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Mac OS X Tiger in a Nutshell by Jason McIntosh, Chuck Toporek, Chris Stone, Andy Lester

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Variables

Preface a variable by a dollar sign ($) to reference its value. You can also optionally enclose it in braces ({ }). You can assign a value to a variable through an equals sign (=) with no whitespace on either side of it:

    $ 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 followed by an index enclosed 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.

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.

Table 5-22. Substitution variables

Variable

Meaning

${var}

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 ...

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