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Advanced Bash−Scripting Guide
An in−depth exploration of the art of shell scripting
Mendel Cooper
<thegrendel@theriver.com>
1.9
21 June 2003
Revision History
Revision 0.1 14 June 2000 Revised by: mc
Initial release.
Revision 0.2 30 October 2000 Revised by: mc
Bugs fixed, plus much additional material and more example scripts.
Revision 0.3 12 February 2001 Revised by: mc
Another major update.
Revision 0.4 08 July 2001 Revised by: mc
More bugfixes, much more material, more scripts − a complete revision and expansion of the book.
Revision 0.5 03 September 2001 Revised by: mc
Major update. Bugfixes, material added, chapters and sections reorganized.
Revision 1.0 14 October 2001 Revised by: mc
Bugfixes, reorganization, material added. Stable release.
Revision 1.1 06 January 2002 Revised by: mc
Bugfixes, material and scripts added.
Revision 1.2 31 March 2002 Revised by: mc
Bugfixes, material and scripts added.
Revision 1.3 02 June 2002 Revised by: mc
'TANGERINE' release: A few bugfixes, much more material and scripts added.
Revision 1.4 16 June 2002 Revised by: mc
'MANGO' release: Quite a number of typos fixed, more material and scripts added.
Revision 1.5 13 July 2002 Revised by: mc
'PAPAYA' release: A few bugfixes, much more material and scripts added.
Revision 1.6 29 September 2002 Revised by: mc
'POMEGRANATE' release: some bugfixes, more material, one more script added.
Revision 1.7 05 January 2003 Revised by: mc
'COCONUT' release: a couple of bugfixes, more material, one more script.
Revision 1.8 10 May 2003 Revised by: mc
'BREADFRUIT' release: a number of bugfixes, more scripts and material.
Revision 1.9 21 June 2003 Revised by: mc
'PERSIMMON' release: bugfixes and more material.
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 snippets 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.
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. See the change log for a revision history.
Dedication
For Anita, the source of all the magic
Table of Contents
Chapter 1. Why Shell Programming?...............................................................................................................1
Chapter 2. Starting Off With a Sha−Bang.......................................................................................................3
2.1. Invoking the script............................................................................................................................5
2.2. Preliminary Exercises.......................................................................................................................5
Part 2. Basics.......................................................................................................................................................6
Chapter 3. Special Characters...........................................................................................................................7
Chapter 4. Introduction to Variables and Parameters..................................................................................23
4.1. Variable Substitution......................................................................................................................23
4.2. Variable Assignment.......................................................................................................................25
4.3. Bash Variables Are Untyped..........................................................................................................26
4.4. Special Variable Types...................................................................................................................28
Chapter 5. Quoting...........................................................................................................................................32
Chapter 6. Exit and Exit Status.......................................................................................................................38
Chapter 7. Tests................................................................................................................................................40
7.1. Test Constructs...............................................................................................................................40
7.2. File test operators............................................................................................................................46
7.3. Comparison operators (binary).......................................................................................................49
7.4. Nested if/then Condition Tests.......................................................................................................54
7.5. Testing Your Knowledge of Tests..................................................................................................54
Chapter 8. Operations and Related Topics....................................................................................................55
8.1. Operators.........................................................................................................................................55
8.2. Numerical Constants.......................................................................................................................61
Part 3. Beyond the Basics.................................................................................................................................63
Chapter 9. Variables Revisited........................................................................................................................64
9.1. Internal Variables............................................................................................................................64
9.2. Manipulating Strings.......................................................................................................................79
9.2.1. Manipulating strings using awk............................................................................................83
9.2.2. Further Discussion.................................................................................................................84
9.3. Parameter Substitution....................................................................................................................84
9.4. Typing variables: declare or typeset...............................................................................................92
9.5. Indirect References to Variables.....................................................................................................94
9.6. $RANDOM: generate random integer............................................................................................96
9.7. The Double Parentheses Construct...............................................................................................101
Chapter 10. Loops and Branches..................................................................................................................103
10.1. Loops..........................................................................................................................................103
10.2. Nested Loops..............................................................................................................................113
10.3. Loop Control...............................................................................................................................114
Advanced Bash−Scripting Guide
i
Table of Contents
Chapter 10. Loops and Branches
10.4. Testing and Branching................................................................................................................117
Chapter 11. Internal Commands and Builtins.............................................................................................124
11.1. Job Control Commands..............................................................................................................144
Chapter 12. External Filters, Programs and Commands...........................................................................148
12.1. Basic Commands........................................................................................................................148
12.2. Complex Commands...................................................................................................................151
12.3. Time / Date Commands..............................................................................................................158
12.4. Text Processing Commands........................................................................................................160
12.5. File and Archiving Commands...................................................................................................176
12.6. Communications Commands......................................................................................................191
12.7. Terminal Control Commands.....................................................................................................195
12.8. Math Commands.........................................................................................................................196
12.9. Miscellaneous Commands..........................................................................................................204
Chapter 13. System and Administrative Commands..................................................................................214
Chapter 14. Command Substitution.............................................................................................................236
Chapter 15. Arithmetic Expansion................................................................................................................241
Chapter 16. I/O Redirection...........................................................................................................................242
16.1. Using exec...................................................................................................................................244
16.2. Redirecting Code Blocks............................................................................................................247
16.3. Applications................................................................................................................................251
Chapter 17. Here Documents.........................................................................................................................253
Chapter 18. Recess Time................................................................................................................................260
Part 4. Advanced Topics.................................................................................................................................261
Chapter 19. Regular Expressions..................................................................................................................262
19.1. A Brief Introduction to Regular Expressions..............................................................................262
19.2. Globbing.....................................................................................................................................265
Chapter 20. Subshells.....................................................................................................................................267
Chapter 21. Restricted Shells.........................................................................................................................270
Chapter 22. Process Substitution...................................................................................................................272
Chapter 23. Functions....................................................................................................................................274
23.1. Complex Functions and Function Complexities.........................................................................276
23.2. Local Variables...........................................................................................................................283
23.2.1. Local variables make recursion possible...........................................................................284
Advanced Bash−Scripting Guide
ii
Table of Contents
Chapter 24. Aliases.........................................................................................................................................286
Chapter 25. List Constructs...........................................................................................................................289
Chapter 26. Arrays.........................................................................................................................................292
Chapter 27. Files.............................................................................................................................................311
Chapter 28. /dev and /proc.............................................................................................................................312
28.1. /dev..............................................................................................................................................312
28.2. /proc............................................................................................................................................312
Chapter 29. Of Zeros and Nulls.....................................................................................................................317
Chapter 30. Debugging...................................................................................................................................320
Chapter 31. Options........................................................................................................................................326
Chapter 32. Gotchas.......................................................................................................................................328
Chapter 33. Scripting With Style..................................................................................................................334
33.1. Unofficial Shell Scripting Stylesheet..........................................................................................334
Chapter 34. Miscellany...................................................................................................................................337
34.1. Interactive and non−interactive shells and scripts......................................................................337
34.2. Shell Wrappers............................................................................................................................338
34.3. Tests and Comparisons: Alternatives..........................................................................................341
34.4. Recursion....................................................................................................................................342
34.5. "Colorizing" Scripts....................................................................................................................343
34.6. Optimizations..............................................................................................................................347
34.7. Assorted Tips..............................................................................................................................348
34.8. Security Issues............................................................................................................................355
34.9. Portability Issues.........................................................................................................................355
34.10. Shell Scripting Under Windows...............................................................................................356
Chapter 35. Bash, version 2...........................................................................................................................357
Chapter 36. Endnotes.....................................................................................................................................362
36.1. Author's Note..............................................................................................................................362
36.2. About the Author........................................................................................................................362
36.3. Tools Used to Produce This Book..............................................................................................362
36.3.1. Hardware...........................................................................................................................362
36.3.2. Software and Printware.....................................................................................................362
36.4. Credits.........................................................................................................................................363
Bibliography....................................................................................................................................................365
Advanced Bash−Scripting Guide
iii
Table of Contents
Appendix A. Contributed Scripts..................................................................................................................370
Appendix B. A Sed and Awk Micro−Primer................................................................................................409
B.1. Sed................................................................................................................................................409
B.2. Awk..............................................................................................................................................412
Appendix C. Exit Codes With Special Meanings.........................................................................................414
Appendix D. A Detailed Introduction to I/O and I/O Redirection.............................................................415
Appendix E. Localization...............................................................................................................................417
Appendix F. History Commands...................................................................................................................419
Appendix G. A Sample .bashrc File..............................................................................................................420
Appendix H. Converting DOS Batch Files to Shell Scripts........................................................................431
Appendix I. Exercises.....................................................................................................................................435
I.1. Analyzing Scripts..........................................................................................................................435
I.2. Writing Scripts...............................................................................................................................436
Appendix J. Copyright...................................................................................................................................442
Advanced Bash−Scripting Guide
iv
Chapter 1. Why Shell Programming?
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.
Writing shell scripts is not hard to learn, 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"
to learn. Most short scripts work right the first time, and debugging even the longer ones is straightforward.
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 shell script, even if slowly, is often a useful first stage in project
development. This way, the structure of the application can be tested and played with, and the major pitfalls
found before proceeding to the final coding in C, C++, Java, or Perl.
Shell scripting hearkens back to the classical 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.
When not to use shell scripts
resource−intensive tasks, especially where speed is a factor (sorting, hashing, etc.)• 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 instead)• complex applications, where structured programming is a necessity (need type−checking of variables,
function prototypes, etc.)
•
mission−critical applications upon which you are betting the ranch, or 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 multi−dimensional arrays• need data structures, such as linked lists or trees• need to generate or manipulate graphics or GUIs• need direct access to system hardware• 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 high−level compiled language such as C, C++, or Java. Even then, prototyping the application as a
shell script might still be a useful development step.
Chapter 1. Why Shell Programming? 1
We will be using Bash, an acronym 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 all flavors of UNIX. Most of the
principles dealt with in this book apply equally well to scripting with other shells, such as the Korn Shell,
from which Bash derives some of its features, [2] 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 −− 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), [3]
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, pdf, or text rendered versions. Be
aware that some of the scripts below 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.
Advanced Bash−Scripting Guide
Chapter 1. Why Shell Programming? 2
Chapter 2. Starting Off With a Sha−Bang
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 the log files in /var/log
# cleanup
# Run as root, of course.
cd /var/log
cat /dev/null > messages
cat /dev/null > wtmp
echo "Logs cleaned up."
There is nothing unusual here, just a set of commands that could just as easily be invoked one by one from the
command line on the console or in an xterm. The advantages of placing the commands in a script go beyond
not having to retype them time and again. The script can easily be modified, customized, or generalized for a
particular application.
Example 2−2. cleanup: An enhanced and generalized version of above script.
#!/bin/bash
# cleanup, version 2
# Run as root, of course.
LOG_DIR=/var/log
ROOT_UID=0 # Only users with $UID 0 have root privileges.
LINES=50 # Default number of lines saved.
E_XCD=66 # Can't change directory?
E_NOTROOT=67 # Non−root exit error.
if [ "$UID" −ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
if [ −n "$1" ]
# Test if command line argument 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=65 # Non−numerical argument (bad arg format)
#
# case "$1" in
Chapter 2. Starting Off With a Sha−Bang 3
# "" ) lines=50;;
# *[!0−9]*) echo "Usage: `basename $0` file−to−cleanup"; exit $E_WRONGARGS;;
# * ) lines=$1;;
# esac
#
#* 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 $E_XCD;
# }
tail −$lines messages > mesg.temp # Saves 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 "Logs cleaned up."
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 variant of the first script keeps the last section
of the message log intact. You will constantly discover ways of refining previously written scripts for
increased effectiveness.
The sha−bang ( #!) 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 [4] "magic number", a special marker that
designates a file type, or in this case an executable shell script (see 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 (line 1 of the script),
ignoring comments. [5]
#!/bin/sh
#!/bin/bash
#!/usr/bin/perl
#!/usr/bin/tcl
#!/bin/sed −f
#!/usr/awk −f
Advanced Bash−Scripting Guide
Chapter 2. Starting Off With a Sha−Bang 4
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. [6] Using #!/bin/sh, the default Bourne Shell in most commercial
variants of UNIX, makes the script portable to non−Linux machines, though you may have to sacrifice a few
Bash−specific features. The script will, however, conform to the POSIX [7] 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.
#! 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. 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 can build a quite
extensive library of nifty routines. As an example, the following script prolog tests whether the script has
been invoked with the correct number of parameters.
if [ $# −ne Number_of_expected args ]
then
echo "Usage: `basename $0` whatever"
exit $WRONG_ARGS
fi
2.1. Invoking the script
Having written the script, you can invoke it by sh scriptname, [8] or alternatively bash scriptname.
(Not recommended is using sh <scriptname>, since this effectively disables reading from stdin within
the script.) Much more convenient is to make the script itself directly executable with a chmod.
Either:
chmod 555 scriptname (gives everyone read/execute permission) [9]
or
chmod +rx scriptname (gives everyone read/execute permission)
chmod u+rx scriptname (gives only the script owner read/execute permission)
Having made the script executable, you may now test it by ./scriptname. [10] If it begins with a
"sha−bang" line, invoking the script calls the correct command interpreter to run it.
As a final step, after testing and debugging, you would likely want to move it to /usr/local/bin (as root,
of course), to make the script available to yourself and all other users as a system−wide executable. The script
could then be invoked by simply typing scriptname [ENTER] from the command line.
2.2. Preliminary Exercises
System administrators often write scripts to automate common tasks. Give several instances where
such scripts would be useful.
1.
Write a script that upon invocation shows the time and date, lists all logged−in users, and gives the
system uptime. The script then saves this information to a logfile.
2.
Advanced Bash−Scripting Guide
Chapter 2. Starting Off With a Sha−Bang 5
Part 2. Basics
Table of Contents
3. Special Characters
4. Introduction to Variables and Parameters
4.1. Variable Substitution
4.2. Variable Assignment
4.3. Bash Variables Are Untyped
4.4. Special Variable Types
5. Quoting
6. Exit and Exit Status
7. Tests
7.1. Test Constructs
7.2. File test operators
7.3. Comparison operators (binary)
7.4. Nested if/then Condition Tests
7.5. Testing Your Knowledge of Tests
8. Operations and Related Topics
8.1. Operators
8.2. Numerical Constants
Part 2. Basics 6
Chapter 3. Special Characters
Special Characters Found In Scripts and Elsewhere
#Comments. Lines beginning with a # (with the exception of #!) are comments.
# This line is a comment.
Comments may also occur at the end of a command.
echo "A comment will follow." # Comment here.
Comments may also follow whitespace at the beginning of a line.
# A tab precedes this comment.
A command may not follow a comment on the same line. There is no method of
terminating the comment, in order for "live code" to begin on the same line. Use a
new line for the next command.
Of course, an escaped # in an echo statement does not begin a comment.
Likewise, a # appears in certain parameter substitution constructs and in
numerical constant expressions.
echo "The # here does not begin a comment."
echo 'The # here does not begin a comment.'
echo The \# here does not begin a comment.
echo The # here begins a comment.
echo ${PATH#*:} # Parameter substitution, not a comment.
echo $(( 2#101011 )) # Base conversion, not a comment.
# Thanks, S.C.
The standard quoting and escape characters (" ' \) escape the #.
Certain pattern matching operations also use the #.
;Command separator. [Semicolon] Permits putting two or more commands on the same line.
echo hello; echo there
Note that the ";" sometimes needs to be escaped.
;; Terminator in a case option. [Double semicolon]
case "$variable" in
abc) echo "$variable = abc" ;;
xyz) echo "$variable = xyz" ;;
esac
Chapter 3. Special Characters 7
.
"dot" command. [period] Equivalent to source (see Example 11−18). This is a bash builtin.
."dot", as a component of a filename. When working with filenames, a dot is the prefix of a
"hidden" file, a file that an ls will not normally show.
bash$ touch .hidden−file
bash$ ls −l
total 10
−rw−r−−r−− 1 bozo 4034 Jul 18 22:04 data1.addressbook
−rw−r−−r−− 1 bozo 4602 May 25 13:58 data1.addressbook.bak
−rw−r−−r−− 1 bozo 877 Dec 17 2000 employment.addressbook
bash$ ls −al
total 14
drwxrwxr−x 2 bozo bozo 1024 Aug 29 20:54 ./
drwx−−−−−− 52 bozo bozo 3072 Aug 29 20:51 ../
−rw−r−−r−− 1 bozo bozo 4034 Jul 18 22:04 data1.addressbook
−rw−r−−r−− 1 bozo bozo 4602 May 25 13:58 data1.addressbook.bak
−rw−r−−r−− 1 bozo bozo 877 Dec 17 2000 employment.addressbook
−rw−rw−r−− 1 bozo bozo 0 Aug 29 20:54 .hidden−file
When considering directory names, a single dot represents the current working directory, and two dots
denote the parent directory.
bash$ pwd
/home/bozo/projects
bash$ cd .
bash$ pwd
/home/bozo/projects
bash$ cd ..
bash$ pwd
/home/bozo/
The dot often appears as the destination (directory) of a file movement command.
bash$ cp /home/bozo/current_work/junk/* .
."dot" character match. When matching characters, as part of a regular expression, a "dot" matches a
single character.
"partial quoting. [double quote] "STRING" preserves (from interpretation) most of the special
characters within STRING. See also Chapter 5.
'full quoting. [single quote] 'STRING' preserves all special characters within STRING. This is a
stronger form of quoting than using ". See also Chapter 5.
,comma operator. The comma operator links together a series of arithmetic operations. All are
evaluated, but only the last one is returned.
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 8
let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2".
\escape. [backslash] \X "escapes" the character X. This has the effect of "quoting" X, equivalent to 'X'.
The \ may be used to quote " and ', so they are expressed literally.
See Chapter 5 for an in−depth explanation of escaped characters.
/Filename path separator. [forward slash] Separates the components of a filename (as in
/home/bozo/projects/Makefile).
This is also the division arithmetic operator.
`command substitution. [backticks] `command` makes available the output of command for setting a
variable. This is also known as backticks or backquotes.
:
null command. [colon] This is the shell equivalent of a "NOP" (no op, a do−nothing operation). It
may be considered a synonym for the shell builtin true. The ":" command is a itself a Bash builtin,
and its exit status is "true" (0).
:
echo $? # 0
Endless loop:
while :
do
operation−1
operation−2
...
operation−n
done
# Same as:
# while true
# do
# ...
# done
Placeholder in if/then test:
if condition
then : # Do nothing and branch ahead
else
take−some−action
fi
Provide a placeholder where a binary operation is expected, see Example 8−2 and default parameters.
: ${username=`whoami`}
# ${username=`whoami`} without the leading : gives an error
# unless "username" is a command or builtin...
Provide a placeholder where a command is expected in a here document. See Example 17−9.
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 9
Evaluate string of variables using parameter substitution (as in Example 9−13).
: ${HOSTNAME?} ${USER?} ${MAIL?}
#Prints error message if one or more of essential environmental variables not set.
Variable expansion / substring replacement.
In combination with the > 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 12−11.
In combination with the >> redirection operator, updates a file access/modification time (: >>
new_file). If the file did not previously exist, creates it. This is equivalent to touch.
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 be appear in a
comment. However, this is not the case with :.
: This is a comment that generates an error, ( if [ $x −eq 3] ).
The ":" also serves as a field separator, in /etc/passwd, and in the $PATH variable.
bash$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games
!
reverse (or negate) the sense of a test or exit status. The ! operator inverts the exit status of the
command to which it is applied (see Example 6−2). It also inverts the meaning of a test operator. This
can, for example, change the sense of "equal" ( = ) to "not−equal" ( != ). The ! operator is a Bash
keyword.
In a different context, the ! also appears in indirect variable references.
In yet another context, from the command line, the ! invokes the Bash history mechanism (see
Appendix F). Note that within a script, the history mechanism is disabled.
*wild card. [asterisk] The * character serves as a "wild card" for filename expansion in globbing. By
itself, it matches every filename in a given directory.
bash$ echo *
abs−book.sgml add−drive.sh agram.sh alias.sh
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 10
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, **, is the exponentiation operator.
?test operator. Within certain expressions, the ? indicates a test for a condition.
In a double parentheses construct, the ? serves as a C−style trinary operator. See Example 9−28.
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.
var1=5
var2=23skidoo
echo $var1 # 5
echo $var2 # 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.
$*, $@ 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 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.
a=123
( a=321; )
echo "a = $a" # a = 123
# "a" within parentheses acts like a local variable.
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 11
array initialization.
Array=(element1 element2 element3)
{xxx,yyy,zzz,...}
Brace expansion.
grep Linux file*.{txt,htm*}
# Finds all instances of the word "Linux"
# in the files "fileA.txt", "file2.txt", "fileR.html", "file−87.htm", etc.
A command may act upon a comma−separated list of file specs within braces. [11] Filename
expansion (globbing) applies to the file specs between the braces.
No spaces allowed within the braces unless the spaces are quoted or escaped.
echo {file1,file2}\ :{\ A," B",' C'}
file1 : A file1 : B file1 : C file2 : A file2 : B file2 :
C
{}
Block of code. [curly brackets] Also referred to as an "inline group", this construct, in effect, creates
an anonymous function. However, unlike a function, the variables in a code block remain visible to
the remainder of the script.
bash$ { local a; a=123; }
bash: local: can only be used in a function
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 "First line in $File is:"
echo "$line1"
echo
echo "Second line in $File is:"
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 12
echo "$line2"
exit 0
Example 3−2. Saving the results 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
{
echo
echo "Archive Description:"
rpm −qpi $1 # Query description.
echo
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
} > "$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. [12]
{} \; 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.
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 13
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 [[ ]] (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 ${Array[1]}
[ ] range of characters.
As part of a regular expression, brackets delineate a range of characters to match.
(( )) integer expansion.
Expand and evaluate integer expression between (( )).
See the discussion on the (( ... )) construct.
> &> >& >> <
redirection.
scriptname >filename redirects the output of scriptname to file filename. Overwrite
filename if it already exists.
command &>filename redirects both the stdout and the stderr of command to filename.
command >&2 redirects stdout of command to stderr.
scriptname >>filename appends the output of scriptname to file filename. If
filename does not already exist, it will be created.
process substitution.
(command)>
<(command)
In a different context, the "<" and ">" characters act as string comparison operators.
In yet another context, the "<" and ">" characters act as integer comparison operators. See also
Example 12−6.
<< redirection used in a here document.
<, >
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 14
ASCII comparison.
veg1=carrots
veg2=tomatoes
if [[ "$veg1" < "$veg2" ]]
then
echo "Although $veg1 precede $veg2 in the dictionary,"
echo "this implies nothing about my culinary preferences."
else
echo "What kind of dictionary are you using, anyhow?"
fi
\<, \> word boundary in a regular expression.
bash$ grep '\<the\>' textfile
|
pipe. Passes the output of previous command to the input 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.
cat $filename | grep $search_word
The output of a command or commands may be piped to a script.
#!/bin/bash
# uppercase.sh : Changes input to uppercase.
tr 'a−z' 'A−Z'
# Letter ranges must be quoted
#+ to prevent filename generation from single−letter filenames.
exit 0
Now, let us pipe the output of ls −l to this script.
bash$ ls −l | ./uppercase.sh
−RW−RW−R−− 1 BOZO BOZO 109 APR 7 19:49 1.TXT
−RW−RW−R−− 1 BOZO BOZO 109 APR 14 16:48 2.TXT
−RW−R−−R−− 1 BOZO BOZO 725 APR 20 20:56 DATA−FILE
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 15
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 # First loop.
do
echo −n "$i "
done & # Run this loop in background.
# Will sometimes execute after second loop.
echo # This 'echo' sometimes will not display.
for i in 11 12 13 14 15 16 17 18 19 20 # Second loop.
do
echo −n "$i "
done
echo # 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
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 16
# 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
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.
COMMAND −[Option1][Option2][...]
ls −al
sort −dfu $filename
set −− $variable
if [ $file1 −ot $file2 ]
then
echo "File $file1 is older than $file2."
fi
if [ "$a" −eq "$b" ]
then
echo "$a is equal to $b."
fi
if [ "$c" −eq 24 −a "$d" −eq 47 ]
then
echo "$c equals 24 and $d equals 47."
fi
−redirection from/to stdin or stdout. [dash]
(cd /source/directory && tar cf − . ) | (cd /dest/directory && tar xpvf −)
# Move entire file tree from one directory to another
# [courtesy Alan Cox <a.cox@swansea.ac.uk>, with a minor change]
# 1) cd /source/directory Source directory, where the files to be moved are.
# 2) && "And−list": if the 'cd' operation successful, then execute the next command.
# 3) tar cf − . The 'c' option 'tar' archiving command creates a new archive,
# the 'f' (file) option, followed by '−' designates the target file as stdout,
# and do it in current directory tree ('.').
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 17
# 4) | Piped to...
# 5) ( ... ) a subshell
# 6) cd /dest/directory Change to the destination directory.
# 7) && "And−list", as above
# 8) tar xpvf − Unarchive ('x'), preserve ownership and file permissions ('p'),
# and send verbose messages to stdout ('v'),
# reading data from stdin ('f' followed by '−').
#
# Note that 'x' is a command, and 'p', 'v', 'f' are options.
# Whew!
# More elegant than, but equivalent to:
# cd source−directory
# tar cf − . | (cd ../target−directory; tar xzf −)
#
# cp −a /source/directory /dest also has same effect.
bunzip2 linux−2.4.3.tar.bz2 | tar xvf −
# −−uncompress tar file−− | −−then pass it to "tar"−−
# If "tar" has not been patched to handle "bunzip2",
# this needs to be done in two discrete steps, using a pipe.
# The purpose of the exercise is to unarchive "bzipped" kernel source.
Note that in this context the "−" is not itself a Bash operator, but rather an option recognized by
certain UNIX utilities that write to stdout, such as tar, cat, etc.
bash$ 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: ASCII text
bash$ file −
#!/bin/bash
standard input: Bourne−Again shell script text executable
Now the command accepts input from stdin and analyzes it.
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 18
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
archive=${1:−$BACKUPFILE}
# If no backup−archive filename specified on command line,
#+ it will default to "backup.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
$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. [dash] cd − changes to the previous working directory. This uses the
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 19
$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.
%modulo. Modulo (remainder of a division) arithmetic operation.
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:
bash$ echo ~nonexistent−user
~nonexistent−user
~+ current working directory. This corresponds to the $PWD internal variable.
~− previous working directory. This corresponds to the $OLDPWD internal variable.
^beginning−of−line. In a regular expression, a "^" addresses the beginning of a line of text.
Control Characters
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 20
change the behavior of the terminal or text display. A control character is a CONTROL + key
combination.
Ctl−C
Terminate a foreground job.
◊
Ctl−D
Log out from a shell (similar to exit).
"EOF" (end of file). This also terminates input from stdin.
◊
Ctl−G
"BEL" (beep).
◊
Ctl−H
Backspace.
#!/bin/bash
# Embedding Ctl−H in a string.
a="^H^H" # Two Ctl−H's (backspaces).
echo "abcdef" # abcdef
echo −n "abcdef$a " # abcd f
# Space at end ^ ^ Backspaces twice.
echo −n "abcdef$a" # abcdef
# No space at end Doesn't backspace (why?).
# Results may not be quite as expected.
echo; echo
◊
Ctl−J
Carriage return.
◊
Ctl−L
Formfeed (clear the terminal screen). This has the same effect as the clear command.
◊
Ctl−M
Newline.
◊
Ctl−U
Erase a line of input.
◊
Ctl−Z
Pause a foreground job.
◊
Whitespace
functions as a separator, separating commands or variables. Whitespace consists of either spaces,
tabs, blank lines, or any combination thereof. 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
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 21
functional sections.
$IFS, the special variable separating fields of input to certain commands, defaults to whitespace.
Advanced Bash−Scripting Guide
Chapter 3. Special Characters 22
Chapter 4. Introduction to Variables and
Parameters
Variables are at the heart of every programming and scripting language. They appear in arithmetic operations
and manipulation of quantities, string parsing, and are indispensable for working in the abstract with symbols
− tokens that represent something else. A variable is nothing more than a location or set of locations in
computer memory holding an item of data.
4.1. Variable Substitution
The name of a variable is a placeholder for its value, the data it holds. Referencing 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. The only time a
variable appears "naked", without the $ prefix, is when declared or assigned, when unset, when
exported, or in the special case of a variable representing a signal (see Example 30−5). 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 alternate form of ${variable}. In contexts where
the $variable syntax causes an error, the longer form may work (see Section 9.3, below).
Example 4−1. Variable assignment and substitution
#!/bin/bash
# Variables: assignment and substitution
a=375
hello=$a
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# No space permitted on either side of = sign when initializing variables.
# If "VARIABLE =value",
#+ script tries to run "VARIABLE" command with one argument, "=value".
# If "VARIABLE= value",
#+ script tries to run "value" command with
#+ the environmental variable "VARIABLE" set to "".
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
echo hello # Not a variable reference, just the string "hello".
Chapter 4. Introduction to Variables and Parameters 23
echo $hello
echo ${hello} # Identical to above.
echo "$hello"
echo "${hello}"
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" give different results.
# Quoting a variable preserves whitespace.
echo
echo '$hello' # $hello
# Variable referencing disabled by single quotes,
#+ which causes the "$" to be interpreted literally.
# Notice the effect of different types of quoting.
hello= # Setting it to a null value.
echo "\$hello (null value) = $hello"
# Note that setting a variable to a null value is not the same as
#+ unsetting it, although the end result is the same (see below).
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# It is permissible to set multiple variables on the same line,
#+ if separated by white space.
# Caution, this may reduce legibility, and may not be portable.
var1=variable1 var2=variable2 var3=variable3
echo
echo "var1=$var1 var2=$var2 var3=$var3"
# May cause problems with older versions of "sh".
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
echo; echo
numbers="one two three"
other_numbers="1 2 3"
# If whitespace within a variable, then quotes necessary.
echo "numbers = $numbers"
echo "other_numbers = $other_numbers" # other_numbers = 1 2 3
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 = $uninitialized_variable"
# It still has a null value.
Advanced Bash−Scripting Guide
Chapter 4. Introduction to Variables and Parameters 24
echo
exit 0
An uninitialized variable has a "null" value − no assigned value at all (not zero!).
Using a variable before assigning a value to it will usually cause problems.
It is nevertheless possible to perform arithmetic operations on an uninitialized
variable.
echo "$uninitialized" # (blank line)
let "uninitialized += 5" # Add 5 to it.
echo "$uninitialized" # 5
# Conclusion:
# An uninitialized variable has no value, however
#+ it acts as if it were 0 in an arithmetic operation.
# This is undocumented (and probably non−portable) behavior.
See also Example 11−19.
4.2. Variable Assignment
=
the assignment operator (no space before & after)
Do not confuse this with = and −eq, which test, rather than assign!
Note that = can be either an assignment or a test operator, depending on context.
Example 4−2. Plain Variable Assignment
#!/bin/bash
# Naked variables
echo
# When is a variable "naked", i.e., lacking the '$' 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 $a."
echo
# In a 'for' loop (really, a type of disguised assignment)
echo −n "Values of \"a\" in the loop are: "
for a in 7 8 9 11
Advanced Bash−Scripting Guide
Chapter 4. Introduction to Variables and Parameters 25
do
echo −n "$a "
done
echo
echo
# In a 'read' statement (also a type of assignment)
echo −n "Enter \"a\" "
read a
echo "The value of \"a\" is now $a."
echo
exit 0
Example 4−3. Variable Assignment, plain and fancy
#!/bin/bash
a=23 # Simple case
echo $a
b=$a
echo $b
# Now, getting a little bit fancier (command substitution).
a=`echo Hello!` # Assigns result of 'echo' command to 'a'
echo $a
# Note that using an exclamation mark (!) in command substitution
#+ will not work from the command line,
#+ since this triggers the Bash "history mechanism."
# Within a script, however, the history functions are disabled.
a=`ls −l` # Assigns result of 'ls −l' command to 'a'
echo $a # Unquoted, however, removes tabs and newlines.
echo
echo "$a" # The quoted variable preserves whitespace.
# (See the chapter on "Quoting.")
exit 0
Variable assignment using the $(...) mechanism (a newer method than backquotes)
# From /etc/rc.d/rc.local
R=$(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 integer operations and comparisons on
variables. The determining factor is whether the value of a variable contains only digits.
Advanced Bash−Scripting Guide
Chapter 4. Introduction to Variables and Parameters 26
Example 4−4. Integer or string?
#!/bin/bash
# int−or−string.sh: Integer or string?
a=2334 # Integer.
let "a += 1"
echo "a = $a " # a = 2335
echo # Integer, still.
b=${a/23/BB} # Substitute "BB" for "23".
# This transforms $b into a string.
echo "b = $b" # b = BB35
declare −i b # Declaring it an integer doesn't help.
echo "b = $b" # b = BB35
let "b += 1" # BB35 + 1 =
echo "b = $b" # b = 1
echo
c=BB34
echo "c = $c" # c = BB34
d=${c/BB/23} # Substitute "23" for "BB".
# This makes $d an integer.
echo "d = $d" # d = 2334
let "d += 1" # 2334 + 1 =
echo "d = $d" # d = 2335
echo
# What about null variables?
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 = $f" # f = 1
echo # Undeclared variable transformed into an integer.
# Variables in Bash are essentially untyped.
exit 0
Untyped variables are both a blessing and a curse. They permit more flexibility in scripting (enough rope to
hang yourself!) and make it easier to grind out lines of code. However, they permit errors to creep in and
encourage sloppy programming habits.
The burden is on the programmer to keep track of what type the script variables are. Bash will not do it for
you.
Advanced Bash−Scripting Guide
Chapter 4. Introduction to Variables and Parameters 27
4.4. Special Variable Types
local variables
variables visible only within a code block or function (see also local variables in functions)
environmental variables
variables that affect the behavior of the shell and user interface
In a more general context, each process has an "environment", that is, a group of
variables that hold information that the process may reference. In this sense, the shell
behaves like any other process.
Every time a shell starts, it creates shell variables that correspond to its own
environmental variables. Updating or adding new environmental variables causes the
shell to update its environment, and all the shell's child processes (the commands it
executes) inherit this environment.
The space allotted to the environment is limited. Creating too many environmental
variables or ones that use up excessive space may cause problems.
bash$ eval "`seq 10000 | sed −e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"
bash$ du
bash: /usr/bin/du: Argument list too long
(Thank you, S. C. 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.
−−−
positional parameters
arguments passed to the script from the command line − $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. [13] 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
Advanced Bash−Scripting Guide
Chapter 4. Introduction to Variables and Parameters 28
MINPARAMS=10
echo
echo "The name of this script is \"$0\"."
# Adds ./ for current directory
echo "The name of this script is \"`basename $0`\"."
# Strips out path name info (see 'basename')
echo
if [ −n "$1" ] # Tested variable is quoted.
then
echo "Parameter #1 is $1" # Need quotes to escape #
fi
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 "Give me at least $MINPARAMS command−line arguments!"
fi
echo
exit 0
The bracket notation for positional parameters leads to a fairly simply way of referencing the last
argument passed to a script on the command line. This also requires indirect referencing.
args=$# # Number of args passed.
lastarg=${!args} # Note that lastarg=${!$#} 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. There must also exist symbolic
links to all the alternate names of the script.
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
Advanced Bash−Scripting Guide
Chapter 4. Introduction to Variables and Parameters 29
this is to append an extra character to both sides of the assignment statement using the
expected positional parameter.
variable1_=$1_
# This will prevent an error, even if positional parameter is absent.
critical_argument01=$variable1_
# The extra character can be stripped off later, if desired, like so.
variable1=${variable1_/_/} # Side effects only if $variable1_ begins with "_".
# This uses one of the parameter substitution templates discussed in Chapter 9.
# 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 $POS_PARAMS_MISSING
fi
−−−
Example 4−6. wh, whois domain name lookup
#!/bin/bash
# Does a 'whois domain−name' lookup on any of 3 alternate servers:
# ripe.net, cw.net, radb.net
# Place this script, named '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−cw
# ln −s /usr/local/bin/wh /usr/local/bin/wh−radb
if [ −z "$1" ]
then
echo "Usage: `basename $0` [domain−name]"
exit 65
fi
case `basename $0` in
# Checks script name and calls proper server
"wh" ) whois $1@whois.ripe.net;;
"wh−ripe") whois $1@whois.ripe.net;;
"wh−radb") whois $1@whois.radb.net;;
"wh−cw" ) whois $1@whois.cw.net;;
* ) echo "Usage: `basename $0` [domain−name]";;
esac
exit 0
−−−
Advanced Bash−Scripting Guide
Chapter 4. Introduction to Variables and Parameters 30
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
# Using 'shift' to step through all the positional parameters.
# Name this script something like shft,
#+ and invoke it with some parameters, for example
# ./shft a b c def 23 skidoo
until [ −z "$1" ] # Until all parameters used up...
do
echo −n "$1 "
shift
done
echo # Extra line feed.
exit 0
The shift command also works on parameters passed to a function. See Example
34−10.
Advanced Bash−Scripting Guide
Chapter 4. Introduction to Variables and Parameters 31
Chapter 5. Quoting
Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in
the string from reinterpretation or expansion by the shell or shell script. (A character is "special" if it has an
interpretation other than its literal meaning, such as the wild card character, *.)
bash$ ls −l [Vv]*
−rw−rw−r−− 1 bozo bozo 324 Apr 2 15:05 VIEWDATA.BAT
−rw−rw−r−− 1 bozo bozo 507 May 4 14:25 vartrace.sh
−rw−rw−r−− 1 bozo bozo 539 Apr 14 17:11 viewdata.sh
bash$ ls −l '[Vv]*'
ls: [Vv]*: No such file or directory
Certain programs and utilities can still reinterpret or expand special characters in a quoted string. This is
an important use of quoting, 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, but not under tcsh.
When referencing a variable, it is generally advisable in enclose it in double quotes (" "). This preserves all
special characters within the variable name, except $, ` (backquote), and \ (escape). [14] 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. [15] An argument enclosed in double quotes presents itself as a
single word, even if it contains whitespace separators.
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, S.C.
Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting is
an issue.
Chapter 5. Quoting 32
Example 5−1. Echoing Weird Variables
#!/bin/bash
# weirdvars.sh: Echoing weird variables.
var="'(]\\{}\$\""
echo $var # '(]\{}$"
echo "$var" # '(]\{}$" Doesn't make a difference.
echo
IFS='\'
echo $var # '(] {}$" \ converted to space.
echo "$var" # '(]\{}$"
# Examples above supplied by S.C.
exit 0
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 Stephane Chazelas.
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
Advanced Bash−Scripting Guide
Chapter 5. Quoting 33
\b
means backspace
\a
means "alert" (beep or flash)
\0xx
translates to the octal ASCII equivalent of 0xx
Example 5−2. Escaped Characters
#!/bin/bash
# escaped.sh: escaped characters
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 BEEP"
echo $'\n' # Newline.
echo $'\a' # Alert (beep).
echo "==============="
echo "QUOTATION MARKS"
# Version 2 and later of Bash permits using the $'\nnn' construct.
# Note that in this case, '\nnn' is an octal value.
echo $'\t \042 \t' # Quote (") framed by tabs.
# It also works with hexadecimal values, in an $'\xhhh' construct.
echo $'\t \x22 \t' # Quote (") framed by tabs.
# Thank you, Greg Keraunen, for pointing this out.
# Earlier Bash versions allowed '\x022'.
echo "==============="
echo
# Assigning ASCII characters to a variable.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
quote=$'\042' # " assigned to a variable.
echo "$quote This is a 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' # 101, 102, 103 are octal A, B, C.
echo $ABC
Advanced Bash−Scripting Guide
Chapter 5. Quoting 34
echo; echo
escape=$'\033' # 033 is octal for escape.
echo "\"escape\" echoes as $escape"
# no visible output.
echo; echo
exit 0
See Example 35−1 for another example of the $' ' string expansion construct.
\"
gives the quote its literal meaning
echo "Hello" # Hello
echo "\"Hello\", he said." # "Hello", he said.
\$
gives the dollar sign its literal meaning (variable name following \$ will not be referenced)
echo "\$variable01" # results in $variable01
\\
gives the backslash its literal meaning
echo "\\" # results in \
The behavior of \ depends on whether it is itself escaped, quoted, or appearing within command
substitution or a here document.
# Simple escaping and quoting
echo \z # z
echo \\z # \z
echo '\z' # \z
echo '\\z' # \\z
echo "\z" # \z
echo "\\z" # \z
# Command substitution
echo `echo \z` # z
echo `echo \\z` # z
echo `echo \\\z` # \z
echo `echo \\\\z` # \z
echo `echo \\\\\\z` # \z
echo `echo \\\\\\\z` # \\z
echo `echo "\z"` # \z
echo `echo "\\z"` # \z
# Here document
cat <<EOF
\z
EOF # \z
cat <<EOF
\\z
EOF # \z
# These examples supplied by Stephane Chazelas.
Advanced Bash−Scripting Guide
Chapter 5. Quoting 35
Elements of a string assigned to a variable may be escaped, but the escape character alone may
not be assigned to a variable.
variable=\
echo "$variable"
# Will not work − gives an error message:
# test.sh: : command not found
# A "naked" escape cannot safely be assigned to a variable.
#
# What actually happens here is that the "\" escapes the newline and
#+ the effect is variable=echo "$variable"
#+ invalid variable assignment
variable=\
23skidoo
echo "$variable" # 23skidoo
# This works, since the second line
#+ is a valid variable assignment.
variable=\
# \^ escape followed by space
echo "$variable" # space
variable=\\
echo "$variable" # \
variable=\\\
echo "$variable"
# Will not work − gives an error message:
# test.sh: \: command not found
#
# First escape escapes second one, but the third one is left "naked",
#+ with same result as first instance, above.
variable=\\\\
echo "$variable" # \\
# Second and fourth escapes escaped.
# This is o.k.
Escaping a space can prevent word splitting in a command's argument list.
file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs−20.7"
# List of files as argument(s) to a command.
# Add two files to the list, and list all.
ls −l /usr/X11R6/bin/xsetroot /sbin/dump $file_list
echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−"
# What happens if we escape a couple of spaces?
ls −l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list
# Error: the first three files concatenated into a single argument to 'ls −l'
# because the two escaped spaces prevent argument (word) splitting.
The escape also provides a means of writing a multi−line command. Normally, each separate line constitutes a
different command, but an escape at the end of a line escapes the newline character, and the command
sequence continues on to the next line.
Advanced Bash−Scripting Guide
Chapter 5. Quoting 36
(cd /source/directory && tar cf − . ) | \
(cd /dest/directory && tar xpvf −)
# Repeating Alan Cox's directory tree copy command,
# but split into two lines for increased legibility.
# As an alternative:
tar cf − −C /source/directory . |
tar xpvf − −C /dest/directory
# See note below.
# (Thanks, Stephane Chazelas.)
If a script line ends with a |, a pipe character, then a \, an escape, is not strictly necessary. It is, however,
good programming practice to always escape the end of a line of code that continues to the following
line.
echo "foo
bar"
#foo
#bar
echo
echo 'foo
bar' # No difference yet.
#foo
#bar
echo
echo foo\
bar # Newline escaped.
#foobar
echo
echo "foo\
bar" # Same here, as \ still interpreted as escape within weak quotes.
#foobar
echo
echo 'foo\
bar' # Escape character \ taken literally because of strong quoting.
#foor\
#bar
# Examples suggested by Stephane Chazelas.
Advanced Bash−Scripting Guide
Chapter 5. Quoting 37
Chapter 6. Exit and Exit Status
...there are dark corners in the Bourne shell, and
people use all of them.
Chet Ramey
The exit command may be used to terminate a script, just as in a C program. It can also return a value, which
is available to the script's parent process.
Every command returns an exit status (sometimes referred to as a return status ). A successful command
returns a 0, while an unsuccessful one returns a non−zero value that usually may be interpreted as an error
code. Well−behaved UNIX commands, programs, and utilities return a 0 exit code upon successful
completion, though there are some exceptions.
Likewise, functions within a script and the script itself return an exit status. The last command executed in the
function or script determines the exit status. Within a script, an exit nnn command may be used to deliver
an nnn exit status to the shell (nnn must be a decimal number in the 0 − 255 range).
When a script ends with an exit that has no parameter, the exit status of the script is the exit status of the
last command executed in the script (not counting the exit).
$? reads the exit status of the last command executed. After a function returns, $? gives the exit status of the
last command executed in the function. This is Bash's way of giving functions a "return value". After a script
terminates, a $? from the command line gives the exit status of the script, that is, the last command executed
in the script, which is, by convention, 0 on success or an integer in the range 1 − 255 on error.
Example 6−1. exit / exit status
#!/bin/bash
echo hello
echo $? # Exit status 0 returned because command executed successfully.
lskdf # Unrecognized command.
echo $? # Non−zero exit status returned because command failed to execute.
echo
exit 113 # Will return 113 to shell.
# To verify this, type "echo $?" after script terminates.
# By convention, an 'exit 0' indicates success,
#+ while a non−zero exit value means an error or anomalous condition.
$? is especially useful for testing the result of a command in a script (see Example 12−27 and Example
12−13).
The !, the logical "not" qualifier, reverses the outcome of a test or command, and this affects its exit
status.
Example 6−2. Negating a condition using !
Chapter 6. Exit and Exit Status 38
true # the "true" builtin.
echo "exit status of \"true\" = $?" # 0
! true
echo "exit status of \"! true\" = $?" # 1
# Note that the "!" needs a space.
# !true leads to a "command not found" error
# Thanks, S.C.
Certain exit status codes have reserved meanings and should not be user−specified in a script.
Advanced Bash−Scripting Guide
Chapter 6. Exit and Exit Status 39
Chapter 7. Tests
Every reasonably complete programming language can test for a condition, then act according to the result of
the test. Bash has the test command, various bracket and parenthesis operators, and the if/then construct.
7.1. Test Constructs
An if/then construct tests whether the exit status of a list of commands is 0 (since 0 means "success"
by UNIX convention), and if so, executes one or more commands.
•
There exists a dedicated command called [ (left bracket special character). It is a synonym for test,
and a builtin for efficiency reasons. This command considers its arguments as comparison expressions
or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for
false).
•
With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons
in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a
command.
Bash sees [[ $a −lt $b ]] as a single element, which returns an exit status.
