NixithOS

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NixOS without systemd: choose s6, dinit, runit, OpenRC or Finit as the init system and service manager. Devices are managed by eudev, logs by s6-log. The default is s6-linux-init with s6-rc.

This is not a rewritten distribution but a layer on top of ordinary nixpkgs (currently the nixos-26.05 branch). The configuration is still a NixOS configuration: the same configuration.nix, the same options, the same service modules. What changes is what runs underneath them.

{ pkgs, ... }: {
  networking.hostName = "box";
  services.openssh.enable = true;
  users.users.alice.isNormalUser = true;

  # your own service is described the same way as in NixOS
  systemd.services.hello = {
    wantedBy = [ "multi-user.target" ];
    serviceConfig.ExecStart = "${pkgs.hello}/bin/hello";
  };
}

$ cat /proc/1/comm
s6-svscan
$ s6-rc -a list | head -3
basic.target
dbus
dbus-log

How it works

In NixOS every daemon, mount point and boot step is described by a systemd unit; two thousand modules are written that way. Throwing them away would be silly, so here systemd.services.* stays the data model: the units are generated by the same code as in NixOS, and then the unitc compiler (it runs only while the system is built) turns the directory of units into an s6-rc service database.

unit	what it becomes in s6-rc
foo.service, Type=oneshot	oneshot foo
foo.service, any other type	longrun foo + its logger foo-log
foo.target	an empty oneshot: a synchronisation point
foo.mount, foo.swap	oneshot running mount / swapon
foo.socket	oneshot: the socket is bound in advance and kept in s6-fdholder; the service takes it on every start
foo.timer	longrun with a scheduler (OnCalendar= and the rest)
dev-….device	oneshot that waits for the device to appear

The process of every service is started by unit-runtime: the environment, User=, directories (RuntimeDirectory= and the like), limits, capabilities, credentials, readiness (Type=notify, forking, dbus), Restart=, ExecStartPre/Post, ExecStop, a cgroup per service.

Two databases instead of one

s6-rc has one kind of dependency: "start after, require, restart together". In systemd, ordering (After=) and requirement (Requires=) are different things. If all the ordering were turned into dependencies, adding any timer during nixith switch would restart half the system, because everything is "after basic.target", and that one is "after timers.target".

So two databases are compiled from the same services:

• compiled-boot: all the ordering is written as dependencies. The system boots from it: s6-rc brings everything up in the right order and as much in parallel as the order allows.
• compiled: only the real dependencies (Requires=, BindsTo=). The system switches to it right after boot (s6-rc-update, without a single restart) and lives on it.

The order between units that are later started or stopped together (systemctl start, switching the configuration, shutdown) is recovered from a "rank": a number that grows along every After=.

More in docs/design.md.

dinit instead of s6

The init system is a setting: nixithos.init = "dinit"; (the installer asks too). dinit is PID 1 and the service manager then, and each user's manager is dinit --user; the units, systemctl (with --user too), journalctl and the logs stay the same. The unit compiler writes dinit's service descriptions from the same graph, and since dinit tells requirement from ordering itself (depends-on, waits-for, after), one set of them does for the boot and for what comes after: no second database. A unit's processes run under unit-supervise, which does for them what s6-supervise does under s6. A switch to the other init system applies the rest of the configuration at once and the init system at the next boot. An installed system changes its init the same way: the line in /etc/nixith/configuration.nix, sudo nixith boot and a reboot (the test init-change takes one installation through dinit, runit, OpenRC and Finit and back to s6).

$ cat /proc/1/comm
dinit
$ dinitctl list | head -3
[[+]     ] boot
[{+}     ] default
[{+}     ] dbus (pid: 612)

runit, the third

nixithos.init = "runit"; makes runit PID 1, with runsvdir over a runsv for every process of the system. runit knows nothing of what a service needs, so unit-rc does (rcctl list, rcctl start …, the commands of dinitctl): it reads the same service descriptions as dinit does, starts a service with what it requires and waits for, runs the oneshots, and has runsv start the processes, under unit-supervise as with dinit. Shutdown is runit's stage 3 after systemctl poweroff has stopped the units in order. The managers of users stay s6 trees. The unit tests run on it as nix-build -A runit.boot and so on.

$ cat /proc/1/comm
runit
$ rcctl list | head -3
[{+}     ] boot
[[+]     ] default
[[+]     ] dbus

This is the beginning of the README. Read the rest at the original. Read the full README ↗