel Cooper .
#+ Released to the Public Domain.
#
# Generates a "PAD" descriptor file for shareware
#+ packages, according to the specifications
#+ of the ASP.
# http://www.asp-shareware.org/pad
#######################################################

# Accepts (optional) save filename as a command-line argument.
if [ -n "$1" ]
then
savefile=$1
else
savefile=save_file.xml
# Default save_file name.
fi

# ===== PAD file headers =====
HDR1=""
HDR2=""
HDR3=""
HDR4="\t1.15"
HDR5="\tPortable Application Description, or PAD
for short, is a data set that is used by shareware authors to
disseminate information to anyone interested in their software products.
To find out more go to http://www.asp-shareware.org/pad"
HDR6=""
# ============================

fill_in ()
{
if [ -z "$2" ]
then
echo -n "$1? "

# Get user input.

Appendix A. Contributed Scripts

694

Advanced Bash-Scripting Guide
else
echo -n "$1 $2? "
fi
read var

# Additional query?

# May paste to fill in field.
# This shows how flexible "read" can be.

if [ -z "$var" ]
then
echo -e "\t\t<$1 />" >>$savefile
# Indent with 2 tabs.
return
else
echo -e "\t\t<$1>$var" >>$savefile
return ${#var}
# Return length of input string.
fi
}
check_field_length () # Check length of program description fields.
{
# $1 = maximum field length
# $2 = actual field length
if [ "$2" -gt "$1" ]
then
echo "Warning: Maximum field length of $1 characters exceeded!"
fi
}
clear
# Clear screen.
echo "PAD File Creator"
echo "--- ---- -------"
echo
# Write File Headers to file.
echo $HDR1 >$savefile
echo $HDR2 >>$savefile
echo $HDR3 >>$savefile
echo -e $HDR4 >>$savefile
echo -e $HDR5 >>$savefile
echo $HDR6 >>$savefile

# Company_Info
echo "COMPANY INFO"
CO_HDR="Company_Info"
echo "<$CO_HDR>" >>$savefile
fill_in
fill_in
fill_in
fill_in
fill_in
fill_in
fill_in
#
#
#
#

Company_Name
Address_1
Address_2
City_Town
State_Province
Zip_Postal_Code
Country

If applicable:
fill_in ASP_Member "[Y/N]"
fill_in ASP_Member_Number
fill_in ESC_Member "[Y/N]"

fill_in Company_WebSite_URL
clear

# Clear screen between sections.

Appendix A. Contributed Scripts

695

Advanced Bash-Scripting Guide
# Contact_Info
echo "CONTACT INFO"
CONTACT_HDR="Contact_Info"
echo "<$CONTACT_HDR>" >>$savefile
fill_in Author_First_Name
fill_in Author_Last_Name
fill_in Author_Email
fill_in Contact_First_Name
fill_in Contact_Last_Name
fill_in Contact_Email
echo -e "\t" >>$savefile
# END Contact_Info
clear
# Support_Info
echo "SUPPORT INFO"
SUPPORT_HDR="Support_Info"
echo "<$SUPPORT_HDR>" >>$savefile
fill_in Sales_Email
fill_in Support_Email
fill_in General_Email
fill_in Sales_Phone
fill_in Support_Phone
fill_in General_Phone
fill_in Fax_Phone
echo -e "\t" >>$savefile
# END Support_Info
echo "" >>$savefile
# END Company_Info
clear
# Program_Info
echo "PROGRAM INFO"
PROGRAM_HDR="Program_Info"
echo "<$PROGRAM_HDR>" >>$savefile
fill_in Program_Name
fill_in Program_Version
fill_in Program_Release_Month
fill_in Program_Release_Day
fill_in Program_Release_Year
fill_in Program_Cost_Dollars
fill_in Program_Cost_Other
fill_in Program_Type "[Shareware/Freeware/GPL]"
fill_in Program_Release_Status "[Beta, Major Upgrade, etc.]"
fill_in Program_Install_Support
fill_in Program_OS_Support "[Win9x/Win2k/Linux/etc.]"
fill_in Program_Language "[English/Spanish/etc.]"
echo; echo
# File_Info
echo "FILE INFO"
FILEINFO_HDR="File_Info"
echo "<$FILEINFO_HDR>" >>$savefile
fill_in Filename_Versioned
fill_in Filename_Previous
fill_in Filename_Generic
fill_in Filename_Long

Appendix A. Contributed Scripts

696

Advanced Bash-Scripting Guide
fill_in
fill_in
fill_in
echo -e
# END

File_Size_Bytes
File_Size_K
File_Size_MB
"\t" >>$savefile
File_Info

clear
# Expire_Info
echo "EXPIRE INFO"
EXPIRE_HDR="Expire_Info"
echo "<$EXPIRE_HDR>" >>$savefile
fill_in Has_Expire_Info "Y/N"
fill_in Expire_Count
fill_in Expire_Based_On
fill_in Expire_Other_Info
fill_in Expire_Month
fill_in Expire_Day
fill_in Expire_Year
echo -e "\t" >>$savefile
# END Expire_Info
clear
# More Program_Info
echo "ADDITIONAL PROGRAM INFO"
fill_in Program_Change_Info
fill_in Program_Specific_Category
fill_in Program_Categories
fill_in Includes_JAVA_VM "[Y/N]"
fill_in Includes_VB_Runtime "[Y/N]"
fill_in Includes_DirectX "[Y/N]"
# END More Program_Info
echo "" >>$savefile
# END Program_Info
clear
# Program Description
echo "PROGRAM DESCRIPTIONS"
PROGDESC_HDR="Program_Descriptions"
echo "<$PROGDESC_HDR>" >>$savefile
LANG="English"
echo "<$LANG>" >>$savefile
fill_in Keywords "[comma + space separated]"
echo
echo "45, 80, 250, 450, 2000 word program descriptions"
echo "(may cut and paste into field)"
# It would be highly appropriate to compose the following
#+ "Char_Desc" fields with a text editor,
#+ then cut-and-paste the text into the answer fields.
echo
echo "
|---------------45 characters---------------|"
fill_in Char_Desc_45
check_field_length 45 "$?"
echo
fill_in Char_Desc_80
check_field_length 80 "$?"

Appendix A. Contributed Scripts

697

Advanced Bash-Scripting Guide
fill_in Char_Desc_250
check_field_length 250 "$?"
fill_in Char_Desc_450
fill_in Char_Desc_2000
echo "" >>$savefile
echo "" >>$savefile
# END Program Description
clear
echo "Done."; echo; echo
echo "Save file is: \""$savefile"\""
exit 0

Example A-39. A man page editor
#!/bin/bash
# maned.sh
# A rudimentary man page editor
#
#
#
#

Version: 0.1 (Alpha, probably buggy)
Author: Mendel Cooper 
Reldate: 16 June 2008
License: GPL3

savefile=
E_NOINPUT=90

# Global, used in multiple functions.
# User input missing (error). May or may not be critical.

# =========== Markup Tags ============ #
TopHeader=".TH"
NameHeader=".SH NAME"
SyntaxHeader=".SH SYNTAX"
SynopsisHeader=".SH SYNOPSIS"
InstallationHeader=".SH INSTALLATION"
DescHeader=".SH DESCRIPTION"
OptHeader=".SH OPTIONS"
FilesHeader=".SH FILES"
EnvHeader=".SH ENVIRONMENT"
AuthHeader=".SH AUTHOR"
BugsHeader=".SH BUGS"
SeeAlsoHeader=".SH SEE ALSO"
BOLD=".B"
# Add more tags, as needed.
# See groff docs for markup meanings.
# ==================================== #
start ()
{
clear
# Clear screen.
echo "ManEd"
echo "-----"
echo
echo "Simple man page creator"
echo "Author: Mendel Cooper"
echo; echo; echo
}
progname ()

Appendix A. Contributed Scripts

698

Advanced Bash-Scripting Guide
{
echo -n "Program name? "
read name
echo -n "Manpage section? [Hit RETURN for default (\"1\") ]
read section
if [ -z "$section" ]
then
section=1
# Most man pages are in section 1.
fi
if [ -n "$name" ]
then
savefile=""$name"."$section""
echo -n "$1 " >>$savefile
name1=$(echo "$name" | tr a-z A-Z)

#

"

Filename suffix = section.

# Change to uppercase,
#+ per man page convention.

echo -n "$name1" >>$savefile
else
echo "Error! No input."
# Mandatory input.
exit $E_NOINPUT
# Critical!
# Exercise: The script-abort if no filename input is a bit clumsy.
#
Rewrite this section so a default filename is used
#+
if no input.
fi
echo -n "

\"$section\"">>$savefile

# Append, always append.

echo
read
echo
echo

-n "Version? "
ver
-n " \"Version $ver \"">>$savefile
>>$savefile

echo
read
echo
echo
echo

-n "Short description [0 - 5 words]? "
sdesc
"$NameHeader">>$savefile
""$BOLD" "$name"">>$savefile
"\- "$sdesc"">>$savefile

}
fill_in ()
{ # This function more or less copied from "pad.sh" script.
echo -n "$2? "
# Get user input.
read var
# May paste (a single line only!) to fill in field.
if [ -n "$var" ]
then
echo "$1 " >>$savefile
echo -n "$var" >>$savefile
else
# Don't append empty field to file.
return $E_NOINPUT # Not critical here.
fi
echo >>$savefile
}

end ()
{
clear

Appendix A. Contributed Scripts

699

Advanced Bash-Scripting Guide
echo
read
if [
exec

-n "Would you like to view the saved man page (y/n)? "
ans
"$ans" = "n" -o "$ans" = "N" ]; then exit; fi
less "$savefile" # Exit script and hand off control to "less" ...
#+ ... which formats for viewing man page source.

}

# ---------------------------------------- #
start
progname "$TopHeader"
fill_in "$SynopsisHeader" "Synopsis"
fill_in "$DescHeader" "Long description"
# May paste in *single line* of text.
fill_in "$OptHeader" "Options"
fill_in "$FilesHeader" "Files"
fill_in "$AuthHeader" "Author"
fill_in "$BugsHeader" "Bugs"
fill_in "$SeeAlsoHeader" "See also"
# fill_in "$OtherHeader" ... as necessary.
end
# ... exit not needed.
# ---------------------------------------- #
#
#+
#
#+
#+

Note that the generated man page will usually
require manual fine-tuning with a text editor.
However, it's a distinct improvement upon
writing man source from scratch
or even editing a blank man page template.

#
#+
#
#
#
#+
#+
#

The main deficiency of the script is that it permits
pasting only a single text line into the input fields.
This may be a long, cobbled-together line, which groff
will automatically wrap and hyphenate.
However, if you want multiple (newline-separated) paragraphs,
these must be inserted by manual text editing on the
script-generated man page.
Exercise (difficult): Fix this!

#
#+
#+
#+

This script is not nearly as elaborate as the
full-featured "manedit" package
http://freshmeat.net/projects/manedit/
but it's much easier to use.

Example A-40. Petals Around the Rose
#!/bin/bash -i
# petals.sh
#########################################################################
# Petals Around the Rose
#
#
#
# Version 0.1 Created by Serghey Rodin
#
# Version 0.2 Modded by ABS Guide Author
#
#
#
# License: GPL3
#
# Used in ABS Guide with permission.
#
# ##################################################################### #
hits=0
WIN=6
ALMOST=5

# Correct guesses.
# Mastered the game.
# One short of mastery.

Appendix A. Contributed Scripts

700

Advanced Bash-Scripting Guide
EXIT=exit

# Give up early?

RANDOM=$$

# Seeds the random number generator from PID of script.

# Bones (ASCII graphics for dice)
bone1[1]="|
|"
bone1[2]="|
o |"
bone1[3]="|
o |"
bone1[4]="| o
o |"
bone1[5]="| o
o |"
bone1[6]="| o
o |"
bone2[1]="|
o
|"
bone2[2]="|
|"
bone2[3]="|
o
|"
bone2[4]="|
|"
bone2[5]="|
o
|"
bone2[6]="| o
o |"
bone3[1]="|
|"
bone3[2]="| o
|"
bone3[3]="| o
|"
bone3[4]="| o
o |"
bone3[5]="| o
o |"
bone3[6]="| o
o |"
bone="+---------+"

# Functions
instructions () {
clear
echo -n "Do you need instructions? (y/n) "; read ans
if [ "$ans" = "y" -o "$ans" = "Y" ]; then
clear
echo -e '\E[34;47m' # Blue type.
#

"cat document"
cat </dev/null # Filter response for digit.
# Otherwise just roll dice again.
if [ "$?" -eq 0 ]
# If-loop #1.
then
if [ "$petal" == "$answer" ]; then
# If-loop #2.
echo -e "\nCorrect. There are $petal petals around the rose.\n"
(( hits++ ))
if [ "$hits" -eq "$WIN" ]; then
# If-loop #3.
echo -e '\E[31;47m' # Red type.
echo -e "\033[1m"
# Bold.
echo "You have unraveled the mystery of the Rose Petals!"
echo "Welcome to the Fellowship of the Rose!!!"
echo "(You are herewith sworn to secrecy.)"; echo
echo -e "\033[0m"
# Turn off red & bold.
break
# Exit!
else echo "You have $hits correct so far."; echo
if [ "$hits" -eq "$ALMOST" ]; then
echo "Just one more gets you to the heart of the mystery!"; echo
fi
fi

# Close if-loop #3.

else
echo -e "\nWrong. There are $answer petals around the rose.\n"
hits=0
# Reset number of correct guesses.
fi
# Close if-loop #2.
echo -n "Hit ENTER for the next roll, or type \"exit\" to end. "
read
if [ "$REPLY" = "$EXIT" ]; then exit
fi
fi
clear
done

# Close if-loop #1.

# End of main (while) loop.

###
exit $?
#
#
#
#
#
#

Resources:
--------1) http://en.wikipedia.org/wiki/Petals_Around_the_Rose
(Wikipedia entry.)
2) http://www.borrett.id.au/computing/petals-bg.htm
(How Bill Gates coped with the Petals Around the Rose challenge.)

Example A-41. Quacky: a Perquackey-type word game
#!/bin/bash
# qky.sh
##############################################################

Appendix A. Contributed Scripts

703

Advanced Bash-Scripting Guide
# QUACKEY: a somewhat simplified version of Perquackey [TM]. #
#
#
# Author: Mendel Cooper 
#
# version 0.1.02
03 May, 2008
#
# License: GPL3
#
##############################################################
WLIST=/usr/share/dict/word.lst
#
^^^^^^^^ Word list file found here.
# ASCII word list, one word per line, UNIX format.
# A suggested list is the script author's "yawl" word list package.
# http://bash.webofcrafts.net/yawl-0.3.2.tar.gz
#
or
# http://ibiblio.org/pub/Linux/libs/yawl-0.3.2.tar.gz
NONCONS=0
CONS=1
SUCCESS=0
NG=1
FAILURE=''
NULL=0
MINWLEN=3
MAXCAT=5
PENALTY=200
total=
E_DUP=70

# Word not constructable from letter set.
# Constructable.

TIMEOUT=10

# Time for word input.

NVLET=10
VULET=13

# 10 letters for non-vulnerable.
# 13 letters for vulnerable (not yet implemented).

#
#
#
#

Zero out value of letter (if found).
Minimum word length.
Maximum number of words in a given category.
General-purpose penalty for unacceptable words.

# Duplicate word error.

declare -a Words
declare -a Status
declare -a Score=( 0 0 0 0 0 0 0 0 0 0 0 )

letters=( a n s r t m l k p r b c i d s i d z e w u e t f
e y e r e f e g t g h h i t r s c i t i d i j a t a o l a
m n a n o v n w o s e l n o s p a q e e r a b r s a o d s
t g t i t l u e u v n e o x y m r k )
# Letter distribution table shamelessly borrowed from "Wordy" game,
#+ ca. 1992, written by a certain fine fellow named Mendel Cooper.
declare -a LS
numelements=${#letters[@]}
randseed="$1"
instructions ()
{
clear
echo "Welcome to QUACKEY, the anagramming word construction game."; echo
echo -n "Do you need instructions? (y/n) "; read ans
if [ "$ans" = "y" -o "$ans" = "Y" ]; then
clear
echo -e '\E[31;47m' # Red foreground. '\E[34;47m' for blue.
cat <> $savefile
echo "Letterset # $randseed (random seed) ">> $savefile
echo -n "Letterset: " >> $savefile
echo "${LS[@]}" >> $savefile
echo "---------" >> $savefile
echo "Words constructed:" >> $savefile
echo "${Words[@]}" >> $savefile
echo >> $savefile
echo "Score: $total" >> $savefile
echo "Statistics for this round saved in \""$savefile"\""
#########################################################

Appendix A. Contributed Scripts

710

Advanced Bash-Scripting Guide
echo "Score for this round: $total"
echo "Words: ${Words[@]}"
}
# ---------#
instructions
seed_random
get_letset
play
end_of_game
# ---------#
exit $?
# TODO:
#
# 1) Clean up code!
# 2) Prettify the display_words () function (maybe with widgets?).
# 3) Improve the time-out ... maybe change to untimed entry,
#+
but with a time limit for the overall round.
# 4) An on-screen countdown timer would be nice.
# 5) Implement "vulnerable" mode of play.
# 6) Improve save-to-file capability (and maybe make it optional).
# 7) Fix bugs!!!
# Reference for more info:
# http://bash.webofcrafts.net/qky.README.html

Example A-42. Nim
#!/bin/bash
# nim.sh: Game of Nim
# Author: Mendel Cooper
# Reldate: 15 July 2008
# License: GPL3
ROWS=5
# Five rows of pegs (or matchsticks).
WON=91
# Exit codes to keep track of wins/losses.
LOST=92
# Possibly useful if running in batch mode.
QUIT=99
peg_msg=
# Peg/Pegs?
Rows=( 0 5 4 3 2 1 )
# Array holding play info.
# ${Rows[0]} holds total number of pegs, updated after each turn.
# Other array elements hold number of pegs in corresponding row.
instructions ()
{
clear
tput bold
echo "Welcome to the game of Nim."; echo
echo -n "Do you need instructions? (y/n) "; read ans
if [ "$ans" = "y" -o "$ans" = "Y" ]; then
clear
echo -e '\E[33;41m' # Yellow fg., over red bg.; bold.
cat < /dev/null
date > /dev/null
grep . < /dev/null
(echo "import time" | python) 2> /dev/null
bc < /dev/null
sed '' < /dev/null
printf '1' > /dev/null
/usr/bin/time false 2> /dev/null
cat < /dev/null
}
cache_progs
# To minimise startup delay.
date +%s.%N | grep -qF 'N' && use_python=1 # If `date` lacks nanoseconds.
now() {
if [ "$use_python" ]; then

Appendix A. Contributed Scripts

716

Advanced Bash-Scripting Guide
echo "import time; print time.time()" 2>/dev/null | python
else
printf "%.2f" `date +%s.%N`
fi
}
fmt_seconds() {
seconds=$1
mins=`echo $seconds/60 | bc`
if [ "$mins" != "0" ]; then
seconds=`echo "$seconds - ($mins*60)" | bc`
echo "$mins:$seconds"
else
echo "$seconds"
fi
}
total() {
end=`now`
total=`echo "$end - $start" | bc`
fmt_seconds $total
}
stop() {
[ "$lapped" ] && lap "$laptime" "display"
total
exit
}
lap() {
laptime=`echo "$1" | sed -n 's/.*real[^0-9.]*\(.*\)/\1/p'`
[ ! "$laptime" -o "$laptime" = "0.00" ] && return
# Signals too frequent.
laptotal=`echo $laptime+0$laptotal | bc`
if [ "$2" = "display" ]; then
lapcount=`echo 0$lapcount+1 | bc`
laptime=`fmt_seconds $laptotal`
echo $laptime "($lapcount)"
lapped="true"
laptotal="0"
fi
}
echo -n "Space for lap | ? for split | Ctrl-C to stop | Space to start...">&2
while true; do
trap true INT QUIT # Set signal handlers.
laptime=`/usr/bin/time -p 2>&1 cat >/dev/null`
ret=$?
trap '' INT QUIT
# Ignore signals within this script.
if [ $ret -eq 1 -o $ret -eq 2 -o $ret -eq 130 ]; then # SIGINT = stop
[ ! "$start" ] && { echo >&2; exit; }
stop
elif [ $ret -eq 3 -o $ret -eq 131 ]; then
# SIGQUIT = lap
if [ ! "$start" ]; then
start=`now` || exit 1
echo >&2
continue
fi
lap "$laptime" "display"
else
# eof = split
[ ! "$start" ] && continue

Appendix A. Contributed Scripts

717

Advanced Bash-Scripting Guide
total
lap "$laptime"

# Update laptotal.

fi
done
exit $?

Example A-44. An all-purpose shell scripting homework assignment solution
#!/bin/bash
# homework.sh: All-purpose homework assignment solution.
# Author: M. Leo Cooper
# If you substitute your own name as author, then it is plagiarism,
#+ possibly a lesser sin than cheating on your homework!
# License: Public Domain
#
#+
#
#

This script may be turned in to your instructor
in fulfillment of ALL shell scripting homework assignments.
It's sparsely commented, but you, the student, can easily remedy that.
The script author repudiates all responsibility!

DLA=1
P1=2
P2=4
P3=7
PP1=0
PP2=8
MAXL=9
E_LZY=99
declare -a L
L[0]="3 4 0 17 29 8 13 18 19 17 20 2 19 14 17 28"
L[1]="8 29 12 14 18 19 29 4 12 15 7 0 19 8 2 0 11 11 24 29 17 4 6 17 4 19"
L[2]="29 19 7 0 19 29 8 29 7 0 21 4 29 13 4 6 11 4 2 19 4 3"
L[3]="19 14 29 2 14 12 15 11 4 19 4 29 19 7 8 18 29"
L[4]="18 2 7 14 14 11 22 14 17 10 29 0 18 18 8 6 13 12 4 13 19 26"
L[5]="15 11 4 0 18 4 29 0 2 2 4 15 19 29 12 24 29 7 20 12 1 11 4 29"
L[6]="4 23 2 20 18 4 29 14 5 29 4 6 17 4 6 8 14 20 18 29"
L[7]="11 0 25 8 13 4 18 18 27"
L[8]="0 13 3 29 6 17 0 3 4 29 12 4 29 0 2 2 14 17 3 8 13 6 11 24 26"
L[9]="19 7 0 13 10 29 24 14 20 26"
declare -a \
alph=( A B C D E F G H I J K L M N O P Q R S T U V W X Y Z . , : ' ' )

pt_lt ()
{
echo -n "${alph[$1]}"
echo -n -e "\a"
sleep $DLA
}
b_r ()
{
echo -e '\E[31;48m\033[1m'
}
cr ()
{
echo -e "\a"

Appendix A. Contributed Scripts

718

Advanced Bash-Scripting Guide
sleep $DLA
}
restore ()
{
echo -e '\033[0m'
tput sgr0
}

# Bold off.
# Normal.

p_l ()
{
for ltr in $1
do
pt_lt "$ltr"
done
}
# ---------------------b_r
for i in $(seq 0 $MAXL)
do
p_l "${L[i]}"
if [[ "$i" -eq "$P1" || "$i" -eq "$P2" || "$i" -eq "$P3" ]]
then
cr
elif [[ "$i" -eq "$PP1" || "$i" -eq "$PP2" ]]
then
cr; cr
fi
done
restore
# ---------------------echo
exit $E_LZY
#
#+
#
#+

A typical example of an obfuscated script that is difficult
to understand, and frustrating to maintain.
In your career as a sysadmin, you'll run into these critters
all too often.

Example A-45. The Knight's Tour
#!/bin/bash
# ktour.sh
# author: mendel cooper
# reldate: 12 Jan 2009
# license: public domain
# (Not much sense GPLing something that's pretty much in the common
#+ domain anyhow.)
###################################################################
#
The Knight's Tour, a classic problem.
#
#
=====================================
#
# The knight must move onto every square of the chess board,
#

Appendix A. Contributed Scripts

719

Advanced Bash-Scripting Guide
# but cannot revisit any square he has already visited.
#
#
#
# And just why is Sir Knight unwelcome for a return visit?
#
# Could it be that he has a habit of partying into the wee hours #
#+ of the morning?
#
# Possibly he leaves pizza crusts in the bed, empty beer bottles #
#+ all over the floor, and clogs the plumbing. . . .
#
#
#
# ------------------------------------------------------------- #
#
#
# Usage: ktour.sh [start-square] [stupid]
#
#
#
# Note that start-square can be a square number
#
#+ in the range 0 - 63 ... or
#
# a square designator in conventional chess notation,
#
# such as a1, f5, h3, etc.
#
#
#
# If start-square-number not supplied,
#
#+ then starts on a random square somewhere on the board.
#
#
#
# "stupid" as second parameter sets the stupid strategy.
#
#
#
# Examples:
#
# ktour.sh 23
starts on square #23 (h3)
#
# ktour.sh g6 stupid
starts on square #46,
#
#
using "stupid" (non-Warnsdorff) strategy. #
###################################################################
DEBUG=
# Set this to echo debugging info to stdout.
SUCCESS=0
FAIL=99
BADMOVE=-999
FAILURE=1
LINELEN=21 # How many moves to display per line.
# ---------------------------------------- #
# Board array params
ROWS=8
# 8 x 8 board.
COLS=8
let "SQUARES = $ROWS * $COLS"
let "MAX = $SQUARES - 1"
MIN=0
# 64 squares on board, indexed from 0 to 63.
VISITED=1
UNVISITED=-1
UNVSYM="##"
# ---------------------------------------- #
# Global variables.
startpos=
# Starting position (square #, 0 - 63).
currpos=
# Current position.
movenum=
# Move number.
CRITPOS=37
# Have to patch for f5 starting position!
declare -i board
# Use a one-dimensional array to simulate a two-dimensional one.
# This can make life difficult and result in ugly kludges; see below.
declare -i moves # Offsets from current knight position.

initialize_board ()
{
local idx

Appendix A. Contributed Scripts

720

Advanced Bash-Scripting Guide
for idx in {0..63}
do
board[$idx]=$UNVISITED
done
}

print_board ()
{
local idx
echo "
_____________________________________"
for row in {7..0}
# Reverse order of rows ...
do
#+ so it prints in chessboard order.
let "rownum = $row + 1"
# Start numbering rows at 1.
echo -n "$rownum |"
# Mark board edge with border and
for column in {0..7}
#+ "algebraic notation."
do
let "idx = $ROWS*$row + $column"
if [ ${board[idx]} -eq $UNVISITED ]
then
echo -n "$UNVSYM
"
##
else
# Mark square with move number.
printf "%02d " "${board[idx]}"; echo -n " "
fi
done
echo -e -n "\b\b\b|" # \b is a backspace.
echo
# -e enables echoing escaped chars.
done
echo "
echo "

-------------------------------------"
a
b
c
d
e
f
g
h"

}

failure()
{ # Whine, then bail out.
echo
print_board
echo
echo
"
Waah!!! Ran out of squares to move to!"
echo -n "
Knight's Tour attempt ended"
echo
" on $(to_algebraic $currpos) [square #$currpos]"
echo
"
after just $movenum moves!"
echo
exit $FAIL
}

xlat_coords ()
# Translate x/y coordinates to board position
{
#+ (board-array element #).
# For user input of starting board position as x/y coords.
# This function not used in initial release of ktour.sh.
# May be used in an updated version, for compatibility with
#+ standard implementation of the Knight's Tour in C, Python, etc.
if [ -z "$1" -o -z "$2" ]
then
return $FAIL

Appendix A. Contributed Scripts

721

Advanced Bash-Scripting Guide
fi
local xc=$1
local yc=$2
let "board_index = $xc * $ROWS + yc"
if [ $board_index -lt $MIN -o $board_index -gt $MAX ]
then
return $FAIL
# Strayed off the board!
else
return $board_index
fi
}

to_algebraic ()
{
if [ -z "$1" ]
then
return $FAIL
fi

# Translate board position (board-array element #)
#+ to standard algebraic notation used by chess players.

local element_no=$1
# Numerical board position.
local col_arr=( a b c d e f g h )
local row_arr=( 1 2 3 4 5 6 7 8 )
let "row_no = $element_no / $ROWS"
let "col_no = $element_no % $ROWS"
t1=${col_arr[col_no]}; t2=${row_arr[row_no]}
local apos=$t1$t2
# Concatenate.
echo $apos
}

from_algebraic ()
{

# Translate standard algebraic chess notation
#+ to numerical board position (board-array element #).
# Or recognize numerical input & return it unchanged.

if [ -z "$1" ]
then
return $FAIL
fi
# If no command-line arg, then will default to random start pos.
local
local
local
local

ix
ix_count=0
b_index
alpos="$1"

# Board index [0-63]

arow=${alpos:0:1} # position = 0, length = 1
acol=${alpos:1:1}
if [[ $arow =~ [[:digit:]] ]]
then
# POSIX char class
if [[ $acol =~ [[:alpha:]] ]]
then return $FAIL
else if [ $alpos -gt $MAX ]
then return $FAIL
else return $alpos
fi
fi

Appendix A. Contributed Scripts

#

Numerical input?

# Number followed by a letter? Illegal!
# Off board?
# Return digit(s) unchanged . . .
#+ if within range.

722

Advanced Bash-Scripting Guide
fi
if [[ $acol -eq $MIN || $acol -gt $ROWS ]]
then
# Outside of range 1 - 8?
return $FAIL
fi
for ix in a b c d e f g h
do # Convert column letter to column number.
if [ "$arow" = "$ix" ]
then
break
fi
((ix_count++))
# Find index count.
done
((acol--))
# Decrementing converts to zero-based array.
let "b_index = $ix_count + $acol * $ROWS"
if [ $b_index -gt $MAX ]
then
return $FAIL
fi

# Off board?

return $b_index
}

generate_moves ()
{
local
local
local
local

# Calculate all valid knight moves,
#+ relative to current position ($1),
#+ and store in ${moves} array.
kt_hop=1
# One square :: short leg of knight move.
kt_skip=2
# Two squares :: long leg of knight move.
valmov=0
# Valid moves.
row_pos; let "row_pos = $1 % $COLS"

let "move1 = -$kt_skip + $ROWS"
# 2 sideways to-the-left, 1 up
if [[ `expr $row_pos - $kt_skip` -lt $MIN ]]
# An ugly, ugly kludge!
then
# Can't move off board.
move1=$BADMOVE
# Not even temporarily.
else
((valmov++))
fi
let "move2 = -$kt_hop + $kt_skip * $ROWS" # 1 sideways to-the-left, 2 up
if [[ `expr $row_pos - $kt_hop` -lt $MIN ]]
# Kludge continued ...
then
move2=$BADMOVE
else
((valmov++))
fi
let "move3 = $kt_hop + $kt_skip * $ROWS" # 1 sideways to-the-right, 2 up
if [[ `expr $row_pos + $kt_hop` -ge $COLS ]]
then
move3=$BADMOVE
else
((valmov++))
fi
let "move4 = $kt_skip + $ROWS"
# 2 sideways to-the-right, 1 up
if [[ `expr $row_pos + $kt_skip` -ge $COLS ]]
then

Appendix A. Contributed Scripts

723

Advanced Bash-Scripting Guide
move4=$BADMOVE
else
((valmov++))
fi
let "move5 = $kt_skip - $ROWS"
# 2 sideways
if [[ `expr $row_pos + $kt_skip` -ge $COLS ]]
then
move5=$BADMOVE
else
((valmov++))
fi
let "move6 = $kt_hop - $kt_skip * $ROWS" # 1 sideways
if [[ `expr $row_pos + $kt_hop` -ge $COLS ]]
then
move6=$BADMOVE
else
((valmov++))
fi
let "move7 = -$kt_hop - $kt_skip * $ROWS" # 1 sideways
if [[ `expr $row_pos - $kt_hop` -lt $MIN ]]
then
move7=$BADMOVE
else
((valmov++))
fi
let "move8 = -$kt_skip - $ROWS"
# 2 sideways
if [[ `expr $row_pos - $kt_skip` -lt $MIN ]]
then
move8=$BADMOVE
else
((valmov++))
fi
# There must be a better way to do this.
local m=( $valmov $move1 $move2
# ${moves[0]} = number of valid
# ${moves[1]} ... ${moves[8]} =
echo "${m[*]}"
# Elements of

to-the-right, 1 dn

to-the-right, 2 dn

to-the-left,

2 dn

to-the-left,

1 dn

$move3 $move4 $move5 $move6 $move7 $move8 )
moves.
possible moves.
array to stdout for capture in a var.

