# This is a trick used to prevent concurrent applications
# from running, but that will work WITHOUT needing pid
# or lock files, and so is kill -9 or power failure safe.
# To work it needs a __DATA__ section to be defined at
# the end of the file.
use Fcntl 'LOCK_EX', 'LOCK_NB';
unless ( flock DATA, LOCK_EX | LOCK_NB ) {
    # An existing instance of this application is already running
    print STDERR "Program is already running\n";
    exit(0);
}

__DATA__
Used as a lock to prevent concurrent execution.
Do not remove this data section.
[download]

That wasn't the variation that I thought I remembered, but it seems to work fine--if not quite how I expected*--under win32.

*It certainly prevents a second copy of the script from being run, but for reasons I dont understand yet, you don't get to see the "Program is already running\n"; message. Leastwise, not without playing games with sleeps and stuff. But it does work to the extent that if I start a script in one session and the switch to another and try again, the second attempt terminates immediately. Quite why I don't see the error message is beyond my understanding at the moment. Whether this would work across logons or userids etc I'm not sure.

I'm seem to recall a version that applied the lock using $0, but maybe I'm wrong. I'll have to super search.

Hmm. Maybe I mis-attributed this. All I can find now is Highlander - allow only one invocation at a time of an expensive CGI script and related articles. I know I saw a mechanism that applied the lock to the script itself somewhere--I was certain it was one of merlyn's.

Merlyn?

Update: Seems I did mis-attribute this--though it's maybe understandable given the stolen name :)

---

Examine what is said, not who speaks.

"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
"Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon