How to make sure only one instance of a Bash script is running at a time?

Question

I want to make a sh script that will only run at most once at any point.

Say, if I exec the script then I go to exec the script again, how do I make it so that if the f

Answer 1

Write the process id into a file and then when a new instance starts, check the file to see if the old instance is still running.