The (( ... )) and let ... constructs also return an exit status of 0 if the arithmetic expressions they
evaluate expand to a non−zero value. These arithmetic expansion constructs may therefore be used to
perform arithmetic comparisons.
let "1<2" returns 0 (as "1<2" expands to "1")
(( 0 && 1 )) returns 1 (as "0 && 1" expands to "0")
•
An if can test any command, not just conditions enclosed within brackets.
if cmp a b &> /dev/null # Suppress output.
then echo "Files a and b are identical."
else echo "Files a and b differ."
fi
if grep −q Bash file
then echo "File contains at least one occurrence of Bash."
fi
if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED
then echo "Command succeeded."
else echo "Command failed."
fi
•
An if/then construct can contain nested comparisons and tests.
if echo "Next *if* is part of the comparison for the first *if*."
if [[ $comparison = "integer" ]]
then (( a < b ))
else
[[ $a < $b ]]
fi
then
echo '$a is less than $b'
fi
•
Chapter 7. Tests 40
This detailed "if−test" explanation courtesy of Stephane Chazelas.
Example 7−1. What is truth?
#!/bin/bash
echo
echo "Testing \"0\""
if [ 0 ] # zero
then
echo "0 is true."
else
echo "0 is false."
fi # 0 is true.
echo
echo "Testing \"1\""
if [ 1 ] # one
then
echo "1 is true."
else
echo "1 is false."
fi # 1 is true.
echo
echo "Testing \"−1\""
if [ −1 ] # minus one
then
echo "−1 is true."
else
echo "−1 is false."
fi # −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 $xyz is null, but...
Advanced Bash−Scripting Guide
Chapter 7. Tests 41
# it's only an uninitialized variable.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
echo "Testing \"−n \$xyz\""
if [ −n "$xyz" ] # More pedantically correct.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
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 # "$false" is false.
# Now, we get the expected result.
echo
exit 0
Exercise. Explain the behavior of Example 7−1, above.
Advanced Bash−Scripting Guide
Chapter 7. Tests 42
if [ condition−true ]
then
command 1
command 2
...
else
# Optional (may be left out if not needed).
# Adds default code block executing if original condition tests false.
command 3
command 4
...
fi
When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both
if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the
same line begins, the old one must terminate.
if [ −x "$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 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
Advanced Bash−Scripting Guide
Chapter 7. Tests 43
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."
fi
echo
if /usr/bin/test −z "$1" # Same result as "test" builtin".
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.
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
Advanced Bash−Scripting Guide
Chapter 7. Tests 44
if [[ −e $file ]]
then
echo "Password file exists."
fi
Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example,
the &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct.
Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly
necessary.
dir=/home/bozo
if cd "$dir" 2>/dev/null; then # "2>/dev/null" hides error message.
echo "Now in $dir."
else
echo "Can't change to $dir."
fi
The "if COMMAND" construct returns the exit status of COMMAND.
Similarly, a condition within test brackets may stand alone without an if, when used in
combination with a list construct.
var1=20
var2=22
[ "$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.
Example 7−3. Arithmetic Tests using (( ))
#!/bin/bash
# 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 )) # true
echo "Exit status of \"(( 5 > 4 ))\" is $?." # 0
(( 5 > 9 )) # false
echo "Exit status of \"(( 5 > 9 ))\" is $?." # 1
Advanced Bash−Scripting Guide
Chapter 7. Tests 45
(( 5 − 5 )) # 0
echo "Exit status of \"(( 5 − 5 ))\" is $?." # 1
(( 5 / 4 )) # Division o.k.
echo "Exit status of \"(( 5 / 4 ))\" is $?." # 0
(( 1 / 2 )) # Division result < 1.
echo "Exit status of \"(( 1 / 2 ))\" is $?." # Rounded off to 0.
# 1
(( 1 / 0 )) 2>/dev/null # Illegal division by 0.
echo "Exit status of \"(( 1 / 0 ))\" is $?." # 1
# What effect does the "2>/dev/null" have?
# What would happen if it were removed?
# Try removing it, then rerunning the script.
exit 0
7.2. File test operators
Returns true if...
−e
file exists
−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 (floppy, cdrom, etc.)
−c
file is a character device (keyboard, modem, sound card, etc.)
−p
file is a pipe
−h
file is a symbolic link
−L
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
Advanced Bash−Scripting Guide
Chapter 7. Tests 46
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. [16] 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. [17] 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,
he can only delete files in it that he owns. This keeps users from inadvertently overwriting or deleting
each other's files in a publicly accessible directory, such as /tmp.
−O
you are owner of file
−G
group−id of file same as yours
−N
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
Advanced Bash−Scripting Guide
Chapter 7. Tests 47
#!/bin/bash
# broken−link.sh
# Written by Lee bigelow <ligelowbee@yahoo.com>
# Used 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. 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 directorys to search
#to current directory. Otherwise set the directorys to search
#to the agrs passed.
####################
[ $# −eq 0 ] && directorys=`pwd` || directorys=$@
#Setup the function linkchk to check the directory it is passed
#for files that are links and don't exist, then print them quoted.
#If one of the elements in the directory is a subdirectory then
#send that send that 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.
################
for directory in $directorys; do
if [ −d $directory ]
then linkchk $directory
else
echo "$directory is not a directory"
echo "Usage: $0 dir1 dir2 ..."
fi
done
exit 0
Example 29−1, Example 10−7, Example 10−3, Example 29−3, and Example A−2 also illustrate uses of the
file test operators.
Advanced Bash−Scripting Guide
Chapter 7. Tests 48
7.3. Comparison operators (binary)
integer comparison
−eq
is equal to
if [ "$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" < "$b"))
<=
is less than or equal to (within double parentheses)
(("$a" <= "$b"))
>
is greater than (within double parentheses)
(("$a" > "$b"))
>=
is greater than or equal to (within double parentheses)
(("$a" >= "$b"))
string comparison
=
Advanced Bash−Scripting Guide
Chapter 7. Tests 49
is equal to
if [ "$a" = "$b" ]
==
is equal to
if [ "$a" == "$b" ]
This is a synonym for =.
[[ $a == z* ]] # true if $a starts with an "z" (pattern matching)
[[ $a == "z*" ]] # true if $a is equal to z*
[ $a == z* ] # file globbing and word splitting take place
[ "$a" == "z*" ] # true if $a is equal to z*
# Thanks, S.C.
!=
is not equal to
if [ "$a" != "$b" ]
This operator uses pattern matching within a [[ ... ]] construct.
<
is less than, in ASCII alphabetical order
if [[ "$a" < "$b" ]]
if [ "$a" \< "$b" ]
Note that the "<" needs to be escaped within a [ ] construct.
>
is greater than, in ASCII alphabetical order
if [[ "$a" > "$b" ]]
if [ "$a" \> "$b" ]
Note that the ">" needs to be escaped within a [ ] construct.
See Example 26−6 for an application of this comparison operator.
−z
string is "null", that is, has zero length
−n
string is not "null".
The −n test absolutely requires that the string be quoted within the test brackets.
Using an unquoted string with ! −z, or even just the unquoted string alone within
test brackets (see Example 7−6) normally works, however, this is an unsafe practice.
Always quote a tested string. [18]
Advanced Bash−Scripting Guide
Chapter 7. Tests 50
Example 7−5. arithmetic and string comparisons
#!/bin/bash
a=4
b=5
# Here "a" and "b" can be treated either as integers or strings.
# There is some blurring between the arithmetic and string comparisons,
#+ since Bash variables are not strongly typed.
# Bash permits integer operations and comparisons on variables
#+ whose value consists of all−integer characters.
# Caution advised.
echo
if [ "$a" −ne "$b" ]
then
echo "$a is not equal to $b"
echo "(arithmetic comparison)"
fi
echo
if [ "$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 $string1 as not null, although it was not initialized.
Advanced Bash−Scripting Guide
Chapter 7. Tests 51
echo
# Lets try it again.
if [ −n "$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, $string1 stands naked.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# This works fine.
# The [ ] test operator alone detects whether the string is null.
# However it is good practice to quote it ("$string1").
#
# As Stephane Chazelas points out,
# if [ $string 1 ] has one argument, "]"
# if [ "$string 1" ] has two arguments, the empty "$string1" and "]"
echo
string1=initialized
if [ $string1 ] # Again, $string1 stands naked.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Again, gives correct result.
# Still, it is better to quote it ("$string1"), because...
string1="a = b"
if [ $string1 ] # Again, $string1 stands naked.
then
echo "String \"string1\" is not null."
else
echo "String \"string1\" is null."
fi
# Not quoting "$string1" now gives wrong result!
exit 0
# Also, thank you, Florian Wisser, for the "heads−up".
Advanced Bash−Scripting Guide
Chapter 7. Tests 52
Example 7−7. zmost
#!/bin/bash
#View gzipped files with 'most'
NOARGS=65
NOTFOUND=66
NOTGZIP=67
if [ $# −eq 0 ] # same effect as: if [ −z "$1" ]
# $1 can exist, but be empty: zmost "" arg2 arg3
then
echo "Usage: `basename $0` filename" >&2
# Error message to stderr.
exit $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 $NOTFOUND
fi
if [ ${filename##*.} != "gz" ]
# Using bracket in variable substitution.
then
echo "File $1 is not a gzipped file!"
exit $NOTGZIP
fi
zcat $1 | most
# Uses the file viewer 'most' (similar to 'less').
# Later versions of 'most' have file decompression capabilities.
# May substitute 'more' or 'less', if desired.
exit $? # Script returns exit status of pipe.
# Actually "exit $?" unnecessary, as the script will, in any case,
# return the exit status of the last command executed.
compound comparison
−a
logical and
exp1 −a exp2 returns true if both exp1 and exp2 are true.
−o
logical or
exp1 −o exp2 returns true if either exp1 or exp2 are true.
These are similar to the Bash comparison operators && and ||, used within double brackets.
Advanced Bash−Scripting Guide
Chapter 7. Tests 53
[[ condition1 && condition2 ]]
The −o and −a operators work with the test command or occur within single test brackets.
if [ "$exp1" −a "$exp2" ]
Refer to Example 8−3 and Example 26−11 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 identical to using the &&
compound comparison operator above.
if [ condition1 ]
then
if [ condition2 ]
then
do−something # But only if both "condition1" and "condition2" valid.
fi
fi
See Example 35−4 for an example of nested if/then condition tests.
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, as the following excerpt shows.
if [ −f $HOME/.Xclients ]; then
exec $HOME/.Xclients
elif [ −f /etc/X11/xinit/Xclients ]; then
exec /etc/X11/xinit/Xclients
else
# failsafe settings. Although we should never get here
# (we provide fallbacks in Xclients as well) it can't hurt.
xclock −geometry 100x100−5+5 &
xterm −geometry 80x50−50+150 &
if [ −f /usr/bin/netscape −a −f /usr/share/doc/HTML/index.html ]; then
netscape /usr/share/doc/HTML/index.html &
fi
fi
Explain the "test" constructs in the above excerpt, then examine the entire file,
/etc/X11/xinit/xinitrc, and analyze the if/then test constructs there. You may need to refer ahead to
the discussions of grep, sed, and regular expressions.
Advanced Bash−Scripting Guide
Chapter 7. Tests 54
Chapter 8. Operations and Related Topics
8.1. Operators
assignment
variable assignment
Initializing or changing the value of a variable
=
All−purpose assignment operator, which works for both arithmetic and string assignments.
var=27
category=minerals # No spaces allowed after the "=".
Do not confuse the "=" assignment operator with the = test operator.
# = as a test operator
if [ "$string1" = "$string2" ]
# if [ "X$string1" = "X$string2" ] is safer,
# to prevent an error message should one of the variables be empty.
# (The prepended "X" characters cancel out.)
then
command
fi
arithmetic operators
+
plus
−
minus
*
multiplication
/
division
**
exponentiation
# Bash, version 2.02, introduced the "**" exponentiation operator.
let "z=5**3"
echo "z = $z" # z = 125
%
modulo, or mod (returns the remainder of an integer division operation)
bash$ echo `expr 5 % 3`
2
This operator finds use in, among other things, generating numbers within a specific range (see
Example 9−23 and Example 9−25) and formatting program output (see Example 26−10 and Example
Chapter 8. Operations and Related Topics 55
A−7). It can even be used to generate prime numbers, (see Example A−17). Modulo turns up
surprisingly often in various 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=65
if [ $# −ne "$ARGS" ]
then
echo "Usage: `basename $0` first−number second−number"
exit $E_BADARGS
fi
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
gcd ()
{
# Arbitrary assignment.
dividend=$1 # It does not matter
divisor=$2 #+ which of the two is larger.
# Why?
remainder=1 # If uninitialized variable used in loop,
#+ it results in an error message
#+ on first pass through loop.
until [ "$remainder" −eq 0 ]
do
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
Advanced Bash−Scripting Guide
Chapter 8. Operations and Related Topics 56
echo; echo "GCD of $1 and $2 = $dividend"; echo
# Exercise :
# −−−−−−−−
# Check command−line arguments to make sure they are integers,
#+ and exit the script with an appropriate error message if not.
exit 0
+=
"plus−equal" (increment variable by a constant)
let "var += 5" results in var being incremented by 5.
−=
"minus−equal" (decrement variable by a constant)
*=
"times−equal" (multiply variable by a constant)
let "var *= 4" results in var being multiplied by 4.
/=
"slash−equal" (divide variable by a constant)
%=
"mod−equal" (remainder of dividing variable by a constant)
Arithmetic operators often occur in an expr or let expression.
Example 8−2. Using Arithmetic Operations
#!/bin/bash
# Counting to 6 in 5 different ways.
n=1; echo −n "$n "
let "n = $n + 1" # let "n = n + 1" also works.
echo −n "$n "
: $((n = $n + 1))
# ":" necessary because otherwise Bash attempts
#+ to interpret "$((n = $n + 1))" as a command.
echo −n "$n "
n=$(($n + 1))
echo −n "$n "
: $[ n = $n + 1 ]
# ":" necessary because otherwise Bash attempts
#+ to interpret "$[ n = $n + 1 ]" as a command.
# Works even if "n" was initialized as a string.
echo −n "$n "
n=$[ $n + 1 ]
# Works even if "n" was initialized as a string.
#* Avoid this type of construct, since it is obsolete and nonportable.
echo −n "$n "; echo
# Thanks, Stephane Chazelas.
Advanced Bash−Scripting Guide
Chapter 8. Operations and Related Topics 57
exit 0
Integer variables in Bash are actually signed long (32−bit) integers, in the range of
−2147483648 to 2147483647. An operation that takes a variable outside these limits
will give an erroneous result.
a=2147483646
echo "a = $a" # a = 2147483646
let "a+=1" # Increment "a".
echo "a = $a" # a = 2147483647
let "a+=1" # increment "a" again, past the limit.
echo "a = $a" # a = −2147483648
# ERROR (out of range)
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.
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 run fast enough to permit its use on the fly.
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"
~
Advanced Bash−Scripting Guide
Chapter 8. Operations and Related Topics 58
bitwise negate
!
bitwise NOT
^
bitwise XOR
^=
"bitwise XOR−equal"
logical operators
&&
and (logical)
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 within [ ... ] construct.
&& may also, depending on context, be used in an and list to concatenate commands.
||
or (logical)
if [ $condition1 ] || [ $condition2 ]
# Same as: if [ $condition1 −o $condition2 ]
# Returns true if either condition1 or condition2 holds true...
if [[ $condition1 || $condition2 ]] # Also works.
# Note that || operator not permitted within [ ... ] construct.
Bash tests the exit status of each statement linked with a logical operator.
Example 8−3. Compound Condition Tests Using && and ||
#!/bin/bash
a=24
b=47
if [ "$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 ']'.
#
# if [[ $a −eq 24 && $b −eq 24 ]] works
# (The "&&" has a different meaning in line 17 than in line 6.)
# Thanks, Stephane Chazelas.
Advanced Bash−Scripting Guide
Chapter 8. Operations and Related Topics 59
if [ "$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 "$b" −eq 47 ]
then
echo "Test #3 succeeds."
else
echo "Test #3 fails."
fi
if [ "$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), but only the last operation is returned.
let "t1 = ((5 + 3, 7 − 1, 15 − 4))"
echo "t1 = $t1" # t1 = 11
Advanced Bash−Scripting Guide
Chapter 8. Operations and Related Topics 60
let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2".
echo "t2 = $t2 a = $a" # t2 = 5 a = 9
The comma operator finds use mainly in for loops. See Example 10−12.
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" # 32
# Nothing out of the ordinary here.
# Octal: numbers preceded by '0' (zero)
let "oct = 032"
echo "octal number = $oct" # 26
# Expresses result in decimal.
# −−−−−−−−− −−−−−− −− −−−−−−−
# Hexadecimal: numbers preceded by '0x' or '0X'
let "hex = 0x32"
echo "hexadecimal number = $hex" # 50
# 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" # 4094
#
# This notation only works for a limited range (2 − 64)
# 10 digits + 26 lowercase characters + 26 uppercase characters + @ + _
echo
echo $((36#zz)) $((2#10101010)) $((16#AF16)) $((53#1aA))
# 1295 170 44822 3375
# Important note:
# −−−−−−−−−−−−−−
Advanced Bash−Scripting Guide
Chapter 8. Operations and Related Topics 61
# Using a digit out of range of the specified base notation
#+ will give an error message.
let "bad_oct = 081"
# numbers.sh: let: oct = 081: value too great for base (error token is "081")
# Octal numbers use only digits in the range 0 − 7.
exit 0 # Thanks, Rich Bartell and Stephane Chazelas, for clarification.
Advanced Bash−Scripting Guide
Chapter 8. Operations and Related Topics 62
Part 3. Beyond the Basics
Table of Contents
9. Variables Revisited
9.1. Internal Variables
9.2. Manipulating Strings
9.3. Parameter Substitution
9.4. Typing variables: declare or typeset
9.5. Indirect References to Variables
9.6. $RANDOM: generate random integer
9.7. The Double Parentheses Construct
10. Loops and Branches
10.1. Loops
10.2. Nested Loops
10.3. Loop Control
10.4. Testing and Branching
11. Internal Commands and Builtins
11.1. Job Control Commands
12. External Filters, Programs and Commands
12.1. Basic Commands
12.2. Complex Commands
12.3. Time / Date Commands
12.4. Text Processing Commands
12.5. File and Archiving Commands
12.6. Communications Commands
12.7. Terminal Control Commands
12.8. Math Commands
12.9. Miscellaneous Commands
13. System and Administrative Commands
14. Command Substitution
15. Arithmetic Expansion
16. I/O Redirection
16.1. Using exec
16.2. Redirecting Code Blocks
16.3. Applications
17. Here Documents
18. Recess Time
Part 3. Beyond the Basics 63
Chapter 9. Variables Revisited
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_VERSINFO[n]
a 6−element array containing version information about the installed release of Bash. This is similar
to $BASH_VERSION, below, but a bit more detailed.
# Bash version info:
for n in 0 1 2 3 4 5
do
echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}"
done
# BASH_VERSINFO[0] = 2 # Major version no.
# BASH_VERSINFO[1] = 05 # Minor version no.
# BASH_VERSINFO[2] = 8 # Patch level.
# BASH_VERSINFO[3] = 1 # Build version.
# BASH_VERSINFO[4] = release # Release status.
# BASH_VERSINFO[5] = i386−redhat−linux−gnu # Architecture
# (same as $MACHTYPE).
$BASH_VERSION
the version of Bash installed on the system
bash$ echo $BASH_VERSION
2.04.12(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.
$DIRSTACK
the top value in the directory stack (affected by pushd and popd)
This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the
directory stack.
Chapter 9. Variables Revisited 64
$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.
$FUNCNAME
name of the current function
xyz23 ()
{
echo "$FUNCNAME now executing." # xyz23 now executing.
}
xyz23
echo "FUNCNAME = $FUNCNAME" # FUNCNAME =
# Null value outside a function.
$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.
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 9−13)
$HOSTNAME
The hostname command assigns the system name at bootup in an init script. However, the
gethostname() function sets the Bash internal variable $HOSTNAME. See also Example 9−13.
$HOSTTYPE
host type
Like $MACHTYPE, identifies the system hardware.
bash$ echo $HOSTTYPE
i686
$IFS
input field separator
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 65
This 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 $IFS. See Example 5−1.
bash$ echo $IFS | cat −vte
$
bash$ bash −c 'set w x y z; IFS=":−;"; echo "$*"'
w:x:y:z
$IFS does not handle whitespace the same as it does other characters.
Example 9−1. $IFS and whitespace
#!/bin/bash
# $IFS treats whitespace differently than other characters.
output_args_one_per_line()
{
for arg
do echo "[$arg]"
done
}
echo; echo "IFS=\" \""
echo "−−−−−−−"
IFS=" "
var=" a b c "
output_args_one_per_line $var # output_args_one_per_line `echo " a b c "`
#
# [a]
# [b]
# [c]
echo; echo "IFS=:"
echo "−−−−−"
IFS=:
var=":a::b:c:::" # Same as above, but substitute ":" for " ".
output_args_one_per_line $var
#
# []
# [a]
# []
# [b]
# [c]
# []
# []
# []
# The same thing happens with the "FS" field separator in awk.
# Thank you, Stephane Chazelas.
echo
exit 0
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 66
(Thanks, S. C., for clarification and examples.)
$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.
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 $PATH definition in
/etc/profile and/or ~/.bashrc (see Chapter 27).
bash$ 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
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 67
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.
$PIPESTATUS
Exit status of last executed foreground pipe. Interestingly enough, this does not give the same result
as the exit status of the last executed command.
bash$ echo $PIPESTATUS
0
bash$ ls −al | bogus_command
bash: bogus_command: command not found
bash$ echo $PIPESTATUS
141
bash$ ls −al | bogus_command
bash: bogus_command: command not found
bash$ echo $?
127
The $PIPESTATUS variable may contain an erroneous 0 value in a login shell.
tcsh% bash
bash$ 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.
$PPID
The $PPID of a process is the process id (pid) of its parent process. [19]
Compare this with the pidof command.
$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 10−29).
$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.
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 68
#!/bin/bash
E_WRONG_DIRECTORY=73
clear # Clear 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.
# Filenames may contain all characters in the 0 − 255 range, except "/".
# Deleting files beginning with weird characters is left as an exercise.
# Various other operations here, as necessary.
echo
echo "Done."
echo "Old files deleted in $TargetDirectory."
echo
exit 0
$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
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
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 69
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
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. If, at the command line, $SHLVL is 1, then in a script it will
increment to 2.
$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.
Unfortunately, this works only while waiting for input at the shell prompt console or
in an xterm. While it would be nice to speculate on the uses of this internal variable
for timed input, for example in combination with read, $TMOUT does not work in that
context and is virtually useless for shell scripting. (Reportedly the ksh version of a
timed read does work.)
Implementing timed input in a script is certainly possible, but may require complex machinations.
One method is to set up a timing loop to signal the script when it times out. This also requires a signal
handling routine to trap (see Example 30−5) the interrupt generated by the timing loop (whew!).
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 70
Example 9−2. Timed Input
#!/bin/bash
# timed−input.sh
# TMOUT=3 useless in a script
TIMELIMIT=3 # Three seconds in this instance, may be set to different value.
PrintAnswer()
{
if [ "$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 14
}
trap Int14Vector 14 # 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 "t−out.sh", below.
# 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
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 71
#!/bin/bash
# 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 (per a suggestion by "syngin seven)
TIMELIMIT=4 # 4 seconds
read −t $TIMELIMIT variable <&1
echo
if [ −z "$variable" ]
then
echo "Timed out, variable still unset."
else
echo "variable = $variable"
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 72
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: Am I root or not?
ROOT_UID=0 # 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 2−2.
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
tcsh% echo $TERM
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 73
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 11−13)
$#
number of command line arguments [20] or positional parameters (see Example 34−2)
$*
All of the positional parameters, seen as a single word
$@
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.
Example 9−6. arglist: Listing arguments with $* and $@
#!/bin/bash
# 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
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
index=1
echo "Listing args with \"\$@\":"
for arg in "$@"
do
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 74
echo "Arg #$index = $arg"
let "index+=1"
done # $@ sees arguments as separate words.
echo "Arg list seen as separate words."
echo
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 "$@" # 1 2 3 4 5
shift
echo "$@" # 2 3 4 5
shift
echo "$@" # 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 12−17 and Example 12−18.
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
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]"
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Chapter 9. Variables Revisited 75
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="$*")'
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 −−−
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Chapter 9. Variables Revisited 76
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
# 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,
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 77
# then "$*" and "$@" do not echo positional params as expected.
mecho () # Echo positional parameters.
{
echo "$1,$2,$3";
}
IFS="" # Set, but empty.
set a b c # Positional parameters.
mecho "$*" # abc,,
mecho $* # a,b,c
mecho $@ # a,b,c
mecho "$@" # a,b,c
# The behavior of $* and $@ when $IFS is empty depends
# on whatever Bash or sh version being run.
# It is therefore inadvisable to depend on this "feature" in a script.
# Thanks, S.C.
exit 0
Other Special Parameters
$−
Flags passed to script (using set). See Example 11−13.
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.
echo −n "PID of \"$COMMAND1\": " >> "$LOG"
${COMMAND1} &
echo $! >> "$LOG"
# PID of "sleep 100": 1506
# Thank you, Jacques Lederer, for suggesting this.
$_
Special variable set to last argument of previous command executed.
Example 9−9. underscore variable
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 78
#!/bin/bash
echo $_ # /bin/bash
# Just called /bin/bash to run the script.
du >/dev/null # So no output from command.
echo $_ # du
ls −al >/dev/null # So no output from command.
echo $_ # −al (last argument)
:
echo $_ # :
$?
Exit status of a command, function, or the script itself (see Example 23−3)
$$
Process id of the script itself. The $$ variable often finds use in scripts to construct "unique" temp file
names (see Example A−14, Example 30−6, Example 12−23, and Example 11−23). This is usually
simpler than invoking mktemp.
9.2. 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 $string
expr "$string" : '.*'
stringZ=abcABC123ABCabc
echo ${#stringZ} # 15
echo `expr length $stringZ` # 15
echo `expr "$stringZ" : '.*'` # 15
Example 9−10. Inserting a blank line between paragraphs in a text file
#!/bin/bash
# paragraph−space.sh
# Inserts a blank line between paragraphs of a single−spaced text file.
# Usage: $0 <FILENAME
MINLEN=45 # May need to change this value.
# Assume lines shorter than $MINLEN characters
#+ terminate a paragraph.
while read line # For as many lines as the input file has...
do
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 79
echo "$line" # Output the line itself.
len=${#line}
if [ "$len" −lt "$MINLEN" ]
then echo # Add a blank line after short line.
fi
done
exit 0
Length of Matching Substring at Beginning of String
expr match "$string" '$substring'
$substring is a regular expression.
expr "$string" : '$substring'
$substring is a regular expression.
stringZ=abcABC123ABCabc
# |−−−−−−|
echo `expr match "$stringZ" 'abc[A−Z]*.2'` # 8
echo `expr "$stringZ" : 'abc[A−Z]*.2'` # 8
Index
expr index $string $substring
Numerical position in $string of first character in $substring that matches.
stringZ=abcABC123ABCabc
echo `expr index "$stringZ" C12` # 6
# C position.
echo `expr index "$stringZ" 1c` # 3
# 'c' (in #3 position) matches before '1'.
This is the near equivalent of strchr() in C.
Substring Extraction
${string:position}
Extracts substring from $string at $position.
If the $string parameter is "*" or "@", then this extracts the positional parameters, [21] starting at
$position.
${string:position:length}
Extracts $length characters of substring from $string at $position.
stringZ=abcABC123ABCabc
# 0123456789.....
# 0−based indexing.
echo ${stringZ:0} # abcABC123ABCabc
echo ${stringZ:1} # bcABC123ABCabc
echo ${stringZ:7} # 23ABCabc
Advanced Bash−Scripting Guide
Chapter 9. Variables Revisited 80
echo ${stringZ:7:3} # 23A
# Three characters of substring.
If the $string parameter is "*" or "@", then this extracts a maximum of $length positional
parameters, starting at $position.
echo ${*:2} # Echoes second and following positional parameters.
echo ${@:2} # Same as above.
echo ${*:2:3} # Echoes three positional parameters, starting at second.
expr substr $string $position $length
Extracts $length characters from $string starting at $position.
stringZ=abcABC123ABCabc
# 123456789......
# 1−based indexing.
echo `expr substr $stringZ 1 2` # ab
echo `expr substr $stringZ 4 3` # ABC
expr match "$string" '\($substring\)'
Extracts $substring at beginning of $string, where $substring is a regular expression.
expr "$string" : '\($substring\)'
Extracts $substring at beginning of $string, where $substring is a regular expression.
stringZ=abcABC123ABCabc
# =======
echo `expr match "$stringZ" '\(.[b−c]*[A−Z]..[0−9]\)'` # abcABC1
echo `expr "$stringZ" : '\(.[b−c]*[A−Z]..[0−9]\)'` # abcABC1
echo `expr "$stringZ" : '\(.......\)'` # abcABC1
# All of the above forms give an identical result.
expr match "$string" '.*\($substring\)'
Extracts $substring at end of $string, where $substring is a regular expression.
expr "$string" : '.*\($substring\)'
Extracts $substring at end of $string, where $substring is a regular expression.
stringZ=abcABC123ABCabc
# ======
echo `expr match "$stringZ" '.*\([A−C][A−C][A−C][a−c]*\)'` # ABCabc
echo `expr "$stringZ" : '.*\(......\)'` # ABCabc
Substring Removal
${string#substring}
Strips shortest match of $substring from front of $string.
${string##substring}
Strips longest match of $substring from front of $string.
stringZ=abcABC123ABCabc
# |−−−−|
# |−−−−−−−−−−|
echo ${stringZ#a*C} # 123ABCabc
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# Strip out shortest match between 'a' and 'C'.
echo ${stringZ##a*C} # abc
# Strip out longest match between 'a' and 'C'.
${string%substring}
Strips shortest match of $substring from back of $string.
${string%%substring}
Strips longest match of $substring from back of $string.
stringZ=abcABC123ABCabc
# ||
# |−−−−−−−−−−−−|
echo ${stringZ%b*c} # abcABC123ABCa
# Strip out shortest match between 'b' and 'c', from back of $stringZ.
echo ${stringZ%%b*c} # a
# Strip out longest match between 'b' and 'c', from back of $stringZ.
Example 9−11. Converting graphic file formats, with filename change
#!/bin/bash
# cvt.sh:
# Converts all the MacPaint image files in a directory to "pbm" format.
# Uses the "macptopbm" binary from the "netpbm" package,
#+ which is maintained by Brian Henderson (bryanh@giraffe−data.com).
# Netpbm is a standard part of most Linux distros.
OPERATION=macptopbm
SUFFIX=pbm # New filename suffix.
if [ −n "$1" ]
then
directory=$1 # If directory name given as a script argument...
else
directory=$PWD # Otherwise use current working directory.
fi
# Assumes all files in the target directory are MacPaint image files,
# + with a ".mac" suffix.
for file in $directory/* # Filename globbing.
do
filename=${file%.*c} # Strip ".mac" suffix off filename
#+ ('.*c' matches everything
#+ between '.' and 'c', inclusive).
$OPERATION $file > "$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
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#+ working directory.
# Modify it to work *only* on files with a ".mac" suffix.
Substring Replacement
${string/substring/replacement}
Replace first match of $substring with $replacement.
${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'.
${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'.
9.2.1. 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 9−12. Alternate ways of extracting substrings
#!/bin/bash
# substring−extraction.sh
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
}
'
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# Piping an empty "echo" to awk gives it dummy input,
#+ and thus makes it unnecessary to supply a filename.
exit 0
9.2.2. Further Discussion
For more on string manipulation in scripts, refer to Section 9.3 and the relevant section of the expr command
listing. For script examples, see:
Example 12−61. Example 9−152. Example 9−163. Example 9−174. Example 9−195.
9.3. 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 ${parameter} form works.
May be used for concatenating variables with strings.
your_id=${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.
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
username0=
# username0 has been declared, but is set to null.
echo "username0 = ${username0−`whoami`}"
# Will not echo.
echo "username1 = ${username1−`whoami`}"
# username1 has not been declared.
# Will echo.
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username2=
# username2 has been declared, but is set to null.
echo "username2 = ${username2:−`whoami`}"
# Will echo because of :− rather than just − in condition test.
exit 0
The default parameter construct finds use in providing "missing" command−line arguments in scripts.
DEFAULT_FILENAME=generic.data
filename=${1:−$DEFAULT_FILENAME}
# If not otherwise specified, the following command block operates
#+ on the file "generic.data".
#
# Commands follow.
See also Example 3−4, Example 29−2, and Example A−7.
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, [22] as above.
echo ${username=`whoami`}
# Variable "username" is now set to `whoami`.
${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 = $a" # a = xyz
param3=123
a=${param3+xyz}
echo "a = $a" # a = xyz
echo
echo "###### \${parameter:+alt_value} ########"
echo
a=${param4:+xyz}
echo "a = $a" # a =
param5=
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a=${param5:+xyz}
echo "a = $a" # a =
# Different result from a=${param5+xyz}
param6=123
a=${param6+xyz}
echo "a = $a" # a = xyz
${parameter?err_msg}, ${parameter:?err_msg}
If parameter set, use it, else print err_msg.
Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is
null, as above.
Example 9−13. Using parameter substitution and error messages
#!/bin/bash
# Check some of the system's environmental variables.
# 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
: ${ZZXy23AB?"ZZXy23AB has not been set."}
# If ZZXy23AB has not been set,
#+ then the script terminates with an error message.
# You can specify the error message.
# : ${ZZXy23AB?"ZZXy23AB has not been set."}
# Same result with: dummy_variable=${ZZXy23AB?}
# dummy_variable=${ZZXy23AB?"ZXy23AB has not been set."}
#
# echo ${ZZXy23AB?} >/dev/null
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echo "You will not see this message, because script terminated above."
HERE=0
exit $HERE # Will *not* exit here.
Example 9−14. 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 12−6). 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.◊ For an array, ${#array[*]} and ${#array[@]} give the number of elements in
the array.
◊
Example 9−15. 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 "Invoke this script with one or more command−line arguments."
exit $E_NO_ARGS
fi
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Chapter 9. Variables Revisited 87
var01=abcdEFGH28ij
echo "var01 = ${var01}"
echo "Length of var01 = ${#var01}"
echo "Number of command−line arguments passed to script = ${#@}"
echo "Number of command−line arguments passed to script = ${#*}"
exit 0
${var#Pattern}, ${var##Pattern}
Remove from $var the shortest/longest part of $Pattern that matches the front end of $var.
A usage illustration from Example A−8:
# Function from "days−between.sh" example.
# Strips leading zero(s) from argument passed.
strip_leading_zero () # Better to strip possible leading zero(s)
{ # from day and/or month
val=${1#0} # since otherwise Bash will interpret them
return $val # as octal values (POSIX.2, sect 2.9.2.1).
}
Another usage illustration:
echo `basename $PWD` # Basename of current working directory.
echo "${PWD##*/}" # Basename of current working directory.
echo
echo `basename $0` # Name of script.
echo $0 # Name of script.
echo "${0##*/}" # Name of script.
echo
filename=test.data
echo "${filename##*.}" # data
# Extension of filename.
${var%Pattern}, ${var%%Pattern}
Remove from $var the shortest/longest part of $Pattern that matches the back end of $var.
Version 2 of Bash adds additional options.
Example 9−16. Pattern matching in parameter substitution
#!/bin/bash
# Pattern matching using the # ## % %% parameter substitution operators.
var1=abcd12345abc6789
pattern1=a*c # * (wild card) matches everything between a − c.
echo
echo "var1 = $var1" # abcd12345abc6789
echo "var1 = ${var1}" # abcd12345abc6789 (alternate form)
echo "Number of characters in ${var1} = ${#var1}"
echo "pattern1 = $pattern1" # a*c (everything between 'a' and 'c')
echo
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echo '${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
pattern2=b*9 # everything between 'b' and '9'
echo "var1 = $var1" # Still abcd12345abc6789
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
# Longest possible match, strips out last 12 characters abcd12345abc6789
# ^^^^ |−−−−−−−−−−−−−|
# Remember, # and ## work from the left end of string,
# % and %% work from the right end.
echo
exit 0
Example 9−17. Renaming file extensions:
#!/bin/bash
# rfe
# −−−
# Renaming file extensions.
#
# rfe old_extension new_extension
#
# Example:
# To rename all *.gif files in working directory to *.jpg,
# rfe gif jpg
ARGS=2
E_BADARGS=65
if [ $# −ne "$ARGS" ]
then
echo "Usage: `basename $0` old_file_suffix new_file_suffix"
exit $E_BADARGS
fi
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
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Chapter 9. Variables Revisited 89
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−15 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.
As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing,
that is, deleted.
Example 9−18. 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, S. C. 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 $path_name` will not work,
#+ but the above expression will.
# (Thanks, S.C.)
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t=${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 = $t"
t=${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 9−19. Matching patterns at prefix or suffix of string
#!/bin/bash
# Pattern replacement at prefix / suffix of string.
v0=abc1234zip1234abc # Original variable.
echo "v0 = $v0" # abc1234zip1234abc
echo
# Match at prefix (beginning) of string.
v1=${v0/#abc/ABCDEF} # abc1234zip1234abc
# |−|
echo "v1 = $v1" # ABCDE1234zip1234abc
# |−−−|
# Match at suffix (end) of string.
v2=${v0/%abc/ABCDEF} # abc1234zip123abc
# |−|
echo "v2 = $v2" # abc1234zip1234ABCDEF
# |−−−−|
echo
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# Must match at beginning / end of string,
#+ otherwise no replacement results.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
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Chapter 9. Variables Revisited 91
v3=${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*}, ${!varprefix@}
Matches all previously declared variables beginning with varprefix.
xyz23=whatever
xyz24=
a=${!xyz*} # Expands to names of declared variables beginning with "xyz".
echo "a = $a" # a = xyz23 xyz24
a=${!xyz@} # Same as above.
echo "a = $a" # a = xyz23 xyz24
# Bash, version 2.04, adds this feature.
9.4. Typing variables: declare or typeset
The declare or typeset builtins (they are exact synonyms) permit restricting the properties of variables. This
is a very weak form of the typing 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
(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.
−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 = $number" # number = 0
# Tries to evaluate "three" as an integer.
Note that certain arithmetic operations are permitted for declared integer variables without the need
for expr or let.
−a array
declare −a indices
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Chapter 9. Variables Revisited 92
The variable indices will be treated as an array.
−f functions
declare −f
A declare −f line with no arguments in a script causes a listing of all the functions previously
defined in that script.
declare −f function_name
A declare −f function_name in a script lists just the function named.
−x export
declare −x var3
This declares a variable as available for exporting outside the environment of the script itself.
var=$value
declare −x var3=373
The declare command permits assigning a value to a variable in the same statement as setting its
properties.
Example 9−20. 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 $var2" # This line will not execute.
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Chapter 9. Variables Revisited 93
exit 0 # Script will not exit here.
9.5. Indirect References to Variables
Assume that the value of a variable is the name of a second variable. Is it somehow possible to retrieve the
value of this second variable from the first one? For example, if a=letter_of_alphabet and
letter_of_alphabet=z, can a reference to a return z? This can indeed be done, and it is called an
indirect reference. It uses the unusual eval var1=\$$var2 notation.
Example 9−21. Indirect References
#!/bin/bash
# Indirect variable referencing.
a=letter_of_alphabet
letter_of_alphabet=z
echo
# Direct reference.
echo "a = $a"
# Indirect reference.
eval a=\$$a
echo "Now a = $a"
echo
# Now, let's try changing the second order reference.
t=table_cell_3
table_cell_3=24
echo "\"table_cell_3\" = $table_cell_3"
echo −n "dereferenced \"t\" = "; eval echo \$$t
# In this simple case,
# eval t=\$$t; echo "\"t\" = $t"
# also works (why?).
echo
t=table_cell_3
NEW_VAL=387
table_cell_3=$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, S.C., for clearing up the above behavior.)
# Another method is the ${!t} notation, discussed in "Bash, version 2" section.
# See also example "ex78.sh".
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Chapter 9. Variables Revisited 94
exit 0
Example 9−22. 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 uses indirect references.
ARGS=2
E_WRONGARGS=65
if [ $# −ne "$ARGS" ] # Check for proper no. of command line args.
then
echo "Usage: `basename $0` filename column−number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
#===== Same as original script, up to this point =====#
# A multi−line awk script is invoked by awk ' ..... '
# Begin awk script.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
awk "
{ total += \$${column_number} # indirect reference
}
END {
print total
}
" "$filename"
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# End awk script.
# Indirect variable reference avoids the hassles
# of referencing a shell variable within the embedded awk script.
# Thanks, Stephane Chazelas.
exit 0
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 35−2) makes indirect
referencing more intuitive.
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Chapter 9. Variables Revisited 95
9.6. $RANDOM: generate random integer
$RANDOM is an internal Bash function (not a constant) that returns a pseudorandom integer in the range 0 −
32767. $RANDOM should not be used to generate an encryption key.
Example 9−23. Generating random numbers
#!/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 −−− $number"
echo
# If you need a random int greater than a lower bound,
# then set up a test to discard all numbers below that.
FLOOR=200
number=0 #initialize
while [ "$number" −le $FLOOR ]
do
number=$RANDOM
done
echo "Random number greater than $FLOOR −−− $number"
echo
# May combine above two techniques to retrieve random number between two limits.
number=0 #initialize
while [ "$number" −le $FLOOR ]
do
number=$RANDOM
let "number %= $RANGE" # Scales $number down within $RANGE.
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Chapter 9. Variables Revisited 96
done
echo "Random number between $FLOOR and $RANGE −−− $number"
echo
# Generate binary choice, that is, "true" or "false" value.
BINARY=2
number=$RANDOM
T=1
let "number %= $BINARY"
# let "number >>= 14" gives a better random distribution
# (right shifts out everything except last binary digit).
if [ "$number" −eq $T ]
then
echo "TRUE"
else
echo "FALSE"
fi
echo
# May generate toss of the dice.
SPOTS=7 # Modulo 7 gives range 0 − 6.
ZERO=0
die1=0
die2=0
# Tosses each die separately, and so gives correct odds.
while [ "$die1" −eq $ZERO ] # Can't have a zero come up.
do
let "die1 = $RANDOM % $SPOTS" # Roll first one.
done
while [ "$die2" −eq $ZERO ]
do
let "die2 = $RANDOM % $SPOTS" # Roll second one.
done
let "throw = $die1 + $die2"
echo "Throw of the dice = $throw"
echo
exit 0
Example 9−24. Picking a random card from a deck
#!/bin/bash
# pick−card.sh
# This is an example of choosing a random element of an array.
# Pick a card, any card.
Suites="Clubs
Diamonds
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Chapter 9. Variables Revisited 97
Hearts
Spades"
Denominations="2
3
4
5
6
7
8
9
10
Jack
Queen
King
Ace"
suite=($Suites) # Read into array variable.
denomination=($Denominations)
num_suites=${#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 $RANDOM.
exit 0
Jipe points out another set of techniques for generating random numbers within a range.
# Generate random number between 6 and 30.
rnumber=$((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))
# Exercise: Try to figure out the pattern here.
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 9−25. Rolling the die with RANDOM
#!/bin/bash
# How random is RANDOM?
RANDOM=$$ # Reseed the random number generator using script process ID.
PIPS=6 # A die has 6 pips.
MAXTHROWS=600 # Increase this, if you have nothing better to do with your time.
throw=0 # Throw count.
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zeroes=0 # Must initialize counts to zero.
ones=0 # since an uninitialized variable is null, not zero.
twos=0
threes=0
fours=0
fives=0
sixes=0
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"
update_count $die1
let "throw += 1"
done
print_result
# 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.
# Exercise (easy):
# −−−−−−−−−−−−−−−
# Rewrite this script to flip a coin 1000 times.
# Choices are "HEADS" or "TAILS".
exit 0
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. (This mirrors the behavior of the
random() function in C.)
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Chapter 9. Variables Revisited 99
Example 9−26. 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 # Setting RANDOM seeds the random number generator.
random_numbers
echo; echo
RANDOM=1 # Same seed for RANDOM...
random_numbers # ...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...
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 device−file provides a means of generating much more "random" pseudorandom
numbers than the $RANDOM variable. dd if=/dev/urandom of=targetfile bs=1
count=XX creates a file of well−scattered pseudorandom numbers. However, assigning these numbers
to a variable in a script requires a workaround, such as filtering through od (as in above example) or
using dd (see Example 12−42).
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Chapter 9. Variables Revisited 100
There are also other means of generating pseudorandom numbers in a script. Awk provides a convenient
means of doing this.
Example 9−27. Pseudorandom numbers, using awk
#!/bin/bash
# random2.sh: Returns a pseudorandom number in the range 0 − 1.
# Uses the awk rand() function.
AWKSCRIPT=' { srand(); print rand() } '
# Command(s) / parameters passed to awk
# Note that srand() reseeds awk's random number generator.
echo −n "Random number between 0 and 1 = "
echo | awk "$AWKSCRIPT"
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
# in the range between 10 and 100.
# 3) Same as exercise #2, above, but generate random integers this time.
9.7. 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−type manipulation of variables in Bash.
Example 9−28. C−type manipulation of variables
#!/bin/bash
# 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"
(( a++ )) # Post−increment 'a', C−style.
echo "a (after a++) = $a"
(( a−− )) # Post−decrement 'a', C−style.
echo "a (after a−−) = $a"
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(( ++a )) # Pre−increment 'a', C−style.
echo "a (after ++a) = $a"
(( −−a )) # Pre−decrement 'a', C−style.
echo "a (after −−a) = $a"
echo
(( t = a<45?7:11 )) # C−style trinary operator.
echo "If a < 45, then t = 7, else t = 11."
echo "t = $t " # Yes!
echo
# −−−−−−−−−−−−−−−−−
# Easter Egg alert!
# −−−−−−−−−−−−−−−−−
# Chet Ramey apparently snuck a bunch of undocumented C−style constructs
#+ into Bash (actually adapted from ksh, pretty much).
# In the Bash docs, Ramey calls ((...)) shell arithmetic,
#+ but it goes far beyond that.
# Sorry, Chet, the secret is now out.
# See also "for" and "while" loops using the ((...)) construct.
# These work only with Bash, version 2.04 or later.
exit 0
See also Example 10−12.
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Chapter 9. Variables Revisited 102
Chapter 10. Loops and Branches
Operations on code blocks are the key to structured, organized shell scripts. Looping and branching constructs
provide the tools for accomplishing this.
10.1. Loops
A loop is a block of code that iterates (repeats) a list of commands as long as the loop control condition is
true.
for loops
for (in) This is the basic looping construct. It differs significantly from its C counterpart.
for arg in [list]
do
command(s)...
done
During each pass through the loop, arg takes on the value of each successive variable
in the list.
for arg in "$var1" "$var2" "$var3" ... "$varN"
# In pass 1 of the loop, $arg = $var1
# In pass 2 of the loop, $arg = $var2
# In pass 3 of the loop, $arg = $var3
# ...
# In pass N of the 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 10−1. Simple for loops
#!/bin/bash
# List the planets.
for planet in Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
do
echo $planet
done
echo
# Entire 'list' enclosed in quotes creates a single variable.
Chapter 10. Loops and Branches 103
for planet in "Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto"
do
echo $planet
done
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
11−13) to force parsing of each [list] element and assignment of each component
to the positional parameters.
