30 1,3,5,7,9,11,13,15,17,19,21,23 * * * echo "run every odd half hour"
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but I agree, 0-23/2 would look nicer in the crontab

-derby

or:

30 * * * * echo "run every half hour"
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And then verify that the hour is not % 2 to run every half hour in the script.

Psudocode:

if (! $currenthour % 2) { 
dostuff; 
} else { 
exit 0; #oopsie it is an even hour just exit
}
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-Waswas

I can do one better :

30 0-23/2 * * * foo

-Matt

I not sure what platform you are on and whether you are looking for a platform independent solution.

I guess for your approach, having an alarm signal for every two hours might make more sense. But I have never tried using alarm for more than 5 minutes and am not even sure if it is supported on Win32.

BTW, this functionality is pretty straightforward to do with crontab in one line. eg: 30 1-23/2 * * * runs it every two hours, 30 min past the hour (at least it does on Linux)

For information about NT At see Schedule Win32 Perl Scripts with AT. I know that you can schedule things like that from GUI, never tried it with the API. For Windows there is also Win32::Scheduler available from http://www.roth.net/perl/packages/.

You should also look through the offerings on CPAN, eg: Schedule::Cron does exactly what you are trying to do.

Just a tongue-tied, twisted, earth-bound misfit. -- Pink Floyd

I wonder why nobody has mentioned it yet, but there is Schedule::Cron, and a nonforking subclass, Schedule::Cron::Nofork (this one is written by me but planned to be included within Schedule::Cron in the next release as far as I gather).

These modules allow you easy scheduling via a permanently running script, and then either forking or running a command, waiting for it to end.

Some code I have running on my PC (under NT) :

#!/usr/bin/perl -w

use strict;
use Schedule::Cron::Nofork;
use File::Spec;
use FindBin;
use Logger qw(my.email@my-work.net);

my $cron = Schedule::Cron::Nofork->new( sub {
  my $self = shift;
  warn "$$ Running $_[0]\n";
  system("start \"Cron: $_[0]\" \"$_[0]\"");
});

warn "Starting cron\n";
$cron->add_entry("0,5,10,15,20,25,30,35,40,45,50,55 18 * * 1-5", RUN =
+> File::Spec->catfile( $FindBin::Bin, "transfer-OPICS-Vertex-53.pl" )
+);
$cron->add_entry("0,5,10,15,20,25,30,35,40,45,50,55 18 * * 1-5", RUN =
+> File::Spec->catfile( $FindBin::Bin, "transfer-OPICS-SapBCA-53.pl" )
+);
$cron->add_entry("0,5,10,15,20,25,30,35,40,45,50,55 18 * * 1-5", RUN =
+> File::Spec->catfile( $FindBin::Bin, "transfer-OPICS-SapFi-53.pl" ))
+;
#$cron->add_entry("* * * * 1-5", RUN => File::Spec->catfile( $FindBin:
+:Bin,"test.pl"));

$cron->run();
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I start the scripts via start, because I don't need and don't trust the built-in forking simulation - it's amazing that it works, but for a simple script that dosen't really need the resource sharing features, I avoid it.

perl -MHTTP::Daemon -MHTTP::Response -MLWP::Simple -e ' ;    # The  
$d = new HTTP::Daemon and fork and getprint $d->url and exit;#spider
($c = $d->accept())->get_request(); $c->send_response( new   #in the
HTTP::Response(200,$_,$_,qq(Just another Perl hacker\n))); ' #  web
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How about scheduling your script to run every two hours (I think that's with */2, to get 0000, 0200, 0400, etc.), then have the script start with a 90 minute sleep?

select(undef, undef, undef, 0.25);
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use Time::HiRes qw(sleep);
sleep(0.25);
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use Time::Local;
use strict;

sub waityourturn {
  my $now = time;
  my $twohours = 60*60*2;
  my $onehour = 60*60;

  my ($sec,$min,$hour,$mday,$mon,$year) = localtime($twohours+$now);

  if ($hour  % 2 == 0) { # if hour is even, let's just add one hour
   ($sec,$min,$hour,$mday,$mon,$year) = localtime($onehour+$now);
  }

  $sec = 0;
  $min = 30;
  my $target_time = timelocal($sec,$min,$hour,$mday,$mon,$year);
  my $naptime = $target_time - time;

  print "I am going to sleep $naptime seconds until ", 
      scalar(localtime($target_time)), "\n";
  sleep($naptime);
}
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open (BAT, ">setup.bat") || die $!;
for (my $i = 2; $i <= 24; $i += 2) {
   print BAT "at $i:00 /every M T W Th F S Su somejob.bat\r\n";
}
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Lighthearted tone aside, I have used perl to generate everything from batch files to 3D Studio MAX scripts to perl modules.