bash flock: exit if can't acquire lock

Question

The following lock mechanism is used for preventing a cron job from running concurrently:

#!/bin/bash

echo \"Before critical section\"
(
    fl         


        

Answer 1

We use exclusive lock on the script file itself, $0 is the name of command file.

exec 200<$0
flock -n 200 || exit 1

The whole solution is in two lines of code. But the trick is to open $0 for reading and then obtain exclusive lock for it.