Example 10−2. 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 $2,000,000 miles from the sun"
#−−−−−−−two tabs−−−concatenate zeroes onto parameter $2
done
# (Thanks, S.C., for additional clarification.)
exit 0
A variable may supply the [list] in a for loop.
Example 10−3. Fileinfo: operating on a file list contained in a variable
#!/bin/bash
# fileinfo.sh
FILES="/usr/sbin/privatepw
/usr/sbin/pwck
/usr/sbin/go500gw
/usr/bin/fakefile
/sbin/mkreiserfs
/sbin/ypbind" # List of files you are curious about.
# Threw in a dummy file, /usr/bin/fakefile.
echo
for file in $FILES
do
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Chapter 10. Loops and Branches 104
if [ ! −e "$file" ] # Check if file exists.
then
echo "$file does not exist."; echo
continue # On to next.
fi
ls −l $file | awk '{ print $9 " file size: " $5 }' # Print 2 fields.
whatis `basename $file` # File info.
echo
done
exit 0
The [list] in a for loop may contain filename globbing, that is, using wildcards for filename
expansion.
Example 10−4. Operating on files with a for loop
#!/bin/bash
# list−glob.sh: Generating [list] in a for−loop using "globbing".
echo
for file in *
do
ls −l "$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
exit 0
Omitting the in [list] part of a for loop causes the loop to operate on $@, the list of arguments
given on the command line to the script. A particularly clever illustration of this is Example A−17.
Example 10−5. Missing in [list] in a for loop
#!/bin/bash
# Invoke both with and without arguments, and see what happens.
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Chapter 10. Loops and Branches 105
for a
do
echo −n "$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
12−39, Example 10−10 and Example 12−33.
Example 10−6. Generating the [list] in a for loop with command substitution
#!/bin/bash
# A for−loop with [list] generated by command substitution.
NUMBERS="9 7 3 8 37.53"
for number in `echo $NUMBERS` # for number in 9 7 3 8 37.53
do
echo −n "$number "
done
echo
exit 0
This is a somewhat more complex example of using command substitution to create the [list].
Example 10−7. 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
E_NOFILE=66
if [ $# −ne 2 ]
then
echo "Usage: `basename $0` string filename"
exit $E_BADARGS
fi
if [ ! −f "$2" ]
then
echo "File \"$2\" does not exist."
exit $E_NOFILE
fi
for word in $( strings "$2" | grep "$1" )
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# 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, the above for−loop 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 10−8. 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 #1 = root
# USER #2 = bin
# USER #3 = daemon
# ...
# USER #30 = bozo
exit 0
A final example of the [list] resulting from command substitution.
Example 10−9. Checking all the binaries in a directory for authorship
#!/bin/bash
# findstring.sh:
# Find a particular string in 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,
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Chapter 10. Loops and Branches 107
#+ 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 0
# Exercise (easy):
# −−−−−−−−−−−−−−−
# Convert this script to taking command−line parameters
#+ for $directory and $fstring.
The output of a for loop may be piped to a command or commands.
Example 10−10. Listing the symbolic links in a directory
#!/bin/bash
# symlinks.sh: Lists symbolic links in a directory.
directory=${1−`pwd`}
# 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.
# As Dominik 'Aeneas' Schnitzer points out,
#+ failing to quote $( find $directory −type l )
#+ will choke on filenames with embedded whitespace.
# Even this will only pick up the first field of each argument.
exit 0
The stdout of a loop may be redirected to a file, as this slight modification to the previous example
shows.
Example 10−11. Symbolic links in a directory, saved to a file
#!/bin/bash
# symlinks.sh: Lists symbolic links in a directory.
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Chapter 10. Loops and Branches 108
OUTFILE=symlinks.list # save file
directory=${1−`pwd`}
# Defaults to current working directory,
#+ if not otherwise specified.
echo "symbolic links in directory \"$directory\"" > "$OUTFILE"
echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−" >> "$OUTFILE"
for file in "$( find $directory −type l )" # −type l = symbolic links
do
echo "$file"
done | sort >> "$OUTFILE" # stdout of loop
# ^^^^^^^^^^^^^ redirected to save file.
exit 0
There is an alternative syntax to a for loop that will look very familiar to C programmers. This
requires double parentheses.
Example 10−12. A C−like for loop
#!/bin/bash
# Two ways to count up to 10.
echo
# Standard syntax.
for a in 1 2 3 4 5 6 7 8 9 10
do
echo −n "$a "
done
echo; echo
# +==========================================+
# Now, let's do the same, using C−like syntax.
LIMIT=10
for ((a=1; a <= LIMIT ; a++)) # Double parentheses, and "LIMIT" with no "$".
do
echo −n "$a "
done # 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++)) # The comma chains together operations.
do
echo −n "$a−$b "
done
echo; echo
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Chapter 10. Loops and Branches 109
exit 0
See also Example 26−10, Example 26−11, and Example A−7.
−−−
Now, a for−loop used in a "real−life" context.
Example 10−13. Using efax in batch mode
#!/bin/bash
EXPECTED_ARGS=2
E_BADARGS=65
if [ $# −ne $EXPECTED_ARGS ]
# Check for proper no. of command line args.
then
echo "Usage: `basename $0` phone# text−file"
exit $E_BADARGS
fi
if [ ! −f "$2" ]
then
echo "File $2 is not a text file"
exit $E_BADARGS
fi
fax make $2 # Create fax formatted files from text files.
for file in $(ls $2.0*) # Concatenate the converted files.
# Uses wild card in variable list.
do
fil="$fil $file"
done
efax −d /dev/ttyS3 −o1 −t "T$1" $fil # Do the work.
# As S.C. points out, the for−loop can be eliminated with
# efax −d /dev/ttyS3 −o1 −t "T$1" $2.0*
# but it's not quite as instructive [grin].
exit 0
while This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is
true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the
number of loop repetitions is not known beforehand.
while [condition]
do
command...
done
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Chapter 10. Loops and Branches 110
As is the case with for/in loops, placing the do on the same line as the condition test requires a
semicolon.
while [condition] ; do
Note that certain specialized while loops, as, for example, a getopts construct, deviate somewhat from
the standard template given here.
Example 10−14. Simple while loop
#!/bin/bash
var0=0
LIMIT=10
while [ "$var0" −lt "$LIMIT" ]
do
echo −n "$var0 " # −n suppresses newline.
var0=`expr $var0 + 1` # var0=$(($var0+1)) also works.
done
echo
exit 0
Example 10−15. Another while loop
#!/bin/bash
echo
while [ "$var1" != "end" ] # while test "$var1" != "end"
do # also works.
echo "Input variable #1 (end to exit) "
read var1 # Not 'read $var1' (why?).
echo "variable #1 = $var1" # Need quotes because of "#".
# If input is 'end', echoes it here.
# Does not test for termination condition until top of loop.
echo
done
exit 0
A while loop may have multiple conditions. Only the final condition determines when the loop
terminates. This necessitates a slightly different loop syntax, however.
Example 10−16. while loop with multiple conditions
#!/bin/bash
var1=unset
previous=$var1
while echo "previous−variable = $previous"
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Chapter 10. Loops and Branches 111
echo
previous=$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 = $var1"
done
# Try to figure out how this all works.
# It's a wee bit tricky.
exit 0
As with a for loop, a while loop may employ C−like syntax by using the double parentheses construct
(see also Example 9−28).
Example 10−17. C−like syntax in a while loop
#!/bin/bash
# wh−loopc.sh: Count to 10 in a "while" loop.
LIMIT=10
a=1
while [ "$a" −le $LIMIT ]
do
echo −n "$a "
let "a+=1"
done # No surprises, so far.
echo; echo
# +=================================================================+
# Now, 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, and no "$" preceding variables.
do
echo −n "$a "
((a += 1)) # let "a+=1"
# Yes, indeed.
# Double parentheses permit incrementing a variable with C−like syntax.
done
echo
# Now, C programmers can feel right at home in Bash.
exit 0
A while loop may have its stdin redirected to a file by a < at its end.
until
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Chapter 10. Loops and Branches 112
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...
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/in loops, placing the do on the same line as the condition test requires a
semicolon.
until [condition−is−true] ; do
Example 10−18. until loop
#!/bin/bash
until [ "$var1" = end ] # Tests condition here, at top of loop.
do
echo "Input variable #1 "
echo "(end to exit)"
read var1
echo "variable #1 = $var1"
done
exit 0
10.2. Nested Loops
A nested loop is a loop within a loop, an inner loop within the body of an outer one. What happens 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 may interrupt this process.
Example 10−19. Nested Loop
#!/bin/bash
# Nested "for" loops.
outer=1 # Set outer loop counter.
# 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.
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Chapter 10. Loops and Branches 113
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 in pass of outer loop.
done
# End of outer loop.
exit 0
See Example 26−6 for an illustration of nested "while" loops, and Example 26−8 to see a "while" loop nested
inside an "until" loop.
10.3. Loop Control
Commands Affecting Loop Behavior
break, continue
The break and continue loop control commands [23] 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 10−20. 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" ]
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 "$a "
done
# Exercise:
# Why does loop print up to 20?
echo; echo
echo Printing Numbers 1 through 20, but something happens after 2.
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 114
##################################################################
# Same loop, but substituting 'break' for 'continue'.
a=0
while [ "$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 10−21. 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 "
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.
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 115
Example 10−22. 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 # outer loop
do
echo; echo −n "Group $outer: "
for inner in 1 2 3 4 5 6 7 8 9 10 # inner loop
do
if [ "$inner" −eq 7 ]
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 "$inner " # 8 9 10 will never echo.
done
done
echo; echo
# Exercise:
# Come up with a meaningful use for "continue N" in a script.
exit 0
Example 10−23. Using "continue N" in an actual task
# Albert Reiner gives an example of how to use "continue N":
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# Suppose I have a large number of jobs that need to be run, with
#+ any data that is to be treated in files of a given name pattern in a
#+ directory. There are several machines that access this directory, and
#+ I want to distribute the work over these different boxen. Then I
#+ usually nohup something like the following on every box:
while true
do
for n in .iso.*
do
[ "$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
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 116
done
# 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.
10.4. 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 equivalent of 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...
;;
"$condition2" )
command...
;;
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 117
esac
Quoting the variables is not mandatory, since word splitting does not take
place.
◊
Each test line ends with a right paren ).◊ Each condition block ends with a double semicolon ;;.◊ The entire case block terminates with an esac (case spelled backwards).◊
Example 10−24. Using case
#!/bin/bash
echo; echo "Hit a key, then hit return."
read Keypress
case "$Keypress" in
[a−z] ) echo "Lowercase letter";;
[A−Z] ) echo "Uppercase letter";;
[0−9] ) echo "Digit";;
* ) echo "Punctuation, whitespace, or other";;
esac # Allows ranges of characters in [square brackets].
# Exercise:
# −−−−−−−−
# As the script stands, # it accepts a single keystroke, then terminates.
# Change the script so it accepts continuous input,
# reports on each keystroke, and terminates only when "X" is hit.
# Hint: enclose everything in a "while" loop.
exit 0
Example 10−25. Creating menus using case
#!/bin/bash
# Crude address database
clear # Clear the screen.
echo " Contact List"
echo " −−−−−−− −−−−"
echo "Choose one of the following persons:"
echo
echo "[E]vans, Roland"
echo "[J]ones, Mildred"
echo "[S]mith, Julie"
echo "[Z]ane, Morris"
echo
read person
case "$person" in
# Note variable is quoted.
"E" | "e" )
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 118
# Accept upper or lowercase input.
echo
echo "Roland Evans"
echo "4321 Floppy 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 "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
echo
# Exercise:
# −−−−−−−−
# Change the script so it accepts continuous input,
#+ 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##*/} <filename>"; exit 65;; # 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;; # Otherwise, $1.
esac
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 119
Example 10−26. Using command substitution to generate the case variable
#!/bin/bash
# Using command substitution to generate a "case" variable.
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
exit 0
A case construct can filter strings for globbing patterns.
Example 10−27. Simple string matching
#!/bin/bash
# match−string.sh: simple string matching
match_string ()
{
MATCH=0
NOMATCH=90
PARAMS=2 # Function requires 2 arguments.
BAD_PARAMS=91
[ $# −eq $PARAMS ] || return $BAD_PARAMS
case "$1" in
"$2") return $MATCH;;
* ) return $NOMATCH;;
esac
}
a=one
b=two
c=three
d=two
match_string $a # wrong number of parameters
echo $? # 91
match_string $a $b # no match
echo $? # 90
match_string $b $d # match
echo $? # 0
exit 0
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 120
Example 10−28. 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."
fi
else
echo "\"$*\" begins with a non−alpha character."
# Also "non−alpha" if no argument passed.
fi
echo
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 121
}
digit_check () # Front−end to isdigit ().
{
if isdigit "$@"
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` # Command substitution.
f=AbcDef
g=27234
h=27a34
i=27.34
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
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 that this may be changed.
Example 10−29. Creating menus using select
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 122
#!/bin/bash
PS3='Choose your favorite vegetable: ' # Sets the prompt string.
echo
select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas"
do
echo
echo "Your favorite veggie is $vegetable."
echo "Yuck!"
echo
break # if no 'break' here, keeps looping forever.
done
exit 0
If in list is omitted, then select uses the list of command line arguments ($@) passed to the script
or to the function in which the select construct is embedded.
Compare this to the behavior of a
for variable [in list]
construct with the in list omitted.
Example 10−30. 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."
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 35−3.
Advanced Bash−Scripting Guide
Chapter 10. Loops and Branches 123
Chapter 11. 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 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.
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.
#!/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 by itself a command,
but part of a larger command structure. [24]
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.
Chapter 11. Internal Commands and Builtins 124
a=`echo "HELLO" | tr A−Z a−z`
See also Example 12−15, Example 12−2, Example 12−32, and Example 12−33.
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 362 Nov 7 2000 seconds.au
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.
Older versions of Bash may not support printf.
Example 11−1. printf in action
#!/bin/bash
# printf demo
PI=3.14159265358979
DecimalConstant=31373
Message1="Greetings,"
Message2="Earthling."
echo
printf "Pi to 2 decimal places = %1.2f" $PI
echo
Advanced Bash−Scripting Guide
Chapter 11. Internal Commands and Builtins 125
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"
Msg=`printf "%s %s \n" $Message1 $Message2`
echo $Msg; echo $Msg
# As it happens, the 'sprintf' function can now be accessed
# as a loadable module to Bash, but this is not portable.
exit 0
Formatting error messages is a useful application of printf
E_BADDIR=65
var=nonexistent_directory
error()
{
printf "$@" >&2
# Formats positional params passed, and sents them to stderr.
echo
exit $E_BADDIR
}
cd $var || error $"Can't cd to %s." "$var"
# Thanks, S.C.
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 26−3).
Example 11−2. Variable assignment, using read
#!/bin/bash
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.
Advanced Bash−Scripting Guide
Chapter 11. Internal Commands and Builtins 126
echo "var1 = $var1"
echo
# A single 'read' statement can set multiple variables.
echo −n "Enter the values of variables 'var2' and 'var3' (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 $REPLY.
Example 11−3. What happens when read has no variable
#!/bin/bash
echo
# −−−−−−−−−−−−−−−−−−−−−−−−−− #
# First code block.
echo −n "Enter a value: "
read var
echo "\"var\" = "$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.
echo
exit 0
Normally, inputting a \ suppresses a newline during input to a read. The −r option causes an
inputted \ to be interpreted literally.
Example 11−4. Multi−line input to read
#!/bin/bash
echo
echo "Enter a string terminated by a \\, then press <ENTER>."
echo "Then, enter a second string, and again press <ENTER>."
read var1 # The "\" suppresses the newline, when reading "var1".
# first line \
# second line
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Chapter 11. Internal Commands and Builtins 127
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 <ENTER>."
read −r var2 # The −r option causes the "\" to be read literally.
# first line \
echo "var2 = $var2"
# var2 = first line \
# Data entry terminates with the first <ENTER>.
echo
exit 0
The read command has some interesting options that permit echoing a prompt and even reading
keystrokes without hitting ENTER.
# Read a keypress without hitting ENTER.
read −s −n1 −p "Hit a key " keypress
echo; echo "Keypress was "\"$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 11−5. 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... "
Advanced Bash−Scripting Guide
Chapter 11. Internal Commands and Builtins 128
# May need to also press ENTER if a key not listed above pressed.
read −n3 key # Read 3 characters.
echo −n "$key" | grep "$arrowup" #Check if character code detected.
if [ "$?" −eq $SUCCESS ]
then
echo "Up−arrow key pressed."
exit $SUCCESS
fi
echo −n "$key" | grep "$arrowdown"
if [ "$?" −eq $SUCCESS ]
then
echo "Down−arrow key pressed."
exit $SUCCESS
fi
echo −n "$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
# Exercises:
# −−−−−−−−−
# 1) Simplify this script by rewriting the multiple "if" tests
#+ as a 'case' construct.
# 2) Add detection of the "Home," "End," "PgUp," and "PgDn" keys.
The −t option to read permits timed input (see Example 9−4).
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
Advanced Bash−Scripting Guide
Chapter 11. Internal Commands and Builtins 129
parameter, then each of these variables gets assigned a successive whitespace−delineated string.
Caution!
Example 11−6. Using read with file redirection
#!/bin/bash
read var1 <data−file
echo "var1 = $var1"
# var1 set to the entire first line of the input file "data−file"
read var2 var3 <data−file
echo "var2 = $var2 var3 = $var3"
# Note non−intuitive behavior of "read" here.
# 1) Rewinds back to the beginning of input file.
# 2) Each variable is now set to a corresponding string,
# separated by whitespace, rather than to an entire line of text.
# 3) The final variable gets the remainder of the line.
# 4) If there are more variables to be set than whitespace−terminated strings
# on the first line of the file, then the excess variables remain empty.
echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−"
# How to resolve the above problem with a loop:
while read line
do
echo "$line"
done <data−file
# Thanks, Heiner Steven for pointing this out.
echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−"
# Use $IFS (Internal File Separator variable) to split a line of input to
# "read", if you do not want the default to be whitespace.
echo "List of all users:"
OIFS=$IFS; IFS=: # /etc/passwd uses ":" for field separator.
while read name passwd uid gid fullname ignore
do
echo "$name ($fullname)"
done </etc/passwd # I/O redirection.
IFS=$OIFS # Restore originial $IFS.
# This code snippet also by Heiner Steven.
# Setting the $IFS variable within the loop itself
#+ eliminates the need for storing the original $IFS
#+ in a temporary variable.
# Thanks, Dim Segebart, for pointing this out.
echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−"
echo "List of all users:"
while IFS=: read name passwd uid gid fullname ignore
do
echo "$name ($fullname)"
done </etc/passwd # I/O redirection.
echo
echo "\$IFS still $IFS"
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Chapter 11. Internal Commands and Builtins 130
exit 0
Piping output to a read, using echo to set variables will fail.
However, piping the output of cat does seem to work.
cat file1 file2 |
while read line
do
echo $line
done
Filesystem
cd The familiar cd change directory command finds use in scripts where execution of a command
requires being in a specified directory.
(cd /source/directory && tar cf − . ) | (cd /dest/directory && tar xpvf −)
[from the previously cited example by Alan Cox]
The −P (physical) option to cd causes it to ignore symbolic links.
cd − changes to $OLDPWD, the previous working directory.
The cd command does not function as expected when presented with two forward
slashes.
bash$ cd //
bash$ pwd
//
The output should, of course, be /. This is a problem both from the command line and
in a script.
pwd Print Working Directory. This gives the user's (or script's) current directory (see Example 11−7). The
effect is identical to reading the value of the builtin variable $PWD.
pushd, popd, dirs
This command set is a mechanism for bookmarking working directories, a means of moving back and
forth through directories in an orderly manner. A pushdown stack is used to keep track of directory
names. Options allow various manipulations of the directory stack.
pushd dir−name pushes the path dir−name onto the directory stack and simultaneously
changes the current working directory to dir−name
popd removes (pops) the top directory path name off the directory stack and simultaneously changes
the current working directory to that directory popped from the stack.
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Chapter 11. Internal Commands and Builtins 131
dirs lists the contents of the directory stack (compare this with the $DIRSTACK variable). A
successful pushd or popd will automatically invoke dirs.
Scripts that require various changes to the current working directory without hard−coding the
directory name changes can make good use of these commands. Note that the implicit $DIRSTACK
array variable, accessible from within a script, holds the contents of the directory stack.
Example 11−7. Changing the current working directory
#!/bin/bash
dir1=/usr/local
dir2=/var/spool
pushd $dir1
# Will do an automatic 'dirs' (list directory stack to stdout).
echo "Now in directory `pwd`." # Uses back−quoted 'pwd'.
# Now, do some stuff in directory 'dir1'.
pushd $dir2
echo "Now in directory `pwd`."
# Now, do some stuff in directory 'dir2'.
echo "The top entry in the DIRSTACK array is $DIRSTACK."
popd
echo "Now back in directory `pwd`."
# Now, do some more stuff in directory 'dir1'.
popd
echo "Now back in original working directory `pwd`."
exit 0
Variables
let The let command carries out arithmetic operations on variables. In many cases, it functions as a less
complex version of expr.
Example 11−8. Letting let do some arithmetic.
#!/bin/bash
echo
let a=11 # Same as 'a=11'
let a=a+5 # Equivalent to let "a = a + 5"
# (double quotes and spaces make it more readable)
echo "11 + 5 = $a"
let "a <<= 3" # Equivalent to let "a = a << 3"
echo "\"\$a\" (=16) left−shifted 3 places = $a"
let "a /= 4" # Equivalent to let "a = a / 4"
echo "128 / 4 = $a"
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let "a −= 5" # Equivalent to let "a = a − 5"
echo "32 − 5 = $a"
let "a = a * 10" # Equivalent to let "a = a * 10"
echo "27 * 10 = $a"
let "a %= 8" # Equivalent to let "a = a % 8"
echo "270 modulo 8 = $a (270 / 8 = 33, remainder $a)"
echo
exit 0
eval
eval arg1 [arg2] ... [argN]
Translates into commands the arguments in a list (useful for code generation within a script).
Example 11−9. Showing the effect of eval
#!/bin/bash
y=`eval ls −l` # Similar to y=`ls −l`
echo $y # but linefeeds removed because "echoed" variable is unquoted.
echo
echo "$y" # Linefeeds preserved when variable is quoted.
echo; echo
y=`eval df` # Similar to y=`df`
echo $y # but linefeeds removed.
# When LF's not preserved, it may make it easier to parse output,
#+ using utilities such as "awk".
exit 0
Example 11−10. Forcing a log−off
#!/bin/bash
y=`eval ps ax | sed −n '/ppp/p' | awk '{ print $1 }'`
# Finding the process number of 'ppp'.
kill −9 $y # Killing it
# Above lines may be replaced by
# kill −9 `ps ax | awk '/ppp/ { print $1 }'
chmod 666 /dev/ttyS3
# Doing a SIGKILL on ppp changes the permissions
# on the serial port. Restore them to previous state.
rm /var/lock/LCK..ttyS3 # Remove the serial port lock file.
exit 0
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Example 11−11. A version of "rot13"
#!/bin/bash
# A version of "rot13" using 'eval'.
# Compare to "rot13.sh" example.
setvar_rot_13() # "rot13" scrambling
{
local varname=$1 varvalue=$2
eval $varname='$(echo "$varvalue" | tr a−z n−za−m)'
}
setvar_rot_13 var "foobar" # Run "foobar" through rot13.
echo $var # sbbone
echo $var | tr a−z n−za−m # foobar
# Back to original variable.
# This example by Stephane Chazelas.
exit 0
Rory Winston contributed the following instance of how useful eval can be.
Example 11−12. Using eval to force variable substitution in a Perl script
In the Perl script "test.pl":
...
my $WEBROOT = <WEBROOT_PATH>;
...
To force variable substitution try:
$export WEBROOT_PATH=/usr/local/webroot
$sed 's/<WEBROOT_PATH>/$WEBROOT_PATH/' < test.pl > out
But this just gives:
my $WEBROOT = $WEBROOT_PATH;
However:
$export WEBROOT_PATH=/usr/local/webroot
$eval sed 's/<WEBROOT_PATH>/$WEBROOT_PATH/' < test.pl > out
# ====
That works fine, and gives the expected substitution:
my $WEBROOT = /usr/local/webroot
The eval command can be risky, and normally should be avoided when there exists a
reasonable alternative. An eval $COMMANDS executes the contents of COMMANDS,
which may contain such unpleasant surprises as rm −rf *. Running an eval on
unfamiliar code written by persons unknown is living dangerously.
set The set command changes the value of internal script variables. One use for this is to toggle option
flags which help determine the behavior of the script. Another application for it is to reset the
positional parameters that a script sees as the result of a command (set `command`). The script
can then parse the fields of the command output.
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Example 11−13. Using set with positional parameters
#!/bin/bash
# script "set−test"
# Invoke this script with three command line parameters,
# for example, "./set−test one two three".
echo
echo "Positional parameters before set \`uname −a\` :"
echo "Command−line argument #1 = $1"
echo "Command−line argument #2 = $2"
echo "Command−line argument #3 = $3"
set `uname −a` # Sets the positional parameters to the output
# of the command `uname −a`
echo $_ # unknown
# Flags set in script.
echo "Positional parameters after set \`uname −a\` :"
# $1, $2, $3, etc. reinitialized to result of `uname −a`
echo "Field #1 of 'uname −a' = $1"
echo "Field #2 of 'uname −a' = $2"
echo "Field #3 of 'uname −a' = $3"
echo −−−
echo $_ # −−−
echo
exit 0
Invoking set without any options or arguments simply lists all the environmental and other variables
that have been initialized.
bash$ set
AUTHORCOPY=/home/bozo/posts
BASH=/bin/bash
BASH_VERSION=$'2.05.8(1)−release'
...
XAUTHORITY=/home/bozo/.Xauthority
_=/etc/bashrc
variable22=abc
variable23=xzy
Using set with the −− option explicitly assigns the contents of a variable to the positional parameters.
When no variable follows the −−, it unsets the positional parameters.
Example 11−14. Reassigning the positional parameters
#!/bin/bash
variable="one two three four five"
set −− $variable
# Sets positional parameters to the contents of "$variable".
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first_param=$1
second_param=$2
shift; shift # Shift past first two positional params.
remaining_params="$*"
echo
echo "first parameter = $first_param" # one
echo "second parameter = $second_param" # two
echo "remaining parameters = $remaining_params" # three four five
echo; echo
# Again.
set −− $variable
first_param=$1
second_param=$2
echo "first parameter = $first_param" # one
echo "second parameter = $second_param" # two
# ======================================================
set −−
# Unsets positional parameters if no variable specified.
first_param=$1
second_param=$2
echo "first parameter = $first_param" # (null value)
echo "second parameter = $second_param" # (null value)
exit 0
See also Example 10−2 and Example 12−40.
unset The unset command deletes a shell variable, effectively setting it to null. Note that this command
does not affect positional parameters.
bash$ unset PATH
bash$ echo $PATH
bash$
Example 11−15. "unsetting" a variable
#!/bin/bash
# unset.sh: Unsetting a variable.
variable=hello # Initialized.
echo "variable = $variable"
unset variable # Unset.
# Same effect as variable=
echo "(unset) variable = $variable" # $variable is null.
exit 0
export
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The export command makes available variables to all child processes of the running script or shell.
Unfortunately, there is no way to export variables back to the parent process, to the process that
called or invoked the script or shell. One important use of export command is in startup files, to
initialize and make accessible environmental variables to subsequent user processes.
Example 11−16. Using export to pass a variable to an embedded awk script
#!/bin/bash
# Yet another version of the "column totaler" script (col−totaler.sh)
# that adds up a specified column (of numbers) in the target file.
# This uses the environment to pass a script variable to 'awk'.
ARGS=2
E_WRONGARGS=65
if [ $# −ne "$ARGS" ] # Check for proper no. of command line args.
then
echo "Usage: `basename $0` filename column−number"
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
#===== Same as original script, up to this point =====#
export column_number
# Export column number to environment, so it's available for retrieval.
# Begin awk script.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
awk '{ total += $ENVIRON["column_number"]
}
END { print total }' $filename
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# End awk script.
# Thanks, Stephane Chazelas.
exit 0
It is possible to initialize and export variables in the same operation, as in export
var1=xxx.
However, as Greg Keraunen points out, in certain situations this may have a different
effect than setting a variable, then exporting it.
bash$ export var=(a b); echo ${var[0]}
(a b)
bash$ var=(a b); export var; echo ${var[0]}
a
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declare, typeset
The declare and typeset commands specify and/or restrict properties of variables.
readonly
Same as declare −r, sets a variable as read−only, or, in effect, as a constant. Attempts to change the
variable fail with an error message. This is the shell analog of the C language const type qualifier.
getopts This powerful tool parses command−line arguments passed to the script. This is the Bash analog of
the getopt external command and the getopt library function familiar to C programmers. It permits
passing and concatenating multiple options [25] and associated arguments to a script (for example
scriptname −abc −e /usr/local).
The getopts construct uses two implicit variables. $OPTIND is the argument pointer (OPTion INDex)
and $OPTARG (OPTion ARGument) the (optional) argument attached to an option. A colon following
the option name in the declaration tags that option as having an associated argument.
A getopts construct usually comes packaged in a while loop, which processes the options and
arguments one at a time, then decrements the implicit $OPTIND variable to step to the next.
The arguments passed from the command line to the script must be preceded
by a minus (−) or a plus (+). It is the prefixed − or + that lets getopts
recognize command−line arguments as options. In fact, getopts will not
process arguments without the prefixed − or +, and will terminate option
processing at the first argument encountered lacking them.
1.
The getopts template differs slightly from the standard while loop, in that it
lacks condition brackets.
2.
The getopts construct replaces the obsolete and less powerful getopt external
command.
3.
while getopts ":abcde:fg" Option
# Initial declaration.
# a, b, c, d, e, f, and g are the options (flags) expected.
# The : after option 'e' shows it will have an argument passed with it.
do
case $Option in
a ) # Do something with variable 'a'.
b ) # Do something with variable 'b'.
...
e) # Do something with 'e', and also with $OPTARG,
# which is the associated argument passed with option 'e'.
...
g ) # Do something with variable 'g'.
esac
done
shift $(($OPTIND − 1))
# Move argument pointer to next.
# All this is not nearly as complicated as it looks <grin>.
Example 11−17. Using getopts to read the options/arguments passed to a script
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Chapter 11. Internal Commands and Builtins 138
#!/bin/bash
# '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
# If an option expects an argument ("flag:"), then it will grab
# whatever is next on the command line.
NO_ARGS=0
E_OPTERROR=65
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, if no argument(s) given.
fi
# Usage: scriptname −options
# Note: dash (−) necessary
while getopts ":mnopq:rs" Option
do
case $Option in
m ) echo "Scenario #1: option −m−";;
n | o ) echo "Scenario #2: option −$Option−";;
p ) echo "Scenario #3: option −p−";;
q ) echo "Scenario #4: option −q−, with argument \"$OPTARG\"";;
# Note that option 'q' must have an associated argument,
# otherwise it falls through to the default.
r | s ) echo "Scenario #5: option −$Option−"'';;
* ) echo "Unimplemented option chosen.";; # DEFAULT
esac
done
shift $(($OPTIND − 1))
# Decrements the argument pointer so it points to next argument.
exit 0
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. This is the shell scripting equivalent of a C/C++
#include directive. It is useful in situations when multiple scripts use a common data file or
function library.
Example 11−18. "Including" a data file
#!/bin/bash
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Chapter 11. Internal Commands and Builtins 139
. 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 11−18, 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."
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 "$1"
# Echo args one at a time, suppressing line feeds.
echo −n " "
# Insert spaces between words.
shift
# Next one.
done
echo
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Chapter 11. Internal Commands and Builtins 140
return 0
}
It is even possible for a script to source itself, though this does not seem to have any practical
applications.
Example 11−19. A (useless) script that sources itself
#!/bin/bash
# self−source.sh: a script sourcing itself "recursively."
# From "Stupid Script Tricks," Volume II.
MAXPASSCNT=100 # Maximum number of execution passes.
echo −n "$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 set $pass_count to 0 if non−initialized.
while [ "$pass_count" −le $MAXPASSCNT ]
do
. $0 # Script "sources" itself, rather than calling itself.
# ./$0 (which would be true recursion) doesn't work here.
done
# What occurs here is not actually recursion,
#+ since the script effectively "expands" itself
#+ (generates a new section of code)
#+ with each pass throught 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 do something useful.
exit Unconditionally terminates a script. 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.
exec
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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. For this reason, if an
exec appears in a script, it would probably be the final command.
Example 11−20. 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 11−21. A script that exec's itself
#!/bin/bash
# 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 0
An exec also serves to reassign file descriptors. exec <zzz−file replaces stdin with the file
zzz−file (see Example 16−1).
The −exec option to find is not the same as the exec shell builtin.
shopt This command permits changing shell options on the fly (see Example 24−1 and Example 24−2). It
often appears in the Bash startup files, but also has its uses in scripts. Needs version 2 or later of Bash.
shopt −s cdspell
# Allows minor misspelling of directory names with 'cd'
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cd /hpme # Oops! Mistyped '/home'.
pwd # /home
# The shell corrected the misspelling.
Commands
true A command that returns a successful (zero) exit status, but does nothing else.
# Endless loop
while true # alias for ":"
do
operation−1
operation−2
...
operation−n
# Need a way to break out of loop.
done
false A command that returns an unsuccessful exit status, but does nothing else.
# Null loop
while false
do
# The following code will not execute.
operation−1
operation−2
...
operation−n
# Nothing happens!
done
type [cmd]
Similar to the which external command, type cmd gives the full pathname to "cmd". Unlike which,
type is a Bash builtin. The useful −a option to type identifies keywords and builtins, and also
locates system commands with identical names.
bash$ type '['
[ is a shell builtin
bash$ type −a '['
[ is a shell builtin
[ is /usr/bin/[
hash [cmds]
Record the path name of specified commands (in the shell hash table), so the shell or script will not
need to search the $PATH on subsequent calls to those commands. When hash is called with no
arguments, it simply lists the commands that have been hashed. The −r option resets the hash table.
help help COMMAND looks up a short usage summary of the shell builtin COMMAND. This is the
counterpart to whatis, but for builtins.
bash$ help exit
exit: exit [n]
Exit the shell with a status of N. If N is omitted, the exit status
is that of the last command executed.
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11.1. Job Control Commands
Certain of the following job control commands take a "job identifier" as an argument. See the table at end of
the chapter.
jobs Lists the jobs running in the background, giving the job number. Not as useful as ps.
It is all too easy to confuse jobs and processes. Certain builtins, such as kill, disown,
and wait accept either a job number or a process number as an argument. The fg, bg
and jobs commands accept only a job number.
bash$ sleep 100 &
[1] 1384
bash $ jobs
[1]+ Running sleep 100 &
"1" is the job number (jobs are maintained by the current shell), and "1384" is the
process number (processes are maintained by the system). To kill this job/process,
either a kill %1 or a kill 1384 works.
Thanks, S.C.
disown Remove job(s) from the shell's table of active jobs.
fg, bg The fg command switches a job running in the background into the foreground. The bg command
restarts a suspended job, and runs it in the background. If no job number is specified, then the fg or bg
command acts upon the currently running job.
wait Stop script execution until all jobs running in background have terminated, or until the job number or
process id specified as an option terminates. Returns the exit status of waited−for command.
You may use the wait command to prevent a script from exiting before a background job finishes
executing (this would create a dreaded orphan process).
Example 11−22. Waiting for a process to finish before proceeding
#!/bin/bash
ROOT_UID=0 # Only users with $UID 0 have root privileges.
E_NOTROOT=65
E_NOPARAMS=66
if [ "$UID" −ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
# "Run along kid, it's past your bedtime."
exit $E_NOTROOT
fi
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if [ −z "$1" ]
then
echo "Usage: `basename $0` find−string"
exit $E_NOPARAMS
fi
echo "Updating 'locate' database..."
echo "This may take a while."
updatedb /usr & # Must be run as root.
wait
# Don't run the rest of the script until 'updatedb' finished.
# You want the the database updated before looking up the file name.
locate $1
# Without the wait command, in the worse case scenario,
# the script would exit while 'updatedb' was still running,
# leaving it as an orphan process.
exit 0
Optionally, wait can take a job identifier as an argument, for example, wait%1 or wait $PPID. See
the job id table.
Within a script, running a command in the background with an ampersand (&) may
cause the script to hang until ENTER is hit. This seems to occur with commands that
write to stdout. It can be a major annoyance.
#!/bin/bash
# test.sh
ls −l &
echo "Done."
bash$ ./test.sh
Done.
[bozo@localhost test−scripts]$ total 1
−rwxr−xr−x 1 bozo bozo 34 Oct 11 15:09 test.sh
_
Placing a wait after the background command seems to remedy this.
#!/bin/bash
# test.sh
ls −l &
echo "Done."
wait
bash$ ./test.sh
Done.
[bozo@localhost test−scripts]$ total 1
−rwxr−xr−x 1 bozo bozo 34 Oct 11 15:09 test.sh
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Redirecting the output of the command to a file or even to /dev/null also takes
care of this problem.
suspendThis has a similar effect to Control−Z, but it suspends the shell (the shell's parent process should
resume it at an appropriate time).
logout Exit a login shell, optionally specifying an exit status.
times Gives statistics on the system time used in executing commands, in the following form:
0m0.020s 0m0.020s
This capability is of very limited value, since it is uncommon to profile and benchmark shell scripts.
kill Forcibly terminate a process by sending it an appropriate terminate signal (see Example 13−4).
Example 11−23. A script that kills itself
#!/bin/bash
# self−destruct.sh
kill $$ # Script kills its own process here.
# Recall that "$$" is the script's PID.
echo "This line will not echo."
# Instead, the shell sends a "Terminated" message to stdout.
exit 0
# After this script terminates prematurely,
#+ what exit status does it return?
#
# sh self−destruct.sh
# echo $?
# 143
#
# 143 = 128 + 15
# TERM signal
kill −l lists all the signals. A kill −9 is a "sure kill", which will usually
terminate a process that stubbornly refuses to die with a plain kill. Sometimes, a kill
−15 works. A "zombie process", that is, a process whose parent has terminated,
cannot be killed (you can't kill something that is already dead), but init will usually
clean it up sooner or later.
command
The command COMMAND directive disables aliases and functions for the command
"COMMAND".
This is one of three shell directives that effect script command processing. The others
are builtin and enable.
builtin Invoking builtin BUILTIN_COMMAND runs the command "BUILTIN_COMMAND" as a shell
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Chapter 11. Internal Commands and Builtins 146
builtin, temporarily disabling both functions and external system commands with the same name.
enable This either enables or disables a shell builtin command. As an example, enable −n kill disables the
shell builtin kill, so that when Bash subsequently encounters kill, it invokes /bin/kill.
The −a option to enable lists all the shell builtins, indicating whether or not they are enabled. The −f
filename option lets enable load a builtin as a shared library (DLL) module from a properly
compiled object file. [26].
autoload
This is a port to Bash of the ksh autoloader. With autoload in place, a function with an "autoload"
declaration will load from an external file at its first invocation. [27] This saves system resources.
Note that autoload is not a part of the core Bash installation. It needs to be loaded in with enable −f
(see above).
Table 11−1. Job Identifiers
Notation Meaning
%N Job number [N]
%S Invocation (command line) of job begins with string S
%?S Invocation (command line) of job contains within it string S
%% "current" job (last job stopped in foreground or started in background)
%+ "current" job (last job stopped in foreground or started in background)
%− Last job
$! Last background process
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Chapter 11. Internal Commands and Builtins 147
Chapter 12. External Filters, Programs and
Commands
Standard UNIX commands make shell scripts more versatile. The power of scripts comes from coupling
system commands and shell directives with simple programming constructs.
12.1. Basic Commands
The first commands a novice learns
ls The basic file "list" command. It is all too easy to underestimate the power of this humble command.
For example, using the −R, recursive option, ls provides a tree−like listing of a directory structure.
Other interesting options are −S, sort listing by file size, −t, sort by file modification time, and −i,
show file inodes (see Example 12−3).
Example 12−1. Using ls to create a table of contents for burning a CDR disk
#!/bin/bash
# burn−cd.sh
# Script to automate burning a CDR.
SPEED=2 # May use higher speed if your hardware supports it.
IMAGEFILE=cdimage.iso
CONTENTSFILE=contents
DEFAULTDIR=/opt # This is the directory containing the data to be burned.
# Make sure it exists.
# Uses Joerg Schilling's "cdrecord" package.
# (http://www.fokus.gmd.de/nthp/employees/schilling/cdrecord.html)
# If this script invoked as an ordinary user, need to suid cdrecord
#+ (chmod u+s /usr/bin/cdrecord, as root).
if [ −z "$1" ]
then
IMAGE_DIRECTORY=$DEFAULTDIR
# Default directory, if not specified on command line.
else
IMAGE_DIRECTORY=$1
fi
# Create a "table of contents" file.
ls −lRF $IMAGE_DIRECTORY > $IMAGE_DIRECTORY/$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 $IMAGFILE $IMAGE_DIRECTORY
echo "Creating ISO9660 file system image ($IMAGEFILE)."
Chapter 12. External Filters, Programs and Commands 148
# Burn the CDR.
cdrecord −v −isosize speed=$SPEED dev=0,0 $IMAGEFILE
echo "Burning the disk."
echo "Please be patient, this will take a while."
exit 0
cat, tac cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it
is commonly used to concatenate files.
cat filename cat file.1 file.2 file.3 > file.123
The −n option to cat inserts consecutive numbers before all lines of the target file(s). The −b option
numbers only the non−blank lines. The −v option echoes nonprintable characters, using ^ notation.
The −s option squeezes multiple consecutive blank lines into a single blank line.
See also Example 12−21 and Example 12−17.
tac, is the inverse of cat, listing a file backwards from its end.
rev reverses each line of a file, and outputs to stdout. This is not the same effect as tac, as it preserves
the order of the lines, but flips each one around.
bash$ cat file1.txt
This is line 1.
This is line 2.
bash$ tac file1.txt
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 12−5).
Particularly useful are the −a archive flag (for copying an entire directory tree) and
the −r and −R recursive flags.
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 9−17 and Example A−3.
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.
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Chapter 12. External Filters, Programs and Commands 149
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.
When used with the recursive flag −r, this command removes files all the way down
the directory tree.
rmdir Remove directory. The directory must be empty of all files, including invisible "dotfiles", [28] for this
command to succeed.
mkdir Make directory, creates a new directory. 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 (see Example 11−10).
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 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.
chattr Change file attributes. This has the same effect as chmod above, but with a different invocation
syntax, and it works only on an ext2 filesystem.
ln Creates links to pre−existings files. Most often used with the −s, symbolic or "soft" link flag. This
permits referencing the linked file by more than one name and is a superior alternative to aliasing (see
Example 4−6).
ln −s oldfile newfile links the previously existing oldfile to the newly created link,
newfile.
man, info
These commands access the manual and information pages on system commands and installed
utilities. When available, the info pages usually contain a more detailed description than do the man
pages.
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Chapter 12. External Filters, Programs and Commands 150
12.2. Complex Commands
Commands for more advanced users
find −exec COMMAND \;
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). If COMMAND contains {}, then find
substitutes the full path name of the selected file for "{}".
bash$ find ~/ −name '*.txt'
/home/bozo/.kde/share/apps/karm/karmdata.txt
/home/bozo/misc/irmeyc.txt
/home/bozo/test−scripts/1.txt
find /home/bozo/projects −mtime 1
# Lists all files in /home/bozo/projects directory tree
#+ that were modified within the last day.
#
# 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 "$DIR" −type f −atime +5 −exec rm {} \;
# Deletes all files in "/home/bozo/junk_files"
#+ that have not been accessed in at least 5 days.
#
# "−type filetype", where
# f = regular file
# d = directory, etc.
# (The 'find' manpage has a complete listing.)
find /etc −exec grep '[0−9][0−9]*[.][0−9][0−9]*[.][0−9][0−9]*[.][0−9][0−9]*' {} \;
# Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files.
# There a few extraneous hits − how can they be filtered out?
# Perhaps by:
find /etc −type f −exec cat '{}' \; | tr −c '.[:digit:]' '\n' \
| grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$'
# [:digit:] is one of the character classes
# introduced with the POSIX 1003.2 standard.
# Thanks, S.C.
The −exec option to find should not be confused with the exec shell builtin.
Example 12−2. Badname, eliminate file names in current directory containing bad characters
and whitespace.
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#!/bin/bash
# Delete filenames in current directory containing bad characters.
for filename in *
do
badname=`echo "$filename" | sed −n /[\+\{\;\"\\\=\?~\(\)\<\>\&\*\|\$]/p`
# Files containing those nasties: + { ; " \ = ? ~ ( ) < > & * | $
rm $badname 2>/dev/null # So error messages deep−sixed.
done
# Now, take care of files containing all manner of whitespace.
find . −name "* *" −exec rm −f {} \;
# The path name of the file that "find" finds replaces the "{}".
# The '\' ensures that the ';' is interpreted literally, as end of command.
exit 0
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# Commands below this line will not execute because of "exit" command.
# An alternative to the above script:
find . −name '*[+{;"\\=?~()<>&*|$ ]*' −exec rm −f '{}' \;
exit 0
# (Thanks, S.C.)
Example 12−3. 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 # Filename arg must be passed to script.
E_WRONGARGS=70
E_FILE_NOT_EXIST=71
E_CHANGED_MIND=72
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
inum=`ls −i | grep "$1" | awk '{print $1}'`
# inum = inode (index node) number of file
# Every file has an inode, a record that hold 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?"
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Chapter 12. External Filters, Programs and Commands 152
exit $E_CHANGED_MIND
;;
*) echo "Deleting file \"$1\".";;
esac
find . −inum $inum −exec rm {} \;
echo "File "\"$1"\" deleted!"
exit 0
See Example 12−22, Example 3−4, and Example 10−9 for scripts using find. Its manpage provides
more detail on this complex and powerful command.
xargs A filter for feeding arguments to a command, and also a tool for assembling the commands
themselves. It breaks a data stream into small enough chunks for filters and commands to process.
Consider it as a powerful replacement for backquotes. In situations where backquotes fail with a too
many arguments error, substituting xargs often works. Normally, xargs reads from stdin or from a
pipe, but it can also be given the output of a file.
