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Comment by simoncion | Hacker News Reader

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...

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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.

> I am curious whether there's a more capable system cron that supports that kind of thing.

I am no cron scholar, nor am I young enough to have the energy required to do an exhaustive survey... so -sadly- I don't know of one. It seems like there'd be one being used internally in at least one business in the world, though, yanno? That just seems like the sort of thing that at least one bored programmer would take a few days to crank out.

> 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.

Oh quite possibly, yeah. But, system crons are quite fine for one-shot things that are re-run on a regular schedule... as well as things that don't have complicated scheduling and/or retry requirements. If you're working on a Big Enterprise Project [0], then you're almost certainly going to have a scheduler inside of you, and -IME- you're very likely to use it to do a lot of that BEP's scheduled tasks. [1]

[0] Or a tiny one wearing the clothes of a BEP

[1] ...if for no other reason than it requires little effort to get the BEP's data and code into its internal scheduler, and can be a huge pain in the ass to get it into an external one.

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