shell script $0,$n,$*,$#,$$,$!,$?,$-,$@

$0 The execution name of this program
$n The nth parameter value of this program, n=1..9
$* All parameters of this program, this option can have more than 9 parameters.
$# number of parameters for this program
$$ The PID of this program (the current number the script is running on)
$! PID of the last background command executed (the number of the last process running in the background)
$? Return value of the last command executed (shows the exit status of the last command. 0 means no error, anything else means error)
$- Displays the current options used by the shell, the same as the set command
$@ is similar to $*, but can be used as an array