Systemd Timers

Systemd timers

This post is to reflect on systemd timers, replacing cron jobs, monitoring, useful references.

Cron jobs

Cron jobs is easy to understand scheduler standard unix scheduler. Common commands are:

• crontab -e edit jobs
• crontab -l list jobs Crontab can be defined in the following locations:
• /etc/cron.d
• /etc/cron.hourly, daily, monthy etc.

  cat /etc/cron.d/e2scrub_all 
  30 3 * * 0 root test -e /run/systemd/system || SERVICE_MODE=1 /usr/lib/arm-linux-gnueabihf/e2fsprogs/e2scrub_all_cron
  10 3 * * * root test -e /run/systemd/system || SERVICE_MODE=1 /sbin/e2scrub_all -A -r
  

  Useful site to test schedules:

• https://crontab.guru/
• https://www.baeldung.com/cron-expressions
• man 5 crontab

Systemd Timers

Systemd timers is a great replacement of cron jobs, however a tad more complicated.

Timer types

• Monotonic - periodic timers without specific time
• Realtime - Start periodically at explicit time

One-off transient timer can be activated as:

• systemd-run --on-active=30 /bin/touch /tmp/foo
• systemd-run --on-active="12h 30m" --unit someunit.service - to run specific unit

Timers live in /var/lib/systemd/timers Timers are designed to work with services. For each .timer file, a matching .service file exists. For example:

Definition

cat ./system/timers.target.wants/test-minute.timer
[Unit]
Description=Test timer: every minute

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


[Install]
WantedBy=timers.target

Service

cat /lib/systemd/system/test-minute.service
[Unit]
Description=test minute timer %a %H

[Service]
Type=oneshot
ExecStart=/bin/true
#User=nobody
Group=systemd-journal

Timer Control

• systemctl list-timers --all - list all timers
• systemctl list-timers - list active timers
• systemctl enable test-minute.timer - enable timer to run including after reboot
• If not activated, timer is shown as dead

  # systemctl status test-minute.timer
  ● test-minute.timer - Test timer: every minute
     Loaded: loaded (/lib/systemd/system/test-minute.timer; enabled; vendor preset: enabled)
     Active: inactive (dead)
    Trigger: n/a
   Triggers: ● test-minute.service
  

• start timer: # systemctl start test-minute.timer
• timer is active now

  # systemctl status test-minute.timer
  ● test-minute.timer - Test timer: every minute
     Loaded: loaded (/lib/systemd/system/test-minute.timer; enabled; vendor preset: enabled)
     Active: active (waiting) since Sun 2023-02-19 13:28:47 PST; 5s ago
    Trigger: Sun 2023-02-19 13:29:00 PST; 6s left
   Triggers: ● test-minute.service
   Feb 19 13:28:47 <host> systemd[1]: Started Test timer: every minute.
  

• systemctl daemon-reload - reload systemd to refresh recently added timers

Monitoring

journalctl -r |grep  test-minute|head
Feb 19 13:32:01 <host> systemd[1]: test-minute.service: Succeeded.
Feb 19 13:31:01 <host> systemd[1]: test-minute.service: Succeeded.

Nits

• Systemd specifiers is a great set of variables that can be used in configuring timers and including into log messages. E.g. the following timer definition results in the logging below:

  Description=test minute timer %a %H
  

  Feb 19 13:34:01 <host> systemd[1]: Finished test minute timer arm <host>.
  Feb 19 13:34:01 <host> systemd[1]: test-minute.service: Succeeded.
  Feb 19 13:34:01 <host> systemd[1]: Starting test minute timer arm <host>...
  

• Explaining timer schedule: systemd-analyze calendar "*-*-* *:*:00"

References

These are great resources about systemd timers:

• Archlinux Timers
• Systemd Specifiers