# Unit handling {#sec-unit-handling}

To figure out what units need to be started/stopped/restarted/reloaded, the
script first checks the current state of the system, similar to what `systemctl
list-units` shows. For each of the units, the script goes through the following
checks:

- Is the unit file still in the new system? If not, **stop** the service unless
  it sets `X-StopOnRemoval` in the `[Unit]` section to `false`.

- Is it a `.target` unit? If so, **start** it unless it sets
  `RefuseManualStart` in the `[Unit]` section to `true` or `X-OnlyManualStart`
  in the `[Unit]` section to `true`. Also **stop** the unit again unless it
  sets `X-StopOnReconfiguration` to `false`.

- Are the contents of the unit files different? They are compared by parsing
  them and comparing their contents. If they are different but only
  `X-Reload-Triggers` in the `[Unit]` section is changed, **reload** the unit.
  The NixOS module system allows setting these triggers with the option
  [systemd.services.\<name\>.reloadTriggers](#opt-systemd.services). There are
  some additional keys in the `[Unit]` section that are ignored as well. If the
  unit files differ in any way, the following actions are performed:

  - `.path` and `.slice` units are ignored. There is no need to restart them
    since changes in their values are applied by systemd when systemd is
    reloaded.

  - `.mount` units are **reload**ed if only their `Options` changed. If anything
    else changed (like `What`), they are **restart**ed unless they are the mount
    unit for `/` or `/nix` in which case they are reloaded to prevent the system
    from crashing. Note that this is the case for `.mount` units and not for
    mounts from `/etc/fstab`. These are explained in [](#sec-switching-systems).

  - `.socket` units are currently ignored. This is to be fixed at a later
    point.

  - The rest of the units (mostly `.service` units) are then **reload**ed if
    `X-ReloadIfChanged` in the `[Service]` section is set to `true` (exposed
    via [systemd.services.\<name\>.reloadIfChanged](#opt-systemd.services)).
    A little exception is done for units that were deactivated in the meantime,
    for example because they require a unit that got stopped before. These
    are **start**ed instead of reloaded.

  - If the reload flag is not set, some more flags decide if the unit is
    skipped. These flags are `X-RestartIfChanged` in the `[Service]` section
    (exposed via
    [systemd.services.\<name\>.restartIfChanged](#opt-systemd.services)),
    `RefuseManualStop` in the `[Unit]` section, and `X-OnlyManualStart` in the
    `[Unit]` section.

  - Further behavior depends on the unit having `X-StopIfChanged` in the
    `[Service]` section set to `true` (exposed via
    [systemd.services.\<name\>.stopIfChanged](#opt-systemd.services)). This is
    set to `true` by default and must be explicitly turned off if not wanted.