APLawrence.com -  Resources for Unix and Linux Systems, Bloggers and the self-employed

Controlling concurrent runs with Perl

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.



Got something to add? Send me email.





(OLDER)    <- More Stuff -> (NEWER)    (NEWEST)   

Printer Friendly Version

-> -> Controlling concurrent runs with Perl


3 comments



Increase ad revenue 50-250% with Ezoic


More Articles by

Find me on Google+

© Anthony Lawrence







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

gravatar
No, it wasn't your script - just a general thing.



Thu Sep 3 20:50:32 2009: 6845   anonymous

gravatar
wow, that's cool indeed ;-)




------------------------
Kerio Samepage


Have you tried Searching this site?

Unix/Linux/Mac OS X support by phone, email or on-site: Support Rates

This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more.

Contact us





There are only two things wrong with C++: The initial concept and the implementation. (Bertrand Meyer)

Being able to break security doesn’t make you a hacker anymore than being able to hotwire cars makes you an automotive engineer. (Eric Raymond)







This post tagged: