Sometimes you have a program that can't be run by more than one person, or one that must run frequently but you don't know for sure how long an instance of it will take.
One way to accomplish that is to use a lock file.
#!/usr/bin/perl5 open(CNT, "/tmp/mylockfile"); flock CNT,2; # program hangs here until other instance is done # .. other processing when it is free # release the lock flock CNT,8; close CNT;
If an instance is running, another instance will be stopped at the "flock CNT,2;" line and won't continue until the first program executes "flock CNT,8;".
But what if you don't want to just hang? An easy way to do that is to write your PID to a file:
#!/usr/bin/perl open (L, "/tmp/mypid.lock"); $pid=<L>; close L; $stat=kill 0, $pid ; chomp $stat; chomp $pid; if ($stat and $pid) { print "\nExiting because $pid exists\n"; exit 1; } open (L, ">/tmp/mypid.lock"); print L "$$\n"; close L; .. other code
The "kill 0" actually doesn't even send a signal; it just checks to see if the process exists. If it does, we print a message and exit; otherwise the code continues.
These are two simple ways to control program execution with Perl.
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More Articles by Anthony Lawrence © 2012-07-14 Anthony Lawrence
FORTRAN—the "infantile disorder"—, by now nearly 20 years old, is hopelessly inadequate for whatever computer application you have in mind today: it is now too clumsy, too risky, and too expensive to use. (Edsger W. Dijkstra)
Mon Dec 12 15:31:40 2005: 1430 bruceg
Thanks, Tony! Did you have me in mind, with my kludgy shell script I sent you a while back? This looks like what I need, intead of the shell madness I was unleashing on that poor server :-)
- Bruce
Tue Dec 13 21:43:01 2005: 1436 TonyLawrence
No, it wasn't your script - just a general thing.
Thu Sep 3 20:50:32 2009: 6845 anonymous
wow, that's cool indeed ;-)
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Controlling concurrent runs with Perl Copyright © December 2005 Tony Lawrence
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