$$ is an internal variable, it stores the process ID of the script itself or the current bash instance.
Here is an example use case
echo $$
The output prompted.
darryl@Darryl-PC$ echo $$
8
Darryl Dias
I’m Darryl. I’m a 3D Artist, Web Developer and Linux enthusiast. On this site I share my insights, tips and tricks, tutorials, methods and best practices. More by Darryl Dias
Updated 