special positional parameters 
--
$0 basename (absoulte command ie: /home/cpfong/bin/test.sh) of the program as it was called. 
$1 .. $9 are the first 9 additional parameters the script was called with. 
$@ is all parameters $1 .. whatever. 
$* is similar, but does not preserve any whitespace. As a general rule, use $@ and avoid $*. 
$# is the number of parameters the script was called with. 

$$ and $! are process numbers. 
$$ is the PID (Process IDentifier) of the currently running shell.
$! is the PID of the last run background process. This is useful to keep track of the process as it gets on with its job.

$ list='a b c d'
$ set -- $list

$ printf %s "$@"	#abcd
$ printf "$1"		#a
$ printf %s "$*"	#a b c d