cron job every 5 hours

[Garrett Cooper]

Fredrik Tolf wrote:
> Olivier Nicole <on at cs.ait.ac.th> writes:
>
>   
>>>     Something like:
>>>
>>>     minute */5 * * * root path/to/scriptname
>>>
>>>     will do the trick.
>>>
>>>     Substitute the * in */5 for your desired start time (* being 0).
>>>
>>> -Garrett
>>>
>>> PS crond won't do 5 hours and every x number of minutes per job (5 hours 
>>> + x mins from end to start), just a flat amount of time (5 hours apart 
>>> from start to start). If you need that type of 'precision', at will 
>>> solve that like Olivier said if you place it at the end of the command.
>>>       
>> I am afraid not.
>>
>> */5 means on every hours that is a multiple of 5, not every five
>>  hours. So it will run every day at hour 0, 5, 10, 15 and 20. Between
>>  hour 20 one day and hour 0 the next day there is only 4 hours, not
>>  the "every 5 hours" requested.
>>     
That's what I meant >_>..
>> Just to confirm that I launched a cron job yesterday:
>>
>> 23 */5 * * * /home/java/on/crontest
>>
>> It ran at 15:23, 20:23 and today at 0:23 and 5:23 and so on:
>>
>> Date: Wed, 18 Jul 2007 05:23:00 +0700 (ICT)
>> From: Olivier Nicole <on at cs.ait.ac.th>
>> To: on at banyan.cs.ait.ac.th
>> Subject: test crontab 5 hours
>> X-Virus-Scanned: on CSIM by amavisd-milter (http://www.amavis.org/)
>>
>> This is a test for crontab
>> [...]
>> Only way to run a job every 5 hours is with at(1).
>>     
>
> I wouldn't go as far as saying the *only* way. You could make the cron
> job run every hour and then have an internal check in it (or using a
> wrapper script that checks it). Kind of like this, maybe?
>
> #!/bin/sh
> unset nogo
> if [ -r /tmp/lastrun ]; then
>     now=`date +%H`
>     if [ $((($now + 24 - `cat /tmp/lastrun`) % 24)) -lt 5 ]; then
>         nogo=y
>     fi
> fi
>
> if [ "$nogo" = y ]; then exit 0; fi
>
> date +%H >/tmp/lastrun
>
> # Do real work here
>   

    If you're going to do it that way, just try something like this:

#!/bin/sh

while [ 1 ]; do
    exec command;
    sleep 1900 # 5 hours => 5*3600;
done

    and set it up as an rc script :).

    Shell scripts with sleep won't give you exactly the 5 hours you 
desire, but should come close (within 1-5 seconds of actual time 
depending on your host PC's precision, and whether or not your RTC 
battery is dead ;)..).

-Garrett