A special variable “$$” can be used with in a shell script to retrieve self process ID (PID). This is useful for monitoring scripts to store their PID in a file for other maintenance script to identify it. The following script shows how it can be done.

#!/bin/bash

# store my PID
echo $$ > /var/run/monitor.pid

while [ 1 ];
do
echo I am monitoring something
sleep 5s
done

Some scripts use their PID to create unique temporary files along with some fixed prefix. The following example script shows how it is done.

#!/bin/bash

TMPFILE=myfile.$$
ls -l > $TMPFILE

However the above method serves basic purpose of creating a unique file name but is not recommended to use. Read the following articles on how to generate unique file names in a bash script.

How to generate random/temporary file name using mcookie command

How to generate temporary file name in bash using RANDOM environment variable