Linux: The Ultimate Step By Guide To Quickly And Easily Learning Linux

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LINUX
The Ultimate Step by Step Guide to Quickly and Easily Learning
Linux
TED DAWSON
© Copyright 2015 by WE CANT BE BEAT LLC
Chapter One: LINUX HISTORY
Chapter Two: LINUX DISTRIBUTION (DISTRO)
Introduction
GUIDE TO CHOOSING DISTRIBUTION
Linux Mint
Ubuntu
Debian GNU/Linux
Mageia
Fedora
openSUSE
Arch Linux
CentOS
PCLinuxOS
Slackware Linux
FreeBSD
Chapter Three: LICENSING
COMMUNITY
DEVELOPMENT
Chapter Four: INSTALLING DEBIAN 8
What is Debian
Customizing your System
Terminal
Install sudo
Set up the network
Setup your hosts file
Log in via SSH!
Installing the basics
Installing MySQL
Setting up MySQL
Chapter Five: INSTALLING CENTOS 7
Installation of CenOS7
Step 1: Download the ISO Image
Step 2: Make a bootable Drive
Step 3: Begin Installation
Step 4: Select Language and Keyboard
Step 5: Change the Installation Destination
Step 6: Select the Partitioning Scheme
Step 7: Create a Swap Space
Step 8: Create a Mountpoint
Step 9: Accept Changes
Step 10: Set Date and Time
Step 11: Begin Installation
Step 12: Set Up Root Password
Step 13: Create a User Account
Step 14: Complete Installation
Change and Set Hostname Command
Method #1: hostnamectl
How do I see the host names?
How do I delete a particular host name?
How do I change host name remotely?
Method #2: nmtui
Method #3: nmcli
To view the host name using nmcli:
To set the host name using nmcli:
Chapter Six: LINUX AND UNIXMAN COMMAND
Syntax
Description
General Options
Main Modes of Operation
Finding Manual Pages
Controlling Formatted Output
Section Numbers
Exit Status
Environment
Files
Examples
Chapter Seven: LINUX DIRECTORY COMMAND
sample outputs
List only files in Unix
Task: Create aliases to save time
Chapter Eight: WORKING WITH FILES
UNIX File Names
Looking at the Contents of Files
Cat Command
More Command
Head Command
Tail Command
Copying, Erasing, Renaming
Copying Files
Erasing Files
Renaming a File
Using the Command Line
Standard Input and Standard Output
Redirection
Using Pipes and Filters
Some Additional File Handling Commands
Word Count
Comparing the Contents of Two Files: the cmp and diff
Commands
Chapter Nine: NAVIGATION AND FILE MANAGEMENT
Prerequisites and Goals
Navigation and Exploration
Finding where you are with the "pwd" command
Looking at the Contents of Directories with "ls"
Moving Around the Filesystem with "cd"
Viewing Files
File and Directory Manipulation
Create a File with "touch"
Create a Directory with "mkdir"
Moving and Renaming Files and Directories with "mv"
Copying Files and Directories with "cp"
Removing Files and Directories with "rm" and "rmdir"
Editing Files
Chapter Ten: UNIX SHELL SCRIPTING
Shell Scripting Introduction
Chapter Eleven: SHELL BASIC OPERATOR
Arithmetic Operators
Relational Operators:
Boolean Operators
String Operators
File Test Operators
Run The .Sh File Shell Script In Linux / Unix
.sh As Root User
chmod Command: Run Shell Script In Linux
Chapter Twelve: SHELL EMBEDDING AND OPTIONS
Shell installing
Backticks
Backticks or single quotes
Shell Alternatives
Practice: Shell Installing
Chapter Thirteen: SHELL HISTORY SEARCH COMMAND
Emacs Line-Edit Mode Command History Searching
fc command
Delete command history
FILE NAME GLOBBING WITH *, ?, [ ]
Chapter fourteen: UNIX - SHELL INPUT/OUTPUT REDIRECTIONS
Chapter Fifteen: UNIX SHELL FUNCTION
Creating Functions
Example
Pass Parameters to a Function
Returning Values from Functions
Example
Nested Functions
Function Call from Prompt
Unix - Pipes and Filters
The grep Command
The sort Command
The pg and more Commands
Chapter Sixteen: UNIX USEFUL COMMAND
Files and Directories
Manipulating data
Messages between Users
Chapter Seventeen: REGULAR EXPRESSION
Invoking sed
The sed General Syntax
Deleting All Lines with sed
The sed Addresses
The sed Address Ranges
The Substitution Command
Substitution Flags
Using an Alternative String Separator
Replacing with Empty Space
Address Substitution
The Matching Command
Using Regular Expression
Using Multiple sed Commands
Back References
Chapter Eighteen: FILE SYSTEM BASICS
Directory Structure
The df Command
The du Command
Mounting the File System
Unmounting the File System
User and Group Quotas
Chapter Nineteen: UNIX-USER ADMINISTRATION
Managing Users and Groups
Modify a Group
Delete a Group:
Create an Account
Modify an Account
Delete an Account
Chapter Twenty: SYSTEM PERFORMANCE
Performance Components
Unix - System Logging
The /etc/syslog.conf file
Logging Actions
Log Rotation
Important Log Locations
Chapter Twenty-one: UNIX SIGNALS AND TRAPS
LIST of Signals
Default Actions
Sending Signals
Trapping Signals
Cleaning Up Temporary Files
Ignoring Signals
Resetting Traps
ABOUT THE AUTHOR
LINKS AND RESOURCES
DEDICATION
SPECIAL THANKS
Chapter One: LINUX HISTORY
The history of Linux began in 1991 with the commencement of a
personal project by Finnish student Linus Torvalds to create a new
free operating system kernel. Since then, the resulting Linux kernel
has been marked by constant growth throughout its history. Since
the initial release of its source code in 1991, it has grown from a
small number of C files under a license prohibiting commercial
distribution to the 3.18 version in 2015 with more than 18 million
lines of source code under the GNU General Public License (GPL).
He wrote the program specifically for the hardware he was using
and independent of an operating system because he wanted to use
the functions of his new PC with an Intel 80386 processor.
Development was done on MINIX using the GNU C compiler. The
GNU C Compiler is still the main choice for compiling Linux today.
The code can be built with other compilers, such as the Intel C
Compiler. As Torvalds wrote in his book ‘Just for Fun,’ he
eventually ended up writing an operating system kernel. On 25
August 1991 (age 21), he announced this new system in a Usenet
posting to the newsgroup "comp.os.minix."
Torvalds first published the Linux kernel under its own license,
which had a restriction on commercial activity. The software used
with the kernel was developed as part of the GNU project licensed
under the GNU GPL, a free software license. The first release of the
Linux kernel, Linux 0.01, included a binary of GNU's Bash shell. In
the "Notes for Linux release 0.01", Torvalds lists the GNU software
that is required to run Linux: Sadly, a kernel by itself gets you
nowhere. To get a working system, you need a shell, compilers, a
library, etc. These are separate parts and may be under stricter (or
even looser) copyright. Most of the tools used with Linux are GNU
software and are under the GNU copyright. These tools are not in
the distribution - ask me (or GNU) for more info. In 1992, he
suggested releasing the kernel under the GNU GPL. He first
announced this decision in the release notes of version 0.12. In the
middle of December 1992, he published version 0.99 using the
GNU GPL. Linux and GNU developers worked to integrate GNU
components with Linux to make a fully functional free operating
system. Torvalds has stated, "Making Linux GPL'd was definitely
the best thing I ever did." Torvalds initially used the designation
“Linux” only for the Linux kernel. The kernel was, however,
frequently used together with other software, especially that of the
GNU project. This quickly became the most popular adoption of
GNU software. In June 1994 in GNU's Bulletin, Linux was referred
to as a "free UNIX clone,” and the Debian project began calling its
product Debian GNU/Linux. In May 1996, Richard Stallman
published the editor Emacs 19.31, in which the type of system was
renamed from Linux to Lignux. This spelling was intended to refer
specifically to the combination of GNU and Linux, but this was
soon abandoned in favor of "GNU/Linux.” This name garnered
varying reactions. The GNU and Debian projects use the name,
although most people simply use the term "Linux" to refer to the
combination.
The largest part of the work on Linux is performed by the
community: the thousands of programmers around the world that
use Linux to send their suggested improvements to the maintainers.
Various companies have also helped not only with the development
of the kernels but also with the writing the body of auxiliary
software, which is distributed with Linux. As of February 2015, over
80% of Linux kernel developers are paid. It is released both by
organized projects such as Debian and by projects connected
directly with companies such as Fedora and openSUSE. The
members of these respective projects meet at various conferences
and fairs, in order to exchange ideas. One of the largest of these
fairs is the LinuxTag in Germany (currently in Berlin), where about
10,000 people assemble annually, in order to discuss Linux and the
projects associated with it. The Open Source Development Lab
(OSDL) was created in the year 2000, and is an independent
nonprofit organization, which pursues the goal of optimizing Linux
for employment in data centers and in the carrier range. It served as
sponsored working premises for Linus Torvalds and also for
Andrew Morton (until the middle of 2006 when Morton transferred
to Google). Torvalds worked full-time on behalf of OSDL,
developing the Linux kernels.
Chronology
1991: The Linux kernel is publicly announced on 25 August by
21-year-old Finnish student Linus Benedict Torvalds.
1992: The Linux kernel is re-licensed under the GNU GPL. The
first Linux distributions are created.
1993: Over 100 developers work on the Linux kernel. With their
assistance, the kernel is adapted to the GNU environment,
which creates a large spectrum of application types for Linux.
The oldest currently (as of 2015) existing Linux distribution,
Slackware, is released for the first time. Later that same year,
the Debian project is established. Today it is the largest
distribution community.
1994: Torvalds judges all components of the kernel to be fully
matured: he releases version 1.0 of Linux. The XFree86 project
contributes a graphical user interface (GUI). Commercial Linux
distribution makers Red Hat and SUSE publish version 1.0 of
their Linux distributions.
1995: Linux is ported to the DEC Alpha and the Sun SPARC
systems. Over the following years, it is ported to an ever-greater
number of platforms.
1996: Version 2.0 of the Linux kernel is released. The kernel can
now serve several processors at the same time using symmetric
multi-processing (SMP), and thereby becomes a serious
alternative for many companies.
1998: Many major companies such as IBM, Compaq, and Oracle
announce their support for Linux. The Cathedral and the
Bazaar were first published as an essay (later as a book),
resulting in Netscape publicly releasing the source code to its
Netscape Communicator web browser suite. Netscape's actions
and crediting of the essay brings Linux's open source
development model to the attention of the popular technical
press. In addition, a group of programmers began developing
the graphical user interface KDE.
1999: A group of developers began work on the graphical
environment GNOME, destined to become a free replacement
for KDE, which at the time depended on the then proprietary,
Qt GUI toolkit. During this year, IBM announced an extensive
project for the support of Linux.
2000: Dell announces that it is now the No. 2 provider of
Linux-based systems worldwide and the first major
manufacturer to offer Linux across its full product line.
2002: The media reported, "Microsoft killed Dell Linux."
2004: The XFree86 team splits up and joins with the existing X
standards body to form the X.Org Foundation, which results in
a substantially faster development of the X server for Linux.
2005: The project openSUSE begins free distribution from
Novell's community. In addition, the project OpenOffice.org
introduces version 2.0 which then started supporting OASIS
OpenDocument standards.
2006: Oracle releases its own distribution of Red Hat Enterprise
Linux. Novell and Microsoft announce cooperation for better
interoperability and mutual patent protection.
2007: Dell starts distributing laptops with Ubuntu pre-installed.
2009: Red Hats market capitalization equals Suns, interpreted
as a symbolic moment for the "Linux-based economy.”
2011: Version 3.0 of the Linux kernel is released.
2012: The aggregate Linux server market revenue exceeds that
of the rest of the UNIX market.
2013: Google's Linux-based Android claims 75% of the
Smartphone market share, in terms of the number of phones
shipped.
2014: Ubuntu claims 22,000,000 users.
2015: Version 4.0 of the Linux kernel is released.
Chapter Two: LINUX DISTRIBUTION (DISTRO)
Introduction
The bewildering choice and the ever increasing number of Linux
distributions can be confusing for those who are new to Linux. This
is why this book was created. It lists 10 Linux distributions (plus an
honorable mention of FreeBSD, by far the most popular of all of the
BSDs), it is considered to be the most widely-used by Linux users
around the world. There are no figures to back up this claim, and
there are many other distributions that might suit your particular
purpose better, but as a general rule, all of these are popular and
have very active forums or mailing lists where you can ask
questions if you get stuck. Ubuntu, Linux Mint, and PCLinuxOS are
considered the easiest for new users who want to get productive in
Linux as soon as possible without having to master all of its
complexities. On the other end of the spectrum, Slackware Linux,
Arch Linux, and FreeBSD are more advanced distributions that
require a deeper understanding, before they can be used effectively.
openSUSE, Fedora, Debian GNU/Linux and Mageia can be
classified as good "middle-road" distributions. CentOS is an
enterprise distribution, suitable for those who prefer stability,
reliability, and long-term support to cutting-edge features and
software.
GUIDE TO CHOOSING DISTRIBUTION
Linux Mint
Linux Mint, a distribution based on Ubuntu, was first launched
in 2006 by Clement Lefebvre, a French-born IT specialist living in
Ireland. Originally maintaining a Linux web site dedicated to
providing help, tips and documentation to new Linux users, the
author saw the potential of developing a Linux distribution that
would address the many usability drawbacks associated with the
generally more technical, mainstream products. Since its
beginnings, the developers have been adding a variety of graphical
"mint" tools for enhanced usability; this includes mintDesktop - a
utility for configuring the desktop environment, mintMenu - a new
and elegant menu structure for easier navigation, mintInstall - an
easy-to-use software installer, and mintUpdate - a software
updater. Mint's reputation for ease of use has been further
enhanced by the inclusion of proprietary and patent-encumbered
multimedia codecs that are often absent from larger distributions
due to potential legal threats. Perhaps one of the best features of
Linux Mint is the fact that the developers listen to the users and are
always fast in implementing good suggestions. While Linux Mint is
available as a free download, the project generates revenue from
donations, advertising, and professional support services.
Pros: Superb collection of "minty" tools developed in-house,
hundreds of user-friendly enhancements, the inclusion of
multimedia codecs, open to users' suggestions.
Cons: The alternative "community" editions do not always include
the latest features; the project does not issue security advisories.
Ubuntu
The launch of Ubuntu was first announced in September 2004.
Although a relative newcomer to the Linux distribution scene, the
project took off like no other, with its mailing lists soon filled with
discussions by eager users and enthusiastic developers. In the years
that followed, Ubuntu grew to become the most popular desktop
Linux distribution and has contributed greatly toward developing
an easy-to-use and free desktop operating system that can compete
well with any of the proprietary ones available on the market. On
the technical side of things, Ubuntu is based on Debian "Sid"
(unstable branch), but with some prominent packages, such as
GNOME, Firefox, and LibreOffice, updated to their latest versions.
It uses a custom user interface called "Unity.” It has a predictable,
6-month release schedule, with an occasional Long Term Support
(LTS) release that is supports security updates for 5 years,
depending on the edition (non-LTS release are supported for 9
months). Other special features of Ubuntu include an installable
live DVD, creative artwork, desktop themes, migration assistant for
Windows users, support for the latest technologies, such as 3D
desktop effects, easy installation of proprietary device drivers for
ATI and NVIDIA graphics cards, wireless networking, and on-
demand support for non-free or patent-encumbered media codecs.
Pros: Fixed release cycle and support period; long-term support
(LTS) variants with 5 years of security updates; novice-friendly; a
wealth of documentation, both official and user-contributed.
Cons: Lacks compatibility with Debian; frequent major changes
tend to drive some users away, the Unity user interface has been
criticized as being more suitable for mobile devices rather than
desktop computers; non-LTS releases come with only 9 months of
security support.
Debian GNU/Linux
Debian GNU/Linux was first announced in 1993. Its founder,
Ian Murdock, envisaged the creation of a completely non-
commercial product developed by hundreds of volunteer
developers. With skeptics far outnumbering optimists at the time, it
seemed destined to disintegrate and collapse, but the reality was
very different. Debian not only survived, it thrived and, in less than
a decade, it became the largest Linux distribution and possibly the
largest collaborative software product ever created! The following
numbers can illustrate the success of Debian GNU/Linux. Over
1,000 volunteer developers develop it, its software repositories
contain close to 50,000 binary packages (compiled for 8 processor
architectures) and is responsible for inspiring over 120 Debian-
based distributions and live CDs. These figures are unmatched by
any other Linux-based operating system. The actual development of
Debian takes place in three main branches (or four if one includes
the bleeding-edge "experimental" branch) of increasing levels of
stability: "unstable" (also known as "sid"), "testing" and "stable.”
However, this lengthy and complex development style has some
drawbacks: the stable releases of Debian are not particularly up-to-
date, and they age rapidly, especially since new stable releases are
only published once every 1 - 3 years. Users who prefer the latest
packages and technologies are forced to use the potentially buggy
Debian testing distributions or unstable branches. The highly
democratic structures of Debian have led to controversial decisions
and have led to infighting among the developers. This has
contributed to stagnation and reluctance to make radical decisions
that would take the project forward.
Pros: Very stable; remarkable quality control; includes over
30,000 software packages; supports more processor architectures
than any other Linux distribution.
Cons: Conservative - due to its support for many processor
architectures, newer technologies are not always included; slow
release cycle (one stable release every 1 - 3 years); discussions on
developer mailing lists and blogs can be uncultured at times.
Mageia
Mageia may be the newest distribution on this list, but its roots
go back to July 1998 when Gaël Duval launched Mandrake Linux.
At the time, it was just a branch of Red Hat Linux with KDE as the
default desktop, better hardware detection, and some user-friendly
features, but it gained instant popularity due to positive reviews in
the media. Mandrake was later turned into a commercial enterprise
and renamed to Mandriva (to avoid some trademark-related hassles
and to celebrate its merger with Brazil's Conectiva) before almost
going bankrupt in 2010. A Russian venture capital firm eventually
saved it, but this came at a cost when the new management decided
to lay off most of the established Mandriva developers in the
company's Paris headquarters. Upon finding themselves out of
work, they decided to form Mageia, a community project that is a
logical continuation of Mandrake and Mandriva, perhaps more so
than Mandriva itself. Mageia is primarily a desktop distribution. Its
best-loved features are cutting-edge software, a superb system
administration suite (Mageia Control Centre), the ability to attract a
large number of volunteer contributors, and extensive
internationalization support. It features one of the easiest, but more
powerful system installers on its installation DVD, while it also
releases a set of live images with both KDE or GNOME desktops
and comprehensive language support, with the ability to install it
onto a hard disk directly from the live desktop session. The
distribution's well-established package management features, with
powerful command-line options and a graphical software
management module, allow easy access to thousands of software
packages. The unique Mageia Control Center continues to improve
with each release, offering a powerful tool for configuring just about
any aspect of their computer without ever reaching for the terminal.
Pros: Beginner-friendly; excellent central configuration utility;
very good out-of-the-box support for dozens of languages;
installable live media.
Cons: Lacks reputation and mindshare following its split from
Mandriva, some concern over the developers' ability to maintain the
distribution long-term on a volunteer basis.
Fedora
Although Fedora was formally only unveiled in September 2004, its
origins date back to 1995 when it was launched by two Linux
visionaries -- Bob Young and Marc Ewing -- under the name of Red
Hat Linux. The company's first product, Red Hat Linux 1.0
"Mother's Day,” was released the same year and was quickly
followed by several bug-fix updates. In 1997, Red Hat introduced its
revolutionary RPM package management system with dependency
resolution and other advanced features which greatly contributed to
the distribution's rapid rise in popularity and its overtaking of
Slackware Linux as the most widely-used Linux distribution in the
world. In later years, Red Hat standardized on a regular, 6-month
release schedule. In 2003, just after the release of Red Hat Linux 9,
the company introduced some radical changes to its product line-
up. It retained the Red Hat trademark for its commercial products,
notably Red Hat Enterprise Linux, and introduced Fedora Core
(later renamed to Fedora), a Red Hat sponsored, but community-
oriented distribution designed for the "Linux hobbyist". After the
initial criticism of the changes, the Linux community accepted the
"new" distribution as the logical continuation of Red Hat Linux. A
few quality releases was all it took for Fedora to regain its former
status as one of the best-loved operating systems on the market. At
the same time, Red Hat quickly became the biggest and most
profitable Linux company in the world, with an innovative product
line-up, excellent customer support, and other popular initiatives,
such as its Red Hat Certified Engineer (RHCE) certification
program. Although Fedora's direction is still largely controlled by
Red Hat, Inc. and the product is sometimes seen -- rightly or
wrongly -- as a test bed for Red Hat Enterprise Linux, there is no
denying that Fedora is one of the most innovative distributions
available today. Its contributions to the Linux kernel, glibc and GCC
are well-known and its more recent integration of SELinux
functionality, virtualization technologies, system service manager,
cutting-edge journaled file systems, and other enterprise-level
features are much appreciated by the company's customers. On a
negative side, Fedora still lacks a clear desktop-oriented strategy
that would make the product easier to use for those beyond the
"Linux hobbyist" target.
Pros: Highly innovative; outstanding security features; a large
number of supported packages; strict adherence to the free software
philosophy; availability of live CDs featuring many popular desktop
environments.
Cons: Fedora's priorities tend to lean towards enterprise features,
rather than desktop usability; some bleeding edge features, such as
early switch to KDE 4 and GNOME 3, occasionally alienate desktop
users.
openSUSE
The beginnings of openSUSE date back to 1992 when four German
Linux enthusiasts -- Roland Dyroff, Thomas Fehr, Hubert Mantel
and Burchard Steinbild -- launched the project under the name of
SUSE (Software und System Entwicklung) Linux. In the early days,
the young company sold sets of floppy disks containing a German
edition of Slackware Linux, but it wasn't long before SUSE Linux
became an independent distribution with the launch of version 4.2
in May 1996. In the following years, the developers adopted the
RPM package management format and introduced YaST, an easy-
to-use graphical system administration tool. Frequent releases,
excellent printed documentation, and easy availability of SUSE
Linux in stores across Europe and North America resulted in
growing popularity of the distribution. SUSE Linux was acquired by
Novell, Inc. in late 2003, and then fell into the hands of Attachmate
in November 2010. Major changes in the development, licensing
and availability of SUSE Linux followed shortly after the first
acquisition - YaST was released under the GPL, the ISO images
were freely distributed from public download servers, and, most
significantly, the development of the distribution was opened to
public participation for the first time. Since the launch of the
openSUSE project and the release of version 10.0 in October 2005,
the distribution became completely free in both senses of the word.
The openSUSE code has become a base system for Novell's
commercial products, first named as Novell Linux, but later
renamed to SUSE Linux Enterprise Desktop and SUSE Linux
Enterprise Server. Today, openSUSE has a large following of
satisfied users. The principal reason for openSUSE receiving high
marks from its users are pleasant and polished desktop
environments (KDE and GNOME), an excellent system
administration utility (YaST), and, for those who buy the boxed
edition, some of the best-printed documentation available.
However, the infamous deal between Novell and Microsoft, which
apparently concedes to Microsoft's argument that it has intellectual
property rights over Linux, has resulted in condemnation by many
Linux personalities and has prompted some users to switch
distributions. Although Novell has downplayed the deal, and
Microsoft has yet to exercise any rights, this issue remains a thorn
in the side of the otherwise very community-friendly Linux
Company.
Pros: Comprehensive and intuitive configuration tool; large
repository of software packages, excellent web site infrastructure,
and printed documentation.
Cons: Novell's patent deal with Microsoft in November 2006
seemingly legitimized Microsoft's intellectual property claims over
Linux; its resource-heavy desktop set-up and graphical utilities are
sometimes seen as "bloated and slow."