}

is_on_board () # Is position actually on the board?
{
if [[ "$1" -lt "$MIN" || "$1" -gt "$MAX" ]]
then
return $FAILURE
else
return $SUCCESS
fi
}

do_move ()
# Move the knight!
{
local valid_moves=0
local aapos
currposl="$1"
lmin=$ROWS
iex=0
squarel=

Appendix A. Contributed Scripts

724

Advanced Bash-Scripting Guide
mpm=
mov=
declare -a p_moves
########################## DECIDE-MOVE #############################
if [ $startpos -ne $CRITPOS ]
then
# CRITPOS = square #37
decide_move
else
# Needs a special patch for startpos=37 !!!
decide_move_patched
# Why this particular move and no other ???
fi
####################################################################
(( ++movenum ))
# Increment move count.
let "square = $currposl + ${moves[iex]}"
##################
DEBUG
###############
if [ "$DEBUG" ]
then debug
# Echo debugging information.
fi
##############################################
if [[ "$square" -gt $MAX || "$square" -lt $MIN ||
${board[square]} -ne $UNVISITED ]]
then
(( --movenum ))
# Decrement move count,
echo "RAN OUT OF SQUARES!!!" #+ since previous one was invalid.
return $FAIL
fi
board[square]=$movenum
currpos=$square
# Update current position.
((valid_moves++));
# moves[0]=$valid_moves
aapos=$(to_algebraic $square)
echo -n "$aapos "
test $(( $Moves % $LINELEN )) -eq 0 && echo
# Print LINELEN=21 moves per line. A valid tour shows 3 complete lines.
return $valid_moves
# Found a square to move to!
}

do_move_stupid()
# Dingbat algorithm,
{
#+ courtesy of script author, *not* Warnsdorff.
local valid_moves=0
local movloc
local squareloc
local aapos
local cposloc="$1"
for movloc in {1..8}
do
# Move to first-found unvisited square.
let "squareloc = $cposloc + ${moves[movloc]}"
is_on_board $squareloc
if [ $? -eq $SUCCESS ] && [ ${board[squareloc]} -eq $UNVISITED ]
then
# Add conditions to above if-test to improve algorithm.
(( ++movenum ))
board[squareloc]=$movenum
currpos=$squareloc
# Update current position.
((valid_moves++));
# moves[0]=$valid_moves
aapos=$(to_algebraic $squareloc)
echo -n "$aapos "

Appendix A. Contributed Scripts

725

Advanced Bash-Scripting Guide
test $(( $Moves % $LINELEN )) -eq 0 && echo
# Print 21 moves/line.
return $valid_moves
# Found a square to move to!
fi
done
return $FAIL
# If no square found in all 8 loop iterations,
#+ then Knight's Tour attempt ends in failure.
# Dingbat algorithm will typically fail after about 30 - 40 moves,
#+ but executes _much_ faster than Warnsdorff's in do_move() function.
}

decide_move ()
# Which move will we make?
{
# But, fails on startpos=37 !!!
for mov in {1..8}
do
let "squarel = $currposl + ${moves[mov]}"
is_on_board $squarel
if [[ $? -eq $SUCCESS && ${board[squarel]} -eq $UNVISITED ]]
then
# Find accessible square with least possible future moves.
# This is Warnsdorff's algorithm.
# What happens is that the knight wanders toward the outer edge
#+ of the board, then pretty much spirals inward.
# Given two or more possible moves with same value of
#+ least-possible-future-moves, this implementation chooses
#+ the _first_ of those moves.
# This means that there is not necessarily a unique solution
#+ for any given starting position.
possible_moves $squarel
mpm=$?
p_moves[mov]=$mpm
if [ $mpm -lt $lmin ]
then #
^^
lmin=$mpm
iex=$mov
fi

# If less than previous minimum ...
# Update minimum.
# Save index.

fi
done
}

decide_move_patched ()
# Decide which move to make,
{ #
^^^^^^^
#+ but only if startpos=37 !!!
for mov in {1..8}
do
let "squarel = $currposl + ${moves[mov]}"
is_on_board $squarel
if [[ $? -eq $SUCCESS && ${board[squarel]} -eq $UNVISITED ]]
then
possible_moves $squarel
mpm=$?
p_moves[mov]=$mpm
if [ $mpm -le $lmin ]
then #
^^

Appendix A. Contributed Scripts

# If less-than-or equal to prev. minimum!

726

Advanced Bash-Scripting Guide
lmin=$mpm
iex=$mov
fi
fi
done

# There has to be a better way to do this.

}

possible_moves ()
{

# Calculate number of possible moves,
#+ given the current position.

if [ -z "$1" ]
then
return $FAIL
fi
local curr_pos=$1
local valid_movl=0
local icx=0
local movl
local sq
declare -a movesloc
movesloc=( $(generate_moves $curr_pos) )
for movl in {1..8}
do
let "sq = $curr_pos + ${movesloc[movl]}"
is_on_board $sq
if [ $? -eq $SUCCESS ] && [ ${board[sq]} -eq $UNVISITED ]
then
((valid_movl++));
fi
done
return $valid_movl

# Found a square to move to!

}

strategy ()
{
echo
if [ -n "$STUPID" ]
then
for Moves in {1..63}
do
cposl=$1
moves=( $(generate_moves $currpos) )
do_move_stupid "$currpos"
if [ $? -eq $FAIL ]
then
failure
fi
done
fi
# Don't need an "else" clause here,
#+ because Stupid Strategy will always fail and exit!
for Moves in {1..63}

Appendix A. Contributed Scripts

727

Advanced Bash-Scripting Guide
do
cposl=$1
moves=( $(generate_moves $currpos) )
do_move "$currpos"
if [ $? -eq $FAIL ]
then
failure
fi
done
echo

# Could have condensed above two do-loops into a single one,
#+ but this would have slowed execution.

print_board
echo
echo "Knight's Tour ends on $(to_algebraic $currpos) [square #$currpos]."
return $SUCCESS
}
debug ()
{
# Enable this by setting DEBUG=1 near beginning of script.
local n
echo
echo
echo
# echo
echo
echo

"================================="
" At move number $movenum:"
" *** possible moves = $mpm ***"
"### square = $square ###"
"lmin = $lmin"
"${moves[@]}"

for n in {1..8}
do
echo -n "($n):${p_moves[n]} "
done
echo
echo "iex = $iex :: moves[iex] = ${moves[iex]}"
echo "square = $square"
echo "================================="
echo
} # Gives pretty complete status after ea. move.

# =============================================================== #
# int main () {
from_algebraic "$1"
startpos=$?
if [ "$startpos" -eq "$FAIL" ]
# Okay even if no $1.
then
#
^^^^^^^^^^^
Okay even if input -lt 0.
echo "No starting square specified (or illegal input)."
let "startpos = $RANDOM % $SQUARES"
# 0 - 63 permissable range.
fi

if [ "$2" = "stupid" ]
then
STUPID=1
echo -n "
### Stupid Strategy ###"
else
STUPID=''
echo -n " *** Warnsdorff's Algorithm ***"

Appendix A. Contributed Scripts

728

Advanced Bash-Scripting Guide
fi

initialize_board
movenum=0
board[startpos]=$movenum
# Mark each board square with move number.
currpos=$startpos
algpos=$(to_algebraic $startpos)
echo; echo "Starting from $algpos [square #$startpos] ..."; echo
echo -n "Moves:"
strategy "$currpos"
echo
exit 0

# return 0;

# }
# End of main() pseudo-function.
# =============================================================== #

# Exercises:
# --------#
# 1) Extend this example to a 10 x 10 board or larger.
# 2) Improve the "stupid strategy" by modifying the
#
do_move_stupid function.
#
Hint: Prevent straying into corner squares in early moves
#
(the exact opposite of Warnsdorff's algorithm!).
# 3) This script could stand considerable improvement and
#
streamlining, especially in the poorly-written
#
generate_moves() function
#
and in the DECIDE-MOVE patch in the do_move() function.
#
Must figure out why standard algorithm fails for startpos=37 ...
#+
but _not_ on any other, including symmetrical startpos=26.
#
Possibly, when calculating possible moves, counts the move back
#+
to the originating square. If so, it might be a relatively easy fix.

Example A-46. Magic Squares
#!/bin/bash
# msquare.sh
# Magic Square generator (odd-order squares only!)
#
#
#
#
#

Author: mendel cooper
reldate: 19 Jan. 2009
License: Public Domain
A C-program by Kwon Young Shin inspired this script.
See http://user.chollian.net/~brainstm/MagicSquare.htm ...

# Definition: A "magic square" is a two-dimensional array
#
of integers in which all the rows, columns,
#
and *long* diagonals add up to the same number.
#
Being "square," the array has the same number
#
of rows and columns.
# An example of a magic square of order 3 is:
#
8 1 6
#
3 5 7
#
4 9 2

Appendix A. Contributed Scripts

729

Advanced Bash-Scripting Guide
# All the rows, columns, and long diagonals add up to 15.

# Globals
EVEN=2
MAXSIZE=31
# 31 rows x 31 cols.
E_usage=90
# Invocation error.
dimension=
declare -i square
usage_message ()
{
echo "Usage: $0 square-size"
echo "
... where \"square-size\" is an ODD integer"
echo "
in the range 3 - 31."
# Actually works for squares up to order 159,
#+ but large squares will not display pretty-printed in a term window.
# Try increasing MAXSIZE, above.
exit $E_usage
}

calculate ()
# Here's where the actual work gets done.
{
local row col index dimadj j k cell_val=1
dimension=$1
let "dimadj = $dimension * 3"; let "dimadj /= 2"

# x 1.5, then truncate.

for ((j=0; j < dimension; j++))
do
for ((k=0; k < dimension; k++))
do # Calculate indices, then convert to 1-dim. array index.
# Bash doesn't support multidimensional arrays. Pity.
let "col = $k - $j + $dimadj"; let "col %= $dimension"
let "row = $j * 2 - $k + $dimension"; let "row %= $dimension"
let "index = $row*($dimension) + $col"
square[$index]=cell_val; ((cell_val++))
done
done
}
# Plain math, no visualization required.

print_square ()
{
local row col idx d1
let "d1 = $dimension - 1"

# Output square, one row at a time.

# Adjust for zero-indexed array.

for row in $(seq 0 $d1)
do
for col in $(seq 0 $d1)
do
let "idx = $row * $dimension + $col"
printf "%3d " "${square[idx]}"; echo -n " "
done
# Displays up to 13-order neatly in 80-column term window.
echo
done

# Newline after each row.

}

Appendix A. Contributed Scripts

730

Advanced Bash-Scripting Guide
#################################################
if [[ -z "$1" ]] || [[ "$1" -gt $MAXSIZE ]]
then
usage_message
fi
let "test_even = $1 % $EVEN"
if [ $test_even -eq 0 ]
then
# Can't handle even-order squares.
usage_message
fi
calculate $1
print_square

# echo "${square[@]}"

# DEBUG

exit $?
#################################################

#
#
#
#
#
#
#
#
#
#
#
#

Exercises:
--------1) Add a function to calculate the sum of each row, column,
and *long* diagonal. The sums must match.
This is the "magic constant" of that particular order square.
2) Have the print_square function auto-calculate how much space
to allot between square elements for optimized display.
This might require parameterizing the "printf" line.
3) Add appropriate functions for generating magic squares
with an *even* number of rows/columns.
This is non-trivial(!).
See the URL for Kwon Young Shin, above, for help.

Example A-47. Fifteen Puzzle
#!/bin/bash
# fifteen.sh
#
#
#
#

Classic
Author:
Lightly
Used in

"Fifteen Puzzle"
Antonio Macchi
edited and commented by ABS Guide author.
ABS Guide with permission. (Thanks!)

# The invention of the Fifteen Puzzle is attributed to either
#+ Sam Loyd or Noyes Palmer Chapman.
# The puzzle was wildly popular in the late 19th-century.
# Object: Rearrange the numbers so they read in order,
#+ from 1 - 15:
________________
#
| 1
2
3
4 |
#
| 5
6
7
8 |
#
| 9 10 11 12 |
#
| 13 14 15
|
#
----------------

#######################
# Constants
#
SQUARES=16
#
FAIL=70
#
E_PREMATURE_EXIT=80 #
#######################

Appendix A. Contributed Scripts

731

Advanced Bash-Scripting Guide

########
# Data #
########
Puzzle=( 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 " " )

#############
# Functions #
#############
function swap
{
local tmp
tmp=${Puzzle[$1]}
Puzzle[$1]=${Puzzle[$2]}
Puzzle[$2]=$tmp
}

function Jumble
{ # Scramble the pieces at beginning of round.
local i pos1 pos2
for i in {1..100}
do
pos1=$(( $RANDOM % $SQUARES))
pos2=$(( $RANDOM % $SQUARES ))
swap $pos1 $pos2
done
}

function PrintPuzzle
{
local i1 i2 puzpos
puzpos=0
clear
echo "Enter

quit

to exit."; echo

# Better that than Ctl-C.

echo ",----.----.----.----."
# Top border.
for i1 in {1..4}
do
for i2 in {1..4}
do
printf "| %2s " "${Puzzle[$puzpos]}"
(( puzpos++ ))
done
echo "|"
# Right-side border.
test $i1 = 4 || echo "+----+----+----+----+"
done
echo "'----'----'----'----'"
# Bottom border.
}

function GetNum
{ # Test for valid input.
local puznum garbage

Appendix A. Contributed Scripts

732

Advanced Bash-Scripting Guide
while true
do
echo "Moves: $moves" # Also counts invalid moves.
read -p "Number to move: " puznum garbage
if [ "$puznum" = "quit" ]; then echo; exit $E_PREMATURE_EXIT; fi
test -z "$puznum" -o -n "${puznum//[0-9]/}" && continue
test $puznum -gt 0 -a $puznum -lt $SQUARES && break
done
return $puznum
}

function GetPosFromNum
{ # $1 = puzzle-number
local puzpos
for puzpos in {0..15}
do
test "${Puzzle[$puzpos]}" = "$1" && break
done
return $puzpos
}

function Move
{ # $1=Puzzle-pos
test $1 -gt 3 && test "${Puzzle[$(( $1 - 4 ))]}" = " "\
&& swap $1 $(( $1 - 4 )) && return 0
test $(( $1%4 )) -ne 3 && test "${Puzzle[$(( $1 + 1 ))]}" = " "\
&& swap $1 $(( $1 + 1 )) && return 0
test $1 -lt 12 && test "${Puzzle[$(( $1 + 4 ))]}" = " "\
&& swap $1 $(( $1 + 4 )) && return 0
test $(( $1%4 )) -ne 0 && test "${Puzzle[$(( $1 - 1 ))]}" = " " &&\
swap $1 $(( $1 - 1 )) && return 0
return 1
}

function Solved
{
local pos
for pos in {0..14}
do
test "${Puzzle[$pos]}" = $(( $pos + 1 )) || return $FAIL
# Check whether number in each square = square number.
done
return 0
# Successful solution.
}

################### MAIN () ########################
moves=0
Jumble
while true
# Loop continuously until puzzle solved.
do
echo; echo
PrintPuzzle
echo
while true

Appendix A. Contributed Scripts

733

Advanced Bash-Scripting Guide
do
GetNum
puznum=$?
GetPosFromNum $puznum
puzpos=$?
((moves++))
Move $puzpos && break
done
Solved && break
done
echo;echo
PrintPuzzle
echo; echo "BRAVO!"; echo
exit 0
####################################################
#
#
#
#+

Exercise:
-------Rewrite the script to display the letters A - O,
rather than the numbers 1 - 15.

Example A-48. The Towers of Hanoi, graphic version
#! /bin/bash
# The Towers Of Hanoi
# Original script (hanoi.bash) copyright (C) 2000 Amit Singh.
# All Rights Reserved.
# http://hanoi.kernelthread.com
#
#
#
#
#+
#
#

###

hanoi2.bash
Version 2.00: modded for ASCII-graphic display.
Version 2.01: fixed no command-line param bug.
Uses code contributed by Antonio Macchi,
with heavy editing by ABS Guide author.
This variant falls under the original copyright, see above.
Used in ABS Guide with Amit Singh's permission (thanks!).

Variables && sanity check

E_NOPARAM=86
E_BADPARAM=87
E_NOEXIT=88
DISKS=${1:-$E_NOPARAM}
Moves=0

###

# Illegal no. of disks passed to script.

# Must specify how many disks.

MWIDTH=7
MARGIN=2
# Arbitrary "magic" constants; work okay for relatively small # of disks.
# BASEWIDTH=51
# Original code.
let "basewidth = $MWIDTH * $DISKS + $MARGIN"
# "Base" beneath rods.
# Above "algorithm" could likely stand improvement.
###
Display variables
###
let "disks1 = $DISKS - 1"
let "spaces1 = $DISKS"
let "spaces2 = 2 * $DISKS"

Appendix A. Contributed Scripts

734

Advanced Bash-Scripting Guide
let "lastmove_t = $DISKS - 1"

# Final move?

declare -a Rod1 Rod2 Rod3
###

#########################

function repeat
local n

{

###

# $1=char $2=number of repetitions
# Repeat-print a character.

for (( n=0; n<$2; n++ )); do
echo -n "$1"
done
}
function FromRod {
local rod summit weight sequence
while true; do
rod=$1
test ${rod/[^123]/} || continue
sequence=$(echo $(seq 0 $disks1 | tac))
for summit in $sequence; do
eval weight=\${Rod${rod}[$summit]}
test $weight -ne 0 &&
{ echo "$rod $summit $weight"; return; }
done
done
}

function ToRod { # $1=previous (FromRod) weight
local rod firstfree weight sequence
while true; do
rod=$2
test ${rod/[^123]} || continue
sequence=$(echo $(seq 0 $disks1 | tac))
for firstfree in $sequence; do
eval weight=\${Rod${rod}[$firstfree]}
test $weight -gt 0 && { (( firstfree++ )); break; }
done
test $weight -gt $1 -o $firstfree = 0 &&
{ echo "$rod $firstfree"; return; }
done
}

function PrintRods {
local disk rod empty fill sp sequence

repeat " " $spaces1
echo -n "|"
repeat " " $spaces2
echo -n "|"
repeat " " $spaces2
echo "|"

Appendix A. Contributed Scripts

735

Advanced Bash-Scripting Guide
sequence=$(echo $(seq 0 $disks1 | tac))
for disk in $sequence; do
for rod in {1..3}; do
eval empty=$(( $DISKS - (Rod${rod}[$disk] / 2) ))
eval fill=\${Rod${rod}[$disk]}
repeat " " $empty
test $fill -gt 0 && repeat "*" $fill || echo -n "|"
repeat " " $empty
done
echo
done
repeat "=" $basewidth
# Print "base" beneath rods.
echo
}

display ()
{
echo
PrintRods
# Get rod-number, summit and weight
first=( `FromRod $1` )
eval Rod${first[0]}[${first[1]}]=0
# Get rod-number and first-free position
second=( `ToRod ${first[2]} $2` )
eval Rod${second[0]}[${second[1]}]=${first[2]}

echo; echo; echo
if [ "${Rod3[lastmove_t]}" = 1 ]
then
# Last move? If yes, then display final position.
echo "+ Final Position: $Moves moves"; echo
PrintRods
fi
}

# From here down, almost the same as original (hanoi.bash) script.
dohanoi() {
# Recursive function.
case $1 in
0)
;;
*)
dohanoi "$(($1-1))" $2 $4 $3
if [ "$Moves" -ne 0 ]
then
echo "+ Position after move $Moves"
fi
((Moves++))
echo -n "
Next move will be: "
echo $2 "-->" $3
display $2 $3
dohanoi "$(($1-1))" $4 $3 $2
;;
esac
}

Appendix A. Contributed Scripts

736

Advanced Bash-Scripting Guide
setup_arrays ()
{
local dim n elem
let "dim1 = $1 - 1"
elem=$dim1
for n in $(seq 0 $dim1)
do
let "Rod1[$elem] = 2 * $n + 1"
Rod2[$n]=0
Rod3[$n]=0
((elem--))
done
}

###

Main

###

setup_arrays $DISKS
echo; echo "+ Start Position"
case $# in
1) case $(($1>0)) in
# Must have at least one disk.
1)
disks=$1
dohanoi $1 1 3 2
#
Total moves = 2^n - 1, where n = number of disks.
echo
exit 0;
;;
*)
echo "$0: Illegal value for number of disks";
exit $E_BADPARAM;
;;
esac
;;
*)
clear
echo "usage: $0 N"
echo "
Where \"N\" is the number of disks."
exit $E_NOPARAM;
;;
esac
exit $E_NOEXIT

# Shouldn't exit here.

# Note:
# Redirect script output to a file, otherwise it scrolls off display.

Example A-49. The Towers of Hanoi, alternate graphic version
#! /bin/bash
# The Towers Of Hanoi
# Original script (hanoi.bash) copyright (C) 2000 Amit Singh.
# All Rights Reserved.
# http://hanoi.kernelthread.com
#
#

hanoi2.bash
Version 2: modded for ASCII-graphic display.

Appendix A. Contributed Scripts

737

Advanced Bash-Scripting Guide
#
#+
#
#

Uses
with
This
Used

code contributed by Antonio Macchi,
heavy editing by ABS Guide author.
variant also falls under the original copyright, see above.
in ABS Guide with Amit Singh's permission (thanks!).

#
Variables
E_NOPARAM=86
E_BADPARAM=87
E_NOEXIT=88
DELAY=2
DISKS=$1
Moves=0

#
# Illegal no. of disks passed to script.
# Interval, in seconds, between moves. Change, if desired.

MWIDTH=7
MARGIN=2
# Arbitrary "magic" constants, work okay for relatively small # of disks.
# BASEWIDTH=51
# Original code.
let "basewidth = $MWIDTH * $DISKS + $MARGIN" # "Base" beneath rods.
# Above "algorithm" could likely stand improvement.
# Display variables.
let "disks1 = $DISKS - 1"
let "spaces1 = $DISKS"
let "spaces2 = 2 * $DISKS"
let "lastmove_t = $DISKS - 1"

# Final move?

declare -a Rod1 Rod2 Rod3
#################

function repeat
local n

{

# $1=char $2=number of repetitions
# Repeat-print a character.

for (( n=0; n<$2; n++ )); do
echo -n "$1"
done
}
function FromRod {
local rod summit weight sequence
while true; do
rod=$1
test ${rod/[^123]/} || continue
sequence=$(echo $(seq 0 $disks1 | tac))
for summit in $sequence; do
eval weight=\${Rod${rod}[$summit]}
test $weight -ne 0 &&
{ echo "$rod $summit $weight"; return; }
done
done
}

function ToRod { # $1=previous (FromRod) weight
local rod firstfree weight sequence

Appendix A. Contributed Scripts

738

Advanced Bash-Scripting Guide
while true; do
rod=$2
test ${rod/[^123]} || continue
sequence=$(echo $(seq 0 $disks1 | tac))
for firstfree in $sequence; do
eval weight=\${Rod${rod}[$firstfree]}
test $weight -gt 0 && { (( firstfree++ )); break; }
done
test $weight -gt $1 -o $firstfree = 0 &&
{ echo "$rod $firstfree"; return; }
done
}

function PrintRods {
local disk rod empty fill sp sequence
tput cup 5 0
repeat " " $spaces1
echo -n "|"
repeat " " $spaces2
echo -n "|"
repeat " " $spaces2
echo "|"
sequence=$(echo $(seq 0 $disks1 | tac))
for disk in $sequence; do
for rod in {1..3}; do
eval empty=$(( $DISKS - (Rod${rod}[$disk] / 2) ))
eval fill=\${Rod${rod}[$disk]}
repeat " " $empty
test $fill -gt 0 && repeat "*" $fill || echo -n "|"
repeat " " $empty
done
echo
done
repeat "=" $basewidth
# Print "base" beneath rods.
echo
}

display ()
{
echo
PrintRods
# Get rod-number, summit and weight
first=( `FromRod $1` )
eval Rod${first[0]}[${first[1]}]=0
# Get rod-number and first-free position
second=( `ToRod ${first[2]} $2` )
eval Rod${second[0]}[${second[1]}]=${first[2]}

if [ "${Rod3[lastmove_t]}" = 1 ]
then
# Last move? If yes, then display final position.
tput cup 0 0
echo; echo "+ Final Position: $Moves moves"
PrintRods

Appendix A. Contributed Scripts

739

Advanced Bash-Scripting Guide
fi
sleep $DELAY
}
# From here down, almost the same as original (hanoi.bash) script.
dohanoi() {
# Recursive function.
case $1 in
0)
;;
*)
dohanoi "$(($1-1))" $2 $4 $3
if [ "$Moves" -ne 0 ]
then
tput cup 0 0
echo; echo "+ Position after move $Moves"
fi
((Moves++))
echo -n "
Next move will be: "
echo $2 "-->" $3
display $2 $3
dohanoi "$(($1-1))" $4 $3 $2
;;
esac
}
setup_arrays ()
{
local dim n elem
let "dim1 = $1 - 1"
elem=$dim1
for n in $(seq 0 $dim1)
do
let "Rod1[$elem] = 2 * $n + 1"
Rod2[$n]=0
Rod3[$n]=0
((elem--))
done
}

###

Main

###

trap "tput cnorm" 0
tput civis
clear
setup_arrays $DISKS
tput cup 0 0
echo; echo "+

Start Position"

case $# in
1) case $(($1>0)) in
# Must have at least one disk.
1)
disks=$1
dohanoi $1 1 3 2
#
Total moves = 2^n - 1, where n = # of disks.
echo

Appendix A. Contributed Scripts

740

Advanced Bash-Scripting Guide
exit 0;
;;
*)
echo "$0: Illegal value for number of disks";
exit $E_BADPARAM;
;;
esac
;;
*)
echo "usage: $0 N"
echo "
Where \"N\" is the number of disks."
exit $E_NOPARAM;
;;
esac
exit $E_NOEXIT
#
#
#
#+
#+

# Shouldn't exit here.

Exercise:
-------There is a minor bug in the script that causes the display of
the next-to-last move to be skipped.
Fix this.

Example A-50. An alternate version of the getopt-simple.sh script
#!/bin/bash
# UseGetOpt.sh
# Author: Peggy Russell 
UseGetOpt
declare
declare
declare
declare
declare

() {
inputOptions
-r E_OPTERR=85
-r ScriptName=${0##*/}
-r ShortOpts="adf:hlt"
-r LongOpts="aoption,debug,file:,help,log,test"

DoSomething () {
echo "The function name is '${FUNCNAME}'"
# Recall that $FUNCNAME is an internal variable
#+ holding the name of the function it is in.
}
inputOptions=$(getopt -o "${ShortOpts}" --long \
"${LongOpts}" --name "${ScriptName}" -- "${@}")
if [[ ($? -ne 0) || ($# -eq 0) ]]; then
echo "Usage: ${ScriptName} [-dhlt] {OPTION...}"
exit $E_OPTERR
fi
eval set -- "${inputOptions}"
# Only for educational purposes. Can be removed.
#----------------------------------------------echo "++ Test: Number of arguments: [$#]"
echo '++ Test: Looping through "$@"'
for a in "$@"; do
echo " ++ [$a]"
done
#-----------------------------------------------

Appendix A. Contributed Scripts

741

Advanced Bash-Scripting Guide
while true; do
case "${1}" in
--aoption | -a) # Argument found.
echo "Option [$1]"
;;
--debug | -d)
# Enable informational messages.
echo "Option [$1] Debugging enabled"
;;
--file | -f)
# Check for optional argument.
case "$2" in
#+ Double colon is optional argument.
"")
# Not there.
echo "Option [$1] Use default"
shift
;;
*) # Got it
echo "Option [$1] Using input [$2]"
shift
;;
esac
DoSomething
;;
--log | -l) # Enable Logging.
echo "Option [$1] Logging enabled"
;;
--test | -t) # Enable testing.
echo "Option [$1] Testing enabled"
;;
--help | -h)
echo "Option [$1] Display help"
break
;;
--)
# Done! $# is argument number for "--", $@ is "--"
echo "Option [$1] Dash Dash"
break
;;
*)
echo "Major internal error!"
exit 8
;;
esac
echo "Number of arguments: [$#]"
shift
done
shift
# Only for educational purposes. Can be removed.
#---------------------------------------------------------------------echo "++ Test: Number of arguments after \"--\" is [$#] They are: [$@]"
echo '++ Test: Looping through "$@"'
for a in "$@"; do
echo " ++ [$a]"

Appendix A. Contributed Scripts

742

Advanced Bash-Scripting Guide
done
#---------------------------------------------------------------------}
################################### M A I N ########################
# If you remove "function UseGetOpt () {" and corresponding "}",
#+ you can uncomment the "exit 0" line below, and invoke this script
#+ with the various options from the command-line.
#------------------------------------------------------------------# exit 0
echo "Test 1"
UseGetOpt -f myfile one "two three" four
echo;echo "Test 2"
UseGetOpt -h
echo;echo "Test 3 - Short Options"
UseGetOpt -adltf myfile anotherfile
echo;echo "Test 4 - Long Options"
UseGetOpt --aoption --debug --log --test --file myfile anotherfile
exit

Example A-51. The version of the UseGetOpt.sh example used in the Tab Expansion appendix
#!/bin/bash
#
#
#+
#

UseGetOpt-2.sh
Modified version of the script for illustrating tab-expansion
of command-line options.
See the "Introduction to Tab Expansion" appendix.

# Possible options: -a -d -f -l -t -h
#+
--aoption, --debug --file --log --test -- help -#

Author of original script: Peggy Russell 

# UseGetOpt () {
declare inputOptions
declare -r E_OPTERR=85
declare -r ScriptName=${0##*/}
declare -r ShortOpts="adf:hlt"
declare -r LongOpts="aoption,debug,file:,help,log,test"
DoSomething () {
echo "The function name is '${FUNCNAME}'"
}
inputOptions=$(getopt -o "${ShortOpts}" --long \
"${LongOpts}" --name "${ScriptName}" -- "${@}")
if [[ ($? -ne 0) || ($# -eq 0) ]]; then
echo "Usage: ${ScriptName} [-dhlt] {OPTION...}"
exit $E_OPTERR
fi
eval set -- "${inputOptions}"

Appendix A. Contributed Scripts

743

Advanced Bash-Scripting Guide

while true; do
case "${1}" in
--aoption | -a) # Argument found.
echo "Option [$1]"
;;
--debug | -d)
# Enable informational messages.
echo "Option [$1] Debugging enabled"
;;
--file | -f)
# Check for optional argument.
case "$2" in
#+ Double colon is optional argument.
"")
# Not there.
echo "Option [$1] Use default"
shift
;;
*) # Got it
echo "Option [$1] Using input [$2]"
shift
;;
esac
DoSomething
;;
--log | -l) # Enable Logging.
echo "Option [$1] Logging enabled"
;;
--test | -t) # Enable testing.
echo "Option [$1] Testing enabled"
;;
--help | -h)
echo "Option [$1] Display help"
break
;;
--)
# Done! $# is argument number for "--", $@ is "--"
echo "Option [$1] Dash Dash"
break
;;
*)
echo "Major internal error!"
exit 8
;;
esac
echo "Number of arguments: [$#]"
shift
done
shift
#

}

exit

Appendix A. Contributed Scripts

744

Advanced Bash-Scripting Guide
Example A-52. Cycling through all the possible color backgrounds
#!/bin/bash
#
#
#
#

show-all-colors.sh
Displays all 256 possible background colors, using ANSI escape sequences.
Author: Chetankumar Phulpagare
Used in ABS Guide with permission.

T1=8
T2=6
T3=36
offset=0
for num1 in {0..7}
do {
for num2 in {0,1}
do {
shownum=`echo "$offset + $T1 * ${num2} + $num1" | bc`
echo -en "\E[0;48;5;${shownum}m color ${shownum} \E[0m"
}
done
echo
}
done
offset=16
for num1 in {0..5}
do {
for num2 in {0..5}
do {
for num3 in {0..5}
do {
shownum=`echo "$offset + $T2 * ${num3} \
+ $num2 + $T3 * ${num1}" | bc`
echo -en "\E[0;48;5;${shownum}m color ${shownum} \E[0m"
}
done
echo
}
done
}
done
offset=232
for num1 in {0..23}
do {
shownum=`expr $offset + $num1`
echo -en "\E[0;48;5;${shownum}m ${shownum}\E[0m"
}
done
echo

Example A-53. Morse Code Practice
#!/bin/bash
# sam.sh, v. 01
# Still Another Morse (code training script)
# With apologies to Sam (F.B.) Morse.
# Author: Mendel Cooper

Appendix A. Contributed Scripts

745

Advanced Bash-Scripting Guide
# License: GPL3
# Reldate: 05/25/11
# Morse code training script.
# Converts arguments to audible dots and dashes.
# Note: lowercase input only at this time.

# Get the wav files from the source tarball:
# http://bash.webofcrafts.net/abs-guide-latest.tar.bz2
DOT='soundfiles/dot.wav'
DASH='soundfiles/dash.wav'
# Maybe move soundfiles to /usr/local/sounds?
LETTERSPACE=300000 # Microseconds.
WORDSPACE=980000
# Nice and slow, for beginners. Maybe 5 wpm?
EXIT_MSG="May the Morse be with you!"
E_NOARGS=75
# No command-line args?

declare -A morse
# Associative array!
# ======================================= #
morse[a]="dot; dash"
morse[b]="dash; dot; dot; dot"
morse[c]="dash; dot; dash; dot"
morse[d]="dash; dot; dot"
morse[e]="dot"
morse[f]="dot; dot; dash; dot"
morse[g]="dash; dash; dot"
morse[h]="dot; dot; dot; dot"
morse[i]="dot; dot;"
morse[j]="dot; dash; dash; dash"
morse[k]="dash; dot; dash"
morse[l]="dot; dash; dot; dot"
morse[m]="dash; dash"
morse[n]="dash; dot"
morse[o]="dash; dash; dash"
morse[p]="dot; dash; dash; dot"
morse[q]="dash; dash; dot; dash"
morse[r]="dot; dash; dot"
morse[s]="dot; dot; dot"
morse[t]="dash"
morse[u]="dot; dot; dash"
morse[v]="dot; dot; dot; dash"
morse[w]="dot; dash; dash"
morse[x]="dash; dot; dot; dash"
morse[y]="dash; dot; dash; dash"
morse[z]="dash; dash; dot; dot"
morse[0]="dash; dash; dash; dash; dash"
morse[1]="dot; dash; dash; dash; dash"
morse[2]="dot; dot; dash; dash; dash"
morse[3]="dot; dot; dot; dash; dash"
morse[4]="dot; dot; dot; dot; dash"
morse[5]="dot; dot; dot; dot; dot"
morse[6]="dash; dot; dot; dot; dot"
morse[7]="dash; dash; dot; dot; dot"
morse[8]="dash; dash; dash; dot; dot"
morse[8]="dash; dash; dash; dash; dot"

Appendix A. Contributed Scripts

746

Advanced Bash-Scripting Guide
# The following must be escaped or quoted.
morse[?]="dot; dot; dash; dash; dot; dot"
morse[.]="dot; dash; dot; dash; dot; dash"
morse[,]="dash; dash; dot; dot; dash; dash"
morse[/]="dash; dot; dot; dash; dot"
morse[\@]="dot; dash; dash; dot; dash; dot"
# ======================================= #
play_letter ()
{
eval ${morse[$1]}
# Play dots, dashes from appropriate sound files.
# Why is 'eval' necessary here?
usleep $LETTERSPACE # Pause in between letters.
}
extract_letters ()
{
local pos=0
local len=1
strlen=${#1}

# Slice string apart, letter by letter.
# Starting at left end of string.
# One letter at a time.

while [ $pos -lt $strlen ]
do
letter=${1:pos:len}
#
^^^^^^^^^^^^
See Chapter 10.1.
play_letter $letter
echo -n "*"
#
Mark letter just played.
((pos++))
done
}
######### Play the sounds ############
dot() { aplay "$DOT" 2&>/dev/null; }
dash() { aplay "$DASH" 2&>/dev/null; }
######################################
no_args ()
{
declare -a usage
usage=( $0 word1 word2 ... )

#

echo "Usage:"; echo
echo ${usage[*]}
for index in 0 1 2 3
do
extract_letters ${usage[index]}
usleep $WORDSPACE
echo -n " "
# Print space between words.
done
echo "Usage: $0 word1 word2 ... "
echo; echo

}

# int main()
# {
clear
# Clear the terminal screen.
echo "
SAM"
echo "Still Another Morse code trainer"
echo "
Author: Mendel Cooper"
echo; echo;

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Advanced Bash-Scripting Guide
if [ -z "$1" ]
then
no_args
echo; echo; echo "$EXIT_MSG"; echo
exit $E_NOARGS
fi
echo; echo "$*"

# Print text that will be played.

until [ -z "$1" ]
do
extract_letters $1
shift
# On to next word.
usleep $WORDSPACE
echo -n " "
# Print space between words.
done
echo; echo; echo "$EXIT_MSG"; echo
exit 0
# }
#
#
#
#+
#

Exercises:
--------1) Have the script accept either lowercase or uppercase words
as arguments. Hint: Use 'tr' . . .
2) Have the script optionally accept input from a text file.

