Variables set automatically by shell:

Variable	Description
$#	Number of command-line arguments.
$-	Options currently in effect (arguments supplied to sh 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.
$n	Individual arguments on command line (positional parameters). The Bourne shell allows only nine parameters to be referenced directly (n = 1-9); the Korn shell allows n to be greater than 9 if specified as ${n}.
$*	All arguments on command line ("$1 $2...").
$@	All arguments on command line, individually quoted ("$1" "$2" ...).

Variables set automatically by Korn shell:

Variable	Description
ERRNO	Error number of last system call that failed.
LINENO	Current line number within the script or function.
OLDPWD	Previous working directory (set by cd).
OPTARG	Name of last option processed by getopts.
OPTIND	Numerical index of OPTARG.
PPID	Process number of this shell's parent.
PWD	Current working directory (set by cd).
RANDOM[=n]	Generate a new random number with each reference; start with integer n, if given.
REPLY	Default reply used by select and read.
SECONDS[=n]	Number of seconds since the shell was started, or, if n is given, number of seconds + n since the shell started.