Keeping a process running with Flock and Cron

We've got a few processes here that aren't system services, but need to be running in the background and should be restarted if they die. However, this method can also be used for a cron that often runs past it's normal re-execution time (say a every 5 min cron that sometimes runs for 7min). It will prevent multiple executions from running simultaneously.

First off, in your crontab, you can add a line like this:

* * * * * flock -x -n /tmp/awesomenessRunning.lock -c "/usr/local/bin/myAwesomeScript.sh" >/dev/null 2>&1

What happens here is fairly straight forward:

• Every minute, flock executes your script in this case "/usr/local/bin/myAwesomeScript.sh"
• flock opens up an exclusive write lock on the lock file, here named "/tmp/awesomenessRunning.lock". When it's done executing, it'll release the lock.
• The next time this cron runs, flock will attempt to again, get an exclusive lock on that lock file... but it can't if that script is still running, so it'll give up and try again next time the cron runs.

Now, generally, if I'm doing this as a systems level item, I'll put the following in a file named for the job or what the job is doing and drop it in /etc/cron.d/. All the files there will get compiled together into the system cron, which helps other admins (or your later self) to find and disable it later. If you do that, remember to stick the user to execute the cron as between the *'s and the flock!