Bash Shell
A shell is a special program which gives you the familiar text mode prompt. You can use it to start other programs and it understands some special commands of its own.
There are several different shell programs in use in Linux. One of them is the Bourne Again Shell (BASH). Bash has more advanced features and it is more user-friendly.
Bash relies on an number of startup files, which comtain bash commands. The shell executes these commands just as if they had been typed at the command prompt. These files include the following:
/etc/profile
/etc/bashrc
~/.bash_profile, ~/.bash_login, and ~/.profile
~/.bashrc
~/.bash_logout
/etc/inputrc
~/.inputrc
Shell Command Syntax
You can enter most commands by typing the command’s name, following by a space and the options and arguments you want to use.
$ command-name options arguments
To create more complex commands, you need to learn the fundamentals of bash’s syntax.
Special Characters and Quoting
Metacharacters
Command Sequences and Groups
Input/Output Redirection
Brace Expansion
Tilde Expansion
Command Substitution
History Expansion
Parameter Expansion
Shell Functions
Job Control
Special Characters and Quoting
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Certain characters have a special meaning to bash. If you want to use these characters in a way other than how bash interprets them, you must enclose them in quotation mark.
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Metacharacters A metacharacter is a special character that functions as a word seperator, as if it were a blank space. However, not all special characters are metacharacters.
example: $ cat ‘test # testing.txt’ Display the text of a file with special character. $ dir “$pwd/chapter(overview)” Specifies a directory to be displayed using command substitution ($pwd). But the double quotation marks protect the paranthesis from interpretation by the shell. $ gunzip linux.tar.gz ; tar -xvf linux.tar Unzips linux.tar.gz and extracts the files from the archive. $ { find linux* ; find unix* ; }| less Locates all the files that start with linux and unix, and pipes the output to the less command. Spaces are needed before the curly braces but not before the vertical bar as it is a metacharacter. |
Command Sequences and Groups
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You can type more than one command on a single line so that they are executed together in the specified sequence from left to right.
example:
$ find README && cat README Displays the README file only if the find command succeeds.
$ find README.bak || cp README README.bak Makes a backup of README only if the find command fails.
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Brace Expansion
With brace expansion, you can generate two or more commands from a single typed command. When bash encounters a brace character, it reads the comma-seperated list inside the braces from left to right. One by one, it uses the strings within the braces to form a series of commands.
example:
$ cp linux/{ppp,mgetty,vgetty} /mnt
This is equivalent to this:
$ cp linux/ppp /mnt
$ cp linux/mgetty /mnt
$ cp linux/vgetty /mnt
Tilde Expansion
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With tilde expansion, you can quickly enter directory names. The following table, Tilde Prefixes, lists the ways you can use tildes to enter directory names.
Tilde Prefixes:
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Command Substitution
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With command substitution, you can insert the output of a command within a command that you are creating.
`command`
example:
$ ls `pwd` Displays the contents of the current directory
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Parameter Expansion
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A shell parameter is an entity that stores a value. Among various types of shell parameters are variables. You can create your own variables and assign values to them. You can also make use of several built-in variables.
Creating and Using Variables
name=value Create a variable called name and assign value to this variable.
$name Insert the value of a name.
Some Built-In Variables:
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History Expansion
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The shell keeps track of the commands you have entered. It does so by creating a history list in which each command is given a unique identifying number. You can use the history list expansion to insert commands from the history list.
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History Command
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The history command displays a numbered list of the commands in the history list. You can specify the maximum number of lines to display. If the history list contains more lines than you specified, you see the most recently used commands.
<<General format>> $ history [option] [lines]
Options:
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Configuring History
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The number of events saved by your system is kept in a special system variable called HISTSIZE. By default this is usually set to 500. You can change this to another number by simply assigning new value to HISTSIZE and exporting it.
example:
$ HISTSIZE=1000 $ export HISTSIZE
or
$ export HISTSIZE=1000
To display the current value of an environment variable, use the echo command. $ echo $HISTSIZE
The actual histroy events are saved in a file whose name is held in a special variable called HISTFILE. By default this file is the .bash_history file in your home directory. However, you can change the file in which history events are saved by assigning its name to the HISTFILE variable.
example:
$ HISTFILE=”.myhistory” ; export HISTFILE
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Shell Functions
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A shell function is a new command with the name you specify. The shell function executes all the commands listed within the curly braces.