Example A-54. Base64 encoding/decoding
#!/bin/bash
# base64.sh: Bash implementation of Base64 encoding and decoding.
#
# Copyright (c) 2011 vladz 
# Used in ABSG with permission (thanks!).
#
# Encode or decode original Base64 (and also Base64url)
#+ from STDIN to STDOUT.
#
#
Usage:
#
#
Encode
#
$ ./base64.sh < binary-file > binary-file.base64
#
Decode
#
$ ./base64.sh -d < binary-file.base64 > binary-file
#
# Reference:
#
#
[1] RFC4648 - "The Base16, Base32, and Base64 Data Encodings"
#
http://tools.ietf.org/html/rfc4648#section-5

# The base64_charset[] array contains entire base64 charset,
# and additionally the character "=" ...
base64_charset=( {A..Z} {a..z} {0..9} + / = )
# Nice illustration of brace expansion.
# Uncomment the ### line below to use base64url encoding instead of
#+ original base64.
### base64_charset=( {A..Z} {a..z} {0..9} - _ = )

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Advanced Bash-Scripting Guide
# Output text width when encoding
#+ (64 characters, just like openssl output).
text_width=64
function display_base64_char {
# Convert a 6-bit number (between 0 and 63) into its corresponding values
#+ in Base64, then display the result with the specified text width.
printf "${base64_charset[$1]}"; (( width++ ))
(( width % text_width == 0 )) && printf "\n"
}
function encode_base64 {
# Encode three 8-bit hexadecimal codes into four 6-bit numbers.
#
We need two local int array variables:
#
c8[]: to store the codes of the 8-bit characters to encode
#
c6[]: to store the corresponding encoded values on 6-bit
declare -a -i c8 c6
# Convert hexadecimal to decimal.
c8=( $(printf "ibase=16; ${1:0:2}\n${1:2:2}\n${1:4:2}\n" | bc) )
#
#+
((
((

Let's play with bitwise operators
(3x8-bit into 4x6-bits conversion).
c6[0] = c8[0] >> 2 ))
c6[1] = ((c8[0] & 3) << 4) | (c8[1] >> 4) ))

# The following operations depend on the c8 element number.
case ${#c8[*]} in
3) (( c6[2] = ((c8[1] & 15) << 2) | (c8[2] >> 6) ))
(( c6[3] = c8[2] & 63 )) ;;
2) (( c6[2] = (c8[1] & 15) << 2 ))
(( c6[3] = 64 )) ;;
1) (( c6[2] = c6[3] = 64 )) ;;
esac
for char in ${c6[@]}; do
display_base64_char ${char}
done
}
function decode_base64 {
# Decode four base64 characters into three hexadecimal ASCII characters.
# c8[]: to store the codes of the 8-bit characters
# c6[]: to store the corresponding Base64 values on 6-bit
declare -a -i c8 c6
# Find decimal value corresponding to the current base64 character.
for current_char in ${1:0:1} ${1:1:1} ${1:2:1} ${1:3:1}; do
[ "${current_char}" = "=" ] && break
position=0
while [ "${current_char}" != "${base64_charset[${position}]}" ]; do
(( position++ ))
done
c6=( ${c6[*]} ${position} )
done
# Let's play with bitwise operators
#+ (4x8-bit into 3x6-bits conversion).
(( c8[0] = (c6[0] << 2) | (c6[1] >> 4) ))

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Advanced Bash-Scripting Guide
# The next operations depends on the
case ${#c6[*]} in
3) (( c8[1] = ( (c6[1] & 15) << 4)
(( c8[2] = (c6[2] & 3) << 6 ));
4) (( c8[1] = ( (c6[1] & 15) << 4)
(( c8[2] = ( (c6[2] & 3) << 6)
esac

c6 elements number.
| (c6[2] >>
unset c8[2]
| (c6[2] >>
| c6[3] ))

2) ))
;;
2) ))
;;

for char in ${c8[*]}; do
printf "\x$(printf "%x" ${char})"
done
}

# main ()
if [ "$1" = "-d" ]; then

# decode

# Reformat STDIN in pseudo 4x6-bit groups.
content=$(cat - | tr -d "\n" | sed -r "s/(.{4})/\1 /g")
for chars in ${content}; do decode_base64 ${chars}; done
else
# Make a hexdump of stdin and reformat in 3-byte groups.
content=$(cat - | xxd -ps -u | sed -r "s/(\w{6})/\1 /g" |
tr -d "\n")
for chars in ${content}; do encode_base64 ${chars}; done
echo
fi

Example A-55. The Gronsfeld Cipher
#!/bin/bash
# gronsfeld.bash
# License: GPL3
# Reldate 06/23/11
#
#
#+
#
#+
#
#+
#

This is an implementation of the Gronsfeld Cipher.
It's essentially a stripped-down variant of the
polyalphabetic Vigenère Tableau, but with only 10 alphabets.
The classic Gronsfeld has a numerical sequence as the key word,
but instead we substitute a letter string, for ease of use.
Allegedly, this cipher was invented by the eponymous Count Gronsfeld
in the 17th Century. It was at one time considered to be unbreakable.
Note that this is ###not### a secure cipher by modern standards.

# Global Variables
Enc_suffix="29378"

#
# Encrypted text output with this 5-digit suffix.
# This functions as a decryption flag,
#+ and when used to generate passwords adds security.
Default_key="gronsfeldk"
# The script uses this if key not entered below
# (at "Keychain").
# Change the above two values frequently
#+ for added security.

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Advanced Bash-Scripting Guide
GROUPLEN=5
# Output in groups of 5 letters, per tradition.
alpha1=( abcdefghijklmnopqrstuvwxyz )
alpha2=( {A..Z} )
# Output in all caps, per tradition.
# Use
alpha2=( {a..z} )
for password generator.
wraplen=26
# Wrap around if past end of alphabet.
dflag=
# Decrypt flag (set if $Enc_suffix present).
E_NOARGS=76
# Missing command-line args?
DEBUG=77
# Debugging flag.
declare -a offsets
# This array holds the numerical shift values for
#+ encryption/decryption.
########Keychain#########
key= ### Put key here!!!
# 10 characters!
#########################

# Function
: ()
{ # Encrypt or decrypt, depending on whether $dflag is set.
# Why ": ()" as a function name? Just to prove that it can be done.
local idx keydx mlen off1 shft
local plaintext="$1"
local mlen=${#plaintext}
for (( idx=0; idx<$mlen; idx++ ))
do
let "keydx = $idx % $keylen"
shft=${offsets[keydx]}
if [ -n "$dflag" ]
then
# Decrypt!
let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) - $shft"
# Shift backward to decrypt.
else
# Encrypt!
let "off1 = $(expr index "${alpha1[*]}" ${plaintext:idx:1}) + $shft"
# Shift forward to encrypt.
test $(( $idx % $GROUPLEN)) = 0 && echo -n " " # Groups of 5 letters.
# Comment out above line for output as a string without whitespace,
# for example, if using the script as a password generator.
fi
((off1--))

# Normalize.

if [ $off1 -lt 0 ]
then
# Catch negative indices.
let "off1 += $wraplen"
fi
((off1 %= $wraplen))

# Wrap around if past end of alphabet.

echo -n "${alpha2[off1]}"
done
if [ -z "$dflag" ]
then
echo " $Enc_suffix"
#
echo "$Enc_suffix" # For password generator.

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Advanced Bash-Scripting Guide
else
echo
fi
} # End encrypt/decrypt function.

# int main () {
# Check
if [ -z
then
echo
exit
fi

if command-line args.
"$1" ]
"Usage: $0 TEXT TO ENCODE/DECODE"
$E_NOARGS

if [ ${!#} == "$Enc_suffix" ]
#
^^^^^ Final command-line arg.
then
dflag=ON
echo -n "+"
# Flag decrypted text with a "+" for easy ID.
fi
if [ -z "$key" ]
then
key="$Default_key"
fi

# "gronsfeldk" per above.

keylen=${#key}
for (( idx=0; idx<$keylen; idx++ ))
do # Calculate shift values for encryption/decryption.
offsets[idx]=$(expr index "${alpha1[*]}" ${key:idx:1})
# Normalize.
((offsets[idx]--)) # Necessary because "expr index" starts at 1,
#+ whereas array count starts at 0.
# Generate array of numerical offsets corresponding to the key.
# There are simpler ways to accomplish this.
done
args=$(echo "$*" | sed -e 's/ //g' | tr A-Z a-z | sed -e 's/[0-9]//g')
# Remove whitespace and digits from command-line args.
# Can modify to also remove punctuation characters, if desired.
# Debug:
# echo "$args"; exit $DEBUG
: "$args"
# Call the function named ":".
# : is a null operator, except . . . when it's a function name!
exit $?

# } End-of-script

#
**************************************************************
#
#
This script can function as a password generator,
#+ with several minor mods, see above.
#
That would allow an easy-to-remember password, even the word
#+ "password" itself, which encrypts to vrgfotvo29378
#+ a fairly secure password not susceptible to a dictionary attack.
#
Or, you could use your own name (surely that's easy to remember!).
#
For example, Bozo Bozeman encrypts to hfnbttdppkt29378.
#
**************************************************************
#

To end this section, a review of the basics . . . and more.
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Advanced Bash-Scripting Guide
Example A-56. Basics Reviewed
#!/bin/bash
# basics-reviewed.bash
# File extension == *.bash == specific to Bash
#
#
#
#
#
#
#

#
#
#
#+

Copyright (c) Michael S. Zick, 2003; All rights reserved.
License: Use in any form, for any purpose.
Revision: $ID$
Edited for layout by M.C.
(author of the "Advanced Bash Scripting Guide")
Fixes and updates (04/08) by Cliff Bamford.

This script tested under Bash versions 2.04, 2.05a and 2.05b.
It may not work with earlier versions.
This demonstration script generates one --intentional-"command not found" error message. See line 436.

# The current Bash maintainer, Chet Ramey, has fixed the items noted
#+ for later versions of Bash.

###-------------------------------------------###
### Pipe the output of this script to 'more' ###
###+ else it will scroll off the page.
###
###
###
### You may also redirect its output
###
###+ to a file for examination.
###
###-------------------------------------------###

# Most of the following points are described at length in
#+ the text of the foregoing "Advanced Bash Scripting Guide."
# This demonstration script is mostly just a reorganized presentation.
#
-- msz
# Variables are not typed unless otherwise specified.
# Variables are named. Names must contain a non-digit.
# File descriptor names (as in, for example: 2>&1)
#+ contain ONLY digits.
# Parameters and Bash array elements are numbered.
# (Parameters are very similar to Bash arrays.)
# A variable name may be undefined (null reference).
unset VarNull
# A variable name may be defined but empty (null contents).
VarEmpty=''
# Two, adjacent, single quotes.
# A variable name may be defined and non-empty.
VarSomething='Literal'
# A variable may contain:
#
* A whole number as a signed 32-bit (or larger) integer
#
* A string

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753

Advanced Bash-Scripting Guide
# A variable may also be an array.
# A string may contain embedded blanks and may be treated
#+ as if it where a function name with optional arguments.
# The names of variables and the names of functions
#+ are in different namespaces.

# A variable may be defined as a Bash array either explicitly or
#+ implicitly by the syntax of the assignment statement.
# Explicit:
declare -a ArrayVar

# The echo command is a builtin.
echo $VarSomething
# The printf command is a builtin.
# Translate %s as: String-Format
printf %s $VarSomething
# No linebreak specified, none output.
echo
# Default, only linebreak output.

# The Bash parser word breaks on whitespace.
# Whitespace, or the lack of it is significant.
# (This holds true in general; there are, of course, exceptions.)

# Translate the DOLLAR_SIGN character as: Content-Of.
# Extended-Syntax way of writing Content-Of:
echo ${VarSomething}
# The ${ ... } Extended-Syntax allows more than just the variable
#+ name to be specified.
# In general, $VarSomething can always be written as: ${VarSomething}.
# Call this script with arguments to see the following in action.

# Outside of double-quotes, the special characters @ and *
#+ specify identical behavior.
# May be pronounced as: All-Elements-Of.
# Without specification of a name, they refer to the
#+ pre-defined parameter Bash-Array.

# Glob-Pattern references
echo $*
echo ${*}

# All parameters to script or function
# Same

# Bash disables filename expansion for Glob-Patterns.
# Only character matching is active.

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Advanced Bash-Scripting Guide

# All-Elements-Of references
echo $@
echo ${@}

# Same as above
# Same as above

# Within double-quotes, the behavior of Glob-Pattern references
#+ depends on the setting of IFS (Input Field Separator).
# Within double-quotes, All-Elements-Of references behave the same.

# Specifying only the name of a variable holding a string refers
#+ to all elements (characters) of a string.

# To specify an element (character) of a string,
#+ the Extended-Syntax reference notation (see below) MAY be used.

# Specifying only the name of a Bash array references
#+ the subscript zero element,
#+ NOT the FIRST DEFINED nor the FIRST WITH CONTENTS element.
# Additional qualification is needed to reference other elements,
#+ which means that the reference MUST be written in Extended-Syntax.
# The general form is: ${name[subscript]}.
# The string forms may also be used: ${name:subscript}
#+ for Bash-Arrays when referencing the subscript zero element.

# Bash-Arrays are implemented internally as linked lists,
#+ not as a fixed area of storage as in some programming languages.

#
#

Characteristics of Bash arrays (Bash-Arrays):
--------------------------------------------

#
#+
#
###
#
#+
###
#
###
#
###
#
#
###
#
###
#
#
#

If not otherwise specified, Bash-Array subscripts begin with
subscript number zero. Literally: [0]
This is called zero-based indexing.
If not otherwise specified, Bash-Arrays are subscript packed
(sequential subscripts without subscript gaps).
Negative subscripts are not allowed.
Elements of a Bash-Array need not all be of the same type.
Elements of a Bash-Array may be undefined (null reference).
That is, a Bash-Array may be "subscript sparse."
Elements of a Bash-Array may be defined and empty (null contents).
Elements of a Bash-Array may contain:
* A whole number as a signed 32-bit (or larger) integer
* A string

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Advanced Bash-Scripting Guide
#
* A string formated so that it appears to be a function name
#
+ with optional arguments
###
#
Defined elements of a Bash-Array may be undefined (unset).
#
That is, a subscript packed Bash-Array may be changed
#
+
into a subscript sparse Bash-Array.
###
#
Elements may be added to a Bash-Array by defining an element
#+ not previously defined.
###
# For these reasons, I have been calling them "Bash-Arrays".
# I'll return to the generic term "array" from now on.
#
-- msz

echo "========================================================="
# Lines 202 - 334 supplied by Cliff Bamford. (Thanks!)
# Demo --- Interaction with Arrays, quoting, IFS, echo, * and @
#+ all affect how things work
ArrayVar[0]='zero'
ArrayVar[1]=one
ArrayVar[2]='two'
ArrayVar[3]='three'
ArrayVar[4]='I am four'
ArrayVar[5]='five'
unset ArrayVar[6]
ArrayValue[7]='seven'
ArrayValue[8]=''
ArrayValue[9]='nine'

echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo

#
#
#
#
#
#
#
#
#
#

0
1
2
3
4
5
6
7
8
9

---

normal
unquoted literal
normal
normal
normal with spaces
normal
undefined
normal
defined but empty
normal

'--- Here is the array we are using for this test'
"ArrayVar[0]='zero'
"ArrayVar[1]=one
"ArrayVar[2]='two'
"ArrayVar[3]='three'
"ArrayVar[4]='I am four'
"ArrayVar[5]='five'
"unset ArrayVar[6]
"ArrayValue[7]='seven'
"ArrayValue[8]=''
"ArrayValue[9]='nine'

#
#
#
#
#
#
#
#
#
#

0
1
2
3
4
5
6
7
8
9

normal"
unquoted literal"
normal"
normal"
normal with spaces"
normal"
undefined"
normal"
defined but empty"
normal"

echo
echo '---Case0: No double-quotes, Default IFS of space,tab,newline ---'
IFS=$'\x20'$'\x09'$'\x0A'
# In exactly this order.
echo 'Here is: printf %q {${ArrayVar[*]}'
printf %q ${ArrayVar[*]}
echo
echo 'Here is: printf %q {${ArrayVar[@]}'
printf %q ${ArrayVar[@]}
echo
echo 'Here is: echo ${ArrayVar[*]}'
echo ${ArrayVar[@]}
echo 'Here is: echo {${ArrayVar[@]}'
echo ${ArrayVar[@]}

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756

Advanced Bash-Scripting Guide
echo
echo '---Case1: Within double-quotes - Default IFS of space-tabnewline ---'
IFS=$'\x20'$'\x09'$'\x0A'
# These three bytes,
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case2: Within double-quotes - IFS is q'
IFS='q'
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case3: Within double-quotes - IFS is ^'
IFS='^'
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case4: Within double-quotes - IFS is ^ followed by
space,tab,newline'
IFS=$'^'$'\x20'$'\x09'$'\x0A'
# ^ + space tab newline
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case6: Within double-quotes - IFS set and empty '
IFS=''
echo 'Here is: printf %q "{${ArrayVar[*]}"'

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757

Advanced Bash-Scripting Guide
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---Case7: Within double-quotes - IFS is unset'
unset IFS
echo 'Here is: printf %q "{${ArrayVar[*]}"'
printf %q "${ArrayVar[*]}"
echo
echo 'Here is: printf %q "{${ArrayVar[@]}"'
printf %q "${ArrayVar[@]}"
echo
echo 'Here is: echo "${ArrayVar[*]}"'
echo "${ArrayVar[@]}"
echo 'Here is: echo "{${ArrayVar[@]}"'
echo "${ArrayVar[@]}"
echo
echo '---End of Cases---'
echo "========================================================="; echo

# Put IFS back to the default.
# Default is exactly these three bytes.
IFS=$'\x20'$'\x09'$'\x0A'
# In exactly this order.
# Interpretation of the above outputs:
#
A Glob-Pattern is I/O; the setting of IFS matters.
###
#
An All-Elements-Of does not consider IFS settings.
###
#
Note the different output using the echo command and the
#+ quoted format operator of the printf command.

# Recall:
#
Parameters are similar to arrays and have the similar behaviors.
###
# The above examples demonstrate the possible variations.
# To retain the shape of a sparse array, additional script
#+ programming is required.
###
# The source code of Bash has a routine to output the
#+ [subscript]=value
array assignment format.
# As of version 2.05b, that routine is not used,
#+ but that might change in future releases.

# The length of a string, measured in non-null elements (characters):
echo
echo '- - Non-quoted references - -'
echo 'Non-Null character count: '${#VarSomething}' characters.'

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758

Advanced Bash-Scripting Guide
# test='Lit'$'\x00''eral'
# echo ${#test}

# $'\x00' is a null character.
# See that?

# The length of an array, measured in defined elements,
#+ including null content elements.
echo
echo 'Defined content count: '${#ArrayVar[@]}' elements.'
# That is NOT the maximum subscript (4).
# That is NOT the range of the subscripts (1 . . 4 inclusive).
# It IS the length of the linked list.
###
# Both the maximum subscript and the range of the subscripts may
#+ be found with additional script programming.
# The length of a string, measured in non-null elements (characters):
echo
echo '- - Quoted, Glob-Pattern references - -'
echo 'Non-Null character count: '"${#VarSomething}"' characters.'
# The length of an array, measured in defined elements,
#+ including null-content elements.
echo
echo 'Defined element count: '"${#ArrayVar[*]}"' elements.'
#
#
#
#+

Interpretation: Substitution does not effect the ${# ... } operation.
Suggestion:
Always use the All-Elements-Of character
if that is what is intended (independence from IFS).

# Define a simple function.
# I include an underscore in the name
#+ to make it distinctive in the examples below.
###
# Bash separates variable names and function names
#+ in different namespaces.
# The Mark-One eyeball isn't that advanced.
###
_simple() {
echo -n 'SimpleFunc'$@
# Newlines are swallowed in
}
#+ result returned in any case.

# The ( ... ) notation invokes a command or function.
# The $( ... ) notation is pronounced: Result-Of.

# Invoke the function _simple
echo
echo '- - Output of function _simple - -'
_simple
# Try passing arguments.
echo
# or
(_simple)
# Try passing arguments.
echo
echo '- Is there a variable of that name? -'
echo $_simple not defined
# No variable by that name.

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Advanced Bash-Scripting Guide
# Invoke the result of function _simple (Error msg intended)
###
$(_simple)
#
#

# Gives an error message:
line 436: SimpleFunc: command not found
---------------------------------------

echo
###
# The first word of the result of function _simple
#+ is neither a valid Bash command nor the name of a defined function.
###
# This demonstrates that the output of _simple is subject to evaluation.
###
# Interpretation:
#
A function can be used to generate in-line Bash commands.

# A simple function where the first word of result IS a bash command:
###
_print() {
echo -n 'printf %q '$@
}
echo '- - Outputs of function _print - -'
_print parm1 parm2
# An Output NOT A Command.
echo
$(_print parm1 parm2)

# Executes: printf %q parm1 parm2
# See above IFS examples for the
#+ various possibilities.

echo
$(_print $VarSomething)
echo

# The predictable result.

# Function variables
# -----------------echo
echo '- - Function variables - -'
# A variable may represent a signed integer, a string or an array.
# A string may be used like a function name with optional arguments.
# set -vx
declare -f funcVar

# Enable if desired
#+ in namespace of functions

funcVar=_print
$funcVar parm1
echo

# Contains name of function.
# Same as _print at this point.

funcVar=$(_print )
$funcVar
$funcVar $VarSomething
echo

# Contains result of function.
# No input, No output.
# The predictable result.

funcVar=$(_print $VarSomething)
$funcVar
echo

# $VarSomething replaced HERE.
# The expansion is part of the
#+ variable contents.

Appendix A. Contributed Scripts

760

Advanced Bash-Scripting Guide
funcVar="$(_print $VarSomething)"
$funcVar
echo
#
#+
#
#

# $VarSomething replaced HERE.
# The expansion is part of the
#+ variable contents.

The difference between the unquoted and the double-quoted versions
above can be seen in the "protect_literal.sh" example.
The first case above is processed as two, unquoted, Bash-Words.
The second case above is processed as one, quoted, Bash-Word.

# Delayed replacement
# ------------------echo
echo '- - Delayed replacement - -'
funcVar="$(_print '$VarSomething')" # No replacement, single Bash-Word.
eval $funcVar
# $VarSomething replaced HERE.
echo
VarSomething='NewThing'
eval $funcVar
echo

# $VarSomething replaced HERE.

# Restore the original setting trashed above.
VarSomething=Literal
#
#+
#
#+

There are a pair of functions demonstrated in the
"protect_literal.sh" and "unprotect_literal.sh" examples.
These are general purpose functions for delayed replacement literals
containing variables.

# REVIEW:
# -----# A string can be considered a Classic-Array of elements (characters).
# A string operation applies to all elements (characters) of the string
#+ (in concept, anyway).
###
# The notation: ${array_name[@]} represents all elements of the
#+ Bash-Array: array_name.
###
# The Extended-Syntax string operations can be applied to all
#+ elements of an array.
###
# This may be thought of as a For-Each operation on a vector of strings.
###
# Parameters are similar to an array.
# The initialization of a parameter array for a script
#+ and a parameter array for a function only differ
#+ in the initialization of ${0}, which never changes its setting.
###
# Subscript zero of the script's parameter array contains
#+ the name of the script.
###
# Subscript zero of a function's parameter array DOES NOT contain

Appendix A. Contributed Scripts

761

Advanced Bash-Scripting Guide
#+ the name of the function.
# The name of the current function is accessed by the $FUNCNAME variable.
###
# A quick, review list follows (quick, not short).
echo
echo
echo
echo
echo
echo
echo

'- - Test (but not change) - -'
'- null reference -'
-n ${VarNull-'NotSet'}' '
${VarNull}
-n ${VarNull:-'NotSet'}' '
${VarNull}

#
#
#
#

NotSet
NewLine only
NotSet
Newline only

echo
echo
echo
echo
echo

'- null contents -'
-n ${VarEmpty-'Empty'}' '
${VarEmpty}
-n ${VarEmpty:-'Empty'}' '
${VarEmpty}

#
#
#
#

Only the space
Newline only
Empty
Newline only

echo '- contents -'
echo ${VarSomething-'Content'}
echo ${VarSomething:-'Content'}

# Literal
# Literal

echo '- Sparse Array -'
echo ${ArrayVar[@]-'not set'}
#
#
#
#
#
#

ASCII-Art time
State
Y==yes,
Unset
Y
Empty
N
Contents N

N==no
:Y
Y
N

${# ... } == 0
${# ... } == 0
${# ... } > 0

# Either the first and/or the second part of the tests
#+ may be a command or a function invocation string.
echo
echo '- - Test 1 for undefined - -'
declare -i t
_decT() {
t=$t-1
}
# Null reference, set: t == -1
t=${#VarNull}
${VarNull- _decT }
echo $t

# Results in zero.
# Function executes, t now -1.

# Null contents, set: t == 0
t=${#VarEmpty}
${VarEmpty- _decT }
echo $t

# Results in zero.
# _decT function NOT executed.

# Contents, set: t == number of non-null characters
VarSomething='_simple'
# Set to valid function name.
t=${#VarSomething}
# non-zero length
${VarSomething- _decT }
# Function _simple executed.
echo $t
# Note the Append-To action.
# Exercise: clean up that example.
unset t
unset _decT
VarSomething=Literal

Appendix A. Contributed Scripts

762

Advanced Bash-Scripting Guide
echo
echo '- - Test and Change - -'
echo '- Assignment if null reference -'
echo -n ${VarNull='NotSet'}' '
# NotSet NotSet
echo ${VarNull}
unset VarNull
echo '- Assignment if null reference -'
echo -n ${VarNull:='NotSet'}' '
# NotSet NotSet
echo ${VarNull}
unset VarNull
echo '- No assignment if null contents -'
echo -n ${VarEmpty='Empty'}' '
# Space only
echo ${VarEmpty}
VarEmpty=''
echo '- Assignment if null contents -'
echo -n ${VarEmpty:='Empty'}' '
echo ${VarEmpty}
VarEmpty=''

# Empty Empty

echo '- No change if already has contents -'
echo ${VarSomething='Content'}
# Literal
echo ${VarSomething:='Content'}
# Literal

# "Subscript sparse" Bash-Arrays
###
# Bash-Arrays are subscript packed, beginning with
#+ subscript zero unless otherwise specified.
###
# The initialization of ArrayVar was one way
#+ to "otherwise specify". Here is the other way:
###
echo
declare -a ArraySparse
ArraySparse=( [1]=one [2]='' [4]='four' )
# [0]=null reference, [2]=null content, [3]=null reference
echo '- - Array-Sparse List - -'
# Within double-quotes, default IFS, Glob-Pattern
IFS=$'\x20'$'\x09'$'\x0A'
printf %q "${ArraySparse[*]}"
echo
# Note that the output does not distinguish between "null content"
#+ and "null reference".
# Both print as escaped whitespace.
###
# Note also that the output does NOT contain escaped whitespace
#+ for the "null reference(s)" prior to the first defined element.
###
# This behavior of 2.04, 2.05a and 2.05b has been reported
#+ and may change in a future version of Bash.
# To output a sparse array and maintain the [subscript]=value
#+ relationship without change requires a bit of programming.
# One possible code fragment:
###

Appendix A. Contributed Scripts

763

Advanced Bash-Scripting Guide
# local l=${#ArraySparse[@]}
# Count of defined elements
# local f=0
# Count of found subscripts
# local i=0
# Subscript to test
(
# Anonymous in-line function
for (( l=${#ArraySparse[@]}, f = 0, i = 0 ; f < l ; i++ ))
do
# 'if defined then...'
${ArraySparse[$i]+ eval echo '\ ['$i']='${ArraySparse[$i]} ; (( f++ )) }
done
)
# The reader coming upon the above code fragment cold
#+ might want to review "command lists" and "multiple commands on a line"
#+ in the text of the foregoing "Advanced Bash Scripting Guide."
###
# Note:
# The "read -a array_name" version of the "read" command
#+ begins filling array_name at subscript zero.
# ArraySparse does not define a value at subscript zero.
###
# The user needing to read/write a sparse array to either
#+ external storage or a communications socket must invent
#+ a read/write code pair suitable for their purpose.
###
# Exercise: clean it up.
unset ArraySparse
echo
echo '- - Conditional alternate (But not change)- -'
echo '- No alternate if null reference -'
echo -n ${VarNull+'NotSet'}' '
echo ${VarNull}
unset VarNull
echo '- No alternate if null reference -'
echo -n ${VarNull:+'NotSet'}' '
echo ${VarNull}
unset VarNull
echo '- Alternate if null contents -'
echo -n ${VarEmpty+'Empty'}' '
echo ${VarEmpty}
VarEmpty=''
echo '- No alternate if null contents -'
echo -n ${VarEmpty:+'Empty'}' '
echo ${VarEmpty}
VarEmpty=''

# Empty

# Space only

echo '- Alternate if already has contents -'
# Alternate literal
echo -n ${VarSomething+'Content'}' '
echo ${VarSomething}
# Invoke function
echo -n ${VarSomething:+ $(_simple) }' '
echo ${VarSomething}
echo

# Content Literal

# SimpleFunc Literal

echo '- - Sparse Array - -'

Appendix A. Contributed Scripts

764

Advanced Bash-Scripting Guide
echo ${ArrayVar[@]+'Empty'}
echo

# An array of 'Empty'(ies)

echo '- - Test 2 for undefined - -'
declare -i t
_incT() {
t=$t+1
}
# Note:
# This is the same test used in the sparse array
#+ listing code fragment.
# Null reference, set: t == -1
t=${#VarNull}-1
${VarNull+ _incT }
echo $t' Null reference'

# Results in minus-one.
# Does not execute.

# Null contents, set: t == 0
t=${#VarEmpty}-1
${VarEmpty+ _incT }
echo $t' Null content'

# Results in minus-one.
# Executes.

# Contents, set: t == (number of non-null characters)
t=${#VarSomething}-1
# non-null length minus-one
${VarSomething+ _incT }
# Executes.
echo $t' Contents'
# Exercise: clean up that example.
unset t
unset _incT
# ${name?err_msg} ${name:?err_msg}
# These follow the same rules but always exit afterwards
#+ if an action is specified following the question mark.
# The action following the question mark may be a literal
#+ or a function result.
###
# ${name?} ${name:?} are test-only, the return can be tested.