The default command for xargs is echo. This means that input piped to xargs may have linefeeds and
other whitespace characters stripped out.
bash$ ls −l
total 0
−rw−rw−r−− 1 bozo bozo 0 Jan 29 23:58 file1
−rw−rw−r−− 1 bozo bozo 0 Jan 29 23:58 file2
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 29 23:58 file2
ls | xargs −p −l gzip gzips every file in current directory, one at a time, prompting before
each operation.
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.)
Example 12−4. Logfile using xargs to monitor system log
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Chapter 12. External Filters, Programs and Commands 153
#!/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 −$LINES /var/log/messages | xargs | fmt −s >>logfile
echo >>logfile
echo >>logfile
exit 0
# Exercise:
# −−−−−−−−
# Modify this script to track changes in /var/log/messages at intervals
#+ of 20 minutes.
# Hint: Use the "watch" command.
Example 12−5. copydir, copying files in current directory to another, using xargs
#!/bin/bash
# Copy (verbose) all files in current directory
# to directory specified on command line.
if [ −z "$1" ] # Exit if no argument given.
then
echo "Usage: `basename $0` directory−to−copy−to"
exit 65
fi
ls . | xargs −i −t cp ./{} $1
# This is the exact equivalent of
# cp * $1
# unless any of the filenames has "whitespace" characters.
exit 0
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.
expr 3 + 5
returns 8
expr 5 % 3
returns 2
expr 5 \* 3
returns 15
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Chapter 12. External Filters, Programs and Commands 154
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 12−6. 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
x=24
y=25
b=`expr $x = $y` # Test equality.
echo "b = $b" # 0 ( $x −ne $y )
echo
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
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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
# Comparison Operators
# −−−−−−−−−− −−−−−−−−−
echo "Comparison Operators"
echo
a=zipper
echo "a is $a"
if [ `expr $a = snap` ]
# Force re−evaluation of variable 'a'
then
echo "a is not zipper"
fi
echo
echo
# String Operators
# −−−−−− −−−−−−−−−
echo "String Operators"
echo
a=1234zipper43231
echo "The string being operated upon is \"$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 \"$b\"."
# The default behavior of the 'match' operations is to
#+ search for the specified match at the ***beginning*** of the string.
#
# uses Regular Expressions
b=`expr match "$a" '[0−9]*'` # Numerical count.
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Chapter 12. External Filters, Programs and Commands 156
echo Number of digits at the beginning of \"$a\" is $b.
b=`expr match "$a" '\([0−9]*\)'` # Note that escaped parentheses
# == == + trigger substring match.
echo "The digits at the beginning of \"$a\" are \"$b\"."
echo
exit 0
The : 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
This example illustrates how expr uses the escaped parentheses −− \( ... \) −− grouping operator in tandem
with regular expression parsing to match a substring.
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Chapter 12. External Filters, Programs and Commands 157
Perl, sed, and awk have far superior string parsing facilities. A short sed or awk "subroutine" within a script
(see Section 34.2) is an attractive alternative to using expr.
See Section 9.2 for more on string operations.
12.3. 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 12−7. 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=`eval date +%s` # The "+%s" option to 'date' is GNU−specific.
filename=$prefix.$suffix
echo $filename
# It's great for creating "unique" 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
zdump Echoes the time in a specified time zone.
bash$ zdump EST
EST Tue Sep 18 22:09:22 2001 EST
time
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Outputs very verbose timing statistics for executing a command.
time ls −l / gives something like this:
0.00user 0.01system 0:00.05elapsed 16%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (149major+27minor)pagefaults 0swaps
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).
at The at job control command executes a given set of commands at a specified time. Superficially, it
resembles crond, 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 noninteractive. 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.
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 30−6.
sleep 3
# Pauses 3 seconds.
The sleep command defaults to seconds, but minute,
hours, or days may also be specified.
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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−grain 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
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.
12.4. Text Processing Commands
Commands affecting text and text files
sort File sorter, 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 10−9,
Example 10−10, and Example A−9.
tsort Topological sort, reading in pairs of whitespace−separated strings and sorting according to input
patterns.
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.
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Chapter 12. External Filters, Programs and Commands 160
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
of a document needs to be examined.
Example 12−8. Word Frequency Analysis
#!/bin/bash
# wf.sh: Crude word frequency analysis on a text file.
# Check for input file on command line.
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
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
########################################################
# main ()
sed −e 's/\.//g' −e 's/ /\
/g' "$1" | tr 'A−Z' 'a−z' | sort | uniq −c | sort −nr
# =========================
# Frequency of occurrence
# Filter out periods and
#+ change space between words to linefeed,
#+ then shift characters to lowercase, and
#+ finally prefix occurrence count and sort numerically.
########################################################
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Chapter 12. External Filters, Programs and Commands 161
# Exercises:
# −−−−−−−−−
# 1) Add 'sed' commands to filter out other punctuation, such as commas.
# 2) Modify to also filter out multiple spaces and other whitespace.
# 3) Add a secondary sort key, so that instances of equal occurrence
#+ are sorted alphabetically.
exit 0
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:
cat /etc/mtab | cut −d ' ' −f1,2
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:
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Chapter 12. External Filters, Programs and Commands 162
# List all the users in /etc/passwd.
FILENAME=/etc/passwd
for user in $(cut −d: −f1 $FILENAME)
do
echo $user
done
# Thanks, Oleg Philon for suggesting this.
cut −d ' ' −f2,3 filename is equivalent to awk −F'[ ]' '{ print $2, $3 }'
filename
See also Example 12−33.
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 this can be changed). It has a
number of interesting options.
Example 12−9. Which files are scripts?
#!/bin/bash
# script−detector.sh: Detects scripts within a directory.
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TESTCHARS=2 # Test first 2 characters.
SHABANG='#!' # 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
Example 12−10. 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.
# 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 <range><action>s. The first and only one is
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Chapter 12. External Filters, Programs and Commands 164
# range action
# 1 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 <range>, 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 <range><action>s (could have been written
# sed −e 's/.* //' −e q):
# range action
# nothing (matches line) s/.* //
# nothing (matches line) 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.
# =================================================================== #
# A simpler altenative to the above 1−line script would be:
# head −c4 /dev/urandom| od −An −tu4
exit 0
See also Example 12−30.
tail lists the end of a file to stdout (the default is 10 lines). Commonly used to keep track of changes to
a system logfile, using the −f option, which outputs lines appended to the file.
Example 12−11. 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
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# /var/log/messages must have world read permission for this to work.
echo "$filename contains tail end of system log."
exit 0
See also Example 12−4, Example 12−30 and Example 30−6.
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, that is, 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.
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.
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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 . # 4
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep −cz '$' # 5
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep −cz '^' # 5
#
printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep −c '$' # 9
# By default, newline chars (\n) separate items to match.
# Note that the −z option is GNU "grep" specific.
# Thanks, S.C.
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 # if grep lookup succeeds
word=Linux
filename=data.file
grep −q "$word" "$filename" # The "−q" option causes nothing to echo to stdout.
if [ $? −eq $SUCCESS ]
then
echo "$word found in $filename"
else
echo "$word not found in $filename"
fi
Example 30−6 demonstrates how to use grep to search for a word pattern in a system logfile.
Example 12−12. Emulating "grep" in a script
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Chapter 12. External Filters, Programs and Commands 167
#!/bin/bash
# grp.sh: Very crude reimplementation of 'grep'.
E_BADARGS=65
if [ −z "$1" ] # Check for argument to script.
then
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.
egrep is the same as grep −E. This uses a somewhat different, extended set of regular
expressions, which can make the search somewhat more flexible.
fgrep is the same as grep −F. It does a literal string search (no regular expressions),
which allegedly speeds things up a bit.
agrep 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.
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.
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Chapter 12. External Filters, Programs and Commands 168
Example 12−13. 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.
wc wc gives a "word count" on a file or I/O stream:
bash $ wc /usr/doc/sed−3.02/README
20 127 838 /usr/doc/sed−3.02/README
[20 lines 127 words 838 characters]
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wc −w gives only the word count.
wc −l gives only the line count.
wc −c 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 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 12−30 and Example 16−7.
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.
Either tr "A−Z" "*" <filename or tr A−Z \* <filename changes all the uppercase
letters in filename to asterisks (writes to stdout). On some systems this may not work, but tr
A−Z '[**]' will.
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The −d option deletes a range of characters.
echo "abcdef" # abcdef
echo "abcdef" | tr −d b−d # aef
tr −d 0−9 <filename
# Deletes all digits from the file "filename".
The −−squeeze−repeats (or −s) option deletes all but the first instance of a string of
consecutive characters. This option is useful for removing excess whitespace.
bash$ echo "XXXXX" | tr −−squeeze−repeats 'X'
X
The −c "complement" option inverts the character set to match. With this option, tr acts only upon
those characters not matching the specified set.
bash$ echo "acfdeb123" | tr −c b−d +
+c+d+b++++
Note that tr recognizes POSIX character classes. [29]
bash$ echo "abcd2ef1" | tr '[:alpha:]' −
−−−−2−−1
Example 12−14. toupper: Transforms a file to all uppercase.
#!/bin/bash
# Changes a file to all uppercase.
E_BADARGS=65
if [ −z "$1" ] # Standard check for command line arg.
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
tr a−z A−Z <"$1"
# Same effect as above, but using POSIX character set notation:
# tr '[:lower:]' '[:upper:]' <"$1"
# Thanks, S.C.
exit 0
Example 12−15. lowercase: Changes all filenames in working directory to lowercase.
#! /bin/bash
#
# Changes every filename in working directory to all lowercase.
#
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Chapter 12. External Filters, Programs and Commands 171
# Inspired by a script of John Dubois,
# which was translated into into Bash by Chet Ramey,
# and considerably simplified by Mendel Cooper, author of this document.
for filename in * # Traverse all files in directory.
do
fname=`basename $filename`
n=`echo $fname | tr A−Z a−z` # Change name to lowercase.
if [ "$fname" != "$n" ] # Rename only files not already lowercase.
then
mv $fname $n
fi
done
exit 0
# Code below this line will not execute because of "exit".
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−#
# To run it, delete script above line.
# The above script will not work on filenames containing blanks or newlines.
# Stephane Chazelas therefore suggests the following alternative:
for filename in * # Not necessary to use basename,
# since "*" won't return any file containing "/".
do n=`echo "$filename/" | tr '[:upper:]' '[:lower:]'`
# POSIX char set notation.
# Slash added so that trailing newlines are not
# removed by command substitution.
# Variable substitution:
n=${n%/} # Removes trailing slash, added above, from filename.
[[ $filename == $n ]] || mv "$filename" "$n"
# Checks if filename already lowercase.
done
exit 0
Example 12−16. du: DOS to UNIX text file conversion.
#!/bin/bash
# du.sh: DOS to UNIX text file converter.
E_WRONGARGS=65
if [ −z "$1" ]
then
echo "Usage: `basename $0` filename−to−convert"
exit $E_WRONGARGS
fi
NEWFILENAME=$1.unx
CR='\015' # Carriage return.
# Lines in a DOS text file end in a CR−LF.
tr −d $CR < $1 > $NEWFILENAME
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Chapter 12. External Filters, Programs and Commands 172
# Delete CR and write to new file.
echo "Original DOS text file is \"$1\"."
echo "Converted UNIX text file is \"$NEWFILENAME\"."
exit 0
Example 12−17. rot13: 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 <filename
# or ./rot13.sh and supply keyboard input (stdin)
cat "$@" | tr 'a−zA−Z' 'n−za−mN−ZA−M' # "a" goes to "n", "b" to "o", etc.
# The 'cat "$@"' construction
#+ permits getting input either from stdin or from files.
exit 0
Example 12−18. Generating "Crypto−Quote" Puzzles
#!/bin/bash
# crypto−quote.sh: Encrypt quotes
# Will encrypt famous quotes in a simple monoalphabetic substitution.
# The result is similar to the "Crypto Quote" puzzles
#+ seen in the Op Ed pages of the Sunday paper.
key=ETAOINSHRDLUBCFGJMQPVWZYXK
# The "key" is nothing more than a scrambled alphabet.
# Changing the "key" changes the encryption.
# The 'cat "$@"' construction gets input either from stdin or from files.
# If using stdin, terminate input with a Control−D.
# Otherwise, specify filename as command−line parameter.
cat "$@" | tr "a−z" "A−Z" | tr "A−Z" "$key"
# | to uppercase | encrypt
# Will work on lowercase, uppercase, or mixed−case quotes.
# Passes non−alphabetic characters through unchanged.
# Try this script with something like
# "Nothing so needs reforming as other people's habits."
# −−Mark Twain
#
# Output is:
# "CFPHRCS QF CIIOQ MINFMBRCS EQ FPHIM GIFGUI'Q HETRPQ."
# −−BEML PZERC
# To reverse the encryption:
# cat "$@" | tr "$key" "A−Z"
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# This simple−minded cipher can be broken by an average 12−year old
#+ using only pencil and paper.
exit 0
tr variants
The tr utility has two historic variants. The BSD version does not use brackets (tr a−z A−Z), but
the SysV one does (tr '[a−z]' '[A−Z]'). The GNU version of tr resembles the BSD one, so
quoting letter ranges within brackets is mandatory.
fold A filter that wraps lines of input to a specified width. This is especially useful with the −s option,
which breaks lines at word spaces (see Example 12−19 and Example A−2).
fmt Simple−minded file formatter, used as a filter in a pipe to "wrap" long lines of text output.
Example 12−19. Formatted file listing.
#!/bin/bash
WIDTH=40 # 40 columns wide.
b=`ls /usr/local/bin` # Get a file listing...
echo $b | fmt −w $WIDTH
# Could also have been done by
# echo $b | fold − −s −w $WIDTH
exit 0
See also Example 12−4.
A powerful alternative to fmt is Kamil Toman's par utility, available from
http://www.cs.berkeley.edu/~amc/Par/.
col This deceptively named filter removes reverse line feeds from an input stream. It also attempts to
replace whitespace with equivalent tabs. The chief use of col is in filtering the output from certain text
processing utilities, such as groff and tbl.
columnColumn formatter. This filter transforms list−type text output into a "pretty−printed" table by inserting
tabs at appropriate places.
Example 12−20. Using column to format a directory listing
#!/bin/bash
# This is a slight modification of the example file in the "column" man page.
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(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG−NAME\n" \
; ls −l | sed 1d) | column −t
# The "sed 1d" in the pipe deletes the first line of output,
#+ which would be "total N",
#+ where "N" is the total number of files found by "ls −l".
# The −t option to "column" pretty−prints a table.
exit 0
colrm Column removal filter. This removes columns (characters) from a file and writes the file, lacking the
range of specified columns, back to stdout. colrm 2 4 <filename removes the second
through fourth characters from each line of the text file filename.
If the file contains tabs or nonprintable characters, this may cause unpredictable
behavior. In such cases, consider using expand and unexpand in a pipe preceding
colrm.
nl Line numbering filter. nl filename lists filename to stdout, but inserts consecutive numbers
at the beginning of each non−blank line. If filename omitted, operates on stdin.
The output of nl is very similar to cat −n, however, by default nl does not list blank lines.
Example 12−21. nl: A self−numbering script.
#!/bin/bash
# This script echoes itself twice to stdout with its lines numbered.
# 'nl' sees this as line 3 since it does not number blank lines.
# 'cat −n' sees the above line as number 5.
nl `basename $0`
echo; echo # Now, let's try it with 'cat −n'
cat −n `basename $0`
# The difference is that 'cat −n' numbers the blank lines.
# Note that 'nl −ba' will also do so.
exit 0
pr Print formatting filter. This will paginate files (or stdout) into sections suitable for hard copy
printing or viewing on screen. Various options permit row and column manipulation, joining lines,
setting margins, numbering lines, adding page headers, and merging files, among other things. The pr
command combines much of the functionality of nl, paste, fold, column, and expand.
pr −o 5 −−width=65 fileZZZ | more gives a nice paginated listing to screen of
fileZZZ with margins set at 5 and 65.
A particularly useful option is −d, forcing double−spacing (same effect as sed −G).
gettext
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A GNU utility for localization and translating the text output of programs into foreign languages.
While primarily intended for C programs, gettext also finds use in shell scripts. See the info page.
iconv A utility for converting file(s) to a different encoding (character set). Its chief use is for localization.
recode Consider this a fancier version of iconv, above. This very versatile utility for converting a file to a
different encoding 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.
Ghostscript (gs) is a GPL−ed Postscript interpreter.
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 (see Example
A−1).
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.
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.
12.5. File and Archiving Commands
Archiving
tar The standard UNIX archiving utility. 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, such as 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). [30]
Some useful tar options:
−c create (a new archive)1.
−x extract (files from existing archive)2.
−−delete delete (files from existing archive)3.
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This option will not work on magnetic tape devices.
−r append (files to existing archive)4.
−A append (tar files to existing archive)5.
−t list (contents of existing archive)6.
−u update archive7.
−d compare archive with specified filesystem8.
−z gzip the archive
(compress or uncompress, depending on whether combined with the −c or −x) option
9.
−j bzip2 the archive10.
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 files in a shell archive are concatenated without compression, and the
resultant archive is essentially a shell script, complete with #!/bin/sh header, and containing all the
necessary unarchiving commands. Shar archives still show up in Internet newsgroups, but otherwise
shar has been pretty well replaced by tar/gzip. The unshar command unpacks shar archives.
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.
An 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
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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.
Example 12−22. Using cpio to move a directory tree
#!/bin/bash
# Copying a directory tree using cpio.
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 man page to decipher these cpio options.
exit 0
rpm2cpio
This command extracts a cpio archive from an rpm one.
Example 12−23. 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.
cpio −−make−directories −F $TEMPFILE −i # Unpacks cpio archive.
rm −f $TEMPFILE # Deletes cpio archive.
exit 0
# Exercise:
# Add check for whether 1) "target−file" exists and
#+ 2) it is really an rpm archive.
# Hint: parse output of 'file' command.
Compression
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 178
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 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.
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 Yet another compression 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 acceptable medium of 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.
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
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Chapter 12. External Filters, Programs and Commands 179
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)
Example 12−24. stripping comments from C program files
#!/bin/bash
# strip−comment.sh: Strips out the comments (/* COMMENT */) in a C program.
E_NOARGS=65
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=`eval file $1 | awk '{ print $2, $3, $4, $5 }'`
# "file $1" echoes file type...
# then awk removes the first field of this, 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 lines with a "*/" or "/*",
# not a desirable result.
exit 0
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# Code below this line will not execute because of 'exit 0' above.
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 180
# Stephane Chazelas suggests the following alternative:
usage() {
echo "Usage: `basename $0` C−program−file" >&2
exit 1
}
WEIRD=`echo −n −e '\377'` # or WEIRD=$'\377'
[[ $# −eq 1 ]] || usage
case `file "$1"` in
*"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
# (lex or yacc perhaps?).
exit 0
which which command−xxx gives the full path to "command−xxx". This is useful for finding out whether a
particular command or utility is installed on the system.
$bash which rm
/usr/bin/rm
whereisSimilar to which, above, whereis command−xxx gives the full path to "command−xxx", but also to
its manpage.
$bash whereis rm
rm: /bin/rm /usr/share/man/man1/rm.1.bz2
whatis whatis filexxx looks up "filexxx" 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 12−25. Exploring /usr/X11R6/bin
#!/bin/bash
# What are all those mysterious binaries in /usr/X11R6/bin?
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 181
DIRECTORY="/usr/X11R6/bin"
# Try also "/bin", "/usr/bin", "/usr/local/bin", etc.
for file in $DIRECTORY/*
do
whatis `basename $file` # Echoes info about the binary.
done
exit 0
# 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 10−3.
vdir Show a detailed directory listing. The effect is similar to ls −l.
This is one of the GNU fileutils.
bash$ vdir
total 10
−rw−r−−r−− 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo
−rw−r−−r−− 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak
−rw−r−−r−− 1 bozo bozo 877 Dec 17 2000 employment.xrolo
bash ls −l
total 10
−rw−r−−r−− 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo
−rw−r−−r−− 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak
−rw−r−−r−− 1 bozo bozo 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
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 10−7 and Example 10−9.
Example 12−26. An "improved" strings command
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 182
#!/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 all the gibberish and noise,
#+ and outputs only recognized words.
# =================================================================
# Standard Check for Script Argument(s)
ARGS=1
E_BADARGS=65
E_NOFILE=66
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 format 1 word per line.
wlist=`strings "$1" | tr A−Z a−z | tr '[:space:]' Z | \
tr −cs '[:alpha:]' Z | tr −s '\173−\377' Z | tr Z ' '`
# 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 pass.
for word in $wlist # Important:
# $wlist must not be quoted here.
# "$wlist" does not work.
# Why?
do
strlen=${#word} # String length.
if [ "$strlen" −lt "$MINSTRLEN" ] # Skip over short strings.
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 183
then
continue
fi
grep −Fw $word "$WORDFILE" # Match whole words only.
done
exit 0
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 spiff, 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.
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 <patch−file
# Takes all the changes listed in 'patch−file'
# and applies them to the files referenced therein.
# This upgrades to a newer version of the package.
Patching the kernel:
cd /usr/src
gzip −cd patchXX.gz | patch −p0
# Upgrading kernel source using 'patch'.
# From the Linux kernel docs "README",
# by anonymous author (Alan Cox?).
The diff command can also recursively compare directories (for the filenames
present).
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 184
bash$ diff −r ~/notes1 ~/notes2
Only in /home/bozo/notes1: file02
Only in /home/bozo/notes1: file03
Only in /home/bozo/notes2: file04
Use zdiff to compare gzipped files.
diff3 An extended version of diff that compares three files at a time. This command returns an exit value of
0 upon successful execution, but unfortunately this gives no information about the results of the
comparison.
bash$ diff3 file−1 file−2 file−3
====
1:1c
This is line 1 of "file−1".
2:1c
This is line 1 of "file−2".
3:1c
This is line 1 of "file−3"
sdiff Compare and/or edit two files in order to merge them into an output file. Because of its interactive
nature, this command would find little use in a script.
cmp The cmp command is a simpler version of diff, above. Whereas diff reports the differences between
two files, cmp merely shows at what point they differ.
Like diff, cmp returns an exit status of 0 if the compared files are identical, and 1 if
they differ. This permits use in a test construct within a shell script.
Example 12−27. Using cmp to compare two files within a script.
#!/bin/bash
ARGS=2 # Two args to script expected.
E_BADARGS=65
E_UNREADABLE=66
if [ $# −ne "$ARGS" ]
then
echo "Usage: `basename $0` file1 file2"
exit $E_BADARGS
fi
if [[ ! −r "$1" || ! −r "$2" ]]
then
echo "Both files to be compared must exist and be readable."
exit $E_UNREADABLE
fi
cmp $1 $2 &> /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.
#
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Chapter 12. External Filters, Programs and Commands 185
# 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◊
−12 suppresses both columns 1 and 2, etc.◊
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−8).
Example 12−28. basename and dirname
#!/bin/bash
a=/home/bozo/daily−journal.txt
echo "Basename of /home/bozo/daily−journal.txt = `basename $a`"
echo "Dirname of /home/bozo/daily−journal.txt = `dirname $a`"
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 186
echo
echo "My own home is `basename ~/`." # Also works with just ~.
echo "The home of my home is `dirname ~/`." # Also works with just ~.
exit 0
split Utility for splitting a file into smaller chunks. Usually used for splitting up large files in order to back
them up on floppies or preparatory to e−mailing or uploading them.
sum, cksum, md5sum
These are utilities for generating checksums. A checksum is a number 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 128−bit md5sum (message digest checksum)
command.
bash$ cksum /boot/vmlinuz
1670054224 804083 /boot/vmlinuz
bash$ md5sum /boot/vmlinuz
0f43eccea8f09e0a0b2b5cf1dcf333ba /boot/vmlinuz
Note that cksum also shows the size, in bytes, of the target file.
Example 12−29. 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.
set_up_database ()
{
echo ""$directory"" > "$dbfile"
# 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" ]
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 187
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."
else
echo "${filename[n]} : CHECKSUM ERROR!"
# File has been changed since last checked.
fi
fi
let "n+=1"
done <"$dbfile" # Read from checksum database file.
}
# =================================================== #
# main ()
if [ −z "$1" ]
then
directory="$PWD" # If not specified,
else #+ use current working directory.
directory="$1"
fi
clear # Clear screen.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
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.
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Chapter 12. External Filters, Programs and Commands 188
echo
# You may wish to redirect the stdout of this script to a file,
#+ especially if the directory checked has many files in it.
# For a much more thorough file integrity check,
#+ consider the "Tripwire" package,
#+ http://sourceforge.net/projects/tripwire/.
exit 0
See also Example A−20 for a creative use of the md5sum command.
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 12−42, 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.
Encoding and Encryption
uuencode
This utility encodes binary files into ASCII characters, making them suitable for transmission in the
body of an e−mail message or in a newsgroup posting.
uudecode
This reverses the encoding, decoding uuencoded files back into the original binaries.
Example 12−30. uudecoding encoded files
#!/bin/bash
lines=35 # Allow 35 lines for the header (very generous).
for File in * # Test all the files in the current working directory...
do
search1=`head −$lines $File | grep begin | wc −w`
search2=`tail −$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:
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 189
# Modify this script to check for a newsgroup header.
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 a
batch by means of a shell script.
crypt At one time, this was the standard UNIX file encryption utility. [31] 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−5).
Miscellaneous
mktempCreate a temporary file with a "unique" filename.
PREFIX=filename
tempfile=`mktemp $PREFIX.XXXXXX`
# ^^^^^^ Need at least 6 placeholders
#+ in the filename template.
echo "tempfile name = $tempfile"
# tempfile name = filename.QA2ZpY
# or something similar...
make Utility for building and compiling binary packages. This can also be used for any set of operations
that is triggered by incremental changes in source files.
The make command checks a Makefile, a list of file dependencies and operations to be carried out.
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 find use
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 a script.
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 190
12.6. Communications Commands
Certain of the following commands find use in chasing spammers, as well as in network data transfer and
analysis.
Information and Statistics
host Searches for information about an Internet host by name or IP address, using DNS.
bash$ host surfacemail.com
surfacemail.com. has address 202.92.42.236
ipcalc Carries out IP address lookups. 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 still has its uses.
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 Similar to nslookup, do an Internet "name server lookup" on a host. May be run either interactively or
noninteractively, i.e., from within a script.
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. IN PTR
;; AUTHORITY SECTION:
6.9.81.in−addr.arpa. 3600 IN SOA ns.eltel.net. noc.eltel.net.
2002031705 900 600 86400 3600
;; Query time: 537 msec
;; SERVER: 135.116.137.2#53(135.116.137.2)
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Chapter 12. External Filters, Programs and Commands 191
;; WHEN: Wed Jun 26 08:35:24 2002
;; MSG SIZE rcvd: 91
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 other machines, either on a local or remote
network. This is a diagnostic tool for testing network connections, and it should be used with caution.
A successful ping returns an exit status of 0. This can be tested for in a script.
bash$ ping localhost
PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data.
Warning: time of day goes back, taking countermeasures.
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
whois Perform a DNS (Domain Name System) lookup. The −h option permits specifying which whois
server to query. See Example 4−6.
finger Retrieve information about users on a network. Optionally, this command can display a user's
~/.plan, ~/.project, and ~/.forward files, if present.
bash$ finger
Login Name Tty Idle Login Time Office Office Phone
bozo Bozo Bozeman tty1 8 Jun 25 16:59
bozo Bozo Bozeman ttyp0 Jun 25 16:59
bozo Bozo Bozeman ttyp1 Jun 25 17:07
bash$ finger bozo
Login: bozo Name: Bozo Bozeman
Directory: /home/bozo Shell: /bin/bash
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
No mail.
No Plan.
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Out of security considerations, many networks disable finger and its associated daemon. [32]
vrfy Verify an Internet e−mail address.
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 17−7, Example A−5, and Example A−14).
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.
cu Call Up a remote system and connect as a simple terminal. This command is part of the uucp
package. It is a sort of dumbed−down version of telnet.
telnet Utility and protocol for connecting to a remote host.
The telnet protocol contains security holes and should therefore probably be avoided.
wget The wget utility non−interactively retrieves or downloads files from a Web or ftp site. It works well
in a script.
wget −p http://www.xyz23.com/file01.html
wget −r ftp://ftp.xyz24.net/~bozo/project_files/ −o $SAVEFILE
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 non−interactively.
lynx −dump http://www.xyz23.com/file01.html >$SAVEFILE
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. Using rcp and similar
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 193
utilities with security implications in a shell script may not be advisable. Consider, instead, using ssh
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.
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.
Mail
mail Send or read e−mail messages.
This stripped−down command−line mail client works fine as a command embedded in a script.
Example 12−31. 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
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.
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Chapter 12. External Filters, Programs and Commands 194
vacation
This utility automatically replies to e−mails that the intended recipient is on vacation and temporarily
unavailable. This runs on a network, in conjunction with sendmail, and is not applicable to a dial−up
POPmail account.
12.7. 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, below. tput reset is the equivalent
of reset, below. tput sgr0 also resets the terminal, but without clearing the screen.
bash$ tput longname
xterm terminal emulator (XFree86 4.0 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.
Note that stty offers a more powerful command set for controlling a terminal.
infocmpThis 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 10−25.
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.
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Chapter 12. External Filters, Programs and Commands 195
12.8. Math Commands
"Doing the numbers"
factor Decompose an integer into prime factors.
bash$ factor 27417
27417: 3 13 19 37
bc Bash can't handle floating point calculations, and it lacks operators for certain important mathematical
functions. Fortunately, bc comes to the rescue.
Not just a versatile, arbitrary precision calculation utility, bc offers many of the facilities of a
programming language.
bc 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 12−32. 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 author of this document).
# http://www.ibiblio.org/pub/Linux/apps/financial/mcalc−1.6.tar.gz [15k]
echo
echo "Given the principal, interest rate, and term of a mortgage,"
echo "calculate the monthly payment."
bottom=1.0
echo
echo −n "Enter principal (no commas) "
read principal
echo −n "Enter interest rate (percent) " # If 12%, enter "12", not ".12".
read interest_r
echo −n "Enter term (months) "
read term
interest_r=$(echo "scale=9; $interest_r/100.0" | bc) # Convert to decimal.
# "scale" determines how many decimal places.
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Chapter 12. External Filters, Programs and Commands 196
interest_rate=$(echo "scale=9; $interest_r/12 + 1.0" | bc)
top=$(echo "scale=9; $principal*$interest_rate^$term" | bc)
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.
# ====================================================================
# 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 a dollar sign in front of amount.
echo
exit 0
# Exercises:
# 1) Filter input to permit commas in principal amount.
# 2) Filter input to permit interest to be entered as percent or decimal.
# 3) If you are really ambitious,
# expand this script to print complete amortization tables.
Example 12−33. Base Conversion
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Chapter 12. External Filters, Programs and Commands 197
:
##########################################################################
# 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 $
##########################################################################
# Description
#
# Changes
# 21−03−95 stv fixed error occuring with 0xb as input (0.2)
##########################################################################
# ==> Used in this document with the script author's permission.
# ==> Comments added by document author.
NOARGS=65
PN=`basename "$0"` # Program name
VER=`echo '$Revision: 1.2 $' | cut −d' ' −f2` # ==> 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
} # ==> Function to print usage message.
Msg () {
for i # ==> in [list] missing.
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
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` # ==> 'bc' is calculator utility.
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Chapter 12. External Filters, Programs and Commands 198
case "$dec" in
[0−9]*) ;; # number ok
*) continue;; # error: ignore
esac
# Print all conversions in one line.
# ==> 'here document' feeds command list to 'bc'.
echo `bc <<!
obase=16; "hex="; $dec
obase=10; "dec="; $dec
obase=8; "oct="; $dec
obase=2; "bin="; $dec
!
` | sed −e 's: : :g'
done
}
while [ $# −gt 0 ]
do
case "$1" in
−−) shift; break;;
−h) Usage;; # ==> Help message.
−*) Usage;;
*) break;; # first number
esac # ==> More error checking for illegal input would be useful.
shift
done
if [ $# −gt 0 ]
then
PrintBases "$@"
else # read from stdin
while read line
do
PrintBases $line
done
fi
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
commands to bc.
variable=`bc << LIMIT_STRING
options
statements
operations
LIMIT_STRING
`
...or...
variable=$(bc << LIMIT_STRING
options
statements
operations
LIMIT_STRING
)
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Chapter 12. External Filters, Programs and Commands 199
Example 12−34. 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...
hyp= # Declare global variable.
hypotenuse () # Calculate hypotenuse of a right triangle.
{
hyp=$(bc −l << EOF
scale = 9
sqrt ( $1 * $1 + $2 * $2 )
EOF
)
# Unfortunately, can't return floating point values from a Bash function.
}
hypotenuse 3.68 7.31
echo "hypotenuse = $hyp" # 8.184039344
exit 0
Example 12−35. Calculating PI
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Chapter 12. External Filters, Programs and Commands 200
#!/bin/bash
# cannon.sh: Approximating PI by firing cannonballs.
# 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 all water, except for the four corners.
# (Think of it as a square with an inscribed circle.)
#
# Let us fire solid iron cannonballs from an old−style cannon
#+ at the square of land.
# All the shots impact somewhere on the plot of land,
#+ either in the lake or on the dry corners.
# Since the lake takes up most of the land area,
#+ most of the shots will SPLASH! into the water.
# Just a few shots will THUD! into solid ground
#+ in the far corners of the land.
#
# If we take enough random, unaimed shots at the plot of land,
#+ Then the ratio of SPLASHES to total shots will approximate
#+ the value of PI/4.
#
# The reason for this is that the cannon is actually shooting
#+ only at the upper right−hand quadrant of the square.
# (The previous explanation was a simplification.)
#
# 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 a few compromises.
# This tends to make the simulation less accurate, unfortunately.
DIMENSION=10000 # Length of each side of the plot of land.
# Also sets ceiling for random integers generated.
MAXSHOTS=1000 # Fire this many shots.
# 10000 or more would be better, but would take too long.
PMULTIPLIER=4.0 # Scaling factor to approximate PI.
get_random ()
{
SEED=$(head −1 /dev/urandom | od −N 1 | awk '{ print $2 }')
RANDOM=$SEED # From "seeding−random.sh"
#+ example script.
let "rnum = $RANDOM % $DIMENSION" # Range less than 10000.
echo $rnum
}
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.
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Chapter 12. External Filters, Programs and Commands 201
# This diminshes the accuracy of the simulation, unfortunately.
}
# main() {
# Initialize variables.
shots=0
splashes=0
thuds=0
Pi=0
while [ "$shots" −lt "$MAXSHOTS" ] # Main loop.
do
xCoord=$(get_random) # Get random X and Y coords.
yCoord=$(get_random)
hypotenuse $xCoord $yCoord # Hypotenuse of right−triangle =
#+ distance.
((shots++))
printf "#%4d " $shots
printf "Xc = %4d " $xCoord
printf "Yc = %4d " $yCoord
printf "Distance = %5d " $distance # Distance from
#+ center of lake,
#+ coordinate (0,0).
if [ "$distance" −le "$DIMENSION" ]
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...
# Probably due to round−off error and imperfect randomness of $RANDOM.
echo
# }
exit 0
# One might well wonder whether a shell script is appropriate for
#+ an application as complex and computation−intensive as a simulation.
#
# There are at least two justifications.
# 1) As a proof of concept: to show it can be done.
# 2) To prototype and test the algorithms before rewriting
#+ it in a compiled high−level language.
dc
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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.
Most persons avoid dc, since it requires non−intuitive RPN input. Yet it has its uses.
Example 12−36. Converting a decimal number to hexadecimal
#!/bin/bash
# hexconvert.sh: Convert a decimal number to hexadecimal.
BASE=16 # Hexadecimal.
if [ −z "$1" ]
then
echo "Usage: $0 number"
exit $E_NOARGS
# Need a command line argument.
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.
# See 'man dc' for other options.
return
}
hexcvt "$1"
exit 0
Studying the info page for dc gives some insight into its intricacies. However, there seems to be a
small, select group of dc wizards who delight in showing off their mastery of this powerful, but
arcane utility.
Example 12−37. Factoring
#!/bin/bash
# factr.sh: Factor a number
MIN=2 # Will not work for number smaller than this.
E_NOARGS=65
E_TOOSMALL=66
if [ −z $1 ]
then
echo "Usage: $0 number"
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Chapter 12. External Filters, Programs and Commands 203
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 line of code written by Michel Charpentier <charpov@cs.unh.edu>.
# Used with permission (thanks).
exit 0
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 12−38. 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 # Script needs sides of triangle passed.
E_BADARGS=65 # 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
echo −n "Hypotenuse of $1 and $2 = "
echo $1 $2 | awk "$AWKSCRIPT"
exit 0
12.9. Miscellaneous Commands
Command that fit in no special category
jot, seq These utilities emit a sequence of integers, with a user−selected increment.
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Chapter 12. External Filters, Programs and Commands 204
The normal 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 12−39. 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' may also take a replaceable parameter.
for a in `seq $COUNT` # or for a in $( seq $COUNT )
do
echo −n "$a "
done # 1 2 3 4 5 ... 80
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
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Chapter 12. External Filters, Programs and Commands 205
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
getopt The getopt command parses command−line options preceded by a dash. This external command
corresponds to the getopts Bash builtin, but it is not nearly as versatile.
Example 12−40. Using getopt to parse command−line options
#!/bin/bash
# Try the following when invoking this script.
# sh ex33a −a
# sh ex33a −abc
# sh ex33a −a −b −c
# sh ex33a −d
# sh ex33a −dXYZ
# sh ex33a −d XYZ
# sh ex33a −abcd
# sh ex33a −abcdZ
# sh ex33a −z
# sh ex33a 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 \"a\"";;
−b) echo "Option \"b\"";;
−c) echo "Option \"c\"";;
−d) echo "Option \"d\" $2";;
*) break;;
esac
shift
done
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Chapter 12. External Filters, Programs and Commands 206
# It is better to use the 'getopts' builtin in a script,
#+ rather than 'getopt'.
# See "ex33.sh".
exit 0
run−parts
The run−parts command [33] executes all the scripts in a target directory, sequentially in
ASCII−sorted filename order. Of course, the scripts need to have execute permission.
The crond 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 may have unintended side−effects.
banner Prints arguments as a large vertical banner to stdout, using an ASCII character (default '#'). This
may be redirected to a printer for hardcopy.
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. [34] These
commands trace the origin of their names to the line printers of another era.
bash$ lp file1.txt or bash lp <file1.txt
It is often useful to pipe the formatted output from pr to lp.
bash$ pr −options file1.txt | lp
Formatting packages, such as groff and Ghostscript may send their output directly to lp.
bash$ groff −Tascii file.tr | lp
bash$ gs −options | lp file.ps
Related commands are lpq, for viewing the print queue, and lprm, for removing jobs from the print
queue.
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Chapter 12. External Filters, Programs and Commands 207
tee [UNIX borrows an idea here from the plumbing trade.]
This is a redirection operator, but with a difference. Like the plumber's "tee," it permits "siponing off"
to a file the output of a command or commands within a pipe, but without affecting the result. This is
useful for printing an ongoing process to a file or paper, perhaps to keep track of it for debugging
purposes.
tee
|−−−−−−> to file
|
===============|===============
command−−−>−−−−|−operator−−>−−−> result of command(s)
===============================
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. [35] Typically, one process writes to the FIFO, and the other reads from it.
See Example A−16.
pathchkThis command checks the validity of a filename. If the filename exceeds the maximum allowable
length (255 characters) or one or more of the directories in its path is not searchable, then an error
message results.
Unfortunately, pathchk does not return a recognizable error code, and it is therefore pretty much
useless in a script. Consider instead the file test operators.
dd This is the somewhat obscure and much feared "data duplicator" command. Originally a utility for
exchanging data on magnetic tapes between UNIX minicomputers and IBM mainframes, this
command still has its uses. The dd command simply copies a file (or stdin/stdout), but with
conversions. Possible conversions are ASCII/EBCDIC, [36] upper/lower case, swapping of byte pairs
between input and output, and skipping and/or truncating the head or tail of the input file. A dd
−−help lists the conversion and other options that this powerful utility takes.
# Exercising 'dd'.
n=3
p=5
input_file=project.txt
output_file=log.txt
dd if=$input_file of=$output_file bs=1 skip=$((n−1)) count=$((p−n+1)) 2> /dev/null
# Extracts characters n to p from file $input_file.
echo −n "hello world" | dd cbs=1 conv=unblock 2> /dev/null
# Echoes "hello world" vertically.
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Chapter 12. External Filters, Programs and Commands 208
# Thanks, S.C.
To demonstrate just how versatile dd is, let's use it to capture keystrokes.
Example 12−41. Capturing Keystrokes
#!/bin/bash
# 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" not specified.
stty "$old_tty_setting" # Restore old terminal settings.
echo "You pressed the \"$keys\" keys."
# Thanks, S.C. 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−6). 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 29−2) and ramdisks
(Example 29−3). 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.
Advanced Bash−Scripting Guide
Chapter 12. External Filters, Programs and Commands 209
Example 12−42. Securely deleting a file
#!/bin/bash
# blotout.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
#+ will not reveal the original file data.
PASSES=7 # Number of file−shredding passes.
BLOCKSIZE=1 # I/O with /dev/urandom requires unit block size,
#+ otherwise you get weird results.
E_BADARGS=70
E_NOT_FOUND=71
E_CHANGED_MIND=72
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}') # Field 5 is file length.
pass_count=1
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
done
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rm −f $file # Finally, delete scrambled and shredded file.
sync # Flush buffers a final time.
echo "File \"$file\" blotted out and deleted."; echo
# 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,
#+ but more efficiently.
# The file cannot not be "undeleted" or retrieved by normal methods.
# However...
#+ this simple method will likely *not* withstand forensic analysis.
# 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
exit 0
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. See Example 9−26 and Example 12−10.
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.
objdump
Displays 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
Disassembly of section .init:
080490bc <.init>:
80490bc: 55 push %ebp
80490bd: 89 e5 mov %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 | sed −e '2p'`
# Uses 'sed' to strip off extraneous characters.
Of course, a script could use md5 for the same purpose.
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Chapter 12. External Filters, Programs and Commands 211
# 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 12−43. Filename generator
#!/bin/bash
# tempfile−name.sh: temp filename generator
BASE_STR=`mcookie` # 32−character magic cookie.
POS=11 # Arbitrary position in magic cookie string.
LEN=5 # 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
exit 0
units This utility converts between different units of measure. While normally invoked in interactive mode,
units may find use in a script.
Example 12−44. 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."
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Chapter 12. External Filters, Programs and Commands 212
# 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 processing filter, [37] 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 12−45. Using m4
#!/bin/bash
# m4.sh: Using the m4 macro processor
# Strings
string=abcdA01
echo "len($string)" | m4 # 7
echo "substr($string,4)" | m4 # A01
echo "regexp($string,[0−1][0−1],\&Z)" | m4 # 01Z
# Arithmetic
echo "incr(22)" | m4 # 23
echo "eval(99 / 3)" | m4 # 33
exit 0
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
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Chapter 12. External Filters, Programs and Commands 213
Chapter 13. System and Administrative Commands
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. [38]
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 [39] and deletes associated files.
The adduser command is a synonym for useradd and is usually a symbolic link to it.
id The id command lists the real and effective user IDs and the group IDs of the current user. 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)
bash$ echo $UID
501
Also see Example 9−5.
Chapter 13. System and Administrative Commands 214
who Show all users logged on to the system.
bash$ who
bozo tty1 Apr 27 17:45
bozo pts/0 Apr 27 17:46
bozo pts/1 Apr 27 17:47
bozo pts/2 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
wShow 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
However...
bash$ su
Password: ......
bash# whoami
root
bash# logname
bozo
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−16.
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.
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Chapter 13. System and Administrative Commands 215
#!/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 or changes a user's password.
The passwd can be used in a script, but should not be.
#!/bin/bash
# set−new−password.sh: Not a good idea.
# This script must be run as root,
#+ or better yet, not run at all.
ROOT_UID=0 # Root has $UID 0.
E_WRONG_USER=65 # Not root?
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."
fi
username=bozo
NEWPASSWORD=security_violation
echo "$NEWPASSWORD" | passwd −−stdin "$username"
# The '−−stdin' option to 'passwd' permits
#+ getting 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
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.
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 little use.
Terminals
tty
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Chapter 13. System and Administrative Commands 216
Echoes the name 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 13−1. 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 erase characters of input.
# Won't work.
echo "Your name is $name."
stty erase '#' # Set "hashmark" (#) as erase character.
echo −n "What is your name? "
read name # Use # to erase last character typed.
echo "Your name is $name."
exit 0
Example 13−2. secret password: Turning off terminal echoing
#!/bin/bash
echo
echo −n "Enter password "
read passwd
echo "password is $passwd"
echo −n "If someone had been looking over your shoulder, "
echo "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
A creative use of stty is detecting a user keypress (without hitting ENTER).
Example 13−3. Keypress detection
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Chapter 13. System and Administrative Commands 217
#!/bin/bash
# keypress.sh: Detect a user keypress ("hot keyboard").
echo
old_tty_settings=$(stty −g) # Save old settings.
stty −icanon
Keypress=$(head −c1) # 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.
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 = <undef>; eol2 = <undef>;
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
wha<ctl−W>I<ctl−H>foo bar<ctl−U>hello world<ENTER>
<ctl−D>
bash$ cat filexxx
hello world
bash$ bash$ wc −c < file
13
The process controlling the terminal receives only 13 characters (12 alphabetic ones, plus a newline),
although the user hit 26 keys.
In non−canonical ("raw") mode, every key hit (including special editing keys such as ctl−H) sends a
character immediately to the controlling process.
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Chapter 13. System and Administrative Commands 218
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.
Stephane Chazelas
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).
setserialSet or display serial port parameters. This command must be run by root user 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 very 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 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 17−2).
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 it.
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
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Chapter 13. System and Administrative Commands 219
hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63
hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4
Information and Statistics
uname Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the −a option, gives
verbose system info (see Example 12−4). The −s option shows only the OS type.
bash$ uname −a
Linux localhost.localdomain 2.2.15−2.5.0 #1 Sat Feb 5 00:13:43 EST 2000 i686 unknown
bash$ uname −s
Linux
arch Show system architecture. Equivalent to uname −m. See Example 10−26.
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 Fri Dec 7 18:43:21 −0700 2001
bin **Never logged in**
daemon **Never logged in**
...
bozo tty1 Sat Dec 8 21:14:29 −0700 2001
bash$ lastlog | grep root
root tty1 Fri Dec 7 18:43:21 −0700 2001
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 USER FD TYPE DEVICE SIZE NODE NAME
init 1 root mem REG 3,5 30748 30303 /sbin/init
init 1 root mem REG 3,5 73120 8069 /lib/ld−2.1.3.so
init 1 root mem REG 3,5 931668 8075 /lib/libc−2.1.3.so
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cardmgr 213 root mem REG 3,5 36956 30357 /sbin/cardmgr
...
strace Diagnostic and debugging tool for tracing system calls and signals. The simplest way of invoking it is
strace COMMAND.
