shell 的$! ,$?, $$,$@

$n        $1 the first parameter,$2 the second...
$#        The number of command-line parameters.
$0        The name of current program.
$?        Last command or function's return value.
$$        The program's PID.
$!        Last program's PID.
$@        Save all the parameters.

almost any shell book will talk about them,from which you can get their detail usages.