# Element operations
# -----------------echo
echo '- - Trailing sub-element selection - -'
#

Strings, Arrays and Positional parameters

# Call this script with multiple arguments
#+ to see the parameter selections.
echo
echo
echo
echo

'- All -'
${VarSomething:0}
${ArrayVar[@]:0}
${@:0}

#
#
#
#

all non-null characters
all elements with content
all parameters with content;
ignoring parameter[0]

echo

Appendix A. Contributed Scripts

765

Advanced Bash-Scripting Guide
echo
echo
echo
echo

'- All after -'
${VarSomething:1}
${ArrayVar[@]:1}
${@:2}

echo
echo '- Range after -'
echo ${VarSomething:4:3}

# all non-null after character[0]
# all after element[0] with content
# all after param[1] with content

# ral
# Three characters after
# character[3]

echo '- Sparse array gotch -'
echo ${ArrayVar[@]:1:2}
# four - The only element with content.
# Two elements after (if that many exist).
# the FIRST WITH CONTENTS
#+ (the FIRST WITH CONTENTS is being
#+ considered as if it
#+ were subscript zero).
# Executed as if Bash considers ONLY array elements with CONTENT
# printf %q "${ArrayVar[@]:0:3}"
# Try this one
# In versions 2.04, 2.05a and 2.05b,
#+ Bash does not handle sparse arrays as expected using this notation.
#
# The current Bash maintainer, Chet Ramey, has corrected this.

echo '- Non-sparse array -'
echo ${@:2:2}
# Two parameters following parameter[1]
# New victims for string vector examples:
stringZ=abcABC123ABCabc
arrayZ=( abcabc ABCABC 123123 ABCABC abcabc )
sparseZ=( [1]='abcabc' [3]='ABCABC' [4]='' [5]='123123' )
echo
echo
echo
echo
echo
echo
echo

'
'
'
'
'
'

-

- Victim string - -'$stringZ'- - '
- Victim array - -'${arrayZ[@]}'- - '
- Sparse array - -'${sparseZ[@]}'- - '
[0]==null ref, [2]==null ref, [4]==null content - '
[1]=abcabc [3]=ABCABC [5]=123123 - '
non-null-reference count: '${#sparseZ[@]}' elements'

echo
echo '- - Prefix sub-element removal - -'
echo '- - Glob-Pattern match must include the first character. - -'
echo '- - Glob-Pattern may be a literal or a function result. - -'
echo

# Function returning a simple, Literal, Glob-Pattern
_abc() {
echo -n 'abc'
}
echo
echo
echo
echo

'- Shortest prefix -'
${stringZ#123}
${stringZ#$(_abc)}
${arrayZ[@]#abc}

# Unchanged (not a prefix).
# ABC123ABCabc
# Applied to each element.

# echo ${sparseZ[@]#abc}
# Version-2.05b core dumps.
# Has since been fixed by Chet Ramey.

Appendix A. Contributed Scripts

766

Advanced Bash-Scripting Guide
# The -it would be nice- First-Subscript-Of
# echo ${#sparseZ[@]#*}
# This is NOT valid Bash.
echo
echo
echo
echo
echo

'- Longest prefix -'
${stringZ##1*3}
${stringZ##a*C}
${arrayZ[@]##a*c}

# Unchanged (not a prefix)
# abc
# ABCABC 123123 ABCABC

# echo ${sparseZ[@]##a*c}
# Version-2.05b core dumps.
# Has since been fixed by Chet Ramey.
echo
echo
echo
echo
echo
echo
echo
echo
echo

'- - Suffix sub-element removal - -'
'- - Glob-Pattern match must include the last character. - -'
'- - Glob-Pattern may be a literal or a function result. - -'
'- Shortest suffix -'
${stringZ%1*3}
${stringZ%$(_abc)}
${arrayZ[@]%abc}

# Unchanged (not a suffix).
# abcABC123ABC
# Applied to each element.

# echo ${sparseZ[@]%abc}
# Version-2.05b core dumps.
# Has since been fixed by Chet Ramey.
# The -it would be nice- Last-Subscript-Of
# echo ${#sparseZ[@]%*}
# This is NOT valid Bash.
echo
echo
echo
echo
echo

'- Longest suffix -'
${stringZ%%1*3}
${stringZ%%b*c}
${arrayZ[@]%%b*c}

# Unchanged (not a suffix)
# a
# a ABCABC 123123 ABCABC a

# echo ${sparseZ[@]%%b*c}
# Version-2.05b core dumps.
# Has since been fixed by Chet Ramey.
echo
echo
echo
echo
echo
echo
echo
echo
echo

'''''''

-

Sub-element replacement - -'
Sub-element at any location in string. - -'
First specification is a Glob-Pattern - -'
Glob-Pattern may be a literal or Glob-Pattern function result. - -'
Second specification may be a literal or function result. - -'
Second specification may be unspecified. Pronounce that'
as: Replace-With-Nothing (Delete) - -'

# Function returning a simple, Literal, Glob-Pattern
_123() {
echo -n '123'
}
echo
echo
echo
echo
echo

'- Replace first occurrence -'
${stringZ/$(_123)/999}
# Changed (123 is a component).
${stringZ/ABC/xyz}
# xyzABC123ABCabc
${arrayZ[@]/ABC/xyz}
# Applied to each element.
${sparseZ[@]/ABC/xyz}
# Works as expected.

echo

Appendix A. Contributed Scripts

767

Advanced Bash-Scripting Guide
echo
echo
echo
echo
echo

'- Delete first occurrence -'
${stringZ/$(_123)/}
${stringZ/ABC/}
${arrayZ[@]/ABC/}
${sparseZ[@]/ABC/}

# The replacement need not be a literal,
#+ since the result of a function invocation is allowed.
# This is general to all forms of replacement.
echo
echo '- Replace first occurrence with Result-Of -'
echo ${stringZ/$(_123)/$(_simple)} # Works as expected.
echo ${arrayZ[@]/ca/$(_simple)}
# Applied to each element.
echo ${sparseZ[@]/ca/$(_simple)}
# Works as expected.
echo
echo
echo
echo
echo
echo

'- Replace all occurrences -'
${stringZ//[b2]/X}
${stringZ//abc/xyz}
${arrayZ[@]//abc/xyz}
${sparseZ[@]//abc/xyz}

echo
echo
echo
echo
echo
echo

'- Delete all occurrences -'
${stringZ//[b2]/}
${stringZ//abc/}
${arrayZ[@]//abc/}
${sparseZ[@]//abc/}

#
#
#
#

X-out b's and 2's
xyzABC123ABCxyz
Applied to each element.
Works as expected.

echo
echo '- - Prefix sub-element replacement - -'
echo '- - Match must include the first character. - -'
echo
echo
echo
echo
echo
echo

'- Replace prefix occurrences -'
${stringZ/#[b2]/X}
# Unchanged (neither is a prefix).
${stringZ/#$(_abc)/XYZ}
# XYZABC123ABCabc
${arrayZ[@]/#abc/XYZ}
# Applied to each element.
${sparseZ[@]/#abc/XYZ}
# Works as expected.

echo
echo
echo
echo
echo
echo

'- Delete prefix occurrences -'
${stringZ/#[b2]/}
${stringZ/#$(_abc)/}
${arrayZ[@]/#abc/}
${sparseZ[@]/#abc/}

echo
echo '- - Suffix sub-element replacement - -'
echo '- - Match must include the last character. - -'
echo
echo
echo
echo
echo
echo

'- Replace suffix occurrences -'
${stringZ/%[b2]/X}
# Unchanged (neither is a suffix).
${stringZ/%$(_abc)/XYZ}
# abcABC123ABCXYZ
${arrayZ[@]/%abc/XYZ}
# Applied to each element.
${sparseZ[@]/%abc/XYZ}
# Works as expected.

echo
echo '- Delete suffix occurrences -'
echo ${stringZ/%[b2]/}
echo ${stringZ/%$(_abc)/}

Appendix A. Contributed Scripts

768

Advanced Bash-Scripting Guide
echo ${arrayZ[@]/%abc/}
echo ${sparseZ[@]/%abc/}
echo
echo '- - Special cases of null Glob-Pattern - -'
echo
echo '- Prefix all -'
# null substring pattern means 'prefix'
echo ${stringZ/#/NEW}
# NEWabcABC123ABCabc
echo ${arrayZ[@]/#/NEW}
# Applied to each element.
echo ${sparseZ[@]/#/NEW}
# Applied to null-content also.
# That seems reasonable.
echo
echo '- Suffix all -'
# null substring pattern means 'suffix'
echo ${stringZ/%/NEW}
# abcABC123ABCabcNEW
echo ${arrayZ[@]/%/NEW}
# Applied to each element.
echo ${sparseZ[@]/%/NEW}
# Applied to null-content also.
# That seems reasonable.
echo
echo '- - Special case For-Each Glob-Pattern - -'
echo '- - - - This is a nice-to-have dream - - - -'
echo
_GenFunc() {
echo -n ${0}
# Illustration only.
# Actually, that would be an arbitrary computation.
}
# All occurrences, matching the AnyThing pattern.
# Currently //*/ does not match null-content nor null-reference.
# /#/ and /%/ does match null-content but not null-reference.
echo ${sparseZ[@]//*/$(_GenFunc)}

# A possible syntax would be to make
#+ the parameter notation used within this construct mean:
#
${1} - The full element
#
${2} - The prefix, if any, to the matched sub-element
#
${3} - The matched sub-element
#
${4} - The suffix, if any, to the matched sub-element
#
# echo ${sparseZ[@]//*/$(_GenFunc ${3})}
# Same as ${1} here.
# Perhaps it will be implemented in a future version of Bash.

exit 0

Appendix A. Contributed Scripts

769

Appendix B. Reference Cards
The following reference cards provide a useful summary of certain scripting concepts. The foregoing text
treats these matters in more depth, as well as giving usage examples.

Table B-1. Special Shell Variables
Variable
$0
$1
$2 - $9
${10}
$#
"$*"

Meaning
Filename of script
Positional parameter #1
Positional parameters #2 - #9
Positional parameter #10
Number of positional parameters
All the positional parameters (as a single
word) *
"$@"
All the positional parameters (as separate
strings)
${#*}
Number of positional parameters
${#@}
Number of positional parameters
$?
Return value
$$
Process ID (PID) of script
$Flags passed to script (using set)
$_
Last argument of previous command
$!
Process ID (PID) of last job run in
background
* Must be quoted, otherwise it defaults to "$@".

Table B-2. TEST Operators: Binary Comparison
Operator

Meaning

Arithmetic Comparison
-eq
Equal to
-ne
-lt
-le
-gt
-ge

Not equal to
Less than
Less than or equal to
Greater than
Greater than or equal to

Appendix B. Reference Cards

----- Operator

Meaning

String Comparison
=
==
!=
\<

Equal to
Equal to
Not equal to
Less than (ASCII) *

\>

Greater than (ASCII) *

-z

String is empty
770

Advanced Bash-Scripting Guide
String is not empty

-n
Arithmetic Comparison within double parentheses (( ... ))
>
Greater than
>=
Greater than or equal to
<
Less than
<=
Less than or equal to
* If within a double-bracket [[ ... ]] test construct, then no escape \ is needed.

Table B-3. TEST Operators: Files
Operator
-e
-f
-d
-h
-L
-b
-c
-p
-S
-t

Tests Whether
File exists
File is a regular file
File is a directory
File is a symbolic link
File is a symbolic link
File is a block device
File is a character device
File is a pipe
File is a socket
File is associated with a terminal

-N
-O
-G

File modified since it was last read
You own the file
Group id of file same as yours

-----

Operator
-s

Tests Whether
File is not zero size

-r
-w
-x

File has read permission
File has write permission
File has execute permission

-g
-u
-k

sgid flag set
suid flag set
"sticky bit" set

F1 -nt F2
F1 -ot F2
F1 -ef F2

File F1 is newer than F2 *
File F1 is older than F2 *
Files F1 and F2 are hard links to the
same file *

!
NOT (inverts sense of above tests)
* Binary operator (requires two operands).

Table B-4. Parameter Substitution and Expansion
Expression
${var}

Meaning
Value of var (same as $var)

${var-DEFAULT} If var not set, evaluate expression as $DEFAULT *
${var:-DEFAULT} If var not set or is empty, evaluate expression as $DEFAULT *
${var=DEFAULT} If var not set, evaluate expression as $DEFAULT *
Appendix B. Reference Cards

771

Advanced Bash-Scripting Guide
${var:=DEFAULT} If var not set, evaluate expression as $DEFAULT *
${var+OTHER}
${var:+OTHER}

If var set, evaluate expression as $OTHER, otherwise as null string
If var set, evaluate expression as $OTHER, otherwise as null string

${var?ERR_MSG} If var not set, print $ERR_MSG and abort script with an exit status of 1.*
${var:?ERR_MSG} If var not set, print $ERR_MSG and abort script with an exit status of 1.*
${!varprefix*} Matches all previously declared variables beginning with varprefix
${!varprefix@} Matches all previously declared variables beginning with varprefix
* If var is set, evaluate the expression as $var with no side-effects.

Table B-5. String Operations
Expression
${#string}

Meaning
Length of $string

${string:position}
${string:position:length}

Extract substring from $string at $position
Extract $length characters substring from $string
at $position [zero-indexed, first character is at
position 0]

${string#substring}

Strip shortest match of $substring from front of
$string
Strip longest match of $substring from front of
$string
Strip shortest match of $substring from back of
$string
Strip longest match of $substring from back of
$string

${string##substring}
${string%substring}
${string%%substring}

${string/substring/replacement}
${string//substring/replacement}
${string/#substring/replacement}
${string/%substring/replacement}

expr match "$string" '$substring'

Appendix B. Reference Cards

Replace first match of $substring with
$replacement
Replace all matches of $substring with
$replacement
If $substring matches front end of $string,
substitute $replacement for $substring
If $substring matches back end of $string,
substitute $replacement for $substring

Length of matching $substring* at beginning of
$string
772

Advanced Bash-Scripting Guide
expr "$string" : '$substring'
expr index "$string" $substring

expr substr $string $position
$length
expr match "$string"
'\($substring\)'
expr "$string" : '\($substring\)'
expr match "$string"
'.*\($substring\)'
expr "$string" :
'.*\($substring\)'
* Where $substring is a Regular Expression.

Length of matching $substring* at beginning of
$string
Numerical position in $string of first character in
$substring* that matches [0 if no match, first
character counts as position 1]
Extract $length characters from $string starting at
$position [0 if no match, first character counts as
position 1]
Extract $substring*, searching from beginning of
$string
Extract $substring* , searching from beginning of
$string
Extract $substring*, searching from end of
$string
Extract $substring*, searching from end of
$string

Table B-6. Miscellaneous Constructs
Expression

Interpretation

Brackets
if [ CONDITION ]
if [[ CONDITION ]]
Array[1]=element1
[a-z]

Test construct
Extended test construct
Array initialization
Range of characters within a Regular Expression

Curly Brackets
${variable}
Parameter substitution
${!variable}
Indirect variable reference
{ command1; command2; . . . commandN; } Block of code
{string1,string2,string3,...}
Brace expansion
{a..z}
Extended brace expansion
{}
Text replacement, after find and xargs

Parentheses
( command1; command2 )
Array=(element1 element2 element3)
result=$(COMMAND)
>(COMMAND)

Appendix B. Reference Cards

Command group executed within a subshell
Array initialization
Command substitution, new style
Process substitution
773

Advanced Bash-Scripting Guide
<(COMMAND)

Process substitution

Double Parentheses
(( var = 78 ))
var=$(( 20 + 5 ))
(( var++ ))
(( var-- ))
(( var0 = var1<98?9:21 ))

Integer arithmetic
Integer arithmetic, with variable assignment
C-style variable increment
C-style variable decrement
C-style trinary operation

Quoting
"$variable"
'string'

"Weak" quoting
'Strong' quoting

Back Quotes
result=`COMMAND`

Command substitution, classic style

Appendix B. Reference Cards

774

Appendix C. A Sed and Awk Micro-Primer
This is a very brief introduction to the sed and awk text processing utilities. We will deal with only a few
basic commands here, but that will suffice for understanding simple sed and awk constructs within shell
scripts.
sed: a non-interactive text file editor
awk: a field-oriented pattern processing language with a C-style syntax
For all their differences, the two utilities share a similar invocation syntax, use regular expressions , read input
by default from stdin, and output to stdout. These are well-behaved UNIX tools, and they work together
well. The output from one can be piped to the other, and their combined capabilities give shell scripts some of
the power of Perl.
One important difference between the utilities is that while shell scripts can easily pass arguments
to sed, it is more complicated for awk (see Example 36-5 and Example 28-2).

C.1. Sed
Sed is a non-interactive [139] stream editor. It receives text input, whether from stdin or from a file,
performs certain operations on specified lines of the input, one line at a time, then outputs the result to
stdout or to a file. Within a shell script, sed is usually one of several tool components in a pipe.
Sed determines which lines of its input that it will operate on from the address range passed to it. [140]
Specify this address range either by line number or by a pattern to match. For example, 3d signals sed to
delete line 3 of the input, and /Windows/d tells sed that you want every line of the input containing a match
to "Windows" deleted.
Of all the operations in the sed toolkit, we will focus primarily on the three most commonly used ones. These
are printing (to stdout), deletion, and substitution.

Table C-1. Basic sed operators
Operator
[address-range]/p
[address-range]/d
s/pattern1/pattern2/

Effect
Print [specified address range]
Delete [specified address range]
Substitute pattern2 for first instance of
pattern1 in a line
[address-range]/s/pattern1/pattern2/ substitute Substitute pattern2 for first instance of
pattern1 in a line, over address-range
[address-range]/y/pattern1/pattern2/ transform replace any character in pattern1 with the
corresponding character in pattern2, over
address-range (equivalent of tr)

Appendix C. A Sed and Awk Micro-Primer

Name
print
delete
substitute

775

Advanced Bash-Scripting Guide
global

g

Operate on every pattern match within each
matched line of input

Unless the g (global) operator is appended to a substitute command, the substitution operates only on the
first instance of a pattern match within each line.
From the command-line and in a shell script, a sed operation may require quoting and certain options.
sed -e '/^$/d' $filename
# The -e option causes the next string to be interpreted as an editing instruction.
# (If passing only a single instruction to sed, the "-e" is optional.)
# The "strong" quotes ('') protect the RE characters in the instruction
#+ from reinterpretation as special characters by the body of the script.
# (This reserves RE expansion of the instruction for sed.)
#
# Operates on the text contained in file $filename.

In certain cases, a sed editing command will not work with single quotes.
filename=file1.txt
pattern=BEGIN
sed "/^$pattern/d" "$filename" # Works as specified.
# sed '/^$pattern/d' "$filename"
has unexpected results.
#
In this instance, with strong quoting (' ... '),
#+
"$pattern" will not expand to "BEGIN".

Sed uses the -e option to specify that the following string is an instruction or set of instructions. If
there is only a single instruction contained in the string, then this may be omitted.
sed -n '/xzy/p'
# The -n option
# Otherwise all
# The -e option

$filename
tells sed to print only those lines matching the pattern.
input lines would print.
not necessary here since there is only a single editing instruction.

Table C-2. Examples of sed operators
Notation
Effect
8d
Delete 8th line of input.
/^$/d
Delete all blank lines.
1,/^$/d
Delete from beginning of input up to, and including first blank line.
/Jones/p
Print only lines containing "Jones" (with -n option).
s/Windows/Linux/
Substitute "Linux" for first instance of "Windows" found in each input line.
s/BSOD/stability/g Substitute "stability" for every instance of "BSOD" found in each input line.
s/ *$//
Delete all spaces at the end of every line.
s/00*/0/g
Compress all consecutive sequences of zeroes into a single zero.
/GUI/d
Delete all lines containing "GUI".
s/GUI//g
Delete all instances of "GUI", leaving the remainder of each line intact.
Substituting a zero-length string for another is equivalent to deleting that string within a line of input. This
leaves the remainder of the line intact. Applying s/GUI// to the line

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Advanced Bash-Scripting Guide
The most important parts of any application are its GUI and sound effects

results in
The most important parts of any application are its

and sound effects

A backslash forces the sed replacement command to continue on to the next line. This has the effect of using
the newline at the end of the first line as the replacement string.
s/^
/g

*/\

This substitution replaces line-beginning spaces with a newline. The net result is to replace paragraph indents
with a blank line between paragraphs.
An address range followed by one or more operations may require open and closed curly brackets, with
appropriate newlines.
/[0-9A-Za-z]/,/^$/{
/^$/d
}

This deletes only the first of each set of consecutive blank lines. That might be useful for single-spacing a text
file, but retaining the blank line(s) between paragraphs.
The usual delimiter that sed uses is /. However, sed allows other delimiters, such as %. This is useful
when / is part of a replacement string, as in a file pathname. See Example 11-9 and Example 16-32.
A quick way to double-space a text file is sed G filename.
For illustrative examples of sed within shell scripts, see:
1. Example 36-1
2. Example 36-2
3. Example 16-3
4. Example A-2
5. Example 16-17
6. Example 16-27
7. Example A-12
8. Example A-16
9. Example A-17
10. Example 16-32
11. Example 11-9
12. Example 16-48
13. Example A-1
14. Example 16-14
15. Example 16-12
16. Example A-10
17. Example 19-12
18. Example 16-19
19. Example A-29
20. Example A-31
21. Example A-24
22. Example A-43
For a more extensive treatment of sed, check the appropriate references in the Bibliography.
Appendix C. A Sed and Awk Micro-Primer

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Advanced Bash-Scripting Guide

C.2. Awk
Awk [141] is a full-featured text processing language with a syntax reminiscent of C. While it possesses an
extensive set of operators and capabilities, we will cover only a few of these here - the ones most useful in
shell scripts.
Awk breaks each line of input passed to it into fields. By default, a field is a string of consecutive characters
delimited by whitespace, though there are options for changing this. Awk parses and operates on each separate
field. This makes it ideal for handling structured text files -- especially tables -- data organized into consistent
chunks, such as rows and columns.
Strong quoting and curly brackets enclose blocks of awk code within a shell script.
# $1 is field #1, $2 is field #2, etc.
echo one two | awk '{print $1}'
# one
echo one two | awk '{print $2}'
# two
# But what is field #0 ($0)?
echo one two | awk '{print $0}'
# one two
# All the fields!

awk '{print $3}' $filename
# Prints field #3 of file $filename to stdout.
awk '{print $1 $5 $6}' $filename
# Prints fields #1, #5, and #6 of file $filename.
awk '{print $0}' $filename
# Prints the entire file!
# Same effect as:
cat $filename . . . or . . . sed '' $filename

We have just seen the awk print command in action. The only other feature of awk we need to deal with here
is variables. Awk handles variables similarly to shell scripts, though a bit more flexibly.
{ total += ${column_number} }

This adds the value of column_number to the running total of total>. Finally, to print "total", there is an
END command block, executed after the script has processed all its input.
END { print total }

Corresponding to the END, there is a BEGIN, for a code block to be performed before awk starts processing
its input.
The following example illustrates how awk can add text-parsing tools to a shell script.

Example C-1. Counting Letter Occurrences
#! /bin/sh

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Advanced Bash-Scripting Guide
#
#
#
#
#
#
#

letter-count2.sh: Counting letter occurrences in a text file.
Script by nyal [nyal@voila.fr].
Used in ABS Guide with permission.
Recommented and reformatted by ABS Guide author.
Version 1.1: Modified to work with gawk 3.1.3.
(Will still work with earlier versions.)

INIT_TAB_AWK=""
# Parameter to initialize awk script.
count_case=0
FILE_PARSE=$1
E_PARAMERR=85
usage()
{
echo "Usage: letter-count.sh file letters" 2>&1
# For example:
./letter-count2.sh filename.txt a b c
exit $E_PARAMERR # Too few arguments passed to script.
}
if [ ! -f "$1" ] ; then
echo "$1: No such file." 2>&1
usage
# Print usage message and exit.
fi
if [ -z "$2" ] ; then
echo "$2: No letters specified." 2>&1
usage
fi
shift
# Letters specified.
for letter in `echo $@`
# For each one . . .
do
INIT_TAB_AWK="$INIT_TAB_AWK tab_search[${count_case}] = \
\"$letter\"; final_tab[${count_case}] = 0; "
# Pass as parameter to awk script below.
count_case=`expr $count_case + 1`
done
# DEBUG:
# echo $INIT_TAB_AWK;
cat $FILE_PARSE |
# Pipe the target file to the following awk script.
#
#
#
#

--------------------------------------------------------------------Earlier version of script:
awk -v tab_search=0 -v final_tab=0 -v tab=0 -v \
nb_letter=0 -v chara=0 -v chara2=0 \

awk \
"BEGIN { $INIT_TAB_AWK } \
{ split(\$0, tab, \"\"); \
for (chara in tab) \
{ for (chara2 in tab_search) \
{ if (tab_search[chara2] == tab[chara]) { final_tab[chara2]++ } } } } \
END { for (chara in final_tab) \
{ print tab_search[chara] \" => \" final_tab[chara] } }"
# ---------------------------------------------------------------------

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Advanced Bash-Scripting Guide
# Nothing all that complicated, just . . .
#+ for-loops, if-tests, and a couple of specialized functions.
exit $?
# Compare this script to letter-count.sh.

For simpler examples of awk within shell scripts, see:
1. Example 15-14
2. Example 20-8
3. Example 16-32
4. Example 36-5
5. Example 28-2
6. Example 15-20
7. Example 29-3
8. Example 29-4
9. Example 11-3
10. Example 16-60
11. Example 9-16
12. Example 16-4
13. Example 10-6
14. Example 36-17
15. Example 11-8
16. Example 36-4
17. Example 16-53
That's all the awk we'll cover here, folks, but there's lots more to learn. See the appropriate references in the
Bibliography.

Appendix C. A Sed and Awk Micro-Primer

780

Appendix D. Exit Codes With Special Meanings
Table D-1. Reserved Exit Codes
Exit Code
Number
1

Meaning

Example

Catchall for general errors

let "var1 = 1/0"

Comments

Miscellaneous errors, such as
"divide by zero" and other
impermissible operations
2
Misuse of shell builtins (according empty_function() {}
Seldom seen, usually defaults
to Bash documentation)
to exit code 1
126
Command invoked cannot execute
Permission problem or
command is not an
executable
127
"command not found"
illegal_command
Possible problem with
$PATH or a typo
128
Invalid argument to exit
exit 3.14159
exit takes only integer args in
the range 0 - 255 (see first
footnote)
128+n
Fatal error signal "n"
kill -9 $PPID of script $? returns 137 (128 + 9)
130
Script terminated by Control-C
Control-C is fatal error signal
2, (130 = 128 + 2, see above)
255*
Exit status out of range
exit -1
exit takes only integer args in
the range 0 - 255
According to the above table, exit codes 1 - 2, 126 - 165, and 255 [142] have special meanings, and should
therefore be avoided for user-specified exit parameters. Ending a script with exit 127 would certainly cause
confusion when troubleshooting (is the error code a "command not found" or a user-defined one?). However,
many scripts use an exit 1 as a general bailout-upon-error. Since exit code 1 signifies so many possible errors,
it is not particularly useful in debugging.

There has been an attempt to systematize exit status numbers (see /usr/include/sysexits.h), but this
is intended for C and C++ programmers. A similar standard for scripting might be appropriate. The author of
this document proposes restricting user-defined exit codes to the range 64 - 113 (in addition to 0, for success),
to conform with the C/C++ standard. This would allot 50 valid codes, and make troubleshooting scripts more
straightforward. [143] All user-defined exit codes in the accompanying examples to this document conform to
this standard, except where overriding circumstances exist, as in Example 9-2.
Issuing a $? from the command-line after a shell script exits gives results consistent with the table above
only from the Bash or sh prompt. Running the C-shell or tcsh may give different values in some cases.

Appendix D. Exit Codes With Special Meanings

781

Appendix E. A Detailed Introduction to I/O and I/O
Redirection
written by Stéphane Chazelas, and revised by the document author

A command expects the first three file descriptors to be available. The first, fd 0 (standard input, stdin), is
for reading. The other two (fd 1, stdout and fd 2, stderr) are for writing.
There is a stdin, stdout, and a stderr associated with each command. ls 2>&1 means temporarily
connecting the stderr of the ls command to the same "resource" as the shell's stdout.
By convention, a command reads its input from fd 0 (stdin), prints normal output to fd 1 (stdout), and
error ouput to fd 2 (stderr). If one of those three fd's is not open, you may encounter problems:
bash$ cat /etc/passwd >&cat: standard output: Bad file descriptor

For example, when xterm runs, it first initializes itself. Before running the user's shell, xterm opens the
terminal device (/dev/pts/ or something similar) three times.
At this point, Bash inherits these three file descriptors, and each command (child process) run by Bash inherits
them in turn, except when you redirect the command. Redirection means reassigning one of the file
descriptors to another file (or a pipe, or anything permissible). File descriptors may be reassigned locally (for
a command, a command group, a subshell, a while or if or case or for loop...), or globally, for the remainder of
the shell (using exec).
ls > /dev/null means running ls with its fd 1 connected to /dev/null.
bash$ lsof -a -p $$ -d0,1,2
COMMAND PID
USER
FD
TYPE DEVICE SIZE NODE NAME
bash
363 bozo
0u
CHR 136,1
3 /dev/pts/1
bash
363 bozo
1u
CHR 136,1
3 /dev/pts/1
bash
363 bozo
2u
CHR 136,1
3 /dev/pts/1

bash$ exec 2> /dev/null
bash$ lsof -a -p $$ -d0,1,2
COMMAND PID
USER
FD
TYPE DEVICE SIZE NODE NAME
bash
371 bozo
0u
CHR 136,1
3 /dev/pts/1
bash
371 bozo
1u
CHR 136,1
3 /dev/pts/1
bash
371 bozo
2w
CHR
1,3
120 /dev/null

bash$ bash -c 'lsof -a -p $$ -d0,1,2' | cat
COMMAND PID USER
FD
TYPE DEVICE SIZE NODE NAME
lsof
379 root
0u
CHR 136,1
3 /dev/pts/1
lsof
379 root
1w FIFO
0,0
7118 pipe
lsof
379 root
2u
CHR 136,1
3 /dev/pts/1

bash$ echo "$(bash -c 'lsof -a -p $$ -d0,1,2' 2>&1)"
COMMAND PID USER
FD
TYPE DEVICE SIZE NODE NAME
lsof
426 root
0u
CHR 136,1
3 /dev/pts/1

Appendix E. A Detailed Introduction to I/O and I/O Redirection

782

Advanced Bash-Scripting Guide
lsof
lsof

426 root
426 root

1w
2w

FIFO
FIFO

0,0
0,0

7520 pipe
7520 pipe

This works for different types of redirection.
Exercise: Analyze the following script.
#! /usr/bin/env bash
mkfifo /tmp/fifo1 /tmp/fifo2
while read a; do echo "FIFO1: $a"; done < /tmp/fifo1 & exec 7> /tmp/fifo1
exec 8> >(while read a; do echo "FD8: $a, to fd7"; done >&7)
exec 3>&1
(
(
(
while read a; do echo "FIFO2: $a"; done < /tmp/fifo2 | tee /dev/stderr \
| tee /dev/fd/4 | tee /dev/fd/5 | tee /dev/fd/6 >&7 & exec 3> /tmp/fifo2
echo 1st,
sleep 1
echo 2nd,
sleep 1
echo 3rd,
sleep 1
echo 4th,
sleep 1
echo 5th,
sleep 1
echo 6th,
sleep 1
echo 7th,
sleep 1
echo 8th,
sleep 1
echo 9th,

to stdout
to stderr >&2
to fd 3 >&3
to fd 4 >&4
to fd 5 >&5
through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5
to fd 6 >&6
to fd 7 >&7
to fd 8 >&8

) 4>&1 >&3 3>&- | while read a; do echo "FD4: $a"; done 1>&3 5>&- 6>&) 5>&1 >&3 | while read a; do echo "FD5: $a"; done 1>&3 6>&) 6>&1 >&3 | while read a; do echo "FD6: $a"; done 3>&rm -f /tmp/fifo1 /tmp/fifo2

# For each command and subshell, figure out which fd points to what.
# Good luck!
exit 0

Appendix E. A Detailed Introduction to I/O and I/O Redirection

783

Appendix F. Command-Line Options
Many executables, whether binaries or script files, accept options to modify their run-time behavior. For
example: from the command-line, typing command -o would invoke command, with option o.

F.1. Standard Command-Line Options
Over time, there has evolved a loose standard for the meanings of command-line option flags. The GNU
utilities conform more closely to this "standard" than older UNIX utilities.
Traditionally, UNIX command-line options consist of a dash, followed by one or more lowercase letters. The
GNU utilities added a double-dash, followed by a complete word or compound word.
The two most widely-accepted options are:
• -h
--help
Help: Give usage message and exit.
• -v
--version
Version: Show program version and exit.
Other common options are:
• -a
--all
All: show all information or operate on all arguments.
• -l
--list
List: list files or arguments without taking other action.
• -o
Output filename
• -q
--quiet
Quiet: suppress stdout.
• -r
-R
Appendix F. Command-Line Options

784

Advanced Bash-Scripting Guide
--recursive
Recursive: Operate recursively (down directory tree).
• -v
--verbose
Verbose: output additional information to stdout or stderr.
• -z
--compress
Compress: apply compression (usually gzip).
However:
• In tar and gawk:
-f
--file
File: filename follows.
• In cp, mv, rm:
-f
--force
Force: force overwrite of target file(s).
Many UNIX and Linux utilities deviate from this "standard," so it is dangerous to assume that a given
option will behave in a standard way. Always check the man page for the command in question when in
doubt.
A complete table of recommended options for the GNU utilities is available at the GNU standards page.

F.2. Bash Command-Line Options
Bash itself has a number of command-line options. Here are some of the more useful ones.
• -c
Read commands from the following string and assign any arguments to the positional parameters.
bash$ bash -c 'set a b c d; IFS="+-;"; echo "$*"'
a+b+c+d

• -r

Appendix F. Command-Line Options

785

Advanced Bash-Scripting Guide
--restricted
Runs the shell, or a script, in restricted mode.
• --posix
Forces Bash to conform to POSIX mode.
• --version
Display Bash version information and exit.
• -End of options. Anything further on the command line is an argument, not an option.

Appendix F. Command-Line Options

786

Appendix G. Important Files
startup files
These files contain the aliases and environmental variables made available to Bash running as a user
shell and to all Bash scripts invoked after system initialization.
/etc/profile
Systemwide defaults, mostly setting the environment (all Bourne-type shells, not just Bash [144])
/etc/bashrc
systemwide functions and aliases for Bash
$HOME/.bash_profile
user-specific Bash environmental default settings, found in each user's home directory (the local
counterpart to /etc/profile)
$HOME/.bashrc
user-specific Bash init file, found in each user's home directory (the local counterpart to
/etc/bashrc). Only interactive shells and user scripts read this file. See Appendix L for a sample
.bashrc file.
logout file
$HOME/.bash_logout
user-specific instruction file, found in each user's home directory. Upon exit from a login (Bash) shell,
the commands in this file execute.
data files
/etc/passwd
A listing of all the user accounts on the system, their identities, their home directories, the groups they
belong to, and their default shell. Note that the user passwords are not stored in this file, [145] but in
/etc/shadow in encrypted form.
system configuration files
/etc/sysconfig/hwconf
Listing and description of attached hardware devices. This information is in text form and can be
extracted and parsed.
bash$ grep -A 5 AUDIO /etc/sysconfig/hwconf
class: AUDIO
bus: PCI
detached: 0
driver: snd-intel8x0
desc: "Intel Corporation 82801CA/CAM AC'97 Audio Controller"
vendorId: 8086

This file is present on Red Hat and Fedora Core installations, but may be missing from
other distros.

Appendix G. Important Files

787

Appendix H. Important System Directories
Sysadmins and anyone else writing administrative scripts should be intimately familiar with the following
system directories.
• /bin
Binaries (executables). Basic system programs and utilities (such as bash).
• /usr/bin [146]
More system binaries.
• /usr/local/bin
Miscellaneous binaries local to the particular machine.
• /sbin
System binaries. Basic system administrative programs and utilities (such as fsck).
• /usr/sbin
More system administrative programs and utilities.
• /etc
Et cetera. Systemwide configuration scripts.
Of particular interest are the /etc/fstab (filesystem table), /etc/mtab (mounted filesystem
table), and the /etc/inittab files.
• /etc/rc.d
Boot scripts, on Red Hat and derivative distributions of Linux.
• /usr/share/doc
Documentation for installed packages.
• /usr/man
The systemwide manpages.
• /dev
Device directory. Entries (but not mount points) for physical and virtual devices. See Chapter 29.
• /proc
Process directory. Contains information and statistics about running processes and kernel parameters.
See Chapter 29.
• /sys
Systemwide device directory. Contains information and statistics about device and device names. This
is newly added to Linux with the 2.6.X kernels.
• /mnt
Mount. Directory for mounting hard drive partitions, such as /mnt/dos, and physical devices. In
newer Linux distros, the /media directory has taken over as the preferred mount point for I/O
Appendix H. Important System Directories

788

Advanced Bash-Scripting Guide
devices.
• /media
In newer Linux distros, the preferred mount point for I/O devices, such as CD/DVD drives or USB
flash drives.
• /var
Variable (changeable) system files. This is a catchall "scratchpad" directory for data generated while
a Linux/UNIX machine is running.
• /var/log
Systemwide log files.
• /var/spool/mail
User mail spool.
• /lib
Systemwide library files.
• /usr/lib
More systemwide library files.
• /tmp
System temporary files.
• /boot
System boot directory. The kernel, module links, system map, and boot manager reside here.
Altering files in this directory may result in an unbootable system.