Arch Linux
The KISS (Keep It Simple, Stupid) philosophy of Arch Linux was
devised around the year 2002 by Judd Vinet, a Canadian computer
science graduate who launched the distribution the same year. For
several years it lived as a marginal project designed for intermediate
and advanced Linux users and only shot to stardom when it began
promoting itself as a "rolling-release" distribution that only needs to
be installed once and is then kept up-to-date thanks to its powerful
package manager and an always fresh software repository. As a
result, Arch Linux "releases" are few and far between and are now
limited to a basic installation DVD that is issued only when
considerable changes in the base system warrant a fresh install.
Besides featuring the much-loved "rolling-release" update
mechanism, Arch Linux is also renowned for its fast and powerful
package manager called "Pacman", the ability to install software
packages from source code, easy creation of binary packages thanks
to its AUR infrastructure, and the ever increasing software
repository of well-tested packages. Its highly regarded
documentation, complemented by the excellent Arch Linux
Handbook makes it possible for even less experienced Linux users
to install and customize the distribution. The powerful tools
available at the user's disposal mean that the distro is infinitely
customizable to the minutest detail and that no two installations
can possibly be the same. On the negative side, any rolling-release
update mechanism has its dangers: a human mistake can creep in, a
library or dependency goes missing, a new version of an application
already in the repository has a yet-to-be-reported critical bug... It is
not unheard of to end up with an unbootable system following a
Pacman upgrade. As such, Arch Linux is the kind of distribution
that requires its users to be alert and to have enough knowledge to
fix any such problems. In addition, the infrequent install media
releases mean that it is sometimes no longer possible to use the old
media to install the distribution due to important system changes
or lack of hardware support in the older Linux kernel.
Pros: Excellent software management infrastructure; unparalleled
customization and tweaking options; superb online documentation.
Cons: Occasional instability and risk of breakdown.
CentOS
Launched in late 2003, CentOS is a community project with the
goals of rebuilding the source code for Red Hat Enterprise Linux
(RHEL) into an installable Linux distribution and to provide timely
security updates for all included software packages. To put in more
bluntly, CentOS is an RHEL clone. The only technical difference
between the two distributions is branding - CentOS replaced all Red
Hat trademarks and logos with its own. Nevertheless, the relations
between Red Hat and CentOS remain amicable, and many CentOS
developers are in active contact with, or even employed directly by,
Red Hat. CentOS is often seen as a reliable server distribution. It
comes with the same well-tested and stable Linux kernel and set of
software packages that form the basis of its parent, Red Hat
Enterprise Linux. Despite being a community project run by
volunteers, it has gained a reputation for being a solid, free
alternative to more costly server products on the market, especially
among experienced Linux system administrators. CentOS is also
suitable as an enterprise desktop solution, specifically where
stability, reliability and long-term support are preferred over latest
software and new features, like RHEL, CentOS includes
approximately 7-10 years of security updates. Despite its
advantages, CentOS might not be the best solution in all
deployment scenarios. Those users who prefer a distribution with
the latest Linux technologies and newest software packages should
look elsewhere. Major CentOS versions, which follow RHEL
versioning, are only released every 2 - 3 years, while "point" releases
(e.g. 5.1) tend to arrive in 6 - 9 month intervals. The point releases
do not usually contain any major features (although they do
sometimes include support for more recent hardware), and only a
handful of software packages may get updated to newer versions.
The Linux kernel, the base system, and most application versions
remain unchanged, but occasionally a newer version of an
important software package (e.g. LibreOffice or Firefox) may be
provided on an experimental basis. As a side project, CentOS also
builds updated packages for the users of its distributions, but the
repositories containing them are not enabled by default as they may
break upstream compatibility.
Pros: Extremely well tested, stable and reliable; free to download
and use; comes with 7+ years of free security updates.
Cons: Lacks latest Linux technologies; occasionally the project fails
to live up its.
PCLinuxOS
Bill “Texstar” Reynolds first introduced PCLinuxOS in 2003.
Prior to creating his own distribution, Texstar was already a well-
known developer in the Mandrake Linux community of users for
building up-to-date RPM packages for the popular distribution and
providing them as a free download. In 2003, he decided to build a
new distribution, initially based on Mandrake Linux, but with
several significant usability improvements. The goals? It should be
beginner-friendly, have out-of-the box support for proprietary
kernel modules, browser plug-in and media codecs, and should
function as a live CD with a simple and intuitive graphical installer.
Several years and development releases later, PCLinuxOS is rapidly
approaching its intended state. In terms of usability, the project
offers out-of-the-box support for many technologies that most
Windows-to-Linux migrants would expect from a new operating
system. On the software side of things, PCLinuxOS is a KDE-
oriented distribution, with a customized and always up-to-date
version of the popular desktop environment. Its growing software
repository contains other desktops and offers a great variety of
desktop packages for many common tasks. For system
configuration, PCLinuxOS has retained much of Mandriva's
excellent Control Centre but has replaced its package management
system with APT and Synaptic, a graphical package management
front-end. On the negative side, PCLinuxOS lacks any form of
roadmap or release goals. Despite growing community involvement,
most development and decision-making remains in the hands of
Texstar, who tends to be conservative when judging the stability of
a release. As a result, the development process of PCLinuxOS is
often arduous. For example, despite frequent calls for a 64-bit
edition, the developers held off producing a 64-bit build until fairly
recently. Furthermore, the project does not provide any security
advisories, relying instead on the users' willingness to keep their
system up-to-date via the included package management tools.
Pros: Out-of-the-box support for graphics drivers, browser plug-
ins, and media codecs; rolling-release update mechanism; up-to-
date software.
Cons: no out-of-the-box support for non-English languages; lacks
release planning and security advisories.
Slackware Linux
Slackware Linux, created by Patrick Volkerding in 1992, is the
oldest surviving Linux distribution. Separated from the now-
discontinued SLS project, Slackware 1.0 came on 24 floppy disks
and was built on top of Linux kernel version 0.99pl11-alpha. It
quickly became the most popular Linux distribution; with some
estimates putting its market share as much as 80% of all Linux
installations in 1995. Its popularity decreased dramatically with the
arrival of Red Hat Linux and other, user-friendlier distributions,
but Slackware Linux still remains a much-appreciated operating
system among the more technically oriented system administrators
and desktop users. Slackware Linux is a highly technical, clean
distribution, with only a limited number of custom utilities. It uses
a simple, text-based system installer and a comparatively primitive
package management system that does not resolve software
dependencies. As a result, Slackware is considered one of the
cleanest and least buggy distributions available today - the lack of
Slackware-specific enhancements reduces the likelihood of new
bugs being introduced into the system. The entire system
configuration is completed by editing text files. There is a saying in
the Linux community that if you learn Red Hat, you'll know Red
Hat, but if you learn Slackware, you'll know Linux. This is
particularly true today when many other Linux distributions keep
developing heavily customized products to meet the needs of less
technical Linux users. While this philosophy of simplicity has its
fans, the fact is that in today's world, Slackware Linux is
increasingly becoming a "core system" upon which new, custom
solutions are built, rather than a complete distribution with a wide
variety of supported software. The only exception is the server
market, where Slackware remains popular, though even here, the
distribution's complex upgrade procedure and lack of officially
supported automated tools for security updates make it increasingly
uncompetitive. Slackware's conservative attitude towards the
system's base components means that it requires much manual
post-installation work before it can be turned into a modern
desktop system.
Pros: Considered highly stable, clean and largely bug-free, strong
adherence to UNIX principles.
Cons: Limited number of officially supported applications;
conservative in terms of base package selection; complex upgrade
procedure.
FreeBSD
FreeBSD, an indirect descendant of AT&T UNIX via the Berkeley
Software Distribution (BSD), has a long and turbulent history
dating back to 1993. Unlike Linux distributions, which are defined
as integrated software solutions consisting of the Linux kernel and
thousands of software applications, FreeBSD is a tightly integrated
operating system built from a BSD kernel and the so-called
"userland" (therefore usable even without extra applications). This
distinction is largely lost once installed on the average computer
system - like many Linux distributions, a large collection of easily
installed, (mostly) open source applications are available for
extending the FreeBSD core, but these are usually provided by
third-party contributors and aren't strictly part of FreeBSD.
FreeBSD has developed a reputation for being a fast, high-
performance and extremely stable operating system, especially
suitable for web serving and similar tasks. Many large web search
engines and organizations with mission-critical computing
infrastructures have deployed and used FreeBSD on their computer
systems for years. Compared to Linux, FreeBSD is distributed
under a much less restrictive license, which allows virtually
unrestricted use and modification of the source code for any
purpose. Even Apple's Mac OS X is known to have been derived
from FreeBSD. Besides the core operating system, the project also
provides over 24,000 software applications in binary and source
code forms for easy installation on top of the core FreeBSD. While
FreeBSD can certainly be used as a desktop operating system,
although it does not compare well to more popular Linux
distributions. The text-mode system installer offers little in terms of
hardware detection or system configuration, leaving much of the
dirty work to the user in a post-installation setup. In terms of
support for modern hardware, FreeBSD generally lags behind
Linux, especially in supporting cutting-edge desktop and laptop
gadgets, such as wireless network cards or digital cameras. Those
users seeking to exploit the speed and stability of FreeBSD on a
desktop or workstation should consider one of the available desktop
FreeBSD projects, rather than FreeBSD itself.
Pros: Fast and stable; availability of over 24,000 software
applications (or "ports") for installation; very good documentation.
Cons: Tends to lag behind Linux in terms of support for new and
exotic hardware, limited availability of commercial applications;
lacks graphical configuration tools.
Chapter Three: LICENSING
Code is contributed to the Linux kernel under a number of
licenses, but all code must be compatible with version 2 of the GNU
(GPLv2), which is the license covering the kernel distribution as a
whole. In practice, that means that all code contributions are
covered either by GPLv2 (with, optionally, language allowing
distribution under later versions of the GPL) or the three-clause
BSD license. Any contributions, which are not covered by a
compatible license, will not be accepted into the kernel. Copyright
assignments are not required (or requested) for code contributed to
the kernel. All code merged into the mainline kernel retains its
original ownership; as a result, the kernel now has thousands of
owners. One implication of this ownership structure is that any
attempt to change the licensing of the kernel is doomed to almost
certain failure. There are few practical scenarios where the
agreement of all copyright holders could be obtained (or their code
removed from the kernel). Therefore, there is no prospect of a
migration to version 3 of the GPL in the foreseeable future. It is
imperative that all code contributed to the kernel be legitimately
free software. For that reason, code from anonymous (or
pseudonymous) contributors will not be accepted.
All contributors are required to "sign off" on their code, stating
that the code can be distributed with the kernel under the GPL.
Code which its owner, has not licensed as free software, or which
risks creating copyright-related problems for the kernel (such as
code which derives from reverse-engineering efforts lacking proper
safeguards) cannot be contributed. Questions about copyright-
related issues are common on Linux development mailing lists.
Such questions will normally receive no shortage of answers, but
one should bear in mind that the people answering those questions
are not lawyers and cannot provide legal advice. If you have legal
questions relating to Linux source code, there is no substitute for
talking with a lawyer who understands this field. Relying on
answers obtained on technical mailing lists is a risky affair.
COMMUNITY
Linux communities come in two basic forms: developer and user.
One of the most compelling features of Linux is that it is accessible
to developers; anybody with the requisite skills can improve Linux
and influence the direction of its development. Proprietary
products cannot offer this kind of openness, which is a
characteristic of the free software process. Developer communities
can volunteer to maintain and support whole distributions, such as
the Debian or Gentoo Projects. Novell and Red Hat also support
community-driven versions of their products, openSUSE and
Fedora, respectively. The improvements to these community distros
are then incorporated into the commercial server and desktop
products. The Linux kernel itself is primarily supported by its
developer community and is one of the largest and most active free
software projects in existence. A typical three-month kernel
development cycle can involve over 1000 developers working for
more than 100 different companies (or for no company at all).
With the growth of Linux has come an increase in the number of
developers (and companies) wishing to participate in its
development. Hardware vendors want to ensure that Linux
supports their products well, making their products attractive to
Linux users. Embedded systems vendors, who use Linux as a
component in an integrated product, want Linux to be as capable
and well suited to the task at hand as possible. Distributors and
other software vendors who base their products on Linux have a
clear interest in the capabilities, performance, and reliability of the
Linux kernel. Other developer communities focus on different
applications and environments that run on Linux, such as Firefox,
OpenOffice, GNOME, and KDE. End users, too, can make valuable
contributions to the development of Linux. With online
communities such as Linux.com, LinuxQuestions, and the many
and varied communities hosted by distributions and applications,
the Linux user base is often a very vocal, but usually positive
advocate and guide for the Linux operating system. The Linux
community is not just a online presence. Local groups known as
Linux Users Groups (LUGs) often meet to discuss issues regarding
the Linux operating system, and provide other local users with free
demonstrations, training, technical support, and install-fests.
DEVELOPMENT
Linux is an operating system comprised of many different
development languages. A very large percentage of the
distributions' code is written in either the C (52.86%) or C++
(25.56%) languages. All of the rest of the code falls into single-digit
percentages, with Java, Perl, and Lisp rounding out the top 5
languages. The Linux kernel itself has an even more dominant C
presence, with over 95 percent of the kernel's code written in that
language. However, other languages make up the kernel as well,
making it more heterogeneous than other operating systems. The
kernel community has evolved its own distinct way of operating
which allows it to function smoothly (and produce a high-quality
product) in an environment where thousands of lines of code are
being changed every day. This means the Linux kernel development
process differs greatly from proprietary development methods.
The kernel's development process may come across as strange
and intimidating to new developers, but there are good reasons and
solid experience behind it. A developer who does not understand
the kernel community's ways (or, worse, who tries to flout or
circumvent them) will become very frustrated . The development
community, while being helpful to those who are trying to learn,
has little time for those who will not listen or who does not care
about the development process. While many Linux developers, still
use text-based tools such as Emacs or Vim to develop their code,
Eclipse, Anjuta, and Netbeans all provide more robust integrated
development environments.
Chapter Four: INSTALLING DEBIAN 8
What is Debian
Debian is an all-volunteer organization dedicated to developing
free software and promoting the ideals of the Free Software
community. The Debian Project began in 1993, when Ian Murdock
issued an open invitation to software developers to contribute to a
complete and coherent software distribution based on the relatively
new Linux kernel. That relatively small band of dedicated
enthusiasts, originally funded by the Free Software Foundation and
influenced by the GNU philosophy, has grown over the years into
an organization of around 1026 Debian Developers.
Debian GNU/Linux
The combination of Debian's philosophy, methodology, GNU
tools, the Linux kernel, and other important free software, form a
unique software distribution called Debian GNU/Linux. This
distribution is made up of a large number of software packages.
Each package in the distribution contains executables, scripts,
documentation, and configuration information, and has
a maintainer who is primarily responsible for keeping the package
up-to-date, tracking bug reports, and communicating with the
upstream author(s) of the packaged software. The extremely large
user base, combined with a bug tracking system ensures that
problems are found and fixed quickly. Debian's attention to detail
allows them to produce a high-quality, stable, and scalable
distribution. Installations can be easily configured to serve many
roles, from stripped-down firewalls to scientific desktop
workstations to high-end network servers. Debian is especially
popular amongst advanced users because of its technical excellence
and its deep commitment to the needs and expectations of the
Linux community. Debian also introduced many features to Linux
that are now commonplace. For example, Debian was the first
Linux distribution to include a package management system for the
easy installation and removal of software. It was also the first Linux
distribution that could be upgraded without requiring re-
installation. Debian continues to be a leader in Linux development.
Its development process is an example of just how well the Open
Source development model can work — even for very complex tasks
such as building and maintaining a complete operating system.
The feature that most distinguishes Debian from other Linux
distributions is its package management system. These tools gives
the administrator of a Debian system complete control over the
packages installed, including the ability to install a single package or
automatically update the entire operating system. Individual
packages can also be protected from being updated. You can even
tell the package management system about software you have
compiled yourself and what dependencies it fulfills. To protect your
system against “Trojan horses” and other malevolent software,
Debian's servers verify that uploaded packages come from
registered Debian maintainers. Debian packagers also take great
care to configure their packages in a secure manner. When security
problems in shipped packages do appear, fixes are usually available
very quickly.
Installing Debian 8 as a Virtual Machine.
First, you will need to download and install VirtualBox, the
Oracle software that allows you to run any OS on… any other OS, as
a virtual environment. This procedure is a lot safer than a dual
boot: no need to fool around with boot sector or disk partitions, all
is virtually created. The counterpart is that the two OS’s share your
computer resources. To avoid any performance issue, we will
choose a lightweight but efficient desktop environment. This book
will use the XFCE Debian edition, that you can download using this
link below:
https://www.debian.org/CD/torrent-cd/
XFCE is not the newest desktop environment, however it is
among the most powerful, lightweight, and customizable, while
being easy to use with its classic desktop metaphor. Imagine an
improved version of Windows XP, a great way to get started with
Linux.
Create your Virtual System
Once installed, start Virtual Box and follow these steps. We will
begin the VM configuration:
New > Name, Type (debian), Version 64
General > Advanced > Activate clipboard and Drag’n’drop
System > check memory, deactivate Floppy, check the nb of
CPU (2)
Display > max video memory, enable 3D acceleration
Storage > select your Debian ISO under “Controller IDE”
Shared Folders > choose which folder to share between the
systems. We will get back to this function later.
We are now ready to run the distribution installation.
Run Debian installer
Select your virtual machine and click the ‘Start’ icon. The
Debian graphic installer is straightforward: just follow the steps,
when asked enter a root password; create your user (name,
username, and password). Once you get to the partitioning, select
‘Guided, use entire disk, all in one partition.’ If you wish to modify the size
of the swap partition, you may select the manual partitioning, or
proceed later with Parted, the partitioning utility tool.
Proxy, part #1: select your network mirror. If you need to configure
a proxy, use the following syntax in the required field:
http://user:pass@host:port/.
Proceed to the installation. Once it has finished, you will have to
choose where to install Grub (the boot loader package): because this
is a VM install, choose /dev/sda (ata-VBOX_HARDDISK). This
would be the tricky part if you were installing on a dual-boot,
system so enjoy the comfort of a VM!
Now reboot the VM.
First Boot and Updates
At startup the Grub menu is displayed, stick with the default entry
and wait for your system to initialize. At XFCE first start, you will
be asked to choose between two panel setups, select the default
config.
Check your time configuration. If incorrect, launch as root
a dpkg-reconfigure tzdata and select your country.
Proxy, part #2 (if needed): during install, you entered your proxy
address for apt connection (repositories connection). It is has been
written inside the file /etc/apt/apt.conf, and can be modified any
time. I use nano as command line text editor, change at will.
Now add the proxy setting for the entire system:
Congratulations, you are now a confirmed Linux user! Now we
can update and install some useful tools, then we will modify the
default UI. To use the full capacity of your VM you need to install a
complementary VirtualBox extension called Guest Additions. To
do so, some dependencies are required.
Check your sources, remove the CD entries, add
the contrib and non-free repos:
Update your system:
Install the Guest Additions requirements
Insert the Guest Additions CD image (Host + D), go to the root of the
mounted drive and execute the program:
Customizing your System
… to get something that looks a little more modern.
Terminal
The terminal is one of the most useful tools on a Linux system.
Spend some time making it pleasant to look at: enable your prompt
color ink .bashrc, add your aliases, and enable colors and auto
completion in root bash.
uncomment the “force_colored_prompt=yes” line
[root] $ nano .bashrc
# set a fancy red prompt
PS1='${debian_chroot:+($debian_chroot)}\[33[01;31m\]\u@\h\[33[00m\]:\
[33[01;34m\]\w\[33[00m\] \$ '
# enable auto-completion
if
[ -f /etc/bash_completion ] && ! shopt -oq posix;
then . /etc/bash_completion
fi
Fonts
Enable font hinting in the Appearance Panel to get the best font
rendering for your system. Settings > Appearance > Fonts > Enable
anti-aliasing + Hinting Slight + Sub-pixel order RGB. This should
be enough in Jessie, no need to create a fonts.conf file or install the
Infinality engine.
I
Themes and icons
GTK Themes, windows decoration and iconsets are the most
important elements to customizing your desktop. Start with
installing some complementary gtk engines that are needed by some
themes:
[root] $ apt-get install gtk2-engines-murrine gtk2-engines-pixbuf dmz-
cursor-theme
Install sudo
Debian doesn’t come with sudo out of the box, this is a handy little
tool for temporarily giving you root access.
Replace USERNAME with your username. The NOPASSWD flag
removes the requirement to enter a password every time you
use sudo. This is not a good idea on a production server!
Reboot your machine to make the changes take effect: $ reboot
From now on all operations that require root should be run with
the sudo prefix, without having to enter the password each time.
Set up the network
Log in to your new VM using the username and password you
chose during installation. We’re going to add a network connection
to your VM that will allow you too easily SSH into the server.
Hit CTRL + X, then Y and ENTER to save changes.
Shutdown the VM.
At the main Virtual Box screen, hit the Settings button. Then select
Network from the list on the left, choose Adapter 2 from the tabs,
check ‘Enable Network Adapter’ and choose ‘Host-only Adapter’ from the
dropdown, then click OK.
What we have done is set up your VM to use a static IP address.
This is a good idea because it allows us to always access our VM
using a single IP address/hostname without having to look it up
each time we boot.
By default, Virtual Box utilizes the 192.168.56.1 address in your
network, and it assigns IP addresses in the 192.168.56.1xx range to
all your VMs.
By editing /etc/network/interfaces we told the OS that it should
expect a network resource to be available at that address.
Setup your hosts file
Now your server is configured lets add the hostname to your hosts
file!
Simply add the following entry into your hosts file. This should be
done on your host machine – be it Windows or Mac OS X or Linux,
not the VM itself.
Keep in mind that for every domain you setup on your VM, you will
need to add it to your hosts file.
Log in via SSH!
Now that you have setup the network adapter in Virtual Box, and
added the correct settings to the VM interfaces file, you’re ready to
actually SSH into your server and begin installing everything! You
may be wondering why you need to SSH and not simply use the VM
window. The reason I do it this way is that the server does not
support copy/paste! There’s a lot of typing ahead and having the
ability to simply copy/paste into your terminal is going to speed
things up quite a bit!
For SSH on Windows, I use KiTTy. It’s an SSH client that adds
some nifty features to PuTTY. There is also Poderosa. For
Mac/Linux just use the terminal! Since you’ve already added the
correct lines to your hosts file, you can set the address to connect to
as debian-vm (or whatever you chose during setup). Make sure to
actually start the VM from VirtualBox before attempting to login.
Just start it, there is no need to login from the VirtualBox server
window.
Installing the basics
First things first, we’re going to install the basic necessities,
like make, curl, wget, as well as the Apache Server, Mercurial, Git
and Subversion:
Edit the new Apache2 config file, $ sudo nano
/etc/apache2/httpd.conf and add
You now have the Apache server up and running! Just point your
browser to http://debian-vm and behold the magic.
Now lets enable Apache’s ModRewrite module:
Installing MySQL
We will be installing MySQL 5.1 from repo, as this is the easiest way
and works fairly well.
On the screens asking for a MySQL password, leave it blank and
hit Enter. Since this is only for a local server there is no point in
setting up a password. Do NOT use a blank password in production
environments.
Setting up MySQL
We need to update the IP address that MySQL will listen to for
connections by editing the my.cnf file.
