Crontab timings confusion

whoasked asked:

Can somebody please eplain why a weekly cron job I’d assumed would run at 06:47 on Sunday morning in fact ran at 00:10 on Tuesday morning?

This is a fairly virgin Debian Stretch box (been up a while waiting for use).

I have the following crontab:

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.


# m h dom mon dow user  command
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

And the following anacrontab:

# /etc/anacrontab: configuration file for anacron

# See anacron(8) and anacrontab(5) for details.


# These replace cron's entries
1       5       cron.daily      run-parts --report /etc/cron.daily
7       10      cron.weekly     run-parts --report /etc/cron.weekly
@monthly        15      cron.monthly    run-parts --report /etc/cron.monthly

/usr/sbin/anacron exists and is executable.


Tue 29 Jan 15:26:23 UTC 2019


15:26:30 up 41 days,  7:03,  2 users,  load average: 0.00, 0.00, 0.00

Syslog shows the weekly cronjob running at 00:10 this morning (Tuesday):

Jan 29 00:10:10 RT-ARCHIVE anacron[48734]: Job `cron.weekly' started
Jan 29 00:10:10 RT-ARCHIVE anacron[49100]: Updated timestamp for job `cron.weekly' to 2019-01-29

Why this time on a Tuesday? If the system had been freshly booted I might understand it playing catchup via anacron, but it hasn’t.

Also, the job didn’t run on the Sunday, just to make that clear also.

I’ve used cron in the past without issue, what am I missing???

My answer:

The three entries in /etc/crontab run only when /usr/sbin/anacrontab is not present or not executable.

Notice that they begin with test -x /usr/sbin/anacrontab. This runs first, then because they are followed by ||, the following runs only when the previous command fails.

As the comment in the anacrontab file also states, it overrides those three entries.

Thus your weekly entry in anacrontab runs every 7 days with a 10 minute delay. Because you didn’t specify a START_HOURS_RANGE, they run at the specified number of minutes past midnight.

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