Unix in a Nutshell, 4th Edition

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 $ 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:

$#	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 line as one string ("$1 $2..."). The values are separated by the first character in IFS.
"$@"	All arguments on command line, individually quoted ("$1" "$2" ...).

Bash and the Korn shell automatically set these additional variables:

$_	Temporary variable; initialized to pathname of script or program being executed. Later, stores the last argument of previous command. Also stores name of matching MAIL file during mail checks. ...