$ sudo nano /etc/mysql/my.cnf
Do a search for bind-address (CTRL + W) and change the setting
to:
Now let’s grant root MySQL user all permissions:
Chapter Five: INSTALLING CENTOS 7
This is a community-supported distribution derived from
sources freely provided to the public by Red Hat for Red Hat
Enterprise Linux (RHEL). As such, CentOS Linux aims to be
functionally and compatible with RHEL. The CentOS Project
mainly changes packages to remove upstream vendor branding and
artwork. CentOS Linux is free and free to redistribute. Each CentOS
version is maintained for up to 10 years (by means of security
updates -- the duration of the support interval by Red Hat has
varied over time with respect to Sources released). A new CentOS
version is released approximately every 2 years and each CentOS
version is periodically updated (roughly every 6 months) to support
newer hardware. This results in a secure, low-maintenance, reliable,
predictable and reproducible Linux environment.
Installation of CenOS7
Finally, the much-awaited CentOS 7 is out. CentOS (Community
Enterprise Operating System) are forked from Red Hat Linux, a
Linux Distro fine tuned for servers. You will learn how to install
Centos 7 in a few easy steps.
Step 1: Download the ISO Image
To get a copy of CentOS 7 download from the link below:
http://mirror.centos.org/centos/7/
CentOS 7 is only available for 64-bit platforms; currently there is no
32-bit ISO image. This is primarily due to the fact that most
production servers are 64-bit.
Step 2: Make a bootable Drive
After you download the ISO image, make a bootable USB drive
using Unetbootin. Alternatively, you can burn a DVD using Brasero
or your favorite CD/DVD burning software.
Step 3: Begin Installation
To begin installation, click on the Install to Hard Drive icon on the
desktop.
Step 4: Select Language and Keyboard
Select your preferred language.
Step 5: Change the Installation Destination
By default, the Anaconda installer will choose automatic
partitioning for your hard disk. Click on the INSTALLATION
DESTINATION icon to change to custom partitioning.
Select the hard drive where you want to install CentOS 7 and under
Other Storage Options, choose ‘I will configure partitioning’ then click Done.
Step 6: Select the Partitioning Scheme
Next, select the partitioning scheme to use for the mountpoints. In
this case, choose Standard Partition.
Step 7: Create a Swap Space
You can create a swap space in one of the partitions and set the
desired capacity, which is dependent on how much RAM, the
system has. Choose the File System for swap space as swap, and select
t h e Reformat option. You can also name your swap space to
whatever name you like but a name like swap is more descriptive.
Step 8: Create a Mountpoint
The next step is to create the mountpoint where the root
partition will be installed. Depending on your requirements, you
might need to put the boot, home and root partitions on different
mountpoints. In this instance, we shall only create one mountpoint
/.
Next, set the Label and Desired Capacity to whatever you wish. A rule of
thumb is to use descriptive names for the label especially if the
computer is to be managed by different system administrators.
Choose the file system as ext4 and select the Reformat option.
Step 9: Accept Changes
After Steps 7 and 8 have successfully completed click on Done
button. A prompt window will appear with a summary of the
changes that will take place. If you are satisfied with them click
Accept Changes.
Step 10: Set Date and Time
Click on the Clock icon under the localization menu and select your
time zone from the map of the world, then click Done.
Step 11: Begin Installation
Click on the Begin Installation button.
Installation will begin immediately and as it proceeds you will need
to set up a user account as well as the root password.
Set
Step 12: Set Up Root Password
Click on the ROOT PASSWORD option and enter a password and
confirmation of the same then click Done.
Step 13: Create a User Account
The next step is to create a user account. Enter the details and if
this is an administrator account, check Make this user administrator
and Require a password to use this account for security purposes.
Step 14: Complete Installation
The installer should finish installing the software and boot loader.
Hopefully, once the install is complete you will get a success
message, after which you can click Quit. Now logout from the live
system and login to your new installation. Finally, once you login to
your CentOS 7 accept the EULA agreement and enjoy!
Change and Set Hostname Command
On a CentOS Linux 7 server, you can use any one of the following
tools to manage hostnames:
hostnamectl: Control the system hostname. This is the
recommended method.
nmtui: Control the system hostname using text user interface
(TUI).
nmcli: Control the system hostname using CLI part of
NetworkManager.
Method #1: hostnamectl
Let us see how to use the hostnamectl command.
How do I see the host names?
Sample outputs:
Static hostname: server1.cyberciti.biz
Pretty hostname: Senator Padmé Amidala's Laptop
Transient hostname: r2-d2
Icon name: computer
Chassis: n/a
Machine ID: b5470b10ccfd49ed8e4a3b0e953a53c3
Boot ID: f79de79e2dac4670bddfe528e826b61f
Virtualization: oracle
Operating System: CentOS Linux 7 (Core)
CPE OS Name: cpe:/o:centos:centos:7
Kernel: Linux 3.10.0-229.1.2.el7.x86_64
Architecture: x86_64
How do I delete a particular host name?
The syntax is:
# hostnamectl set-hostname ""
# hostnamectl set-hostname "" --static
# hostnamectl set-hostname "" –pretty
How do I change host name remotely?
Use any of the following syntax:
# ssh root@server-ip-here hostnamectl set-hostname server1
OR set server1 as host name on a remote server called 192.168.1.42
using ssh:
# hostnamectl set-hostname server1 -H root@192.168.1.42
Method #2: nmtui
You can set host name using the nmtui command, which has text
user interface for new users:
# nmtui
Sample outputs:
Use the Down arrow key > select the ‘Set system hostname’ menu
option > Click the OK button:
You will see the confirmation box as follows:
Finally, restart the hostnamed service by typing the following
command:
# systemctl restart systemd-hostnamed
To verify the changes, enter:
# hostnamectl status
Method #3: nmcli
The nmcli is a command-line tool for controlling Network Manager
and reporting network status.
To view the host name using nmcli:
The syntax is:
# nmcli general hostname
To set the host name using nmcli:
The syntax is:
# nmcli general hostname R2-D2
# nmcli general hostname server42.cyberciti.biz
Finally, restart the systemd-hostnamed service:
# systemctl restart systemd-hostnamed
Chapter Six: LINUX AND UNIXMAN COMMAND
O n Linux and other Unix-like operating systems, m a n is the
interface used to view the system's reference manuals.
Syntax
man [-C file] [-d] [-D] [--warnings[=warnings]] [-R
encoding] [-L locale]
[-m system[,...]] [-M path] [-S list] [-e
extension] [-i|-I]
[--regex|--wildcard] [--names-only] [-a] [-u] [--
no-subpages] [-P pager]
[-r prompt] [-7] [-E encoding] [--no-hyphenation]
[--no-justification]
[-p string] [-t] [-T[device]] [-H[browser]] [-
X[dpi]] [-Z]
[[section] page ...] ...
man -k [apropos options] regexp ...
man -K [-w|-W] [-S list] [-i|-I] [--regex] [section]
term ...
man -f [whatis options] page ...
man -l [-C file] [-d] [-D] [--warnings[=warnings]] [-R
encoding]
[-L locale] [-P pager] [-r prompt] [-7] [-E
encoding] [-p string] [-t]
[-T[device]] [-H[browser]] [-X[dpi]] [-Z] file ...
man -w|-W [-C file] [-d] [-D] page ...
man -c [-C file] [-d] [-D] page ...
man [-hV]
Description
Man is the system's manual viewer; it can be used to display
manual pages, scroll up and down, search for occurrences of
specific text, and other useful functions. Each argument given to
man is normally the name of a program, utility or function. The
pages associated with each of these arguments are then found and
displayed. A section number, if provided, will direct man to look
only in that section of the manual. The default action is to search in
all of the available sections, following a pre-defined order and to
show only the first page found, even if pages exist in several
sections.
General Options
-h, --help Print a help message and
exit.
-V, --version Display version information
and exit.
-C file, --config-
file=file
Use configuration
file file rather than the default
of ~/.manpath.
-d, --debug Print debugging information.
-D, --default This option, when used, is
normally specified as the first
option; it resetsman's
behaviour to its default. Its use
is to reset those options that
may have been set
in $MANOPT. Any options
that follow -D will have their
usual effect.
--
warnings[=warnings]
Enable warnings from
the groff text formatter. This
may be used to perform sanity
checks on the source text of
manual pages. warnings is a
comma-separated list of
warning names; if it is not
supplied, the default is "mac".
See the "Warnings" node in
the groff info page for a list of
available warning names.
Main Modes of Operation
-f, --whatis Equivalent to the whatis command;
displays a short description from the
manual page, if available.
-k, --apropos Equivalent to the apropos command;
Search the short manual page descriptions
for keywords and display any matches.
-K, --global-
apropos
Search for text in all manual pages. This is
a brute-force search, and is likely to take
some time; if you can, you should specify a
section to reduce the number of pages that
need to be searched. Search terms may be
simple strings (the default), or regular
expressions if the --regex option is used.
-l, --local-file Activate 'local' mode. Format and display
local manual files instead of searching
through the system's manual collection.
Each manual page argument will be
interpreted as an nroff source file in the
correct format. No cat file is produced. If a
dash ('-') is listed as one of the arguments,
input will be taken from stdin. When this
option is not used, and man fails to find the
page required, before displaying the error
message it attempts to act as if this option
was supplied, using the name as a filename
and looking for an exact match.
-w, --where, --
location
Don't actually display the manual pages;
instead print the location(s) of the
source nroff files that would be formatted.
-W, --where-
cat, --location-
cat
Don't actually display the manual pages, but
do print the location(s) of the catfiles that
would be displayed. If -w and -W are both
specified, print both, separated by a space.
-c, --catman This option is not for general use and
should only be used by
the catmanprogram.
-R encoding, --
recode=encoding
Instead of formatting the manual page in
the usual way, output its source converted
to the specified encoding. If you already
know the encoding of the source file, you
can also use manconv directly. However,
this option allows you to convert several
manual pages to a single encoding without
having to explicitly state the encoding of
each, provided that they were already
installed in a structure similar to a manual
page hierarchy.
Finding Manual Pages
-L locale, --
locale=locale
man will normally determine your
current locale by a call to the C
functionsetlocale which checks the
values of various environment
variables, possibly
including $LC_MESSAGES and $LANG.
To temporarily override the determined
value, use this option to supply a locale
string directly to man. Note that it will
not take effect until the search for pages
actually begins. Output such as the help
message will always be displayed in the
initially determined locale.
-m system[,...], --
systems=system[,...]
If this system has access to other operating
system's manual pages, they can be accessed
using this option. To search for a manual page
from (for example) the "NewOS" manual page
collection, use the option -m NewOS.
The system specified can be a combination of
comma delimited operating system names. To
include a search of the native operating
system's manual pages, include the system
name man in the argument string. This option
will override the $SYSTEM environment
variable.
-M path, --
manpath=path
Specify an alternate manpath to use. This
option overrides the $MANPATH
environment variable and causes option -m to
be ignored.
A path specified as a manpath must be the root
of a manual page hierarchy structured into
sections as described in the man-db
manual (under "The manual page system"). To
view manual pages outside such hierarchies,
see the -l option.
-S list, -s list, --
sections=list
list is a colon- or comma-separated list of
`order specific' manual sections to search. This
option overrides
the $MANSECT environment variable.
(The –s spelling is for compatibility
with System V.)
-e sub-extension, --
extension=sub-
extension
Some systems incorporate large packages of
manual pages, such as those that accompany
the Tcl package, into the main manual page
hierarchy. To get around the problem of
having two manual pages with the same name
such as exit, the Tcl pages were usually all
assigned to section l (lowercase L). However, it
is now possible to put the pages in the correct
section, and to assign a specific "extension" to
them, in this case, exit (3tcl). Under normal
operation, man will display exit in preference
to exit (3tcl). To negotiate this situation and
to avoid having to know which section the page
you require resides in, it is now possible to
give man a sub-extension string indicating
which package the page must belong to. Using
the above example, supplying the option -e
tcl to man will restrict the search to pages
having an extension of *tcl.
-i, --ignore-case Ignore case when searching for manual pages.
This is the default.
-I, --match-case Search for manual pages case-sensitively.
--regex Show all pages with any part of either their
names or their descriptions matching each
page argument as a regular expression, as
with apropos. Since there is usually no
reasonable way to pick a "best" page when
searching for a regular expression, this option
implies -a.
--wildcard Show all pages with any part of either their
names or their descriptions matching each
page argument using shell-style wildcards, as
with apropos --wildcard. The page
argument must match the entire name or
description, or match on word boundaries in
the description. Since there is usually no
reasonable way to pick a "best" page when
searching for a wildcard, this option implies -a.
--names-only If the --regex or --wildcard option is used,
match only page names, not page descriptions,
as with whatis. Otherwise, this option has no
effect.
-a, --all By default, man will exit after displaying the
most suitable manual page it finds. Using this
option forces man to display all the manual
pages with names that match the search
criteria.
-u, --update This option causes man to perform an inode-
level consistency check on its database caches
to ensure that they are an accurate
representation of the filesystem. It will only
have a useful effect if man is installed with
the setuidbit set.
--no-subpages By default, man will try to interpret pairs of
manual page names given on the command
line as equivalent to a single manual page
name containing a hyphen or an underscore.
This supports the common pattern of
programs that implement a number of
subcommands, allowing them to provide
manual pages for each that can be accessed
using similar syntax as would be used to
invoke the subcommands themselves. For
example, the command:
man -aw git diff
displays the manual page:
/usr/share/man/man1/git-diff.1.gz
To disable this behavior, use the --no-
subpages option.
For example:
man -aw --no-subpages git diff
Will instead show the manual pages for
both git and diff:
/usr/share/man/man1/git.1.gz
/usr/share/man/man3/Git.3pm.gz
/usr/share/man/man1/diff.1.gz
Controlling Formatted Output
-P pager, --
pager=pager
Specify which output pager to
use. By default, man uses pager -
s. This option overrides
the $MANPAGER environment
variable, which in turn overrides
the $PAGER environment
variable. It is not used in
conjunction with -f or -k.
The value may be a simple
command name or a command
with arguments, and may use
shell quoting (backslashes, single
quotes, or double quotes). It may
not use pipes to connect multiple
commands; if you need that, use a
wrapper script, which may take
the file to display either as an
argument or on standard input.
-r prompt, --
prompt=prompt
If a recent version of less is used as
the pager, man will attempt to set its
prompt and some sensible options.
The default prompt looks like:
Manual page name(sec)
line x
where name denotes the manual
page name, sec denotes the section
it was found under and x the current
line number. This is achieved by
using the $LESS environment
variable.
Supplying -r with a string will
override this default. The string may
contain the text $MAN_PN which
will be expanded to the name of the
current manual page and its section
name surrounded by "(" and ")".
The string used to produce the
default could be expressed as;
\ Manual\ page\
\$MAN_PN\ ?ltline\ %lt?
L/%L.:
byte\ %bB?s/%s..?\
(END):?pB\ %pB\\%..
(press h for help or q
to quit)
It is broken into three lines here for
the sake of readability only. For its
meaning see the man page for less.
The shell first evaluates the prompt
string. All double quotes, back-
quotes and backslashes in the
prompt must be escaped by a
preceding backslash. The prompt
string may end in an
escaped $ which may be followed
by further options for less. By
default, man sets the -ix8 options.
If you want to override man's
prompt string processing completely,
use the $MANLESS environment
variable described below.
-7, --ascii When viewing a pure ASCII manual
page on a 7-bit terminal or terminal
emulator, some characters may not
display correctly when using
the latin1device description
with GNU nroff. This option allows
pure ASCII man pages to be
displayed in ASCII with
the latin1 device. It will not translate
any latin1 text. The following table
shows the translations performed:
some parts of it may only be
displayed properly when using
GNU nroff's latin1device. This
option is ignored when using
options -t, -H, -T, or -Z and may be
useless for versions of nroff other
than GNU's.
-E encoding, --
encoding=encoding
Generate output for a character
encoding other than the default. For
backward compatibility, encoding
may be an nroff device such
as ascii, latin1, or utf8 as well as a
true character encoding such
as UTF-8.
--no-hyphenation, -
-nh
Normally, nroff will automatically
hyphenate text at line breaks even in
words that do not contain hyphens,
if it is necessary to do lay out thre
words on a line without excessive
spacing. This option disables
automatic hyphenation, so words
will only be hyphenated if they
already contain hyphens. If you are
writing a man page and simply want
to prevent nroff from hyphenating a
word at an inappropriate point, do
not use this option, but consult
the nroff documentation instead; for
instance, you can put "\%" inside a
word to indicate that it may be
hyphenated at that point, or put
"\%" at the start of a word to
prevent it from being hyphenated.
--no-justification, -
-nj
Normally, nroff will automatically
justify text to both margins. This
option disables full justification,
leaving justified only to the left
margin, sometimes called "ragged-
right" text.
If you are writing a man page and
simply want to prevent nroff from
justifying certain paragraphs, do not
use this option, but consult the nroff
documentation; for instance, you
can use the ".na", ".nf", ".fi", and
".ad" requests to temporarily disable
adjusting and filling.
-p string, --
preprocessor=string
Specify the sequence of
preprocessors to run
before nroff or troff/groff. Not all
installations will have a full set of
preprocessors. Some of the
preprocessors and the letters used to
designate them are: eqn (e), grap (g),
pic (p), tbl (t), vgrind (v), refer (r).
This option overrides the
$MANROFFSEQ environment
variable. zsoelim is always run as
the first preprocessor.
-t, --troff Use groff -mandoc to format the
man page to standard output. This
option is not required in conjunction
with -H, -T, or -Z.
-T[device], --troff-
device[=device]
This option is used to
change groff (or possibly troff's)
output to be suitable for a device
other than the default. It implies -t.
Examples include dvi,
latin1, ps, utf8, X75 and X100.
-H[browser], --
html[=browser]
This option will cause groff to
produce HTML output, and will
display the output in a web browser.
The choice of browser is determined
by the optional browser argument if
one is provided, by
the $BROWSER environment
variable, or by a compile-time
default if that is unset (usually lynx).
This option implies -t, and will only
work with GNU troff.
-X[dpi], --
gxditview[=dpi]
This option displays the output
of groff in a graphical window using
thegxditview program. The dpi (dots
per inch) may be 75, 75-12, 100, or
100-12, defaulting to 75; the -
12 variants use a 12-point base font.
This option implies -T with the X75,
X75-12, X100, or X100-12 devices,
respectively.
-Z, --ditroff groff will run troff and then use an
appropriate post-processor to
produce output suitable for the
chosen device. If groff -
mandoc is groff, this option is
passed to groff and will suppress the
use of a post-processor. It implies -t.
Section Numbers
The section numbers of the manual are listed below. While
reading documentation, if you see a command name followed by a
number in parentheses, the number refers to one of these sections.
For example, man is the documentation of man found in section
number1. Some commands may have documentation in more than
one section, so the numbers after the command name may direct
you to the correct section to find a specific type of information.
The section numbers, and the topics they cover, are as follows:
section # Topic
1Executable programs or shell commands
2System calls (functions provided by
the kernel)
3Library calls (functions within program
libraries)
4Special files (usually found in /dev)
5File formats and conventions
eg /etc/passwd
6Games
7Miscellaneous (including macro packages
and conventions), e.g. man, groff
8System administration commands
(usually only for root)
9Kernel routines [Non standard]
Exit Status
When it terminates, man will return one of the following exit
status:
0 Returned upon successful program
execution.
1Returned if there was a usage, syntax, or
configuration file error.
2Returned if there was an operational error.
3Returned if a child process returned a non-zero
exit status.
16 Returned if one or more of the pages, files, or
keywords searched for did not exist or was not
matched.
Environment
man makes use of the following environment variables:
MANPATH If $MANPATH is set, its value is
used as the path to search for
manual pages.
MANROFFOPT The contents of $MANROFFOPT are
added to the command line every
time man invokes the formatter
(nroff, troff, or groff).
MANROFFSEQ If $MANROFFSEQ is set, its value is
used to determine the set of
preprocessors to pass each manual
page through. The default preprocessor
list is system-dependent.
MANSECT If $MANSECT is set, its value is a
colon-delimited list of sections and it is
used to determine which man sections
to search and in what order.
MANPAGER, PAGER If $MANPAGER or $PAGER is set
($MANPAGER is used in
preference), its value is used as the
name of the program used to display
the man page. By default, pager -s is
used. The value may be a simple
command name or a command with
arguments, and may use shell quoting
(backslashes, single quotes, or double
quotes). It may not use pipes to
connect multiple commands; if you
need that, use a wrapper script, which
may take the file to display either as an
argument or on standard input.
MANLESS If $MANLESS is set, man will not
perform any of its usual processing to
set up a prompt string for
the less pager. Instead, the value of
$MANLESS will be copied verbatim
into $LESS. For example, if you want
to set the prompt string
unconditionally to "my prompt string",
set $MANLESS to ‘-Psmy prompt
string’.
BROWSER If $BROWSER is set, its value is a
colon-delimited list of commands, each
of which in turn is used to try to start a
web browser for man --html. In each
command, %s is replaced by a filename
containing the HTML output
from groff, %% is replaced by a single
percent sign (%), and %c is replaced by
a colon (:).
SYSTEM If $SYSTEM is set, it will have the
same effect as if it had been specified as
the argument to the -m option.
MANOPT If $MANOPT is set, it will be parsed
prior to mans command line and is
expected to be in a similar format. As
all of the other man specific
environment variables can be
expressed as command line options,
and are thus candidates for being
included in $MANOPT it is expected
that they will become obsolete. Note:
all spaces that should be interpreted as
part of an option's argument must be
escaped (preceded with a backslash).
MANWIDTH If $MANWIDTH is set, its value is
used as the line length for which
manual pages should be formatted. If it
is not set, manual pages will be
formatted with a line length
appropriate to the current terminal
(using an ioctl if available, the value
of $COLUMNS, or falling back to 80
characters if neither is
available). cat pages will only be saved
when the default formatting can be
used, that is when the terminal line
length is between 66 and 80
characters.
MAN_KEEP_FORMATTING Normally, when output is not being
directed to a terminal (such as to a file
or a pipe), formatting characters are
discarded to make it easier to read the
result without special tools. However,
if $MAN_KEEP_FORMATTING is
set to any non-empty value, these
formatting characters are retained.
This may be useful for wrappers
around man that can interpret
formatting characters.
MAN_KEEP_STDERR Normally, when output is being
directed to a terminal (usually a pager),
any error output from the command
used to produce formatted versions of
man pages is discarded to avoid
interfering with the pager's display.
Programs such as groff often produce
relatively minor error messages about
typographical problems such as poor
alignment, which are unsightly and
generally confusing when displayed
along with the man page. However,
you might want to see them anyway, so
if $MAN_KEEP_STDERR is set to a
non-empty value, error output will be
displayed as usual.
LANG, LC_MESSAGES Depending on the system and
implementation, either or both
of $LANGand $LC_MESSAGES will
be interrogated for the current message
locale, man will display its messages in
that locale (if available).
Files
These files are used by man:
/etc/manpath.config The man-
db configuration
file.
/usr/share/man A global manual
page hierarchy.
/usr/share/man/index.
(bt|db|dir|pag)
A traditional global
index database
cache.
/var/cache/man/index.
(bt|db|dir|pag)
An FHS compliant
global index
database cache.
Examples
man man
View the man page for the man command.
man --nh --nj man
Chapter Seven: LINUX DIRECTORY COMMAND
Linux or UNIX-like systems use the ls command to list files and
directories. However, ls does not have an option to list only
directories. You can use combination of ls and grep to list directory
names only. You can use the find command too. In this quick
tutorial, you will learn how to list only directories in Linux or
UNIX.
List all directories in Unix
Type the following command:
$ ls -l | grep `^d'
ls -l | egrep `^d'
Try the following ls command to list directories in the
current directory:
$ ls -d */
sample outputs
List only files in Unix
Type the following command:
$ ls -l | egrep -v '^d'
$ ls -l | egrep -v '^d'
The grep command is used to searches input. It will filter
out directory names by matching first character 'd'. To
reverse the effect i.e. just to display files you need to
pass the -v option. It inverts the sense of matching, to
select non-matching lines.
