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Shell programming notes
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Shellprogrammingnotes.txt
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Shellprogrammingnotes.txt
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An Introduction to the
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Linux Command Shell
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For Beginners
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Presented by:
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Victor Gedris
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In Co-Operation With:
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The Ottawa Canada Linux Users Group
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and
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ExitCertifiedCopyright and Redistribution
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This manual was written with the intention of being a helpful guide to Linux users who are trying
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to become familiar with the Bash shell and basic Linux commands. To make this manual useful to
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the widest range of people, I decided to release it under a free documentation license, with the
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hopes that people benefit from it by updating it and re-distributing modified copies. You have
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permission to modify and distribute this document, as specified under the terms of the GNU Free
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Documentation License. Comments and suggestions for improvement may be directed to:
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vic@gedris.org.
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This document was created using an Open Source office application called Open Office. The file
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format is non-proprietary, and the document is also published in various other formats online.
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Updated copies will be available on Vic Gedris' web site [http://vic.dyndns.org/]. For
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more information on Open Office, please visit http://www.openoffice.org/.
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Copyright ++ 2003 Victor Gedris.
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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU
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Free Documentation License, Version 1.1 or any later version published by the Free Software
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Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
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Texts. A copy of the license is available from the Free Software Foundation's website:
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http://www.fsf.org/copyleft/fdl.html
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Document Version: 1.2, 2003-06-251.0
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Introduction
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The purpose of this document is to provide the reader with a fast and simple introduction to using
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the Linux command shell and some of its basic utilities. It is assumed that the reader has zero or
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very limited exposure to the Linux command prompt. This document is designed to accompany an
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instructor-led tutorial on this subject, and therefore some details have been left out. Explanations,
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practical examples, and references to DOS commands are made, where appropriate.
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1.1
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What is a command shell?
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A program that interprets commands
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Allows a user to execute commands by typing them manually at a terminal, or automatically
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in programs called shell scripts.
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A shell is not an operating system. It is a way to interface with the operating system and run
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commands.
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1.2
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What is BASH?
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BASH = Bourne Again SHell
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Bash is a shell written as a free replacement to the standard Bourne Shell (/bin/sh)
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originally written by Steve Bourne for UNIX systems.
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It has all of the features of the original Bourne Shell, plus additions that make it easier to
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program with and use from the command line.
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Since it is Free Software, it has been adopted as the default shell on most Linux systems.
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1.3
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How is BASH different from the DOS command prompt?
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Case Sensitivity: In Linux/UNIX, commands and filenames are case sensitive, meaning
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that typing +,%EXIT"/- instead of the proper #%'exit%$. is a mistake.
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%/)\(** vs. !$*/#)(: In DOS, the forward-slash &$./,"! is the command argument delimiter,
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while the backslash '$,\$,# is a directory separator. In Linux/UNIX, the
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!'.//+) is the directory separator, and the %!(\-/* is an escape character. More
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about these special characters in a minute!
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Filenames: The DOS world uses the .(%eight dot three)*- filename convention, meaning
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that all files followed a format that allowed up to 8 characters in the
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filename, followed by a period (!')dot&**), followed by an option extension,
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up to 3 characters long (e.g. FILENAME.TXT). In UNIX/Linux, there is
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no such thing as a file extension. Periods can be placed at any part of the
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filename, and &"%extensions),! may be interpreted differently by all
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programs, or not at all.1.4
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Special Characters
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Before we continue to learn about Linux shell commands, it is important to know that there are
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many symbols and characters that the shell interprets in special ways. This means that certain
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typed characters: a) cannot be used in certain situations, b) may be used to perform special
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operations, or, c) must be #$.escaped(#* if you want to use them in a normal way.
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Character Description
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\ Escape character. If you want to reference a special character, you must "'"escape!+! it
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with a backslash first.
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Example:
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/
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touch /tmp/filename\*
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Directory separator, used to separate a string of directory names.
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Example:
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/usr/src/linux
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. Current directory. Can also **(hide'#) files when it is the first character in a filename.
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.. Parent directory
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~ User's home directory
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* Represents 0 or more characters in a filename, or by itself, all files in a directory.
