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

(
        if flock -n 9
        then
                echo 'Not doing the critical operation (lock present).'
                exit;
        fi

        # critical section goes here

) 9>'/run/lock/some_lock_file'
rm -f '/run/lock/some_lock_file'

From example in flock(1) man page. Very practical for using in shell scripts.