Task: Create aliases to save time
You can create two aliases to list only directories and files.
alias lf="ls -l | egrep -v '^d'"
alias ldir="ls -l | egrep '^d'"
Put above two aliases in your bash shell startup file:
$ cd
$ vi .bash_profile
Append two lines:
alias lf="ls -l | egrep -v '^d'"
alias ldir="ls -l | egrep '^d'"
Save and close the file.
Now just type lf - to list files and ldir - to list directories only:
$ cd /etc
$ lf
Sample output:
-rw-r--r-- 1 root root 2149 2006-09-04 23:25
adduser.conf
-rw-r--r-- 1 root root 44 2006-09-29 05:11 adjtime
-rw-r--r-- 1 root root 197 2006-09-04 23:48 aliases
-rw------- 1 root root 144 2002-01-18 13:43 at.deny
-rw-r--r-- 1 root root 162 2006-09-22 23:24 aumixrc
-rw-r--r-- 1 root root 28 2006-09-22 23:24 aumixrc1
List directory names only:
$ cd /etc
$ ldir
Sample output:
drwxr-xr-x 4 root root 4096 2006-09-22 16:41 alsa
drwxr-xr-x 2 root root 4096 2006-09-20 20:59
alternatives
drwxr-xr-x 6 root root 4096 2006-09-22 16:41 apm
drwxr-xr-x 3 root root 4096 2006-09-07 02:51 apt
drwxr-xr-x 2 root root 4096 2006-09-08 01:46
bash_completion.dUse find command to list either files or
directories
The find command can be used as follows: to list all directories in
/nas, enter:
find /nas -type d
find /nas -type d -ls
find . -type d -ls
Sample output:
1070785 8 drwxrwxrwt 8 root root 4096 Jul
5 07:12 .
1070797 8 drwx------ 2 root root 4096 Jul
4 07:22 ./orbit-root
1070843 8 drwxr-xr-x 2 root root 4096 Jun
16 18:55 ./w
1070789 8 drwxr-xr-x 10 root root 4096 Jun
17 14:54 ./b
1071340 8 drwxr-xr-x 2 root root 4096 Jun
16 18:55 ./b/init.d
1071581 8 drwxr-xr-x 3 root root 4096 Jun
16 18:55 ./b/bind
1071584 8 drwxr-xr-x 2 root root 4096 Jun
16 18:55 ./b/bind/bak
1071617 8 drwxr-xr-x 2 root root 4096 Jun
16 18:55 ./b/fw
1071628 8 drwxr-xr-x 8 root root 4096 Jun
16 18:55 ./b/scripts
Chapter Eight: WORKING WITH FILES
This chapter will first describe general characteristics of Unix
commands. It will then discuss commands, which are commonly
used to create and manipulate files. A summary of some of the most
commonly used UNIX commands is presented in Command
Comparisons.
UNIX File Names
It is important to understand the rules for creating UNIX files:
UNIX is case sensitive! For example, “fileName” is different from
“filename.” It is recommended that you limit names to the
alphabetic characters, numbers, underscore (_), and dot (.). Dots (.)
used in UNIX filenames are simply characters and not delimiters
between filename components; you may include more than one dot
in a filename. Including a dot as the first character of a filename
makes the file invisible (hidden) to the normal ls command; use the
-a flag of the ls command to display hidden files. Although many
systems will allow more, the recommended length is 14 characters
per file name. Unix shells typically include several important
wildcard characters. The asterisk (*) is used to match 0 or more
character (e.g., abc* will match any file beginning with the letters
abc), the question mark (?) is used to match any single character,
and the left ([) and right (]) square brackets are used to enclose a
string of characters, any one of which is to match. Execute the
following commands and observe the results:
ls m*
ls *.f
ls *.?
ls [a-d]*
Notes for PC users: Unix uses forward slashes ( / ) instead of backslashes
( \ ) for directories
Looking at the Contents of Files
You can examine the contents of files using a variety of
commands. cat, more, pg, head, and tail are described here. Of
course, you can always use an editor; to use vi in “read-only” mode
to examine the contents of the file “argtest”, enter:
vi -R argtest
You can now use the standard vi commands to move through the
file; however, you will not be able to make any changes to the
contents of the file. This option is useful when you simply want to
look at a file and want to guarantee that you make no changes while
doing so.
Use the vi “”” command to exit from the file.
Cat Command
cat is a utility used to conCATenate files. Thus, it can be used to
join files together, but it is perhaps more commonly used to display
the contents of a file on the screen.
Observe the output produced by each of the following commands:
cd; cd xmp
cat cars
cat -vet cars
cat -n cars
The semicolon (;) in the first line of this example is a command
separator which enables entry of more than one command on a line.
When the <Return> key is pressed following this line, the
command cd is issued which changes to your home directory. Then
the command “cd xmp” is issued to change into the subdirectory
“xmp.” Entering this line is equivalent to having entered these
commands sequentially on separate lines. These two commands are
included in the example to guarantee that you are in the
subdirectory containing “cars” and the other example files. You
need not enter these commands if you are already in the “xmp”
directory created when you copied the example file.
The “-vet” option enables display of tab, end-of-line, and other
non-printable characters within a file; the “-n” option numbers
each line as it is displayed.
You can also use the cat command to join files together:
cat page1
cat page2
cat page1 page2 > document
cat document
Note: If the file “document” had previously existed, it will be
replaced by the contents of files “page1” and “page2.”
Cautions to using the cat command: The cat command should
only be used with “text” files; it should not be used to display the
contents of binary (e.g., compiled C or FORTRAN programs).
Unpredictable results may occur, including the termination of your
logon session. Use the command “file *” to display the
characteristics of files within a directory prior to using
the cat command with any unknown file. You can use the od(enter
“man od” for details on use of Octal Dump) command to display the
contents of non-text files. For example, to display the contents of “a.out”
in both hexadecimal and character representation, enter:
od -xc a.out
Warning: cat (and other Unix commands) can destroy files if not
used correctly. For example, as illustrated in the Sobell book, the
cat (also cp and mv) command can overwrite and thus destroy
files. Observe the results of the following command:
cat letter page1 > letter
Typically, UNIX does not return a message when a command
executes successfully. Here the UNIX operating system will attempt
to complete the requested command by first initializing the file
“letter” and then writing the current contents of “letter” (now
nothing) and “page1” into this file. Since “letter” has been
reinitialized and is also named as a source file, an error diagnostic is
generated. Part of the UNIX philosophy is “No news is good news.”
Thus, the appearance of a message is a warning that the command
was not completed successfully.
Now use the “cat” command to individually examine the contents
of the files “letter” and “page1.” Observe that the file “letter” does
not contain the original contents of the files “letter” and “page1” as
was intended.
Use the following command to restore the original file “letter”:
cp ~aixstu00/xmp/letter.
More Command
You may type or browse files using the more command. The
“more” command is useful when examining a large file as it displays
the file contents one page at a time, allowing each page to be
examined at will. As with the man command, you must press the
space bar to proceed to the next screen of the file. On many
systems, pressing the <b> key will enable you to page backwards in
the file. To terminate more at any time, press <q>.
To examine a file with the more command, simply enter:
more file_name
The man command uses the more command to display the manual
pages; thus, the commands you are familiar with using man will
also work with more.
Not all Unix systems include the more command; some implement
the pg command instead. VTAIX includes both
the more and pg commands. When using the p g command,
press <Return> to page down through a file instead of using the
space bar.
Observe the results of entering the following commands:
more argtest
pg argtest
Head Command
The head command is used to display the first few lines of a file.
This command can be useful when you wish to look for specific
information, in the beginning of the file. For example, enter:
head argtest
Tail Command
The tail command is used to display the last lines of a file. This
command can be useful to monitor the status of a program, which
appends output to the end of a file. For example, enter:
tail argtest
Copying, Erasing, Renaming
Warning: The typical Unix operating system provides no ‘unerase’
or ‘undelete’ command. If you mistakenly delete a file you are
dependent upon the backups you or the system administrator have
maintained in order to recover the file. You need to be careful when
using commands like copy and move, which may result in
overwriting existing files. If you are using the C or Korn Shell, you
can create a command alias , which will prompt you for verification
before overwriting files with these commands.
Copying Files
The cp command is used to copy a file or group of files. You have
already seen an example application of the cp command when you
copied the sample files to your userid. Now let’s make a copy of one
of these files. Recall that you can obtain a listing of the files in the
current directory using the ls command. Observe the results from
the following commands:
ls l*
cp letter letter.2
ls l*
Note: Unlike many other operating systems, such as PC/DOS, you
must specify the target with the copy command; it does not assume
the current directory if no “copy-to” target is specified.
Erasing Files
Unix uses the command rm (ReMove) to delete unwanted files. To
remove the file “letter.2” which we have just created, enter:
rm letter.2
Enter the command “ls l*” to display a list of all files beginning
with the letter “l.” Note that letter.2 is no longer present in the
current directory. The remove command can be used with wildcards
in filenames; however, this can be dangerous as you might end up
erasing files you had wanted to keep. It is recommended that you
use the “-i” (interactive) option of rm for wildcard deletes — you
will then be prompted to respond with a “y” or “Y” for each file you
wish to delete.
Renaming a File
The typical Unix operating system utilities do not include a
rename command; however, we can use the mv (MoVe) command
(see for additional uses of this command) to “move” Working with
Directories) a file from one name to another. Observe the results of
the following commands:
ls [d,l]*
mv letter document
ls [d,l]*
mv document letter
ls [d,l]*
Note: The first mv command overwrites the file “document” which
you had created in an earlier exercise by concatenating “page1” and
“page2.” No warning is issued when the m v command is used to
move a file into the name of an existing file. If you would like to be
prompted for confirmation if the mv command were to overwrite an
existing file, use the “-i” (interactive) option of the mv command,
e.g.:
mv -i page1 letter
You will now be told that the file “letter” already exists and you will
be asked if you wish to proceed with the mv command. Answer
anything but “y” or “Y” and the file “letter” will not be overwritten.
Using the Command Line
The command interpreter (shell) provides the mechanism by which
input commands are interpreted and passed to the Unix kernel or
other programs for processing. Observe the results of entering the
following commands:
./filesize
./hobbit
./add2
ls -F
Observe that “filesize” is an executable shell script, which displays
the size of files. Also note that “./hobbit” and “./add2” generate
error diagnostics as there is no command or file with the name
“hobbit” and the file “add2” lacks execute permission.
Standard Input and Standard Output
As you have can see, Unix expects standard input to come from the
keyboard, e.g., enter:
cat
my_text
<Ctrl-D>
Standard output is typically displayed on the terminal screen, e.g.,
enter:
cat cars
Standard error (a listing of program execution error diagnostics) is
typically displayed on the terminal screen, e.g., enter:
ls xyzpqrz
Redirection
As illustrated above, many Unix commands read from standard
input (typically the keyboard) and write to standard output
(typically the terminal screen). The redirection operators enable you
to read input from a file (<) or write program output to a file (>).
When output is redirected to a file, the program output replaces the
original contents of the file if it already exists; to add program
output to the end of an existing file, use the append redirection
operator (>>).
Observe the results of the following command:
./a.out
You will be prompted to enter a Fahrenheit temperature. After
entering a numeric value, a message will be displayed on the screen
informing you of the equivalent Centigrade temperature. In this
example, you entered a numeric value as standard input via the
keyboard and the output of the program was displayed on the
terminal screen.
In the next example, you will read data from a file and have the
result displayed on the screen (standard output):
cat data.in
./a.out < data.in
Now you will read from standard input (keyboard) and write to a
file:
./a.out > data.two
35
cat data.two
Now read from standard input and append the result to the existing
file:
./a.out < data.in >> data.two
As another example of redirection, observe the result of the
following two commands:
ls -la /etc > temp
more temp
Here we have redirected the output of the ls command to the file
“temp” and then used the more command to display the contents of
this file a page at a time. In the next section, we will see how the use
of pipes could simplify this operation.
Using Pipes and Filters
A filter is a Unix program, which accepts input from standard input
and places its output in standard output. Filters add power to the
UNIX system as programs can be written to use the output of
another program as input and create output, which can be used by
yet another program. A pipe (indicated by the symbol “|” — vertical
bar) is used between UNIX commands to indicate that the output
from the first is to be used as input by the second. Compare the
output from the following two commands:
ls -la /etc
ls -la /etc | more
The first command displays of all the files in the in the “/etc”
directory in long format. It is difficult to make use of this
information since it scrolls rapidly across the screen. In the second
line, the results of the ls command are piped into the more
command. We can now examine this information one screen at a
time and can even back up to a prior screen of information if we
wish to. As you became more familiar with UNIX, you will find that
piping output to the more command can be very useful in a variety
of applications.
The sort command can be used to sort the lines in a file in a
desired order. Now enter the following commands and observe the
results:
who
sort cars
who | sort
The who command displays a list of users currently logged onto the
system the sort command enables us to sort the information. The
second command sorts the lines in the file cars alphabetically by
first field and displays the result in standard output. The third
command illustrates how the result of the who command can be
passed to the sort command prior to being displayed. The result is a
listing of logged on users in alphabetical order.
The following example uses the “awk” and “sort” commands to
select and reorganize the output generated by the “ls” command:
ls -l | awk '/:/ {print $5,$9}' | sort -nr
Note: Curly braces do not necessarily display correctly on all
output devices. In the above example, there should be a left curly
brace in front of the word print and a right curly brace following the
number 9.
Observe that the output displays the filesize and filename in
decreasing order of size. Here the ls command first generates a
“long” listing of the files in the current directory, which is piped to
the “awk” utility, whose output is then piped to the “sort”
command.
“awk” is a powerful utility which processes one or more program
lines to find patterns within a file and perform selective actions
based on what is found. Slash (/) characters are used as delimiters
around the pattern to be matched and the action to be taken is
enclosed in curly braces. If no pattern is specified, all lines in the
file are processed and if no action is specified, all lines matching the
specified pattern are output. Since a colon (:) is used here, all lines
containing file information (the time column corresponding to each
file contains a colon) are selected and the information contained in
the 5th and 9th columns are output to the sort command.
Note: If the ls command on your system does not include a
column listing group membership, use {print $4,$8} instead of
the “print” command option of awk listed above.
Here the “sort” command options “-nr” specify that the output
from “awk” be sorted in reverse numeric order, i.e., from largest to
smallest.
The preceding command is somewhat complex and it is easy to
make a mistake in entering it. If this were a command you would
use frequently, we could include it in a shell script as in sample file
“filesize”. To use this shell script, simply enter the command:
./filesize
or
sh filesize
If you examine the contents of this file with
the cat or v i commands, you will see that it contains nothing
more the piping of the ls command to awk and then piping the
output to sort.
The tee utility is used to send output to both a file and the screen:
who | tee who.out | sort
cat who.out
Here you should have observed that a list of logged on users was
displayed on the screen in alphabetical order and that the file
“who.out” contained an unsorted listing of the same userids.
Some Additional File Handling Commands
Word Count
The command w c displays the number of lines, words, and
characters in a file.
To display the number of lines, words, and characters in the file
file_name, enter:
wc file_name
Comparing the Contents of Two Files: the cmp and diff
Commands
T h e c m p a n d diff commands are used to compare files; the
“comp” command is not used to compare files, but to “compose a
message.”
The cmp command can be used for both binary and text files. It
indicates the location (byte and line) where the first difference
between the two files appears.
Th e diff command can be used to compare text files and its
output shows the lines which are different in the two files: a less
than sign (“<“) appears in front of lines from the first file which
differ from those in the second file, a greater than symbol (“>”)
precedes lines from the second file. Matching lines are not
displayed.
Observe the results of the following commands:
cmp page1 page2
diff page1 page2
Lines 1 and 2 of these two files are identical, lines 3 differ by one
character, and page one contains a blank line following line three,
while page2 does not.
Chapter Nine: NAVIGATION AND FILE MANAGEMENT
If you do not have much experience working with Linux systems,
you may be overwhelmed by the prospect of controlling an
operating system from the command line. In this book, we will
attempt to get you up to speed with the basics.
Prerequisites and Goals
In order to follow along with this book, you will need to have
access to a Linux server. You will also want to have a basic
understanding of how the terminal works and what Linux
commands look like. This book covers terminal basics, so you
should check it out if you are new to using terminals. All of the
material in this book can be accomplished with a regular, non-root
(non-administrative) user account. You can learn how to configure
this type of user account by following your distribution's initial
server setup guide (Ubuntu 14.04, CentOS 7). When you are ready
to begin, connect to your Linux server using SSH and continue
below.
Navigation and Exploration
The most fundamental skills you need to master are navigating
the filesystem. We will discuss the tools that allow you to do this in
this section.
Finding where you are with the "pwd" command
When you log onto your server, you are typically dropped into
your user accounts home directory. A home directory is the
directory set aside for your account to store files and create
directories. It is the location in the filesystem where you have full
dominion. To find out where your home directory is in relationship
to the rest of the filesystem, you can use the pwd command. This
command displays the directory that we are currently in:
pwd
You should get back some information that looks like this:
/home/demo
The home directory is named after the user account, so the above
example is what the value would be if you were logged into the
server with an account called demo. This directory is within a
directory called/home, which is itself within the top-level directory,
which is called "root" represented by a single slash "/".
Looking at the Contents of Directories with "ls"
Now that you know how to display the directory that you are in,
we can show you how to look at the contents of a directory.
Currently, your home directory does contain much, so we will go
to another, more populated directory to explore. Type the following
command in your terminal to change directory (we will explain the
details of moving directories in the next section). Afterward, we will
use pwd to confirm that we successfully moved:
cd /usr/share
pwd
/usr/share
Now that we are in a new directory, let us look at what's inside. To
do this, we can use the ls command:
ls
adduser groff pam-
configs
applications grub perl
apport grub-gfxpayload-lists
perl5
apps hal
pixmaps
apt i18n
pkgconfig
aptitude icons
polkit-1
apt-xapian-index info
popularity-contest
. . .
As you can see, there are many items in this directory. We can add
some optional flags to the command to modify the default behavior.
For instance, to list all of the contents in an extended form, we can
use the -l flag (for "long" output):
ls -l
total 440
drwxr-xr-x 2 root root 4096 Apr 17 2014 adduser
drwxr-xr-x 2 root root 4096 Sep 24 19:11
applications
drwxr-xr-x 6 root root 4096 Oct 9 18:16 apport
drwxr-xr-x 3 root root 4096 Apr 17 2014 apps
drwxr-xr-x 2 root root 4096 Oct 9 18:15 apt
drwxr-xr-x 2 root root 4096 Apr 17 2014 aptitude
drwxr-xr-x 4 root root 4096 Apr 17 2014 apt-
xapian-index
drwxr-xr-x 2 root root 4096 Apr 17 2014 awk
. . .
This view gives us plenty of information, most of which looks rather
unusual. The first block describes the file type (if the first column is
a "d" the item is a directory, if it is a "-", it is a normal file) and
permissions. Each subsequent column, separated by white space,
describes the number of hard links, the owner, group owner, item
size, last modification time, and the name of the item. We will
describe some of these at another time, but for now, just know that
you can view this information with the -l flag of ls.
To get a listing of all files, including hidden files and directories,
you can add the -a flag. Since there are no real hidden files in
the /usr/share directory, let's go back to our home directory and
try that command. You can get back to the home directory by
typing cd with no arguments:
cd
ls -a
. .. .bash_logout .bashrc .profile
As you can see, there are three hidden files in this demonstration,
along with . and .., which are special indicators. You will find that
often, configuration files are stored as hidden files.
The dot and double dot entries, are built-in methods of referring to
related directories. The single dot indicates the current directory,
and the double dot indicates this directory's parent directory. This
will come in handy in the next section.
Moving Around the Filesystem with "cd"
We have already changed directories twice to demonstrate some
properties of ls. Let's take a better look at the command here.
Begin by going back to the /usr/share directory by typing this:
cd /usr/share
This is an example of changing a directory by giving an absolute
path. In Linux, every file and directory is under the top-most
directory, which is called the "root" directory, but referred to by a
single leading slash "/". An absolute path indicates the location of a
directory in relation to this top-level directory. This lets us refer to
directories in an unambiguous way from any place in the filesystem.
Every absolute path must begin with a slash.
The alternative is to use relative paths. Relative paths refer to
directories in relation to the current directory. For directories close
to the current directory in the hierarchy, this is usually easier and
shorter. Any directory within the current directory can be
referenced by name without a leading slash. We can change to
the locale directory within /usr/share from our current
location by typing:
cd locale
We can likewise change multiple directory levels with relative paths
by providing the portion of the path that comes after the current
directory's path. From here, we can get to
the LC_MESSAGES directory within the en directory by typing:
cd en/LC_MESSAGES
To move up one directory level, we use the special double dot
indicator we talked about earlier. For instance, we are now in the
/usr/share/locale/en/LC_MESSAGES directory. To move up
one level, we can type:
cd ..
This takes us to the /usr/share/locale/en directory.
A shortcut that you saw earlier that will always take you back to
your home directory is to use cd without providing a directory:
cd
pwd
/home/demo
Viewing Files
In the last section, we learned how to navigate the filesystem. In
this section, we will discuss different ways to view files. In contrast
to some operating systems, Linux and other Unix-like operating
systems rely on plain text files for vast portions of the system. The
main way that we will view files is with the less command. This is
what we call a "pager,” because it allows us to scroll through pages
of a file. While the previous commands immediately executed and
returned you to the command line, less is an application that will
continue to run and occupy the screen until you exit.
We will open the /etc/services file, which is a configuration file
that contains the systems services information:
less /etc/services
The file will be opened in less, allowing you to see the portion of
the document that fits in the terminal window:
# Network services, Internet style
#
# Note that it is presently the policy of IANA to
assign a single well-known
# port number for both TCP and UDP; hence, officially
ports have two entries
# even if the protocol doesn't support UDP operations.
#
# Updated from http://www.iana.org/assignments/port-
numbers and other
# sources like
http://www.freebsd.org/cgi/cvsweb.cgi/src/etc/services
.
# New ports will be added on request if they have been
officially assigned
# by IANA and used in the real-world or are needed by
a debian package.
# If you need a huge list of used numbers please
install the nmap package.
tcpmux 1/tcp # TCP
port service multiplexer
echo 7/tcp
. . .
To scroll, you can use the up and down arrow keys on your
keyboard. To page down one whole screens-worth of information,
you can use either the space bar, the "Page Down" button on your
keyboard, or the CTRL-f shortcut.
To scroll back up, you can use either the "Page Up" button, or
the CTRL-b keyboard shortcut. To search for some text in the
document, you can type a forward slash "/" followed by the search
term. For instance, to search for "mail", we would type: /mail This
will search forward through the document and stop at the first
result. To get to another result, you can type the lower-case n key:
n
To move backwards to the previous result, use a capital N instead:
N
When you wish to exit the less program, you can type q to quit:
q
While we focused on the less tool in this section, there are many
other ways of viewing a file. The cat command displays a file's
contents and returns you to the prompt immediately.
The head command, by default, shows the first 10 lines of a file.
Likewise, the tail command shows the last 10 lines. These
commands display file contents in a way that is useful for "piping"
to other programs. We will discuss this concept in a future guide.
Feel free to see how these commands display
the /etc/services file differently.
File and Directory Manipulation
We learned in the last section how to view a file. In this section,
we will demonstrate how to create and manipulate files and
directories.
Create a File with "touch"
Many commands and programs can create files. The most basic
method of creating a file is with the touch command. This will
create an empty file using the name and location specified.
First, we should make sure we are in our home directory, since this
is a location where we have permission to save files. Then, we can
create a file called file1 by typing:
cd
touch file1
Now, if we view the files in our directory, we can see our newly
created file:
ls
file1
If we use this command on an existing file, the command simply
updates the data our filesystem stores on the time when the file was
last accessed and modified.
