Special variables in the shell

$	File name of the current script
$n	N is a 1-based number, which represents the first argument, and $ $ for the second argument, and ${10} for the tenth argument
$*	All positional parameters (all parameters as a string)
[Email protected]	All positional parameters (each parameter as a separate string)
$#	Number of arguments passed into the script
$_	Last parameter of previous command
$?	The return value of the previous command. Returns 0 if the previous command succeeds, otherwise returns a value other than 0
$$	PID of the current process
$!	PID of the last process running in the background
$-	Displays the options currently used by the shell

Special variables in the shell