Bash - (Environment) Variable

with assignment operator

In the shell command language, a variable is a word consisting of the following parts:

[local] name=[value]

where:

• name is the id of the variable and is not a special parameter
• value is the value of the variable. Default (null string).
• local defines a local variable

If the variable has its integer attribute set, then value is evaluated as an arithmetic expression even if the $((...)) expansion is not used.

All values undergo the following expansion:

• Bash - Tilde (~) Expansion,
• Bash - Parameter Expansion
• What is Variable Expansion in Bash ?,
• Bash - Command substitution,
• Bash - Arithmetic Expression,
• and quote removal

Is not performed:

• Word splitting, with the exception of “$@” See Special Parameters.
• Bash - pathname expansion (Filename expansion).

Assignment statements may also appear as arguments to the:

• alias,
• declare,
• typeset,
• export,
• readonly
• and local builtin commands.

With multiple words

With the read command

read -r var1 var2 var3 <<< "val1 val2 val3"
echo $var1
echo $var2
echo $var3

val1
val2
val3

One

To see an environment variable, you make use of the echo command as :

echo $PATH

/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/root/bin

or with the env command

env | grep PATH

All with env

To list the current values of all environment variables, issue the command

env

HOSTNAME=ebs121.localdomain
TERM=xterm
SHELL=/bin/bash
HISTSIZE=1000
SSH_CLIENT=192.168.2.2 3886 22
SSH_TTY=/dev/pts/1
USER=root
.........

or

$ env | more

All with declare

To list the current values of all environment variables, execute the declare command

declare

To list all names with their definitions, issue the declare command with p

declare -p

declare -- BASH="/bin/bash"
declare -r BASHOPTS="checkwinsize:cmdhist:complete_fullquote:expand_aliases:extglob:extquote:force_fignore:histappend:interactive_comments:login_shell:progcomp:promptvars:sourcepath"
declare -ir BASHPID
declare -A BASH_ALIASES='()'
declare -a BASH_ARGC='()'
..............

A subset

Thanks to the parameter expansion prefix, we can get a list of variable that begins with a prefix.

Example:

Prefix1=Prefix1Value
Prefix2=Prefix2Value
echo ${!Prefix*}

• output:

Prefix1 Prefix2

For a session

The syntax depends of your shell:

• for the Bourne, Bash, or Korn shell:

To add /sbin to the path, type the export command in a console :

export PATH=$PATH:/sbin

or with two variable

$ TMP=/mount_point/tmp
$ TMPDIR=/mount_point/tmp
$ export TMP TMPDIR

• For the C shell:

% setenv TMP /mount_point/tmp
% setenv TMPDIR /mount_point/tmp

Permanently

You must change the shell startup script.

One

unset myEnvironmentVariable

All

With the env utility, you can start with a empty environment with the -i option.

env -i yourScript

Check if unset or empty

• Unset or empty

if [ -z "${var}" ]; then
  echo "var is unset or empty"; 
else 
  echo "var is set to '$var'"; 
fi

• Unset

[ -z ${var} ] && echo "Unset"

See Parameter check for more info.

Check if set

if [[ -n ${var} ]]; then 
  echo "var is set to '$var'"; 
else 
  echo "var is unset";  
fi

See Parameter check for more info.