What is $$ in Bash Shell Script- Special Variable

If you are wondering what $$ (double-dollar) is in a bash shell script, well it is a special variable that can be used to know the Process ID or PID of the current shell.

Let's try running this on a bash shell.

bash_script.sh

#!/bin/bash

echo "The process ID of the current shell is: $$"

Result:

./bash_script.sh 

The process ID of the current shell is: 4181

List of Special Variable that starts with $ and their meaning

Character	Description
$	Variable expansion
$!	Process ID of the most recent background command
$$	Process ID of the current shell
$?	Exit status of the most recently executed command
$#	Number of command-line arguments passed to the script
$*	All command-line arguments as a single string (positional parameters)
$@	All command-line arguments as separate strings (positional parameters)
$0	Name of the script itself (or the shell)
$1, $2, $3, ...	Individual command-line arguments (positional parameters)