On Fri, Oct 1, 2021 at 3:37 PM antlists <antli...@youngman.org.uk> wrote:
> I'm trying to get a systemd unit to fire on boot once a week. Reading up
> on timer units, I can't work out how to get it to work.
>
> This is tied up with my earlier systemd mount post - I've now got that
> sorted - I've got dm-integrity to fire before fstab.
>
> I now want to run lvm snapshot on the first boot of the weekend. Writing
> a unit to do the snapshot seems pretty easy, but obviously I don't want
> it firing every boot, if I stick the date in the volume name I don't
> want it colliding with an earlier run the same day, etc etc.
>
> The question really is - if I have a weekly timer fire and activate the
> unit, is the activation going to survive the reboot to run on the next
> boot?
>
> The problem I'm having is that all the stuff I've seen about timers says
> you have two lines - activate on Saturdays, and activate on boot. Snag
> is, they seem to be independent such that EITHER condition will activate
> the service. As I say, I want BOTH. I don't want the service running
> while the system is up and running.
>
I'm not sure timer units have enough flexibility to do what you want.
I think it would be much simpler to have a Type=oneshot service at boot,
and the Exec= line to call a script. You can store the timestamp of the
last time it was called someplace in the filesystem (say,
/var/lib/my-script or something), and if the timestamp doesn't exists or is
older than one week, the scripts executes lvm snapshot and updates the
timestamp; otherwise it ends without doing anything.
The timer units are very similar to cron, and I believe what you want
cannot be done with cron either; you need special logic and state ("I'm
booting AND haven't run this in at least a week"), so a script is necessary
(I think). Luckly, systemd allows you to smartly manage your scripts and
impose dependencies on them (you need /var in my example, and you can set
it to run before starting X).
Regards.
--
Dr. Canek Peláez Valdés
Profesor de Carrera Asociado C
Departamento de Matemáticas
Facultad de Ciencias
Universidad Nacional Autónoma de México
