Historical origin of cron's day-of-month/weekday behavior?

Tue Oct 18 23:50:39 EDT 2011

It strikes me that this is a limitation of cron, period -- specifically, the
lack of "nth occurrence of day of week" field.  Which is why the paragraph you
quoted ends "One can, however, achieve the desired result by adding a test to
the command...", said test then being illustrated:
       0 4 8-14 * *    test $(date +%u) -eq 6 && echo "2nd Saturday"

The fact that he calls it out, with an example of a work-around, makes me think
that the historical thinking is simple: there wasn't any.  It was -- at least
to Paul Vixie -- an edge case he hadn't considered when he came up with the
initial syntax for crontab, and couldn't gracefully retrofit.  That being said,
everything I just wrote is, obviously, supposition; you could ping Teh Source
[sic]; I imagine he'd enjoy a query as to his rationale: paul-at-vix-dot-com.

-Ken

On Tue, 18 Oct 2011 23:29:03 -0400 John Abreau <jabr at blu.org> wrote

> My experience is exactly the opposite. I've never run into a situation
> you describe, but it would be trivially easy to specify it with two
> cron entries: one specifying Mondays, and the other specifying
> the first of the month.
> 
> On the other hand, my "odd case" that you claim is not particularly
> useful is one that I see all the time. Pretty much every program I've
> ever seen for scheduling events includes this, cron being the
> most notable exception. For the few others that lacked it, I recall
> reading reviews in magazines like PC World giving them bad marks
> for leaving out the feature. This leads me to believe that I'm not the
> only person on the planet who finds this functionality particularly useful.
> 
> If you want other examples other than my user group, my local
> chapter of Toastmasters meets on the first and third Tuesdays
> of every month. At my previous job, we had a monthly staff meeting
> on the fourth Friday of every month. Back when the Boston Computer
> Society still existed, pretty much every BCS SIG would meet once a
> month on the n'th $WEEKDAY of the month. Doesn't GNHLUG
> also schedule its meetings in this manner?
> 
> In any case, my question was about the original author's reasoning.
> I've poked around a bit more and learned that Brian Kernighan
> was the original author, but I still haven't found anything where
> he explains why he chose that behavior.
> 
> Did you read somewhere that your use case was actually what
> Brian had in mind? If so, can you share a link to the article
> or point me to the book or magazine issue that you read it in?
> 
> 
> 
> On Tue, Oct 18, 2011 at 10:45 PM, Ken D'Ambrosio <ken at jots.org> wrote:
> > Not trying to sound snooty, but this seems so obvious to me that I almost
> > wonder if I'm missing the point.  All the other fields deal with different
> > units -- minutes, hours, months.  day-of-month and weekday both apply to
> > days; so, if you wanted something to execute on both Mondays *and* the
> > first day of the month (say, a process that ensured that a given week/month
> > was started afresh), then you'd use both of the fields.  The opposite
case,
> > where things are ANDed -- the intersection of the day of the month with the
> > day of the week -- is an odd enough set that I don't see it being
> > particularly useful. >
> > Have I, perhaps, misunderstood something?
> >
> > $.02, YMMV, etc.,
> >
> > -Ken
> >
> >
> > On Tue, 18 Oct 2011 22:25:31 -0400 John Abreau <jabr at blu.org> wrote
> >
> >> One thing that's always annoyed me about cron is how it handles the
> >> weekday field differently from the other fields. The first four fields,
> >> minute, hour, day-of-month, and month, are logically ANDed, but
> >> the day-of-month and weekday fields are ORed.
> >>
> >> The man page describes the behavior, but does not explain the
> >> reasoning behind it:
> >>
> >> >     Note: The day of a command's execution can be specified by two
> >> > fields     -- day of month, and day of week.  If both fields are
> >> > restricted (ie,     are not *), the command will be run when either
> >> > field matches the     current time.  For example, ''30 4 1,15 * 5''
> >> > would cause a command to     be run at 4:30 am on the 1st and 15th of
> >> > each month, plus every Friday. >>
> >> I've looked for an explanation for this in the past, but I've never
> >> had any luck finding one. Making this a special case makes the code
> >> needlessly more complicated and fragile, and it sacrifices useful
> >> functionality; if the weekday had been ANDed like the other fields,
> >> it would be trivially easy to specify things like "third Wednesday
> >> of the month".
> >>
> >>  I'm unable to find or think of a use case that would make the special
> >> behavior useful, and I have to wonder why it was designed this way.
> >>
> >> Can anyone point me to the original author's thoughts on this?
> >>
> >>
> >>
> >> --
> >> John Abreau / Executive Director, Boston Linux & Unix
> >> Email jabr at blu.org / WWW http://www.abreau.net / PGP-Key-ID 0xD5C7B5D9
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> >
> >
> >
> >
> >
> >
> 
> 
> 
> -- 
> John Abreau / Executive Director, Boston Linux & Unix
> Email jabr at blu.org / WWW http://www.abreau.net / PGP-Key-ID 0xD5C7B5D9
> PGP-Key-Fingerprint 72 FB 39 4F 3C 3B D6 5B E0 C8 5A 6E F1 2C BE 99