What's the best way to make sure only one instance of a Perl program is running?

Question

There are several ways to do this, but I'm not sure which one of them is the best.

Here's what I can think of:

• Look for the process using pgrep.
• Have the script lock itself using flock, and then check if it is locked each time it runs.
• Create a pid file in /var/run/program_name.pid and check for existence, and compare pids if needed.

There are probably more ways to do this. What do you think is the best approach?

Answer 1

There are many ways to do it. PID files are the traditional way to do it. You could also hold a lock on a file, for example the program itself. This small piece of code will do the trick:

use Fcntl ':flock';
open my $self, '<', $0 or die "Couldn't open self: $!";
flock $self, LOCK_EX | LOCK_NB or die "This script is already running";

One advantage over PID files is that files automatically get unlocked when the program exits. It's much easier to implement in a reliable way.