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 truss.
nmap Network port scanner. This command scans a server to locate open ports and the services associated
with those ports. It is an important security tool for locking down a network against hacking attempts.
#!/bin/bash
SERVER=$HOST # localhost.localdomain (127.0.0.1).
PORT_NUMBER=25 # 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
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 used free shared buffers cached
Mem: 30504 28624 1880 15820 1608 16376
−/+ buffers/cache: 10640 19864
Swap: 68540 3128 65412
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.
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Chapter 13. System and Administrative Commands 221
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.
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 1k−blocks Used Available Use% Mounted on
/dev/hda5 273262 92607 166547 36% /
/dev/hda8 222525 123951 87085 59% /home
/dev/hda7 1408796 1075744 261488 80% /usr
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
vmstat Display virtual memory statistics.
bash$ vmstat
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 0 11040 2636 38952 0 0 33 7 271 88 8 3 89
netstat Show current network statistics and information, such as routing tables and active connections. This
utility accesses information in /proc/net (Chapter 28). See Example 28−2.
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netstat −r is equivalent to route.
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
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
hostid Echo a 32−bit hexadecimal numerical identifier for the host machine.
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 report) gives a very detailed rundown on system statistics. This
command is found on some commercial UNIX systems, but is not part of the base Linux distribution.
It is contained in the sysstat utilities package, written by Sebastien Godard.
bash$ sar
Linux 2.4.7−10 (localhost.localdomain) 12/31/2001
10:30:01 AM CPU %user %nice %system %idle
10:40:00 AM all 1.39 0.00 0.77 97.84
10:50:00 AM all 76.83 0.00 1.45 21.72
11:00:00 AM all 1.32 0.00 0.69 97.99
11:10:00 AM all 1.17 0.00 0.30 98.53
11:20:00 AM all 0.51 0.00 0.30 99.19
06:30:00 PM all 100.00 0.00 100.01 0.00
Average: all 1.39 0.00 0.66 97.95
readelf
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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 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 dec hex filename
495971 22496 17392 535859 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 mailing them, as
appropriate. Usually crond 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.
Job Control
ps
Process Statistics: lists currently executing processes by owner and PID (process id). This is usually
invoked with ax options, and may be piped to grep or sed to search for a specific process (see
Example 11−10 and Example 28−1).
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bash$ ps ax | grep sendmail
295 ? S 0:00 sendmail: accepting connections on port 25
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, 2352K buff
Swap: 157208K av, 0K used, 157208K free 37244K cached
PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
848 bozo 17 0 996 996 800 R 5.6 1.2 0:00 top
1 root 8 0 512 512 444 S 0.0 0.6 0:04 init
2 root 9 0 0 0 0 SW 0.0 0.0 0:00 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, snice, and skill.
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 13−4. pidof helps kill a process
#!/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).
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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` ".
# This entire script could be replaced by
# kill $(pidof −x 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.
crond 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.
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!
runlevelShows 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.
Network
ifconfigNetwork interface configuration and tuning utility. It 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.
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[ ${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.
# ...
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 30−6.
route Show info about or make changes to the kernel routing table.
bash$ route
Destination Gateway Genmask Flags MSS Window irtt Iface
pm3−67.bozosisp * 255.255.255.255 UH 40 0 0 ppp0
127.0.0.0 * 255.0.0.0 U 40 0 0 lo
default pm3−67.bozosisp 0.0.0.0 UG 40 0 0 ppp0
chkconfig
Check network configuration. This command lists and manages the network 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 1:off 2:off 3:on 4:on 5:on 6:off
rwhod 0:off 1:off 2:off 3:off 4:off 5: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.
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, using certain of the previously discussed text
processing utilities.
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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 CDROM
mount /mnt/cdrom
# Shortcut, if /mnt/cdrom listed in /etc/fstab
This versatile 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 image before burning it onto a CDR. [40]
Example 13−5. 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.
umountUnmount 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 laptops with swappable floppy
and CDROM drives, this can cause problems, though.
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 12−42) or when the lights begin to flicker.
losetup Sets up and configures loopback devices.
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Example 13−6. Creating a filesystem in a file
SIZE=1000000 # 1 meg
head −c $SIZE < /dev/zero > file # Set up file of designated size.
losetup /dev/loop0 file # Set it up as loopback device.
mke2fs /dev/loop0 # Create filesystem.
mount −o loop /dev/loop0 /mnt # Mount it.
# Thanks, S.C.
mkswapCreates 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 13−7. 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 this document.
# in issue #38 of "Linux Gazette", http://www.linuxgazette.com.
ROOT_UID=0 # This script must be run as root.
E_NOTROOT=67 # Non−root exit error.
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 # Assumes /dev/hdb vacant. Check!
MOUNTPOINT=/mnt/newdisk # Or choose another mount point.
fdisk $NEWDISK
mke2fs −cv $NEWDISK1 # Check for bad blocks & verbose output.
# Note: /dev/hdb1, *not* /dev/hdb!
mkdir $MOUNTPOINT
chmod 777 $MOUNTPOINT # 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
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exit 0
See also Example 13−6 and Example 29−3.
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 | grep 'ount count'
dumpe2fs 1.19, 13−Jul−2000 for EXT2 FS 0.5b, 95/08/09
Mount count: 6
Maximum mount count: 20
hdparmList 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
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. [41] 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.
mkbootdisk
Creates a boot floppy which can be used to bring up the system if, for example, the MBR (master boot
record) becomes corrupted. The mkbootdisk command is actually a Bash script, written by Erik
Troan, in the /sbin directory.
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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
file that controls access to a file, device, or resource. The lock file serves as a flag that this particular
file, device, or resource is in use by a particular process ("busy"), and this permits only restricted
access (or no access) to other processes.
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 Netscape 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 create a lock file that
already exists, the script will likely hang.
Normally, applications create and check for lock files in the /var/lock directory. 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
...
mknod Creates block or character device files (may be necessary when installing new hardware on the
system).
tmpwatch
Automatically deletes files which have not been accessed within a specified period of time. Usually
invoked by crond to remove stale log files.
MAKEDEV
Utility for creating device files. It must be run as root, and in the /dev directory.
root# ./MAKEDEV
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This is a sort of advanced version of mknod.
Backup
dump, restore
The dump command is an elaborate filesystem backup utility, generally used on larger installations
and networks. [42] 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.
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). 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 Chapter 27).
Judicious use of ulimit can protect a system against the dreaded fork bomb.
#!/bin/bash
while 1 # Endless loop.
do
$0 & # This script invokes itself . . .
#+ forks an infinite number of times . . .
#+ until the system freezes up because all resources exhausted.
done # 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 exceeds the preset limit.
umask User file creation 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). [43] 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 Chapter 27).
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
another dangerous command, if misused.
Modules
lsmod List installed kernel modules.
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bash$ lsmod
Module Size Used by
autofs 9456 2 (autoclean)
opl3 11376 0
serial_cs 5456 0 (unused)
sb 34752 0
uart401 6384 0 [sb]
sound 58368 0 [opl3 sb uart401]
soundlow 464 0 [sound]
soundcore 2800 6 [sb sound]
ds 6448 2 [serial_cs]
i82365 22928 2
pcmcia_core 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.
depmodCreates module dependency file, usually invoked from startup script.
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.
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.
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.
ldd Show shared lib dependencies for an executable file.
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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.
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.
Using our knowledge of administrative commands, let us examine a system script. One of the shortest and
simplest to understand scripts is killall, used to suspend running processes at system shutdown.
Example 13−8. 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, <miquels@drinkel.nl.mugnet.org>
# −−> This particular script seems to be Red Hat 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)
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.
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# 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
# −−> using the 'stop' 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 "action" commands to "echos".
Exercise 2. Look at some of the more complex scripts in /etc/rc.d/init.d. See if you can understand
parts 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.
Advanced Bash−Scripting Guide
Chapter 13. System and Administrative Commands 235
Chapter 14. Command Substitution
Command substitution reassigns the output of a command [44] or even multiple commands; it literally plugs
the command output into another context.
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) # The alternative form of command substitution.
echo $textfile_listing
# Same result.
# 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 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`" # This should always work.
# However...
mkdir 'dir with trailing newline
Chapter 14. Command Substitution 236
'
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) # Save old terminal setting.
echo "Hit a key "
stty −icanon −echo # 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.
Thanks, S.C.
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=`<file1` # Set "variable1" to contents of "file1".
variable2=`cat file2` # Set "variable2" to contents of "file2".
# Note 1:
# Removes newlines.
#
# Note 2:
Advanced Bash−Scripting Guide
Chapter 14. Command Substitution 237
# The variables may contain embedded whitespace,
#+ or even (horrors), control characters.
# Excerpts from system file, /etc/rc.d/rc.sysinit
#+ (on a Red Hat Linux installation)
if [ −f /fsckoptions ]; then
fsckoptions=`cat /fsckoptions`
...
fi
#
#
if [ −e "/proc/ide/${disk[$device]}/media" ] ; then
hdmedia=`cat /proc/ide/${disk[$device]}/media`
...
fi
#
#
if [ ! −n "`uname −r | grep −− "−"`" ]; then
ktag="`cat /proc/version`"
...
fi
#
#
if [ $usb = "1" ]; then
sleep 5
mouseoutput=`cat /proc/bus/usb/devices 2>/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 14−1. 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
Advanced Bash−Scripting Guide
Chapter 14. Command Substitution 238
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 14−2. 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" # The 'echo' command is critical
done` #+ to command substitution.
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
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 <stdio.h>
/* "Hello, world." C program */
int main()
{
printf( "Hello, world." );
return (0);
}
bash$ gcc −o hello hello.c
#!/bin/bash
# hello.sh
greeting=`./hello`
echo $greeting
Advanced Bash−Scripting Guide
Chapter 14. Command Substitution 239
bash$ sh hello.sh
Hello, world.
The $(COMMAND) 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.
Examples of command substitution in shell scripts:
Example 10−71. Example 10−262. Example 9−263. Example 12−24. Example 12−155. Example 12−126. Example 12−397. Example 10−138. Example 10−109. Example 12−2410. Example 16−711. Example A−1812. Example 28−113. Example 12−3214. Example 12−3315. Example 12−3416.
Advanced Bash−Scripting Guide
Chapter 14. Command Substitution 240
Chapter 15. Arithmetic Expansion
Arithmetic expansion provides a powerful tool for performing 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 in arithmetic expansion has been superseded by double parentheses $((...))
or the very convenient let construction.
z=$(($z+3))
# $((EXPRESSION)) is arithmetic expansion. # Not to be confused with
#+ command substitution.
let z=z+3
let "z += 3" # Quotes permit the use of spaces and special operators.
# The 'let' operator actually performs arithmetic evaluation,
#+ rather than expansion.
All the above are equivalent. You may use whichever one "rings your chimes".
Examples of arithmetic expansion in scripts:
Example 12−61. Example 10−142. Example 26−13. Example 26−64. Example A−185.
Chapter 15. Arithmetic Expansion 241
Chapter 16. I/O Redirection
There are always three default "files" open, stdin (the keyboard), stdout (the screen), and stderr (error
messages output to the screen). These, and any other open files, can be redirected. Redirection simply means
capturing output from a file, command, program, script, or even code block within a script (see Example 3−1
and Example 3−2) and sending it as input to another file, command, program, or script.
Each open file gets assigned a file descriptor. [45] The file descriptors for stdin, stdout, and stderr are
0, 1, and 2, respectively. For opening additional files, there remain descriptors 3 to 9. It is sometimes useful to
assign one of these additional file descriptors to stdin, stdout, or stderr as a temporary duplicate link.
[46] This simplifies restoration to normal after complex redirection and reshuffling (see Example 16−1).
COMMAND_OUTPUT >
# 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".
#==============================================================================
# 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\"."
Chapter 16. I/O Redirection 242
# These redirection commands automatically "reset" after each line.
# Redirecting stderr, one line at a time.
ERRORFILE=script.errors
bad_command1 2>$ERRORFILE # Error message sent to $ERRORFILE.
bad_command2 2>>$ERRORFILE # Error message appended to $ERRORFILE.
bad_command3 # 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.
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
[j]<>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.
read −n 4 <&3 # Read only 4 characters.
echo −n . >&3 # Write a decimal point there.
exec 3>&− # Close fd 3.
cat File # ==> 1234.67890
# Random access, by golly.
|
# 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".
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Chapter 16. I/O Redirection 243
Multiple instances of input and output redirection and/or pipes can be combined in a single command line.
command < input−file > output−file
command1 | command2 | command3 > output−file
See Example 12−23 and Example A−16.
Multiple output streams may be redirected to one file.
ls −yz >> command.log 2>&1
# Capture result of illegal options "yz" to "ls" in file "command.log".
# Because stderr redirected to the file, any error messages will also be there.
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.
# Redirecting only stderr to a pipe.
exec 3>&1 # Save current "value" of stdout.
ls −l 2>&1 >&3 3>&− | grep bad 3>&− # Close fd 3 for 'grep' (but not 'ls').
# ^^^^ ^^^^
exec 3>&− # Now close it for the remainder of the script.
# Thanks, S.C.
For a more detailed introduction to I/O redirection see Appendix D.
16.1. Using exec
An exec <filename command redirects stdin to a file. From that point on, all stdin comes from that file,
rather than its normal source (usually keyboard input). This provides a method of reading a file line by line
and possibly parsing each line of input using sed and/or awk.
Example 16−1. Redirecting stdin using exec
#!/bin/bash
# Redirecting stdin using 'exec'.
exec 6<&0 # Link file descriptor #6 with stdin.
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Chapter 16. I/O Redirection 244
# Saves stdin.
exec < data−file # stdin replaced by file "data−file"
read a1 # Reads first line of file "data−file".
read a2 # Reads second line of file "data−file."
echo
echo "Following lines read from file."
echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−"
echo $a1
echo $a2
echo; echo; echo
exec 0<&6 6<&−
# Now restore stdin from fd #6, where it had been saved,
#+ and close fd #6 ( 6<&− ) to free it for other processes to use.
#
# <&6 6<&− also works.
echo −n "Enter data "
read b1 # Now "read" functions as expected, reading from normal stdin.
echo "Input read from stdin."
echo "−−−−−−−−−−−−−−−−−−−−−−"
echo "b1 = $b1"
echo
exit 0
Similarly, an exec >filename command redirects stdout to a designated file. This sends all command
output that would normally go to stdout to that file.
Example 16−2. 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"
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Chapter 16. I/O Redirection 245
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 16−3. 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!"
echo "Usage: $0 input−file output−file"
exit $E_FILE_ACCESS
fi # Will exit with same error
#+ even if input file ($1) not specified.
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.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
exec 1>&7 7>&− # Restore stout.
exec 0<&4 4<&− # Restore stdin.
# After restoration, the following line prints to stdout as expected.
echo "File \"$1\" written to \"$2\" as uppercase conversion."
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Chapter 16. I/O Redirection 246
exit 0
16.2. 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 23−7). The < operator at the end of
the code block accomplishes this.
Example 16−4. Redirected while loop
#!/bin/bash
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 ] # Why is variable $name in quotes?
do
read name # Reads from $Filename, rather than stdin.
echo $name
let "count += 1"
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
echo; echo "$count names read"; echo
# 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.
exit 0
Example 16−5. 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.
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Chapter 16. I/O Redirection 247
if [ −z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
exec 3<&0 # Save stdin to file descriptor 3.
exec 0<"$Filename" # Redirect standard input.
count=0
echo
while [ "$name" != Smith ]
do
read name # Reads from redirected stdin ($Filename).
echo $name
let "count += 1"
done <"$Filename" # Loop reads from file $Filename.
# ^^^^^^^^^^^^
exec 0<&3 # Restore old stdin.
exec 3<&− # Close temporary fd 3.
echo; echo "$count names read"; echo
exit 0
Example 16−6. Redirected until loop
#!/bin/bash
# Same as previous example, but with "until" loop.
if [ −z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
# while [ "$name" != Smith ]
until [ "$name" = Smith ] # Change != to =.
do
read name # Reads from $Filename, rather than stdin.
echo $name
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
# Same results as with "while" loop in previous example.
exit 0
Example 16−7. Redirected for loop
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Chapter 16. I/O Redirection 248
#!/bin/bash
if [ −z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
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.
#
# More concise is line_count=$(wc < "$Filename")
for name in `seq $line_count` # Recall that "seq" prints sequence of numbers.
# while [ "$name" != Smith ] −− more complicated than a "while" loop −−
do
read name # Reads from $Filename, rather than stdin.
echo $name
if [ "$name" = Smith ] # Need all this extra baggage here.
then
break
fi
done <"$Filename" # Redirects stdin to file $Filename.
# ^^^^^^^^^^^^
exit 0
We can modify the previous example to also redirect the output of the loop.
Example 16−8. Redirected for loop (both stdin and stdout redirected)
#!/bin/bash
if [ −z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
Savefile=$Filename.new # Filename to save results in.
FinalName=Jonah # Name to terminate "read" on.
line_count=`wc $Filename | awk '{ print $1 }'` # Number of lines in target file.
for name in `seq $line_count`
do
read name
echo "$name"
if [ "$name" = "$FinalName" ]
then
break
Advanced Bash−Scripting Guide
Chapter 16. I/O Redirection 249
fi
done < "$Filename" > "$Savefile" # Redirects stdin to file $Filename,
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^ and saves it to backup file.
exit 0
Example 16−9. Redirected if/then test
#!/bin/bash
if [ −z "$1" ]
then
Filename=names.data # Default, if no filename specified.
else
Filename=$1
fi
TRUE=1
if [ "$TRUE" ] # if true and if : also work.
then
read name
echo $name
fi <"$Filename"
# ^^^^^^^^^^^^
# Reads only first line of file.
# An "if/then" test has no way of iterating unless embedded in a loop.
exit 0
Example 16−10. Data file "names.data" for above examples
Aristotle
Belisarius
Capablanca
Euler
Goethe
Hamurabi
Jonah
Laplace
Maroczy
Purcell
Schmidt
Semmelweiss
Smith
Turing
Venn
Wilson
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.
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16.3. Applications
Clever use of I/O redirection permits parsing and stitching together snippets of command output (see Example
11−6). This permits generating report and log files.
Example 16−11. Logging events
#!/bin/bash
# logevents.sh, by Stephane Chazelas.
# Event logging to a file.
# Must be run as root (for write access in /var/log).
ROOT_UID=0 # Only users with $UID 0 have root privileges.
E_NOTROOT=67 # Non−root exit error.
if [ "$UID" −ne "$ROOT_UID" ]
then
echo "Must be root to run this script."
exit $E_NOTROOT
fi
FD_DEBUG1=3
FD_DEBUG2=4
FD_DEBUG3=5
# Uncomment one of the two lines below to activate script.
# LOG_EVENTS=1
# LOG_VARS=1
log() # Writes time and date to log file.
{
echo "$(date) $*" >&7 # This *appends* the date to the file.
# See below.
}
case $LOG_LEVEL in
1) exec 3>&2 4> /dev/null 5> /dev/null;;
2) exec 3>&2 4>&2 5> /dev/null;;
3) exec 3>&2 4>&2 5>&2;;
*) exec 3> /dev/null 4> /dev/null 5> /dev/null;;
esac
FD_LOGVARS=6
if [[ $LOG_VARS ]]
then exec 6>> /var/log/vars.log
else exec 6> /dev/null # Bury output.
fi
FD_LOGEVENTS=7
if [[ $LOG_EVENTS ]]
then
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# then exec 7 >(exec gawk '{print strftime(), $0}' >> /var/log/event.log)
# Above line will not work in Bash, version 2.04.
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 fd #7.
exit 0
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Chapter 16. I/O Redirection 252
Chapter 17. Here Documents
A here document uses a special form of I/O redirection to feed a command list to an interactive program or
command, such as ftp, telnet, or ex. A "limit string" delineates (frames) the command list. The special symbol
<< designates the limit string. This has the effect of redirecting the output of a file into the program, similar to
interactive−program < command−file, where command−file contains
command #1
command #2
...
The "here document" alternative looks like this:
#!/bin/bash
interactive−program <<LimitString
command #1
command #2
...
LimitString
Choose a limit string sufficiently unusual that it will not occur anywhere in the command list and confuse
matters.
Note that here documents may sometimes be used to good effect with non−interactive utilities and commands.
Example 17−1. dummyfile: Creates a 2−line dummy file
#!/bin/bash
# Non−interactive use of 'vi' to edit a file.
# Emulates 'sed'.
E_BADARGS=65
if [ −z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_BADARGS
fi
TARGETFILE=$1
# Insert 2 lines in file, then save.
#−−−−−−−−Begin here document−−−−−−−−−−−#
vi $TARGETFILE <<x23LimitStringx23
i
This is line 1 of the example file.
This is line 2 of the example file.
^[
ZZ
x23LimitStringx23
#−−−−−−−−−−End here document−−−−−−−−−−−#
# Note that ^[ above is a literal escape
Chapter 17. Here Documents 253
#+ typed by Control−V <Esc>.
# Bram Moolenaar points out that this may not work with 'vim',
#+ because of possible problems with terminal interaction.
exit 0
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.
Example 17−2. broadcast: Sends message to everyone logged in
#!/bin/bash
wall <<zzz23EndOfMessagezzz23
E−mail your noontime orders for pizza to the system administrator.
(Add an extra dollar for anchovy or mushroom topping.)
# Additional message text goes here.
# Note: Comment lines printed by 'wall'.
zzz23EndOfMessagezzz23
# Could have been done more efficiently by
# wall <message−file
# However, saving a message template in a script saves work.
exit 0
Example 17−3. Multi−line message using cat
#!/bin/bash
# 'echo' is fine for printing single line messages,
# but somewhat problematic for for message blocks.
# A 'cat' here document overcomes this limitation.
cat <<End−of−message
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
End−of−message
exit 0
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# Code below disabled, due to "exit 0" above.
# S.C. points out that the following also works.
echo "−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
Advanced Bash−Scripting Guide
Chapter 17. Here Documents 254
This is the last line of the message.
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−"
# However, text may not include double quotes unless they are escaped.
The − option to mark a here document limit string (<<−LimitString) suppresses tabs (but not spaces) in
the output. This may be useful in making a script more readable.
Example 17−4. Multi−line message, with tabs suppressed
#!/bin/bash
# Same as previous example, but...
# The − option to a here document <<−
# suppresses tabs in the body of the document, but *not* spaces.
cat <<−ENDOFMESSAGE
This is line 1 of the message.
This is line 2 of the message.
This is line 3 of the message.
This is line 4 of the message.
This is the last line of the message.
ENDOFMESSAGE
# The output of the script will be flush left.
# Leading tab in each line will not show.
# Above 5 lines of "message" prefaced by a tab, not spaces.
# Spaces not affected by <<− .
exit 0
A here document supports parameter and command substitution. It is therefore possible to pass different
parameters to the body of the here document, changing its output accordingly.
Example 17−5. Here document with parameter substitution
#!/bin/bash
# Another 'cat' here document, using parameter substitution.
# Try it with no command line parameters, ./scriptname
# Try it with one command line parameter, ./scriptname Mortimer
# Try it with one two−word quoted command line parameter,
# ./scriptname "Mortimer Jones"
CMDLINEPARAM=1 # Expect at least command line parameter.
if [ $# −ge $CMDLINEPARAM ]
then
NAME=$1 # If more than one command line param,
# then just take the first.
else
NAME="John Doe" # Default, if no command line parameter.
fi
RESPONDENT="the author of this fine script"
Advanced Bash−Scripting Guide
Chapter 17. Here Documents 255
cat <<Endofmessage
Hello, there, $NAME.
Greetings to you, $NAME, from $RESPONDENT.
# This comment shows up in the output (why?).
Endofmessage
# Note that the blank lines show up in the output.
# So does the "comment".
exit 0
Quoting or escaping the "limit string" at the head of a here document disables parameter substitution within its
body. This has very limited usefulness.
Example 17−6. Parameter substitution turned off
#!/bin/bash
# A 'cat' here document, but with parameter substitution disabled.
NAME="John Doe"
RESPONDENT="the author of this fine script"
cat <<'Endofmessage'
Hello, there, $NAME.
Greetings to you, $NAME, from $RESPONDENT.
Endofmessage
# No parameter substitution when the "limit string" is quoted or escaped.
# Either of the following at the head of the here document would have the same effect.
# cat <<"Endofmessage"
# cat <<\Endofmessage
exit 0
This is a useful script containing a here document with parameter substitution.
Example 17−7. upload: Uploads a file pair to "Sunsite" incoming directory
#!/bin/bash
# upload.sh
# Upload file pair (Filename.lsm, Filename.tar.gz)
# to incoming directory at Sunsite (metalab.unc.edu).
E_ARGERROR=65
if [ −z "$1" ]
then
echo "Usage: `basename $0` filename"
exit $E_ARGERROR
fi
Advanced Bash−Scripting Guide
Chapter 17. Here Documents 256
Filename=`basename $1` # Strips pathname out of file name.
Server="metalab.unc.edu"
Directory="/incoming/Linux"
# These need not be hard−coded into script,
# but may instead be changed to command line argument.
Password="your.e−mail.address" # Change above to suit.
ftp −n $Server <<End−Of−Session
# −n option disables auto−logon
user anonymous "$Password"
binary
bell # Ring 'bell' after each file transfer
cd $Directory
put "$Filename.lsm"
put "$Filename.tar.gz"
bye
End−Of−Session
exit 0
A here document can supply input to a function in the same script.
Example 17−8. Here documents and functions
#!/bin/bash
# here−function.sh
GetPersonalData ()
{
read firstname
read lastname
read address
read city
read state
read zipcode
} # This certainly looks like an interactive function, but...
# Supply input to the above function.
GetPersonalData <<RECORD001
Bozo
Bozeman
2726 Nondescript Dr.
Baltimore
MD
21226
RECORD001
echo
echo "$firstname $lastname"
echo "$address"
echo "$city, $state $zipcode"
echo
Advanced Bash−Scripting Guide
Chapter 17. Here Documents 257
exit 0
It is possible to use : as a dummy command accepting output from a here document. This, in effect, creates an
"anonymous" here document.
Example 17−9. "Anonymous" Here Document
#!/bin/bash
: <<TESTVARIABLES
${HOSTNAME?}${USER?}${MAIL?} # Print error message if one of the variables not set.
TESTVARIABLES
exit 0
A variation of the above technique permits "commenting out" blocks of code.
Example 17−10. Commenting out a block of code
#!/bin/bash
# commentblock.sh
: << COMMENTBLOCK
echo "This line will not echo."
This is a comment line missing the "#" prefix.
This is another comment line missing the "#" prefix.
&*@!!++=
The above line will cause no error message,
because the Bash interpreter will ignore it.
COMMENTBLOCK
echo "Exit value of above \"COMMENTBLOCK\" is $?." # 0
# No error shown.
# The above technique also comes in useful for commenting out
#+ a block of working code for debugging purposes.
# This saves having to put a "#" at the beginning of each line,
#+ then having to go back and delete each "#" later.
: << DEBUGXXX
for file in *
do
cat "$file"
done
DEBUGXXX
exit 0
Yet another twist of this nifty trick makes "self−documenting" scripts possible.
Advanced Bash−Scripting Guide
Chapter 17. Here Documents 258
Example 17−11. A self−documenting script
#!/bin/bash
# self−document.sh: self−documenting script
# Modification of "colm.sh".
DOC_REQUEST=70
if [ "$1" = "−h" −o "$1" = "−−help" ] # Request help.
then
echo; echo "Usage: $0 [directory−name]"; echo
sed −−silent −e '/DOCUMENTATIONXX$/,/^DOCUMENTATION/p' "$0" |
sed −e '/DOCUMENTATIONXX/d'; exit $DOC_REQUEST; fi
: << DOCUMENTATIONXX
List the statistics of a specified directory in tabular format.
−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
The command line parameter gives the directory to be listed.
If no directory specified or directory specified cannot be read,
then list the current working directory.
DOCUMENTATIONXX
if [ −z "$1" −o ! −r "$1" ]
then
directory=.
else
directory="$1"
fi
echo "Listing of "$directory":"; echo
(printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG−NAME\n" \
; ls −l "$directory" | sed 1d) | column −t
exit 0
Here documents create temporary files, but these files are deleted after opening and are not accessible to
any other process.
bash$ bash −c 'lsof −a −p $$ −d0' << EOF
> EOF
lsof 1213 bozo 0r REG 3,5 0 30386 /tmp/t1213−0−sh (deleted)
Some utilities will not work inside a here document.
For those tasks too complex for a "here document", consider using the expect scripting language, which is
specifically tailored for feeding input into interactive programs.
Advanced Bash−Scripting Guide
Chapter 17. Here Documents 259
Chapter 18. 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 you make
the copies, send a 100−line Bash script to the first person
on the list given 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
xbill. 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 18. Recess Time 260
Part 4. 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
19. Regular Expressions
19.1. A Brief Introduction to Regular Expressions
19.2. Globbing
20. Subshells
21. Restricted Shells
22. Process Substitution
23. Functions
23.1. Complex Functions and Function Complexities
23.2. Local Variables
24. Aliases
25. List Constructs
26. Arrays
27. Files
28. /dev and /proc
28.1. /dev
28.2. /proc
29. Of Zeros and Nulls
30. Debugging
31. Options
32. Gotchas
33. Scripting With Style
33.1. Unofficial Shell Scripting Stylesheet
34. Miscellany
34.1. Interactive and non−interactive shells and scripts
34.2. Shell Wrappers
34.3. Tests and Comparisons: Alternatives
34.4. Recursion
34.5. "Colorizing" Scripts
34.6. Optimizations
34.7. Assorted Tips
34.8. Security Issues
34.9. Portability Issues
34.10. Shell Scripting Under Windows
35. Bash, version 2
Part 4. Advanced Topics 261
Chapter 19. Regular Expressions
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.
19.1. A Brief Introduction to Regular Expressions
An expression is a string of characters. Those characters that have 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 for the symbols that follow. Regular Expressions are sets of characters and/or
metacharacters that UNIX endows with special features. [47]
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 substring or an entire string).
The asterisk −− * −− matches any number of repeats of the character string or RE preceding it,
including zero.
"1133*" matches 11 + one or more 3's + possibly other characters: 113,
1133, 111312, and so forth.
•
The dot −− . −− matches any one character, except a newline. [48]
"13." matches 13 + at least one of any character (including a space):
1133, 11333, but not 13 (additional character missing).
•
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.
"^$" matches blank lines.
The ^ and $ are known as anchors, since they indicate or anchor a position within an
RE.
•
Brackets −− [...] −− enclose a set of characters to match in a single RE.
"[xyz]" matches the characters x, y, or z.
"[c−n]" matches any of the characters in the range c to n.
"[B−Pk−y]" matches any of the characters in the ranges B to P and k to y.
"[a−z0−9]" matches any lowercase letter or any digit.
"[^b−d]" matches all characters 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).
•
Chapter 19. Regular Expressions 262
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.
A "\$" reverts back to its literal meaning of "$", rather than its RE meaning of end−of−line. Likewise
a "\\" has the literal meaning of "\".
•
Escaped "angle brackets" −− \<...\> −− mark word boundaries.
The angle brackets must be escaped, since otherwise they have only their literal character meaning.
"\<the\>" matches the word "the", but not the words "them", "there", "other", etc.
bash$ 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 '\<the\>' textfile
This is the only instance of line 2.
•
Extended REs. 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).
•
Advanced Bash−Scripting Guide
Chapter 19. Regular Expressions 263
Curly brackets are not available as an RE in the "classic" (non−POSIX compliant)
version of awk. However, 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 groups of REs. They are useful with the following "|" operator and in
substring extraction using expr.
•
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.
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
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
To see POSIX character classes used in scripts, refer to Example 12−14 and Example
12−15.
•
Advanced Bash−Scripting Guide
Chapter 19. Regular Expressions 264
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−13 and Example A−18 for illustrations of this.
"Sed & Awk", by Dougherty and Robbins gives a very complete and lucid treatment of REs (see the
Bibliography).
19.2. Globbing
Bash itself cannot recognize Regular Expressions. In scripts, commands and utilities, such as sed and awk,
interpret RE's.
Bash does carry out filename expansion, a process known as "globbing", but this does not use the standard RE
set. Instead, globbing recognizes and expands wildcards. Globbing interprets the standard wildcard characters,
* and ?, character lists in square brackets, and certain other special characters (such as ^ for negating the sense
of a match). There are some important limitations on wildcard characters in globbing, however. Strings
containing * will not match filenames that start with a dot, as, for example, .bashrc. [49] Likewise, the ?
has a different meaning in globbing than as part of an RE.
bash$ ls −l
total 2
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 a.1
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 b.1
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 c.1
−rw−rw−r−− 1 bozo bozo 466 Aug 6 17:48 t2.sh
−rw−rw−r−− 1 bozo bozo 758 Jul 30 09:02 test1.txt
bash$ ls −l t?.sh
−rw−rw−r−− 1 bozo bozo 466 Aug 6 17:48 t2.sh
bash$ ls −l [ab]*
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 a.1
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 b.1
bash$ ls −l [a−c]*
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 a.1
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 b.1
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 c.1
bash$ ls −l [^ab]*
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 c.1
−rw−rw−r−− 1 bozo bozo 466 Aug 6 17:48 t2.sh
−rw−rw−r−− 1 bozo bozo 758 Jul 30 09:02 test1.txt
bash$ ls −l {b*,c*,*est*}
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 b.1
−rw−rw−r−− 1 bozo bozo 0 Aug 6 18:42 c.1
−rw−rw−r−− 1 bozo bozo 758 Jul 30 09:02 test1.txt
bash$ echo *
a.1 b.1 c.1 t2.sh test1.txt
bash$ echo t*
t2.sh test1.txt
Even an echo command performs wildcard expansion on filenames.
Advanced Bash−Scripting Guide
Chapter 19. Regular Expressions 265
See also Example 10−4.
Advanced Bash−Scripting Guide
Chapter 19. Regular Expressions 266
Chapter 20. Subshells
Running a shell script launches another instance of the command processor. Just as your commands are
interpreted at the command line prompt, similarly does a script batch process a list of commands in a file.
Each shell script running is, in effect, a subprocess of the parent shell, the one that gives you the prompt at the
console or in an xterm window.
A shell script can also launch subprocesses. These subshells let the script do parallel processing, in effect
executing multiple subtasks simultaneously.
Command List in Parentheses
( 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, local variables.
Example 20−1. Variable scope in a subshell
#!/bin/bash
# subshell.sh
echo
outer_variable=Outer
(
inner_variable=Inner
echo "From subshell, \"inner_variable\" = $inner_variable"
echo "From subshell, \"outer\" = $outer_variable"
)
echo
if [ −z "$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 uninitialized because
# variables defined in a subshell are "local variables".
echo
exit 0
See also Example 32−1.
+
Chapter 20. Subshells 267
Directory changes made in a subshell do not carry over to the parent shell.
Example 20−2. 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)
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
One application of this is testing whether a variable is defined.
if (set −u; : $variable) 2> /dev/null
then
echo "Variable is set."
fi
# Could also be written [[ ${variable−x} != x || ${variable−y} != y ]]
# or [[ ${variable−x} != x$variable ]]
# or [[ ${variable+x} = x ]])
Another application is checking for a lock file:
if (set −C; : > lock_file) 2> /dev/null
then
echo "Another user is already running that script."
exit 65
Advanced Bash−Scripting Guide
Chapter 20. Subshells 268
fi
# Thanks, S.C.
Processes may execute in parallel within different subshells. This permits breaking a complex task into
subcomponents processed concurrently.
Example 20−3. 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 command block between curly braces does not launch a subshell.
{ command1; command2; command3; ... }
Advanced Bash−Scripting Guide
Chapter 20. Subshells 269
Chapter 21. 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.
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 21−1. 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."
SHELL="/bin/ash"
echo
echo "\$SHELL= $SHELL"
echo
echo
Chapter 21. Restricted Shells 270
echo "Attempting to redirect output in restricted mode."
ls −l /usr/bin > bin.files
ls −l bin.files # Try to list attempted file creation effort.
echo
exit 0
Advanced Bash−Scripting Guide
Chapter 21. Restricted Shells 271
Chapter 22. Process Substitution
Process substitution is the counterpart to command substitution. Command substitution sets a
variable to the result of a command, as in dir_contents=`ls −al` or xref=$( grep word datafile). Process
substitution feeds the output of a process to another process (in other words, it sends the results of a command
to another command).
Command substitution template
command within parentheses
>(command)
<(command)
These initiate process substitution. This uses /dev/fd/<n> files to send the results of the process
within parentheses to another process. [50]
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 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/<n>
files, Bash may use temporary files. (Thanks, S.C.)
cat <(ls −l)
# Same as ls −l | cat
sort −k 9 <(ls −l /bin) <(ls −l /usr/bin) <(ls −l /usr/X11R6/bin)
# Lists all the files in the 3 main 'bin' directories, and sorts by filename.
# Note that three (count 'em) distinct commands are fed to 'sort'.
diff <(command1) <(command2) # Gives difference in command output.
tar cf >(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/<n> 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>&−
Chapter 22. Process Substitution 272
exec 3>&−
# Thanks, S.C.
A reader of this document 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)
# 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 S.C. points out, 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 # Same output as above.
Advanced Bash−Scripting Guide
Chapter 22. Process Substitution 273
Chapter 23. 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, 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...
}
Functions are called, triggered, simply by invoking their names.
Example 23−1. Simple function
#!/bin/bash
funky ()
{
echo "This is a funky function."
echo "Now exiting funky function."
} # Function declaration must precede call.
# Now, call the function.
funky
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 # This doesn't help either.
Chapter 23. Functions 274
f1 # 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 permissable.
# Thanks, S.C.
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 # Does nothing, since calling "f1" does not automatically call "f2".
f2 # Now, it's all right to call "f2",
#+ since its definition has been made visible by calling "f1".
# Thanks, S.C.
Function declarations can appear in unlikely places, even where a command would otherwise go.
ls −l | foo() { echo "foo"; } # Permissable, 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.
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Chapter 23. Functions 275
# 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.
# Thanks, S.C.
23.1. 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 23−2. Function Taking Parameters
#!/bin/bash
# Functions and parameters
DEFAULT=default # Default param value.
func2 () {
if [ −z "$1" ] # Is parameter #1 zero length?
then
echo "−Parameter #1 is zero length.−" # Or no parameter passed.
else
echo "−Param #1 is \"$1\".−"
fi
variable=${1−$DEFAULT} # What does
echo "variable = $variable" #+ parameter substitution show?
# −−−−−−−−−−−−−−−−−−−−−−−−−−−
# It distinguishes between
#+ no param and a null param.
if [ "$2" ]
then
echo "−Parameter #2 is \"$2\".−"
fi
return 0
}
echo
echo "Nothing passed."
func2 # Called with no params
echo
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Chapter 23. Functions 276
echo "Zero−length parameter passed."
func2 "" # Called with zero−length param
echo
echo "Null parameter passed."
func2 "$uninitialized_param" # Called with uninitialized param
echo
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 34−10).
In contrast to certain other programming languages, shell scripts normally pass only value parameters to
functions. [51] Variable names (which are actually pointers), if passed as parameters to functions, will be
treated as string literals and cannot be dereferenced. Functions interpret their arguments literally.
Exit and Return
exit status
Functions return a value, called an exit status. 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 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 [52] 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 23−3. Maximum of two numbers
#!/bin/bash
# max.sh: Maximum of two integers.
E_PARAM_ERR=−198 # If less than 2 params passed to function.
EQUAL=−199 # Return value if both params equal.
max2 () # Returns larger of two numbers.
{ # Note: numbers compared must be less than 257.
if [ −z "$2" ]
then
return $E_PARAM_ERR
Advanced Bash−Scripting Guide
Chapter 23. Functions 277
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 $return_val."
fi
exit 0
# Exercise (easy):
# −−−−−−−−−−−−−−−
# Convert this to an interactive script,
#+ that is, have the script ask for input (two numbers).
For a function to return a string or array, use a dedicated variable.
count_lines_in_etc_passwd()
{
[[ −r /etc/passwd ]] && REPLY=$(echo $(wc −l < /etc/passwd))
# If /etc/passwd is readable, set REPLY to line count.
# Returns both a parameter value and status information.
}
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 23−4. Converting numbers to Roman numerals
#!/bin/bash
# Arabic number to Roman numeral conversion
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Chapter 23. Functions 278
# 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"
done
return $number
# Exercise:
# −−−−−−−−
# Explain how this function works.
# Hint: division by successive subtraction.
}
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
Advanced Bash−Scripting Guide
Chapter 23. Functions 279
echo
exit 0
See also Example 10−28.
The largest positive integer a function can return is 256. 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 23−5. Testing large return values in a function
#!/bin/bash
# return−test.sh
# The largest positive value a function can return is 256.
return_test () # Returns whatever passed to it.
{
return $1
}
return_test 27 # o.k.
echo $? # Returns 27.
return_test 256 # Still o.k.
echo $? # Returns 256.
return_test 257 # Error!
echo $? # Returns 1 (return code for miscellaneous error).
return_test −151896 # However, large negative numbers work.
echo $? # Returns −151896.
exit 0
As we have seen, a function can return a large negative value. This also permits returning large
positive integer, using a bit of trickery.
An alternate method of accomplishing this 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 $? # 0
echo "return value = $Return_Val" # 1
Advanced Bash−Scripting Guide
Chapter 23. Functions 280
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
Example 23−6. 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 # Return value if both params equal.
MAXRETVAL=256 # Maximum positive return value from a function.
E_PARAM_ERR=−99999 # Parameter error.
E_NPARAM_ERR=99999 # "Normalized" parameter error.
max2 () # Returns larger of two numbers.
{
if [ −z "$2" ]
then
return $E_PARAM_ERR
fi
if [ "$1" −eq "$2" ]
then
return $EQUAL
else
if [ "$1" −gt "$2" ]
then
retval=$1
else
retval=$2
fi
fi
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
# This is a workaround to enable returning a large integer
# from this function.
if [ "$retval" −gt "$MAXRETVAL" ] # If out of range,
then # then
let "retval = (( 0 − $retval ))" # adjust to a negative value.
# (( 0 − $VALUE )) changes the sign of VALUE.
fi
# Large *negative* return values permitted, fortunately.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
return $retval
}
max2 33001 33997
return_val=$?
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
if [ "$return_val" −lt 0 ] # If "adjusted" negative number,
Advanced Bash−Scripting Guide
Chapter 23. Functions 281
then # then
let "return_val = (( 0 − $return_val ))" # renormalize to positive.
fi # "Absolute value" of $return_val.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
if [ "$return_val" −eq "$E_NPARAM_ERR" ]
then # Parameter error "flag" gets sign changed, too.
echo "Error: Too few parameters."
elif [ "$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
See also Example A−8.
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 23−7. Real name from username
#!/bin/bash
# From username, gets "real name" from /etc/passwd.
ARGCOUNT=1 # Expect one arg.
E_WRONGARGS=65
file=/etc/passwd
pattern=$1
if [ $# −ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` USERNAME"
exit $E_WRONGARGS
fi
file_excerpt () # Scan file for pattern, the print relevant portion of line.
{
while read line # while does not necessarily need "[ condition]"
do
echo "$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
Advanced Bash−Scripting Guide
Chapter 23. Functions 282
# grep PATTERN /etc/passwd | awk −F":" '{ print $5 }'
# 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 alternative, 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 () # This doesn't work.
{
echo $*
} | tr a b # A nested code block is mandatory here.
# Thanks, S.C.
23.2. 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.
Example 23−8. Local variable visibility
#!/bin/bash
func ()
Advanced Bash−Scripting Guide
Chapter 23. Functions 283
{
local loc_var=23 # Declared local.
echo
echo "\"loc_var\" in function = $loc_var"
global_var=999 # Not declared local.
echo "\"global_var\" in function = $global_var"
}
func
# Now, see if local 'a' exists outside function.
echo
echo "\"loc_var\" outside function = $loc_var"
# "loc_var" outside function =
# Nope, $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
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" # global_var =
# Function "func" has not yet been called,
#+ so $global_var is not visible here.
func
echo "global_var = $global_var" # global_var = 37
# Has been set by function call.
23.2.1. Local variables make recursion possible.
Local variables permit recursion, [53] but this practice generally involves much computational overhead and
is definitely not recommended in a shell script. [54]
Example 23−9. Recursion, using a local variable
#!/bin/bash
# factorial
# −−−−−−−−−
# Does bash permit recursion?
# Well, yes, but...
Advanced Bash−Scripting Guide
Chapter 23. Functions 284
# You gotta have rocks in your head to try it.
MAX_ARG=5
E_WRONG_ARGS=65
E_RANGE_ERR=66
if [ −z "$1" ]
then
echo "Usage: `basename $0` number"
exit $E_WRONG_ARGS
fi
if [ "$1" −gt $MAX_ARG ]
then
echo "Out of range (5 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.
let "factorial = $number * $?"
fi
return $factorial
}
fact $1
echo "Factorial of $1 is $?."
exit 0
See also Example A−17 for an example of recursion in a script. Be aware that recursion is resource−intensive
and executes slowly, and is therefore generally not appropriate to use in a script.
Advanced Bash−Scripting Guide
Chapter 23. Functions 285
Chapter 24. 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 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 losing important files.
In a script, aliases have very limited usefulness. It would be quite 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. [55] 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 24−1. Aliases within a script
#!/bin/bash
# Invoke with command line parameter to exercise last section of this script.
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.
# Alias handles concatenated variables, including wild−card o.k.
TRUE=1
Chapter 24. Aliases 286
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 "$0" parameter expands immediately upon declaration of the alias.
exit 0
The unalias command removes a previously set alias.
Example 24−2. unalias: Setting and unsetting an alias
#!/bin/bash
shopt −s expand_aliases # Enables alias expansion.
alias llm='ls −al | more'
llm
echo
unalias llm # Unset alias.
llm
# Error message results, since 'llm' no longer recognized.
exit 0
bash$ ./unalias.sh
total 6
drwxrwxr−x 2 bozo bozo 3072 Feb 6 14:04 .
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Chapter 24. Aliases 287
drwxr−xr−x 40 bozo bozo 2048 Feb 6 14:04 ..
−rwxr−xr−x 1 bozo bozo 199 Feb 6 14:04 unalias.sh
./unalias.sh: llm: command not found
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Chapter 24. Aliases 288
Chapter 25. 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 25−1. 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 "Less than 2 arguments passed to script."
