So, I was tinkering around with my Raspberry
Pi (running Raspbian) over SSH and in doing so
found that when I was looking up info about commands using
man
that although
man
was still running perfectly fine, I was getting this
message:
man: can't resolve /usr/share/man/man6/LS.6.gz: No such file or
directory
The fix is actually a simple
one; just run
sudo mandb which updates man's
database caches. Looking more closely, I found that a mandb
job was actually in cron.daily which means that it should run daily.
However, when I checked the syslog with:
grep cron.daily /var/log/syslog
there were no results indicating it hadn't actually run! The reason for this is that my Pi's not switched on long enough for the cron job to run. I usually leave it on for a couple of hours and then shutdown once I've finished with it.
Installing anacron is the solution!
If you install anacron:
sudo apt-get
install anacron, you should find an
/etc/anacrontab
containing lines like:
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
This means that anacron is going to run the jobs in
/etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly.
It should do this at boot up by default (you can check /etc/init.d/anacron).
There's then a user-specified delay before anacron runs each task; here,
it's 5 minutes for /etc/cron.daily, 10 minutes for /etc/cron.weekly
and 15 minutes for /etc/cron.monthly.
If you then restart and wait five minutes or so, then do:
grep cron.daily /var/log/syslog
should show lines like:
Jan 31 10:28:02 raspberrypi anacron[2105]: Will run job `cron.daily' in
5 min.
Jan 31 10:33:21 raspberrypi anacron[2105]: Job `cron.daily' started
Jan 31 10:33:21 raspberrypi anacron[2692]: Updated timestamp for
job `cron.daily' to 2014-01-31
Jan 31 10:33:29 raspberrypi anacron[2105]: Job `cron.daily' terminated
This shows that anacron is doing exactly what we want: running the cron.daily job five minutes after we boot. It wasn't that obvious to me how the existing cron scheduler interacts with anacron (nor that easy to search for) but this site gave a very clear and helpful explanation of how the two interoperate.