We can also create multiple files at the same time. We can use
absolute paths as well. For instance, if our user account is
called demo, we could type:
touch /home/demo/file2 /home/demo/file3
ls
file1 file2 file3
Create a Directory with "mkdir"
Similar to the touch command, the mkdir command allows us to
create empty directories.
For instance, to create a directory within our home directory
called test, we could type:
cd
mkdir test
We can make a directory within the test directory
called example by typing:
mkdir test/example
For the above command to work, the test directory must already
exist. To tell mkdir that it should create any directories necessary
to construct a given directory path, you can use the -p option. This
allows you to create nested directories in one step. We can create a
directory structure that looks like some/other/directories by
typing:
mkdir -p some/other/directories
The command will make the some directory first, then it will create
the other directory in the some directory. Finally, it will create
the directories directory in the other directory.
Moving and Renaming Files and Directories with "mv"
We can move a file to a new location using the mv command. For
instance, we can move file1 into the test directory by typing:
mv file1 test
For this command, we list all items that we want to move, with the
location to move them to. We can move that file back to our home
directory by using the special dot reference to refer to our current
directory. We should make sure we are in our home directory, and
then execute the command:
cd
mv test/file1 .
This may seem unintuitive at first, but the mv command is also
used to rename files and directories. In essence, moving and
renaming are both just adjusting the location and name for an
existing item.
So to rename the test directory to testing, we could type:
mv test testing
Note: It is important to realize that your Linux system will not
prevent you from certain destructive actions. If you are renaming a
file and choose a name that already exists, the previous file will
b e overwritten by the file you are moving. There is no way to
recover the previous file if you accidentally overwrite it.
Copying Files and Directories with "cp"
With the mv command, we could move or rename a file or directory,
but we could not duplicate it. The c p command can make a new
copy of an existing item.
For instance, we can copy file3 to a new file called file4:
cp file3 file4
Unlike the mv operation, after which file3 would no longer exist,
we now have both file3 and file4.
Note: As with the mv command, it is possible to overwrite a file if
you are not careful about the filename you are using as the target of
the operation. For instance, if file4 already existed in the above
example, its contents would be completely replaced by the contents
of file3.
In order to copy directories, you must include the -r option in the
command. This stands for "recursive,” as it copies the directory,
plus all of the directory's contents. This option is necessary with
directories, regardless of whether the directory is empty. For
instance, to copy the some directory structure to a new structure
called again, we could type:
cp -r some again
Unlike with files, with which an existing destination would lead to
an overwrite, if the target is an existing directory, the file or
directory is copied into the target:
cp file1 again
This will create a new copy of file1 and place it inside of
the again directory.
Removing Files and Directories with "rm" and "rmdir"
To delete a file, you can use the rm command.
Note: Be extremely careful when using any destructive command
like rm. There is no "undo" command for these actions so it is
possible to accidentally destroy important files permanently.
To remove a regular file, just pass it to the rm command:
cd
rm file4
Likewise, to remove empty directories, we can use
the rmdir command. This will only succeed if there is nothing in
the directory in question. For instance, to remove
the example directory within the testing directory, we can type:
rmdir testing/example
If you wish to remove a non-empty directory, you will have to use
the rm command again. This time, you will have to pass the -
r option, which removes all of the directory's contents recursively,
plus the directory itself.
For instance, to remove the again directory and everything within
it, we can type:
rm -r again
Once again, it is worth reiterating that these are permanent actions.
Be entirely sure that the command you typed is the one that you
wish to execute.
Editing Files
Currently, we know how to manipulate files as objects, but we
have not learned how to actually edit and add content to them.
The nano command is one of the simplest command-line Linux
text editors, and is a great starting point for beginners. It operates
somewhat similarly to the less program we discussed earlier, in
that it occupies the entire terminal for the duration of its use.
The nano editor can open existing files, or create a file. If you want
to create a new file, you can give it a name when you call
the nano editor, or later on, when you wish to save your content.
We can open the file1 file for editing by typing:
cd
nano file1
The nano application will open the file (which is currently blank).
The interface looks something like this:
GNU nano 2.2.6 File: file1
[ Read 0 lines ]
^G Get Help ^O WriteOut ^R Read File ^Y Prev
Page ^K Cut Text ^C Cur Pos
^X Exit ^J Justify ^W Where Is ^V Next
Page ^U UnCut Text ^T To Spell
Along the top, we have the name of the application and the name of
the file we are editing. In the middle, the content of the file,
currently blank, is displayed. Along the bottom, we have a number
of key combinations that indicate some basic controls for the editor.
For each of these, the ^ character means the CTRL key.
To get help from within the editor, type:
CTRL-G
When you are finished browsing the help, type CTRL-X to get back
to your document.
Type in or modify any text you would like. For this example, we will
just type these two sentences:
Hello there.
Here is some text.
To save type:
CTRL-O
This is the letter "o,” not a zero. It will ask you to confirm the name
of the file you wish to save to:
File Name to Write: file1
^G Get Help M-D DOS Format M-A
Append M-B Backup File
^C Cancel M-M Mac Format M-P
Prepend
As you can see, the options at the bottom have also changed. These
are contextual, meaning they will change depending on what you
are trying to do. If file1 is still the file you wish to write to, hit
"ENTER.”
If we make additional changes and wish to save the file and exit the
program, we will see a similar prompt. Add a new line, and then try
to exit the program by typing:
CTRL-X
If you have not saved after making your modification, you will be
asked whether you wish to save the modifications you made:
Save modified buffer (ANSWERING "No" WILL DESTROY
CHANGES) ?
Y Yes
N No ^C Cancel
You can type "Y" to save your changes, "N" to discard your changes
and exit, or "CTRL-C" to cancel the exit operation. If you choose to
save, you will be given the same file prompt that you received
before, confirming that you want to save the changes to the same
file. Press ENTER to save the file and exit the editor.
You can see the contents of the file you created using either
the cat command to display the contents, or the less command
to open the file for viewing. After viewing with less, remember
that you should hit q to get back to the terminal.
less file1
Hello there.
Here is some text.
Another line.
Chapter Ten: UNIX SHELL SCRIPTING
Time is precious. It does not make sense to waste time typing a
frequently used sequence of commands at a command prompt,
more especially if they are abnormally long or complex. Scripting is
a way by which one can alleviate this problem by automating these
command sequences in order to make life at the shell easier and
more productive. Scripting is all about making the computer, the
tool, do the work. By the end of this tutorial you will have a good
understanding of the kind of scripting languages available for Unix
and how to apply them to your problems. UNIX contains many
wonderful and strange commands that can be very useful in the
world of scripting, the more tools you know and the better you
know them, the more use you will find for them. Most of the Unix
commands and many of the built-in commands have man
pages; man pages contain the usage instructions pertaining to the
parent tool. They are not always very clear and may require reading
several times. In order to access a man page in Unix the following
command sequence is applied:
man command
If a man page exists for the command specified the internal viewer
will be invoked and you will be able to read about the various
options and usage instructions.
Shell Scripting Introduction
UNIX uses shells to accept commands given by the user; there
are quite a few different shells available. The most commonly used
shells are SH (Bourne SHell) CSH (C SHell) and KSH (Korn SHell),
most of the other shells you encounter will be variants of these
shells and will share the same syntax, KSH is based on SH as is
BASH (Bourne again shell). TCSH (Extended C SHell) is based on
CSH.
The various shells all have built in functions which allow for the
creation of shell scripts, that is, the stringing together of shell
commands and constructs to automate repetitive tasks in order to
make life easier for the user.
With all these different shells available, what shell should we use?
This is debatable. For the purpose of this tutorial we will be using
SH because it is practically guaranteed to be available on most Unix
systems and be supported by the SH based shells. Your default shell
may not be SH. Fortunately we do not have to be using a specific
shell in order to exploit its features because we can specify the shell
we want to interpret our shell script within the script itself by
including the following in the first line.
#!/path/to/shell
Usually anything following (#) is interpreted as a comment and
ignored but if it occurs on the first line with a (!) following it is
treated as being special and the filename following the (!) is
considered to point to the location of the shell that should interpret
the script.
When a script is "executed”, it is being interpreted by an invocation
of the shell that is running it. Hence, the shell is said to be running
non-interactively, when the shell is used "normally" it is said to be
running interactively.
Note: There are many variations of the basic commands and extra
information which is too specific to be mentioned in this short
tutorial, you should read the man page for your shell to get a more
comprehensive idea of the options available to you. This tutorial
will concentrate on highlighting the most often used and useful
commands and constructs.
Command Redirection and Pipelines
By default a normal command accepts input from standard input,
which we abbreviate to stdin, standard input is the command line
in the form of arguments passed to the command. By default a
normal command directs its output to standard output, which we
abbreviate to stdout, standard output is usually the console display.
For some commands this may be the desired action but other times
we may wish to get our input for a command from somewhere
other than stdin and direct our output to somewhere other than
stdout. This is done by redirection:
We use > to redirect stdout to a file, for instance, if we wanted to
redirect a directory listing generated by the ls we could do the
following:
ls > file
We use < to specify that we want the command immediately before
the redirection symbol to get its input from the source specified
immediately after the symbol, for instance, we could redirect the
input to grep (which searches for strings within files) so that it
comes from a file like this:
grep searchterm < file
We use >> to append stdout to a file, for example, if we wanted to
append the date to the end of a file we would redirect the output
from date like so:
date >> file
One can redirect standard error (stderr) to a file by using 2>, if we
wanted to redirect the standard error from commandA to a file we
would use:
commmandA 2>
Pipes are another form of redirection that are used to chain
commands so that powerful composite commands can be
constructed, the pipe symbol '|' takes the stdout from the command
preceding it and redirects it to the command following it:
ls -l | grep searchword | sort -r
The example above firsts requests a long (-l directory listing of the
current directory using the l s command, the output from this is
then piped to grep which filters out all the listings containing the
searchword and then finally pipes this through to sort which then
sorts the output in reverse (-r, sort then passes the output on
normally to stdout.
Variables
When a script starts, all environment variables are turned into shell
variables. New variables can be instantiated like this:
name=value
You must do it exactly like that, with no spaces, the name must only
be made up of alphabetic characters, numeric characters and
underscores; it cannot begin with a numeric character. You should
avoid using keywords like for or anything like that, the interpreter
will let you use them but doing so can lead to obfuscated code ;)
Variables are referenced like this: $name, here is an example:
#!/bin/sh
msg1=Hello
msg2=There!
echo $msg1 $msg2
This would echo "Hello There!" to the console display, if you want
to assign a string to a variable and the string contains spaces you
should enclose the string in double quotes ("), the double quotes tell
the shell to take the contents literally and ignore keywords,
however, a few keywords are still processed. You can still
use $ within a (") quoted string to include variables:
#!/bin/sh
msg1="one"
msg2="$msg1 two"
msg3="$msg2 three"
echo $msg3
Would echo "one two three" to the screen. The escape character can
also be used within a double quoted section to output special
characters, the escape character is "\", it outputs the character
immediately following it literally so \\ would output \. A special
case is when the escape character is followed by a newline; the shell
ignores the newline character, which allows the spreading of long
commands that must be executed on a single line in reality over
multiple lines within the script. The escape character can be used
anywhere, except within single quotes.
Surrounding anything with single quotes causes it to be treated as
literal text that will be passed on exactly as intended, this can be
useful for sending command sequences to other files in order to
create new scripts because the text between the single quotes will
remain untouched. For example:
#!/bin/sh
echo 'msg="Hello World!"' > hello
echo 'echo $msg' >> hello
chmod 700 hello
./hello
This would cause "msg="Hello World!" to be echoed and redirected
to the file hello, "echo $msg" would then be echoed and redirected
to the file hello but this time appended to the end. The chmod line
changes the file permissions of hello so that we can execute it. The
final line executes hello causing it output "Hello World.” If we had
not used literal quotes we would not have had to use escape
characters to ensure that ($) and (") were echoed to the file, this
makes the code a little clearer.
A variable may be referenced like so ${VARIABLENAME}, this
allows one to place characters immediately preceding the variable
like ${VARIABLENAME}aaa without the shell interpreting aaa as
being part of the variable name.
Command Line Arguments
Command line arguments are treated as special variables within the
script, the reason I am calling them variables is because they can be
changed with the shift command. The command line arguments
are enumerated in the following
manner $0, $1, $2, $3, $4, $5, $6, $7, $8 and $9. $0 is special in
that it corresponds to the name of the script itself. $1 is the first
argument, $2 is the second argument and so on. To reference after
the ninth argument you must enclose the number in brackets like
this ${nn}. You can use the shift command to shift the arguments 1
variable to the left so that $2 becomes $1, $1 becomes$0 and so
o n , $0 gets scrapped because it has nowhere to go, this can be
useful to process all the arguments using a loop, using one variable
to reference the first argument and shifting until you have
exhausted the arguments list.
As well as the command line arguments there are some special
built-in variables:
$# represents the parameter count. Useful for controlling loop
constructs that need to process each parameter.
$@ expands to all the parameters separated by spaces. Useful
for passing all the parameters to some other function or
program.
$- expands to the flags(options) the shell was invoked with.
Useful for controlling program flow based on the flags set.
$$ expands to the process id of the shell innovated to run the
script. Useful for creating unique temporary filenames relative
to this instantiation of the script.
Note: The command line arguments will be referred to as
parameters from now on, this is because SH also allows the
definition of functions which can take parameters and when called
th e $nfamily will be redefined, hence these variables are always
parameters, its just that in the case of the parent script the
parameters are passed via the command line. One exception
is $0which is always set to the name of the parent script regardless
of whether it is inside a function or not.
Command Substitution
In the words of the SH manual “Command substitution allows the
output of a command to be substituted in place of the command
name itself”. There are two ways this can be done. The first is to
enclose the command like this:
$(command)
The second is to enclose the command in back quotes like this:
`command`
The command will be executed in a sub-shell environment and the
standard output of the shell will replace the command substitution
when the command finishes.
Arithmetic Expansion
Arithmetic expansion is also allowed and comes in the form:
$((expression))
The value of the expression will replace the substitution. Eg:
!#/bin/sh
echo $((1 + 3 + 4))
Will echo "8" to stdout
Control Constructs
The flow of control within SH scripts is done via four main
constructs; if... then... elif…, else…, do..., while…, for… and case….
If.. Then.. Elif.. Else
This construct takes the following generic form; the parts enclosed
within ([) and (]) are optional:
if list
then list
[elif list
then list] ...
[else list]
fi
When a Unix command exits it exits with what is known as an exit
status, this indicates to anyone who wants to know the degree of
success a command, usually when a command executes without
error it terminates with an exit status of zero. An exit status of some
other value would indicates that an error has occurred, the details
of which are specific to the command. The commands' man pages
detail the exit status messages.
A list is defined in the SH as "a sequence of zero or more
commands separated by newlines, semicolons, or ampersands,
and optionally terminated by one of these three characters.” Hence
in the generic definition of the if above the list will determine which
of the execution paths the script takes. For example, there is a
command called test on UNIX, which evaluates an expression
and if it evaluates true will return zero and will return one
otherwise, this is how we can test conditions in the list part(s) of
the if construct because test is a command.
We do not actually have to type the test command directly into
the list to use it; it can be implied by encasing the test case within
([) and (]) characters, as illustrated by the following (silly) example:
#!/bin/sh
if [ "$1" = "1" ]
then
echo "The first choice is nice"
elif [ "$1" = "2" ]
then
echo "The second choice is just as nice"
elif [ "$1" = "3" ]
then
echo "The third choice is excellent"
else
echo "I see you were wise enough not to choose"
echo "You win"
fi
What this example does is compare the first parameter (command
line argument in this case) with the strings "1", "2" and "3"
using tests' (=) test which compares two strings for equality, if any
of them match it prints out the corresponding message. If none of
them match it prints out the final case. OK the example is silly and
actually flawed (the user still wins even if they type in (4) or
something) but it illustrates how the if statement works.
Notice that there are spaces between (if) and ([), ([) and the test and
the test and (]), these spaces must be present otherwise the shell
will complain. There must also be spaces between the operator and
operands of the test otherwise it will not work properly. Notice how
it starts with (if) and ends with (fi), also, notice how (then) is on a
separate line to the test above it and that (else) does not require a
(then) statement. You must construct this construct exactly like this
for it to work properly.
It is also possible to integrate logical AND and OR into the testing,
by using two tests separated by either "&&" or "||" respectively. For
example, we could replace the third test case in the example above
with:
elif [ "$1" = "3"] || [ "$1" = "4" ]
then echo "The third choi...
The script would print out "The third choice is excellent" if the first
parameter was either "3" OR "4". To illustrate the use of "&&" we
could replace the third test case with:
elif [ "$1" = "3"] || [ "$2" = "4" ]
then echo "The third choi...
The script would print out "The third choice is excellent" if and only
if the first parameter was "3" AND the second parameter was "4".
"&&" and "||" are both lazily evaluating which means that in the
case of "&&", if the first test fails it won’t bother evaluating the
second because the list will only be true if they BOTH pass and
since one has already failed there is no point wasting time
evaluating the second. In the case of "||" if the first test passes it
won’t bother evaluating the second test because we only
need ONE of the tests to pass for the whole list to pass. See
the test man page for the list of tests possible (other than the
string equality test mentioned here).
Do...While
The Do...While takes the following generic form:
while list
do list
done
In the words of the SH manual "The two lists are executed
repeatedly while the exit status of the first list is zero." There is a
variation on this that uses until in place of while which
executes until the exit status of the first list is zero. Here is an
example use of the while statement:
#!/bin/sh
count=$1 # Initialise count to first parameter
while [ $count -gt 0 ] # while count is greater than 10 do
do
echo $count seconds till supper time!
count=$(expr $count -1) # decrement count by 1
sleep 1 # sleep for a second using the Unix sleep command
done
echo Supper time!!, YEAH!! # were finished
If called from the command line with an argument of 4 this script
will output
4 seconds till supper time!
3 seconds till supper time!
2 seconds till supper time!
1 seconds till supper time!
Supper time!!, YEAH!!
You can see that this time we have used the -gt of
the test command implicitly called by '[' and ']', which stands for
greater than. Pay careful attention to the formatting and spacing.
For
The syntax of the for command is:
for variable in word ...
do list
done
The SH manual states “The words are expanded, and then the list
is executed repeatedly with the variable set to each word in turn.”
A word is essentially some other variable that contains a list of
values of some sort, the for construct assigns each of the values in
the word to variable and then variable can be used within the body
of the construct, upon completion of the body variable will be
assigned the next value in word until there are no more values
in word. This example should make this clearer:
#!/bin/sh
fruitlist="Apple Pear Tomato Peach Grape"
for fruit in $fruitlist
do
if [ "$fruit" = "Tomato" ] || [ "$fruit" = "Peach" ]
then
echo "I like ${fruit}es"
else
echo "I like ${fruit}s"
fi
done
In this example, fruitlist is word, fruit is variable and the body of
the statement outputs how much this person loves various fruits
but includes an if... then… else statement to deal with the correct
addition of letters to describe the plural version of the fruit, notice
that the variable fruit was expressed like ${fruit} because otherwise
the shell would have interpreted the preceding letter(s) as being
part of the variable and echoed nothing because we have not
defined the variables fruits and fruites When executed this script
will output:
I like Apples
I like Pears
I like Tomatoes
I like Peachs
I like Grapes
Within the for construct, do and done may be replaced by '{' and '}'.
Case
The case construct has the following syntax:
case word in
pattern) list ;;
...
esac
An example of this should make things clearer:
!#/bin/sh
case $1
in
1) echo 'First Choice';;
2) echo 'Second Choice';;
*) echo 'Other Choice';;
esac
"1", "2" and "*" are patterns, word is compared to each pattern and
if a match is found the body of the corresponding pattern is
executed, we have used "*" to represent everything, since this is
checked last we will still catch "1" and "2" because they are checked
first. In our example word is "$1", the first parameter, hence if the
script is ran with the argument "1" it will output "First Choice", "2"
"Second Choice" and anything else "Other Choice". In this example
we compared against numbers (essentially still a string comparison
however) but the pattern can be more complex, see the SH man
page for more information.
Functions
The syntax of an SH function is defined as follows:
name ( ) command
It is usually laid out like this:
name() {
commands
}
A function will return with a default exit status of zero, one can
return different exit statuses by using the notation return exit
status. Variables can be defined locally within a function
using local name=value. The example below shows the use of a user
defined increment function:
Increment Function Example
#!/bin/sh
inc() { # The increment is
defined first so we can use it
echo $(($1 + $2)) # We echo the result
of the first parameter plus the second parameter
}
# We check to see
that all the command line arguments are present
if [ "$1" "" ] || [ "$2" = "" ] || [ "$3" = "" ]
then
echo USAGE:
echo " counter startvalue incrementvalue
endvalue"
else
count=$1 # Rename are
variables with clearer names
value=$2
end=$3
while [ $count -lt $end ] # Loop while count
is less than end
do
echo $count
count=$(inc $count $value) # Call increment
with count and value as parameters
done # so that count is
incremented by value
fi
inc() {
echo $(($1 + $2))
}
The function is defined and opened with inc() {, the line echo $(($1
+ $2)) uses the notation for arithmetic expression substitution
which is $((expression)) to enclose the expression, $1 + $2 which
adds the first and second parameters passed to the function
together, the echo bit at the start echoes them to standard output,
we can catch this value by assigning the function call to a variable,
as is illustrated by the function call.
count=$(inc $count $value)
We use command substitution which substitutes the value of a
command to substitute the value of the function call whereupon it
is assigned to the count variable. The command within the
command substitution block is inc $count $value, the last two
values being its parameters. Which are then referenced from within
the function using $1 and $2. We could have used the other
command substitution notation to call the function if we had
wanted:
count=`inc $count $value`
Here is another example illustrating the scope of variables:
Variable Scope, Example
#!/bin/sh
inc() {
local value=4
echo "value is $value within the function\\n"
echo "\\b\$1 is $1 within the function"
}
value=5
echo value is $value before the function
echo "\$1 is $1 before the function"
echo
echo -e $(inc $value)
echo
echo value is $value after the function
echo "\$1 is $1 after the function"
inc() {
local value=4
echo "value is $value within the function\\n"
echo "\\b\$1 is $1 within the function"
}
We assign a local value to the variable value of 4. The next three
lines construct the output, remember that this is being echoed to a
buffer and will be replace the function call with all that was passed
to stdout within the function when the function exits. So, the
calling code will be replaced with whatever we direct to standard
output within the function. The function is called like this:
echo -e $(inc $value)
We have passed the option -e to the echo command which causes
it to process C-style backslash escape characters, so we can process
any backslash escape characters which the string generated by the
function call contains. If we just echo the lines we want to be
returned by the function it will not pass the newline character onto
the buffer even if we explicitly include it with an escape character
reference so what we do is actually include the sequence of
characters that will produce a new line within the string so that
when it is echoed by the calling code with the -e the escape
characters will be processed and the newlines will be placed where
we want them.
echo "value is $value within the function\\n"
Notice how the newline has been inserted with \\n, the first two
backslashes indicate that we want to echo a backslash because
within double quotes a backslash indicates to process the next
character literally, we have to do this because we are only between
double quotes and not the literal-text single quotes. If we had used
single quotes we would had have to echo the bit with the newline in
separately from the bit that contains $value otherwise $value would
not be expanded.
echo "\\b\$1 is $1 within the function"
This is our second line, and is contained within double quotes so
that the variable $1 will be expanded, \\b is included so
that \b will be placed in the echoed line and our calling code
processes this as a backspace character. If we do not do that the
shell prefixes a space to the second line, the backspace removes this
space.
