Linux in a Nutshell, Fifth Edition

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Chapter 6: The Bash Shell and Korn Shell

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

Built-in Shell Variables

Built-in variables are automatically set by the shell and are typically used inside

shell scripts. Built-in variables can make use of the variable substitution patterns

shown previously. Note that the $S is not actually part of the variable name,

although the variable is always referenced this way. The following are available in

any Bourne-compatible shell.

Bash and the Korn shell automatically set these additional variables.

ksh93 automatically sets these additional variables. Variables whose names

contain “.” must be enclosed in braces when referenced—e.g., ${.sh.edchar}.

Variable Meaning

$# Number of command-line arguments.

$- Options currently in effect (arguments supplied on command line or to set).

$? Exit value of last executed command.

$$ Process number of current process.

$! Process number of last background command.

$0 First word; that is, command name. This will have the full pathname if it was found via a PATH

search.

$n Individual arguments on command line (positional parameters). The Bourne shell allows only nine

parameters to be referenced directly (n = 1–9); Bash and the Korn shell allow n to be greater than

9 if specified as ${n}.

$*, $@ All arguments on command line ($1 $2 …).

"$*" All arguments on command ...