# At least one of the chained commands returns false.
fi
# Note that "if [ ! −z $1 ]" works, but its supposed equivalent,
# if [ −n $1 ] does not. However, quoting fixes this.
# if [ −n "$1" ] works. Careful!
# It is best to always quote tested variables.
# This accomplishes the same thing, using "pure" if/then statements.
if [ ! −z "$1" ]
then
echo "Argument #1 = $1"
fi
if [ ! −z "$2" ]
then
echo "Argument #2 = $2"
echo "At least 2 arguments passed to script."
else
echo "Less than 2 arguments passed to script."
fi
# It's longer and less elegant than using an "and list".
exit 0
Example 25−2. Another command−line arg test using an "and list"
Chapter 25. List Constructs 289
#!/bin/bash
ARGS=1 # Number of arguments expected.
E_BADARGS=65 # Exit value if incorrect number of args passed.
test $# −ne $ARGS && echo "Usage: `basename $0` $ARGS argument(s)" && exit $E_BADARGS
# If condition−1 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 $?" after script termination.
Of course, an and list can also set variables to a default value.
arg1=$@ # Set $arg1 to command line arguments, if any.
[ −z "$arg1" ] && arg1=DEFAULT
# 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 25−3. Using "or lists" in combination with an "and list"
#!/bin/bash
# delete.sh, not−so−cunning file deletion utility.
# Usage: delete filename
E_BADARGS=65
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 \"$file\" not found. \
Cowardly refusing to delete a nonexistent file."
# AND LIST, to give error message if file not present.
# Note echo message continued on to a second line with an escape.
[ ! −f "$file" ] || (rm −f $file; echo "File \"$file\" deleted.")
# OR LIST, to delete file if present.
# Note logic inversion above.
Advanced Bash−Scripting Guide
Chapter 25. List Constructs 290
# AND LIST executes on true, OR LIST on false.
exit 0
If the first command in an "or list" returns true, it will execute.
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 extensive debugging.
false && true || echo false # false
# Same result as
( false && true ) || echo false # false
# But *not*
false && ( true || echo false ) # (nothing echoed)
# Note left−to−right grouping and evaluation of statements,
#+ since the logic operators "&&" and "||" have equal precedence.
# It's best to avoid such complexities, unless you know what you're doing.
# Thanks, S.C.
See Example A−8 and Example 7−4 for illustrations of using an and / or list to test variables.
Advanced Bash−Scripting Guide
Chapter 25. List Constructs 291
Chapter 26. 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 (find the contents of) an array element, use curly bracket notation, that
is, ${variable[xx]}.
Example 26−1. 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 o.k.
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.
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".
# Another way of assigning array variables...
# array_name=( XXX YYY ZZZ ... )
area2=( zero one two three four )
Chapter 26. Arrays 292
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
Bash permits array operations on variables, even if the variables are not explicitly
declared as arrays.
string=abcABC123ABCabc
echo ${string[@]} # abcABC123ABCabc
echo ${string[*]} # abcABC123ABCabc
echo ${string[0]} # abcABC123ABCabc
echo ${string[1]} # No output!
# Why?
echo ${#string[@]} # 1
# One element in the array.
# The string itself.
# Thank you, Michael Zick, for pointing this out.
Once again this demonstrates that Bash variables are untyped.
Example 26−2. Formatting a poem
#!/bin/bash
# poem.sh
# Lines of the poem (single stanza).
Line[1]="I do not know which to prefer,"
Line[2]="The beauty of inflections"
Line[3]="Or the beauty of innuendoes,"
Line[4]="The blackbird whistling"
Line[5]="Or just after."
# Attribution.
Attrib[1]=" Wallace Stevens"
Advanced Bash−Scripting Guide
Chapter 26. Arrays 293
Attrib[2]="\"Thirteen Ways of Looking at a Blackbird\""
for index in 1 2 3 4 5 # Five lines.
do
printf " %s\n" "${Line[index]}"
done
for index in 1 2 # Two attribution lines.
do
printf " %s\n" "${Attrib[index]}"
done
exit 0
Array variables have a syntax all their own, and even standard Bash commands and operators have special
options adapted for array use.
array=( zero one two three four five )
echo ${array[0]} # zero
echo ${array:0} # zero
# Parameter expansion of first element.
echo ${array:1} # ero
# Parameter expansion of first element,
#+ starting at position #1 (2nd character).
echo ${#array} # 4
# Length of first element of array.
array2=( [0]="first element" [1]="second element" [3]="fourth element" )
echo ${array2[0]} # first element
echo ${array2[1]} # second element
echo ${array2[2]} #
# Skipped in initialization, therefore null.
echo ${array2[3]} # fourth element
In an array context, some Bash builtins have a slightly altered meaning. For example, unset deletes array
elements, or even an entire array.
Example 26−3. Some special properties of arrays
#!/bin/bash
declare −a colors
# Permits declaring an array without specifying its size.
echo "Enter your favorite colors (separated from each other by a space)."
read −a colors # Enter at least 3 colors to demonstrate features below.
# Special option to 'read' command,
#+ allowing assignment of elements in an array.
echo
element_count=${#colors[@]}
Advanced Bash−Scripting Guide
Chapter 26. Arrays 294
# Special syntax to extract number of elements in array.
# element_count=${#colors[*]} works also.
#
# The "@" variable allows word splitting within quotes
#+ (extracts variables separated by whitespace).
index=0
while [ "$index" −lt "$element_count" ]
do # List all the elements in the array.
echo ${colors[$index]}
let "index = $index + 1"
done
# Each array element listed on a separate line.
# If this is not desired, use echo −n "${colors[$index]} "
#
# Doing it with a "for" loop instead:
# for i in "${colors[@]}"
# do
# echo "$i"
# done
# (Thanks, S.C.)
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 26−4. Of empty arrays and empty elements
#!/bin/bash
# empty−array.sh
# Thanks to Stephane Chazelas for the original example,
#+ and to Michael Zick for extending it.
# An empty array is not the same as an array with empty elements.
Advanced Bash−Scripting Guide
Chapter 26. Arrays 295
array0=( first second third )
array1=( '' ) # "array1" has one empty element.
array2=( ) # No elements... "array2" is empty.
echo
ListArray()
{
echo
echo "Elements in array0: ${array0[@]}"
echo "Elements in array1: ${array1[@]}"
echo "Elements in array2: ${array2[@]}"
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
echo "Number of elements in array0 = ${#array0[*]}" # 3
echo "Number of elements in array1 = ${#array1[*]}" # 1 (surprise!)
echo "Number of elements in array2 = ${#array2[*]}" # 0
}
# ===================================================================
ListArray
# Try extending those arrays
# Adding an element to an array.
array0=( "${array0[@]}" "new1" )
array1=( "${array1[@]}" "new1" )
array2=( "${array2[@]}" "new1" )
ListArray
# or
array0[${#array0[*]}]="new2"
array1[${#array1[*]}]="new2"
array2[${#array2[*]}]="new2"
ListArray
# When extended as above; arrays are 'stacks'
# The above is the 'push'
# The stack 'height' is:
height=${#array2[@]}
echo
echo "Stack height for array2 = $height"
# The 'pop' is:
unset array2[${#array2[@]}−1] # Arrays are zero based
height=${#array2[@]}
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 #
declare −a array3=( ${array0[@]:1:2} )
echo
Advanced Bash−Scripting Guide
Chapter 26. Arrays 296
echo "Elements in array3: ${array3[@]}"
# Works like a string (array of characters)
# Try some other "string" forms
# Replacement
declare −a array4=( ${array0[@]/second/2nd} )
echo
echo "Elements in array4: ${array4[@]}"
# Replace all matching wildcarded string
declare −a array5=( ${array0[@]//new?/old} )
echo
echo "Elements in array5: ${array5[@]}"
# Just when you are getting the feel for this...
declare −a array6=( ${array0[@]#*new} )
echo # This one might surprise you
echo "Elements in array6: ${array6[@]}"
declare −a array7=( ${array0[@]#new1} )
echo # After array6 this should not be a surprise
echo "Elements in array7: ${array7[@]}"
# Which looks a lot like...
declare −a 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.
# Question, are those strings hard or soft quotes?
zap='new*'
declare −a array9=( ${array0[@]/$zap/} )
echo
echo "Elements in array9: ${array9[@]}"
# Just when you thought you where still in Kansas...
declare −a array10=( ${array0[@]#$zap} )
echo
echo "Elements in array10: ${array10[@]}"
# Compare array7 with array10
# Compare array8 with array9
# Answer, must be soft quotes.
exit 0
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[@]}" )
Advanced Bash−Scripting Guide
Chapter 26. Arrays 297
# or
array2="${array1[@]}"
# Adding an element to an array.
array=( "${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" | tr '\n' ' '`) # Loads contents
# of $filename into array1.
# list file to stdout.
# change linefeeds in file to spaces.
echo ${array1[@]} # List the array.
# 1 a b c 2 d e fg
#
# Each whitespace−separated "word" in the file
#+ has been assigned to an element of the array.
element_count=${#array1[*]}
echo $element_count # 8
Clever scripting makes it possible to add array operations.
Example 26−5. 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
echo −n 'eval '
Advanced Bash−Scripting Guide
Chapter 26. Arrays 298
echo −n "$2" # Destination name
echo −n '=( ${'
echo −n "$1" # Source name
echo −n '[@]} )'
# That could all be a single command.
# Matter of style only.
}
declare −f CopyArray # Function "Pointer"
CopyArray=CpArray_Mac # 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 = ${before[@]}"
exit 0
−−
Arrays permit deploying old familiar algorithms as shell scripts. Whether this is necessarily a good idea is left
to the reader to decide.
Example 26−6. An old friend: The Bubble Sort
Advanced Bash−Scripting Guide
Chapter 26. Arrays 299
#!/bin/bash
# bubble.sh: Bubble sort, of sorts.
# Recall the algorithm for a bubble sort. In this particular version...
# With each successive pass through the array to be sorted,
#+ compare two adjacent elements, and swap them if out of order.
# At the end of the first pass, the "heaviest" element has sunk to bottom.
# At the end of the second pass, the next "heaviest" one has sunk next to bottom.
# And so forth.
# This means that each successive pass needs to traverse less of the array.
# You will therefore notice a speeding up in the printing of the later passes.
exchange()
{
# Swaps two members of the array.
local temp=${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 ] # Beginning of outer loop
do
index=0 # 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.
Advanced Bash−Scripting Guide
Chapter 26. Arrays 300
# if [[ ${Countries[$index]} > ${Countries[`expr $index + 1`]} ]]
#+ also works.
then
exchange $index `expr $index + 1` # Swap.
fi
let "index += 1"
done # End of inner loop
let "comparisons −= 1" # Since "heaviest" element bubbles to bottom,
#+ we need do one less comparison each pass.
echo
echo "$count: ${Countries[@]}" # Print resultant array at end of each pass.
echo
let "count += 1" # Increment pass count.
done # 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.
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} ${AnArray[@]:6:5} )
# Array has two elements, each of which is in turn an array.
echo "Current directory and date of last status change:"
echo "${SubArray[@]}"
exit 0
−−
Embedded arrays in combination with indirect references create some fascinating possibilities
Example 26−7. 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.
Advanced Bash−Scripting Guide
Chapter 26. Arrays 301
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[*]"
local ${!TEST2} # Indirect reference (as above).
echo "Array element VAR1_1: $VAR1_1 from ARRAY21"
}
print
echo
exit 0
# As the author of the script notes,
#+ "you can easily expand it to create named−hashes in bash."
# (Difficult) exercise for the reader: implement this.
Advanced Bash−Scripting Guide
Chapter 26. Arrays 302
−−
Arrays enable implementing a shell script version of the Sieve of Eratosthenes. Of course, a
resource−intensive application of this nature should really be written in a compiled language, such as C. It
runs excruciatingly slowly as a script.
Example 26−8. Complex array application: Sieve of Eratosthenes
#!/bin/bash
# sieve.sh
# Sieve of Eratosthenes
# Ancient algorithm for finding prime numbers.
# This runs a couple of orders of magnitude
# slower than the equivalent C program.
LOWER_LIMIT=1 # Starting with 1.
UPPER_LIMIT=1000 # Up to 1000.
# (You may set this higher... if you have time on your hands.)
PRIME=1
NON_PRIME=0
let SPLIT=UPPER_LIMIT/2
# Optimization:
# Need to test numbers only halfway to upper limit.
declare −a Primes
# Primes[] is an array.
initialize ()
{
# Initialize the array.
i=$LOWER_LIMIT
until [ "$i" −gt "$UPPER_LIMIT" ]
do
Primes[i]=$PRIME
let "i += 1"
done
# Assume all array members guilty (prime)
# until proven innocent.
}
print_primes ()
{
# Print out the members of the Primes[] array tagged as prime.
i=$LOWER_LIMIT
until [ "$i" −gt "$UPPER_LIMIT" ]
do
if [ "${Primes[i]}" −eq "$PRIME" ]
then
printf "%8d" $i
Advanced Bash−Scripting Guide
Chapter 26. Arrays 303
# 8 spaces per number gives nice, even columns.
fi
let "i += 1"
done
}
sift () # Sift out the non−primes.
{
let i=$LOWER_LIMIT+1
# We know 1 is prime, so 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
}
# Invoke the functions sequentially.
initialize
sift
print_primes
# This is what they call structured programming.
echo
exit 0
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
# Code below line will not execute.
# This improved version of the Sieve, by Stephane Chazelas,
# executes somewhat faster.
# Must invoke with command−line argument (limit of primes).
UPPER_LIMIT=$1 # From command line.
let SPLIT=UPPER_LIMIT/2 # Halfway to max number.
Advanced Bash−Scripting Guide
Chapter 26. Arrays 304
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 ${Primes[*]}
exit 0
Compare this array−based prime number generator with an alternative that does not use arrays, Example
A−17.
−−
Arrays lend themselves, to some extent, to emulating data structures for which Bash has no native support.
Example 26−9. Emulating a push−down stack
#!/bin/bash
# stack.sh: push−down stack simulation
# Similar to the CPU stack, a push−down stack stores data items
#+ sequentially, but releases them in reverse order, last−in first−out.
BP=100 # Base Pointer of stack array.
# Begin at element 100.
SP=$BP # Stack Pointer.
# Initialize it to "base" (bottom) of stack.
Data= # Contents of stack location.
# Must use local variable,
#+ because of limitation on function return range.
declare −a stack
push() # Push item on stack.
{
if [ −z "$1" ] # Nothing to push?
then
return
fi
let "SP −= 1" # Bump stack pointer.
stack[$SP]=$1
return
Advanced Bash−Scripting Guide
Chapter 26. Arrays 305
}
pop() # Pop item off stack.
{
Data= # Empty out data item.
if [ "$SP" −eq "$BP" ] # Stack empty?
then
return
fi # This also keeps SP from getting past 100,
#+ i.e., prevents a runaway stack.
Data=${stack[$SP]}
let "SP += 1" # Bump stack pointer.
return
}
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; push $value1
value2=skidoo; push $value2
value3=FINAL; push $value3
pop # FINAL
status_report
pop # skidoo
status_report
pop # 23
status_report # Last−in, first−out!
# Notice how the stack pointer decrements with each push,
#+ and increments with each pop.
echo
# =======================================================
# Exercises:
Advanced Bash−Scripting Guide
Chapter 26. Arrays 306
# −−−−−−−−−
# 1) Modify the "push()" function to permit pushing
# + multiple element on the stack with a single function call.
# 2) Modify the "pop()" function to permit popping
# + multiple element from the stack with a single function call.
# 3) Using this script as a jumping−off point,
# + write a stack−based 4−function calculator.
exit 0
−−
Fancy manipulation of array "subscripts" may require intermediate variables. For projects involving this,
again consider using a more powerful programming language, such as Perl or C.
Example 26−10. Complex array application: Exploring a weird mathematical series
#!/bin/bash
# Douglas Hofstadter's notorious "Q−series":
# Q(1) = Q(2) = 1
# Q(n) = Q(n − Q(n−1)) + Q(n − Q(n−2)), for n>2
# This is a "chaotic" integer series with strange and unpredictable behavior.
# The first 20 terms of the series are:
# 1 1 2 3 3 4 5 5 6 6 6 8 8 8 10 9 10 11 11 12
# See Hofstadter's book, "Goedel, Escher, Bach: An Eternal Golden Braid",
# p. 137, ff.
LIMIT=100 # Number of terms to calculate
LINEWIDTH=20 # Number of terms printed per line
Q[1]=1 # First two terms of series are 1.
Q[2]=1
echo
echo "Q−series [$LIMIT terms]:"
echo −n "${Q[1]} " # Output first two terms.
echo −n "${Q[2]} "
for ((n=3; n <= $LIMIT; n++)) # C−like loop conditions.
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.
let "n1 = $n − 1" # n−1
let "n2 = $n − 2" # n−2
t0=`expr $n − ${Q[n1]}` # n − Q[n−1]
t1=`expr $n − ${Q[n2]}` # n − Q[n−2]
T0=${Q[t0]} # Q[n − Q[n−1]]
T1=${Q[t1]} # Q[n − Q[n−2]]
Advanced Bash−Scripting Guide
Chapter 26. Arrays 307
Q[n]=`expr $T0 + $T1` # Q[n − Q[n−1]] + Q[n − Q[n−2]]
echo −n "${Q[n]} "
if [ `expr $n % $LINEWIDTH` −eq 0 ] # Format output.
then # mod
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.
−−
Bash supports only one−dimensional arrays, however a little trickery permits simulating multi−dimensional
ones.
Example 26−11. Simulating a two−dimensional array, then tilting it
#!/bin/bash
# Simulating a two−dimensional array.
# A two−dimensional array stores rows sequentially.
Rows=5
Columns=5
declare −a alpha # char alpha [Rows] [Columns];
# Unnecessary declaration.
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
local row=`expr $rc / $Columns`
local column=`expr $rc % $Rows`
let "index = $row * $Rows + $column"
alpha[$index]=$i # alpha[$row][$column]
let "rc += 1"
done
# Simpler would be
# declare −a alpha=( A B C D E F G H I J K L M N O P Q R S T U V W X Y )
# but this somehow lacks the "flavor" of a two−dimensional array.
}
print_alpha ()
{
Advanced Bash−Scripting Guide
Chapter 26. Arrays 308
local row=0
local index
echo
while [ "$row" −lt "$Rows" ] # Print out in "row major" order −
do # columns vary
# while row (outer loop) remains the same.
local column=0
while [ "$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 () # Filter out negative array indices.
{
echo −n " " # Provides the tilt.
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][$column]
fi
}
rotate () # Rotate the array 45 degrees
{ # ("balance" it on its lower lefthand corner).
local row
local column
for (( row = Rows; row > −Rows; row−− )) # Step through the array backwards.
do
for (( column = 0; column < Columns; column++ ))
do
if [ "$row" −ge 0 ]
then
let "t1 = $column − $row"
let "t2 = $column"
else
let "t1 = $column"
Advanced Bash−Scripting Guide
Chapter 26. Arrays 309
let "t2 = $column + $row"
fi
filter $t1 $t2 # Filter out negative array indices.
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).
}
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−#
load_alpha # Load the array.
print_alpha # Print it out.
rotate # Rotate it 45 degrees counterclockwise.
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−#
# This is a rather contrived, not to mention kludgy simulation.
#
# Exercises:
# −−−−−−−−−
# 1) Rewrite the array loading and printing functions
# + in a more intuitive and elegant fashion.
#
# 2) Figure out how the array rotation functions work.
# Hint: think about the implications of backwards−indexing an array.
exit 0
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−11.
Advanced Bash−Scripting Guide
Chapter 26. Arrays 310
Chapter 27. 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 [56])
/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 G for a sample
.bashrc file.
logout file
$HOME/.bash_logout
user−specific instruction file, found in each user's home directory. Upon exit from a login (Bash)
shell, the commands in this file execute.
Chapter 27. Files 311
Chapter 28. /dev and /proc
A Linux or UNIX machine typically has two special−purpose directories, /dev and /proc.
28.1. /dev
The /dev directory contains entries for the physical devices that may or may not be present in the hardware.
[57] The hard drive partitions containing the mounted filesystem(s) have entries in /dev, as a simple df
shows.
bash$ df
Filesystem 1k−blocks Used Available Use%
Mounted on
/dev/hda6 495876 222748 247527 48% /
/dev/hda1 50755 3887 44248 9% /boot
/dev/hda8 367013 13262 334803 4% /home
/dev/hda5 1714416 1123624 503704 70% /usr
Among other things, the /dev directory also 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. [58] This enables
mounting an entire filesystem within a single large file. See Example 13−6 and Example 13−5.
A few of the pseudo−devices in /dev have other specialized uses, such as /dev/null, /dev/zero and
/dev/urandom.
28.2. /proc
The /proc directory is actually a pseudo−filesystem. The files in the /proc directory 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
Chapter 28. /dev and /proc 312
bash$ cat /proc/interrupts
CPU0
0: 84505 XT−PIC timer
1: 3375 XT−PIC keyboard
2: 0 XT−PIC cascade
5: 1 XT−PIC soundblaster
8: 1 XT−PIC rtc
12: 4231 XT−PIC PS/2 Mouse
14: 109373 XT−PIC ide0
NMI: 0
ERR: 0
bash$ cat /proc/partitions
major minor #blocks name rio rmerge rsect ruse wio wmerge wsect wuse running use aveq
3 0 3007872 hda 4472 22260 114520 94240 3551 18703 50384 549710 0 111550 644030
3 1 52416 hda1 27 395 844 960 4 2 14 180 0 800 1140
3 2 1 hda2 0 0 0 0 0 0 0 0 0 0 0
3 4 165280 hda4 10 0 20 210 0 0 0 0 0 210 210
...
bash$ cat /proc/loadavg
0.13 0.42 0.27 2/44 1119
Shell scripts may extract data from certain of the files in /proc. [59]
bash$ cat /proc/filesystems | grep iso9660
iso9660
kernel_version=$( awk '{ print $3 }' /proc/version )
CPU=$( awk '/model name/ {print $4}' < /proc/cpuinfo )
if [ $CPU = Pentium ]
then
run_some_commands
...
else
run_different_commands
...
fi
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.
Advanced Bash−Scripting Guide
Chapter 28. /dev and /proc 313
Example 28−1. Finding the process associated with a PID
#!/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 # Error message >stderr.
exit $E_WRONGARGS
fi
pidno=$( 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 $1" filters out this possibility.
if [ −z "$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 [ ! −r "/proc/$1/$PROCFILE" ] # Check for read permission.
then
echo "Process $1 running, but..."
echo "Can't get read permission on /proc/$1/$PROCFILE."
exit $E_NOPERMISSION # Ordinary user can't access some files in /proc.
fi
# 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 $11}' )
#
# /proc/pid−number/exe is a symbolic link
# to the complete path name of the invoking process.
if [ −e "$exe_file" ] # If /proc/pid−number/exe exists...
Advanced Bash−Scripting Guide
Chapter 28. /dev and /proc 314
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 $1 −d txt | sed −ne 's/^n//p'
# Additional commentary by Stephane Chazelas.
exit 0
Example 28−2. On−line connect status
#!/bin/bash
PROCNAME=pppd # ppp daemon
PROCFILENAME=status # Where to look.
NOTCONNECTED=65
INTERVAL=2 # Update every 2 seconds.
pidno=$( 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 $NOTCONNECTED
else
echo "Connected."; echo
fi
while [ true ] # Endless loop, script can be improved here.
do
if [ ! −e "/proc/$pidno/$PROCFILENAME" ]
# While process running, then "status" file exists.
then
echo "Disconnected."
exit $NOTCONNECTED
fi
Advanced Bash−Scripting Guide
Chapter 28. /dev and /proc 315
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.
In general, it is dangerous to write to the files in /proc, as this can corrupt the filesystem or crash the
machine.
Advanced Bash−Scripting Guide
Chapter 28. /dev and /proc 316
Chapter 29. Of Zeros and Nulls
/dev/zero and /dev/null
Uses of /dev/null
Think of /dev/null as a "black hole". It is the nearest equivalent to a write−only file. Everything
written to it disappears forever. Attempts to read or output from it result in nothing. Nevertheless,
/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 12−2).
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 2−2):
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 Web commercial sites):
Example 29−1. Hiding the cookie jar
if [ −f ~/.netscape/cookies ] # Remove, if exists.
then
rm −f ~/.netscape/cookies
fi
ln −s /dev/null ~/.netscape/cookies
Chapter 29. Of Zeros and Nulls 317
# 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 file, but it actually contains nulls (numerical zeros, not
the ASCII kind). Output written to it disappears, and it is fairly difficult to actually read the nulls in
/dev/zero, though it can be done with od or a hex editor. The chief use for /dev/zero is in
creating an initialized dummy file of specified length intended as a temporary swap file.
Example 29−2. Setting up a swapfile using /dev/zero
#!/bin/bash
# Creating a swapfile.
# This script must be run as root.
ROOT_UID=0 # Root has $UID 0.
E_WRONG_USER=65 # Not root?
FILE=/swap
BLOCKSIZE=1024
MINBLOCKS=40
SUCCESS=0
if [ "$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 # Must be at least 40 blocks long.
fi
echo "Creating swap file of size $blocks blocks (KB)."
dd if=/dev/zero of=$FILE bs=$BLOCKSIZE count=$blocks # Zero out file.
mkswap $FILE $blocks # Designate it a swap file.
swapon $FILE # Activate swap file.
echo "Swap file created and activated."
exit $SUCCESS
Advanced Bash−Scripting Guide
Chapter 29. Of Zeros and Nulls 318
Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose,
such as mounting a filesystem on a loopback device (see Example 13−6) or securely deleting a file
(see Example 12−42).
Example 29−3. Creating a ramdisk
#!/bin/bash
# ramdisk.sh
# A "ramdisk" is a segment of system RAM memory
#+ that acts as if it were a filesystem.
# Its advantage is very fast access (read/write time).
# Disadvantages: volatility, loss of data on reboot or powerdown.
# less RAM available to system.
#
# What good is a ramdisk?
# Keeping a large dataset, such as a table or dictionary on ramdisk
#+ speeds up data lookup, since memory access is much faster than disk access.
E_NON_ROOT_USER=70 # Must run as root.
ROOTUSER_NAME=root
MOUNTPT=/mnt/ramdisk
SIZE=2000 # 2K blocks (change as appropriate)
BLOCKSIZE=1024 # 1K (1024 byte) block size
DEVICE=/dev/ram0 # First ram device
username=`id −nu`
if [ "$username" != "$ROOTUSER_NAME" ]
then
echo "Must be root to run \"`basename $0`\"."
exit $E_NON_ROOT_USER
fi
if [ ! −d "$MOUNTPT" ] # Test whether mount point already there,
then #+ so no error if this script is run
mkdir $MOUNTPT #+ multiple times.
fi
dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE # Zero out RAM device.
mke2fs $DEVICE # Create an ext2 filesystem on it.
mount $DEVICE $MOUNTPT # Mount it.
chmod 777 $MOUNTPT # So ordinary user can access ramdisk.
# However, must be root to unmount it.
echo "\"$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 again to set up ramdisk.
# Remounting /mnt/ramdisk without the other steps will not work.
exit 0
Advanced Bash−Scripting Guide
Chapter 29. Of Zeros and Nulls 319
Chapter 30. Debugging
The Bash shell contains no debugger, nor even any debugging−specific commands or constructs. [60] 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 30−1. A buggy script
#!/bin/bash
# ex74.sh
# This is a buggy script.
a=37
if [$a −gt 27 ]
then
echo $a
fi
exit 0
Output from script:
./ex74.sh: [37: command not found
What's wrong with the above script (hint: after the if)?
Example 30−2. Missing keyword
#!/bin/bash
# missing−keyword.sh: What error message will this generate?
for a in 1 2 3
do
echo "$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.
What if the script executes, but does not work as expected? This is the all too familiar logic error.
Chapter 30. Debugging 320
Example 30−3. test24, another buggy script
#!/bin/bash
# This is supposed to delete all filenames in current directory
#+ containing embedded spaces.
# It doesn't work. Why not?
badname=`ls | grep ' '`
# echo "$badname"
rm "$badname"
exit 0
Try to find out what's wrong with Example 30−3 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,
It bombs with a "syntax error" message, or1. It runs, but does not work as expected (logic error).2. It runs, works as expected, but has nasty side effects (logic bomb).3.
Tools for debugging non−working scripts include
echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of
what is going on.
1.
using the tee filter to check processes or data flows at critical points.2. 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.
3.
Advanced Bash−Scripting Guide
Chapter 30. Debugging 321
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 at each attempt to use an undeclared variable.
Using an "assert" function to test a variable or condition at critical points in a script. (This is an idea
borrowed from C.)
Example 30−4. Testing a condition with an "assert"
#!/bin/bash
# assert.sh
assert () # If condition false,
{ #+ exit from script with error message.
E_PARAM_ERR=98
E_ASSERT_FAILED=99
if [ −z "$2" ] # Not enough parameters passed.
then
return $E_PARAM_ERR # No damage done.
fi
lineno=$2
if [ ! $1 ]
then
echo "Assertion failed: \"$1\""
echo "File \"$0\", line $lineno"
exit $E_ASSERT_FAILED
# else
# return
# and continue executing script.
fi
}
a=5
b=4
condition="$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.
# ...
echo "This statement echoes only if the \"assert\" does not fail."
# ...
# Some more commands.
exit 0
4.
trapping at exit.
The exit command in a script triggers a signal 0, terminating the process, that is, the script itself. [61]
It is often useful to trap the exit, forcing a "printout" of variables, for example. The trap must be the
5.
Advanced Bash−Scripting Guide
Chapter 30. Debugging 322
first command in the script.
Trapping signals
trap Specifies an action on receipt of a signal; also useful for debugging.
A signal is simply 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.
trap '' 2
# Ignore interrupt 2 (Control−C), with no action specified.
trap 'echo "Control−C disabled."' 2
# Message when Control−C pressed.
Example 30−5. Trapping at exit
#!/bin/bash
trap 'echo Variable Listing −−− a = $a b = $b' EXIT
# EXIT is the name of the signal generated upon exit from a script.
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 30−6. Cleaning up after Control−C
#!/bin/bash
# logon.sh: A quick 'n dirty script to check whether you are on−line yet.
TRUE=1
LOGFILE=/var/log/messages
# Note that $LOGFILE must be readable (chmod 644 /var/log/messages).
TEMPFILE=temp.$$
# Create a "unique" temp file name, using process id of the script.
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
Advanced Bash−Scripting Guide
Chapter 30. Debugging 323
while [ $TRUE ] #Endless loop.
do
tail −$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" ] # Quotes necessary because of possible spaces.
then
echo "On−line"
rm −f $TEMPFILE # Clean up temp file.
exit $ONLINE
else
echo −n "." # −n option to echo suppresses newline,
# so you get continuous rows of dots.
fi
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, as 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 −1 "$LOGFILE" | grep −q "$KEYWORD"
do echo −n .
sleep $CHECK_INTERVAL
done
echo "On−line"
# Exercise: Discuss the strengths and weaknesses
# of each of these various approaches.
Advanced Bash−Scripting Guide
Chapter 30. Debugging 324
The DEBUG argument to trap causes a specified action to execute after every command in a script. This
permits tracing variables, for example.
Example 30−7. 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."
# The "trap 'commands' DEBUG" construct would be more useful
# in the context of a complex script,
# where placing multiple "echo $variable" statements might be
# clumsy and time−consuming.
# Thanks, Stephane Chazelas for the pointer.
exit 0
trap '' SIGNAL (two adjacent apostrophes) disables SIGNAL for the remainder of the script. trap
SIGNAL restores the functioning of SIGNAL once more. This is useful to protect a critical portion of a
script from an undesirable interrupt.
trap '' 2 # Signal 2 is Control−C, now disabled.
command
command
command
trap 2 # Reenables Control−C
Advanced Bash−Scripting Guide
Chapter 30. Debugging 325
Chapter 31. Options
Options are settings that change shell and/or script behavior.
The set command enables options within a script. At the point in the script where you want the options to take
effect, use set −o option−name or, in short form, set −option−abbrev. These two forms are equivalent.
#!/bin/bash
set −o verbose
# Echoes all commands before executing.
#!/bin/bash
set −v
# Exact same effect as above.
To disable an option within a script, use set +o option−name or set +option−abbrev.
#!/bin/bash
set −o verbose
# Command echoing on.
command
...
command
set +o verbose
# Command echoing off.
command
# Not echoed.
set −v
# Command echoing on.
command
...
command
set +v
# Command echoing off.
command
exit 0
An alternate method of enabling options in a script is to specify them immediately following the #! script
header.
#!/bin/bash −x
#
# Body of script follows.
Chapter 31. Options 326
It is also possible to enable script options from the command line. Some options that will not work with set
are available this way. Among these are −i, force script to run interactive.
bash −v script−name
bash −o verbose script−name
The following is a listing of some useful options. They may be specified in either abbreviated form or by
complete name.
Table 31−1. bash options
Abbreviation Name Effect
−C noclobber Prevent overwriting of files by redirection (may be overridden by >|)
−D (none) List double−quoted strings prefixed by $, but do not execute commands in script
−a allexport Export all defined variables
−b notify Notify when jobs running in background terminate (not of much use in a script)
−c ... (none) Read commands from ...
−f noglob Filename expansion (globbing) disabled
−i interactive Script runs in interactive mode
−p privileged Script runs as "suid" (caution!)
−r restricted Script runs in restricted mode (see Chapter 21).
−u nounset Attempt to use undefined variable outputs error message, and forces an exit
−v verbose Print each command to stdout before executing it
−x xtrace Similar to −v, but expands commands
−e errexit Abort script at first error (when a command exits with non−zero status)
−n noexec Read commands in script, but do not execute them (syntax check)
−s stdin Read commands from stdin
−t (none) Exit after first command
−(none) End of options flag. All other arguments are positional parameters.
−− (none) Unset positional parameters. If arguments given (−− arg1 arg2), positional
parameters set to arguments.
Advanced Bash−Scripting Guide
Chapter 31. Options 327
Chapter 32. Gotchas
Turandot: Gli enigmi sono tre, la morte una!
Caleph: No, no! Gli enigmi sono tre, una la vita!
Puccini
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 o.k.
# However... using just the underscore will not work.
_=25
echo $_ # $_ is a special variable set to last arg of last command.
xyz((!*=value2 # Causes severe problems.
Using a hyphen or other reserved characters in a variable name.
var−1=23
# Use 'var_1' 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".
let c = $a − $b # 'let c=$a−$b' or 'let "c = $a − $b"' are correct.
if [ $a −le 5] # if [ $a −le 5 ] is correct.
# if [ "$a" −le 5 ] is even better.
# [[ $a −le 5 ]] also works.
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
Chapter 32. Gotchas 328
echo "uninitialized_var = $uninitialized_var"
# uninitialized_var =
Mixing up = and −eq in a test. Remember, = is for comparing literal variables and −eq for integers.
if [ "$a" = 273 ] # Is $a an integer or string?
if [ "$a" −eq 273 ] # If $a is an integer.
# Sometimes you can mix up −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
Mixing up integer and string comparison operators.
#!/bin/bash
# bad−op.sh
number=1
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: 5: No such file or directory
Sometimes variables within "test" brackets ([ ]) need to be quoted (double quotes). Failure to do so may cause
unexpected behavior. See Example 7−6, Example 16−4, and Example 9−6.
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.
Advanced Bash−Scripting Guide
Chapter 32. Gotchas 329
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, 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.
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 # Script changes itself to DOS format.
chmod 755 $0 # 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 may 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.
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
Advanced Bash−Scripting Guide
Chapter 32. Gotchas 330
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.
Example 32−1. 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"
# End subshell
)
echo
echo "inner_variable outside subshell = $inner_variable" # Unset.
echo "outer_variable outside subshell = $outer_variable" # Unchanged.
echo
exit 0
Piping echooutput 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 11−14).
Example 32−2. Piping the output of echo to a read
#!/bin/bash
# badread.sh:
# Attempting to use 'echo and 'read'
#+ to assign variables non−interactively.
a=aaa
b=bbb
c=ccc
Advanced Bash−Scripting Guide
Chapter 32. Gotchas 331
echo "one two three" | read a b c
# Try to reassign a, b, and c.
echo
echo "a = $a" # a = aaa
echo "b = $b" # b = bbb
echo "c = $c" # c = ccc
# Reassignment failed.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# Try the following alternative.
var=`echo "one two three"`
set −− $var
a=$1; b=$2; c=$3
echo "−−−−−−−"
echo "a = $a" # a = one
echo "b = $b" # b = two
echo "c = $c" # c = three
# Reassignment 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 # Starting all over again.
b=bbb
c=ccc
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
Using "suid" commands within scripts is risky, as it may compromise system security. [62]
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 machine. Such
scripts often employ GNU commands and filters which have greater functionality than their generic UNIX
counterparts. This is particularly true of such text processing utilites as tr.
Advanced Bash−Scripting Guide
Chapter 32. Gotchas 332
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
Advanced Bash−Scripting Guide
Chapter 32. Gotchas 333
Chapter 33. 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 intended as an Official Shell Scripting Stylesheet.
33.1. 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. #
#************************************************#
BADDIR=65 # No such directory.
projectdir=/home/bozo/projects # Directory to clean up.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
# cleanup_pfiles () #
# Removes all files in designated directory. #
# Parameter: $target_directory #
# Returns: 0 on success, $BADDIR if something went wrong. #
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
cleanup_pfiles ()
{
if [ ! −d "$1" ] # Test if target directory exists.
then
echo "$1 is not a directory."
return $BADDIR
fi
rm −f "$1"/*
return 0 # Success.
}
cleanup_pfiles $projectdir
exit 0
Be sure to put the #!/bin/bash at the beginning of the first line of the script, preceding any comment
headers.
•
Avoid using "magic numbers", [63] that is, "hard−wired" literal constants. Use meaningful variable•
Chapter 33. Scripting With Style 334
names instead. This makes the script easier to understand and permits making changes and updates
without breaking the application.
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 # Only line that needs to be changed.
if [ −f "$LOGFILE" ]
then
...
fi
Choose descriptive names for variables and functions.
fl=`ls −al $dirname` # Cryptic.
file_listing=`ls −al $dirname` # Better.
MAXVAL=10 # All caps used for a script constant.
while [ "$index" −le "$MAXVAL" ]
...
E_NOTFOUND=75 # Uppercase for an errorcode,
# and name begins with "E_".
if [ ! −e "$filename" ]
then
echo "File $filename not found."
exit $E_NOTFOUND
fi
MAIL_DIRECTORY=/var/spool/mail/bozo # Uppercase for an environmental variable.
export MAIL_DIRECTORY
GetAnswer () # Mixed case works well for a function.
{
prompt=$1
echo −n $prompt
read answer
return $answer
}
GetAnswer "What is your favorite number? "
favorite_number=$?
echo $favorite_number
_uservariable=23 # Permissable, 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=65
...
...
exit $E_WRONG_ARGS
•
Advanced Bash−Scripting Guide
Chapter 33. Scripting With Style 335
See also Appendix C.
Break complex scripts into simpler modules. Use functions where appropriate. See Example 35−4.• 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
Advanced Bash−Scripting Guide
Chapter 33. Scripting With Style 336
Chapter 34. Miscellany
Nobody really knows what the Bourne shell's
grammar is. Even examination of the source code
is little help.
Tom Duff
34.1. 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
# Stephane Chazelas (thanks again).
Let us consider an interactive script to be one that requires input from the user, usually with read statements
(see Example 11−2). "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 <file). These particular workarounds make possible general purpose scripts that run
in either interactive or non−interactive modes.
If a script needs to test whether it is running in an interactive shell, it is simply a matter of finding whether the
prompt variable, $PS1 is set. (If the user is being prompted for input, then the script needs to display a
prompt.)
if [ −z $PS1 ] # no prompt?
then
# non−interactive
...
else
Chapter 34. Miscellany 337
# interactive
...
fi
Alternatively, the script can test for the presence of option "i" in the $− flag.
case $− in
*i*) # interactive shell
;;
*) # non−interactive shell
;;
# (Courtesy of "UNIX F.A.Q.," 1993)
Scripts may be forced to run in interactive mode with the −i option or with a #!/bin/bash −i
header. Be aware that this can cause erratic script behavior or show error messages even when no error is
present.
34.2. Shell Wrappers
A "wrapper" is a shell script that embeds a system command or utility, that saves a set of parameters passed to
that command. Wrapping a script around a complex command line simplifies invoking it. This is expecially
useful with sed and awk.
A sed or awk script would normally be invoked from the command line by a sed −e 'commands' or
awk 'commands'. Embedding such a script in a Bash script permits calling it more simply, and makes it
"reusable". This also enables combining the functionality of sed and awk, for example piping the output of a
set of sed commands to awk. As a saved executable file, you can then repeatedly invoke it in its original form
or modified, without the inconvenience of retyping it on the command line.
Example 34−1. shell wrapper
#!/bin/bash
# This is a simple script that removes blank lines from a file.
# No argument checking.
#
# You might wish to add something like:
# if [ −z "$1" ]
# then
# echo "Usage: `basename $0` target−file"
# exit 65
# fi
# Same as
# sed −e '/^$/d' filename
# invoked from the command line.
sed −e /^$/d "$1"
# The '−e' means an "editing" command follows (optional here).
# '^' is the beginning of line, '$' is the end.
# This match lines with nothing between the beginning and the end,
#+ blank lines.
Advanced Bash−Scripting Guide
Chapter 34. Miscellany 338
# The 'd' is the delete command.
# Quoting the command−line arg permits
#+ whitespace and special characters in the filename.
exit 0
Example 34−2. A slightly more complex shell wrapper
#!/bin/bash
# "subst", a script that substitutes one pattern for
# another in a file,
# i.e., "subst Smith Jones letter.txt".
ARGS=3
E_BADARGS=65 # Wrong number of arguments passed to script.
if [ $# −ne "$ARGS" ]
# Test number of arguments to script (always a good idea).
then
echo "Usage: `basename $0` old−pattern new−pattern filename"
exit $E_BADARGS
fi
old_pattern=$1
new_pattern=$2
if [ −f "$3" ]
then
file_name=$3
else
echo "File \"$3\" does not exist."
exit $E_BADARGS
fi
# Here is where the heavy work gets done.
sed −e "s/$old_pattern/$new_pattern/g" $file_name
# 's' is, of course, the substitute command in sed,
# and /pattern/ invokes address matching.
# The "g", or global flag causes substitution for *every*
# occurence of $old_pattern on each line, not just the first.
# Read the literature on 'sed' for a more in−depth explanation.
exit 0 # Successful invocation of the script returns 0.
Example 34−3. A shell wrapper around an awk script
#!/bin/bash
# Adds up a specified column (of numbers) in the target file.
ARGS=2
E_WRONGARGS=65
if [ $# −ne "$ARGS" ] # Check for proper no. of command line args.
then
echo "Usage: `basename $0` filename column−number"
Advanced Bash−Scripting Guide
Chapter 34. Miscellany 339
exit $E_WRONGARGS
fi
filename=$1
column_number=$2
# Passing shell variables to the awk part of the script is a bit tricky.
# See the awk documentation for more details.
# A multi−line awk script is invoked by awk ' ..... '
# Begin awk script.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−
awk '
{ 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 in shell scripts, and there may even be some substance to the claim that Perl
can totally replace shell scripting (though the author of this document remains skeptical).
Example 34−4. 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 "==============================================================="
Advanced Bash−Scripting Guide
Chapter 34. Miscellany 340
echo "However, the script may also contain shell and system commands."
exit 0
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 34−5. Bash and Perl scripts combined
#!/bin/bash
# bashandperl.sh
echo "Greetings from the Bash part of the script."
# More Bash commands may follow here.
exit 0
# End of Bash part of the script.
# =======================================================
#!/usr/bin/perl
# This part of the script must be invoked with −x option.
print "Greetings from the Perl part of the script.\n";
# 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.
34.3. 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" # "and list"
/bin/test "$a" −lt 16 && echo "yes, $a < 16"
[ "$a" −lt 16 ] && echo "yes, $a < 16"
[[ $a −lt 16 ]] && echo "yes, $a < 16" # Quoting variables within
(( a < 16 )) && echo "yes, $a < 16" # [[ ]] 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.
Advanced Bash−Scripting Guide
Chapter 34. Miscellany 341
# Thank you, S.C.
34.4. Recursion
Can a script recursively call itself? Indeed.
Example 34−6. 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 $MAXVAL.
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
Example 34−7. A (useful) script that recursively calls itself
#!/bin/bash
# pb.sh: phone book
# Written by Rick Boivie, and used with permission.
# Modifications by document author.
MINARGS=1 # Script needs at least one argument.
DATAFILE=./phonebook
PROGNAME=$0
E_NOARGS=70 # No arguments error.
if [ $# −lt $MINARGS ]; then
echo "Usage: "$PROGNAME" data"
exit $E_NOARGS
fi
if [ $# −eq $MINARGS ]; then
grep $1 "$DATAFILE"
else
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Chapter 34. Miscellany 342
( shift; "$PROGNAME" $* ) | grep $1
# Script recursively calls itself.
fi
exit 0 # Script exits here.
# 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 Franciso, SF 94338 (415) 501−1631
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
$bash pb.sh Roe
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
$bash pb.sh Roe Sam
Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444−5678
# When more than one argument passed to script,
#+ prints *only* the line(s) containing all the arguments.
Too many levels of recursion can exhaust the script's stack space, causing a segfault.
34.5. "Colorizing" Scripts
The ANSI [64] 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 34−8. 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
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."
echo "vans, Roland" # "[E]vans, Roland"
echo −en '\E[47;35m'"\033[1mJ\033[0m" # Magenta
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tput sgr0
echo "ones, Mildred"
echo −en '\E[47;32m'"\033[1mS\033[0m" # Green
tput sgr0
echo "mith, Julie"
echo −en '\E[47;31m'"\033[1mZ\033[0m" # Red
tput sgr0
echo "ane, Morris"
echo
read person
case "$person" in
# Note variable is quoted.
"E" | "e" )
# Accept upper or lowercase input.
echo
echo "Roland Evans"
echo "4321 Floppy 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
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.
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Chapter 34. Miscellany 344
bash$ echo −e "\033[1mThis is bold text.\033[0m"
A similar escape sequence switches on the underline attribute (on an rxvt and 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
The tput sgr0 restores the terminal settings to normal. Omitting this lets all subsequent output from that
particular terminal remain blue.
Use the following template for writing colored text on a colored background.
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 34−1. Numbers Representing Colors in Escape Sequences
Color Foreground Background
black 30 40
red 31 41
green 32 42
yellow 33 43
blue 34 44
magenta 35 45
cyan 36 46
white 37 47
Example 34−9. Echoing colored text
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Chapter 34. Miscellany 345
#!/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.
message=${1:−$default_msg} # Defaults to default message.
color=${2:−$black} # Defaults to black, if not specified.
echo −e "$color"
echo "$message"
Reset # 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.