The output from this script called with 2 as the first argument is:
value is 5 before the function
$1 is 2 before the function
value is 4 within the function
$1 is 5 within the function
value is 5 after the function
$1 is 2 after the function
Tip: You can use ". DIRECTORY/common.sh" to import
functions from a script called common.sh in DIRECTORY, a quick
example is shown below, first is test.sh:
#!/bin/sh
. ./common.sh
if [ "$1" = "" ]; then
echo USAGE:
echo "sh test.sh type"
exit
fi
if `validtype $1`; then
echo Valid type
else
echo Invalid type
fi
Here is common.sh:
#!/bin/sh
validtype() {
if [ "$1" = "TYPEA" ] ||
[ "$1" = "TYPEB" ] ||
[ "$1" = "TYPEC" ] ||
[ "$1" = "TYPED" ] ||
[ "$1" = "TYPEE" ];
then
exit 0
else
exit 1
fi
}
Chapter Eleven: SHELL BASIC OPERATOR
Each shell supports various operators. This chapter is based on
default shell (Bourne) so we are going to cover all the important
Bourne Shell operators in this tutorial.
We will discuss the following operators −
Arithmetic Operators.
Relational Operators.
Boolean Operators.
String Operators.
File Test Operators.
The Bourne shell did not originally have any mechanism to perform
simple arithmetic but it uses external programs, either awk or the
must simpler program expr.
Here is simple example to add two numbers –
#!/bin/sh
val=`expr 2 + 2`
echo "Total value : $val"
This produces the following result −
Total value : 4
Note:
There must be spaces between operators and expressions for
example 2+2 is not correct, it should be written as 2 + 2.
Complete expressions should be enclosed between ``, inverted
commas.
Arithmetic Operators
The following are arithmetic operators supported by the Bourne
Shell.
Assume variable ‘a’ holds 10 and variable ‘b’ holds 20 then −
Example, using all the arithmetic operators −
#!/bin/sh
a=10
b=20
val=`expr $a + $b`
echo "a + b : $val"
val=`expr $a - $b`
echo "a - b : $val"
val=`expr $a \* $b`
echo "a * b : $val"
val=`expr $b / $a`
echo "b / a : $val"
val=`expr $b % $a`
echo "b % a : $val"
if [ $a == $b ]
then
echo "a is equal to b"
fi
if [ $a != $b ]
then
echo "a is not equal to b"
fi
This produces the following result −
a + b : 30
a - b : -10
a * b : 200
b / a : 2
b % a : 0
a is not equal to b
Note:
There must be spaces between operators and expressions for
example 2+2 is not correct, whereas it should be written as 2 +
2.
Complete expression should be enclosed between ``, inverted
commas.
You should use \ on the * symbol for multiplication.
The if...then...fi statement is a decision-making statement,
which will be explained in the next chapter.
Operator Description Example
+Addition - Adds values on
either side of the operator
`expr $a +
$b` will give
30
-Subtraction - Subtracts
right hand operand from
left hand operand
`expr $a -
$b` will give
-10
*Multiplication - Multiplies
values on either side of the
operator
`expr $a \*
$b` will give
200
/Division - Divides left hand
operand by right hand
operand
`expr $b /
$a` will give
2
%Modulus - Divides left
hand operand by right
hand operand and returns
remainder
`expr $b %
$a` will give
0
=Assignment - Assign right
operand in left operand
a=$b would
assign value
of b into a
== Equality - Compares two
numbers, if both are same
then returns true.
[ $a == $b ]
would
return false.
!= Not Equality - Compares
two numbers, if both are
different then returns true.
[ $a != $b ]
would
return true.
It is very important to note that all the conditional expressions
should be put inside square braces with one spaces around them,
for example [ $a == $b ] is correct whereas [$a==$b] is incorrect.
All the arithmetical calculations are done using long integers.
Relational Operators:
Bourne Shell supports the following relational operators, which are
specific to numeric values. These operators will not work for string
values unless their value is numeric. For example, the following
operators check a relation between 10 and 20 as well as in between
"10" and "20" but not in between "ten" and "twenty". Assume
variable a holds 10 and variable b holds 20 then −
This example uses all the relational operators −
#!/bin/sh
a=10
b=20
if [ $a -eq $b ]
then
echo "$a -eq $b : a is equal to b"
else
echo "$a -eq $b: a is not equal to b"
fi
if [ $a -ne $b ]
then
echo "$a -ne $b: a is not equal to b"
else
echo "$a -ne $b : a is equal to b"
fi
if [ $a -gt $b ]
then
echo "$a -gt $b: a is greater than b"
else
echo "$a -gt $b: a is not greater than b"
fi
if [ $a -lt $b ]
then
echo "$a -lt $b: a is less than b"
else
echo "$a -lt $b: a is not less than b"
fi
if [ $a -ge $b ]
then
echo "$a -ge $b: a is greater or equal to b"
else
echo "$a -ge $b: a is not greater or equal to b"
fi
if [ $a -le $b ]
then
echo "$a -le $b: a is less or equal to b"
else
echo "$a -le $b: a is not less or equal to b"
fi
This produces the following result −
10 -eq 20: a is not equal to b
10 -ne 20: a is not equal to b
10 -gt 20: a is not greater than b
10 -lt 20: a is less than b
10 -ge 20: a is not greater or equal to b
10 -le 20: a is less or equal to b
Note:
There must be spaces between operators and expressions for
example 2+2 is not correct, whereas it should be written as 2 +
2.
if...then...else...fi statement is a decision making statement
which will be explained in the next chapter.
Operator Description Example
-eq Checks if the value of two
operands are equal, if yes then
condition becomes true.
[ $a -eq
$b ] is not
true.
-ne Checks if the value of two
operands are equal, if values
are not equal then condition
becomes true.
[ $a -ne
$b ] is
true.
-gt Checks if the value of left
operand is greater than the
value of right operand, if yes
then condition becomes true.
[ $a -gt $b
] is not
true.
-lt Checks if the value of left
operand is less than the value
of right operand, if yes then
condition becomes true.
[ $a -lt $b
] is true.
-ge Checks if the value of left
operand is greater than or
equal to the value of right
operand, if yes then condition
becomes true.
[ $a -ge $b
] is not
true.
-le Checks if the value of left
operand is less than or equal to
the value of right operand, if
yes then condition becomes
true.
[ $a -le $b
] is true.
It is very important to note that all the conditional expressions
should be put inside square braces with one spaces around them,
for example [ $a <= $b ] is correct whereas [$a <= $b] is incorrect.
Boolean Operators
The Bourne Shell supports the following boolean operators.
Assume variable ‘a’ holds 10 and variable ‘b’ holds 20 then −
This example uses all the boolean operators −
#!/bin/sh
a=10
b=20
if [ $a != $b ]
then
echo "$a != $b : a is not equal to b"
else
echo "$a != $b: a is equal to b"
fi
if [ $a -lt 100 -a $b -gt 15 ]
then
echo "$a -lt 100 -a $b -gt 15 : returns true"
else
echo "$a -lt 100 -a $b -gt 15 : returns false"
fi
if [ $a -lt 100 -o $b -gt 100 ]
then
echo "$a -lt 100 -o $b -gt 100 : returns true"
else
echo "$a -lt 100 -o $b -gt 100 : returns false"
fi
if [ $a -lt 5 -o $b -gt 100 ]
then
echo "$a -lt 100 -o $b -gt 100 : returns true"
else
echo "$a -lt 100 -o $b -gt 100 : returns false"
fi
This produces the following result −
10 != 20 : a is not equal to b
10 -lt 100 -a 20 -gt 15 : returns true
10 -lt 100 -o 20 -gt 100 : returns true
10 -lt 5 -o 20 -gt 100 : returns false
Note:
There must be spaces between operators and expressions for
example 2+2 is not correct, whereas it should be written as 2 +
2.
Operator Description Example
!This is a logical negation.
This inverts a true condition
into false and vice versa.
[ ! false ]
is true.
-o This is a logical OR. If one of
the operands were true then
condition would be true.
[ $a -lt 20
-o $b -gt
100 ] is
true.
-a This is a logical AND. If both
the operands were true then
condition would be true
otherwise, it would be false.
[ $a -lt 20
-a $b -gt
100 ] is
false.
String Operators
The Bourne Shell supports the following string operators.
Assume variable ‘a’ holds "abc" and variable ‘b’ holds "efg" then −
This example uses all the string operators −
#!/bin/sh
a="abc"
b="efg"
if [ $a = $b ]
then
echo "$a = $b : a is equal to b"
else
echo "$a = $b: a is not equal to b"
fi
if [ $a != $b ]
then
echo "$a != $b : a is not equal to b"
else
echo "$a != $b: a is equal to b"
fi
if [ -z $a ]
then
echo "-z $a : string length is zero"
else
echo "-z $a : string length is not zero"
fi
if [ -n $a ]
then
echo "-n $a : string length is not zero"
else
echo "-n $a : string length is zero"
fi
if [ $a ]
then
echo "$a : string is not empty"
else
echo "$a : string is empty"
fi
This produces the following result −
abc = efg: a is not equal to b
abc != efg : a is not equal to b
-z abc : string length is not zero
-n abc : string length is not zero
abc : string is not empty
Note:
There must be spaces between operators and expressions for
example 2+2 is not correct, whereas it should be written as 2 +
2.
Operator Description Example
=Checks if the value
of two operands is
equal, if yes then
condition becomes
true.
[ $a = $b ]
is not
true.
!= Checks if the value
of two operands
are equal, if values
are not equal then
condition becomes
true.
[ $a != $b
] is true.
-z Checks if the given
string operand size
is zero. If it is zero
length then it
returns true.
[ -z $a ] is
not true.
-n Checks if the
given string
operand size is
non-zero. If it
is non-zero
length then it
returns true.
[ -z $a ] is not
false.
str Check if str is
not the empty
string. If it is
empty then it
returns false.
[ $a ] is not false.
File Test Operators
The following operators test various properties associated with a
Unix file.
Assume a variable file holds an existing file name "test" whose
size is 100 bytes and has read, write and execute permissions −
This example uses all the file test operators −
#!/bin/sh
file="/var/www/tutorialspoint/unix/test.sh"
if [ -r $file ]
then
echo "File has read access"
else
echo "File does not have read access"
fi
if [ -w $file ]
then
echo "File has write permission"
else
echo "File does not have write permission"
fi
if [ -x $file ]
then
echo "File has execute permission"
else
echo "File does not have execute permission"
fi
if [ -f $file ]
then
echo "File is an ordinary file"
else
echo "This is special file"
fi
if [ -d $file ]
then
echo "File is a directory"
else
echo "This is not a directory"
fi
if [ -s $file ]
then
echo "File size is zero"
else
echo "File size is not zero"
fi
if [ -e $file ]
Operator Description Example
-b file Checks if file is a block special
file if yes then condition
becomes true.
[ -b $file ]
is false.
-c file Checks if file is a character
special file if yes then condition
becomes true.
[ -c $file ]
is false.
-d file Check if file is a directory if yes
then condition becomes true.
[ -d $file ]
is not
true.
-f file Check if file is an ordinary file as
opposed to a directory or special
file if yes then condition
[ -f $file ]
is true.
then
echo "File exists"
else
echo "File does not exist"
fi
This produces the following result −
File has read access
File has write permission
File has execute permission
File is an ordinary file
This is not a directory
File size is zero
File exists
Note:
There must be spaces between operators and expressions for
example 2+2 is not correct, whereas it should be written as 2 +
2.
Run The .Sh File Shell Script In
Linux / Unix
After downloading the software, the
becomes true.
-g file Checks if file has its set group
ID (SGID) bit set if yes then
condition becomes true.
[ -g $file ]
is false.
-k file Checks if file has its sticky bit
set if yes then condition
becomes true.
[ -k $file ]
is false.
-p file Checks if file is a named pipe if
yes then condition becomes
true.
[ -p $file ]
is false.
-t file Checks if file descriptor is open
and associated with a terminal if
yes then condition becomes
true.
[ -t $file ]
is false.
-u file Checks if file has its set user id
(SUID) bit set if yes then
condition becomes true.
[ -u $file ]
is false.
-r file Checks if file is readable if yes
then condition becomes true.
[ -r $file ]
is true.
-w file Checks if file is writable if yes
then condition becomes true.
[ -w $file ]
is true.
-x file Checks if file is execute if yes
then condition becomes true.
[ -x $file ]
is true.
-s file Checks if file has size greater
than 0 if yes then condition
becomes true.
[ -s $file ]
is true.
-e file Checks if file exists. Is true even
if file is a directory but exists.
[ -e $file ]
is true.
.sh file is nothing but the shell script to
install a given application or to perform
other tasks under UNIX like operating
systems. The easiest way to run .sh shell
script in Linux or UNIX is to type either
of the following commands. Open the
terminal (your shell prompt) and type
the command:
sh file.sh
OR
bash file.sh
.sh As Root User
Sometimes you will need to install an
application that requires root level
privileges.
Root access is disabled by default on many Linux and UNIX like
systems. Simply use sudo or su as follows:
sudo bash filename.sh
Type your password. Another option is to use the su command as
follows to become superuser:
su -
Type root user password and finally run your script:
bash filename.sh
chmod Command: Run Shell Script In Linux
Another recommended option is to set an executable permission
using the chmod command as follows:
chmod +x file.sh
Now you can run the .sh file.
./file.sh
Chapter Twelve: SHELL EMBEDDING AND OPTIONS
This chapter investigates tyke shells, implanted shells and shell
alternatives.
Shell installing
Shells can be installed to work on a command line; the command
line can bring forth new procedures containing a fork of the present
shell. You can use variables to demonstrate that the new shells are
made. The variable $var1 just exists in the (interim) sub shell.
[paul@RHELv4u3 gen]$ reverberation $var1
[paul@RHELv4u3 gen]$ reverberation $(var1=5;echo
$var1)
5
[paul@RHELv4u3 gen]$ reverberation $var1
[paul@RHELv4u3 gen]$
You can install a shell into an inserted shell; this is called settled
implanting of shells.
paul@deb503:~$ A=shell
paul@deb503:~$ reverberation $C$B$A $(B=sub;echo
$C$B$A; reverberation $(C=sub;echo $C$B$A))
shell subshell subsubshell
Backticks
Single installation can change your present index and uses
backticks instead of dollar section to implant.
[paul@RHELv4u3 ~]$ reverberation `cd/and so forth; ls
- d * | grep pass`
passwd-passwd.OLD
[paul@RHELv4u3 ~]$
The backticks /() and the $() documentation can be used to
implant a shell.
Backticks or single quotes
Putting the installing between backticks utilizes one character not
exactly the dollar and bracket combo. Be cautious on the other
hand, backticks are frequently mistaken for single quotes.
The specialized contrast in the middle of " and ` is critical!
[paul@RHELv4u3 gen]$ reverberation `var1=5;echo $var1`
[paul@RHELv4u3 gen]$ reverberation 'var1=5;echo $var1'
var1=5;echo $var1
[paul@RHELv4u3 gen]$
Shell Alternatives
Both set and unset are built-in shell commands, which are used
to set alternatives to the bash shell. The following case will clear up
this. The shell will treat unset variables as an unimportant variable.
By setting the - u alternative, the shell will treat any reference to
unset variables as an error.
[paul@RHEL4b ~]$ reverberation $var123
[paul@RHEL4b ~]$ set - u
[paul@RHEL4b ~]$ reverberation $var123
- bash: var123: unbound variable
[paul@RHEL4b ~]$ set +u
[paul@RHEL4b ~]$ reverberation $var123
[paul@RHEL4b ~]$
To rundown all the set choices for your shell, use reverberation $-.
The noclobber (or - C).
[paul@RHEL4b ~]$ reverberation $-
himBH
[paul@RHEL4b ~]$ set - C ; set - u
[paul@RHEL4b ~]$ reverberation $-
himuBCH
[paul@RHEL4b ~]$ set +C ; set +u
[paul@RHEL4b ~]$ reverberation $-
himBH
[paul@RHEL4b ~]$
When writing a set without alternatives, you get a rundown of all
variables without capacity, when the shell is in posix mode, you can
set bash in posix mode writing set - o posix.
Practice: Shell Installing
1. Discover the rundown of shell choices in the man page of bash.
What is the contrast between situated - u and set - o nounset?
2. Actuate nounset in your shell. Test if it demonstrates an error
message when using non-existing variables.
3. Deactivate nounset.
4. Execute disc/var and ls in an installed shell.
The reverberation command is expected to demonstrate the after
effects of the ls command. Precluding will cause the shell to
execute the first document as a command.
5. Add the variable embvar into an inserted shell and
reverberation it. Does the variable exist in your present shell
now?
6. Clarify what "set - x" does. Can this be useful?
(Optional)
7. Given the accompanying screenshot, add four characters to the
command so that the aggregate yield is FirstMiddleLast.
[paul@RHEL4b ~]$ reverberate First; resound Middle;
reverberate Last.
8. Display a long listing (ls - l) of the passwd command
utilizing the which command inside an installed shell.
15.4. Arrangement: shell inserting
1. Discover the rundown of shell alternatives in the man page of
bash. What is the distinction between situated
u and set - o nounset?
Check the man of bash and look for nounset - both mean the
same thing.
2. Add nounset to your shell. Test that it demonstrates a lapse
message when utilizing nonexisting variables.
set - u
Or
set - o nounset
Both lines have the same impact.
3. Deactivate nounset.
set +u
Or
set +o nounset
4. Execute album/var and ls in an implanted shell.
reverberation $(cd/var ; ls)
The reverberation command is used to demonstrate the
consequence of the ls command. Precluding will make the shell
to execute the first document as a command.
5. Make the variable embvar in an implanted shell and
reverberation it. Does the variable exist in your present shell?
reverberation $(embvar=emb;echo $embvar) ; reverberation
$embvar #the last resound fizzles
$embvar does not exist in your present shell
6. Clarify what "set - x" does. Is this useful?
It shows shell development for investigating your command.
(Optional)
7. Given the accompanying screenshot, add four characters to the
command line so that the aggregate yield is FirstMiddleLast.
[paul@RHEL4b ~]$ reverberate First; resound Middle;
reverberate Last
reverberation - n First; resound - n Middle; resound Last
8. Show a long posting (ls - l) of the passwd command using
the “which command” inside an implanted shell.
ls - l $(which passwd
Almost all-modern shell allows you to search command history if
enabled by the user. History command can display the history list
with line numbers, listed with a * have been modified by user.
Chapter Thirteen: SHELL HISTORY SEARCH COMMAND
Type history at a shell prompt:
$ history
Output:
6 du -c
7 du -ch
8 ls [01-15]*-2008
9 ls -ld [01-15]*-2008
10 ls -ld [1-15]*-2008
11 ls -ld [0]*-2008
12 ls -ld [01]*-2008
13 rm -vrf [01]*-2008
14 du -ch
15 ls
16 cd
17 umount /mnt
18 df -H
19 vnstat
20 yum update
21 vnstat -m
22 vnstat -m -i eth0
....
...
996 ping router.nixcraft.in
997 ssh vivek@p1.vpn.nixcraft.in
998 alias
999 ~/scripts/clean.rss --fetch
1000 vnstat
1001 ~/scripts/clean.rss --update
To search for a particular command, enter:
$ history | grep command-name
$ history | egrep -i 'scp|ssh|ftp'
Emacs Line-Edit Mode Command History Searching
To find the command containing string, hit [CTRL]+[r] followed by
search string:
(reverse-i-search):
To show the previous command, hit [CTRL] + [p]. You can also use
up arrow key.
CTRL-p
To show the next command, hit [CTRL] +[n]. You can also use
down arrow key.
CTRL-n
fc command
fc stands for either the find command or the fix command. For
example to list the last 10 commands, enter:
$ fc -l 10
to list commands 130 through 150, enter:
$ fc -l 130 150
to list all commands since the last command beginning with ssh,
enter:
$ fc -l ssh
You can edit commands 1 through 5 using vi text editor, enter:
$ fc -e vi 1 5
Delete command history
The -c option causes the history list to be cleared by deleting all of
the entries:
$ history -c
FILE NAME GLOBBING WITH *, ?, [ ]
Sometimes you want to manipulate a group of files, e.g., delete
all of them, without having to perform the command on each file
separately. For example, suppose we want to delete all of the .c files
in a directory. A wildcard is a pattern, which matches something
else. Two commonly used *nix wildcards are * (star)
and ? (question mark).
Star (*) means zero or more characters
Question Mark (?) means exactly one character
Brackets ([]) represent a set of characters
Commands involving filenames specified with wildcards are
expanded by the shell (this is called globbing after the name of a
former program called glob which used to do this outside the
shell).
The '*'
' file*' will match any filename which starts with the characters
"file", and then is followed by zero or more occurrences of any
character.
Examples
Suppose Fred's home directory contains the files,
file01.cpp
file02.cpp
file03.cpp
file1.cpp
file01.o
file02.o
file03.o
 file1.o
To delete all of the .c files, type,
$ rm *.c
To delete file01.cpp and file01.o,
$ rm file01.*
The '?'
'?' Represents a single character.
Examples
Consider again Fred's home directory from the previous example.
Delete file01.o, file02.o and file03.o, but not file1.o,
$ rm file??.o
Delete file01.o, but not file01.cpp,
$ rm file01.?
The '[ ]' Glob
A set of characters can be specified with brackets [ ]. '[ab]' means
the single character can be ‘a ‘OR ‘b’. Ranges can also be specified
(ex: '[1-57-9]' represents 1-5 OR 7-9).
Examples
Delete file02.cpp and file03.cpp from Fred's directory,
$ rm file0[23].cpp
This will delete any files that start with f or F (remember linux is
case sensitive),
$rm [fF]*
To delete all files that start with the string "file" followed by a
single letter type,
$ rm file[a-zA-Z]
The a-z and A-Z in the last example means all the letters in the
range lowercase a-z or uppercase A-Z.
There is much more to wildcard matching, but this is enough to get
you started.
More Examples
Remove all files that are exactly 1 character,
$rm ?
Let's say you have a directory named '9-15-2007-Backup-Really-
Long-Name-blah...' Rather than typing the whole name, you could
just type a subset of the string and use it with the cd command.
$ cd 9-15-2007*
If you have multiple folders that start with 9-15-2007, your
directory will be changed to the first one alphabetically.
You can use file name globbing on most commands that accept files
as arguments.
Chapter fourteen: UNIX - SHELL INPUT/OUTPUT REDIRECTIONS
Time is precious. It does not make sense to waste time typing a
frequently used sequence of commands at a command prompt,
more especially if they are abnormally long or complex. Scripting is
a way by which one can alleviate this problem by automating these
command sequences in order to make life at the shell easier and
more productive. Scripting is all about making the computer, the
tool, do the work. By the end of this tutorial you will have a good
understanding of the kind of scripting languages available for Unix
and how to apply them to your problems. UNIX contains many
wonderful and strange commands that can be very useful in the
world of scripting, the more tools you know and the better you
know them, the more use you will find for them. Most of the Unix
commands and many of the built-in commands have man
pages; man pages contain the usage instructions pertaining to the
parent tool. They are not always very clear and may require reading
several times. In order to access a man page in Unix the following
command sequence is applied
Chapter Fifteen: UNIX SHELL FUNCTION
Functions enable you to break down the overall functionality of a
script into smaller, logical subsections, which are then called upon
to perform their individual tasks when needed. Using functions to
perform repetitive tasks is an excellent way to create code reuse.
Code reuse is an important part of modern object-oriented
programming principles. Shell functions are similar to subroutines,
procedures, and functions in other programming languages.
Creating Functions
To declare a function, simply use the following syntax −
function_name () {
list of commands
}
The name of your function is function_name, and that is what you
will use to call it from elsewhere in your scripts. The function name
must be followed by parentheses, which are followed by a list of
commands enclosed within braces.
