Re: [gentoo-user] Daylight time change and cron run twice

[Michael George]

Well, this is what I initially thought would happen.  However, my
crontab indicates that cron.daily would be removed at 3:09am.  But at
2am EDT, the time changed back to 1am EST, never having reached 3:09am
EDT.  Then after an hour it would be 2am EST, or "2am again".  See here:
http://www.webexhibits.org/daylightsaving/b2.html

But 3:09am would never have been reached last night during EDT, only at
EST, so I don't expect something after 3am would ever suffer from the
change.

What am I missing?

On Sun, Nov 04, 2012 at 04:36:02PM +0200, Alan McKinnon wrote:
> On Sun, 4 Nov 2012 08:58:54 -0500
> Michael George <george@××××××××××.com> wrote:
>
> > Last night was our change from EDT to EST.  So we went through the
> > 1am-2am hour twice.  My crontab has the following:
> >
> > # for vixie cron
> > # $Header:
> > # /var/cvsroot/gentoo-x86/sys-process/vixie-cron/files/crontab-3.0.1-r4,v
> > # 1.3 2011/09/20 15:13:51 idl0r Exp $
> >
> > # Global variables
> > SHELL=/bin/bash
> > PATH=/sbin:/bin:/usr/sbin:/usr/bin
> > MAILTO=root
> > HOME=/
> >
> > # check scripts in cron.hourly, cron.daily, cron.weekly and
> > cron.monthly 59  *  * * *    root    rm
> > -f /var/spool/cron/lastrun/cron.hourly 9  3  * * *     root    rm
> > -f /var/spool/cron/lastrun/cron.daily 19 4  * * 6     root    rm
> > -f /var/spool/cron/lastrun/cron.weekly 29 5  1 * *     root    rm
> > -f /var/spool/cron/lastrun/cron.monthly */10  *  * * *  root    test
> > -x /usr/sbin/run-crons && /usr/sbin/run-crons
> >
> > So the cron.daily file should be removed at 3:09am -- it should only
> > run once as the 2am-forward time only occurred once.
> >
> > However, this morning I have two sets of email from my daily jobs
> > running.  At this time:
> >
> > Date: Sun,  4 Nov 2012 02:20:05 -0500 (EST)
> >
> > and this time:
> >
> > Date: Sun,  4 Nov 2012 03:10:04 -0500 (EST)
> >
> > Why would it have run twice?
>
>
> This is clearly documented in the man pages for Vixie cron.
>
> cron is utterly unaware of the vagaries and stupidities that humans get
> up to recording the passage of time. If your crontab is this:
>
> 0 0 * * * <something>
>
> then when the clock says it is 0 minutes past the 0 hour, the cron will
> run.
>
> That time happened twice. Therefore the cron ran twice.
>
> You can't really blame cron for not picking up that an event that is
> guaranteed to happen once in a period actually happened twice in a
> period. There comes a time when a developer has to step back and say
>
> "To hell with that, it's out of contract spec and actually not my
> problem!"
>
> There are other crons that avoid this problem by taking a completely
> different approach. You configure them by declaring you want a job to
> run in a given period of time. When daylight saving kicks in, it sees a
> job has already run and doesn't do it again. But you have to lose the
> ability to specify the exact time a job must run at to do it this way.
>
>
> --
> Alan McKinnon
> alan.mckinnon@×××××.com
>
>

-- 
-M

Rident stolidi verba Latina.
                        -Ovid