Shell Built in Variables

Shell Built in Variables	Meaning
$#	Number of command line arguments. Useful to test no. of command line args in shell script.
$*	All arguments to shell
$@	Same as above
$-	Option supplied to shell
$$	PID of shell
$!	PID of last started background process (started with &)

$? - (1) If return value is zero (0), command is successful.
(2) If return value is nonzero, command is not successful or some sort of error executing command/shell script.
This value is know as Exit Status.