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

flock -n -e 200 || exit 1

flock -n tells you it failed by returning a failure code (something other than zero). You could instead do set -e at the top of your script to make it exit when it sees any unchecked error.

Depending on your application, you might want to exit 0 to indicate success when the lock can't be acquired.

Answer 2

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.