Chapter 14. shell variables

Table of Contents

$ dollar sign
case sensitive
creating variables
quotes
set
unset
$PS1
$PATH
env
export
delineate variables
unbound variables
practice: shell variables
solution: shell variables

In this chapter we learn to manage environment variables in the shell. These variables are often needed by applications.

$ dollar sign

Another important character interpreted by the shell is the dollar sign $. The shell will look for an environment variable named like the string following the dollar sign and replace it with the value of the variable (or with nothing if the variable does not exist).

These are some examples using $HOSTNAME, $USER, $UID, $SHELL, and $HOME.

[paul@RHELv4u3 ~]$ echo This is the $SHELL shell
This is the /bin/bash shell
[paul@RHELv4u3 ~]$ echo This is $SHELL on computer $HOSTNAME
This is /bin/bash on computer RHELv4u3.localdomain
[paul@RHELv4u3 ~]$ echo The userid of $USER is $UID
The userid of paul is 500
[paul@RHELv4u3 ~]$ echo My homedir is $HOME
My homedir is /home/paul

case sensitive

This example shows that shell variables are case sensitive!

[paul@RHELv4u3 ~]$ echo Hello $USER
Hello paul
[paul@RHELv4u3 ~]$ echo Hello $user
Hello

creating variables

This example creates the variable $MyVar and sets its value. It then uses echo to verify the value.

[paul@RHELv4u3 gen]$ MyVar=555
[paul@RHELv4u3 gen]$ echo $MyVar
555
[paul@RHELv4u3 gen]$

quotes

Notice that double quotes still allow the parsing of variables, whereas single quotes prevent this.

[paul@RHELv4u3 ~]$ MyVar=555
[paul@RHELv4u3 ~]$ echo $MyVar
555
[paul@RHELv4u3 ~]$ echo "$MyVar"
555
[paul@RHELv4u3 ~]$ echo '$MyVar'
$MyVar

The bash shell will replace variables with their value in double quoted lines, but not in single quoted lines.

paul@laika:~$ city=Burtonville
paul@laika:~$ echo "We are in $city today."
We are in Burtonville today.
paul@laika:~$ echo 'We are in $city today.'
We are in $city today. 

set

You can use the set command to display a list of environment variables. On Ubuntu and Debian systems, the set command will also list shell functions after the shell variables. Use set | more to see the variables then.

unset

Use the unset command to remove a variable from your shell environment.

[paul@RHEL4b ~]$ MyVar=8472
[paul@RHEL4b ~]$ echo $MyVar
8472
[paul@RHEL4b ~]$ unset MyVar
[paul@RHEL4b ~]$ echo $MyVar

[paul@RHEL4b ~]$

$PS1

The $PS1 variable determines your shell prompt. You can use backslash escaped special characters like \u for the username or \w for the working directory. The bash manual has a complete reference.

In this example we change the value of $PS1 a couple of times.

paul@deb503:~$ PS1=prompt
prompt
promptPS1='prompt '
prompt 
prompt PS1='> '
> 
> PS1='\u@\h$ '
paul@deb503$ 
paul@deb503$ PS1='\u@\h:\W$'
paul@deb503:~$

To avoid unrecoverable mistakes, you can set normal user prompts to green and the root prompt to red. Add the following to your .bashrc for a green user prompt:

# color prompt by paul
RED='\[\033[01;31m\]'
WHITE='\[\033[01;00m\]'
GREEN='\[\033[01;32m\]'
BLUE='\[\033[01;34m\]'
export PS1="${debian_chroot:+($debian_chroot)}$GREEN\u$WHITE@$BLUE\h$WHITE\w\$ "

$PATH

The $PATH variable is determines where the shell is looking for commands to execute (unless the command is builtin or aliased). This variable contains a list of directories, separated by colons.

[[paul@RHEL4b ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:

The shell will not look in the current directory for commands to execute! (Looking for executables in the current directory provided an easy way to hack PC-DOS computers). If you want the shell to look in the current directory, then add a . at the end of your $PATH.

[paul@RHEL4b ~]$ PATH=$PATH:.
[paul@RHEL4b ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:.
[paul@RHEL4b ~]$

Your path might be different when using su instead of su - because the latter will take on the environment of the target user. The root user typically has /sbin directories added to the $PATH variable.

[paul@RHEL3 ~]$ su
Password: 
[root@RHEL3 paul]# echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin
[root@RHEL3 paul]# exit
[paul@RHEL3 ~]$ su -
Password: 
[root@RHEL3 ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:
[root@RHEL3 ~]#

env

The env command without options will display a list of exported variables. The difference with set with options is that set lists all variables, including those not exported to child shells.

