Systemd/timers

Timers are systemd unit files whose name ends in .timer that control .service files or events. Timers can be used as an alternative to cron. Timers have built-in support for calendar time events, monotonic time events, and can be run asynchronously.

Configuration

Following example timer runs a systemd unit every 5 minutes which invokes a bash script

systemd.timers."hello-world" = {
  wantedBy = [ "timers.target" ];
    timerConfig = {
      OnBootSec = "5m";
      OnUnitActiveSec = "5m";
      Unit = "hello-world.service";
    };
};

systemd.services."hello-world" = {
  script = ''
    set -eu
    ${pkgs.coreutils}/bin/echo "Hello World"
  '';
  serviceConfig = {
    Type = "oneshot";
    User = "root";
  };
};

The following example would starts once a day (at 12:00am). When activated, it triggers the service immediately if it missed the last start time (option Persistent=true), for example due to the system being powered of

...
  serviceConfig = {
      OnCalendar = "daily";
      Persistent = true; 
  };
};

Some more examples can be found here and here.

Usage

List active timers and their current state

systemctl list-timers

Manually run service once for testing purpose

systemctl start hello-world