Appendix H. Important System Directories

789

Appendix I. An Introduction to Programmable
Completion
The programmable completion feature in Bash permits typing a partial command, then pressing the [Tab] key
to auto-complete the command sequence. [147] If multiple completions are possible, then [Tab] lists them all.
Let's see how it works.
bash$ xtra[Tab]
xtraceroute
xtrapin
xtraceroute.real xtrapinfo
xtrapchar
xtrapout

bash$ xtrac[Tab]
xtraceroute

xtrapproto
xtrapreset
xtrapstats

xtraceroute.real

bash$ xtraceroute.r[Tab]
xtraceroute.real

Tab completion also works for variables and path names.
bash$ echo $BASH[Tab]
$BASH
$BASH_COMPLETION
$BASH_SUBSHELL
$BASH_ARGC
$BASH_COMPLETION_DIR $BASH_VERSINFO
$BASH_ARGV
$BASH_LINENO
$BASH_VERSION
$BASH_COMMAND
$BASH_SOURCE

bash$ echo /usr/local/[Tab]
bin/
etc/
include/ libexec/ sbin/
doc/
games/
lib/
man/
share/

src/

The Bash complete and compgen builtins make it possible for tab completion to recognize partial parameters
and options to commands. In a very simple case, we can use complete from the command-line to specify a
short list of acceptable parameters.
bash$
bash$
bash$
bash$

touch sample_command
touch file1.txt file2.txt file2.doc file30.txt file4.zzz
chmod +x sample_command
complete -f -X '!*.txt' sample_command

bash$ ./sample[Tab][Tab]
sample_command
file1.txt
file2.txt
file30.txt

The -f option to complete specifies filenames, and -X the filter pattern.

For anything more complex, we could write a script that specifies a list of acceptable command-line
parameters. The compgen builtin expands a list of arguments to generate completion matches.

Appendix I. An Introduction to Programmable Completion

790

Advanced Bash-Scripting Guide
Let us take a modified version of the UseGetOpt.sh script as an example command. This script accepts a
number of command-line parameters, preceded by either a single or double dash. And here is the
corresponding completion script, by convention given a filename corresponding to its associated command.

Example I-1. Completion script for UseGetOpt.sh
# file: UseGetOpt-2
# UseGetOpt-2.sh parameter-completion
_UseGetOpt-2 ()
# By convention, the function name
{
#+ starts with an underscore.
local cur
# Pointer to current completion word.
# By convention, it's named "cur" but this isn't strictly necessary.
COMPREPLY=()
# Array variable storing the possible completions.
cur=${COMP_WORDS[COMP_CWORD]}

#
#
#
#

case "$cur" in
-*)
COMPREPLY=( $( compgen -W '-a -d -f -l -t -h --aoption --debug \
--file --log --test --help --' -- $cur ) );;
Generate the completion matches and load them into $COMPREPLY array.
xx) May add more cases here.
yy)
zz)
esac
return 0

}
complete -F _UseGetOpt-2 -o filenames ./UseGetOpt-2.sh
#
^^ ^^^^^^^^^^^^ Invokes the function _UseGetOpt-2.

Now, let's try it.
bash$ source UseGetOpt-2
bash$ ./UseGetOpt-2.sh -[Tab]
---aoption --debug
-a
-d
-f

--file
-h

--help
-l

--log
-t

--test

bash$ ./UseGetOpt-2.sh --[Tab]
---aoption --debug

--file

--help

--log

--test

We begin by sourcing the "completion script." This sets the command-line parameters. [148]
In the first instance, hitting [Tab] after a single dash, the output is all the possible parameters preceded by one
or more dashes. Hitting [Tab] after two dashes gives the possible parameters preceded by two or more dashes.
Now, just what is the point of having to jump through flaming hoops to enable command-line tab completion?
It saves keystrokes. [149]
-Resources:
Appendix I. An Introduction to Programmable Completion

791

Advanced Bash-Scripting Guide
Bash programmable completion project
Mitch Frazier's Linux Journal article, More on Using the Bash Complete Command
Steve's excellent two-part article, "An Introduction to Bash Completion": Part 1 and Part 2

Appendix I. An Introduction to Programmable Completion

792

Appendix J. Localization
Localization is an undocumented Bash feature.
A localized shell script echoes its text output in the language defined as the system's locale. A Linux user in
Berlin, Germany, would get script output in German, whereas his cousin in Berlin, Maryland, would get
output from the same script in English.
To create a localized script, use the following template to write all messages to the user (error messages,
prompts, etc.).
#!/bin/bash
# localized.sh
# Script by Stéphane Chazelas,
#+ modified by Bruno Haible, bugfixed by Alfredo Pironti.
. gettext.sh
E_CDERROR=65
error()
{
printf "$@" >&2
exit $E_CDERROR
}
cd $var || error "`eval_gettext \"Can\'t cd to \\\$var.\"`"
# The triple backslashes (escapes) in front of $var needed
#+ "because eval_gettext expects a string
#+ where the variable values have not yet been substituted."
#
-- per Bruno Haible
read -p "`gettext \"Enter the value: \"`" var
# ...

#
#

-----------------------------------------------------------------Alfredo Pironti comments:

# This script has been modified to not use the $"..." syntax in
#+ favor of the "`gettext \"...\"`" syntax.
# This is ok, but with the new localized.sh program, the commands
#+ "bash -D filename" and "bash --dump-po-string filename"
#+ will produce no output
#+ (because those command are only searching for the $"..." strings)!
# The ONLY way to extract strings from the new file is to use the
# 'xgettext' program. However, the xgettext program is buggy.
# Note that 'xgettext' has another bug.
#
# The shell fragment:
#
gettext -s "I like Bash"
# will be correctly extracted, but . . .
#
xgettext -s "I like Bash"
# . . . fails!
# 'xgettext' will extract "-s" because
#+ the command only extracts the
#+ very first argument after the 'gettext' word.

Appendix J. Localization

793

Advanced Bash-Scripting Guide
#
#
#
#
#+
#
#
#+
#
#+
#+
#
#
#
#+

Escape characters:
To localize a sentence like
echo -e "Hello\tworld!"
you must use
echo -e "`gettext \"Hello\\tworld\"`"
The "double escape character" before the `t' is needed because
'gettext' will search for a string like: 'Hello\tworld'
This is because gettext will read one literal `\')
and will output a string like "Bonjour\tmonde",
so the 'echo' command will display the message correctly.
You may not use
echo "`gettext -e \"Hello\tworld\"`"
due to the xgettext bug explained above.

# Let's localize the following shell fragment:
#
echo "-h display help and exit"
#
# First, one could do this:
#
echo "`gettext \"-h display help and exit\"`"
# This way 'xgettext' will work ok,
#+ but the 'gettext' program will read "-h" as an option!
#
# One solution could be
#
echo "`gettext -- \"-h display help and exit\"`"
# This way 'gettext' will work,
#+ but 'xgettext' will extract "--", as referred to above.
#
# The workaround you may use to get this string localized is
#
echo -e "`gettext \"\\0-h display help and exit\"`"
# We have added a \0 (NULL) at the beginning of the sentence.
# This way 'gettext' works correctly, as does 'xgettext.'
# Moreover, the NULL character won't change the behavior
#+ of the 'echo' command.
# -----------------------------------------------------------------bash$ bash -D localized.sh
"Can't cd to %s."
"Enter the value: "

This lists all the localized text. (The -D option lists double-quoted strings prefixed by a $, without executing
the script.)
bash$ bash --dump-po-strings localized.sh
#: a:6
msgid "Can't cd to %s."
msgstr ""
#: a:7
msgid "Enter the value: "
msgstr ""

The --dump-po-strings option to Bash resembles the -D option, but uses gettext "po" format.
Bruno Haible points out:
Starting with gettext-0.12.2, xgettext -o - localized.sh is recommended instead of bash
--dump-po-strings localized.sh, because xgettext . . .

Appendix J. Localization

794

Advanced Bash-Scripting Guide
1. understands the gettext and eval_gettext commands (whereas bash --dump-po-strings understands only
its deprecated $"..." syntax)
2. can extract comments placed by the programmer, intended to be read by the translator.
This shell code is then not specific to Bash any more; it works the same way with Bash 1.x and other
/bin/sh implementations.
Now, build a language.po file for each language that the script will be translated into, specifying the
msgstr. Alfredo Pironti gives the following example:
fr.po:
#: a:6
msgid "Can't cd to $var."
msgstr "Impossible de se positionner dans le repertoire $var."
#: a:7
msgid "Enter the value: "
msgstr "Entrez la valeur : "
#
#+
#+
#+

The string are dumped with the variable names, not with the %s syntax,
similar to C programs.
This is a very cool feature if the programmer uses
variable names that make sense!

Then, run msgfmt.
msgfmt -o localized.sh.mo fr.po
Place the resulting localized.sh.mo file in the /usr/local/share/locale/fr/LC_MESSAGES
directory, and at the beginning of the script, insert the lines:
TEXTDOMAINDIR=/usr/local/share/locale
TEXTDOMAIN=localized.sh

If a user on a French system runs the script, she will get French messages.
With older versions of Bash or other shells, localization requires gettext, using the -s option. In this
case, the script becomes:

#!/bin/bash
# localized.sh
E_CDERROR=65
error() {
local format=$1
shift
printf "$(gettext -s "$format")" "$@" >&2
exit $E_CDERROR
}
cd $var || error "Can't cd to %s." "$var"
read -p "$(gettext -s "Enter the value: ")" var
# ...

The TEXTDOMAIN and TEXTDOMAINDIR variables need to be set and exported to the environment. This
should be done within the script itself.
Appendix J. Localization

795

Advanced Bash-Scripting Guide
--This appendix written by Stéphane Chazelas, with modifications suggested by Alfredo Pironti, and by Bruno
Haible, maintainer of GNU gettext.

Appendix J. Localization

796

Appendix K. History Commands
The Bash shell provides command-line tools for editing and manipulating a user's command history. This is
primarily a convenience, a means of saving keystrokes.
Bash history commands:
1. history
2. fc
bash$ history
1 mount /mnt/cdrom
2 cd /mnt/cdrom
3 ls
...

Internal variables associated with Bash history commands:
1. $HISTCMD
2. $HISTCONTROL
3. $HISTIGNORE
4. $HISTFILE
5. $HISTFILESIZE
6. $HISTSIZE
7. $HISTTIMEFORMAT (Bash, ver. 3.0 or later)
8. !!
9. !$
10. !#
11. !N
12. !-N
13. !STRING
14. !?STRING?
15. ^STRING^string^
Unfortunately, the Bash history tools find no use in scripting.
#!/bin/bash
# history.sh
# A (vain) attempt to use the 'history' command in a script.
history

# No output.

var=$(history); echo "$var"

# $var is empty.

# History commands disabled within a script.
bash$ ./history.sh
(no output)

The Advancing in the Bash Shell site gives a good introduction to the use of history commands in Bash.

Appendix K. History Commands

797

Appendix L. Sample .bashrc and .bash_profile
Files
The ~/.bashrc file determines the behavior of interactive shells. A good look at this file can lead to a
better understanding of Bash.
Emmanuel Rouat contributed the following very elaborate .bashrc file, written for a Linux system. He
welcomes reader feedback on it.
Study the file carefully, and feel free to reuse code snippets and functions from it in your own .bashrc file
or even in your scripts.

Example L-1. Sample .bashrc file
#=============================================================
#
# PERSONAL $HOME/.bashrc FILE for bash-3.0 (or later)
# By Emmanuel Rouat 
#
# Last modified: Sun Nov 30 16:27:45 CET 2008
# This file is read (normally) by interactive shells only.
# Here is the place to define your aliases, functions and
# other interactive features like your prompt.
#
# The majority of the code here assumes you are on a GNU
# system (most likely a Linux box) and is based on code found
# on Usenet or internet. See for instance:
#
# http://tldp.org/LDP/abs/html/index.html
# http://www.caliban.org/bash/
# http://www.shelldorado.com/scripts/categories.html
# http://www.dotfiles.org/
#
# This bashrc file is a bit overcrowded -- remember it is just
# just an example. Tailor it to your needs.
#
#
#=============================================================
# --> Comments added by HOWTO author.

#------------------------------------------------------------# Source global definitions (if any)
#-------------------------------------------------------------

if [ -f /etc/bashrc ]; then
. /etc/bashrc
# --> Read /etc/bashrc, if present.
fi
#------------------------------------------------------------# Automatic setting of $DISPLAY (if not set already).
# This works for linux - your mileage may vary. ...
# The problem is that different types of terminals give
# different answers to 'who am i' (rxvt in particular can be

Appendix L. Sample .bashrc and .bash_profile Files

798

Advanced Bash-Scripting Guide
# troublesome).
# I have not found a 'universal' method yet.
#------------------------------------------------------------function get_xserver ()
{
case $TERM in
xterm )
XSERVER=$(who am i | awk '{print $NF}' | tr -d ')''(' )
# Ane-Pieter Wieringa suggests the following alternative:
# I_AM=$(who am i)
# SERVER=${I_AM#*(}
# SERVER=${SERVER%*)}
XSERVER=${XSERVER%%:*}
;;
aterm | rxvt)
# Find some code that works here. ...
;;
esac
}
if [ -z ${DISPLAY:=""} ]; then
get_xserver
if [[ -z ${XSERVER} || ${XSERVER} == $(hostname) || \
${XSERVER} == "unix" ]]; then
DISPLAY=":0.0"
# Display on local host.
else
DISPLAY=${XSERVER}:0.0 # Display on remote host.
fi
fi
export DISPLAY
#------------------------------------------------------------# Some settings
#------------------------------------------------------------ulimit -S -c 0
set -o notify
set -o noclobber
set -o ignoreeof
set -o nounset
#set -o xtrace
# Enable
shopt -s
shopt -s
shopt -s
shopt -s
shopt -s
shopt -s
shopt -s
shopt -s
shopt -s

# Don't want any coredumps.

# Useful for debuging.

options:
cdspell
cdable_vars
checkhash
checkwinsize
sourcepath
no_empty_cmd_completion
cmdhist
histappend histreedit histverify
extglob
# Necessary for programmable completion.

# Disable options:
shopt -u mailwarn
unset MAILCHECK

# Don't want my shell to warn me of incoming mail.

export TIMEFORMAT=$'\nreal %3R\tuser %3U\tsys %3S\tpcpu %P\n'

Appendix L. Sample .bashrc and .bash_profile Files

799

Advanced Bash-Scripting Guide
export HISTTIMEFORMAT="%H:%M > "
export HISTIGNORE="&:bg:fg:ll:h"
export HOSTFILE=$HOME/.hosts
# Put list of remote hosts in ~/.hosts ...

#------------------------------------------------------------# Greeting, motd etc...
#------------------------------------------------------------# Define some colors first:
red='\e[0;31m'
RED='\e[1;31m'
blue='\e[0;34m'
BLUE='\e[1;34m'
cyan='\e[0;36m'
CYAN='\e[1;36m'
NC='\e[0m'
# No Color
# --> Nice. Has the same effect as using "ansi.sys" in DOS.

# Looks best on a terminal with black background.....
echo -e "${CYAN}This is BASH ${RED}${BASH_VERSION%.*}\
${CYAN} - DISPLAY on ${RED}$DISPLAY${NC}\n"
date
if [ -x /usr/games/fortune ]; then
/usr/games/fortune -s
# Makes our day a bit more fun.... :-)
fi
function _exit()
# Function to run upon exit of shell.
{
echo -e "${RED}Hasta la vista, baby${NC}"
}
trap _exit EXIT

#------------------------------------------------------------# Shell Prompt
#-------------------------------------------------------------

if [[ "${DISPLAY%%:0*}" != "" ]]; then
HILIT=${red}
# remote machine: prompt will be partly red
else
HILIT=${cyan} # local machine: prompt will be partly cyan
fi
# --> Replace instances of \W with \w in prompt functions below
#+ --> to get display of full path name.
function fastprompt()
{
unset PROMPT_COMMAND
case $TERM in
*term | rxvt )
PS1="${HILIT}[\h]$NC \W > \[\033]0;\${TERM} [\u@\h] \w\007\]" ;;
linux )
PS1="${HILIT}[\h]$NC \W > " ;;
*)
PS1="[\h] \W > " ;;
esac
}

Appendix L. Sample .bashrc and .bash_profile Files

800

Advanced Bash-Scripting Guide

_powerprompt()
{
LOAD=$(uptime|sed -e "s/.*: \([^,]*\).*/\1/" -e "s/ //g")
}
function powerprompt()
{
PROMPT_COMMAND=_powerprompt
case $TERM in
*term | rxvt )
PS1="${HILIT}[\A - \$LOAD]$NC\n[\u@\h \#] \W > \
\[\033]0;\${TERM} [\u@\h] \w\007\]" ;;
linux )
PS1="${HILIT}[\A - \$LOAD]$NC\n[\u@\h \#] \W > " ;;
* )
PS1="[\A - \$LOAD]\n[\u@\h \#] \W > " ;;
esac
}
powerprompt

# This is the default prompt -- might be slow.
# If too slow, use fastprompt instead. ...

#===============================================================
#
# ALIASES AND FUNCTIONS
#
# Arguably, some functions defined here are quite big.
# If you want to make this file smaller, these functions can
# be converted into scripts and removed from here.
#
# Many functions were taken (almost) straight from the bash-2.04
# examples.
#
#===============================================================
#------------------# Personnal Aliases
#------------------alias rm='rm -i'
alias cp='cp -i'
alias mv='mv -i'
# -> Prevents accidentally clobbering files.
alias mkdir='mkdir -p'
alias
alias
alias
alias
alias
alias
alias

h='history'
j='jobs -l'
which='type -a'
..='cd ..'
path='echo -e ${PATH//:/\\n}'
libpath='echo -e ${LD_LIBRARY_PATH//:/\\n}'
print='/usr/bin/lp -o nobanner -d $LPDEST'
# Assumes LPDEST is defined (default printer)
alias pjet='enscript -h -G -fCourier9 -d $LPDEST'
# Pretty-print using enscript
alias du='du -kh'
alias df='df -kTh'

# Makes a more readable output.

Appendix L. Sample .bashrc and .bash_profile Files

801

Advanced Bash-Scripting Guide
#------------------------------------------------------------# The 'ls' family (this assumes you use a recent GNU ls)
#------------------------------------------------------------alias ll="ls -l --group-directories-first"
alias ls='ls -hF --color' # add colors for filetype recognition
alias la='ls -Al'
# show hidden files
alias lx='ls -lXB'
# sort by extension
alias lk='ls -lSr'
# sort by size, biggest last
alias lc='ls -ltcr'
# sort by and show change time, most recent last
alias lu='ls -ltur'
# sort by and show access time, most recent last
alias lt='ls -ltr'
# sort by date, most recent last
alias lm='ls -al |more'
# pipe through 'more'
alias lr='ls -lR'
# recursive ls
alias tree='tree -Csu'
# nice alternative to 'recursive ls'
# If your version of 'ls' doesn't support --group-directories-first try this:
# function ll(){ ls -l "$@"| egrep "^d" ; ls -lXB "$@" 2>&-| \
#
egrep -v "^d|total "; }

#------------------------------------------------------------# tailoring 'less'
#------------------------------------------------------------alias more='less'
export PAGER=less
export LESSCHARSET='latin1'
export LESSOPEN='|/usr/bin/lesspipe.sh %s 2>&-'
# Use this if lesspipe.sh exists
export LESS='-i -N -w -z-4 -g -e -M -X -F -R -P%t?f%f \
:stdin .?pb%pb\%:?lbLine %lb:?bbByte %bb:-...'

#------------------------------------------------------------# spelling typos - highly personnal and keyboard-dependent :-)
#------------------------------------------------------------alias
alias
alias
alias
alias

xs='cd'
vf='cd'
moer='more'
moew='more'
kk='ll'

#------------------------------------------------------------# A few fun ones
#-------------------------------------------------------------

function xtitle()
# Adds some text in the terminal frame.
{
case "$TERM" in
*term | rxvt)
echo -n -e "\033]0;$*\007" ;;
*)
;;
esac
}
# aliases that use xtitle
alias top='xtitle Processes on $HOST && top'
alias make='xtitle Making $(basename $PWD) ; make'

Appendix L. Sample .bashrc and .bash_profile Files

802

Advanced Bash-Scripting Guide
alias ncftp="xtitle ncFTP ; ncftp"
# .. and functions
function man()
{
for i ; do
xtitle The $(basename $1|tr -d .[:digit:]) manual
command man -F -a "$i"
done
}

#------------------------------------------------------------# Make the following commands run in background automatically:
#------------------------------------------------------------function te() # Wrapper around xemacs/gnuserv ...
{
if [ "$(gnuclient -batch -eval t 2>&-)" == "t" ]; then
gnuclient -q "$@";
else
( xemacs "$@" &);
fi
}
function soffice() { command soffice "$@" & }
function firefox() { command firefox "$@" & }
function xpdf() { command xpdf "$@" & }

#------------------------------------------------------------# File & string-related functions:
#-------------------------------------------------------------

# Find a file with a pattern in name:
function ff() { find . -type f -iname '*'$*'*' -ls ; }
# Find a file with pattern $1 in name and Execute $2 on it:
function fe()
{ find . -type f -iname '*'${1:-}'*' -exec ${2:-file} {} \;

; }

# Find a pattern in a set of files and highlight them:
# (needs a recent version of egrep)
function fstr()
{
OPTIND=1
local case=""
local usage="fstr: find string in files.
Usage: fstr [-i] \"pattern\" [\"filename pattern\"] "
while getopts :it opt
do
case "$opt" in
i) case="-i " ;;
*) echo "$usage"; return;;
esac
done
shift $(( $OPTIND - 1 ))
if [ "$#" -lt 1 ]; then
echo "$usage"
return;
fi

Appendix L. Sample .bashrc and .bash_profile Files

803

Advanced Bash-Scripting Guide
find . -type f -name "${2:-*}" -print0 | \
xargs -0 egrep --color=always -sn ${case} "$1" 2>&- | more
}
function cuttail() # cut last n lines in file, 10 by default
{
nlines=${2:-10}
sed -n -e :a -e "1,${nlines}!{P;N;D;};N;ba" $1
}
function lowercase() # move filenames to lowercase
{
for file ; do
filename=${file##*/}
case "$filename" in
*/*) dirname==${file%/*} ;;
*) dirname=.;;
esac
nf=$(echo $filename | tr A-Z a-z)
newname="${dirname}/${nf}"
if [ "$nf" != "$filename" ]; then
mv "$file" "$newname"
echo "lowercase: $file --> $newname"
else
echo "lowercase: $file not changed."
fi
done
}

function swap() # Swap 2 filenames around, if they exist
{
#(from Uzi's bashrc).
local TMPFILE=tmp.$$
[ $# -ne 2 ] && echo "swap: 2 arguments needed" && return 1
[ ! -e $1 ] && echo "swap: $1 does not exist" && return 1
[ ! -e $2 ] && echo "swap: $2 does not exist" && return 1
mv "$1" $TMPFILE
mv "$2" "$1"
mv $TMPFILE "$2"
}
function extract()
#
{
if [ -f $1 ] ; then
case $1 in
*.tar.bz2)
*.tar.gz)
*.bz2)
*.rar)
*.gz)
*.tar)
*.tbz2)
*.tgz)
*.zip)
*.Z)
*.7z)
*)
esac
else

Handy Extract Program.

tar xvjf $1
;;
tar xvzf $1
;;
bunzip2 $1
;;
unrar x $1
;;
gunzip $1
;;
tar xvf $1
;;
tar xvjf $1
;;
tar xvzf $1
;;
unzip $1
;;
uncompress $1
;;
7z x $1
;;
echo "'$1' cannot be extracted via >extract<" ;;

Appendix L. Sample .bashrc and .bash_profile Files

804

Advanced Bash-Scripting Guide
echo "'$1' is not a valid file"
fi
}
#------------------------------------------------------------# Process/system related functions:
#-------------------------------------------------------------

function my_ps() { ps $@ -u $USER -o pid,%cpu,%mem,bsdtime,command ; }
function pp() { my_ps f | awk '!/awk/ && $0~var' var=${1:-".*"} ; }

function killps()
# Kill by process name.
{
local pid pname sig="-TERM"
# Default signal.
if [ "$#" -lt 1 ] || [ "$#" -gt 2 ]; then
echo "Usage: killps [-SIGNAL] pattern"
return;
fi
if [ $# = 2 ]; then sig=$1 ; fi
for pid in $(my_ps| awk '!/awk/ && $0~pat { print $1 }' pat=${!#} ) ; do
pname=$(my_ps | awk '$1~var { print $5 }' var=$pid )
if ask "Kill process $pid <$pname> with signal $sig?"
then kill $sig $pid
fi
done
}
function my_ip() # Get IP adresses.
{
MY_IP=$(/sbin/ifconfig ppp0 | awk '/inet/ { print $2 } ' | \
sed -e s/addr://)
MY_ISP=$(/sbin/ifconfig ppp0 | awk '/P-t-P/ { print $3 } ' | \
sed -e s/P-t-P://)
}
function ii()
# Get current host related info.
{
echo -e "\nYou are logged on ${RED}$HOST"
echo -e "\nAdditionnal information:$NC " ; uname -a
echo -e "\n${RED}Users logged on:$NC " ; w -h
echo -e "\n${RED}Current date :$NC " ; date
echo -e "\n${RED}Machine stats :$NC " ; uptime
echo -e "\n${RED}Memory stats :$NC " ; free
my_ip 2>&- ;
echo -e "\n${RED}Local IP Address :$NC" ; echo ${MY_IP:-"Not connected"}
echo -e "\n${RED}ISP Address :$NC" ; echo ${MY_ISP:-"Not connected"}
echo -e "\n${RED}Open connections :$NC "; netstat -pan --inet;
echo
}
#------------------------------------------------------------# Misc utilities:
#------------------------------------------------------------function repeat()
# Repeat n times command.
{
local i max
max=$1; shift;
for ((i=1; i <= max ; i++)); do # --> C-like syntax
eval "$@";

Appendix L. Sample .bashrc and .bash_profile Files

805

Advanced Bash-Scripting Guide
done
}

function ask()
# See 'killps' for example of use.
{
echo -n "$@" '[y/n] ' ; read ans
case "$ans" in
y*|Y*) return 0 ;;
*) return 1 ;;
esac
}
function corename()
# Get name of app that created a corefile.
{
for file ; do
echo -n $file : ; gdb --core=$file --batch | head -1
done
}

#=========================================================================
# PROGRAMMABLE COMPLETION - ONLY SINCE BASH-2.04
# Most are taken from the bash 2.05 documentation and from Ian McDonald's
# 'Bash completion' package (http://www.caliban.org/bash/#completion).
# You will in fact need bash more recent than 3.0 for some features.
#=========================================================================
if [ "${BASH_VERSION%.*}" \< "3.0" ]; then
echo "You will need to upgrade to version 3.0 \
for full programmable completion features."
return
fi
shopt -s extglob
#set +o nounset

# Necessary,
# otherwise some completions will fail.

complete
complete
complete
complete
complete
complete
complete

-A
-A
-A
-A
-A
-A
-A

hostname
export
variable
enabled
alias
function
user

rsh rcp telnet rlogin r ftp ping disk
printenv
export local readonly unset
builtin
alias unalias
function
su mail finger

complete
complete
complete
complete

-A
-A
-A
-A

helptopic help
# Currently, same as builtins.
shopt
shopt
stopped -P '%' bg
job -P '%'
fg jobs disown

complete -A directory
complete -A directory

mkdir rmdir
-o default cd

# Compression
complete -f -o
complete -f -o
complete -f -o
complete -f -o
complete -f -o
complete -f -o

-X
-X
-X
-X
-X
-X

default
default
default
default
default
default

'*.+(zip|ZIP)'
'!*.+(zip|ZIP)'
'*.+(z|Z)'
'!*.+(z|Z)'
'*.+(gz|GZ)'
'!*.+(gz|GZ)'

zip
unzip
compress
uncompress
gzip
gunzip

Appendix L. Sample .bashrc and .bash_profile Files

806

Advanced Bash-Scripting Guide
complete -f -o default -X '*.+(bz2|BZ2)' bzip2
complete -f -o default -X '!*.+(bz2|BZ2)' bunzip2
complete -f -o default -X '!*.+(zip|ZIP|z|Z|gz|GZ|bz2|BZ2)' extract

# Documents - Postscript,pdf,dvi.....
complete -f -o default -X '!*.+(ps|PS)' gs ghostview ps2pdf ps2ascii
complete -f -o default -X '!*.+(dvi|DVI)' dvips dvipdf xdvi dviselect dvitype
complete -f -o default -X '!*.+(pdf|PDF)' acroread pdf2ps
complete -f -o default -X \
'!*.@(@(?(e)ps|?(E)PS|pdf|PDF)?(.gz|.GZ|.bz2|.BZ2|.Z))' gv ggv
complete -f -o default -X '!*.texi*' makeinfo texi2dvi texi2html texi2pdf
complete -f -o default -X '!*.tex' tex latex slitex
complete -f -o default -X '!*.lyx' lyx
complete -f -o default -X '!*.+(htm*|HTM*)' lynx html2ps
complete -f -o default -X \
'!*.+(doc|DOC|xls|XLS|ppt|PPT|sx?|SX?|csv|CSV|od?|OD?|ott|OTT)' soffice
# Multimedia
complete -f -o default -X \
'!*.+(gif|GIF|jp*g|JP*G|bmp|BMP|xpm|XPM|png|PNG)' xv gimp ee gqview
complete -f -o default -X '!*.+(mp3|MP3)' mpg123 mpg321
complete -f -o default -X '!*.+(ogg|OGG)' ogg123
complete -f -o default -X \
'!*.@(mp[23]|MP[23]|ogg|OGG|wav|WAV|pls|m3u|xm|mod|s[3t]m|it|mtm|ult|flac)' xmms
complete -f -o default -X \
'!*.@(mp?(e)g|MP?(E)G|wma|avi|AVI|asf|vob|VOB|bin|dat|vcd|\
ps|pes|fli|viv|rm|ram|yuv|mov|MOV|qt|QT|wmv|mp3|MP3|ogg|OGG|\
ogm|OGM|mp4|MP4|wav|WAV|asx|ASX)' xine

complete -f -o default -X '!*.pl'

perl perl5

# This is a 'universal' completion function - it works when commands have
# a so-called 'long options' mode , ie: 'ls --all' instead of 'ls -a'
# Needs the '-o' option of grep
# (try the commented-out version if not available).
# First, remove '=' from completion word separators
# (this will allow completions like 'ls --color=auto' to work correctly).
COMP_WORDBREAKS=${COMP_WORDBREAKS/=/}

_get_longopts()
{
#$1 --help | sed -e '/--/!d' -e 's/.*--\([^[:space:].,]*\).*/--\1/'| \
#grep ^"$2" |sort -u ;
$1 --help | grep -o -e "--[^[:space:].,]*" | grep -e "$2" |sort -u
}
_longopts()
{
local cur
cur=${COMP_WORDS[COMP_CWORD]}
case "${cur:-*}" in
-*)
;;
*)
return ;;
esac

Appendix L. Sample .bashrc and .bash_profile Files

807

Advanced Bash-Scripting Guide
case "$1" in
\~*)
eval cmd="$1" ;;
*)
cmd="$1" ;;
esac
COMPREPLY=( $(_get_longopts ${1} ${cur} ) )
}
complete
complete

-o default -F _longopts configure bash
-o default -F _longopts wget id info a2ps ls recode

_tar()
{
local cur ext regex tar untar
COMPREPLY=()
cur=${COMP_WORDS[COMP_CWORD]}
# If we want an option, return the possible long options.
case "$cur" in
-*)
COMPREPLY=( $(_get_longopts $1 $cur ) ); return 0;;
esac
if [ $COMP_CWORD -eq 1 ]; then
COMPREPLY=( $( compgen -W 'c t x u r d A' -- $cur ) )
return 0
fi
case "${COMP_WORDS[1]}" in
?(-)c*f)
COMPREPLY=( $( compgen -f $cur ) )
return 0
;;
+([^Izjy])f)
ext='tar'
regex=$ext
;;
*z*f)
ext='tar.gz'
regex='t\(ar\.\)\(gz\|Z\)'
;;
*[Ijy]*f)
ext='t?(ar.)bz?(2)'
regex='t\(ar\.\)bz2\?'
;;
*)
COMPREPLY=( $( compgen -f $cur ) )
return 0
;;
esac
if [[ "$COMP_LINE" == tar*.$ext' '* ]]; then
# Complete on files in tar file.
#
# Get name of tar file from command line.
tar=$( echo "$COMP_LINE" | \
sed -e 's|^.* \([^ ]*'$regex'\) .*$|\1|' )
# Devise how to untar and list it.
untar=t${COMP_WORDS[1]//[^Izjyf]/}
COMPREPLY=( $( compgen -W "$( echo $( tar $untar $tar \
2>/dev/null ) )" -- "$cur" ) )

Appendix L. Sample .bashrc and .bash_profile Files

808

Advanced Bash-Scripting Guide
return 0
else
# File completion on relevant files.
COMPREPLY=( $( compgen -G $cur\*.$ext ) )
fi
return 0
}
complete -F _tar -o default tar
_make()
{
local mdef makef makef_dir="." makef_inc gcmd cur prev i;
COMPREPLY=();
cur=${COMP_WORDS[COMP_CWORD]};
prev=${COMP_WORDS[COMP_CWORD-1]};
case "$prev" in
-*f)
COMPREPLY=($(compgen -f $cur ));
return 0
;;
esac;
case "$cur" in
-*)
COMPREPLY=($(_get_longopts $1 $cur ));
return 0
;;
esac;
# make reads `GNUmakefile', then `makefile', then `Makefile'
if [ -f ${makef_dir}/GNUmakefile ]; then
makef=${makef_dir}/GNUmakefile
elif [ -f ${makef_dir}/makefile ]; then
makef=${makef_dir}/makefile
elif [ -f ${makef_dir}/Makefile ]; then
makef=${makef_dir}/Makefile
else
makef=${makef_dir}/*.mk
# Local convention.
fi

# Before we scan for targets, see if a Makefile name was
# specified with -f ...
for (( i=0; i < ${#COMP_WORDS[@]}; i++ )); do
if [[ ${COMP_WORDS[i]} == -f ]]; then
# eval for tilde expansion
eval makef=${COMP_WORDS[i+1]}
break
fi
done
[ ! -f $makef ] && return 0
# deal with included Makefiles
makef_inc=$( grep -E '^-?include' $makef | \
sed -e "s,^.* ,"$makef_dir"/," )
for file in $makef_inc; do
[ -f $file ] && makef="$makef $file"
done

Appendix L. Sample .bashrc and .bash_profile Files

809

Advanced Bash-Scripting Guide

# If we have a partial word to complete, restrict completions to
# matches of that word.
if [ -n "$cur" ]; then gcmd='grep "^$cur"' ; else gcmd=cat ; fi
COMPREPLY=( $( awk -F':' '/^[a-zA-Z0-9][^$#\/\t=]*:([^=]|$)/ \
{split($1,A,/ /);for(i in A)print A[i]}' \
$makef 2>/dev/null | eval $gcmd ))
}
complete -F _make -X '+($*|*.[cho])' make gmake pmake

_killall()
{
local cur prev
COMPREPLY=()
cur=${COMP_WORDS[COMP_CWORD]}
# get a list of processes (the first sed evaluation
# takes care of swapped out processes, the second
# takes care of getting the basename of the process)
COMPREPLY=( $( /usr/bin/ps -u $USER -o comm | \
sed -e '1,1d' -e 's#[]\[]##g' -e 's#^.*/##'| \
awk '{if ($0 ~ /^'$cur'/) print $0}' ))
return 0
}
complete -F _killall killall killps

#
#
#
#

A meta-command completion function for commands like sudo(8), which need to
first complete on a command, then complete according to that command's own
completion definition - currently not quite foolproof,
but still quite useful (By Ian McDonald, modified by me).

