The entry of the shell script

shell script parameters can be as many as possible, but only the first 9 can be accessed, using the shift command can change this limit. The parameter starts at the first and ends at the Nineth one.
$ A program name
$n Nth parameter value, n=1..9
$* all command-line arguments
[email protected] All command line arguments, if it is enclosed in quotation marks, like "[email protected]", each parameter is also quoted separately
$# number of command line arguments
$$ Process ID (PID) of the current process
$! Process ID of the most recent background process
$? Exit status of the most recently used command

$$ is the id,$ of the script process? Similar to the FETCH function return value in C

The entry of the shell script