Linux in a Nutshell, Third Edition

Variables

Variables are prefaced by a dollar sign ($) and optionally enclosed in braces ({}). You can assign a value to a variable through an equals sign (=); no whitespace can appear on either side of the equals sign:

$ TMP=temp.file

By default, variables are seen only within the shell itself; to pass variables to other programs invoked within the shell, see the export built-in command.

If subscripted by brackets ([]), the variable is considered an array variable. For instance:

$ DIR_LIST[0]=src
$ DIR_LIST[1]=headers
$ ls ${DIR_LIST[1]}

The contents of headers are listed. Many substitutions and commands in this chapter handle arrays by operating on each element separately.

This subsection describes:

• Variable substitution

• Built-in shell variables

Variable Substitution

In the following substitutions, braces ({ }) are optional, except when needed to separate a variable name from following characters that would otherwise be considered part of the name.

Variable	Meaning
${ var }	The value of variable var.
$0	Name of the program.
${ n }	Individual arguments on command line (positional parameters); 1 ≤ n ≤ 9.
$#	Number of arguments on command line.
$*	All arguments on command line.
$@	Same as $* but contents are split into words when the variable is enclosed in double quotes.
$$	Process number of current shell; useful as part of a filename for creating temporary files with unique names.
$?	Exit status of last command (normally 0 for success).
$!	Process number of most recently issued ...