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simoncion • yesterday at 10:44 AM • 1 reply • view on HN

This was fun to cook up and may (or may not!) break if one's locale changes:

  # Run a command on the last day of the month. Only starts checking at midnight towards the end of the month. Assumes GNU date, which is fair if we're discussing a Linux-only cron-alike.
  0 0 28-31 * * if [ $(date +\%d) -eq $(date --date="$(date +\%m)/1 + 1 month - 1 day" +\%d) ]; then /usr/local/bin/runCommand.sh; fi
I bet one could do something similar to determine if we're at the "last $NAMED_WEEKDAY in the month" by counting ahead a week and seeing if the month name changes.

If I were doing this for real, I'd either switch to a more capable cron, or take a serious try at the date math and then wrap it up as a standalone helper. Or I guess I'd look to see if someone already built that helper. ...I guess...

Replies

harshreality • yesterday at 7:32 PM

I am curious whether there's a more capable system cron that supports that kind of thing. Quartz job scheduler (java), AWS, and CF obviously wouldn't qualify. I think this is only possible if you're using a heavyweight job scheduler like those. Or... systemd.

It ultimately doesn't matter "for real", because almost nobody without some horrific legacy system to integrate with would need to use anything more than lists, ranges, and increments. There's a reason nobody's added these features to system crons. Clever trigger times for events that don't really need to be triggered at clever times... sure, but lacking that capability wouldn't change anything, the trigger would just be a more boring one.

Events like "last Friday of the month" or "nearest weekday to the 1st of the month" or even "Friday the 13th" are more for business logic, not system crontabs.