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Today we’ll be covering shell variables, search path, and aliases.

Shell Variables

Variables serve a number of purposes in programming environments, as well as Linux. You can use them to store data that will be called later, and come in handy when you’re not sure what the value will be, but still want to setup a macro to run certain behaviors.

You can define variables and their values by assigning them:

$ MYVAR = 3

To refer to a value, simply place a dollar sign in front of the variable name:

$ echo $MYVAR
3

Some variables are standard and commonly defined by your shell upon login.

Variable	Meaning
DISPLAY	The name of your X window display
HOME	The name of your home directory
LOGNAME	Your login name
MAIL	Path to your incoming mailbox
OLDPWD	Your shell’s previous directory
PATH	Your shell search path: directories seperated by colons
PWD	Your shell’s current directory
SHELL	The path to your shell, e.g., /bin/bash
TERM	The type of your terminal, e.g., xterm or vt100
USER	Your login name

To see a shell’s variables, run:

$ printenv

The scope of the variable (i.e., which programs know about it) is, by default, the shell in which it’s defined. To make a variable and its value available to other programs your shell invokes (i.e., subshells), use the export command:

$ export MYVAR

or the shorthand:

$ export MYVAR=3

Your variable is now called an environment variable, since it’s available to other programs in your shell’s “environment.” To make a specific value available to a specific program just once, prepend variable=value to the command line:

$ echo $HOME
/home/george
$ HOME=/home/sally echo “My home is $HOME”
My home is /home/sally
$ echo $HOME
/home/george

As you can see, the original value is unaffected.

Search Path

A very important variable is PATH, which instructs the shell where to find programs. When you type any command:

$ who

the shell has to find the program(s) in question. It consults the value of PATH, which is a sequence of directories separated by colons:

$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/george/bin


and looks for the who command in each of these directories. If it finds who (say, /usr/bin/who), it runs the command. Otherwise, it reports:

bash: who: command not found

To add directories to your shell’s search path temporarily, modify its PATH variable. For example, to append /usr/sbin to your shell’s search path:

$ PATH=$PATH:/usr/sbin
$ echo $PATH
/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/home/george/bin:/usr/sbin

To make this change permanent, modify the PATH variable in your startup file ~/.bash_profile, then log out and log back in.

Aliases

The built-in command alias defines a convenient shorthand for a longer command, to save typing. For example:

$ alias ll='ls -l'

defines a new command ll that runs ls -l:

$ ll
total 436
-rw-r--r-- 1 george 3584 Oct 11 14:59 file1
-rwxr-xr-x 1 george 72 Aug 6 23:04 file2
...


Define aliases in your ~/.bashrc file to be available whenever you log in. To see all your aliases, type alias. If aliases don’t seem powerful enough for you (since they have no parameters or branching), run info bash, and read up on “shell functions”.

That’s it for today. Tomorrow, I’ll cover Input/output redirection, Pipes, Combining commands, Quoting, Escaping, Command-line editing, Command history, and Filename completion. There will be a lot of content covered tomorrow, so be sure not to miss it!

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Tags: command line, linux

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