Shell Built in Variables

http://www.freeos.com/guides/lsst/misc.htm#variexceans

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 &)

Also note that you can't assigne the new value to command line arguments i.e positional parameters. So following all statements in shell script are invalid:
$1 = 5
$2 = "My Name"