Shell variables are a vital part to shell programming. You can assign a value to variables and print it out like this example:
user@host user=fred user@host echo $user fred
Note that there must be no spaces on either side of the "=". We can also assign the value of variables to other variables:
user@host user2=$user user@host echo $user2 fred
A variable can be set to null like this:
user@host user=
Here's an oddity about assigning variables that shows just how file system orientated the command shell is:
user@host files=/etc/ho* user@host echo $files /etc/host.conf /etc/hostname /etc/hosts /etc/hosts.allow /etc/hosts.deny user@host echo "$files" /etc/ho*
Looking at the first output, you might think that assigning "/etc/ho*" to a variable has expanded the answer to a directory listing of all files in /etc/ beginning with "ho". The second printout show that the variable is not set to a directory listing, so it's actually the echo command which is doing the file expansion. All we can learn from this is to be careful when playing with variables - if you're not sure what they are, put quotes around them!
You can see what environment variables are set by typing "env". Environment variables are used for all sorts of wonderful things. For example "PATH" is used to tell the shell where to find executable programs.
In this example we set our PATH variable to NULL to show that the shell cannot find the "uname" program without it (unless we specify the full path). Setting PATH to something that includes the path of our command helps the shell to locate "uname" again.
user@host PATH= user@host uname sh: uname: not found user@host /bin/uname Linux user@host PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin user@host uname Linux
If you want to make a variable accessible to shells run from within your shell, you need to use the "export" command. An example is the best way of demonstrating this. So, in this example we set the variable named "animal" to the value "goose". We can use this variable locally, but if we try and execute this command from another shell (using "sh -c command_string" to invoke a new shell and run command_string) we see that the new shell cannot see the variable. Exporting the variable makes it available to the new shell.
user@host animal=goose user@host echo $animal goose user@host sh -c 'echo $animal' user@host export animal user@host sh -c 'echo $animal' goose
The shell also sets some special variables, which are explained here.