Example
Example using function −
#!/bin/sh
# Define your function here
Hello () {
echo "Hello World"
}
# Invoke your function
Hello
Output
$./test.sh
Hello World
$
Pass Parameters to a Function
You can define a function, which would accept parameters while
calling those functions. These parameters would be represented by
$1, $2 and so on. The following is an example where we pass two
parameters Zara and Ali and then we capture and print these
parameters in the function.
#!/bin/sh
# Define your function here
Hello () {
echo "Hello World $1 $2"
}
# Invoke your function
Hello Zara Ali
Output
$./test.sh
Hello World Zara Ali
$
Returning Values from Functions
If you execute an exit command from inside a function, its effect
is not only to terminate execution of the function but also of the
shell program that called the function. If you instead want to just
terminate execution of the function, then there is a way to come
out of a defined function. Based on the situation you can return
any value from your function using the return command whose
syntax is as follows −
return code
Here code can be anything you choose, but obviously you should
choose something that is meaningful or useful in the context of
your script as a whole.
Example
Following function returns a value 1 −
#!/bin/sh
# Define your function here
Hello () {
echo "Hello World $1 $2"
return 10
}
# Invoke your function
Hello Zara Ali
# Capture value returned by last command
ret=$?
echo "Return value is $ret"
Output
$./test.sh
Hello World Zara Ali
Return value is 10
$
Nested Functions
One of the more interesting features of functions is that they can
call themselves as well as call other functions. A function that calls
itself is known as a recursive function.
The following example demonstrates a nesting of two functions −
#!/bin/sh
# Calling one function from another
number_one () {
echo "This is the first function speaking..."
number_two
}
number_two () {
echo "This is now the second function speaking..."
}
# Calling function one.
number_one
Output
This is the first function speaking...
This is now the second function speaking...
Function Call from Prompt
You can put definitions for commonly used functions inside
y o u r .profile so they will be available whenever you log in.
Alternatively, you can group the definitions in a file, say test.sh,
and then execute the file in the current shell by typing −
$. test.sh
This has the effect of causing any functions defined inside test.sh to
be read in and defined to the current shell as follows −
$ number_one
This is the first function speaking...
This is now the second function speaking...
$
To remove the definition of a function from the shell, you use the
unset command with the .f option. This is the same command you
use to remove the definition of a variable to the shell.
$unset .f function_name
Unix - Pipes and Filters
You can connect two commands together so that the output
from one program becomes the input of the next program. Two or
more commands connected in this way form a pipe. To make a
pipe, put a vertical bar (|) on the command line between two
commands. When a program takes its input from another program,
performs some operation on that input, and writes the result to the
standard output, it is referred to as a filter.
The grep Command
Th e grep program searches a file or files for lines that have a
certain pattern. The syntax is −
$grep pattern file(s)
The name "grep" derives from the ed (a UNIX line editor)
command g/re/p, which means, "globally search for a regular
expression and print all lines containing it." A regular expression is
either plain text (a word, for example) and/or special characters
used for pattern matching. The simplest use of grep is to look for a
pattern consisting of a single word. It can be used in a pipe so that
only those lines of the input files containing a given string are sent
to the standard output. If you do not give grep a filename to read,
it reads its standard input; that is the way all filter programs work
−
Option Description
-v Print all lines that do not match pattern.
-n Print the matched line and its line
number.
-l Print only the names of files with
matching lines (letter "l")
-c Print only the count of matching lines.
-i Match either upper- or lowercase.
$ls -l | grep "Aug"
-rw-rw-rw- 1 john doc 11008 Aug 6 14:10 ch02
-rw-rw-rw- 1 john doc 8515 Aug 6 15:30 ch07
-rw-rw-r-- 1 john doc 2488 Aug 15 10:51 intro
-rw-rw-r-- 1 carol doc 1605 Aug 23 07:35 macros
$
There are various options, which you can
use along with grep command –
Next, let's use a regular expression that
t ells grep to find lines with "carol",
followed by zero or more other characters
abbreviated in a regular expression as
".*"), then followed by "Aug". Here we are
using -i option to have case insensitive search −
$ls -l | grep -i "carol.*aug"
-rw-rw-r-- 1 carol doc 1605 Aug 23 07:35 macros
$
The sort Command
T h e sort command arranges lines of text alphabetically or
numerically. The example below sorts the lines in the food file −
$sort food
Afghani Cuisine
Bangkok Wok
Big Apple Deli
Isle of Java
Option Description
-n Sort numerically (example: 10 will
sort after 2), ignore blanks and tabs.
-r Reverse the order of sort.
-f Sort upper- and lowercase together.
+x Ignore first x fields when sorting.
Mandalay
Sushi and Sashimi
Sweet Tooth
Tio Pepe's Peppers
$
The sort command arranges lines of text
alphabetically by default. There are many
options that control the sorting −
More than two commands may be linked
by a pipe. Taking a previous pipe example
using grep, we can further sort the files modified in August by
order of size.
The following pipe consists of the commands ls,
grep, and sort −
$ls -l | grep "Aug" | sort +4n
-rw-rw-r-- 1 carol doc 1605 Aug 23 07:35 macros
-rw-rw-r-- 1 john doc 2488 Aug 15 10:51 intro
-rw-rw-rw- 1 john doc 8515 Aug 6 15:30 ch07
-rw-rw-rw- 1 john doc 11008 Aug 6 14:10 ch02
$
This pipe sorts all files in your directory modified in August by
order of size, and prints them to the terminal screen. The sort
option +4n skips four fields (fields are separated by blanks) then
sort the lines in numeric order.
The pg and more Commands
A long output would normally zip by you on the screen, but if you
run text through more or pg as a filter, the display stops after each
screen of text. Let us assume that you have a long directory listing.
To make it easier to read the sorted listing, pipe the output
through more as follows −
$ls -l | grep "Aug" | sort +4n | more
-rw-rw-r-- 1 carol doc 1605 Aug 23 07:35 macros
-rw-rw-r-- 1 john doc 2488 Aug 15 10:51 intro
-rw-rw-rw- 1 john doc 8515 Aug 6 15:30 ch07
-rw-rw-r-- 1 john doc 14827 Aug 9 12:40 ch03
.
.
.
-rw-rw-rw- 1 john doc 16867 Aug 6 15:56 ch05
--More--(74%)
The screen will fill up with one screen of text consisting of lines
sorted by order of file size. At the bottom of the screen is
the more prompt where you can type a command to move through
the sorted text. When you are done with this screen, you can use
any of the commands listed in the discussion of the more program.
Command Description
awk Pattern scanning and processing
language
cmp Compare the contents of two files
comm. Compare sorted data
cut Cut out selected fields of each line
of a file
Chapter Sixteen: UNIX USEFUL COMMAND
This quick guide lists commands, including a syntax and brief
description. For more detailed information check out commands
man page.
$man command
Files and Directories
These commands allow you to create directories and handle files.
Command Description
cat Display File Contents
cd Changes Directory to dirname
chgrp Change file group
chmod Changing Permissions
cp Copy source file into
destination
file Determine file type
find Find files
grep Search files for regular
expressions.
head Display first few lines of a file
ln Create softlink on oldname
ls Display information about file
type.
mkdir Create a new directory dirname
more Display data in paginated form.
mv Move (Rename) a oldname to
newname.
pwd Print current working directory.
rm Remove (Delete) filename
rmdir Delete an existing directory
provided it is empty.
tail Prints last few lines in a file.
touch Update access and modification
time of a file.
Manipulating data
The contents of files can be compared and
altered with the following commands.
diff Differential file comparator
expand Expand tabs to spaces
join Join files on some common field
Perl Data manipulation language
sed Stream text editor
sort Sort file data
split Split file into smaller files
tr Translate characters
uniq Report repeated lines in a file
wc Count words, lines, and characters
vi Opens vi text editor
vim Opens vim text editor
fmt Simple text formatter
spell Check text for spelling error
ispell Check text for spelling error
Emacs GNU project Emacs
ex, edit Line editor
Emacs GNU project Emacs
Command Description
Compressed Files
Files may be compressed to save space.
Compressed files can be created and
examined −
Command Description
gompress Compress files
gunzip Uncompress gzipped files
gzip GNU alternative compression
method
uncompress Uncompress files
unzip List, test and extract
compressed files in a ZIP
archive
zcat Cat a compressed file
zcmp Compare compressed files
zdiff Compare compressed files
zmore File perusal filter for crt
viewing of compressed text
Getting Information
Various Unix manuals and documentation are available on-line.
The following Shell commands give information −
Command Description
apropos Locate commands by keyword
lookup
info Displays command information
pages online
man Displays manual pages online
whatis Search the whatis database for
complete words.
yelp GNOME help viewer
Network Communication
The following commands are used to send and receive files from a
local UNIX hosts to the remote hosts around the world.
ftp File transfer program
rcp Remote file copy
rlogin Remote login to a UNIX host
rsh Remote shell
tftp Trivial file transfer program
telnet Make terminal connection to
another host
ssh Secure shell terminal or command
connection
scp Secure shell remote file copy
sftp secure shell file transfer program
Some of these commands may be
restricted on your computer for security
reasons.
Messages between Users
The UNIX system supports on-screen
messaging to other users and worldwide
electronic mail −
Command Description
Evolution GUI mail handling tool on
Linux
mail Simple send or read mail
program
nesg Permit or deny messages
parcel Send files to another user
Pine Vdu-based mail utility
Talk Talk to another user
write Write message to another
user
Programming Utilities
The following programming tools and languages are available based
on which version of Unix you are using
Command Description
dbx Sun debugger
gdb GNU debugger
make Maintain program groups
and compile programs.
nm Print program's name list
size Print program's sizes
strip Remove symbol table and
relocation bits
cb C program beautifier
cc ANSI C compiler for Suns
SPARC systems
Ctrace C program debugger
Gcc GNU ANSI C Compiler
Indent Indent and format C program
Command Description
chfn Change your finger information
chgrp Change the group ownership of
a file
chown Change owner
date Print the date
determin Automatically find terminal type
du Print amount of disk usage
echo Echo arguments to the standard
options
exit Quit the system
finger Print information about logged-
in users
groupadd Create a user group
Show group
memberships
homequota Show quota and file usage
iostat Report I/O statistics
kill Send a signal to a process
source
Bc Interactive arithmetic
language processor
Gcl GNU Common Lisp
Perl General purpose language
Php Web page embedded
language
Py Python language interpreter
Asp Web page embedded
language
CC C++ compiler for Suns
SPARC systems
g++ GNU C++ Compiler
Javac JAVA compiler
appletvieweir JAVA applet viewer
Netbeans Java integrated development
environment on Linux
Sqlplus Run the Oracle SQL
interpreter
Sqlldr Run the Oracle SQL data
loader
MySql Run the mysql SQL
interpreter
Misc Commands
These commands list or alter system information −
td>groups
last Show last logins of users
logout log off UNIX
lun List user names or login ID
netstat Show network status
passwd Change user password
passwd Change your login password
printenv Display value of a shell variable
ps Display the status of current
processes
ps Print process status statistics
quota –v Display disk usage and limits
reset Reset terminal mode
script Keep script of terminal session
script Save the output of a command
or process
setenv Set environment variables
sty Set terminal options
time Time a command
top Display all system processes
tset Set terminal mode
tty Print current terminal name
umask Show the permissions that are
given to view files by default
uname Display name of the current
system
uptime Get the system up time
useradd Create a user account
users Print names of logged in users
vmstat Report virtual memory statistics
wShow what logged in users are
doing
who List logged in users
Chapter Seventeen: REGULAR EXPRESSION
A regular expression is a string that can be used to describe
several sequences of characters. Regular expressions are used by
several different Unix commands, including ed, sed, awk,
grep, and, to a more limited extent, vi. Here sed stands
f o r stream editor a stream oriented editor, which was created
exclusively for executing scripts. Thus, all the input you feed into it
passes through and goes to STDOUT and it does not change the
input file.
Invoking sed
Before we start, let us take make sure you have a local copy of
/etc/passwd text file to work with sed. As mentioned previously,
sed can be invoked by sending data through a pipe to it as follows
−
$ cat /etc/passwd | sed
Usage: sed [OPTION]... {script-other-script}
[input-file]...
-n, --quiet, --silent
suppress automatic printing of pattern
space
-e script, --expression=script
...............................
Th e cat command dumps the contents of /etc/passwd to sed
through the pipe into sed's pattern space. The pattern space is the
internal work buffer that sed uses to do its work.
The sed General Syntax
General syntax for sed
/pattern/action
Here, pattern is a regular expression, and action is one of the
commands given in the following table. If pattern is
omitted, action is performed for every line as we have seen above.
The slash characters ( /) that surround the pattern are required
because they are used as delimiters.
Range Description
PPrints the line
DDeletes the line
s/pattern1/pattern2/ Substitutes the first
occurrence of pattern1 with
pattern2.
Deleting All Lines with sed
Invoke sed again, but this time tell sed to use the editing
command delete line, denoted by the single letter d −
$ cat /etc/passwd | sed 'd'
$
Instead of invoking sed by sending a file to it through a pipe, you
can instruct sed to read the data from a file, as in the following
example. The following command does exactly the same thing as
the previous example, without the cat command −
$ sed -e 'd' /etc/passwd
$
The sed Addresses
sed also understands something called addresses. Addresses are
either particular locations in a file or a range where a particular
editing command should be applied. When sed encounters no
addresses, it performs its operations on every line in the file.
The following command adds a basic address to the sed command
you have been using –
$ cat /etc/passwd | sed '1d' |more
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
$
Notice that the number 1 is added before the delete edit command.
This tells sed to perform the editing command on the first line of
the file. In this example, sed will delete the first line of
/etc/password and print the rest of the file.
The sed Address Ranges
So, what if you want to remove more than one line from a file? You
can specify an address range with sed as follows −
$ cat /etc/passwd | sed '1, 5d' |more
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
$
The above command will be applied on all the lines starting from 1
through 5. Therefore, it deletes the first five lines.
Try out the following address ranges –
Range Description
'4,10d' Lines starting from 4th till 10th are
deleted
'10,4d' Only 10th line is deleted, because sed
does not work in reverse direction.
'4,+5d' This will match line 4 in the file, delete
that line, continue to delete the next five
lines, and then cease its deletion and print
the rest
'2,5!d' This will deleted everything except
starting from 2nd till 5th line.
'1~3d' This deletes the first line, steps over the
next three lines, and then deletes the
fourth line. Sed continues applying this
pattern until the end of the file.
'2~2d' This tells sed to delete the second line,
step over the next line, delete the next
line, and repeat until the end of the file is
reached.
'4,10p' Lines starting from 4th till 10th are
printed
'4,d' This would generate syntax error.
',10d' This would also generate syntax error.
Note: While using p action, you should use - n option to avoid
repetition of line printing. Check the difference between these two
commands −
$ cat /etc/passwd | sed -n '1,3p'
Check the above command without -n as follows −
$ cat /etc/passwd | sed '1,3p'
The Substitution Command
The substitution command, denoted by s, will substitute any
string that you specify with any other string that you specify. To
substitute one string with another, you need to have a way of
telling sed where your first string ends and the substitution string
begins. This is traditionally done by bookending the two strings
with the forward slash (/) character. The following command
substitutes the first occurrence on a line of the string root with the
string amrood.
$ cat /etc/passwd | sed 's/root/amrood/'
amrood:x:0:0:root user:/root:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
..........................
It is very important to note that sed substitutes only the first
occurrence on a line. If the string root occurs more than once on a
line, only the first match will be replaced. To tell sed to do a global
substitution, add the letter g to the end of the command as follows
−
$ cat /etc/passwd | sed 's/root/amrood/g'
amrood:x:0:0:amrood user:/amrood:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
Flag Description
g Replace all matches, not just the
first match.
NUMBER Replace only NUMBERth match.
p If substitution was made, print
pattern space.
w
FILENAME
If substitution was made, write
result to FILENAME.
I or i Match in a case-insensitive
manner.
M or m In addition to the normal behavior
of the special regular expression
characters ^ and $, this flag causes
^ to match the empty string after a
newline and $ to match the empty
string before a newline.
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
...........................
Substitution Flags
There are a number of other useful flags
that can be passed in addition to the g
flag, and you can specify more than one
at a time.
Using an Alternative String
Separator
You may find yourself having to do a
substitution on a string that includes
the forward slash character. In this case,
you can specify a different separator by
providing the designated character after
the s.
$ cat /etc/passwd | sed 's:/root:/amrood:g'
amrood:x:0:0:amrood user:/amrood:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
In the above example we used: as delimiter instead of slash /
because we were trying to search /root instead of simple root.
Replacing with Empty Space
Use an empty substitution string to delete the root string from the
/etc/passwd file entirely −
$ cat /etc/passwd | sed 's/root//g'
:x:0:0::/:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
Address Substitution
If you want to substitute the string ‘sh’ with the string ‘quiet’ on
line 10, you can specify it as follows −
$ cat /etc/passwd | sed '10s/sh/quiet/g'
root:x:0:0:root user:/root:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/quiet
Similarly, to do an address range substitution, you could do
something like this −
$ cat /etc/passwd | sed '1,5s/sh/quiet/g'
root:x:0:0:root user:/root:/bin/quiet
daemon:x:1:1:daemon:/usr/sbin:/bin/quiet
bin:x:2:2:bin:/bin:/bin/quiet
sys:x:3:3:sys:/dev:/bin/quiet
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
As you can see from the output, the first five lines had the string
‘sh’ changed to ‘quiet,’ but the rest of the lines were left untouched.
The Matching Command
You would use p option along with -n option to print all the
matching lines as follows −
$ cat testing | sed -n '/root/p'
root:x:0:0:root user:/root:/bin/sh
[root@ip-72-167-112-17 amrood]# vi testing
root:x:0:0:root user:/root:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
Using Regular Expression
While matching patterns, you can use regular expressions, which
provide greater flexibility. Check the following example, which
matches all the lines starting with daemon and then deletes them −
$ cat testing | sed '/^daemon/d'
root:x:0:0:root user:/root:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
Example that deletes all lines ending with sh −
$ cat testing | sed '/sh$/d'
sync:x:4:65534:sync:/bin:/bin/sync
The following table lists four special characters that are very useful
in regular expressions.
Character Description
^Matches the beginning of lines.
$Matches the end of lines.
.Matches any single character.
*Matches zero or more occurrences of
the previous character
[chars] Matches any one of the characters
Expression Description
/a.c/ Matches lines that contain strings
such as a+c, a-c, abc, match, and
a3c, whereas the pattern
/a*c/ Matches the same strings along
with strings such as ace, yacc, and
arctic.
/[tT]he/ Matches the string The and the:
/^$/ Matches Blank lines
/^.*$/ Matches an entire line whatever it
is.
/ */ Matches one or more spaces
/^$/ Matches Blank lines
Set Description
[a-z] Matches a single lowercase letter
[A-Z] Matches a single uppercase letter
[a-zA-
Z]
Matches a single letter
[0-9] Matches a single number
[a-zA-
Z0-9]
Matches a single letter or number
given in chars, where chars is a
sequence of characters. You can use
the - character to indicate a range of
characters.
Matching Characters
Expressions to demonstrate the use of
metacharacters.
The following
table shows some
frequently used
sets of characters
−
Character Class Keywords
Some special keywords are commonly available to regexps,
especially GNU utilities that employ regexps. These are very useful
for sed regular expressions as they simplify things and enhance
readability.
For example, the characters a through z as well as the characters A
through Z constitute one such class of characters that has the
keyword [[:alpha:]] Using the alphabet character class keyword,
this command prints only those lines in the /etc/syslog.conf file
that start with a letter of the alphabet −
$ cat /etc/syslog.conf | sed -n '/^[[:alpha:]]/p'
authpriv.* /var/log/secure
mail.* -/var/log/maillog
Character
Class
Description
[[:alnum:]] Alphanumeric [a-z A-Z 0-9]
[[:alpha:]] Alphabetic [a-z A-Z]
[[:blank:]] Blank characters (spaces or tabs)
[[:cntrl:]] Control characters
[[:digit:]] Numbers [0-9]
[[:graph:]] Any visible characters (excludes
whitespace)
[[:lower:]] Lowercase letters [a-z]
[[:print:]] Printable characters (noncontrol
characters)
[[:punct:]] Punctuation characters
[[:space:]] Whitespace
[[:upper:]] Uppercase letters [A-Z]
[[:xdigit:]] Hex digits [0-9 a-f A-F]
cron.* /var/log/cron
uucp,news.crit /var/log/spooler
local7.* /var/log/boot.log
C omplete list of available character class
keywords in GNU sed.
Ampersand Referencing
The sed metacharacter & represents the
contents of the pattern that was matched.
For instance, you have a file called
phone.txt full of phone numbers −
5555551212
5555551213
5555551214
6665551215
6665551216
7775551217
You want to make the area code (the first three digits) surrounded
by parentheses for easier reading. To do this, you can use the
ampersand replacement character, like so −
$ sed -e 's/^[[:digit:]][[:digit:]][[:digit:]]/(&)/g'
phone.txt
(555)5551212
(555)5551213
(555)5551214
(666)5551215
(666)5551216
(777)5551217
Here you are matching the first 3 digits and then using & replacing
those 3 digits with surrounding parentheses.
Using Multiple sed Commands
You can use multiple sed commands in a single sed command as
follows −
$ sed -e 'command1' -e 'command2' ... -e 'commandN' files
Here command1 through commandN are sed commands of the
type discussed previously. These commands are applied to each line
in the list of files given by files. Using the same mechanism, we can
write above phone number example as follows –
$ sed -e 's/^[[:digit:]]\{3\}/(&)/g' \
-e 's/)[[:digit:]]\{3\}/&-/g'
phone.txt
(555)555-1212
(555)555-1213
(555)555-1214
(666)555-1215
(666)555-1216
(777)555-1217
Note − In the above example, instead of repeating the character
class keyword [[:digit:]] three times, you replaced it with \{3\},
which means to match the preceding regular expression three
times. Here I used \ to give line break you should remove this
before running this command.
Back References
The ampersand metacharacter is useful, but even more useful is the
ability to define specific regions in a regular expression so you can
reference them in your replacement strings. By defining specific
parts of a regular expression, you can then refer back to those parts
with a special reference character. To do back references, you have
to first define a region and then refer back to that region. To define
a region you insert backslash parentheses around each region of
interest. The first region that you surround with backslashes is
then referenced by \1, the second region by \2, and so on.
Assuming phone.txt has the following text −
(555)555-1212
(555)555-1213
(555)555-1214
(666)555-1215
(666)555-1216
(777)555-1217
Now try the following command −
$ cat phone.txt | sed 's/\(.*)\)\(.*-\)\(.*$\)/Area \
code: \1 Second: \2 Third: \3/'
Area code: (555) Second: 555- Third: 1212
Area code: (555) Second: 555- Third: 1213
Area code: (555) Second: 555- Third: 1214
Area code: (666) Second: 555- Third: 1215
Area code: (666) Second: 555- Third: 1216
Area code: (777) Second: 555- Third: 1217
Note: In the above example each regular expression inside the
parenthesis would be back referenced by \1, \2 and so on. Here I
used \ to give line break you should remove this before running
this command.
Chapter Eighteen: FILE SYSTEM BASICS
A file system is a logical collection of files on a partition or disk.
A partition is a container for information and can span an entire
hard drive if desired. Your hard drive can have various partitions
which usually contain only one file system, such as one file system
housing the / file system or another containing the /home file
system. One file system per partition allows for the logical
maintenance and management of differing file systems. Everything
in Unix is considered to be a file, including physical devices such as
DVD-ROMs, USB devices, floppy drives, and so forth.
Directory Structure
Unix uses a hierarchical file system structure, much like an upside-
down tree, with root (/) at the base of the file system and all other
directories spreading from there. A UNIX file system is a collection
of files and directories that have the following properties −
It has a root directory (/) that contains other files and
directories.
Each file or directory is uniquely identified by its name, the
directory in which it resides, and a unique identifier, typically
called an inode.
