Exercise: Timers and one-off jobs

Timers and one-off jobs

• Write a oneshot service and a timer and enable the timer.
• Check calendar expressions with systemd-analyze.
• Schedule and remove an at job.

Start the exercise

On workstation, start the exercise. It removes the hello units and the at package of an earlier run from servera.

[student@workstation ~]$ lab start sa-timers

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

The service

On servera as root (sudo -i), create /etc/systemd/system/hello.service: a oneshot service that logs hello from the timer with the tag hello-timer (use logger). Start it by hand and look in the journal.

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ sudo -i
[root@servera ~]# cat > /etc/systemd/system/hello.service <<'EOT'
[Unit]
Description=Write a hello line to the journal

[Service]
Type=oneshot
ExecStart=/usr/bin/logger -t hello-timer "hello from the timer"
EOT
[root@servera ~]# systemctl daemon-reload
[root@servera ~]# systemctl start hello.service
[root@servera ~]# journalctl -t hello-timer --no-pager | tail -1 | cut -c1-110
Oct 03 17:53:40 servera.lab.example.com hello-timer[1301]: hello from the timer

The timer

Create hello.timer that fires every minute (OnCalendar=*-*-* *:*:00) with Persistent=true and WantedBy=timers.target. Enable and start it, and list it.

[root@servera ~]# cat > /etc/systemd/system/hello.timer <<'EOT'
[Unit]
Description=Run hello every minute

[Timer]
OnCalendar=*-*-* *:*:00
Persistent=true

[Install]
WantedBy=timers.target
EOT
[root@servera ~]# systemctl daemon-reload
[root@servera ~]# systemctl enable --now hello.timer
Created symlink /etc/systemd/system/timers.target.wants/hello.timer → /etc/systemd/system/hello.timer.
[root@servera ~]# systemctl list-timers hello.timer --no-pager
NEXT                        LEFT     LAST PASSED UNIT        ACTIVATES
Sat 2026-10-03 17:54:00 UTC 26s left -    -      hello.timer hello.service

1 timers listed.

Watch it run

Wait a minute or two, then show how many times the message appeared and when the timer last fired.

[root@servera ~]# sleep 100
[root@servera ~]# journalctl -t hello-timer --no-pager | grep -c "hello from the timer"
3
[root@servera ~]# systemctl status hello.timer --no-pager | sed -n 1,6p
● hello.timer - Run hello every minute
     Loaded: loaded (/etc/systemd/system/hello.timer; enabled; preset: disabled)
     Active: active (waiting) since Sat 2026-10-03 17:53:33 UTC; 1min 5s ago
      Until: Sat 2026-10-03 17:53:33 UTC; 1min 5s ago
    Trigger: Sat 2026-10-03 17:55:00 UTC; 21s left
   Triggers: ● hello.service

Your count depends on how long you waited. Each run is an ordinary service run, so the output is in the journal under the unit.

Test calendar expressions

Ask systemd-analyze calendar when Mon *-*-* 09:30 and *:0/15 next elapse.

[root@servera ~]# systemd-analyze calendar 'Mon *-*-* 09:30'
  Original form: Mon *-*-* 09:30
Normalized form: Mon *-*-* 09:30:00
    Next elapse: Mon 2026-10-05 09:30:00 UTC
       From now: 1 day 15h left
[root@servera ~]# systemd-analyze calendar '*:0/15' | sed -n 1,4p
  Original form: *:0/15
Normalized form: *-*-* *:00/15:00
    Next elapse: Sat 2026-10-03 18:00:00 UTC
       From now: 9min left

*:0/15 means "every hour, starting at minute 0, every 15 minutes".

An at job

Install at, enable atd, and as student schedule echo at job ran > /home/student/at.out for one minute from now. Check the queue, wait, and read the file. Then schedule another one for 23:59 and delete it.

[root@servera ~]# dnf install -y at > /dev/null && systemctl enable --now atd
[root@servera ~]# su - student -c 'echo "echo at job ran > /home/student/at.out" | at now + 1 minute; atq'
warning: commands will be executed using /bin/sh
job 1 at Sat Oct  3 17:53:00 2026
1	Sat Oct  3 17:53:00 2026 a student
[root@servera ~]# sleep 65; cat /home/student/at.out
at job ran
[root@servera ~]# su - student -c 'echo "touch /tmp/never" | at 23:59; atq; atrm 2; atq'
job 2 at Sat Oct  3 23:59:00 2026
2	Sat Oct  3 23:59:00 2026 a student

After atrm 2 the queue is empty and the job never runs.

Grade and finish

Check your work with lab grade sa-timers, then put the exercise away with lab finish sa-timers.