How systemd timer work?

Question

I have myscript.service and I want this service to start every hour. So I wrote myscript.timer

Description=My script timer

[Timer]
OnCalendar=hourly
;OnCalendar=*-*-* 0/2:00:00

[Install]
WantedBy=timers.target

I started systemctl enable --now myscript.timer, then systemctl status myscript.timer And I got

[user@localhost ~]$ sudo systemctl status myscript.timer
● myscript.timer - My script timer
     Loaded: loaded (/etc/systemd/system/myscript.timer; enabled; vendor preset: disabled)
     Active: active (waiting) since Tue 2020-09-01 12:10:54 +05; 4s ago
    Trigger: Tue 2020-09-01 16:31:29 +05; 4h 20min left
   Triggers: ● myscript.service

Sep 01 12:10:54 localhost.localdomain systemd[1]: Started My script timer.

And I can't understand why it's not trigger in an hour?

Answer 1

Unfortunately, I'm not reproducing the issue you're seeing. Judging by the commented ;OnCalendar=, you've been changing the field. Are you sure that you used systemctl daemon-reload between the edit and starting the timer?

When I test it out on my system I see:

$ systemctl --user cat mytime.timer
# /home/stew/.config/systemd/user/mytime.timer
[Unit]
Description=Test timer

[Timer]
OnCalendar=hourly


$ systemctl --user start mytime.timer
$ systemctl --user status mytime.timer
● mytime.timer - Test timer
     Loaded: loaded (/home/stew/.config/systemd/user/mytime.timer; static)
     Active: active (waiting) since Tue 2020-09-01 09:49:14 CEST; 7s ago
    Trigger: Tue 2020-09-01 10:00:00 CEST; 10min left
   Triggers: ● mytime.service

Sep 01 09:49:14 stewbian systemd[1691]: Started Test timer.

Then I waited 10m for the first timer to expire and got:

$ journalctl --user -u mytime.timer -u mytime.service
-- Logs begin at Mon 2020-07-06 04:41:08 CEST, end at Tue 2020-09-01 10:00:00 CEST. --
Sep 01 09:49:14 stewbian systemd[1691]: Started Test timer.
Sep 01 10:00:00 stewbian systemd[1691]: Starting mytime.service...
Sep 01 10:00:00 stewbian systemd[1691]: mytime.service: Succeeded.
Sep 01 10:00:00 stewbian systemd[1691]: Finished mytime.service.


$ systemctl --user status mytime.timer
● mytime.timer - Test timer
     Loaded: loaded (/home/stew/.config/systemd/user/mytime.timer; static)
     Active: active (waiting) since Tue 2020-09-01 09:49:14 CEST; 10min ago
    Trigger: Tue 2020-09-01 11:00:00 CEST; 59min left
   Triggers: ● mytime.service

Sep 01 09:49:14 stewbian systemd[1691]: Started Test timer.

In this case, I used OnCalendar=hourly. The first trigger was at the start of the next hour. The second trigger is set for the start of the following hour.

---

Since I suspect the issue is a daemon-reload, I tried to reproduce your problem by changing OnCalendar=. I found:

1. If I use systemctl daemon-reload the change is applied
2. If I systemctl stop then systemctl start, the change is applied, even without a daemon-reload.
3. If I systemctl start without stopping the previous timer, the change is not applied and I get a warning about this:

$ systemctl --user start mytime.timer
Warning: The unit file, source configuration file or drop-ins of mytime.timer
  changed on disk. Run 'systemctl --user daemon-reload' to reload units.