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Linux in a Nutshell, Fourth Edition
book

Linux in a Nutshell, Fourth Edition

by Ellen Siever, Stephen Figgins, Aaron Weber
June 2003
Beginner to intermediate content levelBeginner to intermediate
944 pages
43h 1m
English
O'Reilly Media, Inc.
Content preview from Linux in a Nutshell, Fourth Edition

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 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.

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 issued background command.

$-

Current ...

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Publisher Resources

ISBN: 0596004826