Bash - 04 - Variables and Aliases

In Bash, variables can be defined from the command line and used later in the same shell session or from within a script. All variables are typically capitalized but not a requirement.

When a variable is available throughout the environment for use, not just one session or script, it is called an environment variable.

env - Environment Variables

The env command can be used to see all the environment variables defined (these can vary across distributions).

sujith@sujith-Latitude-7490:~/Desktop$ env
SHELL=/bin/bash
SESSION_MANAGER=local/sujith-Latitude-7490:@/tmp/.ICE-unix/1537,unix/sujith-Latitude-7490:/tmp/.ICE-unix/1537
QT_ACCESSIBILITY=1
PWD=/home/sujith/Desktop
LOGNAME=sujith
HOME=/home/sujith
USERNAME=sujith
IM_CONFIG_PHASE=1
LANG=en_US.UTF-8
USER=sujith
GNOME_TERMINAL_SERVICE=:1.132
DISPLAY=:0
SHLVL=1
PATH=/home/sujith/.nvm/versions/node/v20.17.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/snap/bin
GDMSESSION=ubuntu
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus
NVM_BIN=/home/sujith/.nvm/versions/node/v20.17.0/bin
_=/usr/bin/env
OLDPWD=/home/sujith

These environment variables can also be viewed by using echo where string is the name of the environment variable. To output the value of a variable, use the $ sign before the variable name.

sujith@sujith-Latitude-7490:~/Desktop$ echo $USER; echo $SHELL; echo $PWD; echo $OLDPWD;
sujith
/bin/bash
/home/sujith/Desktop
/home/sujith

Common Bash Environment Variables

• DESKTOP_SESSION: Name of the desktop GUI (e.g., gnome)
• HISTSIZE: Size of the history list (e.g., 1000)
• LANG: The specified language and character encoding (e.g., en_US.UTF-8)
• MAIL: Location of the user’s main mail storage
• OLDPWD: The previous working directory (prior to the current one)
• PATH: List of directories to search for executable programs
• PS1: Defines the user prompt
• SHELL: The user’s default shell

PATH is a crucial environment variable in Linux. It contains a list of directories in which the shell searches for executable programs. Most are found in /user/bin /user/sbin

PATH=/home/sujith/.nvm/versions/node/v20.17.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/snap/bin

Without the PATH variable, you would need to specify the full path to the executable before running any command.

Defining Variables

To store a value in a variable, use an assignment statement. The syntax is VAR=VALUE with no spaces around the equal sign. We can define new variables or redefine existing ones.

$ FIRST=Sujith
$ LAST=kumar
$ echo $FIRST $LAST
Sujith Kumar

Using Single Quotes:
When using single quotes (' '), the Bash interpreter treats the content literally and does not expand the value of variables.
So it will not apply $ to retrieve the value from variables.

$ FULL_NAME='$FIRST $LAST'
$ echo $FULL_NAME
$FIRST $LAST

Using Double Quotes:
If there is any blank space in the Value being assigned, then value has to be in " "
When using double quotes (" "), the Bash interpreter will expand variables.

$ FULL_NAME="$FIRST $LAST"
$ echo $FULL_NAME
Sujith Kumar

If a command like pwd is stored in a variable and called, Bash will execute it.

unset variable will remove the variable from the list of variables.
readonly variable will not allow the variable to be reassigned.

$ psite=~/Desktop/website/site
$ cd $psite

$ pnote=~/Desktop/obsid/notes
$ cd $pnote

$ size=`wc -c < file.text`

The Shell Order of Processing can be:

• Parsing : Breaking command into words with space or tab as delimiter.
• Variable evaluation : All $ prefixed strings are evaluated as variables, unless quoted or escaped.
• Command Substitution : Any command surrounded by back quotes are executed by the shell which then replaces the standard output of the command into the command line.
• Redirection : Looks for < > >> to open files they point to.
• Wildcard Interpretation : Replacing with matching file names.
• PATH Evaluation : Searching for the command in directories.

Escape Character (\):
The escape character is used to tell the Bash interpreter to treat the following character literally and not interpret it.
\$$AMOUNT escapes the first $, so it is not interpreted as a variable.

Other Escape Characters:

• \\ : outputs a backslash (\)
• \b : backspace
• \n : newline
• \t : tab
• \! , \$ , \& , \; , \' , \" : escaping special symbols

Putting a string of command in ' ' single quotes will also work as escaping all the characters in it.

[!Important] All variables and aliases defined in the terminal in a session in that environment is temporary by default and will get erased after the terminal session ends. To make it permanent, they need to be defined in the configuration files that are loaded when the shell starts like ~/.bashrc, for system wide environment variables like /etc/environemnt, /etc/bash.bashrc

Aliases

An alias is used to define a shortcut for a command.
Similar to defining a variable, name can be assigned to a command to shorten its execution.

To define an alias, use an assignment statement:
alias name="command"
The name is the alias, and the command is the Linux command it represents.

alias ..="cd .."
alias ~="cd ~"
alias lss=less
alias sl=ls
alias rm='rm -i'  # -i for interactive mode

rm is given in single quotes to input the interactive version.

alias psite="cd ~/Desktop/Sites/Personal"

You can view all the predefined aliases by typing alias:

alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias l='ls -CF'
alias la='ls -A'
alias ll='ls -alF'
alias ls='ls --color=auto'
alias alert='notify-send --urgency=low -i "$([ $? = 0 ] && echo terminal || echo error)" "$(history|tail -n1|sed -e '\''s/^\s*[0-9]\+\s*//;s/[;&|]\s*alert$//'\'')"'

To remove an alias, use the unalias command:

unalias ll