[Chicago-talk] pid files and whatchamacalits.

Steven Lembark lembark at wrkhors.com
Tue Dec 9 22:32:15 CST 2003

-- "Dooley, Michael" <Dooley.Michael at con-way.com>

> I have a perl script that runs via cron.
> outside of checking for a pid file does anyone have any cleaner ways of
> checking to see if the script is already running?
> I was looking at Proc::PID_File. but I would like to try and stay with the
> default install of perl.
> This is perl, v5.8.0 built for sun4-solaris
> Is below a standard way to do this?
> if (-e "/tmp/pid.file") {
> exit;
> } else {
> open (PID, ">/tmp/pid.file");
> print PID $$;
> close PID;
> }

Logic race: you can have multiple proc's checking the file's
existance and blowing up trying to create the file...

Check File::Temp for a better way (hands back a path and
open FD in one pass).

	use File::Temp;
	use File::Basename;

	my $base = basename $0;

	if( my($fh,$path) = tempfile $base . "XXX", DIR => '/var/run' )
		print $fh $$
		close $fh;

		print STDERR "Pidfile: $fh";


	unlink $path;

This'll give you a guaranteed pidfile.

If what you really want is a semaphore file then consider
using either a directory (mkdir is a system call on *NIX
and is therefore atomic) or a samaphore to test for the
running program.

Steven Lembark                               2930 W. Palmer
Workhorse Computing                       Chicago, IL 60647
                                            +1 888 359 3508

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