run cron every 105 minutes - linux

Question

I'm not great with Linux, my apologies if this is easy.
I need to run 7 different scripts via cron at 15 minute intervals. I have worked out that each script needs to run every 105 minutes (every 1 hour & 45 minutes) as follows :
Script 1 Script 2 Script 3 Script 4 Script 5 Script 6 Script 7
00:15 00:30 00:45 01:00 01:15 01:30 01:45
02:00 02:15 02:30 02:45 03:00 03:15 03:30
etc...
Now I know I can't just put this:
*/105 * * * * something
Because the minutes will only go up to 59.
Other than writing each one in a line separately, is there an easy way to do this?
Any assistance would be greatly appreciated.

Answer 1

No, cron can't do that directly.
What you can do is have a cron job that runs every 15 minutes, invoking a wrapper script. The wrapper script can query the current time and decide which of your 7 scripts to execute.
Doing the math is left as an exercise. Don't assume that your wrapper will start exactly at the top of the minute; there can be some delay between the time a cron job is scheduled to execute and the time it actually runs, depending on system load.

Answer 2

at is a good candidate for this problem.
Write a wrapper (or build into your script):
#!/bin/bash
at -f $0 now + 105 minutes
<program>
Then use at -f <script> <time> for each instance.
Or generate the crontab entries using a script (my first draft was a bit too long to be a oneliner):
#!/usr/bin/perl -Ws
use strict;
use DateTime;
our ($period, $offset, $command);
my ($offh, $offm) = split(/:/, $offset || '0:00');
my %crontab = ();
for (my $i = 0; $i < 1440; $i += $period) {
my ($h,$m) = split /:/, 'DateTime'->today->set('hour', $offh, 'minute', $offm)->add('minutes', $i)->hms;
$crontab{$m}{$h}++;
}
printf("%02d %-15s * * *\t%s\n", $_, join(',', sort keys %{$crontab{$_}}), $command) for sort keys %crontab;
Usage:
$ ./gencrontab -period=105 -offset=1:30 -command=myprogram
00 05,12,19 * * * myprogram
15 00,03,10,17 * * * myprogram
30 01,08,15,22 * * * myprogram
45 06,13,20 * * * myprogram