in reply to Re: Using perl as a cron script?
in thread Using perl as a cron script?

Remember to take into account the execution time of the code though...I don't know how heavy the functionality is, but if it takes a while to execute; your start times will gradually become offset. For example, if your processing takes 2 minutes and you sleep for 3 hours starting at noon...
12:00 execute 1
15:02 execute 2
18:04 execute 3
etc...

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This may be acceptable but if your caveat is that your script starts every three hours this method is probably not what you're looking for. Perhaps a shell script Dmn wrapper that executes your script in the background without waiting for it to complete to start the sleep cycle would be more effective.
-Jason

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Re:x3 Using perl as a cron script? (dynamic sleeping)
by grinder (Bishop) on Jun 28, 2002 at 20:38 UTC
    if it takes a while to execute; your start times will gradually become offset. For example, if your processing takes 2 minutes and you sleep for 3 hours starting at noon...

    If start time drift is a problem, you can stay in sync by simply reading the current time and then calculating how long you want to sleep, in order to wake up at the desired time.

    The following snippet does just that. My internal mail server (running Bloatus Notes) has to be shut down for backups. Rather than sending mail (created by other night-time batch jobs) to the external server, I just sleep until a point in time when I know it will be available.

    use Time::Local;

# sleep a while until the mail server comes back up
my @later_bits   = (localtime)[0..5];
@later_bits[2,1] = (8, 15); # 08:15
my $later        = timelocal( @later_bits );
my $duration     = $later > $now ? $later - $now : 0;
sleep $duration;

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    Modifying this to allow for sleeping up until midday modulo 3 is left as an exercise to the reader. Note that this technique lets you do cool stuff like "run every 8 minutes", which is not possible with standard cron.

    update: while I do use Paul Vixie's cron when I can, it's not always present. On Solaris, for instance, you can only specify comma-separated values (at least on the O/S version I use at the moment).

    I would be hesitant of of using */8 in a crontab entry, because 60 is not evenly divisible by 8. I've never bothered to see what it would do, but I have a sneaking suspicion that it would fire off your script at 0, 8, 16 ... 48 and 56 minutes of the hour. Thus, at the end of the hour the script will have two runs 4 minutes apart.


    print@_{sort keys %_},$/if%_=split//,'= & *a?b:e\f/h^h!j+n,o@o;r$s-t%t#u'
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[d/l]
      What is a "standard cron" in your books?

      My Vixie Cron (standard on Linux?) has allowed me to put */8 in the minutes field for ages...

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        Clarification. By "my" I mean the copy of cron that is on my machine. I didn't write it!
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