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Chapter 34. Miscellany 346
# 2) Add options for colored backgrounds.
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 greatly compromises the usefulness of "colorizing" scripts,
and possibly relegates this technique to the status of a gimmick or even a "toy".
Moshe Jacobson's color utility (http://runslinux.net/projects/color) considerably simplifies using ANSI escape
sequences. It substitutes a clean and logical syntax for the clumsy constructs just discussed.
34.6. 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 structured, coherent form, 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 drastically reduce the execution time of a script, see
Example 12−32.
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Chapter 34. Miscellany 347
34.7. Assorted Tips
To keep a record of which user scripts have run during a particular sesssion 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.
date>> $SAVE_FILE #Date and time.
echo $0>> $SAVE_FILE #Script name.
echo>> $SAVE_FILE #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?
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*.
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");.
•
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 # Root has $UID 0.
E_NOTROOT=101 # Not root user error.
MAXRETVAL=256 # Maximum (positive) return value of a function.
SUCCESS=0
FAILURE=−1
# Functions
Usage () # "Usage:" message.
{
if [ −z "$1" ] # No arg passed.
then
•
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Chapter 34. Miscellany 348
msg=filename
else
msg=$@
fi
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
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 () # Absolute value.
{ # Caution: Max return value = 256.
E_ARGERR=−999999
if [ −z "$1" ] # Need arg passed.
then
return $E_ARGERR # Obvious error value returned.
fi
if [ "$1" −ge 0 ] # If non−negative,
then #
absval=$1 # stays as−is.
else # Otherwise,
let "absval = (( 0 − $1 ))" # change sign.
fi
return $absval
}
tolower () # Converts string(s) passed as argument(s)
{ #+ to lowercase.
if [ −z "$1" ] # If no argument(s) passed,
Advanced Bash−Scripting Guide
Chapter 34. Miscellany 349
then #+ send error message
echo "(null)" #+ (C−style void−pointer error message)
return #+ and return from function.
fi
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
#
# 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 wildcards.
#+ 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"
•
A particularly clever use of if−test constructs is commenting out blocks of code.
#!/bin/bash
COMMENT_BLOCK=
# Try setting the above variable to something or other
#+ for an unpleasant surprise.
if [ $COMMENT_BLOCK ]; then
Comment block −−
=================================
This is a comment line.
This is another comment line.
This is yet another comment line.
=================================
echo "This will not echo."
Comment blocks are error−free! Whee!
fi
echo "No more comments, please."
•
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Chapter 34. Miscellany 350
exit 0
Compare this with using here documents to comment out code blocks.
Using the $? exit status variable, a script may test if a parameter contains only digits, so it can be
treated as an integer.
#!/bin/bash
SUCCESS=0
E_BADINPUT=65
test "$1" −ne 0 −o "$1" −eq 0 2>/dev/null
# An integer is either equal to 0 or not equal to 0.
# 2>/dev/null suppresses error message.
if [ $? −ne "$SUCCESS" ]
then
echo "Usage: `basename $0` integer−input"
exit $E_BADINPUT
fi
let "sum = $1 + 25" # Would give error if $1 not integer.
echo "Sum = $sum"
# 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 the "return value", and
assign this to a variable.
Example 34−10. Return value trickery
#!/bin/bash
# multiplication.sh
multiply () # Multiplies params passed.
{ # Will accept a variable number of args.
local product=1
until [ −z "$1" ] # Until uses up arguments passed...
do
let "product *= $1"
shift
done
echo $product # Will not echo to stdout,
} #+ since this will be assigned to a variable.
mult1=15383; mult2=25211
val1=`multiply $mult1 $mult2`
echo "$mult1 X $mult2 = $val1"
# 387820813
mult1=25; mult2=5; mult3=20
val2=`multiply $mult1 $mult2 $mult3`
echo "$mult1 X $mult2 X $mult3 = $val2"
•
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Chapter 34. Miscellany 351
# 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 () # Capitalizes initial character
{ #+ of argument string(s) passed.
string0="$@" # Accepts multiple arguments.
firstchar=${string0:0:1} # First character.
string1=${string0:1} # Rest of string(s).
FirstChar=`echo "$firstchar" | tr a−z A−Z`
# Capitalize first character.
echo "$FirstChar$string1" # Output to stdout.
}
newstring=`capitalize_ichar "each sentence should start with a capital letter."`
echo "$newstring" # Each sentence should start with a capital letter.
It is even possible for a function to "return" multiple values with this method.
Example 34−11. 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` # Assigns output of function.
sum=`echo "$retval" | awk '{print $1}'` # Assigns first field.
product=`echo "$retval" | awk '{print $2}'` # Assigns second field.
echo "$first + $second = $sum"
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Chapter 34. Miscellany 352
echo "$first * $second = $product"
echo
exit 0
Next in our bag of trick 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 34−12. 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.
#
# Note that 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.
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Chapter 34. Miscellany 353
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[@]}"
# NULL VALUE since this is a variable local to the function.
echo
exit 0
For a more elaborate example of passing arrays to functions, see Example A−11.
Using the double parentheses construct, it is possible to use C−like syntax for setting and
incrementing variables and in for and while loops. See Example 10−12 and Example 10−17.
•
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 34−13. Fun with anagrams
#!/bin/bash
# agram.sh: Playing games with anagrams.
# Find anagrams of...
LETTERSET=etaoinshrdlu
anagram "$LETTERSET" | # Find all anagrams of the letterset...
grep '.......' | # With at least 7 letters,
grep '^is' | # starting with 'is'
grep −v 's$' | # no plurals
grep −v 'ed$' # no past tense verbs
# Uses "anagram" utility
#+ that is part of the author's "yawl" word list package.
# http://ibiblio.org/pub/Linux/libs/yawl−0.2.tar.gz
exit 0 # End of code.
bash$ sh agram.sh
islander
isolate
isolead
isotheral
See also Example 28−2, Example 12−18, and Example A−10.
•
Use "anonymous here documents" to comment out blocks of code, to save having to individually
comment out each line with a #. See Example 17−10.
•
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 # First choice.
PlanB=command2 # Fallback option.
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Chapter 34. Miscellany 354
command_test=$(whatis "$CMD" | grep 'nothing appropriate')
# If 'command1' not found on system , 'whatis' will return
#+ "command1: nothing appropriate."
if [[ −z "$command_test" ]] # Check whether command present.
then
$CMD option1 option2 # Run command1 with options.
else # Otherwise,
$PlanB #+ run command2.
fi
The run−parts command is handy for running a set of command scripts in sequence, particularly in
combination with cron or at.
•
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.
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).
For more effective 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).
•
34.8. Security Issues
A brief warning about script security is appropriate. 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 to easy for even a novice,
a "script kiddie", to write one. [65]
Here is yet another reason to learn scripting. Being able to look at and understand scripts may protect your
system from being hacked or damaged.
34.9. Portability Issues
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. Even lacking this measure, 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.
Advanced Bash−Scripting Guide
Chapter 34. Miscellany 355
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• Certain string manipulation operations• Process substitution• Bash−specific builtins•
See the Bash F.A.Q. for a complete listing.
34.10. 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.
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Chapter 34. Miscellany 356
Chapter 35. Bash, version 2
The current version of Bash, the one you have running on your machine, is actually version 2.XX.Y.
bash$ echo $BASH_VERSION
2.05.8(1)−release
This update of the classic Bash scripting language added array variables, [66] string and parameter expansion,
and a better method of indirect variable references, among other features.
Example 35−1. 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'
echo $'Three form feeds \f \f \f'
echo $'10 newlines \n\n\n\n\n\n\n\n\n\n'
exit 0
Example 35−2. 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 greatly superior to the old "eval var1=\$$var2"
echo
t=table_cell_3
table_cell_3=24
echo "t = ${!t}" # t = 24
table_cell_3=387
echo "Value of t changed to ${!t}" # 387
# This is useful for referencing members of an array or table,
# or for simulating a multi−dimensional array.
# An indexing option would have been nice (sigh).
Chapter 35. Bash, version 2 357
exit 0
Example 35−3. Simple database application, using indirect variable referencing
#!/bin/bash
# resistor−inventory.sh
# Simple database application using indirect variable referencing.
# ============================================================== #
# Data
B1723_value=470 # ohms
B1723_powerdissip=.25 # watts
B1723_colorcode="yellow−violet−brown" # color bands
B1723_loc=173 # where they are
B1723_inventory=78 # how many
B1724_value=1000
B1724_powerdissip=.25
B1724_colorcode="brown−black−red"
B1724_loc=24N
B1724_inventory=243
B1725_value=10000
B1725_powerdissip=.25
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
echo
echo "Catalog number $catalog_number:"
echo "There are ${!Inv} of [${!Val} ohm / ${!Pdissip} watt] resistors in stock."
echo "These are located in bin # ${!Loc}."
echo "Their color code is \"${!Ccode}\"."
break
done
echo; echo
# Exercise:
# −−−−−−−−
Advanced Bash−Scripting Guide
Chapter 35. Bash, version 2 358
# Rewrite this script using arrays, rather than indirect variable referencing.
# Which method is more straightforward and intuitive?
# 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 35−4. Using arrays and other miscellaneous trickery to deal four random hands from a deck
of cards
#!/bin/bash
# May need to be invoked with #!/bin/bash2 on older machines.
# Cards:
# 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 and more intuitive
# with a single, 3−dimensional array.
# Perhaps a future version of Bash will support multidimensional arrays.
initialize_Deck ()
{
i=$LOWER_LIMIT
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 ()
Advanced Bash−Scripting Guide
Chapter 35. Bash, version 2 359
{
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"
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.
{
seed=`eval date +%s`
let "seed %= 32766"
RANDOM=$seed
}
deal_cards ()
{
echo
cards_picked=0
while [ "$cards_picked" −le $UPPER_LIMIT ]
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).
let "u %= $CARDS_IN_SUIT"
if [ "$u" −eq 0 ] # Nested if/then condition test.
then
echo
echo
fi
# Separate hands.
let "cards_picked += 1"
Advanced Bash−Scripting Guide
Chapter 35. Bash, version 2 360
fi
done
echo
return 0
}
# Structured programming:
# entire program logic modularized in functions.
#================
seed_random
initialize_Deck
initialize_Suits
initialize_Cards
deal_cards
exit 0
#================
# Exercise 1:
# Add comments to thoroughly document this script.
# Exercise 2:
# Revise the script 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.
Advanced Bash−Scripting Guide
Chapter 35. Bash, version 2 361
Chapter 36. Endnotes
36.1. Author's Note
How did I come to write a Bash scripting book? It's a strange tale. It seems that a couple of years back, I
needed to learn shell scripting −− and what better way to do that than to read a good book on the subject? I
was looking to buy a tutorial and reference covering all aspects of the subject. I was looking for a book that
would take difficult concepts, turn them inside out, and explain them in excruciating detail with
well−commented examples. [67] In fact, I was looking for this very book, or something much like it.
Unfortunately, it didn't exist, and if I wanted it, I'd have to write it. And so, here we are, folks.
This reminds me of the apocryphal story about the mad professor. Crazy as a loon, the fellow was. At the
sight of a book, any book −− at the library, at a bookstore, anywhere −− he would become totally obsessed
with the idea that he could have written it, should have written it, and done a better job of it to boot. He would
thereupon rush home and proceed to do just that, write a book with the very same title. When he died some
years later, he allegedly had several thousand books to his credit, probably putting even Asimov to shame.
The books might not have been any good, who knows, but does that really matter? Here's a fellow who lived
his dream, even if he was obsessed by it, driven by it, and I can't help admiring the old coot...
36.2. About the Author
Who is this guy anyhow?
The author claims no credentials or special qualifications, other than a compulsion to write. [68] This book is
somewhat of a departure from his other major work, HOW−2 Meet Women: The Shy Man's Guide to
Relationships. He has also written the Software−Building HOWTO.
A Linux user since 1995 (Slackware 2.2, kernel 1.2.1), the author has emitted a few software truffles,
including the cruft one−time pad encryption utility, the mcalc mortgage calculator, the judge Scrabble®
adjudicator, and the yawl word gaming list package. He got his start in programming using FORTRAN IV on
a CDC 3800, but is not the least bit nostalgic for those days.
Living in a secluded desert community with wife and dog, he cherishes human frailty.
36.3. Tools Used to Produce This Book
36.3.1. Hardware
A used IBM Thinkpad, model 760XL laptop (P166, 104 meg RAM) running Red Hat 7.1/7.3. Sure, it's slow
and has a funky keyboard, but it beats the heck out of a No. 2 pencil and a Big Chief tablet.
36.3.2. Software and Printware
Bram Moolenaar's powerful SGML−aware vim text editor.i. OpenJade, a DSSSL rendering engine for converting SGML documents into other formats.ii. Norman Walsh's DSSSL stylesheets.iii. DocBook, The Definitive Guide, by Norman Walsh and Leonard Muellner (O'Reilly, ISBNiv.
Chapter 36. Endnotes 362
1−56592−580−7). This is the standard reference for anyone attempting to write a document in
Docbook SGML format.
36.4. Credits
Community participation made this project possible. The author gratefully acknowledges that writing this
book would have been an impossible task without help and feedback from all you people out there.
Philippe Martin translated 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 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.
Stephane Chazelas sent a long list of corrections, additions, and example scripts. More than a contributor, he
has, in effect, taken on the role of editor for this document. Merci beaucoup!
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. 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−18).
Likewise, thanks to Michel Charpentier for permission to use his dc factoring script (Example 12−37).
Kudos to Noah Friedman for permission to use his string function script (Example A−19).
Emmanuel Rouat suggested corrections and additions on command substitution and aliases. He also
contributed a very nice sample .bashrc file (Appendix G).
Heiner Steven kindly gave permission to use his base conversion script, Example 12−33. He also made a
number of corrections and many helpful suggestions. Special thanks.
Rick Boivie contributed the delightfully recursive pb.sh script (Example 34−7) and suggested performance
improvements for the monthlypmt.sh script (Example 12−32).
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
provided other examples of this.
Marc−Jano Knopp sent corrections on DOS batch files.
Advanced Bash−Scripting Guide
Chapter 36. Endnotes 363
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 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", Emilio Conti, Dennis Leeuw, and David Lawyer (himself
an author of 4 HOWTOs).
My gratitude to Chet Ramey and Brian Fox for writing Bash, an elegant and powerful scripting tool.
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 large extent, enabled the publication of this book.
Thanks most of all to my wife, Anita, for her encouragement and emotional support.
Advanced Bash−Scripting Guide
Chapter 36. Endnotes 364
Bibliography
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.
*
Dale Dougherty and Arnold Robbins, Sed and Awk, 2nd edition, O'Reilly and Associates, 1997,
1−156592−225−5.
To unfold the full power of shell scripting, you need at least a passing familiarity with sed and awk. This is
the standard tutorial. It includes an excellent introduction to "regular expressions". Read this book.
*
Aeleen Frisch, Essential System Administration, 3rd edition, O'Reilly and Associates, 2002, 0−596−00343−9.
This excellent sys admin manual has a decent introduction to shell scripting for sys administrators and does a
nice job of explaining the startup and initialization scripts. The long overdue third edition of this classic has
finally been released.
*
Stephen Kochan and Patrick Woods, Unix Shell Programming, Hayden, 1990, 067248448X.
The standard reference, though a bit dated by now.
*
Neil Matthew and Richard Stones, Beginning Linux Programming, Wrox Press, 1996, 1874416680.
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.
*
Bibliography 365
David Medinets, Unix Shell Programming Tools, McGraw−Hill, 1999, 0070397333.
Good info on 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 somewhat deficient in coverage on programming topics
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, 2nd edition, O'Reilly and Associates,
Random House, 1997, 1−56592−260−3.
Contains a couple of sections of very informative in−depth articles on shell programming, but falls short of
being a tutorial. It reproduces much of the regular expressions tutorial from the Dougherty and Robbins book,
above.
*
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).
*
Arnold Robbins, Bash Reference Card, SSC, 1998, 1−58731−010−5.
Excellent Bash pocket reference (don't leave home without it). A bargain at $4.95, but also available for free
download on−line in pdf format.
Advanced Bash−Scripting Guide
Bibliography 366
*
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 this document.
*
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.
*
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, even has a Bash section.
*
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.
*
The O'Reilly books on Perl. (Actually, any O'Reilly books.)
−−−
Advanced Bash−Scripting Guide
Bibliography 367
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 F.A.Q.
Ed Schaefer's Shell Corner in Unix Review.
Example shell scripts at Lucc's Shell Scripts .
Example shell scripts at SHELLdorado .
Example shell scripts at Noah Friedman's script site.
Steve Parker's Shell Programming Stuff.
Example shell scripts at SourceForge Snippet Library − shell scrips.
Giles Orr's Bash−Prompt HOWTO.
Very nice sed, awk, and regular expression tutorials at The UNIX Grymoire.
Eric Pement's sed resources page.
The GNU gawk reference manual (gawk is the extended GNU version of awk available on Linux and BSD
systems).
Trent Fisher's groff tutorial.
Advanced Bash−Scripting Guide
Bibliography 368
Mark Komarinski's Printing−Usage HOWTO.
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.
Rocky Bernstein is in the process of developing a "full−fledged" debugger for Bash.
−−−
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.
The comp.os.unix.shell newsgroup.
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.
Advanced Bash−Scripting Guide
Bibliography 369
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. manview: Viewing formatted manpages
#!/bin/bash
# manview.sh: Formats the source of a man page for viewing.
# This is useful when writing man page source and you want to
#+ look at the intermediate results on the fly while working on it.
E_WRONGARGS=65
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.
exit 0
Example A−2. mailformat: Formatting an e−mail message
#!/bin/bash
# mail−format.sh: Format e−mail messages.
# Gets rid of carets, tabs, also fold 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
Appendix A. Contributed Scripts 370
else
echo "File \"$1\" does not exist."
exit $E_NOFILE
fi
# =================================================================
MAXWIDTH=70 # Width to fold long lines to.
# Delete carets and tabs at beginning of lines,
#+ then fold lines to $MAXWIDTH characters.
sed '
s/^>//
s/^ *>//
s/^ *//
s/ *//
' $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 Windows utility with similar functionality.
#
# An nice set of text processing utilities and an efficient
#+ scripting language provide an alternative to bloated executables.
exit 0
Example A−3. rn: A simple−minded file rename utility
This script is a modification of Example 12−15.
#! /bin/bash
#
# 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=65
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.
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 371
let "number += 1"
fi
done
if [ "$number" −eq "$ONE" ] # For correct grammar.
then
echo "$number file renamed."
else
echo "$number files renamed."
fi
exit 0
# Exercises:
# −−−−−−−−−
# What type of files will this not work on?
# How can this be fixed?
#
# Rewrite this script to process all the files in a directory
#+ containing spaces in their names, and to rename them,
#+ substituting an underscore for each space.
Example A−4. blank−rename: renames filenames containing blanks
This is an even simpler−minded version of previous script.
#! /bin/bash
# blank−rename.sh
#
# Substitutes underscores for blanks in all the filenames in a directory.
ONE=1 # For getting singular/plural right (see below).
number=0 # Keeps track of how many files actually renamed.
FOUND=0 # 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 # Strip off path.
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" ] # For correct grammar.
then
echo "$number file renamed."
else
echo "$number files renamed."
fi
exit 0
Example A−5. encryptedpw: Uploading to an ftp site, using a locally encrypted password
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 372
#!/bin/bash
# Example "ex72.sh" modified to use encrypted password.
# Note that this is still somewhat insecure,
#+ since the decrypted password is sent in the clear.
# Use something like "ssh" if this is a concern.
E_BADARGS=65
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://metalab.unc.edu /pub/Linux/utils/file
#+ cruft−0.2.tar.gz [16k]
ftp −n $Server <<End−Of−Session
user $Username $Password
binary
bell
cd $Directory
put $Filename
bye
End−Of−Session
# −n option to "ftp" disables auto−logon.
# "bell" rings 'bell' after each file transfer.
exit 0
Example A−6. copy−cd: Copying a data CD
#!/bin/bash
# copy−cd.sh: copying a data CD
CDROM=/dev/cdrom # CD ROM device
OF=/home/bozo/projects/cdimage.iso # output file
# /xxxx/xxxxxxx/ Change to suit your system.
BLOCKSIZE=2048
SPEED=2 # May use higher speed if supported.
echo; echo "Insert source CD, but do *not* mount it."
echo "Press ENTER when ready. "
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 373
read ready # Wait for input, $ready not used.
echo; echo "Copying the source CD to $OF."
echo "This may take a while. Please be patient."
dd if=$CDROM of=$OF bs=$BLOCKSIZE # Raw device copy.
echo; echo "Remove data CD."
echo "Insert blank CDR."
echo "Press ENTER when ready. "
read ready # Wait for input, $ready not used.
echo "Copying $OF to CDR."
cdrecord −v −isosize speed=$SPEED dev=0,0 $OF
# Uses Joerg Schilling's "cdrecord" package (see its docs).
# http://www.fokus.gmd.de/nthp/employees/schilling/cdrecord.html
echo; echo "Done copying $OF to CDR on device $CDROM."
echo "Do you want to erase the image file (y/n)? " # Probably a huge file.
read answer
case "$answer" in
[yY]) rm −f $OF
echo "$OF erased."
;;
*) echo "$OF not erased.";;
esac
echo
# Exercise:
# Change the above "case" statement to also accept "yes" and "Yes" as input.
exit 0
Example A−7. Collatz series
#!/bin/bash
# collatz.sh
# The notorious "hailstone" or Collatz series.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# 1) Get the integer "seed" from the command line.
# 2) NUMBER <−−− seed
# 3) Print NUMBER.
# 4) If NUMBER is even, divide by 2, or
# 5)+ if odd, multiply by 3 and add 1.
# 6) NUMBER <−−− result
# 7) Loop back to step 3 (for specified number of iterations).
#
# The theory is that every sequence,
#+ no matter how large the initial value,
#+ eventually settles down to repeating "4,2,1..." cycles,
#+ even after fluctuating through a wide range of values.
#
# This is an instance of an "iterate",
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 374
#+ an operation that feeds its output back into the input.
# Sometimes the result is a "chaotic" series.
MAX_ITERATIONS=200
# For large seed numbers (>32000), increase 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
let "remainder = h % 2"
if [ "$remainder" −eq 0 ] # Even?
then
let "h /= 2" # Divide by 2.
else
let "h = h*3 + 1" # Multiply by 3 and add 1.
fi
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 mathematical function,
#+ see "Computers, Pattern, Chaos, and Beauty", by Pickover, p. 185 ff.,
#+ as listed in the bibliography.
exit 0
Example A−8. days−between: Calculate number of 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
ARGS=2 # Two command line parameters expected.
E_PARAM_ERR=65 # Param error.
REFYR=1600 # Reference year.
CENTURY=100
DIY=365
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 375
ADJ_DIY=367 # Adjusted for leap year + fraction.
MIY=12
DIM=31
LEAPCYCLE=4
MAXRETVAL=256 # Largest permissable
# positive return value from a function.
diff= # Declare global variable for date difference.
value= # Declare global variable for absolute value.
day= # Declare globals for day, month, year.
month=
year=
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 $E_PARAM_ERR
}
Parse_Date () # Parse date from command line params.
{
month=${1%%/**}
dm=${1%/**} # Day and month.
day=${dm#*/}
let "year = `basename $1`" # Not a filename, but works just the same.
}
check_date () # Checks for invalid date(s) passed.
{
[ "$day" −gt "$DIM" ] || [ "$month" −gt "$MIY" ] || [ "$year" −lt "$REFYR" ] && Param_Error
# Exit script on bad value(s).
# Uses "or−list / and−list".
#
# Exercise: Implement more rigorous date checking.
}
strip_leading_zero () # Better to strip possible leading zero(s)
{ # from day and/or month
val=${1#0} # since otherwise Bash will interpret them
return $val # as octal values (POSIX.2, sect 2.9.2.1).
}
day_index () # Gauss' Formula:
{ # Days from Jan. 3, 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
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 376
let "year −= $REFYR"
let "indexyr = $year / $CENTURY"
let "Days = $DIY*$year + $year/$LEAPCYCLE − $indexyr + $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day − $DIM"
# For an in−depth explanation of this algorithm, see
# http://home.t−online.de/home/berndt.schwerdtfeger/cal.htm
if [ "$Days" −gt "$MAXRETVAL" ] # If greater than 256,
then # then change to negative value
let "dindex = 0 − $Days" # which can be returned from function.
else let "dindex = $Days"
fi
return $dindex
}
calculate_difference () # Difference between to day indices.
{
let "diff = $1 − $2" # Global variable.
}
abs () # Absolute value
{ # Uses global "value" variable.
if [ "$1" −lt 0 ] # If negative
then # then
let "value = 0 − $1" # change sign,
else # else
let "value = $1" # leave it alone.
fi
}
if [ $# −ne "$ARGS" ] # Require two command line params.
then
Param_Error
fi
Parse_Date $1
check_date $day $month $year # See if valid date.
strip_leading_zero $day # Remove any leading zeroes
day=$? # on day and/or month.
strip_leading_zero $month
month=$?
day_index $day $month $year
date1=$?
abs $date1 # Make sure it's positive
date1=$value # by getting absolute value.
Parse_Date $2
check_date $day $month $year
strip_leading_zero $day
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 377
day=$?
strip_leading_zero $month
month=$?
day_index $day $month $year
date2=$?
abs $date2 # Make sure it's positive.
date2=$value
calculate_difference $date1 $date2
abs $diff # Make sure it's positive.
diff=$value
echo $diff
exit 0
# Compare this script with the implementation of Gauss' Formula in C at
# http://buschencrew.hypermart.net/software/datedif
Example A−9. Make a "dictionary"
#!/bin/bash
# makedict.sh [make dictionary]
# Modification of /usr/sbin/mkdict 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 lexicographic research.
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 $* | # Contents of specified files to stdout.
tr A−Z a−z | # Convert to lowercase.
tr ' ' '\012' | # New: change spaces to newlines.
# tr −cd '\012[a−z][0−9]' | # Get rid of everything non−alphanumeric
#+ (original script).
tr −c '\012a−z' '\012' | # Rather than deleting
#+ now change non−alpha to newlines.
sort | # $SORT options unnecessary now.
uniq | # Remove duplicates.
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 378
grep −v '^#' | # Delete lines beginning with a hashmark.
grep −v '^$' # Delete blank lines.
exit 0
Example A−10. Soundex conversion
#!/bin/bash
# soundex.sh: Calculate "soundex" code for names
# =======================================================
# Soundex script
# by
# Mendel Cooper
# thegrendel@theriver.com
# 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/
# =======================================================
ARGCOUNT=1 # Need name as argument.
E_WRONGARGS=70
if [ $# −ne "$ARGCOUNT" ]
then
echo "Usage: `basename $0` name"
exit $E_WRONGARGS
fi
assign_value () # Assigns numerical value
{ #+ to letters of name.
val1=bfpv # 'b,f,p,v' = 1
val2=cgjkqsxz # 'c,g,j,k,q,s,x,z' = 2
val3=dt # etc.
val4=l
val5=mn
val6=r
# Exceptionally clever use of 'tr' follows.
# Try to figure out what is going on here.
value=$( 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 letter values.
# Remove duplicate numbers, except when separated by vowels.
# Ignore vowels, except as separators, so delete them last.
# Ignore 'w' and 'h', even as separators, so delete them first.
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 379
#
# The above command substitution lays more pipe than a plumber <g>.
}
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 $prefix0 | tr a−z A−Z`
# Uppercase 1st letter of soundex.
let "char_pos += 1" # Bump character position to 2nd letter of name.
name1=${name:$char_pos}
# ++++++++++++++++++++++++++ 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` # First letter of name, lowercased.
assign_value $name
s1=$value
assign_value $name1
s2=$value
assign_value $char1
s3=$value
s3=9$s3 # If first letter of name is a vowel
#+ or 'w' or 'h',
#+ then its "value" will be null (unset).
#+ Therefore, set it to 9, an otherwise
#+ unused value, which can be tested for.
if [[ "$s1" −ne "$s2" || "$s3" −eq 9 ]]
then
suffix=$s2
else
suffix=${s2:$char_pos}
fi
# ++++++++++++++++++++++ end Exception Patch +++++++++++++++++++++++++++++++++
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 380
padding=000 # Use at most 3 zeroes to pad.
soun=$prefix$suffix$padding # Pad with zeroes.
MAXLEN=4 # Truncate to maximum of 4 chars.
soundex=${soun:0:$MAXLEN}
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.
#
#
# 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−11. "Game of Life"
#!/bin/bash
# life.sh: "Life in the Slow Lane"
# ##################################################################### #
# 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 #
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 381
# 4*5 #
# 678 #
# #
# 2) A living cell with either 2 or 3 living neighbors remains alive. #
# 3) A dead cell with 3 living neighbors becomes alive (a "birth"). #
SURVIVE=2 #
BIRTH=3 #
# 4) All other cases result in dead cells. #
# ##################################################################### #
startfile=gen0 # Read the starting generation from the file "gen0".
# Default, if no other file specified when invoking script.
#
if [ −n "$1" ] # Specify another "generation 0" file.
then
if [ −e "$1" ] # Check for existence.
then
startfile="$1"
fi
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 will cause very slow execution).
ROWS=10
COLS=10
GENERATIONS=10 # How many generations to cycle through.
# Adjust this upwards,
#+ if you have time on your hands.
NONE_ALIVE=80 # Exit status on premature bailout,
#+ if no cells left alive.
TRUE=0
FALSE=1
ALIVE=0
DEAD=1
avar= # Global; holds current generation.
generation=0 # Initialize generation count.
# =================================================================
let "cells = $ROWS * $COLS"
# How many cells.
declare −a initial # Arrays containing "cells".
declare −a current
display ()
{
alive=0 # How many cells "alive".
# Initially zero.
declare −a arr
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 382
arr=( `echo "$1"` ) # Convert passed arg to array.
element_count=${#arr[*]}
local i
local rowcheck
for ((i=0; i<$element_count; i++))
do
# Insert newline at end of each row.
let "rowcheck = $i % ROWS"
if [ "$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 and change underscores to spaces.
done
return
}
IsValid () # Test whether cell coordinate valid.
{
if [ −z "$1" −o −z "$2" ] # Mandatory arguments missing?
then
return $FALSE
fi
local row
local lower_limit=0 # Disallow negative coordinate.
local upper_limit
local left
local right
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 * $ROWS" # Left limit.
let "right = $left + $COLS − 1" # Right limit.
if [ "$1" −lt "$left" −o "$1" −gt "$right" ]
then
return $FALSE # Beyond row boundary.
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 383
fi
return $TRUE # Valid coordinate.
}
IsAlive () # Test whether cell is alive.
# Takes array, cell number, state of cell as arguments.
{
GetCount "$1" $2 # Get alive cell count in neighborhood.
local nhbd=$?
if [ "$nhbd" −eq "$BIRTH" ] # Alive in any case.
then
return $ALIVE
fi
if [ "$3" = "." −a "$nhbd" −eq "$SURVIVE" ]
then # Alive only if previously alive.
return $ALIVE
fi
return $DEAD # Default.
}
GetCount () # Count live cells in passed cell's neighborhood.
# Two arguments needed:
# $1) variable holding array
# $2) cell number
{
local cell_number=$2
local array
local top
local center
local bottom
local r
local row
local i
local t_top
local t_cen
local t_bot
local count=0
local ROW_NHBD=3
array=( `echo "$1"` )
let "top = $cell_number − $COLS − 1" # Set up cell neighborhood.
let "center = $cell_number − 1"
let "bottom = $cell_number + $COLS − 1"
let "r = $cell_number / $ROWS"
for ((i=0; i<$ROW_NHBD; i++)) # Traverse from left to right.
do
let "t_top = $top + $i"
let "t_cen = $center + $i"
let "t_bot = $bottom + $i"
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 384
let "row = $r" # Count center row of neighborhood.
IsValid $t_cen $row # Valid cell position?
if [ $? −eq "$TRUE" ]
then
if [ ${array[$t_cen]} = "$ALIVE1" ] # Is it alive?
then # Yes?
let "count += 1" # Increment count.
fi
fi
let "row = $r − 1" # Count top row.
IsValid $t_top $row
if [ $? −eq "$TRUE" ]
then
if [ ${array[$t_top]} = "$ALIVE1" ]
then
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]} # Is cell alive?
if [ $? −eq "$ALIVE" ]
then # If alive, then
array[$i]=. #+ represent the cell as a period.
else
array[$i]="_" # Otherwise underscore
fi #+ (which will later be converted to space).
let "i += 1"
done
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 385
# let "generation += 1" # Increment generation count.
# 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' |\
sed −e 's/\./\. /g' −e 's/_/_ /g'` )
# Delete lines containing '#' comment character.
# Remove linefeeds and insert space between elements.
clear # Clear screen.
echo # Title
echo "======================="
echo " $GENERATIONS generations"
echo " of"
echo "\"Life in the Slow Lane\""
echo "======================="
# −−−−−−−− Display first generation. −−−−−−−−
Gen0=`echo ${initial[@]}`
display "$Gen0" # Display only.
echo; echo
echo "Generation $generation −− $alive alive"
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
let "generation += 1" # Increment generation count.
echo
# −−−−−−− Display second generation. −−−−−−−
Cur=`echo ${initial[@]}`
next_gen "$Cur" # Update & display.
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
let "generation += 1" # Increment generation count.
# −−−−−− Main loop for displaying subsequent generations −−−−−−
while [ "$generation" −le "$GENERATIONS" ]
do
Cur="$avar"
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 386
next_gen "$Cur"
let "generation += 1"
done
# ==============================================================
echo
exit 0
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# 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.
Example A−12. Data file for "Game of Life"
# 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.
__.__..___
___._.____
____.___..
_._______.
____._____
..__...___
____._____
___...____
__.._..___
_..___..__
+++
The following two scripts are by Mark Moraes of the University of Toronto. See the enclosed file
"Moraes−COPYRIGHT" for permissions and restrictions.
Example A−13. 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
# 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 '/^[ ]*$/d'
# −−> Delete empty lines and all lines until
# −−> first one beginning with white space.
else
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 387
# ==> If command line args present, then work on files named.
for i do
sed −e '1,/^$/d' −e '/^[ ]*$/d' $i
# −−> Ditto, as above.
done
fi
# ==> 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?
Example A−14. ftpget: Downloading files via ftp
#! /bin/sh
# $Id: ftpget,v 1.2 91/05/07 21:15:43 moraes Exp $
# Script to perform batch anonymous ftp. Essentially converts a list of
# of command line arguments into input to ftp.
# Simple, and quick − written as a companion to ftplist
# −h specifies the remote host (default prep.ai.mit.edu)
# −d specifies the remote directory to cd to − you can provide a sequence
# of −d options − they will be cd'ed to in turn. If the paths are relative,
# make sure you get the sequence right. Be careful with relative paths −
# there are far too many symlinks nowadays.
# (default is the ftp login directory)
# −v turns on the verbose option of ftp, and shows all responses from the
# ftp server.
# −f remotefile[:localfile] gets the remote file into localfile
# −m pattern does an mget with the specified pattern. Remember to quote
# shell characters.
# −c does a local cd to the specified directory
# For example,
# ftpget −h expo.lcs.mit.edu −d contrib −f xplaces.shar:xplaces.sh \
# −d ../pub/R3/fixes −c ~/fixes −m 'fix*'
# will get xplaces.shar from ~ftp/contrib on expo.lcs.mit.edu, and put it in
# xplaces.sh in the current working directory, and get all fixes from
# ~ftp/pub/R3/fixes and put them in the ~/fixes directory.
# Obviously, the sequence of the options is important, since the equivalent
# commands are executed by ftp in corresponding order
#
# Mark Moraes (moraes@csri.toronto.edu), Feb 1, 1989
# ==> Angle brackets changed to parens, so Docbook won't get indigestion.
#
# ==> These comments added by author of this document.
# PATH=/local/bin:/usr/ucb:/usr/bin:/bin
# export PATH
# ==> Above 2 lines from original script probably superfluous.
TMPFILE=/tmp/ftp.$$
# ==> Creates temp file, using process id of script ($$)
# ==> to construct filename.
SITE=`domainname`.toronto.edu
# ==> 'domainname' similar to 'hostname'
# ==> May rewrite this to parameterize this for general use.
usage="Usage: $0 [−h remotehost] [−d remotedirectory]... [−f remfile:localfile]... \
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 388
[−c localdirectory] [−m filepattern] [−v]"
ftpflags="−i −n"
verbflag=
set −f # So we can use globbing in −m
set x `getopt vh:d:c:m:f: $*`
if [ $? != 0 ]; then
echo $usage
exit 65
fi
shift
trap 'rm −f ${TMPFILE} ; exit' 0 1 2 3 15
echo "user anonymous ${USER−gnu}@${SITE} > ${TMPFILE}"
# ==> Added quotes (recommended in complex echoes).
echo binary >> ${TMPFILE}
for i in $* # ==> Parse command line args.
do
case $i in
−v) verbflag=−v; echo hash >> ${TMPFILE}; shift;;
−h) remhost=$2; shift 2;;
−d) echo cd $2 >> ${TMPFILE};
if [ x${verbflag} != x ]; then
echo pwd >> ${TMPFILE};
fi;
shift 2;;
−c) echo lcd $2 >> ${TMPFILE}; shift 2;;
−m) echo mget "$2" >> ${TMPFILE}; shift 2;;
−f) f1=`expr "$2" : "\([^:]*\).*"`; f2=`expr "$2" : "[^:]*:\(.*\)"`;
echo get ${f1} ${f2} >> ${TMPFILE}; shift 2;;
−−) shift; break;;
esac
done
if [ $# −ne 0 ]; then
echo $usage
exit 65 # ==> Changed from "exit 2" to conform with standard.
fi
if [ x${verbflag} != x ]; then
ftpflags="${ftpflags} −v"
fi
if [ x${remhost} = x ]; then
remhost=prep.ai.mit.edu
# ==> Rewrite to match your favorite ftp site.
fi
echo quit >> ${TMPFILE}
# ==> All commands saved in tempfile.
ftp ${ftpflags} ${remhost} < ${TMPFILE}
# ==> Now, tempfile batch processed by ftp.
rm −f ${TMPFILE}
# ==> Finally, tempfile deleted (you may wish to copy it to a logfile).
# ==> Exercises:
# ==> −−−−−−−−−
# ==> 1) Add error checking.
# ==> 2) Add bells & whistles.
+
Antek Sawicki contributed the following script, which makes very clever use of the parameter substitution
operators discussed in Section 9.3.
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 389
Example A−15. password: Generating random 8−character passwords
#!/bin/bash
# May need to be invoked with #!/bin/bash2 on older machines.
#
# Random password generator for bash 2.x by Antek Sawicki <tenox@tenox.tc>,
# who generously gave permission to the document author to use it here.
#
# ==> Comments added by document author ==>
MATRIX="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
LENGTH="8"
# ==> May change 'LENGTH' for longer password, of course.
while [ "${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 Section 3.3.1
# ==> and following 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 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−16. fifo: Making daily backups, using named pipes
#!/bin/bash
# ==> Script by James R. Van Zandt, and used here with his permission.
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 390
# ==> 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?
exit 0
+
Stephane Chazelas contributed the following script to demonstrate that generating prime numbers does not
require arrays.
Example A−17. 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 uses the more intuitive method of testing each candidate number
#+ for factors (divisors), using the "%" modulo operator.
LIMIT=1000 # Primes 2 − 1000
Primes()
{
(( n = $1 + 1 )) # Bump to next integer.
shift # Next parameter in list.
# echo "_n=$n i=$i_"
if (( n == LIMIT ))
then echo $*
return
fi
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 391
for i; do # "i" gets set to "@", previous values of $n.
# echo "−n=$n i=$i−"
(( i * i > n )) && break # Optimization.
(( n % i )) && continue # Sift out non−primes using modulo operator.
Primes $n $@ # Recursion inside loop.
return
done
Primes $n $@ $n # Recursion outside loop.
# Successively accumulate positional parameters.
# "$@" is the accumulating list of primes.
}
Primes 1
exit 0
# Uncomment lines 17 and 25 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, for faster execution.
+
Jordi Sanfeliu gave permission to use his elegant tree script.
Example A−18. tree: Displaying a directory tree
#!/bin/sh
# @(#) tree 1.1 30/11/95 by Jordi Sanfeliu
# email: mikaku@fiwix.org
#
# Initial version: 1.0 30/11/95
# Next version : 1.1 24/02/97 Now, with symbolic links
# Patch by : Ian Kjos, to support unsearchable dirs
# email: beth13@mail.utexas.edu
#
# Tree is a tool for view the directory tree (obvious :−) )
#
# ==> 'Tree' script used here with the permission of its author, Jordi Sanfeliu.
# ==> Comments added by the author of this document.
# ==> Argument quoting added.
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 directory level.
while [ $zz != $deep ] # Keep track of inner nested loop.
do
echo −n "| " # ==> Display vertical connector symbol,
# ==> with 2 spaces & no line feed in order to indent.
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 392
zz=`expr $zz + 1` # ==> Increment zz.
done
if [ −L "$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 "+−−−$dir" # ==> Display horizontal connector symbol...
# ==> and print directory name.
if cd "$dir" ; then # ==> If can move to subdirectory...
deep=`expr $deep + 1` # ==> Increment depth.
search # with recursivity ;−)
# ==> Function calls itself.
numdirs=`expr $numdirs + 1` # ==> Increment directory count.
fi
fi
fi
done
cd .. # ==> Up one directory level.
if [ "$deep" ] ; then # ==> If depth = 0 (returns TRUE)...
swfi=1 # ==> set flag showing that search is done.
fi
deep=`expr $deep − 1` # ==> Decrement depth.
}
# − Main −
if [ $# = 0 ] ; then
cd `pwd` # ==> No args to script, then use current working directory.
else
cd $1 # ==> Otherwise, move to indicated directory.
fi
echo "Initial directory = `pwd`"
swfi=0 # ==> Search finished flag.
deep=0 # ==> Depth of listing.
numdirs=0
zz=0
while [ "$swfi" != 1 ] # While flag not set...
do
search # ==> Call function after initializing variables.
done
echo "Total directories = $numdirs"
exit 0
# ==> Challenge: try to figure out exactly how this script works.
Noah Friedman gave permission to use his string function script, which essentially reproduces some of the
C−library string manipulation functions.
Example A−19. string functions: C−like string functions
#!/bin/bash
# string.bash −−− bash emulation of string(3) library routines
# Author: Noah Friedman <friedman@prep.ai.mit.edu>
# ==> Used with his kind permission in this document.
# Created: 1992−07−01
# Last modified: 1993−09−29
# Public domain
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 393
# 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 $a
# => foobar
#
#:end docstring:
###;;;autoload ==> Autoloading of function commented out.
function strcat ()
{
local s1_val s2_val
s1_val=${!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} # ==> indirect variable expansion
s2_val=${!s2}
if [ ${#s2_val} −gt ${n} ]; then
s2_val=${s2_val:0:$n} # ==> substring extraction
fi
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 394
eval "$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
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.
}
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 395
#: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 $s2
#
# Strcspn returns the length of the maximum initial segment of string s1,
# which consists entirely of characters not from string s2.
#:end docstring:
###;;;autoload
function strcspn ()
{
# Unsetting IFS allows whitspace to be handled as normal chars.
local IFS=
local result="${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}"
}
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 396
#: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 ()
{
:
}
#: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
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 397
echo "Testing \"strlen\" function:"
str=123456789
echo "\"str\" = $str"
echo −n "Length of \"str\" = "
strlen str
echo
# Exercise:
# −−−−−−−−
# Add code to test all the other string functions above.
exit 0
Michael Zick's complex array example uses the md5sum check sum command to encode directory
information.
Example A−20. Directory information
#! /bin/bash
# directory−info.sh
# Parses and lists directory information.
# NOTE: Change lines 273 and 353 per "README" file.
# Michael Zick is the author of this script.
# Used here with his permission.
# Controls
# If overridden by command arguments, they must be in the order:
# Arg1: "Descriptor Directory"
# Arg2: "Exclude Paths"
# Arg3: "Exclude Directories"
#
# Environment Settings override Defaults.
# Command arguments override Environment Settings.
# Default location for content addressed file descriptors.
MD5UCFS=${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.
: << LSfieldsDoc
# # # # # List Filesystem Directory Information # # # # #
#
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 398
# ListDirectory "FileGlob" "Field−Array−Name"
# or
# ListDirectory −of "FileGlob" "Field−Array−Filename"
# '−of' meaning 'output to filename'
# # # # #
String format description based on: ls (GNU fileutils) version 4.0.36
Produces a line (or more) formatted:
inode permissions hard−links owner group ...
32736 −rw−−−−−−− 1 mszick mszick
size day month date hh:mm:ss year path
2756608 Sun Apr 20 08:53:06 2003 /home/mszick/core
Unless it is formatted:
inode permissions hard−links owner group ...
266705 crw−rw−−−− 1 root uucp
major minor day month date hh:mm:ss year path
4, 68 Sun Apr 20 09:27:33 2003 /dev/ttyS4
NOTE: that pesky comma after the major number
NOTE: the 'path' may be multiple fields:
/home/mszick/core
/proc/982/fd/0 −> /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
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 399
'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 # Default return in variable
# OLD_IFS=$IFS # Using BASH default ' \t\n'
case "$#" in
3) case "$1" in
−of) of=1 ; shift ;;
* ) return 1 ;;
esac ;;
2) : ;; # Poor man's "continue"
*) return 1 ;;
esac
# 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) 2>/dev/null
return $? # Exit status of the let thread
fi
}
# # # # # Index Filesystem Directory Information # # # # #
#
# IndexList "Field−Array−Name" "Index−Array−Name"
# or
# IndexList −if Field−Array−Filename Index−Array−Name
# IndexList −of Field−Array−Name Index−Array−Filename
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 400
# IndexList −if −of Field−Array−Filename Index−Array−Filename
# # # # #
: << IndexListDoc
Walk an array of directory fields produced by ListDirectory
Having suppressed the line breaks in an otherwise line oriented
report, build an index to the array element which starts each line.
Each line gets two index entries, the first element of each line
(inode) and the element that holds the pathname of the file.
The first index entry pair (Line−Number==0) are informational:
Index−Array−Name[0] : Number of "Lines" indexed
Index−Array−Name[1] : "Current Line" pointer into Index−Array−Name
The following index pairs (if any) hold element indexes into
the Field−Array−Name per:
Index−Array−Name[Line−Number * 2] : The "inode" field element.
NOTE: This distance may be either +11 or +12 elements.
Index−Array−Name[(Line−Number * 2) + 1] : The "pathname" element.
NOTE: This distance may be a variable number of elements.
Next line index pair for Line−Number+1.
IndexListDoc
IndexList()
{
local −a LIST # Local of listname passed
local −a −i INDEX=( 0 0 ) # Local of index to return
local −i Lidx Lcnt
local −i if=0 of=0 # Default to variable names
case "$#" in # Simplistic option testing
0) return 1 ;;
1) return 1 ;;
2) : ;; # Poor man's continue
3) case "$1" in
−if) if=1 ;;
−of) of=1 ;;
* ) return 1 ;;
esac ; shift ;;
4) if=1 ; of=1 ; shift ; shift ;;
*) return 1
esac
# Make local copy of list
case "$if" in
0) eval LIST=\( \"\$\{$1\[@\]\}\" \) ;;
1) LIST=( $(cat $1) ) ;;
esac
# Grok (grope?) the array
Lcnt=${#LIST[@]}
Lidx=0
until (( Lidx >= Lcnt ))
do
if IsNumber ${LIST[$Lidx]}
then
local −i inode name
local ft
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 401
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
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[@]}" > "$2" ;;
esac
return 0 # What could go wrong?
}
# # # # # Content Identify File # # # # #
#
# DigestFile Input−Array−Name Digest−Array−Name
# or
# DigestFile −if Input−FileName Digest−Array−Name
# # # # #
# Here document used as a comment block.
: <<DigestFilesDoc
The key (no pun intended) to a Unified Content File System (UCFS)
is to distinguish the files in the system based on their content.
Distinguishing files by their name is just, so, 20th Century.
The content is distinguished by computing a checksum of that content.
This version uses the md5sum program to generate a 128 bit checksum
representative of the file's contents.
There is a chance that two files having different content might
generate the same checksum using md5sum (or any checksum). Should
that become a problem, then the use of md5sum can be replace by a
cyrptographic signature. But until then...
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 402
The md5sum program is documented as outputting three fields (and it
does), but when read it appears as two fields (array elements). This
is caused by the lack of whitespace between the second and third field.
So this function gropes the md5sum output and returns:
[0] 32 character checksum in hexidecimal (UCFS filename)
[1] Single character: ' ' text file, '*' binary file
[2] Filesystem (20th Century Style) name
Note: That name may be the character '−' indicating STDIN read.
DigestFilesDoc
DigestFile()
{
local if=0 # Default, variable name
local −a T1 T2
case "$#" in
3) case "$1" in
−if) if=1 ; shift ;;
* ) return 1 ;;
esac ;;
2) : ;; # Poor man's "continue"
*) return 1 ;;
esac
case $if in
0) eval T1=\( \"\$\{$1\[@\]\}\" \)
T2=( $(echo ${T1[@]} | md5sum −) )
;;
1) T2=( $(md5sum $1) )
;;
esac
case ${#T2[@]} in
0) return 1 ;;
1) return 1 ;;
2) case ${T2[1]:0:1} in # SanScrit−2.0.5
\*) T2[${#T2[@]}]=${T2[1]:1}
T2[1]=\*
;;
*) T2[${#T2[@]}]=${T2[1]}
T2[1]=" "
;;
esac
;;
3) : ;; # Assume it worked
*) return 1 ;;
esac
local −i len=${#T2[0]}
if [ $len −ne 32 ] ; then return 1 ; fi
eval $2=\( \"\$\{T2\[@\]\}\" \)
}
# # # # # Locate File # # # # #
#
# LocateFile [−l] FileName Location−Array−Name
# or
# LocateFile [−l] −of FileName Location−Array−FileName
# # # # #
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 403
# A file location is Filesystem−id and inode−number
# Here document used as a comment block.
: <<StatFieldsDoc
Based on stat, version 2.2
stat −t and stat −lt fields
[0] name
[1] Total size
File − number of bytes
Symbolic link − string length of pathname
[2] Number of (512 byte) blocks allocated
[3] File type and Access rights (hex)
[4] User ID of owner
[5] Group ID of owner
[6] Device number
[7] Inode number
[8] Number of hard links
[9] Device type (if inode device) Major
[10] Device type (if inode device) Minor
[11] Time of last access
May be disabled in 'mount' with noatime
atime of files changed by exec, read, pipe, utime, mknod (mmap?)
atime of directories changed by addition/deletion of files
[12] Time of last modification
mtime of files changed by write, truncate, utime, mknod
mtime of directories changed by addtition/deletion of files
[13] Time of last change
ctime reflects time of changed inode information (owner, group
permissions, link count
−*−*− Per:
Return code: 0
Size of array: 14
Contents of array
Element 0: /home/mszick
Element 1: 4096
Element 2: 8
Element 3: 41e8
Element 4: 500
Element 5: 500
Element 6: 303
Element 7: 32385
Element 8: 22
Element 9: 0
Element 10: 0
Element 11: 1051221030
Element 12: 1051214068
Element 13: 1051214068
For a link in the form of linkname −> 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
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 404
Note: 'mount −l' does return the label and could return the UUID
[3] Maximum length of filenames
[4] Filesystem type
[5] Total blocks in the filesystem
[6] Free blocks
[7] Free blocks for non−root user(s)
[8] Block size of the filesystem
[9] Total inodes
[10] 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
Element 7: 2146050
Element 8: 4096
Element 9: 1311552
Element 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\[@\]\}\" \) ;;
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 405
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)
# 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
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 406
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
Stephane Chazelas demonstrates object−oriented programming in a Bash script.
Example A−21. Object−oriented database
#!/bin/bash
# obj−oriented.sh: Object−oriented programming in a shell script.
# Script by Stephane Chazelas.
person.new() # Looks almost like a class declaration in C++.
{
local obj_name=$1 name=$2 firstname=$3 birthdate=$4
eval "$obj_name.set_name() {
eval \"$obj_name.get_name() {
echo \$1
}\"
}"
eval "$obj_name.set_firstname() {
eval \"$obj_name.get_firstname() {
echo \$1
}\"
}"
eval "$obj_name.set_birthdate() {
eval \"$obj_name.get_birthdate() {
echo \$1
}\"
eval \"$obj_name.show_birthdate() {
echo \$(date −d \"1/1/1970 0:0:\$1 GMT\")
}\"
eval \"$obj_name.get_age() {
echo \$(( (\$(date +%s) − \$1) / 3600 / 24 / 365 ))
}\"
}"
$obj_name.set_name $name
$obj_name.set_firstname $firstname
$obj_name.set_birthdate $birthdate
}
echo
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 407
person.new self Bozeman Bozo 101272413
# Create an instance of "person.new" (actually passing args to the function).
self.get_firstname # Bozo
self.get_name # Bozeman
self.get_age # 28
self.get_birthdate # 101272413
self.show_birthdate # Sat Mar 17 20:13:33 MST 1973
echo
# typeset −f
# to see the created functions (careful, it scrolls off the page).
exit 0
Advanced Bash−Scripting Guide
Appendix A. Contributed Scripts 408
Appendix B. 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−like syntax
For all their differences, the two utilities share a similar invocation syntax, both use regular expressions , both
read input by default from stdin, and both output to stdout. These are well−behaved UNIX tools, and
they work together well. The output from one can be piped into 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 34−3 and Example 9−22).
B.1. Sed
Sed is a non−interactive line 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. [69] 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 B−1. Basic sed operators
Operator Name Effect
[address−range]/p print Print [specified address range]
[address−range]/d delete Delete [specified address
range]
s/pattern1/pattern2/ substitute 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
Appendix B. A Sed and Awk Micro−Primer 409
address−range (equivalent
of tr)
gglobal 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 option may be omitted.
sed −n '/xzy/p' $filename
# The −n option tells sed to print only those lines matching the pattern.
# Otherwise all input lines would print.
# The −e option not necessary here since there is only a single editing instruction.
Table B−2. Examples
Notation Effect
8d Delete 8th line of input.
/^$/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.
Advanced Bash−Scripting Guide
Appendix B. A Sed and Awk Micro−Primer 410
/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
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
The backslash represents a newline as a substitution character. In this special case, the replacement expression
continues on the next line.
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.
A quick way to double−space a text file is sed G filename.
For illustrative examples of sed within shell scripts, see:
Example 34−11. Example 34−22. Example 12−23. Example A−34. Example 12−125. Example 12−206. Example A−137. Example A−188. Example 12−249. Example 10−910. Example 12−3311. Example A−212. Example 12−1013. Example 12−814. Example A−1115. Example 17−1116.
Advanced Bash−Scripting Guide
Appendix B. A Sed and Awk Micro−Primer 411
For a more extensive treatment of sed, check the appropriate references in the Bibliography.
B.2. Awk
Awk 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 couple of these here − the ones most useful
for shell scripting.
Awk breaks each line of input passed to it into fields. By default, a field is a string of consecutive characters
separated by whitespace, though there are options for changing the delimiter. Awk parses and operates on
each separate field. This makes awk ideal for handling structured text files, especially tables, data organized
into consistent chunks, such as rows and columns.
Strong quoting (single quotes) and curly brackets enclose segments of awk code within a shell script.
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.
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.
For examples of awk within shell scripts, see:
Example 11−101. Example 16−72. Example 12−243. Example 34−34. Example 9−225. Example 11−166. Example 28−17. Example 28−28. Example 10−39. Example 12−4210. Example 9−2611. Example 12−312. Example 9−1213. Example 34−1114. Example 10−815.
Advanced Bash−Scripting Guide
Appendix B. A Sed and Awk Micro−Primer 412
That's all the awk we'll cover here, folks, but there's lots more to learn. See the appropriate references in the
Bibliography.
Advanced Bash−Scripting Guide
Appendix B. A Sed and Awk Micro−Primer 413
Appendix C. Exit Codes With Special Meanings
Table C−1. "Reserved" Exit Codes
Exit Code
Number Meaning Example Comments
1catchall for general errors let "var1 = 1/0" miscellaneous errors, such as "divide
by zero"
2misuse of shell builtins, according to
Bash documentation Seldom seen, usually defaults to exit
code 1
126 command invoked cannot execute permission problem or command is
not an executable
127 "command not found" 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
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 table, exit codes 1 − 2, 126 − 165, and 255 [70] have special meanings, and should therefore
be avoided as user−specified exit parameters. Ending a script with exit 127 would certainly cause confusion
when troubleshooting (is the error 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, this might
not add any additional ambiguity, but, on the other hand, it probably would not be very informative either.
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.
All user−defined exit codes in the accompanying examples to this document now conform to this standard,
except where overriding circumstances exist, as in Example 9−2.
Issuing a $? from the command line after a shell script exits gives results consistent with the table above
only from the Bash or sh prompt. Running the C−shell or tcsh may give different values in some cases.
Appendix C. Exit Codes With Special Meanings 414
Appendix D. A Detailed Introduction to I/O and I/O
Redirection
written by Stephane Chazelas, and revised by the document author
A command expects the first three file descriptors to be available. The first, fd 0 (standard input, stdin), is
for reading. The other two (fd 1, stdout and fd 2, stderr) are for writing.
There is a stdin, stdout, and a stderr associated with each command. ls 2>&1 means temporarily
connecting the stderr of the ls command to the same "resource" as the shell's stdout.
By convention, a command reads its input from fd 0 (stdin), prints normal output to fd 1 (stdout), and
error ouput to fd 2 (stderr). If one of those three fd's is not open, you may encounter problems:
bash$ 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/<n> 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 D. A Detailed Introduction to I/O and I/O Redirection 415
lsof 426 root 1w FIFO 0,0 7520 pipe
lsof 426 root 2w FIFO 0,0 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, to stdout
sleep 1
echo 2nd, to stderr >&2
sleep 1
echo 3rd, to fd 3 >&3
sleep 1
echo 4th, to fd 4 >&4
sleep 1
echo 5th, to fd 5 >&5
sleep 1
echo 6th, through a pipe | sed 's/.*/PIPE: &, to fd 5/' >&5
sleep 1
echo 7th, to fd 6 >&6
sleep 1
echo 8th, to fd 7 >&7
sleep 1
echo 9th, 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: $a"; done 3>&−
rm −f /tmp/fifo1 /tmp/fifo2
# For each command and subshell, figure out which fd points to what.
exit 0
Advanced Bash−Scripting Guide
Appendix D. A Detailed Introduction to I/O and I/O Redirection 416
Appendix E. Localization
Localization is an undocumented Bash feature.
A localized shell script echoes its text output in the language defined as the system's locale. A Linux user in
Berlin, Germany, would get script output in German, whereas his cousin in Berlin, Maryland, would get
output from the same script in English.
To create a localized script, use the following template to write all messages to the user (error messages,
prompts, etc.).
#!/bin/bash
# localized.sh
E_CDERROR=65
error()
{
printf "$@" >&2
exit $E_CDERROR
}
cd $var || error $"Can't cd to %s." "$var"
read −p $"Enter the value: " var
# ...
bash$ 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.
Now, build a language.po file for each language that the script will be translated into, specifying the
msgstr. As an example:
fr.po:
#: a:6
msgid "Can't cd to %s."
msgstr "Impossible de se positionner dans le répertoire %s."
#: a:7
msgid "Enter the value: "
msgstr "Entrez la valeur : "
Appendix E. Localization 417
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 "$(gettext −s "Enter the value: ")" var
# ...
The TEXTDOMAIN and TEXTDOMAINDIR variables need to be exported to the environment.
−−−
This appendix written by Stephane Chazelas.
Advanced Bash−Scripting Guide
Appendix E. Localization 418
Appendix F. History Commands
The Bash shell provides command−line tools for editing and manipulating a user's command history. This is
primarily a convenience, a means of saving keystrokes.
Bash history commands:
history1. fc2.
bash$ history
1 mount /mnt/cdrom
2 cd /mnt/cdrom
3 ls
...
Internal variables associated with Bash history commands:
$HISTCMD1. $HISTCONTROL2. $HISTIGNORE3. $HISTFILE4. $HISTFILESIZE5. $HISTSIZE6. !!7. !$8. !#9. !N10. !−N11. !STRING12. !?STRING?13. ^STRING^string^14.
Unfortunately, the Bash history tools find no use in scripting.
#!/bin/bash
# history.sh
# Attempt to use 'history' command in a script.
history
# Script produces no output.
# History commands do not work within a script.
bash$ ./history.sh
(no output)
Appendix F. History Commands 419
Appendix G. A Sample .bashrc File
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 G−1. Sample .bashrc file
#===============================================================
#
# PERSONAL $HOME/.bashrc FILE for bash−2.05a (or later)
#
# Last modified: Tue Apr 15 20:32:34 CEST 2003
#
# 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.
#
# This file was designed (originally) for Solaris but based
# on Redhat's default .bashrc file
# −−> Modified for Linux.
# The majority of the code you'll find here is based on code found
# on Usenet (or internet).
# This bashrc file is a bit overcrowded − remember it is just
# just an example. Tailor it to your needs
#
#
#===============================================================
# −−> Comments added by HOWTO author.
# −−> And then edited again by ER :−)
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# 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'......
# I have not found a 'universal' method yet
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
function get_xserver ()
{
case $TERM in
xterm )
Appendix G. A Sample .bashrc File 420
XSERVER=$(who am i | awk '{print $NF}' | tr −d ')''(' )
XSERVER=${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 # Don't want any coredumps
set −o notify
set −o noclobber
set −o ignoreeof
set −o nounset
#set −o xtrace # useful for debuging
# Enable options:
shopt −s cdspell
shopt −s cdable_vars
shopt −s checkhash
shopt −s checkwinsize
shopt −s mailwarn
shopt −s sourcepath
shopt −s no_empty_cmd_completion # bash>=2.04 only
shopt −s cmdhist
shopt −s histappend histreedit histverify
shopt −s extglob # necessary for programmable completion
# Disable options:
shopt −u mailwarn
unset MAILCHECK # I don't want my shell to warn me of incoming mail
export TIMEFORMAT=$'\nreal %3R\tuser %3U\tsys %3S\tpcpu %P\n'
export HISTIGNORE="&:bg:fg:ll:h"
export HOSTFILE=$HOME/.hosts # Put a 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'
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 421
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 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#$HOST}" != ":0.0" && "${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
}
function powerprompt()
{
_powerprompt()
{
LOAD=$(uptime|sed −e "s/.*: \([^,]*\).*/\1/" −e "s/ //g")
}
PROMPT_COMMAND=_powerprompt
case $TERM in
*term | rxvt )
PS1="${HILIT}[\A \$LOAD]$NC\n[\h \#] \W > \[\033]0;\${TERM} [\u@\h] \w\007\]" ;;
linux )
PS1="${HILIT}[\A − \$LOAD]$NC\n[\h \#] \w > " ;;
* )
PS1="[\A − \$LOAD]\n[\h \#] \w > " ;;
esac
}
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 422
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
# (ie 'lowercase') but my workstation has 512Meg of RAM, so .....
# If you want to make this file smaller, these functions can
# be converted into scripts.
#
# 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 h='history'
alias j='jobs −l'
alias r='rlogin'
alias which='type −all'
alias ..='cd ..'
alias path='echo −e ${PATH//:/\\n}'
alias print='/usr/bin/lp −o nobanner −d $LPDEST' # Assumes LPDEST is defined
alias pjet='enscript −h −G −fCourier9 −d $LPDEST' # Pretty−print using enscript
alias background='xv −root −quit −max −rmode 5' # Put a picture in the background
alias du='du −kh'
alias df='df −kTh'
# The 'ls' family (this assumes you use the GNU ls)
alias la='ls −Al' # show hidden files
alias ls='ls −hF −−color' # add colors for filetype recognition
alias lx='ls −lXB' # sort by extension
alias lk='ls −lSr' # sort by size
alias lc='ls −lcr' # sort by change time
alias lu='ls −lur' # sort by access time
alias lr='ls −lR' # recursive ls
alias lt='ls −ltr' # sort by date
alias lm='ls −al |more' # pipe through 'more'
alias tree='tree −Csu' # nice alternative to 'ls'
# tailoring 'less'
alias more='less'
export PAGER=less
export LESSCHARSET='latin1'
export LESSOPEN='|/usr/bin/lesspipe.sh %s 2>&−' # Use this if lesspipe.sh exists
export LESS='−i −N −w −z−4 −g −e −M −X −F −R −P%t?f%f \
:stdin .?pb%pb\%:?lbLine %lb:?bbByte %bb:−...'
# spelling typos − highly personnal :−)
alias xs='cd'
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 423
alias vf='cd'
alias moer='more'
alias moew='more'
alias kk='ll'
#−−−−−−−−−−−−−−−−
# a few fun ones
#−−−−−−−−−−−−−−−−
function xtitle ()
{
case "$TERM" in
*term | rxvt)
echo −n −e "\033]0;$*\007" ;;
*)
;;
esac
}
# aliases...
alias top='xtitle Processes on $HOST && top'
alias make='xtitle Making $(basename $PWD) ; make'
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
}
function ll(){ ls −l "$@"| egrep "^d" ; ls −lXB "$@" 2>&−| egrep −v "^d|total "; }
function te() # wrapper around xemacs/gnuserv
{
if [ "$(gnuclient −batch −eval t 2>&−)" == "t" ]; then
gnuclient −q "$@";
else
( xemacs "$@" &);
fi
}
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# File & strings related functions:
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# Find a file with a pattern in name:
function ff() { find . −type f −iname '*'$*'*' −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 pattern in a set of filesand highlight them:
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 "$opt" in
i) case="−i " ;;
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 424
*) echo "$usage"; return;;
esac
done
shift $(( $OPTIND − 1 ))
if [ "$#" −lt 1 ]; then
echo "$usage"
return;
fi
local SMSO=$(tput smso)
local RMSO=$(tput rmso)
find . −type f −name "${2:−*}" −print0 | xargs −0 grep −sn ${case} "$1" 2>&− | \
sed "s/$1/${SMSO}\0${RMSO}/gI" | more
}
function cuttail() # cut last n lines in file, 10 by default
{
nlines=${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
{
local TMPFILE=tmp.$$
mv "$1" $TMPFILE
mv "$2" "$1"
mv $TMPFILE "$2"
}
#−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−
# 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:−".*"} ; }
# This function is roughly the same as 'killall' on linux
# but has no equivalent (that I know of) on Solaris
function killps() # kill by process name
{
local pid pname sig="−TERM" # default signal
if [ "$#" −lt 1 ] || [ "$#" −gt 2 ]; then
echo "Usage: killps [−SIGNAL] pattern"
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 425
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
}
# Misc utilities:
function repeat() # repeat n times command
{
local i max
max=$1; shift;
for ((i=1; i <= max ; i++)); do # −−> C−like syntax
eval "$@";
done
}
function ask()
{
echo −n "$@" '[y/n] ' ; read ans
case "$ans" in
y*|Y*) return 0 ;;
*) return 1 ;;
esac
}
#=========================================================================
#
# PROGRAMMABLE COMPLETION − ONLY SINCE BASH−2.04
# Most are taken from the bash 2.05 documentation and from Ian McDonalds
# 'Bash completion' package (http://www.caliban.org/bash/index.shtml#completion)
# You will in fact need bash−2.05a for some features
#
#=========================================================================
if [ "${BASH_VERSION%.*}" \< "2.05" ]; then
echo "You will need to upgrade to version 2.05 for programmable completion"
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 426
return
fi
shopt −s extglob # necessary
set +o nounset # otherwise some completions will fail
complete −A hostname rsh rcp telnet rlogin r ftp ping disk
complete −A export printenv
complete −A variable export local readonly unset
complete −A enabled builtin
complete −A alias alias unalias
complete −A function function
complete −A user su mail finger
complete −A helptopic help # currently same as builtins
complete −A shopt shopt
complete −A stopped −P '%' bg
complete −A job −P '%' fg jobs disown
complete −A directory mkdir rmdir
complete −A directory −o default cd
# Compression
complete −f −o default −X '*.+(zip|ZIP)' zip
complete −f −o default −X '!*.+(zip|ZIP)' unzip
complete −f −o default −X '*.+(z|Z)' compress
complete −f −o default −X '!*.+(z|Z)' uncompress
complete −f −o default −X '*.+(gz|GZ)' gzip
complete −f −o default −X '!*.+(gz|GZ)' gunzip
complete −f −o default −X '*.+(bz2|BZ2)' bzip2
complete −f −o default −X '!*.+(bz2|BZ2)' bunzip2
# Postscript,pdf,dvi.....
complete −f −o default −X '!*.ps' gs ghostview ps2pdf ps2ascii
complete −f −o default −X '!*.dvi' dvips dvipdf xdvi dviselect dvitype
complete −f −o default −X '!*.pdf' acroread pdf2ps
complete −f −o default −X '!*.+(pdf|ps)' gv
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
# Multimedia
complete −f −o default −X '!*.+(jp*g|gif|xpm|png|bmp)' xv gimp
complete −f −o default −X '!*.+(mp3|MP3)' mpg123 mpg321
complete −f −o default −X '!*.+(ogg|OGG)' ogg123
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'
_get_longopts ()
{
$1 −−help | sed −e '/−−/!d' −e 's/.*−−\([^[:space:].,]*\).*/−−\1/'| \
grep ^"$2" |sort −u ;
}
_longopts_func ()
{
case "${2:−*}" in
−*) ;;
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 427
*) return ;;
esac
case "$1" in
\~*) eval cmd="$1" ;;
*) cmd="$1" ;;
esac
COMPREPLY=( $(_get_longopts ${1} ${2} ) )
}
complete −o default −F _longopts_func configure bash
complete −o default −F _longopts_func wget id info a2ps ls recode
_make_targets ()
{
local mdef makef gcmd cur prev i
COMPREPLY=()
cur=${COMP_WORDS[COMP_CWORD]}
prev=${COMP_WORDS[COMP_CWORD−1]}
# if prev argument is −f, return possible filename completions.
# we could be a little smarter here and return matches against
# `makefile Makefile *.mk', whatever exists
case "$prev" in
−*f) COMPREPLY=( $(compgen −f $cur ) ); return 0;;
esac
# if we want an option, return the possible posix options
case "$cur" in
−) COMPREPLY=(−e −f −i −k −n −p −q −r −S −s −t); return 0;;
esac
# make reads `makefile' before `Makefile'
if [ −f makefile ]; then
mdef=makefile
elif [ −f Makefile ]; then
mdef=Makefile
else
mdef=*.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 [[ ${COMP_WORDS[i]} == −*f ]]; then
eval makef=${COMP_WORDS[i+1]} # eval for tilde expansion
break
fi
done
[ −z "$makef" ] && makef=$mdef
# if we have a partial word to complete, restrict completions to
# matches of that word
if [ −n "$2" ]; then gcmd='grep "^$2"' ; else gcmd=cat ; fi
# if we don't want to use *.mk, we can take out the cat and use
# test −f $makef and input redirection
COMPREPLY=( $(cat $makef 2>/dev/null | awk 'BEGIN {FS=":"} /^[^.# ][^=]*:/ {print $1}' | tr −s ' ' '\012' | sort −u | eval $gcmd ) )
}
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 428
complete −F _make_targets −X '+($*|*.[cho])' make gmake pmake
# cvs(1) completion
_cvs ()
{
local cur prev
COMPREPLY=()
cur=${COMP_WORDS[COMP_CWORD]}
prev=${COMP_WORDS[COMP_CWORD−1]}
if [ $COMP_CWORD −eq 1 ] || [ "${prev:0:1}" = "−" ]; then
COMPREPLY=( $( compgen −W 'add admin checkout commit diff \
export history import log rdiff release remove rtag status \
tag update' $cur ))
else
COMPREPLY=( $( compgen −f $cur ))
fi
return 0
}
complete −F _cvs cvs
_killall ()
{
local cur prev
COMPREPLY=()
cur=${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 $0}' ))
return 0
}
complete −F _killall killall killps
# A meta−command completion function for commands like sudo(8), which need to
# first complete on a command, then complete according to that command's own
# completion definition − currently not quite foolproof (e.g. mount and umount
# don't work properly), but still quite useful − By Ian McDonald, modified by me.
_my_command()
{
local cur func cline cspec
COMPREPLY=()
cur=${COMP_WORDS[COMP_CWORD]}
if [ $COMP_CWORD = 1 ]; then
COMPREPLY=( $( compgen −c $cur ) )
elif complete −p ${COMP_WORDS[1]} &>/dev/null; then
cspec=$( complete −p ${COMP_WORDS[1]} )
if [ "${cspec%%−F *}" != "${cspec}" ]; then
# complete −F <function>
#
# COMP_CWORD and COMP_WORDS() are not read−only,
# so we can set them before handing off to regular
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 429
# completion routine
# set current token number to 1 less than now
COMP_CWORD=$(( $COMP_CWORD − 1 ))
# get function name
func=${cspec#*−F }
func=${func%% *}
# get current command line minus initial command
cline="${COMP_LINE#$1 }"
# split current command line tokens into array
COMP_WORDS=( $cline )
$func $cline
elif [ "${cspec#*−[abcdefgjkvu]}" != "" ]; then
# complete −[abcdefgjkvu]
#func=$( echo $cspec | sed −e 's/^.*\(−[abcdefgjkvu]\).*$/\1/' )
func=$( echo $cspec | sed −e 's/^complete//' −e 's/[^ ]*$//' )
COMPREPLY=( $( eval compgen $func $cur ) )
elif [ "${cspec#*−A}" != "$cspec" ]; then
# complete −A <type>
func=${cspec#*−A }
func=${func%% *}
COMPREPLY=( $( compgen −A $func $cur ) )
fi
else
COMPREPLY=( $( compgen −f $cur ) )
fi
}
complete −o default −F _my_command nohup exec eval trace truss strace sotruss gdb
complete −o default −F _my_command command type which man nice
# Local Variables:
# mode:shell−script
# sh−shell:bash
# End:
Advanced Bash−Scripting Guide
Appendix G. A Sample .bashrc File 430
Appendix H. Converting DOS Batch Files to Shell
Scripts
Quite a number of programmers learned scripting on a PC running DOS. Even the crippled DOS batch file
language allowed writing some fairly powerful scripts and applications, though they often required extensive
kludges and workarounds. Occasionally, the need still arises to convert an old DOS batch file to a UNIX shell
script. This is generally not difficult, as DOS batch file operators are only a limited subset of the equivalent
shell scripting ones.
Table H−1. Batch file keywords / variables / operators, and their shell equivalents
Batch File Operator Shell Script Equivalent Meaning
%$ command−line parameter prefix
/− command option flag
\/ directory path separator
== = (equal−to) string comparison test
!==! != (not equal−to) string comparison test
|| pipe
@set +v do not echo current command
** filename "wild card"
>> file redirection (overwrite)
>> >> file redirection (append)
<< redirect stdin
%VAR% $VAR environmental variable
REM # comment
NOT ! negate following test
NUL /dev/null "black hole" for burying command output
ECHO echo echo (many more option in Bash)
ECHO. echo echo blank line
ECHO OFF set +v do not echo command(s) following
FOR %%VAR IN (LIST) DO for var in [list]; do "for" loop
:LABEL none (unnecessary) label
GOTO none (use a function) jump to another location in the script
PAUSE sleep pause or wait an interval
CHOICE case or select menu choice
IF if if−test
IF EXIST FILENAME if [ −e filename ] test if file exists
IF !%N==! if [ −z "$N" ] if replaceable parameter "N" not present
CALL source or . (dot operator) "include" another script
COMMAND /C source or . (dot operator) "include" another script (same as CALL)
SET export set an environmental variable
Appendix H. Converting DOS Batch Files to Shell Scripts 431
SHIFT shift left shift command−line argument list
SGN −lt or −gt sign (of integer)
ERRORLEVEL $? exit status
CON stdin "console" (stdin)
PRN /dev/lp0 (generic) printer device
LPT1 /dev/lp0 first printer device
COM1 /dev/ttyS0 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 H−2. DOS Commands and Their UNIX Equivalents
DOS Command UNIX Equivalent Effect
ASSIGN ln link file or directory
ATTRIB chmod change file permissions
CD cd change directory
CHDIR cd change directory
CLS clear clear screen
COMP diff, comm, cmp file compare
COPY cp file copy
Ctl−C Ctl−C break (signal)
Ctl−Z Ctl−D EOF (end−of−file)
DEL rm delete file(s)
DELTREE rm −rf delete directory recursively
DIR ls −l directory listing
ERASE rm delete file(s)
EXIT exit exit current process
FC comm, cmp file compare
FIND grep find strings in files
MD mkdir make directory
MKDIR mkdir make directory
MORE more text file paging filter
MOVE mv move
PATH $PATH path to executables
REN mv rename (move)
RENAME mv rename (move)
RD rmdir remove directory
RMDIR rmdir remove directory
SORT sort sort file
TIME date display system time
TYPE cat output file to stdout
Advanced Bash−Scripting Guide
Appendix H. Converting DOS Batch Files to Shell Scripts 432
XCOPY cp (extended) file copy
Virtually all UNIX and shell operators and commands have many more options and enhancements than
their DOS and batch file equivalents. Many DOS batch files rely on auxiliary utilities, such as ask.com,
a crippled counterpart to read.
DOS supports a very limited and incompatible subset of filename wildcard expansion, recognizing only
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 H−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.
Example H−2. viewdata.sh: Shell Script Conversion of VIEWDATA.BAT
#!/bin/bash
# Conversion of VIEWDATA.BAT to shell script.
DATAFILE=/home/bozo/datafiles/book−collection.data
ARGNO=1
# @ECHO OFF Command unnecessary here.
if [ $# −lt "$ARGNO" ] # IF !%1==! GOTO VIEWDATA
then
less $DATAFILE # TYPE C:\MYDIR\BOOKLIST.TXT | MORE
else
grep "$1" $DATAFILE # FIND "%1" C:\MYDIR\BOOKLIST.TXT
fi
exit 0 # :EXIT0
Advanced Bash−Scripting Guide
Appendix H. Converting DOS Batch Files to Shell Scripts 433
# GOTOs, labels, smoke−and−mirrors, and flimflam unnecessary.
# The converted script is short, sweet, and clean,
# which is more than can be said for the original.
Ted Davis' Shell Scripts on the PC site has a set of comprehensive tutorials on the old−fashioned art of batch
file programming. Certain of his ingenious techniques could conceivably have relevance for shell scripts.
Advanced Bash−Scripting Guide
Appendix H. Converting DOS Batch Files to Shell Scripts 434
Appendix I. Exercises
I.1. Analyzing Scripts
Examine the following script. Run it, then explain what it does. Annotate the script, then rewrite it in a more
compact and elegant manner.
#!/bin/bash
MAX=10000
for((nr=1; nr<$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
continue
fi
break # What heppens when you comment out this line? Why?
done
echo "Number = $nr"
exit 0
−−−
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 I. Exercises 435
Analyze Example A−11, and reorganize it in a simplified and more logical style. See how many of its
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.
I.2. Writing Scripts
Write a script to carry out each of the following tasks.
Easy
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 floppy, then press ENTER. Finally, save
the file to the floppy.
Converting for loops to while and until loops
Convert the for loops in Example 10−1 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
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.
BackupArchive 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.
Primes Print (to stdout) all prime numbers between 60000 and 63000. The output should be nicely formatted
in columns (hint: use printf).
Advanced Bash−Scripting Guide
Appendix I. Exercises 436
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.
Intermediate
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.
Safe Delete
Write, as a script, a "safe" delete command, srm.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 /home/username/trash directory. At invocation, the script checks the "trash"
directory for files older than 48 hours and deletes them.
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 23−4.
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 four decimal places.
Hint: pipe the coefficients to bc, using the well−known formula, x = ( −B +/− sqrt( B^2 − 4AC ) ) /
2A.
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
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.
Alphabetizing a String
Alphabetize (in ASCII order) an arbitrary string read from the command line.
ParsingParse /etc/passwd, and output its contents in nice, easy−to−read tabular form.
Pretty−Printing a Data File
Certain database and spreadsheet packages use save−files with comma−separated values (CSVs).
Other applications often need to parse these files.
Given a data file with comma−separated fields, of the form:
Advanced Bash−Scripting Guide
Appendix I. Exercises 437
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, by adjusting the word spacing right−justify each
line to a user−specified line−width and 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.
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.
Difficult
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 should
be flagged. Write this data as neatly formatted records in a logfile.
Strip Comments
Strip all comments from a shell script whose name is specified on the command line. Note that the "#!
line" must not be stripped out.
HTML Conversion
Convert a given text file to HTML. This non−interactive script automatically inserts all appropriate
HTML tags into a file specified as an argument.
Strip HTML Tags
Strip all 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.
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.
As necessary, use the appropriate network analysis commands.
Morse Code
Convert a text file to Morse code. Each character of the text file will be represented as a
corresponding Morse code group of dots and dashes (underscores), separated by whitespace from the
next. For example, "script" ===> "... _._. ._. .. .__. _".
Hex Dump
Do a hex(adecimal) dump on a binary file specified as an argument. 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.
Emulating a Shift Register
Advanced Bash−Scripting Guide
Appendix I. Exercises 438
Using Example 26−9 as an inspiration, write a script that emulates a 64−bit shift register as an array.
Implement functions to load the register, shift left, and shift right. Finally, write a function that
interprets the register contents as eight 8−bit ASCII characters.
Determinant
Solve a 4 x 4 determinant.
Hidden Words
Write a "word−find" puzzle generator, a script that hides 10 input words in a 10 x 10 matrix of
random letters. The words may be hidden across, down, or diagonally.
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.
Fog Index
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.
Choose a section of the text at least 100 words in length.1. Count the number of sentences (a portion of a sentence truncated by the boundary of the text
section counts as one).
2.
Find the average number of words per sentence.
AVE_WDS_SEN = TOTAL_WORDS / SENTENCES
3.
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
4.
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 ) )
5.
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 12−35, 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 to it, and the angle of the needle to that crack. You may use bc to handle the calculations.
Playfair Cipher
Advanced Bash−Scripting Guide
Appendix I. Exercises 439
Implement the Playfair (Wheatstone) Cipher in a script.
The Playfair Cipher encrypts text by substitution of each 2−letter "digram" (grouping). Traditionally,
one would use a 5 x 5 letter scrambled alphabet code key square for the encryption and decryption.
C O D E S
A B F G H
I K L M N
P Q R T U
V W X Y Z
Each letter of the alphabet appears once, except "I" also represents
"J". The arbitrarily chosen key word, "CODES" comes first, then all the
rest of the alphabet, 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 (wraps around left to beginning of row)
E −−> S
=========================================================================
To decrypt encrypted text, reverse the above procedure under cases #1
Advanced Bash−Scripting Guide
Appendix I. Exercises 440
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 Cryptoanalysis" (1939), gives a
fairly detailed rundown on the Playfair Cipher and its solution methods.
This script will have three main sections
Generating the "key square", based on a user−input keyword.I. Encrypting a "plaintext" message.II. Decrypting encrypted text.III.
The script will make extensive use of arrays and functions.
−−
Please do not send the author your solutions to these exercises. There are better ways to impress him with
your cleverness, such as submitting bugfixes and suggestions for improving this book.
Advanced Bash−Scripting Guide
Appendix I. Exercises 441
Appendix J. Copyright
The "Advanced Bash−Scripting Guide" is copyright, (c) 2000, by Mendel Cooper. 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 prohibited without the explicit permission of the copyright holder.
B. Distribution of the work or derivative of the work in any standard
(paper) book form is prohibited unless prior permission is obtained from
the copyright holder.
Provision A, above, explicitly prohibits the insertion of company logos or navigation bars into the document,
but with the following exemptions.
Non−profit organizations, such as the Linux Documentation Project and Sunsite.1. "Pure−play" Linux distributors, such as Debian, Red Hat, Mandrake, and others.2.
Essentially, you may freely distribute this book in unaltered electronic form. You must obtain the author's
permission to distribute a substantially modified version or derivative work. The purpose of this restriction is
to preserve the artistic integrity of this document and to prevent "forking".
These are very liberal terms, and they should not hinder any legitimate distribution or use of this book. The
author especially encourages the use of this book for instructional purposes.
The commercial print rights to this book are available. Please contact the author if interested.
The author produced this book in a manner consistent with the spirit of the LDP Manifesto.
Hyun Jin Cha has done a Korean translation of version 1.0.11 of this book. Spanish, Portuguese, French,
German, Italian, and Chinese translations are underway. 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.
Linux is a trademark registered to Linus Torvalds.
Unix and UNIX are trademarks registered to the Open Group.
MS Windows is a trademark registered to the Microsoft Corp.
All other commercial trademarks mentioned in the body of this work are registered to their respective
owners.
Notes
[1] These are referred to as builtins, features internal to the shell.
[2] Many of the features of ksh88, and even a few from the updated ksh93 have been merged into Bash.
Appendix J. Copyright 442
[3] 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 follow this guideline.
[4] Some flavors of UNIX (those based on 4.2BSD) take a four−byte magic number, requiring a blank after
the !, #! /bin/sh.
[5] 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.
[6] 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=65
echo "This line will never print (betcha!)."
exit $WHATEVER # Doesn't matter. The script will not exit here.
Also, try starting a README file with a #!/bin/more, and making it executable. The result is a
self−listing documentation file.
[7] Portable Operating System Interface, an attempt to standardize UNIX−like OSes.
[8] Caution: invoking a Bash script by sh scriptname turns off Bash−specific extensions, and the
script may therefore fail to execute.
[9] A script needs read, as well as execute permission for it to run, since the shell needs to be able to read
it.
[10] Why not simply invoke the script with scriptname? If the directory you are in ($PWD) is where
scriptname is located, why doesn't this work? This fails because, for security reasons, the current
directory, "." is not included in a user's $PATH. It is therefore necessary to explicitly invoke the script
in the current directory with a ./scriptname.
[11] The shell does the brace expansion. The command itself acts upon the result of the expansion.
[12] Exception: a code block in braces as part of a pipe may be 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" # Will not work.
# Thanks, S.C.
Advanced Bash−Scripting Guide
Appendix J. Copyright 443
[13] The process calling the script sets the $0 parameter. By convention, this parameter is the name of the
script. See the manpage for execv.
[14] Encapsulating "!" within double quotes gives an error when used from the command line. Apparently
this is interpreted as a history command. Within a script, though, this problem does not occur.
Of more concern is the inconsistent behavior of "\" within double quotes.
bash$ echo hello\!
hello!
bash$ echo "hello\!"
hello\!
bash$ echo −e x\ty
xty
bash$ echo −e "x\ty"
x y
(Thank you, Wayne Pollock, for pointing this out.)
[15] "Word splitting", in this context, means dividing a character string into a number of separate and
discrete arguments.
[16] Be aware that suid binaries may open security holes and that the suid flag has no effect on shell scripts.
[17] On modern UNIX systems, the sticky bit is no longer used for files, only on directories.
[18] 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).
[19] The pid of the currently running script is $$, of course.
[20] The words "argument" and "parameter" are often used interchangeably. In the context of this document,
they have the same precise meaning, that of a variable passed to a script or function.
[21] This applies to either command line arguments or parameters passed to a function.
[22] If $parameter is null in a non−interactive script, it will terminate with a 127 exit status (the Bash error
code code for "command not found").
[23] These are shell builtins, whereas other loop commands, such as while and case, are keywords.
[24] An exception to this is the time command, listed in the official Bash documentation as a keyword.
[25] A 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.
[26] The C source for a number of loadable builtins is typically found in the
/usr/share/doc/bash−?.??/functions directory.
Note that the −f option to enable is not portable to all systems.
[27] The same effect as autoload can be achieved with typeset −fu.
[28]
Advanced Bash−Scripting Guide
Appendix J. Copyright 444
These are files whose names begin with a dot, such as ~/.Xdefaults. Such filenames do not show
up in a normal ls listing, 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.
[29] This is only true of the GNU version of tr, not the generic version often found on commercial UNIX
systems.
[30] A tar czvf archive_name.tar.gz * will include dotfiles in directories below the current working
directory. This is an undocumented GNU tar "feature".
[31] 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.
[32] 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.
The word "daemon" means ghost in Greek, and there is certainly something mysterious, almost
supernatural, about the way UNIX daemons silently wander about behind the scenes, carrying out their
appointed tasks.
[33] This is actually a script adapted from the Debian Linux distribution.
[34] The print queue is the group of jobs "waiting in line" to be printed.
[35] For an excellent overview of this topic, see Andy Vaught's article, Introduction to Named Pipes, in the
September, 1997 issue of Linux Journal.
[36] EBCDIC (pronounced "ebb−sid−ic") is an acronym for Extended Binary Coded Decimal Interchange
Code. This is an IBM data format no longer in much use. 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.
[37] A macro is a symbolic constant that expands into a command string or a set of operations on
parameters.
[38] This is the case on a Linux machine or a UNIX system with disk quotas.
[39] The userdel command will fail if the particular user being deleted is still logged on.
[40] For more detail on burning CDRs, see Alex Withers' article, Creating CDs, in the October, 1999 issue
of Linux Journal.
[41] The −c option to mke2fs also invokes a check for bad blocks.
[42] Operators of single−user Linux systems generally prefer something simpler for backups, such as tar.
[43] NAND is the logical "not−and" operator. Its effect is somewhat similar to subtraction.
[44] For purposes of command substitution, a command may be an external system command, an internal
scripting builtin, or even a script function.
[45] 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 version of a file pointer. It is analogous to a file handle in C.
[46] 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.
[47] The simplest type of Regular Expression is a character string that retains its literal meaning, not
Advanced Bash−Scripting Guide
Appendix J. Copyright 445
containing any metacharacters.
[48] 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 # Here Document
line1
line2
EOF
# OUTPUT:
# [line1
# line2]
echo
awk '{ $0=$1 "\n" $2; if (/line.1/) {print}}' << EOF
line 1
line 2
EOF
# OUTPUT:
# line
# 1
# Thanks, S.C.
exit 0
[49] Filename expansion can match dotfiles, but only if the pattern explicitly includes the dot.
~/[.]bashrc # Will not expand to ~/.bashrc
~/?bashrc # Neither will this.
# Wild cards and metacharacters will not expand to a dot in globbing.
~/.[b]ashrc # Will expand to ~./bashrc
~/.ba?hrc # Likewise.
~/.bashr* # Likewise.
# Setting the "dotglob" option turns this off.
# Thanks, S.C.
[50] This has the same effect as a named pipe (temp file), and, in fact, named pipes were at one time used in
process substitution.
[51] Indirect variable references (see Example 35−2) provide a clumsy sort of mechanism for passing
variable pointers to functions.
#!/bin/bash
ITERATIONS=3 # How many times to get input.
icount=1
my_read () {
# Called with my_read varname,
# outputs the previous value between brackets as the default value,
# then asks for a new value.
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Appendix J. Copyright 446
local local_var
echo −n "Enter a value "
eval 'echo −n "[$'$1'] "' # Previous value.
read local_var
[ −n "$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
[52] The return command is a Bash builtin.
[53] Herbert Mayer defines recursion as "...expressing an algorithm by using a simpler version of that same
algorithm..." A recursive function is one that calls itself.
[54] Too many levels of recursion may crash a script with a segfault.
#!/bin/bash
recursive_function ()
{
(( $1 < $2 )) && f $(( $1 + 1 )) $2;
# As long as 1st parameter is less than 2nd,
#+ increment 1st and recurse.
}
recursive_function 1 50000 # Recurse 50,000 levels!
# Segfaults, of course.
# Recursion this deep might cause even a C program to segfault,
#+ by using up all the memory allotted to the stack.
# Thanks, S.C.
exit 0 # This script will not exit normally.
[55] However, aliases do seem to expand positional parameters.
[56] This does not apply to csh, tcsh, and other shells not related to or descended from the classic Bourne
shell (sh).
[57] 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.
[58] A block device reads and/or writes data in chunks, or blocks, in contrast to a character device, which
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Appendix J. Copyright 447
acesses data in character units. Examples of block devices are a hard drive and CD ROM drive. An
example of a character device is a keyboard.
[59] Certain system commands, such as procinfo, free, vmstat, lsdev, and uptime do this as well.
[60] Rocky Bernstein's Bash debugger partially makes up for this lack.
[61] By convention, signal 0 is assigned to exit.
[62] Setting the suid permission on the script itself has no effect.
[63] In this context, " magic numbers" have an entirely different meaning than the magic numbers used to
designate file types.
[64] ANSI is, of course, the acronym for the American National Standards Institute.
[65] See Marius van Oers' article, Unix Shell Scripting Malware, and also the Denning reference in the
bibliography.
[66] Chet Ramey promises associative arrays (a Perl feature) in a future Bash release.
[67] This is the notorious "flog it to death" technique.
[68] Those who can, do. Those who can't... get an MCSE.
[69] If no address range is specified, the default is all lines.
[70] Out of range exit values can result in unpredictable exit codes. For example, exit 3809 gives an exit
code of 225.
Advanced Bash−Scripting Guide
Appendix J. Copyright 448

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