wiki/notebook/linux.systemd.timers.org

systemd timers

• Timer management
• Timedate formats

You can schedule tasks with timers. If systemd is compiled with the feature, it makes cron unnecessary.

Timer management

In a fully-installed systemd-enabled system, there are multiple ways to manage your timers.

While managing them is practically the same as any other units (see systemd), there are timer-specific ways to manage them easier.

• systemctl list-timers is the go-to command that displays an overview of all active timers. Just give the --all flag to list all timers including disabled timers.
• systemctl status ${TIMER_UNIT} is another way for a specific timer unit. It gives the same details as the list-timers subcommand so you'll rarely use this subcommand in practice.

Timedate formats

systemd has different ways to denote time.

• Timespans denote the duration — e.g., 100 seconds, 5M 3w.
• Timestamps refer to a specific point in time — e.g., 2021-04-02, today, now.
• Calendar events can refer to more than one point of time — e.g., *-*-4/2, Sun,Wed,Fri *-1/2-1/8.

To find more details about time notation, you can view the systemd.time.7 manual page.

Here's an example of setting a timer for an example backup service. The following timer unit sets it to execute every day at 18:00.

[Unit]
Description=A deduplicated backup from my computer
Documentation=man:borg(1) https://borgbackup.readthedocs.io/

[Timer]
Unit=borg-backup.service
OnCalendar=*-*-* 18:00:00
Persistent=true

[Install]
WantedBy=graphical.target

If the timer unit is started, this will trigger borg-backup.service from the load path. But you can omit it if you named the timer unit file similarly (e.g., borg-backup.timer with borg-backup.service).

You can find more information about it from the systemd.timer.5 manual page. Furthermore, systemd has a testing tool for time with systemd-analyze {timespan,timestamp,calendar}.

printf "Timespan example:\n"
printf "..............\n"
systemd-analyze timespan 4000min
printf "..............\n\n"

printf "Timestamp example:\n"
printf "..............\n"
systemd-analyze timestamp 2021-07-01
printf "..............\n\n"

printf "Calendar example:\n"
printf "..............\n"
systemd-analyze calendar "*-1/4-5 0/2:00:00"
printf "..............\n\n"

Timespan example:
..............
Original: 4000min
      μs: 240000000000
   Human: 2d 18h 40min
..............

Timestamp example:
..............
  Original form: 2021-07-01
Normalized form: Thu 2021-07-01 00:00:00 PST
       (in UTC): Wed 2021-06-30 16:00:00 UTC
   UNIX seconds: @1625068800
       From now: 11h ago
..............

Calendar example:
..............
  Original form: *-1/4-5 0/2:00:00
Normalized form: *-01/4-05 00/2:00:00
    Next elapse: Sun 2021-09-05 00:00:00 PST
       (in UTC): Sat 2021-09-04 16:00:00 UTC
       From now: 2 months 4 days left
..............