_meta_comp()
{
local cur func cline cspec
COMPREPLY=()
cur=${COMP_WORDS[COMP_CWORD]}
cmdline=${COMP_WORDS[@]}
if [ $COMP_CWORD = 1 ]; then
COMPREPLY=( $( compgen -c $cur ) )
else
cmd=${COMP_WORDS[1]}
# Find command.
cspec=$( complete -p ${cmd} )
# Find spec of that command.
# COMP_CWORD and COMP_WORDS() are not read-only,
# so we can set them before handing off to regular
# completion routine:
# Get current command line minus initial command,
cline="${COMP_LINE#$1 }"
# split current command line tokens into array,

Appendix L. Sample .bashrc and .bash_profile Files

810

Advanced Bash-Scripting Guide
COMP_WORDS=( $cline )
# set current token number to 1 less than now.
COMP_CWORD=$(( $COMP_CWORD - 1 ))
# If current arg is empty, add it to COMP_WORDS array
# (otherwise that information will be lost).
if [ -z $cur ]; then COMP_WORDS[COMP_CWORD]="" ; fi
if [ "${cspec%%-F *}" != "${cspec}" ]; then
# if -F then get function:
func=${cspec#*-F }
func=${func%% *}
eval $func $cline
# Evaluate it.
else
func=$( echo $cspec | sed -e 's/^complete//' -e 's/[^ ]*$//' )
COMPREPLY=( $( eval compgen $func $cur ) )
fi
fi
}

complete -o default -F _meta_comp nohup \
eval exec trace truss strace sotruss gdb
complete -o default -F _meta_comp command type which man nice time
#
#
#
#

Local Variables:
mode:shell-script
sh-shell:bash
End:

And, here is a snippet from Andrzej Szelachowski's instructive .bash_profile file.

Example L-2. .bash_profile file
# From Andrzej Szelachowski's ~/.bash_profile:

# Note that a variable may require special treatment
#+ if it will be exported.
DARKGRAY='\e[1;30m'
LIGHTRED='\e[1;31m'
GREEN='\e[32m'
YELLOW='\e[1;33m'
LIGHTBLUE='\e[1;34m'
NC='\e[m'
PCT="\`if [[ \$EUID -eq 0 ]]; then T='$LIGHTRED' ; else T='$LIGHTBLUE'; fi;
echo \$T \`"
#
#+
#+
#
#+
#+

For "literal" command substitution to be assigned to a variable,
use escapes and double quotes:
PCT="\` ... \`" . . .
Otherwise, the value of PCT variable is assigned only once,
when the variable is exported/read from .bash_profile,
and it will not change afterwards even if the user ID changes.

PS1="\n$GREEN[\w] \n$DARKGRAY($PCT\t$DARKGRAY)-($PCT\u$DARKGRAY)-($PCT\!
$DARKGRAY)$YELLOW-> $NC"

Appendix L. Sample .bashrc and .bash_profile Files

811

Advanced Bash-Scripting Guide
# Escape a variables whose value changes:
#
if [[ \$EUID -eq 0 ]],
# Otherwise the value of the EUID variable will be assigned only once,
#+ as above.
#
#+
#
#+
#

When a variable is assigned, it should be called escaped:
echo \$T,
Otherwise the value of the T variable is taken from the moment the PCT
variable is exported/read from .bash_profile.
So, in this example it would be null.

#
#
#

When a variable's value contains a semicolon it should be strong quoted:
T='$LIGHTRED',
Otherwise, the semicolon will be interpreted as a command separator.

#

Variables PCT and PS1 can be merged into a new PS1 variable:

PS1="\`if [[ \$EUID -eq 0 ]]; then PCT='$LIGHTRED';
else PCT='$LIGHTBLUE'; fi;
echo '\n$GREEN[\w] \n$DARKGRAY('\$PCT'\t$DARKGRAY)-\
('\$PCT'\u$DARKGRAY)-('\$PCT'\!$DARKGRAY)$YELLOW-> $NC'\`"
# The trick is to use strong quoting for parts of old PS1 variable.

Appendix L. Sample .bashrc and .bash_profile Files

812

Appendix M. Converting DOS Batch Files to Shell
Scripts
Quite a number of programmers learned scripting on a PC running DOS. Even the crippled DOS batch file
language allowed writing some fairly powerful scripts and applications, though they often required extensive
kludges and workarounds. Occasionally, the need still arises to convert an old DOS batch file to a UNIX shell
script. This is generally not difficult, as DOS batch file operators are only a limited subset of the equivalent
shell scripting ones.

Table M-1. Batch file keywords / variables / operators, and their shell equivalents
Batch File Operator
%
/
\
==
!==!

Shell Script Equivalent
$
/
=
!=

|
@
*
>
>>
<
%VAR%
REM
NOT
NUL

|
set +v
*
>
>>
<
$VAR
#
!
/dev/null

ECHO

echo

ECHO.
ECHO OFF

echo
set +v

FOR %%VAR IN (LIST) DO
:LABEL
GOTO

for var in [list]; do
none (unnecessary)
none (use a function)

PAUSE
CHOICE
IF

sleep
case or select
if

Appendix M. Converting DOS Batch Files to Shell Scripts

Meaning
command-line parameter prefix
command option flag
directory path separator
(equal-to) string comparison test
(not equal-to) string comparison
test
pipe
do not echo current command
filename "wild card"
file redirection (overwrite)
file redirection (append)
redirect stdin
environmental variable
comment
negate following test
"black hole" for burying
command output
echo (many more option in
Bash)
echo blank line
do not echo command(s)
following
"for" loop
label
jump to another location in the
script
pause or wait an interval
menu choice
if-test

813

Advanced Bash-Scripting Guide
IF EXIST FILENAME
IF !%N==!

if [ -e filename ]
if [ -z "$N" ]

CALL
COMMAND /C

source or . (dot operator)
source or . (dot operator)

SET
SHIFT

export
shift

SGN
ERRORLEVEL
CON
PRN
LPT1
COM1

-lt or -gt
$?
stdin
/dev/lp0
/dev/lp0
/dev/ttyS0

test if file exists
if replaceable parameter "N" not
present
"include" another script
"include" another script (same
as CALL)
set an environmental variable
left shift command-line
argument list
sign (of integer)
exit status
"console" (stdin)
(generic) printer device
first printer device
first serial port

Batch files usually contain DOS commands. These must be translated into their UNIX equivalents in order to
convert a batch file into a shell script.

Table M-2. DOS commands and their UNIX equivalents
DOS Command
ASSIGN

UNIX Equivalent
ln

ATTRIB

chmod

CD
CHDIR
CLS
COMP
COPY
Ctl-C
Ctl-Z
DEL
DELTREE

cd
cd
clear
diff, comm, cmp
cp
Ctl-C
Ctl-D
rm
rm -rf

DIR
ERASE
EXIT

ls -l
rm
exit

FC
FIND
MD

comm, cmp
grep
mkdir

Effect
link file or
directory
change file
permissions
change directory
change directory
clear screen
file compare
file copy
break (signal)
EOF (end-of-file)
delete file(s)
delete directory
recursively
directory listing
delete file(s)
exit current
process
file compare
find strings in files
make directory

Appendix M. Converting DOS Batch Files to Shell Scripts

814

Advanced Bash-Scripting Guide
MKDIR
MORE

mkdir
more

MOVE
PATH
REN
RENAME
RD
RMDIR
SORT
TIME

mv
$PATH
mv
mv
rmdir
rmdir
sort
date

TYPE

cat

XCOPY

cp

make directory
text file paging
filter
move
path to executables
rename (move)
rename (move)
remove directory
remove directory
sort file
display system
time
output file to
stdout
(extended) file
copy

Virtually all UNIX and shell operators and commands have many more options and enhancements than
their DOS and batch file counterparts. Many DOS batch files rely on auxiliary utilities, such as ask.com,
a crippled counterpart to read.
DOS supports only a very limited and incompatible subset of filename wild-card expansion, recognizing
just the * and ? characters.
Converting a DOS batch file into a shell script is generally straightforward, and the result ofttimes reads better
than the original.

Example M-1. VIEWDATA.BAT: DOS Batch File
REM VIEWDATA
REM INSPIRED BY AN EXAMPLE IN "DOS POWERTOOLS"
REM
BY PAUL SOMERSON

@ECHO OFF
IF !%1==! GOTO VIEWDATA
REM IF NO COMMAND-LINE ARG...
FIND "%1" C:\BOZO\BOOKLIST.TXT
GOTO EXIT0
REM PRINT LINE WITH STRING MATCH, THEN EXIT.
:VIEWDATA
TYPE C:\BOZO\BOOKLIST.TXT | MORE
REM SHOW ENTIRE FILE, 1 PAGE AT A TIME.
:EXIT0

The script conversion is somewhat of an improvement. [150]

Appendix M. Converting DOS Batch Files to Shell Scripts

815

Advanced Bash-Scripting Guide
Example M-2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
#!/bin/bash
# viewdata.sh
# Conversion of VIEWDATA.BAT to shell script.
DATAFILE=/home/bozo/datafiles/book-collection.data
ARGNO=1
# @ECHO OFF

Command unnecessary here.

if [ $# -lt "$ARGNO" ]
then
less $DATAFILE
else
grep "$1" $DATAFILE
fi

# IF !%1==! GOTO VIEWDATA

exit 0

# :EXIT0

# TYPE C:\MYDIR\BOOKLIST.TXT | MORE
# FIND "%1" C:\MYDIR\BOOKLIST.TXT

# GOTOs, labels, smoke-and-mirrors, and flimflam unnecessary.
# The converted script is short, sweet, and clean,
#+ which is more than can be said for the original.

Ted Davis' Shell Scripts on the PC site has a set of comprehensive tutorials on the old-fashioned art of batch
file programming. Certain of his ingenious techniques could conceivably have relevance for shell scripts.

Appendix M. Converting DOS Batch Files to Shell Scripts

816

Appendix N. Exercises
The exercises that follow test and extend your knowledge of scripting. Think of them as a challenge, as an
entertaining way to take you further along the stony path toward UNIX wizardry.

On a dingy side street in a run-down section of Hoboken, New Jersey,
there sits a nondescript squat two-story brick building with an inscriptio
incised on a marble plate in its wall:
Bash Scripting Hall of Fame.
Inside, among various dusty uninteresting exhibits is a corroding,
cobweb-festooned brass plaque inscribed with a short, very short
list of those few persons who have successfully mastered the material
in the Advanced Bash Scripting
Guide, as evidenced by their performance
on the following Exercise sections.
(Alas, the author of the ABS
Guide is not represented among the exhibits.
This is possibly due to malicious rumors about lack of
credentials and
deficient scripting skills.)

N.1. Analyzing Scripts
Examine the following script. Run it, then explain what it does. Annotate the script and rewrite it in a more
compact and elegant manner.
#!/bin/bash
MAX=10000

for((nr=1; nr<$MAX; nr++))
do
let "t1 = nr % 5"
if [ "$t1" -ne 3 ]
then
continue
fi
let "t2 = nr % 7"
if [ "$t2" -ne 4 ]
then
continue
fi
let "t3 = nr % 9"
if [ "$t3" -ne 5 ]
then

Appendix N. Exercises

817

Advanced Bash-Scripting Guide
continue
fi
break

# What happens when you comment out this line? Why?

done
echo "Number = $nr"

exit 0

--Explain what the following script does. It is really just a parameterized command-line pipe.
#!/bin/bash
DIRNAME=/usr/bin
FILETYPE="shell script"
LOGFILE=logfile
file "$DIRNAME"/* | fgrep "$FILETYPE" | tee $LOGFILE | wc -l
exit 0

--Examine and explain the following script. For hints, you might refer to the listings for find and stat.
#!/bin/bash
# Author: Nathan Coulter
# This code is released to the public domain.
# The author gave permission to use this code snippet in the ABS Guide.
find -maxdepth 1 -type f -printf '%f\000' | {
while read -d $'\000'; do
mv "$REPLY" "$(date -d "$(stat -c '%y' "$REPLY") " '+%Y%m%d%H%M%S'
)-$REPLY"
done
}
# Warning: Test-drive this script in a "scratch" directory.
# It will somehow affect all the files there.

--A reader sent in the following code snippet.
while read LINE
do
echo $LINE
done < `tail -f /var/log/messages`

He wished to write a script tracking changes to the system log file, /var/log/messages. Unfortunately,
the above code block hangs and does nothing useful. Why? Fix this so it does work. (Hint: rather than
redirecting the stdin of the loop, try a pipe.)
---

Appendix N. Exercises

818

Advanced Bash-Scripting Guide
Analyze the following "one-liner" (here split into two lines for clarity) contributed by Rory Winston:
export SUM=0; for f in $(find src -name "*.java");
do export SUM=$(($SUM + $(wc -l $f | awk '{ print $1 }'))); done; echo $SUM

Hint: First, break the script up into bite-sized sections. Then, carefully examine its use of double-parentheses
arithmetic, the export command, the find command, the wc command, and awk.
--Analyze Example A-10, and reorganize it in a simplified and more logical style. See how many of the
variables can be eliminated, and try to optimize the script to speed up its execution time.
Alter the script so that it accepts any ordinary ASCII text file as input for its initial "generation". The script
will read the first $ROW*$COL characters, and set the occurrences of vowels as "living" cells. Hint: be sure to
translate the spaces in the input file to underscore characters.

N.2. Writing Scripts
Write a script to carry out each of the following tasks.
EASY
Self-reproducing Script
Write a script that backs itself up, that is, copies itself to a file named backup.sh.
Hint: Use the cat command and the appropriate positional parameter.
Home Directory Listing
Perform a recursive directory listing on the user's home directory and save the information to a file.
Compress the file, have the script prompt the user to insert a USB flash drive, then press ENTER.
Finally, save the file to the flash drive after making certain the flash drive has properly mounted by
parsing the output of df. Note that the flash drive must be unmounted before it is removed.
Converting for loops to while and until loops
Convert the for loops in Example 11-1 to while loops. Hint: store the data in an array and step through
the array elements.
Having already done the "heavy lifting," now convert the loops in the example to until loops.
Changing the line spacing of a text file
Write a script that reads each line of a target file, then writes the line back to stdout, but with an
extra blank line following. This has the effect of double-spacing the file.
Include all necessary code to check whether the script gets the necessary command-line argument (a
filename), and whether the specified file exists.
When the script runs correctly, modify it to triple-space the target file.
Finally, write a script to remove all blank lines from the target file, single-spacing it.
Backwards Listing
Write a script that echoes itself to stdout, but backwards.
Automatically Decompressing Files
Appendix N. Exercises

819

Advanced Bash-Scripting Guide
Given a list of filenames as input, this script queries each target file (parsing the output of the file
command) for the type of compression used on it. Then the script automatically invokes the
appropriate decompression command (gunzip, bunzip2, unzip, uncompress, or whatever). If a target
file is not compressed, the script emits a warning message, but takes no other action on that particular
file.
Unique System ID
Generate a "unique" 6-digit hexadecimal identifier for your computer. Do not use the flawed hostid
command. Hint: md5sum /etc/passwd, then select the first 6 digits of output.
Backup
Archive as a "tarball" (*.tar.gz file) all the files in your home directory tree
(/home/your-name) that have been modified in the last 24 hours. Hint: use find.
Optional: you may use this as the basis of a backup script.
Checking whether a process is still running
Given a process ID (PID) as an argument, this script will check, at user-specified intervals, whether
the given process is still running. You may use the ps and sleep commands.
Primes
Print (to stdout) all prime numbers between 60000 and 63000. The output should be nicely
formatted in columns (hint: use printf).
Lottery Numbers
One type of lottery involves picking five different numbers, in the range of 1 - 50. Write a script that
generates five pseudorandom numbers in this range, with no duplicates. The script will give the
option of echoing the numbers to stdout or saving them to a file, along with the date and time the
particular number set was generated. (If your script consistently generates winning lottery numbers,
then you can retire on the proceeds and leave shell scripting to those of us who have to work for a
living.)
INTERMEDIATE
Integer or String
Write a script function that determines if an argument passed to it is an integer or a string. The
function will return TRUE (0) if passed an integer, and FALSE (1) if passed a string.
Hint: What does the following expression return when $1 is not an integer?
expr $1 + 0
ASCII to Integer
The atoi function in C converts a string character to an integer. Write a shell script function that
performs the same operation. Likewise, write a shell script function that does the inverse, mirroring
the C itoa function which converts an integer into an ASCII character.
Managing Disk Space
List, one at a time, all files larger than 100K in the /home/username directory tree. Give the user
the option to delete or compress the file, then proceed to show the next one. Write to a logfile the
names of all deleted files and the deletion times.
Banner
Simulate the functionality of the deprecated banner command in a script.
Removing Inactive Accounts
Inactive accounts on a network server waste disk space and may become a security risk. Write an
administrative script (to be invoked by root or the cron daemon) that checks for and deletes user
accounts that have not been accessed within the last 90 days.
Enforcing Disk Quotas
Appendix N. Exercises

820

Advanced Bash-Scripting Guide
Write a script for a multi-user system that checks users' disk usage. If a user surpasses a preset limit
(500 MB, for example) in her /home/username directory, then the script automatically sends her a
"pigout" warning e-mail.
The script will use the du and mail commands. As an option, it will allow setting and enforcing quotas
using the quota and setquota commands.
Logged in User Information
For all logged in users, show their real names and the time and date of their last login.
Hint: use who, lastlog, and parse /etc/passwd.
Safe Delete
Implement, as a script, a "safe" delete command, sdel.sh. Filenames passed as command-line
arguments to this script are not deleted, but instead gzipped if not already compressed (use file to
check), then moved to a ~/TRASH directory. Upon invocation, the script checks the ~/TRASH
directory for files older than 48 hours and permanently deletes them. (An better alternative might be
to have a second script handle this, periodically invoked by the cron daemon.)
Extra credit: Write the script so it can handle files and directories recursively. This would give it the
capability of "safely deleting" entire directory structures.
Making Change
What is the most efficient way to make change for $1.68, using only coins in common circulations
(up to 25c)? It's 6 quarters, 1 dime, a nickel, and three cents.
Given any arbitrary command-line input in dollars and cents ($*.??), calculate the change, using the
minimum number of coins. If your home country is not the United States, you may use your local
currency units instead. The script will need to parse the command-line input, then change it to
multiples of the smallest monetary unit (cents or whatever). Hint: look at Example 24-8.
Quadratic Equations
Solve a quadratic equation of the form Ax^2 + Bx + C = 0. Have a script take as arguments the
coefficients, A, B, and C, and return the solutions to five decimal places.
Hint: pipe the coefficients to bc, using the well-known formula, x = ( -B +/- sqrt( B^2 4AC ) ) / 2A.
Table of Logarithms
Using the bc and printf commands, print out a nicely-formatted table of eight-place natural logarithms
in the interval between 0.00 and 100.00, in steps of .01.
Hint: bc requires the -l option to load the math library.
Unicode Table
Using Example S-1 as a template, write a script that prints to a file a complete Unicode table.
Hint: Use the -e option to echo: echo -e '\uXXXX', where XXXX is the Unicode numerical character
designation. This requires version 4.2 or later of Bash.
Sum of Matching Numbers
Find the sum of all five-digit numbers (in the range 10000 - 99999) containing exactly two out of the
following set of digits: { 4, 5, 6 }. These may repeat within the same number, and if so, they count
once for each occurrence.
Some examples of matching numbers are 42057, 74638, and 89515.
Lucky Numbers

Appendix N. Exercises

821

Advanced Bash-Scripting Guide
A lucky number is one whose individual digits add up to 7, in successive additions. For example,
62431 is a lucky number (6 + 2 + 4 + 3 + 1 = 16, 1 + 6 = 7). Find all the lucky numbers between 1000
and 10000.
Craps
Borrowing the ASCII graphics from Example A-40, write a script that plays the well-known gambling
game of craps. The script will accept bets from one or more players, roll the dice, and keep track of
wins and losses, as well as of each player's bankroll.
Tic-tac-toe
Write a script that plays the child's game of tic-tac-toe against a human player. The script will let the
human choose whether to take the first move. The script will follow an optimal strategy, and therefore
never lose. To simplify matters, you may use ASCII graphics:
o | x |
---------| x |
---------| o |
Your move, human (row, column)?

Alphabetizing a String
Alphabetize (in ASCII order) an arbitrary string read from the command-line.
Parsing
Parse /etc/passwd, and output its contents in nice, easy-to-read tabular form.
Logging Logins
Parse /var/log/messages to produce a nicely formatted file of user logins and login times. The
script may need to run as root. (Hint: Search for the string "LOGIN.")
Pretty-Printing a Data File
Certain database and spreadsheet packages use save-files with the fields separated by commas,
commonly referred to as comma-separated values or CSVs. Other applications often need to parse
these files.
Given a data file with comma-separated fields, of the form:
Jones,Bill,235 S. Williams St.,Denver,CO,80221,(303) 244-7989
Smith,Tom,404 Polk Ave.,Los Angeles,CA,90003,(213) 879-5612
...

Reformat the data and print it out to stdout in labeled, evenly-spaced columns.
Justification
Given ASCII text input either from stdin or a file, adjust the word spacing to right-justify each line
to a user-specified line-width, then send the output to stdout.
Mailing List
Using the mail command, write a script that manages a simple mailing list. The script automatically
e-mails the monthly company newsletter, read from a specified text file, and sends it to all the
addresses on the mailing list, which the script reads from another specified file.
Generating Passwords
Generate pseudorandom 8-character passwords, using characters in the ranges [0-9], [A-Z], [a-z].
Each password must contain at least two digits.
Monitoring a User
You suspect that one particular user on the network has been abusing her privileges and possibly
attempting to hack the system. Write a script to automatically monitor and log her activities when
she's signed on. The log file will save entries for the previous week, and delete those entries more
than seven days old.
Appendix N. Exercises

822

Advanced Bash-Scripting Guide
You may use last, lastlog, and lastcomm to aid your surveillance of the suspected fiend.
Checking for Broken Links
Using lynx with the -traversal option, write a script that checks a Web site for broken links.
DIFFICULT
Testing Passwords
Write a script to check and validate passwords. The object is to flag "weak" or easily guessed
password candidates.
A trial password will be input to the script as a command-line parameter. To be considered
acceptable, a password must meet the following minimum qualifications:
◊ Minimum length of 8 characters
◊ Must contain at least one numeric character
◊ Must contain at least one of the following non-alphabetic characters: @, #, $, %, &, *, +, -, =
Optional:
◊ Do a dictionary check on every sequence of at least four consecutive alphabetic characters in
the password under test. This will eliminate passwords containing embedded "words" found
in a standard dictionary.
◊ Enable the script to check all the passwords on your system. These do not reside in
/etc/passwd.
This exercise tests mastery of Regular Expressions.
Cross Reference
Write a script that generates a cross-reference (concordance) on a target file. The output will be a
listing of all word occurrences in the target file, along with the line numbers in which each word
occurs. Traditionally, linked list constructs would be used in such applications. Therefore, you should
investigate arrays in the course of this exercise. Example 16-12 is probably not a good place to start.
Square Root
Write a script to calculate square roots of numbers using Newton's Method.
The algorithm for this, expressed as a snippet of Bash pseudo-code is:
# (Isaac) Newton's Method for speedy extraction
#+ of square roots.
guess = $argument
# $argument is the number to find the square root of.
# $guess is each successive calculated "guess" -- or trial solution -#+ of the square root.
# Our first "guess" at a square root is the argument itself.
oldguess = 0
# $oldguess is the previous $guess.
tolerance = .000001
# To how close a tolerance we wish to calculate.
loopcnt = 0
# Let's keep track of how many times through the loop.
# Some arguments will require more loop iterations than others.

Appendix N. Exercises

823

Advanced Bash-Scripting Guide
while [ ABS( $guess $oldguess ) -gt $tolerance ]
#
^^^^^^^^^^^^^^^^^^^^^^^ Fix up syntax, of course.
#
#+
#
#+

"ABS" is a (floating point) function to find the absolute value
of the difference between the two terms.
So, as long as difference between current and previous
trial solution (guess) exceeds the tolerance, keep looping.

do
oldguess = $guess
#
#
#
#
#
#+
#+
#
#+
#+
#

# Update $oldguess to previous $guess.

=======================================================
guess = ( $oldguess + ( $argument / $oldguess ) ) / 2.0
= 1/2 ( ($oldguess **2 + $argument) / $oldguess )
equivalent to:
= 1/2 ( $oldguess + $argument / $oldguess )
that is, "averaging out" the trial solution and
the proportion of argument deviation
(in effect, splitting the error in half).
This converges on an accurate solution
with surprisingly few loop iterations . . .
for arguments > $tolerance, of course.
=======================================================

(( loopcnt++ ))
done

# Update loop counter.

It's a simple enough recipe, and seems at first glance easy enough to convert into a working Bash
script. The problem, though, is that Bash has no native support for floating point numbers. So, the
script writer needs to use bc or possibly awk to convert the numbers and do the calculations. It could
get rather messy . . .
Logging File Accesses
Log all accesses to the files in /etc during the course of a single day. This information should
include the filename, user name, and access time. If any alterations to the files take place, that will be
flagged. Write this data as tabular (tab-separated) formatted records in a logfile.
Monitoring Processes
Write a script to continually monitor all running processes and to keep track of how many child
processes each parent spawns. If a process spawns more than five children, then the script sends an
e-mail to the system administrator (or root) with all relevant information, including the time, PID of
the parent, PIDs of the children, etc. The script appends a report to a log file every ten minutes.
Strip Comments
Strip all comments from a shell script whose name is specified on the command-line. Note that the
initial #! line must not be stripped out.
Strip HTML Tags
Strip all the HTML tags from a specified HTML file, then reformat it into lines between 60 and 75
characters in length. Reset paragraph and block spacing, as appropriate, and convert HTML tables to
their approximate text equivalent.
XML Conversion
Convert an XML file to both HTML and text format.
Optional: A script that converts Docbook/SGML to XML.
Chasing Spammers
Write a script that analyzes a spam e-mail by doing DNS lookups on the IP addresses in the headers to
identify the relay hosts as well as the originating ISP. The script will forward the unaltered spam
message to the responsible ISPs. Of course, it will be necessary to filter out your own ISP's IP
address, so you don't end up complaining about yourself.
Appendix N. Exercises

824

Advanced Bash-Scripting Guide
As necessary, use the appropriate network analysis commands.
For some ideas, see Example 16-41 and Example A-28.
Optional: Write a script that searches through a list of e-mail messages and deletes the spam
according to specified filters.
Creating man pages
Write a script that automates the process of creating man pages.
Given a text file which contains information to be formatted into a man page, the script will read the
file, then invoke the appropriate groff commands to output the corresponding man page to stdout.
The text file contains blocks of information under the standard man page headings, i.e., NAME,
SYNOPSIS, DESCRIPTION, etc.
Example A-39 is an instructive first step.
Hex Dump
Do a hex(adecimal) dump on a binary file specified as an argument to the script. The output should be
in neat tabular fields, with the first field showing the address, each of the next 8 fields a 4-byte hex
number, and the final field the ASCII equivalent of the previous 8 fields.
The obvious followup to this is to extend the hex dump script into a disassembler. Using a lookup
table, or some other clever gimmick, convert the hex values into 80x86 op codes.
Emulating a Shift Register
Using Example 27-15 as an inspiration, write a script that emulates a 64-bit shift register as an array.
Implement functions to load the register, shift left, shift right, and rotate it. Finally, write a function
that interprets the register contents as eight 8-bit ASCII characters.
Calculating Determinants
Write a script that calculates determinants [151] by recursively expanding the minors. Use a 4 x 4
determinant as a test case.
Hidden Words
Write a "word-find" puzzle generator, a script that hides 10 input words in a 10 x 10 array of random
letters. The words may be hidden across, down, or diagonally.
Optional: Write a script that solves word-find puzzles. To keep this from becoming too difficult, the
solution script will find only horizontal and vertical words. (Hint: Treat each row and column as a
string, and search for substrings.)
Anagramming
Anagram 4-letter input. For example, the anagrams of word are: do or rod row word. You may use
/usr/share/dict/linux.words as the reference list.
Word Ladders
A "word ladder" is a sequence of words, with each successive word in the sequence differing from the
previous one by a single letter.
For example, to "ladder" from mark to vase:
mark --> park --> part --> past --> vast --> vase
^
^
^
^
^

Write a script that solves word ladder puzzles. Given a starting and an ending word, the script will list
all intermediate steps in the "ladder." Note that all words in the sequence must be legitimate
dictionary words.
Fog Index
Appendix N. Exercises

825

Advanced Bash-Scripting Guide
The "fog index" of a passage of text estimates its reading difficulty, as a number corresponding
roughly to a school grade level. For example, a passage with a fog index of 12 should be
comprehensible to anyone with 12 years of schooling.
The Gunning version of the fog index uses the following algorithm.
1. Choose a section of the text at least 100 words in length.
2. Count the number of sentences (a portion of a sentence truncated by the boundary of the text
section counts as one).
3. Find the average number of words per sentence.
AVE_WDS_SEN = TOTAL_WORDS / SENTENCES
4. Count the number of "difficult" words in the segment -- those containing at least 3 syllables.
Divide this quantity by total words to get the proportion of difficult words.
PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS
5. The Gunning fog index is the sum of the above two quantities, multiplied by 0.4, then
rounded to the nearest integer.
G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN + PRO_DIFF_WORDS ) )
Step 4 is by far the most difficult portion of the exercise. There exist various algorithms for estimating
the syllable count of a word. A rule-of-thumb formula might consider the number of letters in a word
and the vowel-consonant mix.
A strict interpretation of the Gunning fog index does not count compound words and proper nouns as
"difficult" words, but this would enormously complicate the script.
Calculating PI using Buffon's Needle
The Eighteenth Century French mathematician de Buffon came up with a novel experiment.
Repeatedly drop a needle of length n onto a wooden floor composed of long and narrow parallel
boards. The cracks separating the equal-width floorboards are a fixed distance d apart. Keep track of
the total drops and the number of times the needle intersects a crack on the floor. The ratio of these
two quantities turns out to be a fractional multiple of PI.
In the spirit of Example 16-50, write a script that runs a Monte Carlo simulation of Buffon's Needle.
To simplify matters, set the needle length equal to the distance between the cracks, n = d.
Hint: there are actually two critical variables: the distance from the center of the needle to the nearest
crack, and the inclination angle of the needle to that crack. You may use bc to handle the calculations.
Playfair Cipher
Implement the Playfair (Wheatstone) Cipher in a script.
The Playfair Cipher encrypts text by substitution of digrams (2-letter groupings). It is traditional to
use a 5 x 5 letter scrambled-alphabet key square for the encryption and decryption.
C
A
I
P
V

O
B
K
Q
W

D
F
L
R
X

E
G
M
T
Y

S
H
N
U
Z

Each letter of the alphabet appears once, except "I" also represents
"J". The arbitrarily chosen key word, "CODES" comes first, then all

Appendix N. Exercises

826

Advanced Bash-Scripting Guide
the rest of the alphabet, in order from left to right, skipping letters
already used.
To encrypt, separate the plaintext message into digrams (2-letter
groups). If a group has two identical letters, delete the second, and
form a new group. If there is a single letter left over at the end,
insert a "null" character, typically an "X."
THIS IS A TOP SECRET MESSAGE
TH IS IS AT OP SE CR ET ME SA GE

For each digram, there are three possibilities.
----------------------------------------------1) Both letters will be on the same row of the key square:
For each letter, substitute the one immediately to the right, in that
row. If necessary, wrap around left to the beginning of the row.
or
2) Both letters will be in the same column of the key square:
For each letter, substitute the one immediately below it, in that
row. If necessary, wrap around to the top of the column.
or
3) Both letters will form the corners of a rectangle within the key square:
For each letter, substitute the one on the other corner the rectangle
which lies on the same row.

The "TH" digram falls under case #3.
G H
M N
T U
(Rectangle with "T" and "H" at corners)
T --> U
H --> G

The "SE" digram falls under case #1.
C O D E S
(Row containing "S" and "E")
S --> C
E --> S

(wraps around left to beginning of row)

=========================================================================
To decrypt encrypted text, reverse the above procedure under cases #1
and #2 (move in opposite direction for substitution). Under case #3,
just take the remaining two corners of the rectangle.

Helen Fouche Gaines' classic work, ELEMENTARY CRYPTANALYSIS (1939), gives a
fairly detailed description of the Playfair Cipher and its solution methods.

This script will have three main sections
I. Generating the key square, based on a user-input keyword.
Appendix N. Exercises

827

Advanced Bash-Scripting Guide
II. Encrypting a plaintext message.
III. Decrypting encrypted text.
The script will make extensive use of arrays and functions. You may use Example A-55 as an
inspiration.
-Please do not send the author your solutions to these exercises. There are more appropriate ways to impress
him with your cleverness, such as submitting bugfixes and suggestions for improving the book.

Appendix N. Exercises

828

Appendix O. Revision History
This document first appeared as a 60-page HOWTO in the late spring
of 2000. Since then, it has gone through quite a number of updates
and revisions. This book could not have been written without the
assistance of the Linux community, and especially of the volunteers
of the Linux Documentation Project.

