Re: question about cron and anacron
On Fri, Apr 13, 2007 at 13:09:43 -0600, Paul E Condon wrote:
> When I moved to Etch, I noticed something strange, and
> am looking for an explanation. My cron job for doing
> daily backups started doing the job at the wrong time.
>
> It turns out that the move to Etch somehow installed
> anacron on a system that is supposed to be left on and
> it was anacron that was actually running the script.
>
> But how? I look at the code in /etc/crontab and in
> /etc/cron.daily/0anacron and I think I see that it
> is impossible for anacron to be run on a daily basis:
>
> /etc/anacron contains a line:
> 25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
Are you sure this snippet is from /etc/anacrontab? It looks like a line
from /etc/crontab to me. (I cannot find "/etc/anacron" anywhere in
Debian, so I assume you mean "...tab".)
> This line invokes run-parts only if anacron is _not_ installed. And,
>
> $ ls /etc/cron.daily/
> 00test-env 10chkpnt aptitude exim4-base logrotate modutils standard
> 0anacron apt bsdmainutils find man-db samba sysklogd
>
> And,
> script 0anacron contains:
> #!/bin/sh
> #
> # anacron's cron script
> #
> # This script updates anacron time stamps. It is called through run-parts
> # either by anacron itself or by cron.
> #
> # The script is called "0anacron" to assure that it will be executed
> # _before_ all other scripts.
>
> test -x /usr/sbin/anacron || exit 0
> anacron -u cron.daily
> <end>
>
> But how is this script executed usefully? Since cron doesn't do
> run-parts /etc/cron.daily if anacron is installed?
>
> Further, it appears that cron.daily scripts _are_ being run daily, ---
> at 7:35am. Why 7:35am? Where is that time set? It is _not_ a time
> that is in /var/spool/anacron. I can't think of anywhere else to look.
Do you by any chance boot the system around 7:30am every day? Anacron's
default delay for the daily jobs is 5 minutes.
> There is something, somewhere, that is doing this. Where?
This is how I understand it:
/etc/init.d/anacron starts anacron as a daemon in the background during
boot (unless the system is running on batteries at that moment). This
daemon handles all the jobs specified in /etc/anacrontab. The default
configuration causes anacron to take over all daily, weekly and monthly
cronjobs. Anacron determines how many days have passed since each of
these jobs was run the last time. If this period is longer than what is
specified in anacrontab then anacron will start the job after a certain
delay (which is also defined in anacrontab). The "0anacron" jobs make
sure that the timestamps in /var/spool/anacron get updated whenever
cronjobs are executed.
The "test -x /usr/sbin/anacron" statements in /etc/crontab make sure
that cron does not interfere with anacron, even though cron still takes
care of the hourly jobs itself.
--
Regards,
Florian
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