<<General format>>
[ function ] name { command1; command2;…;} Creates a function called name that includes the specified commands.
name () { command1; command2;…;} Creates a function called name that includes the specified commands.
example:
$ list () { ls -al | less; } Creates a shell function called list that executes the ls command and pipes its output to less.
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Shell Configuration Files
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Several configuration files support how your shell behaves. Some of these files are executed for every user and every shell. Others are specific to the particular user that creates the configuration file. These files are:
/etc/profile
This file sets up user environment information for every user. It is executed when you first log in and the shell starts. This file provides values for your path and maximum file size that you can create. It also sets environment variables for such things as location of your mailbox and size of your history files. Finally, /etc/profile gathers shell settings from configuration files in the /etc/profile.d directory.
/etc/bashrc
This file is executed for every user that runs the bash shell. It is read each time a bash shell is opened. It sets the default prompt and may add one or more aliases. Values in this file can be overridden by information in each user’s ~/.bashrc file.
~/.bash_profile
This file is used by each user to enter information that is specific to their own use of the shell. It is executed only once, when the user logs in. By default its sets a few environment variables and executes the user’s .bashrc file.
~/.bashrc
This file contains the bash information that is specific to your bash shells. It is read when you log in and also each time you open a new bash shell. This is the best location to add environment variables and aliases so that your shell picks them up.
~/.bash_logout
This file executes each time you log out (exit the last bash shell). By default, it simply clears your screen.
To change the /etc/profile or /etc/bashrc files, you must be the root user. Any user can change the information in $HOME/.bash_profile, $HOME/.bashrc, and $HOME/.bash_logout files in their own home directories. |
Setting Your Prompt
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Your prompt consists of a set of characters that appear each time the shell is ready to accept a command. Exactly what that prompt contains is determined by the value in PS1 environment variable. Here is an example of the prompt.
[kundan@linuxnepal tmp]$
If you were to change directories, the tmp name would change to the name of the new current working directory. Likewise, if you were to log in as a different user or to a different host, that information would change.
You can use several special characters to include different information in your prompt.
Some of the examples are:
\! Shows the current command history number. This includes all previous commands stored for your username.
\# Shows the command number of the current command. This includes only the commands for the active shell.
\$ Shows the standard user prompt ($) or root prompt (#), depending on which user you are.
\W Shows only the current working directory base name. For example, if the current directory was /var/spool/mail, this value would simply appear as mail.
\\ Shows a backslash.
\d Displays the day, month, and the number of the date. eg: Fri Jun 11
\h Shows the host name of the computer running the shell.
\n newline
\nnn Shows the character that relates to the octal number replacing nnn.
\s Displays the current shell name. bash for bash shell.
\t Prints the current time in hours, minutes, and seconds. eg: 12:10:44
\u Prints the current user name.
\w Displays the full path to the current working directory. |
Adding Environment Variables
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You can add your own environment variables into your .bashrc file. These can help make working with the shell more efficient and effective. You can update your $PATH environment variables like:
PATH=$PATH:/myprogs/bin ; export $PATH
This example first reads all the current path directories into the new PATH ($PATH), adds the /myprogs/bin directory, and then exports the new PATH.
You can create your own environment variables to provide shortcuts in your work. Choose any name that is not being used and assign a useful value for it.
PF=/usr/share/project/info ; export PF
You can change to this directory by issuing command as:
$ cd $PF
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Adding Aliases
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Setting aliases can save you even more typing than setting environment variables. With aliases, you can have a string of characters execute an entire command line. You can add and list aliases with the alias command.
example:
alias p=’pwd ; ls -alF’ alias rm=’rm -i’
In the first command the letter p is assigned to run the command pwd, and then to run ls -alF to print the current directory and list its contents. The second example runs the rm command with the -i option each time you type rm.
When you are in the shell, you can check which aliases are set by simply typing the alias command.
If you want to remove an alias, you can use the unalias command. If the alias is set in a configuration file like .bashrc, it will be set again when you open another shell. eg:
To remove alias named p $ unalias p
To remove all of your aliases $ unalias -a |