Meaning of variables $$, $ A, etc. in shell scripts

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.
$# the number of parameters for this program
$$ the PID of this program (the current process ID number of the script run)
$! PID of the previous background instruction (process ID number of the last process running in the background)
$? Executes the return value of the previous instruction (displays the exit status of the last command. 0 means no error, any other value indicates an error)
$-shows the current options used by the shell, same as the SET command function
[email protected] is similar to $*, but can be used as an array

Example:

1 #!/bin/bash 2 # 3 printf "The complete list is %s\n" "$$" 4 printf "The complete list is %s\n" "$!" 5 printf "The complete list is %s\n" "$?" 6 printf "The complete list is %s\n" "$*" 7 printf "The complete list is %s\n" "[email protected]" 8 printf "The complete list is %s\n" "$#" 9 printf "The complete list is %s\n" "$0"10 printf "The complete list is %s\n" "$1"11 printf "The complete list is %s\n" "$2

Results:

[[email protected] ~]$ bash params.sh 123456 QQThe complete list is 24249The complete list isThe complete list is 0The complete list is 123456 QQThe complete list is 123456The complete list is QQThe complete list is 2The complete list is params.shThe complete list is 123456The complete list is QQ

Meaning of variables $$, $ A, etc. in shell scripts