Make yourself at home

Easy Setup

By Heike Jurzik

3pod, Fotolia

If you enter a command like ls in the shell, the shell knows that you want to call the /bin/ls program. Bash knows where on the filesystem to look for executables, so there is no need to type in the full path. This path is defined in what is known as an environmental variable. Variables of this kind are also used to modify the appearance of the shell prompt, set the time zone and the user's home directory, and other things.

Note that because bash (the "Bourne Again Shell") has become the default shell on Linux, all of the examples in this article assume bash as the command-line interpreter.

An environmental variable has three parts: the name, an assignment operator (=), and a value:

variable=value

The name can include upper- and lowercase letters, underlines, and numbers. The only restriction is that a table can't start with a number. If the value of the variable contains blanks or nonstandard characters, you have to double-quote your input:

LESS="-X"

Every user on a Linux system has a place he or she can call home; bash provides a fully furnished environment for every single account, but the appearance depends on the distribution you use. To discover which variables your home uses, type env (for "environment"). Listing 1 shows an example.

01 huhn@asteroid:~$ env
02 TERM=xterm
03 SHELL=/bin/bash
04 USER=huhn
05 LS_COLORS=no=00:fi=00:...
06 PATH=/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games:/home/huhn/bin
07 IRCSERVER=irc.freenode.net
08 LANG=de_DE@euro
09 ...

Some of the variables in Listing 1 can be set by the user. Other shell variables are predefined but can be modified as needed. The convention is to use uppercase letters. Table 1 gives an overview of common variables for bash.

The echo command allows you to discover the value of a variable. The command outputs everything to its right place at the command line:

huhn@asteroid:~$echo Huhu
Huhu

In addition to this, echo displays the value of the variable if you type a dollar sign in front of the variable's name:

huhn@asteroid:~$echo $LANG
de_DE@euro
huhn@asteroid:~$echo $PS1
\u@\h:\w\$

The shell replaces the variables $LANG and $PS1 with their respective values and then calls echo to output the text.

Even if you have everything set up just the way you like it, it is nice to have a change from the daily grind. You might want to refresh your language skills, so you could use the following command line to launch the Firefox browser in French, even though your environment normally speaks English:

LANG=FR firefox

Figure 1 shows how to temporarily move to a different language environment. Before calling the program, the example shows how to output a list of the locales your machine supports with the locale -a command.

Figure 1: You can change the variable temporarily without worrying about the aftereffects.

Change

As you have seen, you can easily change a variable by entering its name, the assignment operator, and the value. Variables defined in this way are only valid for the current terminal session. To export the settings, you need the export command. This makes the settings accessible to child processes and subshells.

For example, you can set the IRCNICK, IRCNAME, and IRCSERVER variables for the current bash to tell command line-based IRC clients such as Irssi or BitchX which nick and name to use to automatically log on to a specific IRC server. If you then launch a second shell from the first one and try to output the values of the new variables, you won't see a thing. Quit the second shell by typing exit (Ctrl+D), and export the new variables. Now open a new shell from the first one, and you will see that it has inherited variables, which are now available in the subshell (Figure 2).

Figure 2: The export command is used to export new variables to subshell environments.

The export builtin has more tricks up its sleeve. In combination with the -p option, or when called without any parameters, it displays a list of all exported variables. The -n option deletes a variable from the list. Listing 2 gives a couple of examples.

01 huhn@asteroid:~$ export
02 declare -x CHARSET="latin1"
03 declare -x LS_COLORS="no=00:fi=00:..."
04 declare -x IRCSERVER="irc.freenode.net"
05 ...
06 huhn@asteroid:~$ export -n IRCSERVER

Any variables that you export would only be effective in the current shell and its child processes. To set up a variable permanently, you need to modify your bash configuration file and reload your environment. To do so, add your new export commands to one of the bash startup files, such as ~/.bashrc (Figure 3), and then parse the file once you've finished your home improvements:

source ~/.bashrc

Listing 3 shows a couple of practical environmental variables, including comments, that should make you feel right at home.

Figure 3: The bashrc file contains configuration settings for the bash environment.

01 # prevents "less" from leaving an empty
02 # screen at the end of the output:
03
04 export LESS="-X"
05
06 # adds a touch of color: user prompt in
07 # green, root prompt in red:
08
09 if [ $(id -u) = 0 ] ; then
10         COLOR1='\[\033[00;31m\]'
11 else
12         COLOR1='\[\033[01;32m\]'
13 fi
14
15 COLOR2='\[\033[00;33m\]'
16 COLOR3='\[\033[00;37m\]'
17 PS1=$COLOR1'[\u@\h'$COLOR2' \W'$COLOR1']'$COLOR3'\$ '
18 PS2=$COLOR1'>'$COLOR3' '
19
20 # I want the CET time zone on my server in Canada:
21
22 export TZ=CET
23
24 # I want ispell to use the correct character set
25 # and the correct dictionary:
26
27 export DICTIONARY=ngerman
28 export CHARSET=latin1