Systemd timer units

pdo debian eng sw systemd

These are the notes of a training course on systemd I gave as part of my work with Truelite.

.timer units

Configure activation of other units (usually a .service unit) at some given time.

The functionality is similar to cron, with more features and a finer time granularity. For example, in Debian Stretch apt has a timer for running apt update which runs at a random time to distribute load on servers:

# /lib/systemd/system/apt-daily.timer
[Unit]
Description=Daily apt download activities
After=network-online.target
Wants=network-online.target

[Timer]
OnCalendar=*-*-* 6,18:00
RandomizedDelaySec=12h
Persistent=true

[Install]
WantedBy=timers.target

The corresponding apt-daily.service file then only runs when the system is on mains power, to avoid unexpected batter drains for systems like laptops:

# /lib/systemd/system/apt-daily.service
[Unit]
Description=Daily apt download activities
Documentation=man:apt(8)
ConditionACPower=true

[Service]
Type=oneshot
ExecStart=/usr/lib/apt/apt.systemd.daily update

Note that if you want to schedule tasks with an accuracy under a minute (for example to play a beep every 10 seconds when running on battery), you need to also configure AccuracySec= for the timer to a delay shorter than the default 1 minute.

This is how to make your computer beep when on battery:

# /etc/systemd/system/beep-on-battery.timer
[Unit]
Description=Beeps every 10 seconds

[Install]
WantedBy=timers.target

[Timer]
AccuracySec=1s
OnUnitActiveSec=10s

# /etc/systemd/system/beep-on-battery.service
[Unit]
Description=Beeps when on battery
ConditionACPower=false

[Service]
Type=oneshot
ExecStart=/usr/bin/aplay /tmp/beep.wav

See: