~monk d4vis
#!/usr/bin/fnord

You could just sleep for 10800 seconds at the end of an infinite loop. If demands are placed on the system while your script is sleeping, it will be paged out, so no harm there. You should just take care to close all your file or socket descriptors before sleeping.

Also, you could set it up to listen for SIGINT signals, and exit gracefully upon reception of such a signal.

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...
[download]

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

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;
[download]

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'

while (1) {
   # do stuff
   sleep(60*60*3);   # sleep for three hours
}
[download]

Sorry, I did not really answer the question. I would suggest that you set you script in a crontab. You could also have your program run and wait for localtime to equal the time you are waiting for. Another way if it does not need to be precise is you can do sleeps and check for the time if it is in your window run task if not go back to sleep.

Sparky
FMTEYEWTK

If Schedule::Cron won't do what you're looking for (I suspect it probably will, but I've never used it), maybe you could use something like this:

#! /usr/bin/perl

use strict;
use warnings ;
use diagnostics ;

sub schedule (&@)
{
    my ( $condition, $exec_stmt ) = @_ ;
    if ( &{$condition} )
    {
        my $pid = fork() ;
        if ( !$pid )
        {
            exec( $exec_stmt ) ;
        }
    }
}

while ( 1 )
{
    my ( $min, $hour, $mday, $mon, $year, $wday ) = ( localtime )[1..6
+] ;
    $year += 1900 ;

    # Run everyday at 12:00
    schedule { $hour == 12 && $min == 00 } '/usr/bin/some/prog' ;

    # Run every three hours
    schedule { $hour % 3 == 0 } '/usr/sbin/some/other/prog' ;

    # Every Tuesday, plus the 1st & 15th of every month at midnight.
    schedule {
           ( $hour == 0 && $min == 0 ) &&
           ( ( $wday == 2 ) || ( $mday == 1 || $mday == 15 ) )
             } '/usr/local/bin/yet/another/prog' ;

    sleep 60 ;
}
[download]

Thanks DamnDirtyApe, very useful script. I have a question, if the program sleep for 60 seconds, what prevents to run the same progran more than one time (since "hour%3 ==0" conditions is testing every 60 seconds ) ?

This post is almost 20 years old and DamnDirtyApe hasn't been here for 12 years.

But you are right, you need to check the minute too.

And you should be aware that this is a hack, rounding errors could easily lead to skipped jobs.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

This is somebody else's machine, they might not like persistent processes. On same machines it is possible to RLIMIT the total CPU time a process may take...

--
perl -pew "s/\b;([mnst])/'$1/g"

With these assumptions, this means that you would have to keep a telnet or ssh session open to the remote server in order to keep the Perl script running. As many people will find, telnet and/or ssh are not the best ways to keep a persistent connection: especially telnet sessions have a tendency to hang at unexpected moments. In this case, if it happens to hang while the script is running, the execution of the script would be terminated, resulting in no crontab running.

Just my 10 cents (yes, the price of opinions has been increased by 8 cents)

You don't have to keep a session going. On Unix just "nohup perlprog.pl &" which will start a process immune to hangups and automatically sends all output to nohup.out, see your man or info page for details.