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Example:
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?
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Represents a single character in a filename.
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Example:
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[ ]
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cd /var/log ; less messages
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Command separator as above, but only runs the second command if the first one
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finished without errors.
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Example:
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&
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more < phonenumbers.txt
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Command separator. Allows you to execute multiple commands on a single line.
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Example:
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&&
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echo $#'Mary 555-1234,/" >> phonenumbers.txt
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Redirect a file as input to a program.
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Example:
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;
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ls > myfiles.txt
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Redirect the output of a command onto the end of an existing file.
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Example:
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<
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ls | more
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Redirect output of a command into a new file. If the file already exists, over-write it.
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Example:
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>>
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hello[0-2].txt represents the names hello0.txt,
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hello1.txt, and hello2.txt
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-,*Pipe),&. Redirect the output of one command into another command.
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Example:
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>
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hello?.txt can represent hello1.txt, helloz.txt, but not
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hello22.txt
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Can be used to represent a range of values, e.g. [0-9], [A-Z], etc.
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Example:
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pic*2002 can represent the files pic2002, picJanuary2002,
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picFeb292002, etc.
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cd /var/logs && less messages
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Execute a command in the background, and immediately get your shell back.
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Example:
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find / -name core > /tmp/corefiles.txt &1.5
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Executing Commands
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The Command PATH:
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Most common commands are located in your shell's *.)PATH(&$, meaning that you can just
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type the name of the program to execute it.
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Example: Typing ).* ls"/& will execute the *!' ls"') command.
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Your shell's ###PATH)&" variable includes the most common program locations, such as
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/bin, /usr/bin, /usr/X11R6/bin, and others.
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To execute commands that are not in your current PATH, you have to give the complete
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location of the command.
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Examples:
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/home/bob/myprogram
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./program (Execute a program in the current directory)
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~/bin/program (Execute program from a personal bin directory)
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Command Syntax
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Commands can be run by themselves, or you can pass in additional arguments to make them do
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different things. Typical command syntax can look something like this:
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command [-argument] [-argument] [--argument] [file]
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Examples:
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ls List files in current directory
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ls -l Lists files in *$-long+,) format
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ls -l --color As above, with colourized output
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cat filename Show contents of a file
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cat -n filename Show contents of a file, with line numbers2.0
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Getting Help
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When you're stuck and need help with a Linux command, help is usually only a few keystrokes
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away! Help on most Linux commands is typically built right into the commands themselves,
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available through online help programs (!-.man pages"-+ and %-*info pages&/)), and of course online.
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2.1
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Using a Command's Built-In Help
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Many commands have simple #)!help)"( screens that can be invoked with special command flags.
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These flags usually look like "$*-h$%' or ,&*--help!"..
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Example:
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2.2
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grep --help
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Online Manuals: !)#Man Pages.*/
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The best source of information for most commands can be found in the online manual pages,
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known as *.-man pages$%, for short. To read a command's man page, type $',man command$!-.
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Examples: man ls
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man man
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Get help on the #))ls$-- command.
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A manual about how to use the manual!
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To search for a particular word within a man page, type +!*/word,"+. To quit from a man page, just
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type the *"%Q-!. key.
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Sometimes, you might not remember the name of Linux command and you need to search for it.
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For example, if you want to know how to change a file's permissions, you can search the man page
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descriptions for the word $"%permission!,) like this:
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man -k permission
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If you look at the output of this command, you will find a line that looks something like:
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chmod
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(1)
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- change file access permissions
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Now you know that $/'chmod/*. is the command you were looking for. Typing %-(man chmod-!, will
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show you the chmod command's manual page!
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2.3
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Info Pages
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Some programs, particularly those released by the Free Software Foundation, use info pages as
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their main source of online documentation. Info pages are similar to man page, but instead of
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being displayed on one long scrolling screen, they are presented in shorter segments with links to
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other pieces of information. Info pages are accessed with the *#,info$/% command, or on some
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Linux distributions, ')'pinfo#-. (a nicer info browser).
