$! ,$?, $$,$@在shell中代表什么

from:http://blog.chinaunix.net/uid-639516-id-2692347.html

$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.