Variables

A shell exposes and uses environment variables to:

  • alter its behavior

  • alter the behavior of some external commands

  • store data

With bash, variables:

  • don’t have a data type in the first place

  • only exist in the current shell (if nothing is done to change that)

Variables: listing all of them

Internal commands export and set without parameters print the list of variables defined in the current shell

$> export
...
declare -x SHELL="/bin/bash"
declare -x SHLVL="1"
declare -x SSH_AGENT_PID="1338"
declare -x USER="luisgustavo.nardin"
...
$>

  • set also prints user defined shell functions

Variables: storing data

Variables are a nice way to store data that is not too large

$> MY_VAR="I store text in this variable"
$> NUM=123
$> TMP_DIR=/tmp
$> SONG=The\ fool\ on\ the\ hill
$>

  • Assignment is made with the = sign
    no space allowed before or after this character!

  • If the data you want to store in a variable contains special characters, you have to put your data in double quotes, or you may escape all the special characters by preceding them with \ (backslash) (kbd:[AltGr+8] or kbd:[⇪+⌥+L])

Variables: printing their values

To use a variable (to get its value), it must be prefixed with the $ character

$> echo $SHELL
/bin/bash
$> echo $MY_VAR
I store text in this variable
$> echo $SONG
The fool on the hill
$> echo $NUM
123
$>

  • echo is a command that may be used to print the content (the value) of a variable

Variables: from the shell to other programs

To make a variable available to other commands launched by the shell or to subshells, you need to export it

$> bash -c "echo \$MY_VAR"

$> export MY_VAR
$> bash -c "echo \$MY_VAR"
I store text in this variable
$>

  • An export only affects the current shell and its subshells

  • It is not possible to export a variable to the parent shell

  • Once the variable is exported, any change to its value is automatically exported (however, the value in the already spawned subshells will not change)

Variables: from the shell to other programs

You can temporarily change the value or set a variable for the next call to an external command

$> echo $LANG
fr_FR.utf8
$> man 3 printf
## la page de manuel s'affiche en français
$> LANG=en_GB man 3 printf
## la page de manuel s'affiche, pour cette fois, en anglais
$> echo $LANG
fr_FR.utf8
$>

Variables: altering their contents

You may temporarily alter the content of a variable.

The main operators are:

  • # or ## to remove a prefix

  • % or %% to remove a suffix

  • / or // to substitute a text with another

$> VAR="hello world.txt"
$> echo ${VAR#hel}
lo world.txt
$> echo ${VAR%.txt}
hello world
$> echo ${VAR/txt/c/}
hello world.c
$> echo ${VAR//o/00}
hell00 w00rld.txt
$>

More on that in the EXPANSION / Parameter Expansion section of the bash manual page.

Arithmetics

$> VAL=123
$> ((VAL++))
$> echo $VAL
124
$> echo $((VAL/2))
62
$> echo $VAL
124
$> RES=$((VAL/=2))
$> echo "RES=$RES, VAL=$VAL"
RES=62, VAL=62
$>

When a variable stores an integer, it is possible to make some computations on it using the (( …​ )) construct.

When the variable does not contain digits, bash make its 0.

References

‡ : read thoses pages on your own operating system, not on the Internet!