Here is the e-mail to the LDP requesting permission to submit version 0.1.
From thegrendel@theriver.com Sat Jun 10 09:05:33 2000 -0700
Date: Sat, 10 Jun 2000 09:05:28 -0700 (MST)
From: "M. Leo Cooper" 
X-Sender: thegrendel@localhost
To: ldp-discuss@lists.linuxdoc.org
Subject: Permission to submit HOWTO
Dear HOWTO Coordinator,
I am working on and would like to submit to the LDP a HOWTO on the subject
of "Bash Scripting" (shell scripting, using 'bash'). As it happens,
I have been writing this document, off and on, for about the last eight
months or so, and I could produce a first draft in ASCII text format in
a matter of just a few more days.
I began writing this out of frustration at being unable to find a
decent book on shell scripting. I managed to locate some pretty good
articles on various aspects of scripting, but nothing like a complete,
beginning-to-end tutorial. Well, in keeping with my philosophy, if all
else fails, do it yourself.
As it stands, this proposed "Bash-Scripting HOWTO" would serve as a
combination tutorial and reference, with the heavier emphasis on the
tutorial. It assumes Linux experience, but only a very basic level
of programming skills. Interspersed with the text are 79 illustrative
example scripts of varying complexity, all liberally commented. There
are even exercises for the reader.
At this stage, I'm up to 18,000+ words (124k), and that's over 50 pages of
text (whew!).

I haven't mentioned that I've previously authored an LDP HOWTO, the
"Software-Building HOWTO", which I wrote in Linuxdoc/SGML. I don't know
if I could handle Docbook/SGML, and I'm glad you have volunteers to do
the conversion. You people seem to have gotten on a more organized basis
these last few months. Working with Greg Hankins and Tim Bynum was nice,
but a professional team is even nicer.
Anyhow, please advise.

Mendel Cooper
thegrendel@theriver.com

Table O-1. Revision History
Release Date

Comments

Appendix O. Revision History

829

Advanced Bash-Scripting Guide
0.1
0.2
0.3
0.4
0.5
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
3.0
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.1
4.2
4.3
5.0
5.1
5.2
5.3

14 Jun 2000
30 Oct 2000
12 Feb 2001
08 Jul 2001
03 Sep 2001
14 Oct 2001
06 Jan 2002
31 Mar 2002
02 Jun 2002
16 Jun 2002
13 Jul 2002
29 Sep 2002
05 Jan 2003
10 May 2003
21 Jun 2003
24 Aug 2003
14 Sep 2003
31 Oct 2003
03 Jan 2004
25 Jan 2004
15 Feb 2004
15 Mar 2004
18 Apr 2004
11 Jul 2004
03 Oct 2004
14 Nov 2004
06 Feb 2005
20 Mar 2005
08 May 2005
05 Jun 2005
28 Aug 2005
23 Oct 2005
26 Feb 2006
15 May 2006
18 Jun 2006
08 Oct 2006
10 Dec 2006
29 Apr 2007
24 Jun 2007
10 Nov 2007
16 Mar 2008
11 May 2008

Initial release.
Bugs fixed, plus much additional material and more example scripts.
Major update.
Complete revision and expansion of the book.
Major update: Bugfixes, material added, sections reorganized.
Stable release: Bugfixes, reorganization, material added.
Bugfixes, material and scripts added.
Bugfixes, material and scripts added.
TANGERINE release: A few bugfixes, much more material and scripts added.
MANGO release: A number of typos fixed, more material and scripts.
PAPAYA release: A few bugfixes, much more material and scripts added.
POMEGRANATE release: Bugfixes, more material, one more script.
COCONUT release: A couple of bugfixes, more material, one more script.
BREADFRUIT release: A number of bugfixes, more scripts and material.
PERSIMMON release: Bugfixes, and more material.
GOOSEBERRY release: Major update.
HUCKLEBERRY release: Bugfixes, and more material.
CRANBERRY release: Major update.
STRAWBERRY release: Bugfixes and more material.
MUSKMELON release: Bugfixes.
STARFRUIT release: Bugfixes and more material.
SALAL release: Minor update.
MULBERRY release: Minor update.
ELDERBERRY release: Minor update.
LOGANBERRY release: Major update.
BAYBERRY release: Bugfix update.
BLUEBERRY release: Minor update.
RASPBERRY release: Bugfixes, much material added.
TEABERRY release: Bugfixes, stylistic revisions.
BOXBERRY release: Bugfixes, some material added.
POKEBERRY release: Bugfixes, some material added.
WHORTLEBERRY release: Bugfixes, some material added.
BLAEBERRY release: Bugfixes, some material added.
SPICEBERRY release: Bugfixes, some material added.
WINTERBERRY release: Major reorganization.
WAXBERRY release: Minor update.
SPARKLEBERRY release: Important update.
INKBERRY release: Bugfixes, material added.
SERVICEBERRY release: Major update.
LINGONBERRY release: Minor update.
SILVERBERRY release: Important update.
GOLDENBERRY release: Minor update.

Appendix O. Revision History

830

Advanced Bash-Scripting Guide
5.4
5.5
5.6
6.0
6.1
6.2
6.3
6.4

21 Jul 2008
23 Nov 2008
26 Jan 2009
23 Mar 2009
30 Sep 2009
17 Mar 2010
30 Apr 2011
30 Aug 2011

ANGLEBERRY release: Major update.
FARKLEBERRY release: Minor update.
WORCESTERBERRY release: Minor update.
THIMBLEBERRY release: Major update.
BUFFALOBERRY release: Minor update.
ROWANBERRY release: Minor update.
SWOZZLEBERRY release: Major update.
VORTEXBERRY release: Minor update.

Appendix O. Revision History

831

Appendix P. Download and Mirror Sites
The latest update of this document, as an archived, bzip2-ed "tarball" including both the SGML source and
rendered HTML, may be downloaded from the author's home site). A pdf version is also available. There is
likewise an epub version, courtesy of Craig Barnes. The change log gives a detailed revision history. The
ABS Guide even has its own freshmeat.net page to keep track of major updates, user comments, and
popularity ratings for the project.
The legacy hosting site for this document is the Linux Documentation Project, which maintains many other
Guides and HOWTOs as well.
Many thanks to Ronny Bangsund for donating server space to host this project.

Appendix P. Download and Mirror Sites

832

Appendix Q. To Do List
• A comprehensive survey of incompatibilities between Bash and the classic Bourne shell.
• Same as above, but for the Korn shell (ksh).

Appendix Q. To Do List

833

Appendix R. Copyright
The Advanced Bash Scripting Guide is copyright © 2000, by Mendel Cooper. The author also asserts
copyright on all previous versions of this document. [152]
This blanket copyright recognizes and protects the rights of all contributors to this document.
This document may only be distributed subject to the terms and conditions set forth in the Open Publication
License (version 1.0 or later), http://www.opencontent.org/openpub/. The following license options also
apply.
A.

Distribution of substantively modified versions of this document
is permitted only under the following provisions.

A1. The modified document must clearly indicate that it is derivative
of the original Advanced Bash Scripting Guide, and the original
author, Mendel Cooper, must be listed as the primary author.
A2. The modified or derivative document must clearly indicate which portions
of the text differ or deviate from the original document. A notice must
be present, stating that the original author does not necessarily
endorse the changes to the original.
A3. The modified or derivative document must be distributed under this
same license, and the original author's copyright, as applicable,
may not be modified.
A4. This License Appendix is invariant, may not be modified, and may not
be omitted from any otherwise modified variants or derivatives of
this document.

B.

This document, or any modified or derivative version thereof, may
NOT be distributed encrypted or with any form of DRM (Digital Rights
Management) or content-control mechanism embedded in it. Nor may this
document or any derivative thereof be bundled with other DRM-ed works.

C.

If this document (or any previous version or derivative thereof)
is made available on a Web or ftp site, then the file(s) must be
publicly accessible. No password or other access restrictions to
its download may be imposed.

D.

Distribution of the original work in any standard (paper) book form
requires permission from the copyright holder.

E.

In the event that the author or maintainer of this document cannot
be contacted, the Linux Documentation Project is authorized to
take over custodianship of the document and name a new maintainer,
who would then have the right to update and modify the document.

Without explicit written permission from the author, distributors and publishers (including on-line publishers)
are prohibited from imposing any additional conditions, strictures, or provisions on this document, any
previous versions, or any derivative versions. As of this update, the author asserts that he has not entered into
any contractual obligations that would alter the foregoing declarations.
Essentially, you may freely distribute this book or any derivative thereof in electronic form.

Appendix R. Copyright

834

Advanced Bash-Scripting Guide
If you display or distribute this document, any previous versions thereof, or any derivatives thereof under any
license except the one above, then you are required to obtain the author's written permission. Failure to do so
may terminate your distribution rights.
Additionally, the following waiver applies:
By copying or distributing this book you WAIVE THE RIGHT
to use the materials within, or any portion thereof, in a patent or copyright
lawsuit against the Open Source community, its developers, its
distributors, or against any of its associated software or documentation
including, but not limited to, the Linux kernel, Open Office, Samba,
and Wine. You further WAIVE THE RIGHT to use any of the materials within
this book in testimony or depositions as a plaintiff's "expert witness" in
any lawsuit against the Open Source community, any of its developers, its
distributors, or any of its associated software or documentation.

These are very liberal terms, and they should not hinder any legitimate distribution or use of this book. The
author especially encourages its use for classroom and instructional purposes.
Certain of the scripts contained in this document are, where noted, in the Public Domain. These scripts
are exempt from the foregoing license and copyright restrictions.
The commercial print and other rights to this book are available. Please contact the author if interested. As of
this date, August 2011, limited print rights (Lulu edition) have been granted to Steve Glines and to no one
else.
The author produced this book in a manner consistent with the spirit of the LDP Manifesto.

Linux is a trademark registered to Linus Torvalds.
Fedora is a trademark registered to Red Hat.
Unix and UNIX are trademarks registered to the Open Group.
MS Windows is a trademark registered to the Microsoft Corp.
Solaris is a trademark registered to Oracle, Inc.
OSX is a trademark registered to Apple, Inc.
Yahoo is a trademark registered to Yahoo, Inc.
Pentium is a trademark registered to Intel, Inc.
Thinkpad is a trademark registered to Lenovo, Inc.
Scrabble is a trademark registered to Hasbro, Inc.
Librie, PRS-500, and PRS-505 are trademarks registered to Sony, Inc.
All other commercial trademarks mentioned in the body of this work are registered to their respective
owners.

Appendix R. Copyright

835

Advanced Bash-Scripting Guide
Hyun Jin Cha has done a Korean translation of version 1.0.11 of this book. Spanish, Portuguese, French,
German, Italian, Russian, Czech, Chinese, Indonesian, Dutch, and Romanian translations are also available or
in progress. If you wish to translate this document into another language, please feel free to do so, subject to
the terms stated above. The author wishes to be notified of such efforts.

For those readers who absolutely insist on making a donation to the author, you may contribute a small
amount via Paypal to my e-mail address, . Note that this is
emphatically not necessary, or even recommended. This book is a free and freely distributed document for
the use and enjoyment of the Linux community.

Appendix R. Copyright

836

Appendix S. ASCII Table
By tradition, a book of this sort has an ASCII Table appendix. This book does not. Instead, here is a short
shell script that generates a complete ASCII table and writes it to the file ASCII.txt.

Example S-1. A script that generates an ASCII table
#!/bin/bash
# ascii.sh
# ver. 0.2, reldate 26 Aug 2008
# Patched by ABS Guide author.
# Original script by Sebastian Arming.
# Used with permission (thanks!).
exec >ASCII.txt

# Save stdout to file,
#+ as in the example scripts
#+ reassign-stdout.sh and upperconv.sh.

MAXNUM=256
COLUMNS=5
OCT=8
OCTSQU=64
LITTLESPACE=-3
BIGSPACE=-5
i=1 # Decimal counter
o=1 # Octal counter
while [ "$i" -lt "$MAXNUM" ]; do # We don't have to count past 400 octal.
paddi="
$i"
echo -n "${paddi: $BIGSPACE} "
# Column spacing.
paddo="00$o"
#
echo -ne "\\${paddo: $LITTLESPACE}"
# Original.
echo -ne "\\0${paddo: $LITTLESPACE}" # Fixup.
#
^
echo -n "
"
if (( i % $COLUMNS == 0)); then
# New line.
echo
fi
((i++, o++))
# The octal notation for 8 is 10, and 64 decimal is 100 octal.
(( i % $OCT == 0))
&& ((o+=2))
(( i % $OCTSQU == 0)) && ((o+=20))
done
exit $?
# Compare this script with the "pr-asc.sh" example.
# This one handles "unprintable" characters.
# Exercise:
# Rewrite this script to use decimal numbers, rather than octal.

Appendix S. ASCII Table

837

Advanced Bash-Scripting Guide

Index
This index / glossary / quick-reference lists many of the important topics covered in the text. Terms are
arranged in approximate ASCII sorting order, modified as necessary for enhanced clarity.
Note that commands are indexed in Part 4.
***
^ (caret)
• Beginning-of-line, in a Regular Expression
•^
^^
Uppercase conversion in parameter substitution
~ Tilde
• ~ home directory, corresponds to $HOME
• ~/ Current user's home directory
• ~+ Current working directory
• ~- Previous working directory
= Equals sign
• = Variable assignment operator
• = String comparison operator
== String comparison operator
• =~ Regular Expression match operator
Example script
< Left angle bracket
• Is-less-than
String comparison
Integer comparison within double parentheses
• Redirection
< stdin
<< Here document
<<< Here string

Appendix S. ASCII Table

838

Advanced Bash-Scripting Guide
<> Opening a file for both reading and writing
> Right angle bracket
• Is-greater-than
String comparison
Integer comparison, within double parentheses
• Redirection
> Redirect stdout to a file
>> Redirect stdout to a file, but append
i>&j Redirect file descriptor i to file descriptor j
>&j Redirect stdout to file descriptor j
>&2 Redirect stdout of a command to stderr
2>&1 Redirect stderr to stdout
&> Redirect both stdout and stderr of a command to a file
:> file Truncate file to zero length
| Pipe, a device for passing the output of a command to another command or to the shell
|| Logical OR test operator
- (dash)
• Prefix to default parameter, in parameter substitution
• Prefix to option flag
• Indicating redirection from stdin or stdout
• -- (double-dash)
Prefix to long command options
C-style variable decrement within double parentheses
; (semicolon)
• As command separator
• \; Escaped semicolon, terminates a find command
• ;; Double-semicolon, terminator in a case option
Required when ...
do keyword is on the first line of loop
Appendix S. ASCII Table

839

Advanced Bash-Scripting Guide
terminating curly-bracketed code block
• ;;& ;& Terminators in a case option (version 4+ of Bash).
: Colon
• :> filename Truncate file to zero length
• null command, equivalent to the true Bash builtin
• Used in an anonymous here document
• Used as a function name
! Negation operator, inverts exit status of a test or command
• != not-equal-to String comparison operator
? (question mark)
• Match zero or one characters, in an Extended Regular Expression
• Single-character wild card, in globbing
• In a C-style Trinary operator
// Double forward slash, behavior of cd command toward
. (dot / period)
• . Load a file (into a script), equivalent to source command
• . Match single character, in a Regular Expression
• . Current working directory
./ Current working directory
• .. Parent directory
' ... ' (single quotes) strong quoting
" ... " (double quotes) weak quoting
• Double-quoting the backslash (\) character
,
• Comma operator
•,
,,
Lowercase conversion in parameter substitution
() Parentheses
• ( ... ) Command group; starts a subshell
• ( ... ) Enclose group of Extended Regular Expressions
• >( ... )
Appendix S. ASCII Table

840

Advanced Bash-Scripting Guide
<( ... ) Process substitution
• ... ) Terminates test-condition in case construct
• (( ... )) Double parentheses, in arithmetic expansion
[ Left bracket, test construct
[ ]Brackets
• Array element
• Enclose character set to match in a Regular Expression
• Test construct
[[ ... ]] Double brackets, extended test construct
$ Anchor, in a Regular Expression
$ Prefix to a variable name
$( ... ) Command substitution, setting a variable with output of a command, using parentheses notation
` ... ` Command substitution, using backquotes notation
$[ ... ] Integer expansion (deprecated)
${ ... } Variable manipulation / evaluation
• ${var} Value of a variable
• ${#var} Length of a variable
• ${#@}
${#*} Number of positional parameters
• ${parameter?err_msg} Parameter-unset message
• ${parameter-default}
${parameter:-default}
${parameter=default}
${parameter:=default} Set default parameter
• ${parameter+alt_value}
${parameter:+alt_value}
Alternate value of parameter, if set
• ${!var}
Indirect referencing of a variable, new notation
• ${!#}
Final positional parameter. (This is an indirect reference to $#.)
• ${!varprefix*}
Appendix S. ASCII Table

841

Advanced Bash-Scripting Guide
${!varprefix@}
Match names of all previously declared variables beginning with varprefix
• ${string:position}
${string:position:length} Substring extraction
• ${var#Pattern}
${var##Pattern} Substring removal
• ${var%Pattern}
${var%%Pattern} Substring removal
• ${string/substring/replacement}
${string//substring/replacement}
${string/#substring/replacement}
${string/%substring/replacement} Substring replacement
$' ... ' String expansion, using escaped characters.
\ Escape the character following
• \< ... \> Angle brackets, escaped, word boundary in a Regular Expression
• \{ N \} "Curly" brackets, escaped, number of character sets to match in an Extended RE
• \; Semicolon, escaped, terminates a find command
• \$$ Indirect reverencing of a variable, old-style notation
• Escaping a newline, to write a multi-line command
&
• &> Redirect both stdout and stderr of a command to a file
• >&j Redirect stdout to file descriptor j
>&2 Redirect stdout of a command to stderr
• i>&j Redirect file descriptor i to file descriptor j
2>&1 Redirect stderr to stdout
• Closing file descriptors
n<&- Close input file descriptor n
0<&-, <&- Close stdin
n>&- Close output file descriptor n
1>&-, >&- Close stdout
• && Logical AND test operator
• Command & Run job in background

Appendix S. ASCII Table

842

Advanced Bash-Scripting Guide
# Hashmark, special symbol beginning a script comment
#! Sha-bang, special string starting a shell script
* Asterisk
• Wild card, in globbing
• Any number of characters in a Regular Expression
• ** Exponentiation, arithmetic operator
• ** Extended globbing file-match operator
% Percent sign
• Modulo, division-remainder arithmetic operation
• Substring removal (pattern matching) operator
+ Plus sign
• Character match, in an extended Regular Expression
• Prefix to alternate parameter, in parameter substitution
• ++ C-style variable increment, within double parentheses
***
Shell Variables
$_ Last argument to previous command
$- Flags passed to script, using set
$! Process ID of last background job
$? Exit status of a command
$@ All the positional parameters, as separate words
$* All the positional parameters, as a single word
$$ Process ID of the script
$# Number of arguments passed to a function, or to the script itself
$0 Filename of the script
$1 First argument passed to script
$9 Ninth argument passed to script
Table of shell variables
******
Appendix S. ASCII Table

843

Advanced Bash-Scripting Guide
-a Logical AND compound comparison test
Address database, script example
Advanced Bash Scripting Guide, where to download
Alias
• Removing an alias, using unalias
Anagramming
And list
• To supply default command-line argument
And logical operator &&
Angle brackets, escaped, \< . . . \> word boundary in a Regular Expression
Anonymous here document, using :
Archiving
• rpm
• tar
Arithmetic expansion
• exit status of
• variations of
Arithmetic operators
• combination operators, C-style
+= -= *= /= %=
In certain contexts, += can also function as a string concatenation operator.
Arrays
• Associative arrays
• Bracket notation
• Concatenating, example script
• Copying
• Declaring
declare -a array_name
• Embedded arrays
Appendix S. ASCII Table

844

Advanced Bash-Scripting Guide
• Empty arrays, empty elements, example script
• Indirect references
• Initialization
array=( element1 element2 ... elementN)
Example script
Using command substitution
• Loading a file into an array
• Multidimensional, simulating
• Nesting and embedding
• Notation and usage
• Number of elements in
${#array_name[@]}
${#array_name[*]}
• Operations
• Passing an array to a function
• As return value from a function
• Special properties, example script
• String operations, example script
• unset deletes array elements
Arrow keys, detecting
ASCII
• Definition
• Script to generate ASCII table
awk field-oriented text processing language
• rand(), random function
• String manipulation
• Using export to pass a variable to an embedded awk script
***
Backquotes, used in command substitution
Base conversion, example script
Bash
• Bad scripting practices
• Basics reviewed, script example
• Command-line options
Table
Appendix S. ASCII Table

845

Advanced Bash-Scripting Guide
• Features that classic Bourne shell lacks
• Internal variables
• Version 2
• Version 3
• Version 4
Version 4.1
Version 4.2
.bashrc
$BASH_SUBSHELL
Basic commands, external
Batch files, DOS
Batch processing
bc, calculator utility
• In a here document
• Template for calculating a script variable
Bibliography
Bison utility
Bitwise operators
• Example script
Block devices
• testing for
Blocks of code
• Redirection
Script example: redirecting output of a a code block
Brace expansion
• Extended, {a..z}
• With increment and zero-padding (new feature in Bash, version 4)
Brackets, [ ]
• Array element
Appendix S. ASCII Table

846

Advanced Bash-Scripting Guide
• Enclose character set to match in a Regular Expression
• Test construct
Brackets, curly, {}, used in
• Code block
• find
• Extended Regular Expressions
• Positional parameters
• xargs
break loop control command
• Parameter (optional)
Builtins in Bash
• Do not fork a subprocess
***
case construct
• Command-line parameters, handling
• Globbing, filtering strings with
cat, concatentate file(s)
• Abuse of
• cat scripts
• Less efficient than redirecting stdin
• Piping the output of, to a read
• Uses of
Character devices
• testing for
Checksum
Child processes
Colon, : , equivalent to the true Bash builtin
Colorizing scripts
• Cycling through the background colors, example script
• Table of color escape sequences
• Template, colored text on colored background
Comma operator, linking commands or operations
Appendix S. ASCII Table

847

Advanced Bash-Scripting Guide
Command-line options
command_not_found_handle () builtin error-handling function (version 4+ of Bash)
Command substitution
• $( ... ), preferred notation
• Backquotes
• Extending the Bash toolset
• Invokes a subshell
• Nesting
• Removes trailing newlines
• Setting variable from loop output
• Word splitting
Comment headers, special purpose
Commenting out blocks of code
• Using an anonymous here document
• Using an if-then construct
Communications and hosts
Compound comparison operators
Compression utilities
• bzip2
• compress
• gzip
• zip
continue loop control command
Control characters
• Control-C, break
• Control-D, terminate / log out / erase
• Control-G, BEL (beep)
• Control-H, rubout
• Control-J, newline
• Control-M, carriage return
Coprocesses
cron, scheduling daemon
C-style syntax , for handling variables
Crossword puzzle solver
Appendix S. ASCII Table

848

Advanced Bash-Scripting Guide
Cryptography
Curly brackets {}
• in find command
• in an Extended Regular Expression
• in xargs
***
Daemons, in UNIX-type OS
date
dc, calculator utility
dd, data duplicator command
• Conversions
• Copying raw data to/from devices
• File deletion, secure
• Keystrokes, capturing
• Options
• Random access on a data stream
• Swapfiles, initializing
• Thread on www.linuxquestions.org
Debugging scripts
• Tools
• Trapping at exit
• Trapping signals
Decimal number, Bash interprets numbers as
declare builtin
• options
case-modification options (version 4+ of Bash)
Default parameters
/dev directory
• /dev/null pseudo-device file
• /dev/urandom pseudo-device file, generating pseudorandom numbers with
• /dev/zero, pseudo-device file
Device file

Appendix S. ASCII Table

849

Advanced Bash-Scripting Guide
dialog, utility for generating dialog boxes in a script
$DIRSTACK directory stack
Disabled commands, in restricted shells
do keyword, begins execution of commands within a loop
done keyword, terminates a loop
DOS batch files, converting to shell scripts
DOS commands, UNIX equivalents of (table)
dot files, "hidden" setup and configuration files
Double brackets [[ ... ]] test construct
• and evaluation of octal/hex constants
Double parentheses (( ... )) arithmetic expansion/evaluation construct
Double quotes " ... " weak quoting
• Double-quoting the backslash (\) character
Double-spacing a text file, using sed
***
-e File exists test
echo
• Feeding commands down a pipe
• Setting a variable using command substitution
• /bin/echo, external echo command
elif, Contraction of else and if
else
Encrypting files, using openssl
esac, keyword terminating case construct
Environmental variables
-eq , is-equal-to integer comparison test
Eratosthenes, Sieve of, algorithm for generating prime numbers
Appendix S. ASCII Table

850

Advanced Bash-Scripting Guide
Escaped characters, special meanings of
• Within $' ... ' string expansion
• Used with Unicode characters
/etc/fstab (filesystem mount) file
/etc/passwd (user account) file
$EUID, Effective user ID
eval, Combine and evaluate expression(s), with variable expansion
• Effects of, Example script
• Forces reevaluation of arguments
• And indirect references
• Risk of using
• Using eval to convert array elements into a command list
• Using eval to select among variables
Evaluation of octal/hex constants within [[ ... ]]
exec command, using in redirection
Exercises
Exit and Exit status
• exit command
• Exit status (exit code, return status of a command)
Table, Exit codes with special meanings
Anomalous
Out of range
Pipe exit status
Specified by a function return
Successful, 0
/usr/include/sysexits.h, system file listing C/C++ standard exit codes
Export, to make available variables to child processes
• Passing a variable to an embedded awk script
expr, Expression evaluator

Appendix S. ASCII Table

851

Advanced Bash-Scripting Guide
• Substring extraction
• Substring index (numerical position in string)
• Substring matching
Extended Regular Expressions
• ? (question mark) Match zero / one characters
• ( ... ) Group of expressions
• \{ N \} "Curly" brackets, escaped, number of character sets to match
• + Character match
***
factor, decomposes an integer into its prime factors
• Application: Generating prime numbers
false, returns unsuccessful (1) exit status
Field, a group of characters that comprises an item of data
Files / Archiving
File descriptors
• Closing
n<&- Close input file descriptor n
0<&-, <&- Close stdin
n>&- Close output file descriptor n
1>&-, >&- Close stdout
• File handles in C, similarity to
File encryption
find
• {} Curly brackets
• \; Escaped semicolon
Filter
• Using - with file-processing utility as a filter
• Feeding output of a filter back to same filter
Floating point numbers, Bash does not recognize
fold, a filter to wrap lines of text
Appendix S. ASCII Table

852

Advanced Bash-Scripting Guide
Forking a child process
for loops
Functions
• Arguments passed referred to by position
• Capturing the return value of a function using echo
• Colon as function name
• Definition must precede first call to function
• Exit status
• Local variables
and recursion
• Passing an array to a function
• Passing pointers to a function
• Positional parameters
• Recursion
• Redirecting stdin of a function
• return
Multiple return values from a function, example script
Returning an array from a function
Return range limits, workarounds
• Shift arguments passed to a function
• Unusual function names
***
Games and amusements
• Anagrams
• Anagrams, again
• Crossword puzzle solver
• Crypto-Quotes
• Dealing a deck of cards
• Fifteen Puzzle
• Horse race
• Knight's Tour
• "Life" game
• Magic Squares
• Music-playing script
• Nim
• Pachinko
• Perquackey
• Petals Around the Rose
• Podcasting
• Poem
• Towers of Hanoi
Appendix S. ASCII Table

853

Advanced Bash-Scripting Guide
Graphic version
Alternate graphic version
getopt, external command for parsing script command-line arguments
• Emulated in a script
getopts, Bash builtin for parsing script command-line arguments
• $OPTIND / $OPTARG
Global variable
Globbing, filename expansion
• Wild cards
• Will not match dot files
Golden Ratio (Phi)
-ge , greater-than or equal integer comparison test
-gt , greater-than integer comparison test
groff, text markup and formatting language
Gronsfeld cipher
$GROUPS, Groups user belongs to
gzip, compression utility
***
Hashing, creating lookup keys in a table
• Example script
head, echo to stdout lines at the beginning of a text file
help, gives usage summary of a Bash builtin
Here documents
• Anonymous here documents, using :
Commenting out blocks of code
Self-documenting scripts
• bc in a here document
Appendix S. ASCII Table

854

Advanced Bash-Scripting Guide
• cat scripts
• Command substitution
• ex scripts
• Function, supplying input to
• Here strings
Calculating the Golden Ratio
Prepending text
As the stdin of a loop
Using read
• Limit string
! as a limit string
Closing limit string may not be indented
Dash option to limit string, <<-LimitString
• Literal text output, for generating program code
• Parameter substitution
Disabling parameter substitution
• Passing parameters
• Temporary files
• Using vi non-interactively
History commands
$HOME, user's home directory
Homework assignment solver
$HOSTNAME, system host name
***
$Id parameter, in rcs (Revision Control System)
if [ condition ]; then ... test construct
• if-grep, if and grep in combination
Fixup for if-grep test
$IFS, Internal field separator variable
• Defaults to whitespace
Integer comparison operators
Appendix S. ASCII Table

855

Advanced Bash-Scripting Guide
in, keyword preceding [list] in a for loop
Initialization table, /etc/inittab
Inline group, i.e., code block
Interactive script, test for
I/O redirection
Indirect referencing of variables
• New notation, introduced in version 2 of Bash ( example script)
iptables, packet filtering and firewall utility
• Usage example
• Example script
Iteration
***
Job IDs, table
jot, Emit a sequence of integers. Equivalent to seq.
• Random sequence generation
Just another Bash hacker!
***
Keywords
kill, terminate a process by process ID
• Options (-l, -9)
killall, terminate a process by name
killall script in /etc/rc.d/init.d
***
lastpipe shell option
-le , less-than or equal integer comparison test
let, setting and carrying out arithmetic operations on variables

Appendix S. ASCII Table

856

Advanced Bash-Scripting Guide
• C-style increment and decrement operators
Limit string, in a here document
$LINENO, variable indicating the line number where it appears in a script
Link, file (using ln command)
• Invoking script with multiple names, using ln
• symbolic links, ln -s
List constructs
• And list
• Or list
Local variables
• and recursion
Localization
Logical operators (&&, ||, etc.)
Logout file, the ~/.bash_logout file
Loopback device, mounting a file on a block device
Loops
• break loop control command
• continue loop control command
• C-style loop within double parentheses
for loop
while loop
• do (keyword), begins execution of commands within a loop
• done (keyword), terminates a loop
• for loops
for arg in [list]; do
Command substitution to generate [list]
Filename expansion in [list]
Multiple parameters in each [list] element
Omitting [list], defaults to positional parameters

Appendix S. ASCII Table

857

Advanced Bash-Scripting Guide
Parameterizing [list]
Redirection
• in, (keyword) preceding [list] in a for loop
• Nested loops
• Running a loop in the background, script example
• Semicolon required, when do is on first line of loop
for loop
while loop
• until loop
until [ condition-is-true ]; do
• while loop
while [ condition ]; do
Function call inside test brackets
Multiple conditions
Omitting test brackets
Redirection
while read construct
• Which type of loop to use
Loopback devices
• In /dev directory
• Mounting an ISO image
-lt , less-than integer comparison test
***
m4, macro processing language
$MACHTYPE, Machine type
Magic number, marker at the head of a file indicating the file type
Makefile, file containing the list of dependencies used by make command
man, manual page (lookup)
• Man page editor (script)
mapfile builtin, loads an array with a text file
Appendix S. ASCII Table

858

Advanced Bash-Scripting Guide
Math commands
Meta-meaning
Morse code training script
Modulo, arithmetic remainder operator
• Application: Generating prime numbers
Mortgage calculations, example script
***
-n String not null test
Named pipe, a temporary FIFO buffer
• Example script
nc, netcat, a network toolkit for TCP and UDP ports
-ne, not-equal-to integer comparison test
Negation operator, !, reverses the sense of a test
netstat, Network statistics
Network programming
nl, a filter to number lines of text
Noclobber, -C option to Bash to prevent overwriting of files
NOT logical operator, !
null variable assignment, avoiding
***
-o Logical OR compound comparison test
Obfuscation
• Colon as function name
• Homework assignment
• Just another Bash hacker!
octal, base-8 numbers
od, octal dump
Appendix S. ASCII Table

859

Advanced Bash-Scripting Guide
$OLDPWD Previous working directory
openssl encryption utility
Operator
• Definition of
• Precedence
Options, passed to shell or script on command line or by set command
Or list
Or logical operator, ||
***
Parameter substitution
• ${parameter+alt_value}
${parameter:+alt_value}
Alternate value of parameter, if set
• ${parameter-default}
${parameter:-default}
${parameter=default}
${parameter:=default}
Default parameters
• ${!varprefix*}
${!varprefix@}
Parameter name match
• ${parameter?err_msg}
Parameter-unset message
• ${parameter}
Value of parameter
• Case modification (version 4+ of Bash).
• Script example
• Table of parameter substitution
Parent / child process problem, a child process cannot export variables to a parent process
Parentheses
Appendix S. ASCII Table

860

Advanced Bash-Scripting Guide
• Command group
• Enclose group of Extended Regular Expressions
• Double parentheses, in arithmetic expansion
$PATH, the path (location of system binaries)
• Appending directories to $PATH using the += operator.
Perl, programming language
• Combined in the same file with a Bash script
• Embedded in a Bash script
Perquackey-type anagramming game (Quackey script)
Petals Around the Rose
PID, Process ID, an identification number assigned to a running process.
Pipe, | , a device for passing the output of a command to another command or to the shell
• Avoiding unnecessary commands in a pipe
• Comments embedded within
• Exit status of a pipe
• Pipefail, set -o pipefail option to indicate exit status within a pipe
• $PIPESTATUS, exit status of last executed pipe
• Piping output of a command to a script
• Redirecting stdin, rather than using cat in a pipe
Pitfalls
• - (dash) is not redirection operator
• // (double forward slash), behavior of cd command toward
• #!/bin/sh script header disables extended Bash features
• Abuse of cat
• CGI programming, using scripts for
• Closing limit string in a here document, indenting
• DOS-type newlines (\r\n) crash a script
• Double-quoting the backslash (\) character
• eval, risk of using
• Execute permission lacking for commands within a script
• Exit status, anomalous
• Exit status of arithmetic expression not equivalent to an error code
• Export problem, child process to parent process
• Extended Bash features not available
• Failing to quote variables within test brackets
• GNU command set, in cross-platform scripts
• let misuse: attempting to set string variables
• Multiple echo statements in a function whose output is captured
• null variable assignment
• Numerical and string comparison operators not equivalent
Appendix S. ASCII Table

861

Advanced Bash-Scripting Guide
= and -eq not interchangeable
• Omitting terminal semicolon, in a curly-bracketed code block
• Piping
echo to a loop
echo to read (however, this problem can be circumvented)
tail -f to grep
• Preserving whitespace within a variable, unintended consequences
• suid commands inside a script
• Undocumented Bash features, danger of
• Uninitialized variables
• Variable names, inappropriate
• Variables in a subshell, scope limited
• Subshell in while-read loop
• Whitespace, misuse of
Pointers
• and file descriptors
• and functions
• and indirect references
• and variables
Portability issues in shell scripting
• Setting path and umask
• A test suite script (Bash versus classic Bourne shell)
• Using whatis
Positional parameters
• $@, as separate words
• $*, as a single word
• in functions
POSIX, Portable Operating System Interface / UNIX
• --posix option
• 1003.2 standard
• Character classes
$PPID, process ID of parent process
Precedence, operator
Prepending lines at head of a file, script example
Prime numbers

