Cron job execute every 45 days possible? (approx. 1 and a half months)

cron

After reading the crontab documentation, I can't think of a way of notating the following schedule: execute script exactly once every 45 days.

Does anyone know of am elegant solution? Thank you.

Best Answer

Because a period of 45 days does not divide nicely into the weeks, months, or years of the Gregorian calendar there is no way to restrict which dates your script runs on and still have it execute on all of the days which you want.

Because of that you will need to have your command run every single day and have the command itself return without doing anything on 44 out of every 45 days.

For example if you want the script to run at 03:17 the command could look like this:

17 3 * * * /usr/local/bin/your-script

And inside the script you could do this:

#!/bin/bash
if [[ $[($(date +%s)/86400)%45] != 0 ]]
then
    return
fi

real things happen here

The != 0 part can use a different number depending on which day you want it to run the first time. And you can choose to put that extra logic in the crontab rather than in the script, though I personally find it more readable to do it in the script.

Notice that if your timezone uses DST you may need to tweak the above to avoid the schedule shifting by a day when the clock is adjusted. The exact details depend on which timezone and which time of day you choose to run your script.

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