By convention, the root directory has an inode number of 2
and the lost+found directory has an inode number of 3. Inode
numbers 0 and 1 are not used. File inode numbers can be seen
by specifying the -i option to ls command.
It is self-contained. There are no dependencies between one
file system and any other.
The directories have specific purposes and generally hold the same
types of information for easily locating files. The following are the
directories that exist on the major versions of Unix −
Directory Description
/This is the root directory, which
should contain only the
directories needed at the top level
of the file structure.
/bin This is where the executable files
are located. They are available to
all users.
/dev These are device drivers.
/etc Supervisor directory commands,
configuration files, disk
configuration files, valid user lists,
groups, ethernet, hosts, where to
send critical messages.
/lib Contains shared library files and
sometimes other kernel-related
files.
/boot Contains files for booting the
system.
/home Contains the home directory for
users and other accounts.
/mnt Used to mount other temporary
file systems, such as cdrom and
floppy for the CD-ROM drive and
floppy diskette drive, respectively.
/proc Contains all processes marked as
a file by process number or other
information that is dynamic to the
system.
/tmp Holds temporary files used
between system boots.
/usr Used for miscellaneous purposes,
can be used by many users.
Includes administrative
commands, shared files, library
files, and others.
/var Typically contains variable-length
files such as log and print files and
other file types that may contain a
variable amount of data.
/sbin Contains binary (executable) files,
usually for system administration.
For
example fdisk and ifconfig utlities.
/kernel Contains kernel files
Navigating the File System
Now that you understand the basics of the file system, you can
begin navigating to the files you need. The following are commands
you will use to navigate the system −
Command Description
cat
filename
Displays a filename.
cd
dirname
Moves you to the directory
identified.
cp file1
file2
Copies one file/directory to
specified location.
file
filename
Identifies the file type (binary,
text, etc).
find
filename
dir
Finds a file/directory.
head
filename
Shows the beginning of a file.
less
filename
Browses through a file from end
or beginning.
ls
dirname
Shows the contents of the
directory specified.
mkdir
dirname
Creates the specified directory.
more
filename
Browses through a file from
beginning to end.
mv file1
file2
Moves the location of or
renames a file/directory.
pwd Shows the current directory the
user is in.
rm
filename
Removes a file.
rmdir
dirname
Removes a directory.
tail
filename
Shows the end of a file.
touch
filename
Creates a blank file or modifies
an existing file.s attributes.
whereis
filename
Shows the location of a file.
which
filename
Shows the location of a file if it is
in your PATH.
You can use Manpage Help to check complete syntax for each of
these commands.
The df Command
The first way to manage your partition space is with the df (disk
free) command. The command df -k (disk free) displays the disk
space usage in kilobytes, as shown below –
$df -k
Filesystem 1K-blocks Used Available Use%
Mounted on
/dev/vzfs 10485760 7836644 2649116 75% /
/devices 0 0 0 0%
/devices
$
Some of the directories, such as /devices, shows 0 in the kbytes,
used, and avail columns as well as 0% for capacity. These are
special (or virtual) file systems, and although they reside on the
disk under /, by themselves they do not take up disk space.
The df -k output is generally the same on all Unix systems. Here's
what it usually includes −
Column Description
Filesystem The physical file system
name.
Kbytes Total kilobytes of space
available on the storage
medium.
Used Total kilobytes of space used
(by files).
Avail Total kilobytes available for
use.
Capacity Percentage of total space
used by files.
Mounted on What the file system is
mounted on.
You can use the -h (human readable) option to display the output
in a format that shows the size in easier-to-understand notation.
The du Command
The du (disk usage) command enables you to specify directories to
show disk space usage on a particular directory. This command is
helpful if you want to determine how much space a particular
directory is using. The following command will display the number
of blocks used by each directory. A single block may take either 512
Bytes or 1 Kilo Byte depending on your system.
$du /etc
10 /etc/cron.d
126 /etc/default
6 /etc/dfs
...
$
The -h option makes the output easier to comprehend −
$du -h /etc
5k /etc/cron.d
63k /etc/default
3k /etc/dfs
...
$
Mounting the File System
A file system must be mounted in order to be usable by the system.
To see what is currently mounted (available for use) on your
system, use this command −
$ mount
/dev/vzfs on / type reiserfs (rw,usrquota,grpquota)
proc on /proc type proc (rw,nodiratime)
devpts on /dev/pts type devpts (rw)
$
The /mnt directory, by Unix convention, is where temporary
mounts (such as CD-ROM drives, remote network drives, and
floppy drives) are located. If you need to mount a file system, you
can use the mount command with the following syntax −
mount -t file_system_type device_to_mount
directory_to_mount_to
For example, if you want to mount a CD-ROM to the directory
/mnt/cdrom, for example, you can type −
$ mount -t iso9660 /dev/cdrom /mnt/cdrom
This assumes that your CD-ROM device is called /dev/cdrom and
that you want to mount it to /mnt/cdrom. Refer to the mount man
page for more specific information or type mount -h at the
command line for help information. After mounting, you can use
t h e cd command to navigate the newly available file system
through the mountpoint you just created.
Unmounting the File System
To unmount (remove) the file system from your system, use
the umount command by identifying the mountpoint or device.
For example, to unmount cdrom, use the following command −
$ umount /dev/cdrom
The mount command enables you to access your file systems, but
on most modern Unix systems, the automount function makes this
process invisible to the user and requires no intervention.
User and Group Quotas
User and group quotas provide the mechanisms by which the
amount of space used by a single user or all users within a specific
group can be limited to a value defined by the administrator.
Quotas operate around two limits that allow the user to take some
action if the amount of space, or number of disk blocks start to
reach the administrator defined limits −
Soft Limit − If the user exceeds the limit defined, there is a
grace period that allows the user to free up some space.
Hard Limit − When the hard limit is reached, regardless of
the grace period, no further files or blocks can be allocated.
There are a number of commands to administer quotas −
Command Description
quota Displays disk usage and limits for a
user of group.
edquota This is a quota editor. Users or
Groups quota can be edited using
this command.
quotacheck Scan a file system for disk usage,
create, check and repair quota files
setquota This is also a command line quota
editor.
quotaon This announces to the system that
disk quotas should be enabled on
one or more file systems.
quotaoff This announces to the system that
disk quotas should be disabled off
one or more file systems.
repquota This prints a summary of the disc
usage and quotas for the specified
file systems
Chapter Nineteen: UNIX-USER ADMINISTRATION
There are three types of accounts on Unix system −
Root account − also called superuser and has complete and
unfettered control of the system. A superuser can run any
commands without any restriction. This user should be
assumed as a system administrator.
System accounts − System accounts are those needed for
the operation of system-specific components for example mail
accounts and the sshd accounts. These accounts are usually
needed for some specific function on your system, and any
modifications to them could adversely affect the system.
User accounts − User accounts provide interactive access to
the system for users and groups of users. General users are
typically assigned to these accounts and usually have limited
access to critical system files and directories.
UNIX supports a concept of Group Account , which logically
groups a number of accounts. Every account would be a part of any
group account. Unix groups plays important role in handling file
permissions and process management.
Managing Users and Groups
There are three main user administration files −
/etc/passwd: − Keeps user account and password
information. This file holds the majority of information about
accounts on the Unix system.
/etc/shadow: − Holds the encrypted password of the
corresponding account. Not all systems support this file.
/etc/group: − Contains group information for each account.
/etc/gshadow: − Contains secure group account
information.
Check all the above files using cat command.
Commands available on the majority of Unix systems to create and
manage accounts and groups –
Command Description
useradd Adds accounts to the system.
usermod Modifies account attributes.
userdel Deletes accounts from the
system.
groupadd Adds groups to the system.
groupmod Modifies group attributes.
groupdel Removes groups from the
system.
Create a Group
You need to create groups before creating any account otherwise
you would have to use existing groups on your system. You have all
the groups listed in /etc/groups file. All the default groups would
be system account specific groups and it is not recommended to use
them for ordinary accounts. Use the following syntax to create a
new group account −
groupadd [-g gid [-o]] [-r] [-f] groupname
Here are the details of the parameters:
Option Description
-g GID The numerical value of the group's
ID.
-o This option permits to add group
with non-unique GID
-r This flag instructs groupadd to add
a system account
-f This option causes to just exit with
success status if the specified
group already exists. With -g, if
specified GID already exists, other
(unique) GID is chosen
groupname Actual group name to be created.
If you do not specify any parameters then the system will use the
default values.
The following example will create a developers group with default
values, which is acceptable for most administrators.
$ groupadd developers
Modify a Group
To modify a group, use the groupmod syntax −
$ groupmod -n new_modified_group_name old_group_name
To change the developers_2 group name to developer, type −
$ groupmod -n developer developer_2
Here is how you would change the financial GID to 545 −
$ groupmod -g 545 developer
Delete a Group:
To delete an existing group, all you need are the groupdel
command and the group name. To delete the developer group, the
command is −
$ groupdel developer
This removes only the group, not any files associated with that
group. The files are still accessible by their owners.
Create an Account
Let us see how to create a new account on your Unix system. Use
the syntax to create a user's account −
useradd -d homedir -g groupname -m -s shell -u userid
accountname
Available Parameters −
Option Description
-d homedir Specifies home directory for the
account.
-g
groupname
Specifies a group account for this
account.
-m Creates the home directory if it
doesn't exist.
-s shell Specifies the default shell for
this account.
-u userid You can specify a user id for this
account.
accountname Actual account name to be
created
If you do not specify a parameter then system will use the default
values. The useradd command modifies the /etc/passwd,
/etc/shadow, and /etc/group files and creates a home directory.
The following is an example which will create an
account mcmohd setting its home directory to /home/mcmohd and
group as developers. This user would have Korn Shell assigned to
it.
$ useradd -d /home/mcmohd -g developers -s /bin/ksh mcmohd
Before issuing the above command, make sure you already have
a developers group created using groupadd command. Once an
account is created you can set its password using
the passwd command as follows −
$ passwd mcmohd20
Changing password for user mcmohd20.
New UNIX password:
Retype new UNIX password:
passwd: all authentication tokens updated successfully.
When you type passwd accountname, it gives you option to change
the password provided you are superuser otherwise you would be
able to change just your password using the same command but
without specifying your account name.
Modify an Account
T h e usermod command enables you to make changes to an
existing account from the command line. It uses the same
arguments as the useradd command, plus the -l argument,
which allows you to change the account name. For example, to
change the account name mcmohd to mcmohd20 and to change
home directory accordingly, you would need to issue following
command −
$ usermod -d /home/mcmohd20 -m -l mcmohd mcmohd20
Delete an Account
The userdel command can be used to delete an existing user.
This is a very dangerous command if not used with caution. There
is only one argument or option available for the command: -r, for
removing the account's home directory and mail file. For example,
to remove account mcmohd20, you would need to issue following
command −
$ userdel -r mcmohd20
If you want to keep the home directory for backup purposes, omit
the -r option. You can remove the home directory as needed at a
later time.
Chapter Twenty: SYSTEM PERFORMANCE
The purpose of this tutorial is to introduce the performance
analyst to some of the free tools available to monitor and manage
performance on UNIX systems, and to provide a guideline on how
to diagnose and fix performance problems in a Unix environment.
UNIX has following major resource types that need to be
monitored and tuned −
CPU
Memory
Disk space
Communications lines
I/O Time
Network Time
Applications programs
Performance Components
There are five major components where total system time goes –
Component Description
User state CPU The actual amount of
time the CPU spends
running the users
program in the user state.
It includes time spent
executing library calls, but
does not include time
spent in the kernel on its
behalf.
System state CPU This is the amount of
time the CPU spends in
the system state on behalf
of this program. All I/O
routines require kernel
services. The programmer
can affect this value by
the use of blocking for
Command Description
nice/renice Run a program with modified
scheduling priority
netstat Print network connections,
routing tables, interface statistics,
masquerade connections, and
multicast memberships
time Time a simple command or give
resource usage
uptime System Load Average
ps Report a snapshot of the current
processes.
vmstat Report virtual memory statistics
gprof Display call graph profile data
prof Process Profiling
top Display system tasks
I/O transfers.
I/O Time and
Network Time
These are the amount of
time spent moving data
and servicing I/O
requests
Virtual Memory
Performance
This includes context
switching and swapping.
Application
Program
Time spent running other
programs - when the
system is not servicing
this application because
another application
currently has the CPU.
Performance Tools
Unix provides the following important
tools to measure and fine-tune Unix
system performance −
Unix - System Logging
UNIX systems have a very flexible and
powerful logging system, which enables
you to record almost anything and then
manipulate the logs to retrieve the information you require. Many
versions of UNIX provide a general-purpose logging facility
called syslog. Individual programs that need to have information
logged send the information to syslog.
Unix syslog is a host-configurable, uniform system logging
facility. The system uses a centralized system logging process that
runs the program /etc/syslogd or /etc/syslog.
The operation of the system logger is quite straightforward.
Programs send their log entries to syslogd, which consults the
configuration file /etc/syslogd.conf or /etc/syslog and, when a
Term Description
Facility The identifier used to describe the
application or process that submitted the
log message. Examples are mail, kernel,
and ftp.
Priority An indicator of the importance of the
message. Levels are defined within syslog
as guidelines, from debugging information
to critical events.
Selector A combination of one or more facilities and
levels. When an incoming event matches a
selector, an action is performed.
Action What happens to an incoming message that
matches a selector. Actions can write the
message to a log file, echo the message to a
console or other device, write the message
to a logged in user, or send the message
along to another syslog server.
Facility Description
auth Activity related to requesting name
and password (getty, su, login).
authpriv Same as auth but logged to a file
that can only be read by selected
users.
console Used to capture messages that
would generally be directed to the
system console.
cron Messages from the cron system
scheduler.
daemon System daemon catchall.
ftp Messages relating to the ftp
daemon.
kern Kernel messages.
local0.local7 Local facilities defined per site.
lpr Messages from the line printing
system.
mail Messages relating to the mail
system.
mark Pseudo-event used to generate
timestamps in log files.
news Messages relating to network news
protocol (nntp).
ntp Messages relating to network time
protocol.
user Regular user processes.
uucp UUCP subsystem.
Priority Description
emerg Emergency condition, such as an
imminent system crash, usually
broadcast to all users.
alert Condition that should be corrected
immediately, such as a corrupted
system database.
crit Critical condition, such as a hardware
error.
err Ordinary error.
warning Warning.
notice Condition that is not an error, but
possibly should be handled in a special
match is found, writes the log message to the desired log file.
There are four basic syslog terms that
you should understand −
Syslog Facilities
Here are the
available
facilities for
the selector.
Not all
facilities are
present on all
versions of
UNIX.
Syslog
Priorities
The syslog
priorities are
summarized in
the following table −
The combination of facilities and levels
enables you to be discerning about what
is logged and where that information
goes. As each program sends its
way.
info Informational message.
debug Messages that are used when
debugging programs.
none Pseudo level used to specify not to log
messages.
messages dutifully to the system logger,
the logger makes decisions on what to
keep track of and what to discard based
on the levels defined in the selector.
When you specify a level, the system will keep track of everything at
that level and higher.
The /etc/syslog.conf file
The /etc/syslog.conf file controls where messages are logged. A
typical syslog.conf file might look like this −
*.err;kern.debug;auth.notice /dev/console
daemon,auth.notice /var/log/messages
lpr.info /var/log/lpr.log
mail.* /var/log/mail.log
ftp.* /var/log/ftp.log
auth.* @prep.ai.mit.edu
auth.* root,amrood
netinfo.err /var/log/netinfo.log
install.* /var/log/install.log
*.emerg *
*.alert |program_name
mark.* /dev/console
Each line of the file contains two parts −
A message selector that specifies which kind of messages to
log. For example, all error messages or all debugging messages
from the kernel.
An action field that says what should be done with the
message. For example, put it in a file or send the message to a
user's terminal.
Following are the notable points for the above configuration −
Message selectors have two parts: a facility and a priority. For
example,kern.debug selects all debug messages (the priority)
generated by the kernel (the facility).
Message selector kern.debug selects all priorities that are
greater than debug.
An asterisk in place of either the facility or the priority
indicates "all." For example, *.debug means all debug
messages, while kern.* means all messages generated by the
kernel.
You can also use commas to specify multiple facilities. Two or
more selectors can be grouped together by using a semicolon.
Logging Actions
The action field specifies one of five actions −
Log messages to a file or a device. For example,
/var/log/lpr.log or /dev/console.
Send a message to a user. You can specify multiple usernames
by separating them with commas (e.g., root, amrood).
Send a message to all users. In this case, the action field
consists of an asterisk (e.g., *).
Pipe the message to a program. In this case, the program is
specified after the UNIX pipe symbol (|).
Send the message to the syslog on another host. In this case,
the action field consists of a hostname, preceded by an at sign
(e.g., @tutorialspoint.com)
The logger Command
UNIX provides the logger command, which is extremely useful
for system logging. The logger command sends logging messages
to the syslogd daemon, and consequently provokes system logging.
This means we can check from the command line at any time using
t h e syslogd daemon and its configuration. The logger
command provides a method for adding one-line entries to the
system log file from the command line.
The format of the command is −
logger [-i] [-f file] [-p priority] [-t tag] [message]...
Here is the detail of the parameters –
Option Description
-f
filename
Use the contents of file filename as the
message to log.
-i Log the process ID of the logger process
with each line.
-p
priority
Enter the message with the specified
priority (specified selector entry); the
message priority can be specified
numerically, or as a facility.priority pair.
The default priority is user.notice.
-t tag Mark each line added to the log with the
specified tag.
message The string arguments whose contents are
concatenated together in the specified
order, separated by the space
You can use Manpage Help to check complete syntax for this
command.
Log Rotation
Log files have the propensity to grow very fast and consume large
Application Directory
httpd /var/log/httpd
samba /var/log/samba
cron /var/log/
mail /var/log/
mysql /var/log/
amounts of disk space. To enable log rotations, most distributions
use tools such as newsyslog or logrotate. These tools should be
called at frequent intervals using the cron daemon. Check the man
pages for newsyslog or logrotate for more details.
Important Log Locations
All the system applications create their
log files in /var/log and its sub-
directories. Here are few important
applications and their corresponding log
directories −
Chapter Twenty-one: UNIX SIGNALS AND TRAPS
Signals are software interrupts sent to a program to indicate that
an important event has occurred. The events can vary from user
requests to illegal memory access errors. Some signals, such as the
interrupt signal, indicate that a user has asked the program to do
something that is not in the usual flow of control.
The following are some of the more common signals you might
encounter and want to use in your programs −
Signal
Name
Signal
Number
Description
SIGHUP 1 Hang up detected on
controlling terminal or
death of controlling process
SIGINT 2 Issued if the user sends an
interrupt signal (Ctrl + C).
SIGQUIT 3 Issued if the user sends a
quit signal (Ctrl + D).
SIGFPE 8 Issued if an illegal
mathematical operation is
attempted
SIGKILL 9 If a process gets this signal
it must quit immediately
and will not perform any
cleanup operations
SIGALRM 14 Alarm Clock signal (used
for timers)
SIGTERM 15 Software termination signal
(sent by kill by default).
LIST of Signals
There is an easy way to list all the signals supported by your
system. Just issue the kill –l command and it will display all the
supported signals −
$ kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL
5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE
9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2
13) SIGPIPE 14) SIGALRM 15) SIGTERM 16)
SIGSTKFLT
17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU
25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28)
SIGWINCH
29) SIGIO 30) SIGPWR 31) SIGSYS 34)
SIGRTMIN
35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38)
SIGRTMIN+4
39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42)
SIGRTMIN+8
43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46)
SIGRTMIN+12
47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50)
SIGRTMAX-14
51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54)
SIGRTMAX-10
55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58)
SIGRTMAX-6
59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62)
SIGRTMAX-2
63) SIGRTMAX-1 64) SIGRTMAX
The actual list of signals varies between Solaris, HP-UX, and Linux.
Default Actions
Every signal has a default action associated with it. The default
action for a signal is the action that a script or program performs
when it receives a signal.
Some of the possible default actions are −
Terminate the process.
Ignore the signal.
Dump core. This creates a file called core containing the
memory image of the process when it received the signal.
Stop the process.
Continue a stopped process.
Sending Signals
There are several methods for delivering signals to a program or
script. One of the most common is for a user to type CONTROL-C
or the INTERRUPT key while a script is executing.
When you press the Ctrl+C key a SIGINT is sent to the script and
as per the defined default action script terminates.
The other common method for delivering signals is to use the kill
command whose syntax is as follows −
$ kill -signal pid
Here signal is either the number or name of the signal to deliver
and pid is the process ID that the signal should be sent to. For
Example −
$ kill -1 1001
Sends the HUP or hang-up signal to the program that is running
with process ID 1001. To send a kill signal to the same process use
the following command −
$ kill -9 1001
This would kill the process running with process ID 1001.
Trapping Signals
When you press the Ctrl+C or Break key at your terminal during
execution of a shell program, normally that program is immediately
terminated, and your command prompt returned. This may not
always be desirable. For instance, you may end up leaving a bunch
of temporary files that will not get cleaned up.
Trapping these signals is quite easy, the trap command has the
following syntax −
$ trap commands signals
Here command can be any valid Unix command, or even a user-
defined function, and signal can be a list of any number of signals
you want to trap.
There are three common uses for trap in shell scripts −
Clean up temporary files
Ignore signals
Cleaning Up Temporary Files
As an example of the trap command, the following shows how you
can remove files and then exit if someone tries to abort the program
from the terminal −
$ trap "rm -f $WORKDIR/work1$$ $WORKDIR/dataout$$; exit"
2
From the point in the shell program that this trap is executed, the
two files work1$$ anddataout$$ will be automatically removed if
signal number 2 is received by the program. Therefore, if the user
interrupts execution of the program after this trap is executed, you
can be assured that these two files will be cleaned up.
T h e exit command that follows the r m is necessary because
without it execution would continue in the program at the point
that it left off when the signal was received.
Signal number 1 is generated for hangup: Either someone
intentionally hangs up the line or the line gets accidentally
disconnected. You can modify the preceding trap to also remove
the two specified files in this case by adding signal number 1 to the
list of signals −$ trap "rm $WORKDIR/work1$$
$WORKDIR/dataout$$; exit" 1 2. Now these files will be removed
if the line gets hung up or if the Ctrl+C key gets pressed. The
commands specified to trap must be enclosed in quotes if they
contain more than one command. Also, note that the shell scans
the command line at the time that the trap command gets executed
and also when one of the listed signals is received.
So in the preceding example, the value of WORKDIR and $$ will be
substituted at the time that the trap command is executed. If you
wanted this substitution to occur at the time that either signal 1 or
2 was received you can put the commands inside single quotes −
$ trap 'rm $WORKDIR/work1$$ $WORKDIR/dataout$$; exit' 1 2
Ignoring Signals
If the command listed for trap is null, the specified signal will be
ignored when received. For example, the command −
$ trap '' 2
Specifies that the interrupt signal be ignored. You might want to
ignore certain signals when performing some operations that you
do not want interrupted. You can specify multiple signals to be
ignored as follows −
$ trap '' 1 2 3 15
Note that the first argument must be specified for a signal to be
ignored and is not equivalent to writing the following, which has a
separate meaning of its own −
$ trap 2
If you ignore a signal, all subshells also ignore that signal. However,
if you specify an action to be taken on receipt of a signal, all
subshells will still take the default action on receipt of that signal.
Resetting Traps
After you have changed the default action to be taken on receipt of
a signal, you can change it back again with trap if you simply omit
the first argument; so.
$ trap 1 2
Resets the action to be taken on receipt of signals 1 or 2 back to the
default.
DEDICATION
This book is dedicated to……My Wife and Son
SPECIAL THANKS
To my Wife Katherine and my son Adam

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