Re: Cron Jobs That Run For Too Long

On Unix, there are three common ways to solve this.

The first way is to have a lockfile that's always around. When your script starts, it tries to lock that file with flock; if it's already locked it exits, otherwise it does its work. The OS will automatically remove the lock when the process exits, even if it crashes or is killed.

The second way is to write the PID of the process into the lockfile. Then when another copy starts up, it checks if the lockfile exists, and if so it reads the PID and verifies that that PID is still running and is actually itself. If it finds that the process is no longer running, it just creates a new lock file and goes on. When the process finishes it removes the lockfile.

The third way is to get a list of all processes, and see if any of those are executing that script. If so, exit.

I generally favor the first of these, because it's easy and efficient.

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Re^2: Cron Jobs That Run For Too Long by ruoso (Curate) on Dec 20, 2005 at 22:07 UTC
Just a note, adding to what sgifford said... I've seen some scripts around listing /proc to check if the process is still running. I prefer to use a kill 0 on the proccess... And I, personally, prefer the second way because it can be used for lockfiles even if the only place you can write the lockfile can't provide locking, or the locking isn't reliable (NFS) daniel	[reply]
Re^3: Cron Jobs That Run For Too Long by adamk (Chaplain) on Dec 20, 2005 at 23:22 UTC
But wait, it gets better again! You know how you can have that nifty __DATA__ block at the end of your script? It turns out you can lock that too :) I've used this a number of times and it works just great. #!/usr/bin/perl use strict; use Fcntl 'LOCK_EX', 'LOCK_NB'; unless ( flock DATA, LOCK_EX \| LOCK_NB ) { print STDERR "Found duplicate script run. Stopping\n"; exit(0); } ... 1; ### DO NOT REMOVE THE FOLLOWING LINES ### __DATA__ This exists to allow the locking code at the beginning of the file to work. DO NOT REMOVE THESE LINES!	[reply]
Re^4: Cron Jobs That Run For Too Long by LanX (Saint) on Apr 23, 2021 at 16:02 UTC
(For the record if someone reads this thread again) Cool trick, but strongly discouraged on Windows. See Re^2: File lock demo and other links in that thread for explanation. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply]
Re^5: Cron Jobs That Run For Too Long by Bod (Parson) on Apr 23, 2021 at 17:52 UTC
Re^6: Cron Jobs That Run For Too Long by LanX (Saint) on Apr 23, 2021 at 19:21 UTC
Some notes below your chosen depth have not been shown here
Re^4: Cron Jobs That Run For Too Long by astroboy (Chaplain) on Dec 23, 2005 at 18:40 UTC
hey, that's really nice.	[reply]
Re^4: Cron Jobs That Run For Too Long by Anonymous Monk on Jul 12, 2007 at 15:36 UTC
this works great!!! can you give explenations on the __DATA__ part? I dont have a clue what is this for... Roy	[reply]
Re^5: Cron Jobs That Run For Too Long by marto (Cardinal) on Jul 12, 2007 at 15:46 UTC
Re^4: Cron Jobs That Run For Too Long by Anonymous Monk on Oct 21, 2016 at 19:08 UTC
11 years later this is still brilliant.	[reply]