Linux in a Nutshell, Third Edition

Variables

This subsection describes the following:

• Variable substitution

• Variable modifiers

• Predefined shell variables

• Formatting for the prompt variable

• Sample .cshrc file

• Environment 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	Description
${ var }	The value of variable var.
${ var [ i ]}	Select word or words in position i of var. i can be a single number, a range m-n, a range -n (missing m implies 1), a range m- (missing n implies all remaining words), or * (select all words). i also can be a variable that expands to one of these values.
${# var }	The number of words in var.
${#argv}	The number of arguments.
$0	Name of the program.
${argv[ n ]}	Individual arguments on command line (positional parameters); 1 ≤ n ≤ 9.
${ n }	Same as ${argv[ n ]}.
${argv[*]}	All arguments on command line.
$*	Same as {$argv[*]}.
$argv[$#argv]	The last argument.
${? var }	Return 1 if var is set, 0 if var is not set.
$$	Process number of current shell; useful as part of a filename for creating temporary files with unique names.
${?name}	Return 1 if name is set, 0 if not.
$?0	Return 1 if input filename is known, 0 if not.

Examples

Sort the third through last arguments and save the output in a file whose name is unique to this process:

                  sort $argv[3-] > tmp.$$

Process .cshrc commands only if the shell is interactive (i.e., ...