Working with Systemd Timers

+1 for NixOS. I used to find systemd a bit intimidating, but working with NixOS has made it all fit together somehow.

As an example, this is a systemd timer I have that periodically runs rclone to sync photos to Backblaze:

    systemd.services.rclone-photos-sync = {
      serviceConfig.Type = "oneshot";
      path = [ pkgs.rclone ];
      script = ''
        rclone \
          --config ${config.age.secrets."rclone.conf".path} \
          --bwlimit 20M --transfers 16 \
          sync /mnt/photos/originals/ photos:
      '';
      unitConfig = {
        RequiresMountsFor = "/mnt/photos";
      };
    };
    systemd.timers.rclone-photos-sync = {
      timerConfig = {
        # Every 2 hours.
        OnCalendar = "00/2:00:00";
        RandomizedDelaySec = "5m";
        Persistent = true;
        Unit = "rclone-photos-sync.service";
      };
      partOf = [ "rclone-photos-sync.service" ];
      wantedBy = [ "timers.target" ];
    };

I'm convinced that the people who hate systemd are a small minority. They also happen to be fairly vocal, which I think makes them seem outsized.

I was initially against systemd (around the time Debian started discussing using it) for non-technical reasons: the project seemed to have some very negative aspects in terms of how feedback and concerns are addressed. I don't know for sure but I strongly suspect this was the core issue a good chunk of those "against" were concerned about: the way the project is run.

Having used it for several major releases of Debian technically I'm happy to have it - it's an improvement over the sysv init & crontabs. I'm not completely in love with journal compared to regular logs but I can see some advantages.

Systemd hater here. My stance is and always was: if you love systemd, go nuts with it. I'm not looking to reach into your bathroom to set your shower temperature.

But I don't want to have to run it or even think about it. Lennart has other plans, hence his campaign to get distros to require it and software up the stack to hard-depend on it. I don't like its design, I want to opt out of it in favor of something better, but one of the development effort's goals is making that extremely difficult for modern Linux systems. That's my issue with it.

But really it macht nichs for me right now because I run Void, btw.

I actually disagree with simplicity. cron does weird things with the environment and it can be hard to correctly reproduce that environment when creating a job and debugging. Conversely, systemd timers are very easy to manually trigger and otherwise behave as a normal oneshot service.

I found the implicit column-based configuration layout of cron rather cubersome. I prefer the key-value configuration layout of systemd timers.

There is also one huge disadvantage of cron: it requires root. Users can create and run systemd timers without root/sudo.

And quite frankly, your experience is likely due to your personal knowledge bias. Systemd timers are quite easy to debug, in fact easier than cron in my opinion. systemctl list-timers lists all the timers, when they last ran, when they will run next, and you can use other commands to inspect each timer in detail and all associated logs.

In contrast, I find cron much harder to debug. Starting with the first problem, you must first figure out what cron is running, as different implementations have different behavior!

> I guess it's just inherent to what that unit type does, and the fact that both units are named backup?

Correct. If there's no `Unit=` specified in the timer unit, it defaults to the service unit of the same name. See https://www.freedesktop.org/software/systemd/man/latest/syst...

This is also a common pattern for the socket units of socket activated services

Using a modern CoW filesystem lets you level this up to just a snapshot command in a pacman pre-upgrade hook.

IMO Systemd Timers provide much better control over how the cron job is run.

Need to distribute lots of cron jobs evenly to avoid overloading the system? Use RandomizedDelaySec.

Some cron jobs are flaky and you want to re-run if it fails? Add Restart=on-failure to corresponding service.

Some cron jobs conflict with each other? Set Conflicts=foo.service or maybe Before & After.

Sure, all above are possible with shell scripts. But systemd provides a standard, reliable way to define these properties.

> Much more significantly, the amount of setup involved in a systemd timer is way higher than putting a line in a crontab

Counterpoint: this requires root, while you can edit and run systemd timers without root. Thus it's also generally more secure, both when creating the job and every time it runs.

> Systemd involves writing a 7 line timer unit file, an additional 5 line service file, running a daemon-reload, then enabling the timer. It turns what's usually a 10 second mindless task into a much more involved procedure. That can be worth it if there are material benefits from it, but I'm not really seeing them here.

You could write a script to automate it, if it's such a big deal. Creating timers/cronjobs isn't something that needs to be done often enough for this to matter.

If it is, another counterpoint: systemd supports creating transient timers, which you could do programmatically.

And final counterpoint: you can make systemd understand crontab and convert it into timers (systemd-crontab-generator)

It would be cool if someone made a crontab interface to systemd timers. Use crontab syntax but generate systemd boilerplate with sane defaults.

I pair crons with healthchecks.io