Appendix S. ASCII Table

862

Advanced Bash-Scripting Guide
• Generating primes using the factor command
• Generating primes using the modulo operator
• Sieve of Eratosthenes, example script
printf, formatted print command
/proc directory
• Running processes, files describing
• Writing to files in /proc, warning
Process
• Child process
• Parent process
• Process ID (PID)
Process substitution
• To compare contents of directories
• To supply stdin of a command
• Template
• while-read loop without a subshell
Programmable completion (tab expansion)
Prompt
• $PS1, Main prompt, seen at command line
• $PS2, Secondary prompt
Pseudo-code, as problem-solving method
$PWD, Current working directory
***
Quackey, a Perquackey-type anagramming game (script)
Question mark, ?
• Character match in an Extended Regular Expression
• Single-character wild card, in globbing
• In a C-style Trinary operator
Quoting
• Character string
• Variables
within test brackets
Appendix S. ASCII Table

863

Advanced Bash-Scripting Guide
• Whitespace, using quoting to preserve
***
Random numbers
• /dev/urandom
• rand(), random function in awk
• $RANDOM, Bash function that returns a pseudorandom integer
• Random sequence generation, using date command
• Random sequence generation, using jot
• Random string, generating
rcs
read, set value of a variable from stdin
• Detecting arrow keys
• Options
• Piping output of cat to read
• "Prepending" text
• Problems piping echo to read
• Redirection from a file to read
• $REPLY, default read variable
• Timed input
• while read construct
readline library
Recursion
• Demonstration of
• Factorial
• Fibonacci sequence
• Local variables
• Script calling itself recursively
• Towers of Hanoi
Redirection
• Code blocks
• exec filename
• read input redirected from a file
• stderr to stdout

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2>&1
• stdin / stdout, using • stdinof a function
• stdout to a file
> ... >>
• stdout to file descriptor j
>&j
• file descriptori to file descriptor j
i>&j
• stdout of a command to stderr
>&2
• stdout and stderr of a command to a file
&>
• tee, redirect to a file output of command(s) partway through a pipe
Reference Cards
• Miscellaneous constructs
• Parameter substitution/expansion
• Special shell variables
• String operations
• Test operators
Binary comparison
Files
Regular Expressions
• ^ (caret) Beginning-of-line
• $ (dollar sign) Anchor
• . (dot) Match single character
• * (asterisk) Any number of characters
• [ ] (brackets) Enclose character set to match
• \ (backslash) Escape, interpret following character literally
• \< ... \> (angle brackets, escaped) Word boundary
• Extended REs
+ Character match
\{ \} Escaped "curly" brackets
[: :] POSIX character classes
$REPLY, Default value associated with read command

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Restricted shell, shell (or script) with certain commands disabled
return, command that terminates a function
run-parts
• Running scripts in sequence, without user intervention
***
Scope of a variable, definition
Script options, set at command line
Scripting routines, library of useful definitions and functions
Secondary prompt, $PS2
Security issues
• nmap, network mapper / port scanner
• sudo
• suid commands inside a script
• Viruses, trojans, and worms in scripts
• Writing secure scripts
sed, pattern-based programming language
• Table, basic operators
• Table, examples of operators
select, construct for menu building
• in list omitted
Semaphore
Semicolon required, when do keyword is on first line of loop
• When terminating curly-bracketed code block
seq, Emit a sequence of integers. Equivalent to jot.
set, Change value of internal script variables
• set -u, Abort script with error message if attempting to use an undeclared variable.
Shell script, definition of
Shell wrapper, script embedding a command or utility

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shift, reassigning positional parameters
$SHLVL, shell level, depth to which the shell (or script) is nested
shopt, change shell options
Signal, a message sent to a process
Simulations
• Brownian motion
• Galton board
• Horserace
• Life, game of
• PI, approximating by firing cannonballs
• Pushdown stack
Single quotes (' ... ') strong quoting
Socket, a communication node associated with an I/O port
Sorting
• Bubble sort
• Insertion sort
source, execute a script or, within a script, import a file
• Passing positional parameters
Spam, dealing with
• Example script
• Example script
• Example script
• Example script
Special characters
Stack
• Definition
• Emulating a push-down stack, example script
Standard Deviation, example script
Startup files, Bash
stdin and stdout
Stopwatch, example script
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Strings
• =~ String match operator
• Comparison
• Length
${#string}
• Manipulation
• Manipulation, using awk
• Null string, testing for
• Protecting strings from expansion and/or reinterpretation, script example
Unprotecting strings, script example
• strchr(), equivalent of
• strlen(), equivalent of
• strings command, find printable strings in a binary or data file
• Substring extraction
${string:position}
${string:position:length}
Using expr
• Substring index (numerical position in string)
• Substring matching, using expr
• Substring removal
${var#Pattern}
${var##Pattern}
${var%Pattern}
${var%%Pattern}
• Substring replacement
${string/substring/replacement}
${string//substring/replacement}
${string/#substring/replacement}
${string/%substring/replacement}
Script example
• Table of string/substring manipulation and extraction operators
Strong quoting ' ... '
Stylesheet for writing scripts

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Subshell
• Command list within parentheses
• Variables, $BASH_SUBSHELL and $SHLVL
• Variables in a subshell
scope limited, but ...
... can be accessed outside the subshell?
su Substitute user, log on as a different user or as root
suid (set user id) file flag
• suid commands inside a script, not advisable
Symbolic links
Swapfiles
***
Tab completion
Table lookup, script example
tail, echo to stdout lines at the (tail) end of a text file
tar, archiving utility
tee, redirect to a file output of command(s) partway through a pipe
Terminals
• setserial
• setterm
• stty
• tput
• wall
test command
• Bash builtin
• external command, /usr/bin/test (equivalent to /usr/bin/[)
Test constructs
Test operators
• -a Logical AND compound comparison
• -e File exists
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• -eq is-equal-to (integer comparison)
• -f File is a regular file
• -ge greater-than or equal (integer comparison)
• -gt greater-than (integer comparison)
• -le less-than or equal (integer comparison)
• -lt less-than (integer comparison)
• -n not-zero-length (string comparison)
• -ne not-equal-to (integer comparison)
• -o Logical OR compound comparison
• -u suid flag set, file test
• -z is-zero-length (string comparison)
• = is-equal-to (string comparison)
== is-equal-to (string comparison)
• < less-than (string comparison)
• < less-than, (integer comparison, within double parentheses)
• <= less-than-or-equal, (integer comparison, within double parentheses)
• > greater-than (string comparison)
• > greater-than, (integer comparison, within double parentheses)
• >= greater-than-or-equal, (integer comparison, within double parentheses)
• || Logical OR
• && Logical AND
• ! Negation operator, inverts exit status of a test
!= not-equal-to (string comparison)
• Tables of test operators
Binary comparison
File
Text and text file processing
Time / Date
Timed input
• Using read -t
• Using stty
• Using timing loop
• Using $TMOUT
Tips and hints for Bash scripts
• Array, as return value from a function
• Capturing the return value of a function, using echo
• CGI programming, using scripts for
• Comment blocks
Using anonymous here documents

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Using if-then constructs
• Comment headers, special purpose
• C-style syntax , for manipulating variables
• Double-spacing a text file
• Filenames prefixed with a dash, removing
• Filter, feeding output back to same filter
• Function return value workarounds
• if-grep test fixup
• Library of useful definitions and functions
• null variable assignment, avoiding
• Passing an array to a function
• $PATH, appending to, using the += operator.
• Prepending lines at head of a file
• Progress bar template
• Pseudo-code
• rcs
• Redirecting a test to /dev/null to suppress output
• Running scripts in sequence without user intervention, using run-parts
• Script as embedded command
• Script portability
Setting path and umask
Using whatis
• Setting script variable to a block of embedded sed or awk code
• Subshell variable, accessing outside the subshell
• Testing a variable to see if it contains only digits
• Testing whether a command exists, using type
• Tracking script usage
• while-read loop without a subshell
• Widgets, invoking from a script
$TMOUT, Timeout interval
Token, a symbol that may expand to a keyword or command
tput, terminal-control command
tr, character translation filter
• DOS to Unix text file conversion
• Options
• Soundex, example script
• Variants
Trap, specifying an action upon receipt of a signal
Trinary operator, C-style, var>10?88:99
• in double-parentheses construct
• in let construct
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true, returns successful (0) exit status
typeset builtin
• options
***
$UID, User ID number
unalias, to remove an alias
uname, output system information
Unicode, encoding standard for representing letters and symbols
Uninitialized variables
uniq, filter to remove duplicate lines from a sorted file
unset, delete a shell variable
until loop
until [ condition-is-true ]; do
***
Variables
• Array operations on
• Assignment
Script example
Script example
Script example
• Bash internal variables
• Block of sed or awk code, setting a variable to
• C-style increment/decrement/trinary operations
• Change value of internal script variables using set
• declare, to modify the properties of variables
• Deleting a shell variable using unset
• Environmental
• Expansion / Substring replacement operators
• Indirect referencing
eval variable1=\$$variable2
Newer notation
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${!variable}
• Integer
• Integer / string (variables are untyped)
• Length
${#var}
• Lvalue
• Manipulating and expanding
• Name and value of a variable, distinguishing between
• Null string, testing for
• Null variable assignment, avoiding
• Quoting
within test brackets
to preserve whitespace
• rvalue
• Setting to null value
• In subshell not visible to parent shell
• Testing a variable if it contains only digits
• Typing, restricting the properties of a variable
• Undeclared, error message
• Uninitialized
• Unquoted variable, splitting
• Unsetting
• Untyped
***
wait, suspend script execution
• To remedy script hang
Weak quoting " ... "
while loop
while [ condition ]; do
• C-style syntax
• Calling a function within test brackets
• Multiple conditions
• Omitting test brackets
• while read construct
Avoiding a subshell
Whitespace, spaces, tabs, and newline characters
• $IFS defaults to
• Inappropriate use of
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• Preceding closing limit string in a here document, error
• Preceding script comments
• Quoting, to preserve whitespace within strings or variables
• [:space:], POSIX character class
who, information about logged on users
•w
• whoami
• logname
Widgets
Wild card characters
• Asterisk *
• In [list] constructs
• Question mark ?
• Will not match dot files
Word splitting
• Definition
• Resulting from command substitution
Wrapper, shell
***
xargs, Filter for grouping arguments
• Curly brackets
• Limiting arguments passed
• Options
• Processes arguments one at a time
• Whitespace, handling
***
yes
• Emulation
***
-z String is null
Zombie, a process that has terminated, but not yet been killed by its parent

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Notes
[1]
[2]
[3]
[4]
[5]

[6]
[7]
[8]

These are referred to as builtins, features internal to the shell.
Although recursion is possible in a shell script, it tends to be slow and its implementation is often an
ugly kludge.
An acronym is a word formed by pasting together the initial letters of the words in a tongue-tripping
phrase. This is a harmful and subversive practice that surely deserves severe punishment.
Many of the features of ksh88, and even a few from the updated ksh93 have been merged into Bash.
By convention, user-written shell scripts that are Bourne shell compliant generally take a name with a
.sh extension. System scripts, such as those found in /etc/rc.d, do not conform to this
nomenclature.
More commonly seen in the literature as she-bang or sh-bang. This derives from the concatenation of
the tokens sharp (#) and bang (!).
Some flavors of UNIX (those based on 4.2 BSD) allegedly take a four-byte magic number, requiring a
blank after the ! -- #! /bin/sh. According to Sven Mascheck this is probably a myth.
The #! line in a shell script will be the first thing the command interpreter (sh or bash) sees. Since this
line begins with a #, it will be correctly interpreted as a comment when the command interpreter finally
executes the script. The line has already served its purpose - calling the command interpreter.
If, in fact, the script includes an extra #! line, then bash will interpret it as a comment.
#!/bin/bash
echo "Part 1 of script."
a=1
#!/bin/bash
# This does *not* launch a new script.
echo "Part 2 of script."
echo $a # Value of $a stays at 1.

[9]

This allows some cute tricks.
#!/bin/rm
# Self-deleting script.
# Nothing much seems to happen when you run this... except that the file disappears.
WHATEVER=85
echo "This line will never print (betcha!)."
exit $WHATEVER

# Doesn't matter. The script will not exit here.
# Try an echo $? after script termination.
# You'll get a 0, not a 85.

Also, try starting a README file with a #!/bin/more, and making it executable. The result is a
self-listing documentation file. (A here document using cat is possibly a better alternative -- see
Example 19-3).
[10] Portable Operating System Interface, an attempt to standardize UNIX-like OSes. The POSIX
specifications are listed on the Open Group site.
[11] To avoid this possibility, a script may begin with a #!/bin/env bash sha-bang line. This may be useful
on UNIX machines where bash is not located in /bin
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[12] If Bash is your default shell, then the #! isn't necessary at the beginning of a script. However, if
launching a script from a different shell, such as tcsh, then you will need the #!.
[13] Caution: invoking a Bash script by sh scriptname turns off Bash-specific extensions, and the
script may therefore fail to execute.
[14] A script needs read, as well as execute permission for it to run, since the shell needs to be able to read
it.
[15] Why not simply invoke the script with scriptname? If the directory you are in ($PWD) is where
scriptname is located, why doesn't this work? This fails because, for security reasons, the current
directory (./) is not by default included in a user's $PATH. It is therefore necessary to explicitly
invoke the script in the current directory with a ./scriptname.
[16] An operator is an agent that carries out an operation. Some examples are the common arithmetic
operators, + - * /. In Bash, there is some overlap between the concepts of operator and keyword.
[17]
American Standard Code for Information Interchange. This is a system for encoding text characters
(alphabetic, numeric, and a limited set of symbols) as 7-bit numbers that can be stored and manipulated
by computers. Many of the ASCII characters are represented on a standard keyboard.
[18]
A PID, or process ID, is a number assigned to a running process. The PIDs of running processes may
be viewed with a ps command.

Definition: A process is a currently executing command (or program), sometimes referred to as a
job.
[19] The shell does the brace expansion. The command itself acts upon the result of the expansion.
[20] Exception: a code block in braces as part of a pipe may run as a subshell.
ls | { read firstline; read secondline; }
# Error. The code block in braces runs as a subshell,
#+ so the output of "ls" cannot be passed to variables within the block.
echo "First line is $firstline; second line is $secondline" # Won't work.
# Thanks, S.C.

[21] Even as in olden times a philtre denoted a potion alleged to have magical transformative powers, so
does a UNIX filter transform its target in (roughly) analogous fashion. (The coder who comes up with a
"love philtre" that runs on a Linux machine will likely win accolades and honors.)
[22] Bash stores a list of commands previously issued from the command-line in a buffer, or memory space,
for recall with the builtin history commands.
[23] A linefeed (newline) is also a whitespace character. This explains why a blank line, consisting only of a
linefeed, is considered whitespace.
[24] Technically, the name of a variable is called an lvalue, meaning that it appears on the left side of an
assignment statment, as in VARIABLE=23. A variable's value is an rvalue, meaning that it appears on
the right side of an assignment statement, as in VAR2=$VARIABLE.
A variable's name is, in fact, a reference, a pointer to the memory location(s) where the actual data
associated with that variable is kept.
[25] Note that functions also take positional parameters.
[26] The process calling the script sets the $0 parameter. By convention, this parameter is the name of the
script. See the manpage (manual page) for execv.
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From the command-line, however, $0 is the name of the shell.
bash$ echo $0
bash
tcsh% echo $0
tcsh

[27] If the the script is sourced or symlinked, then this will not work. It is safer to check $BASH_Source.
[28] Unless there is a file named first in the current working directory. Yet another reason to quote.
(Thank you, Harald Koenig, for pointing this out.
[29]
Encapsulating "!" within double quotes gives an error when used from the command line. This is
interpreted as a history command. Within a script, though, this problem does not occur, since the Bash
history mechanism is disabled then.
Of more concern is the apparently inconsistent behavior of \ within double quotes, and especially
following an echo -e command.
bash$ echo hello\!
hello!
bash$ echo "hello\!"
hello\!

bash$
>
bash$
>
bash$
a
bash$
\a

echo \
echo "\"
echo \a
echo "\a"

bash$ echo x\ty
xty
bash$ echo "x\ty"
x\ty
bash$ echo -e x\ty
xty
bash$ echo -e "x\ty"
x
y

Double quotes following an echo sometimes escape \. Moreover, the -e option to echo causes the "\t"
to be interpreted as a tab.

[30]
[31]
[32]
[33]

(Thank you, Wayne Pollock, for pointing this out, and Geoff Lee and Daniel Barclay for explaining it.)
"Word splitting," in this context, means dividing a character string into separate and discrete
arguments.
In those instances when there is no return terminating the function.
A token is a symbol or short string with a special meaning attached to it (a meta-meaning). In Bash,
certain tokens, such as [ and . (dot-command), may expand to keywords and commands.
Per the 1913 edition of Webster's Dictionary:

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Deprecate
...
To
to
to
to
to
to

pray against, as an evil;
seek to avert by prayer;
desire the removal of;
seek deliverance from;
express deep regret for;
disapprove of strongly.

[34] Be aware that suid binaries may open security holes. The suid flag has no effect on shell scripts.
[35] On Linux systems, the sticky bit is no longer used for files, only on directories.
[36] As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ -n
"$string" -o "$a" = "$b" ] may cause an error with some versions of Bash if $string is
empty. The safe way is to append an extra character to possibly empty variables, [ "x$string" !=
x -o "x$a" = "x$b" ] (the "x's" cancel out).
[37] In a different context, += can serve as a string concatenation operator. This can be useful for modifying
environmental variables.
[38] Side effects are, of course, unintended -- and usually undesirable -- consequences.
[39] Precedence, in this context, has approximately the same meaning as priority
[40] A stack register is a set of consecutive memory locations, such that the values stored (pushed) are
retrieved (popped) in reverse order. The last value stored is the first retrieved. This is sometimes called
a LIFO (last-in-first-out) or pushdown stack.
[41] The PID of the currently running script is $$, of course.
[42] Somewhat analogous to recursion, in this context nesting refers to a pattern embedded within a larger
pattern. One of the definitions of nest, according to the 1913 edition of Webster's Dictionary, illustrates
this beautifully: "A collection of boxes, cases, or the like, of graduated size, each put within the one
next larger."
[43] The words "argument" and "parameter" are often used interchangeably. In the context of this document,
they have the same precise meaning: a variable passed to a script or function.
[44] Within a script, inside a subshell, $$ returns the PID of the script, not the subshell.
[45] In this context, typing a variable means to classify it and restrict its properties. For example, a variable
declared or typed as an integer is no longer available for string operations.
declare -i intvar
intvar=23
echo "$intvar"
# 23
intvar=stringval
echo "$intvar"
# 0

[46] True "randomness," insofar as it exists at all, can only be found in certain incompletely understood
natural phenomena, such as radioactive decay. Computers only simulate randomness, and
computer-generated sequences of "random" numbers are therefore referred to as pseudorandom.
[47] The seed of a computer-generated pseudorandom number series can be considered an identification
label. For example, think of the pseudorandom series with a seed of 23 as Series #23.
A property of a pseurandom number series is the length of the cycle before it starts repeating itself. A
good pseurandom generator will produce series with very long cycles.
[48] This applies to either command-line arguments or parameters passed to a function.
[49]
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[50]
[51]
[52]
[53]

Note that $substring and $replacement may refer to either literal strings or variables,
depending on context. See the first usage example.
If $parameter is null in a non-interactive script, it will terminate with a 127 exit status (the Bash error
code for "command not found").
Iteration: Repeated execution of a command or group of commands, usually -- but not always, while a
given condition holds, or until a given condition is met.
These are shell builtins, whereas other loop commands, such as while and case, are keywords.
Pattern-match lines may also start with a ( left paren to give the layout a more structured appearance.
case $( arch ) in
# $( arch ) returns machine architecture.
( i386 ) echo "80386-based machine";;
# ^
^
( i486 ) echo "80486-based machine";;
( i586 ) echo "Pentium-based machine";;
( i686 ) echo "Pentium2+-based machine";;
(
* ) echo "Other type of machine";;
esac

[54] For purposes of command substitution, a command may be an external system command, an internal
scripting builtin, or even a script function.
[55] In a more technically correct sense, command substitution extracts the stdout of a command, then
assigns it to a variable using the = operator.
[56] In fact, nesting with backticks is also possible, but only by escaping the inner backticks, as John
Default points out.
word_count=` wc -w \`echo * | awk '{print $8}'\` `

[57] As Nathan Coulter points out, "while forking a process is a low-cost operation, executing a new
program in the newly-forked child process adds more overhead."
[58] An exception to this is the time command, listed in the official Bash documentation as a keyword
("reserved word").
[59] Note that let cannot be used for setting string variables.
[60] To Export information is to make it available in a more general context. See also scope.
[61] An option is an argument that acts as a flag, switching script behaviors on or off. The argument
associated with a particular option indicates the behavior that the option (flag) switches on or off.
[62] Technically, an exit only terminates the process (or shell) in which it is running, not the parent process.
[63] Unless the exec is used to reassign file descriptors.
[64]
Hashing is a method of creating lookup keys for data stored in a table. The data items themselves are
"scrambled" to create keys, using one of a number of simple mathematical algorithms (methods, or
recipes).
An advantage of hashing is that it is fast. A disadvantage is that collisions -- where a single key maps to
more than one data item -- are possible.
For examples of hashing see Example A-20 and Example A-21.
[65] The readline library is what Bash uses for reading input in an interactive shell.
[66] This only applies to child processes, of course.
[67] The C source for a number of loadable builtins is typically found in the
/usr/share/doc/bash-?.??/functions directory.
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Note that the -f option to enable is not portable to all systems.
[68] The same effect as autoload can be achieved with typeset -fu.
[69]
Dotfiles are files whose names begin with a dot, such as ~/.Xdefaults. Such filenames do not
appear in a normal ls listing (although an ls -a will show them), and they cannot be deleted by an
accidental rm -rf *. Dotfiles are generally used as setup and configuration files in a user's home
directory.
[70] This particular feature may not yet be implemented in the version of the ext2/ext3 filesystem installed
on your system. Check the documentation for your Linux distro.
[71] And even when xargs is not strictly necessary, it can speed up execution of a command involving
batch-processing of multiple files.
[72] This is only true of the GNU version of tr, not the generic version often found on commercial UNIX
systems.
[73] An archive, in the sense discussed here, is simply a set of related files stored in a single location.
[74] A tar czvf ArchiveName.tar.gz * will include dotfiles in subdirectories below the current
working directory. This is an undocumented GNU tar "feature."
[75] The checksum may be expressed as a hexadecimal number, or to some other base.
[76] For even better security, use the sha256sum, sha512, and sha1pass commands.
[77] This is a symmetric block cipher, used to encrypt files on a single system or local network, as opposed
to the public key cipher class, of which pgp is a well-known example.
[78] Creates a temporary directory when invoked with the -d option.
[79]
A daemon is a background process not attached to a terminal session. Daemons perform designated
services either at specified times or explicitly triggered by certain events.

[80]
[81]
[82]
[83]

The word "daemon" means ghost in Greek, and there is certainly something mysterious, almost
supernatural, about the way UNIX daemons wander about behind the scenes, silently carrying out their
appointed tasks.
This is actually a script adapted from the Debian Linux distribution.
The print queue is the group of jobs "waiting in line" to be printed.
For an excellent overview of this topic, see Andy Vaught's article, Introduction to Named Pipes, in the
September, 1997 issue of Linux Journal.
EBCDIC (pronounced "ebb-sid-ick") is an acronym for Extended Binary Coded Decimal Interchange
Code, an obsolete IBM data format. A bizarre application of the conv=ebcdic option of dd is as a
quick 'n easy, but not very secure text file encoder.
cat $file | dd conv=swab,ebcdic > $file_encrypted
# Encode (looks like gibberish).
# Might as well switch bytes (swab), too, for a little extra obscurity.
cat $file_encrypted | dd conv=swab,ascii > $file_plaintext
# Decode.

[84] A macro is a symbolic constant that expands into a command string or a set of operations on
parameters. Simply put, it's a shortcut or abbreviation.
[85] This is the case on a Linux machine or a UNIX system with disk quotas.
[86] The userdel command will fail if the particular user being deleted is still logged on.
[87]
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[88]
[89]
[90]
[91]

[92]
[93]

For more detail on burning CDRs, see Alex Withers' article, Creating CDs, in the October, 1999 issue
of Linux Journal.
The -c option to mke2fs also invokes a check for bad blocks.
Since only root has write permission in the /var/lock directory, a user script cannot set a lock file
there.
Operators of single-user Linux systems generally prefer something simpler for backups, such as tar.
As of the version 4 update of Bash, the -f and -c options take a block size of 512 when in POSIX
mode. Additionally, there are two new options: -b for socket buffer size, and -T for the limit on the
number of threads.
NAND is the logical not-and operator. Its effect is somewhat similar to subtraction.
In Bash and other Bourne shell derivatives, it is possible to set variables in a single command's
environment.
var1=value1 var2=value2 commandXXX
# $var1 and $var2 set in the environment of 'commandXXX' only.

[94] The killall system script should not be confused with the killall command in /usr/bin.
[95] A meta-meaning is the meaning of a term or expression on a higher level of abstraction. For example,
the literal meaning of regular expression is an ordinary expression that conforms to accepted usage.
The meta-meaning is drastically different, as discussed at length in this chapter.
[96] Since sed, awk, and grep process single lines, there will usually not be a newline to match. In those
cases where there is a newline in a multiple line expression, the dot will match the newline.
#!/bin/bash
sed -e 'N;s/.*/[&]/' << EOF
line1
line2
EOF
# OUTPUT:
# [line1
# line2]

# Here Document

echo
awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF
line 1
line 2
EOF
# OUTPUT:
# line
# 1

# Thanks, S.C.
exit 0

[97] Filename expansion means expanding filename patterns or templates containing special characters. For
example, example.??? might expand to example.001 and/or example.txt.
[98] A wild card character, analogous to a wild card in poker, can represent (almost) any other character.
[99] Filename expansion can match dotfiles, but only if the pattern explicitly includes the dot as a literal
character.
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~/[.]bashrc
~/?bashrc

#
#
#
#+

Will not expand to ~/.bashrc
Neither will this.
Wild cards and metacharacters will NOT
expand to a dot in globbing.

~/.[b]ashrc
~/.ba?hrc
~/.bashr*

#
#
#

Will expand to ~/.bashrc
Likewise.
Likewise.

# Setting the "dotglob" option turns this off.
# Thanks, S.C.

[100] Except, as Dennis Benzinger points out, if using <<- to suppress tabs.
[101] By convention in UNIX and Linux, data streams and peripherals (device files) are treated as files, in a
fashion analogous to ordinary files.
[102] A file descriptor is simply a number that the operating system assigns to an open file to keep track of it.
Consider it a simplified type of file pointer. It is analogous to a file handle in C.
[103] Using file descriptor 5 might cause problems. When Bash creates a child process, as with
exec, the child inherits fd 5 (see Chet Ramey's archived e-mail, SUBJECT: RE: File descriptor 5 is
held open). Best leave this particular fd alone.
[104] An external command invoked with an exec does not (usually) fork off a subprocess / subshell.
[105] This has the same effect as a named pipe (temp file), and, in fact, named pipes were at one time used in
process substitution.
[106] The return command is a Bash builtin.
[107] However, as Thomas Braunberger points out, a local variable declared in a function is also visible to
functions called by the parent function.
#!/bin/bash
function1 ()
{
local func1var=20
echo "Within function1, \$func1var = $func1var."
function2
}
function2 ()
{
echo "Within function2, \$func1var = $func1var."
}
function1
exit 0

# Output of the script:
# Within function1, $func1var = 20.
# Within function2, $func1var = 20.

This is documented in the Bash manual:

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[108]
[109]
[110]
[111]
[112]

"Local can only be used within a function; it makes the variable name have a visible scope restricted to
that function and its children." [emphasis added] The ABS Guide author considers this behavior to be a
bug.
Otherwise known as redundancy.
Otherwise known as tautology.
Otherwise known as a metaphor.
Otherwise known as a recursive function.
Too many levels of recursion may crash a script with a segfault.
#!/bin/bash
#
#

Warning: Running this script could possibly lock up your system!
If you're lucky, it will segfault before using up all available memory.

recursive_function ()
{
echo "$1"
# Makes the function do something, and hastens the segfault.
(( $1 < $2 )) && recursive_function $(( $1 + 1 )) $2;
# As long as 1st parameter is less than 2nd,
#+ increment 1st and recurse.
}
recursive_function 1 50000 # Recurse 50,000 levels!
# Most likely segfaults (depending on stack size, set by ulimit -m).
# Recursion this deep might cause even a C program to segfault,
#+ by using up all the memory allotted to the stack.

echo "This will probably not print."
exit 0 # This script will not exit normally.
#

Thanks, Stéphane Chazelas.

[113] ... as the first word of a command string. Obviously, an alias is only meaningful at the beginning of a
command.
[114] However, aliases do seem to expand positional parameters.
[115] The entries in /dev provide mount points for physical and virtual devices. These entries use very little
drive space.
Some devices, such as /dev/null, /dev/zero, and /dev/urandom are virtual. They are not
actual physical devices and exist only in software.
[116] A block device reads and/or writes data in chunks, or blocks, in contrast to a character device, which
acesses data in character units. Examples of block devices are hard drives, CDROM drives, and flash
drives. Examples of character devices are keyboards, modems, sound cards.
[117] Of course, the mount point /mnt/flashdrive must exist. If not, then, as root, mkdir
/mnt/flashdrive.
To actually mount the drive, use the following command: mount /mnt/flashdrive
Newer Linux distros automount flash drives in the /media directory without user intervention.
[118] Certain system commands, such as procinfo, free, vmstat, lsdev, and uptime do this as well.
[119] By convention, signal 0 is assigned to exit.
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[120] Setting the suid permission on the script itself has no effect in Linux and most other UNIX flavors.
[121] In this context, "magic numbers" have an entirely different meaning than the magic numbers used to
designate file types.
[122] Quite a number of Linux utilities are, in fact, shell wrappers. Some examples are
/usr/bin/pdf2ps, /usr/bin/batch, and /usr/bin/xmkmf.
[123] ANSI is, of course, the acronym for the American National Standards Institute. This august body
establishes and maintains various technical and industrial standards.
[124] This usually means liberal use of functions.
[125] See Marius van Oers' article, Unix Shell Scripting Malware, and also the Denning reference in the
bibliography.
[126] Or, better yet, #!/bin/env sh.
[127] To be more specific, Bash 4+ has limited support for associative arrays. It's a bare-bones
implementation, and it lacks the much of the functionality of such arrays in other programming
languages.
[128] Copyright 1995-2009 by Chester Ramey.
[129] This only works with pipes and certain other special files.
[130] But only in conjunction with readline, i.e., from the command-line.
[131] And while you're at it, consider fixing the notorious piped read problem.
[132] This is the notorious flog it to death technique that works so well with slow learners, eccentrics, odd
ducks, fools and geniuses.
[133] In fact, he is a school dropout with no formal credentials or professional experience whatsoever. None.
Aside from the ABS Guide, his major claim to fame is a First Place in the sack race at the Colfax
Elementary School Field Day in June, 1958.
[134] Those who can, do. Those who can't . . . get an MCSE.
[135] Sometimes it seems as if he has spent his entire life flouting conventional wisdom and defying the
sonorous Voice of Authority: "Hey, you can't do that!"
[136] E-mails from certain spam-infested TLDs (61, 202, 211, 218, 220, etc.) will be trapped by spam filters
and deleted unread.
[137] Well, if you absolutely insist, you can try modifying Example A-44 to suit your purposes.
[138] It was hard to resist the obvious pun. No slight intended, since the book is a pretty decent introduction
to the basic concepts of shell scripting.
[139] Sed executes without user intervention.
[140] If no address range is specified, the default is all lines.
[141] Its name derives from the initials of its authors, Aho, Weinberg, and Kernighan.
[142] Out of range exit values can result in unexpected exit codes. An exit value greater than 255 returns an
exit code modulo 256. For example, exit 3809 gives an exit code of 225 (3809 % 256 = 225).
[143] An update of /usr/include/sysexits.h allocates previously unused exit codes from 64 - 78. It
may be anticipated that the range of unallotted exit codes will be further restricted in the future. The
author of this document will not do fixups on the scripting examples to conform to the changing
standard. This should not cause any problems, since there is no overlap or conflict in usage of exit
codes between compiled C/C++ binaries and shell scripts.
[144] This does not apply to csh, tcsh, and other shells not related to or descended from the classic Bourne
shell (sh).
[145] In older versions of UNIX, passwords were stored in /etc/passwd, and that explains the name of
the file.
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[146] Some early UNIX systems had a fast, small-capacity fixed disk (containing /, the root partition), and a
second drive which was larger, but slower (containing /usr and other partitions). The most frequently
used programs and utilities therefore resided on the small-but-fast drive, in /bin, and the others on the
slower drive, in /usr/bin.

[147]
[148]

[149]
[150]

[151]

This likewise accounts for the split between /sbin and /usr/sbin, /lib and /usr/lib, etc.
This works only from the command line, of course, and not within a script.
Normally the default parameter completion files reside in either the /etc/profile.d directory or
in /etc/bash_completion. These autoload on system startup. So, after writing a useful
completion script, you might wish to move it (as root, of course) to one of these directories.
It has been extensively documented that programmers are willing to put in long hours of effort in order
to save ten minutes of "unnecessary" labor. This is known as optimization.
Various readers have suggested modifications of the above batch file to prettify it and make it more
compact and efficient. In the opinion of the ABS Guide author, this is wasted effort. A Bash script can
access a DOS filesystem, or even an NTFS partition (with the help of ntfs-3g) to do batch or scripted
operations.
For all you clever types who failed intermediate algebra, a determinant is a numerical value associated
with a multidimensional matrix (array of numbers).
For the simple case of a 2 x 2 determinant:
|a
|b

b|
a|

The solution is a*a - b*b, where "a" and "b" represent numbers.

[152] The author intends that this book be released into the Public Domain after a period of 14 years from
initial publication, that is, in 2014. In the early years of the American republic this was the duration
statutorily granted to a copyrighted work.

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