But env can also be used to start a clean shell (a shell without any inherited environment). The env -i command clears the environment for the subshell.

Notice in this screenshot that bash will set the $SHELL variable on startup.

[paul@RHEL4b ~]$ bash -c 'echo $SHELL $HOME $USER'
/bin/bash /home/paul paul
[paul@RHEL4b ~]$ env -i bash -c 'echo $SHELL $HOME $USER'
/bin/bash
[paul@RHEL4b ~]$

You can use the env command to set the $LANG, or any other, variable for just one instance of bash with one command. The example below uses this to show the influence of the $LANG variable on file globbing (see the chapter on file globbing).

[paul@RHEL4b test]$ env LANG=C bash -c 'ls File[a-z]'
Filea  Fileb
[paul@RHEL4b test]$ env LANG=en_US.UTF-8 bash -c 'ls File[a-z]'
Filea  FileA  Fileb  FileB
[paul@RHEL4b test]$

export

You can export shell variables to other shells with the export command. This will export the variable to child shells.

[paul@RHEL4b ~]$ var3=three
[paul@RHEL4b ~]$ var4=four
[paul@RHEL4b ~]$ export var4
[paul@RHEL4b ~]$ echo $var3 $var4
three four
[paul@RHEL4b ~]$ bash
[paul@RHEL4b ~]$ echo $var3 $var4
four

But it will not export to the parent shell (previous screenshot continued).

[paul@RHEL4b ~]$ export var5=five
[paul@RHEL4b ~]$ echo $var3 $var4 $var5
four five
[paul@RHEL4b ~]$ exit
exit
[paul@RHEL4b ~]$ echo $var3 $var4 $var5
three four
[paul@RHEL4b ~]$

delineate variables

Until now, we have seen that bash interprets a variable starting from a dollar sign, continuing until the first occurrence of a non-alphanumeric character that is not an underscore. In some situations, this can be a problem. This issue can be resolved with curly braces like in this example.

[paul@RHEL4b ~]$ prefix=Super
[paul@RHEL4b ~]$ echo Hello $prefixman and $prefixgirl
Hello  and
[paul@RHEL4b ~]$ echo Hello ${prefix}man and ${prefix}girl
Hello Superman and Supergirl
[paul@RHEL4b ~]$

unbound variables

The example below tries to display the value of the $MyVar variable, but it fails because the variable does not exist. By default the shell will display nothing when a variable is unbound (does not exist).

[paul@RHELv4u3 gen]$ echo $MyVar
				
[paul@RHELv4u3 gen]$

There is, however, the nounset shell option that you can use to generate an error when a variable does not exist.

paul@laika:~$ set -u
paul@laika:~$ echo $Myvar
bash: Myvar: unbound variable
paul@laika:~$ set +u
paul@laika:~$ echo $Myvar

paul@laika:~$

In the bash shell set -u is identical to set -o nounset and likewise set +u is identical to set +o nounset.

practice: shell variables

1. Use echo to display Hello followed by your username. (use a bash variable!)

2. Create a variable answer with a value of 42.

3. Copy the value of $LANG to $MyLANG.

4. List all current shell variables.

5. List all exported shell variables.

6. Do the env and set commands display your variable ?

6. Destroy your answer variable.

7. Create two variables, and export one of them.

8. Display the exported variable in an interactive child shell.

9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore.

10. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt.

solution: shell variables

1. Use echo to display Hello followed by your username. (use a bash variable!)

echo Hello $USER

2. Create a variable answer with a value of 42.

answer=42

3. Copy the value of $LANG to $MyLANG.

MyLANG=$LANG

4. List all current shell variables.

set
set|more on Ubuntu/Debian

5. List all exported shell variables.

env
export
declare -x

6. Do the env and set commands display your variable ?

env | more
set | more

6. Destroy your answer variable.

unset answer

7. Create two variables, and export one of them.

var1=1; export var2=2

8. Display the exported variable in an interactive child shell.

bash
echo $var2

9. Create a variable, give it the value 'Dumb', create another variable with value 'do'. Use echo and the two variables to echo Dumbledore.

varx=Dumb; vary=do
echo ${varx}le${vary}re
solution by Yves from Dexia : echo $varx'le'$vary're'
solution by Erwin from Telenet : echo "$varx"le"$vary"re

10. Find the list of backslash escaped characters in the manual of bash. Add the time to your PS1 prompt.

PS1='\t \u@\h \W$ '