`environment.systemPackages` doesn't make the service available to
users or the system.

In order to enable a systemd *system* service with provided upstream
package, use (e.g):

```nix
{
  systemd.packages = [ pkgs.packagekit ];
}
```

Usually NixOS modules written by the community do the above, plus take
care of other details. If a module was written for a service you are
interested in, you'd probably need only to use
`services.#name#.enable = true;`. These services are defined in
Nixpkgs' [ `nixos/modules/` directory
](https://github.com/NixOS/nixpkgs/tree/master/nixos/modules). In case
the service is simple enough, the above method should work, and start
the service on boot.

*User* systemd services on the other hand, should be treated
differently. Given a package that has a systemd unit file at
`#pkg-out#/lib/systemd/user/`, using [](#opt-systemd.packages) will
make you able to start the service via `systemctl --user start`, but it
won't start automatically on login. However, You can imperatively
enable it by adding the package's attribute to
[](#opt-systemd.packages) and then do this (e.g):

```ShellSession
$ mkdir -p ~/.config/systemd/user/default.target.wants
$ ln -s /run/current-system/sw/lib/systemd/user/syncthing.service ~/.config/systemd/user/default.target.wants/
$ systemctl --user daemon-reload
$ systemctl --user enable syncthing.service
```

If you are interested in a timer file, use `timers.target.wants` instead
of `default.target.wants` in the 1st and 2nd command.

Using `systemctl --user enable syncthing.service` instead of the above,
will work, but it'll use the absolute path of `syncthing.service` for
the symlink, and this path is in `/nix/store/.../lib/systemd/user/`.
Hence [garbage collection](#sec-nix-gc) will remove that file and you
will wind up with a broken symlink in your systemd configuration, which
in turn will not make the service / timer start on login.

## Template units {#sect-nixos-systemd-template-units}

systemd supports templated units where a base unit can be started multiple
times with a different parameter. The syntax to accomplish this is
`service-name@instance-name.service`. Units get the instance name passed to
them (see `systemd.unit(5)`). NixOS has support for these kinds of units and
for template-specific overrides. A service needs to be defined twice, once
for the base unit and once for the instance. All instances must include
`overrideStrategy = "asDropin"` for the change detection to work. This
example illustrates this:
```nix
{
  systemd.services = {
    "base-unit@".serviceConfig = {
      ExecStart = "...";
      User = "...";
    };
    "base-unit@instance-a" = {
      overrideStrategy = "asDropin"; # needed for templates to work
      wantedBy = [ "multi-user.target" ]; # causes NixOS to manage the instance
    };
    "base-unit@instance-b" = {
      overrideStrategy = "asDropin"; # needed for templates to work
      wantedBy = [ "multi-user.target" ]; # causes NixOS to manage the instance
      serviceConfig.User = "root"; # also override something for this specific instance
    };
  };
}
```