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For example:
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info df
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Loads the !"/df,)# info page.3.0
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Navigating the Linux Filesystem
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The Linux filesystem is a tree-like hierarchy hierarchy of directories and files. At the base of the
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filesystem is the %%"/$+# directory, otherwise known as the )/'root!,/ (not to be confused with the root
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user). Unlike DOS or Windows filesystems that have multiple $-"roots()!, one for each disk drive, the
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Linux filesystem mounts all disks somewhere underneath the / filesystem. The following table
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describes many of the most common Linux directories.
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3.1
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The Linux Directory Layout
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Directory
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Description
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The nameless base of the filesystem. All other directories, files, drives, and
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devices are attached to this root. Commonly (but incorrectly) referred to as
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the ,/"slash*"& or ()./.&( directory. The !"$/),& is just a directory separator, not a
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directory itself.
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/bin Essential command binaries (programs) are stored here (bash, ls, mount,
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tar, etc.)
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/boot Static files of the boot loader.
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/dev Device files. In Linux, hardware devices are acceessd just like other files, and
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they are kept under this directory.
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/etc Host-specific system configuration files.
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/home Location of users' personal home directories (e.g. /home/susan).
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/lib Essential shared libraries and kernel modules.
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/proc Process information pseudo-filesystem. An interface to kernel data structures.
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/root The root (superuser) home directory.
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/sbin Essential system binaries (fdisk, fsck, init, etc).
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/tmp Temporary files. All users have permission to place temporary files here.
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/usr The base directory for most shareable, read-only data (programs, libraries,
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documentation, and much more).
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/usr/bin Most user programs are kept here (cc, find, du, etc.).
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/usr/include Header files for compiling C programs.
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/usr/lib Libraries for most binary programs.
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/usr/local $&&Locally*-) installed files. This directory only really matters in environments
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where files are stored on the network. Locally-installed files go in
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/usr/local/bin, /usr/local/lib, etc.). Also often used for
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software packages installed from source, or software not officially shipped
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with the distribution.
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/usr/sbin Non-vital system binaries (lpd, useradd, etc.)
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/usr/share Architecture-independent data (icons, backgrounds, documentation, terminfo,
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man pages, etc.).
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/usr/src Program source code. E.g. The Linux Kernel, source RPMs, etc.
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/usr/X11R6 The X Window System.
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/var Variable data: mail and printer spools, log files, lock files, etc.3.2
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Commands for Navigating the Linux Filesystems
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The first thing you usually want to do when learning about the Linux filesystem is take some time
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to look around and see what's there! These next few commands will: a) Tell you where you are,
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b) take you somewhere else, and c) show you what's there. The following table describes the basic
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operation of the pwd, cd, and ls commands, and compares them to certain DOS commands that
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you might already be familiar with.
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Linux Command
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DOS Command
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Description
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pwd cd )).Print Working Directory+!'.
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location in the directory tree.
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cd cd, chdir #$-Change Directory$&.. When typed all by itself, it
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returns you to your home directory.
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cd directory cd directory Change into the specified directory
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Example: cd /usr/src/linux
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cd ~
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cd ..
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Shows the current
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name.
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*-.~($+ is an alias for your home directory. It can be
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used as a shortcut to your '+-home",!, or other
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directories relative to your home.
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cd..
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cd -
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Move up one directory. For example, if you are in
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/home/vic and you type (/&cd ..%)+, you will end
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up in /home.
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Return to previous directory. An easy way to get
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back to your previous location!
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ls dir /w
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ls directory dir directory List the files in the specified directory.
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List all files in the current directory, in column
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format.
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Example: ls /var/log
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ls -l dir List files in %#$long#%$ format, one file per line. This
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also shows you additional info about the file, such
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as ownership, permissions, date, and size.
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ls -a dir /a List all files, including )'.hidden&#$ files. Hidden files
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are those files that begin with a +,)."+,, e.g. The
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.bash_history file in your home directory.
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ls -ld
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directory
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A ,*.long!+* list of "*)directory.&*, but instead of showing
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the directory contents, show the directory's detailed
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information. For example, compare the output of
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the following two commands:
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ls -l /usr/bin
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ls -ld /usr/bin
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ls /usr/bin/d*
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dir d*.*
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