.. _sec-building-parts:

Building Specific Parts of NixOS
--------------------------------

With the command :command:`nix-build`, you can build specific parts
of your NixOS configuration. This is done as follows:
::

    $ cd */path/to/nixpkgs/nixos*$ nix-build -A config.*option*

where *option* is a NixOS option with type
“derivation” (i.e. something that can be built). Attributes of interest
include:

``system.build.toplevel``

    The top-level option that builds the entire NixOS system. Everything else
    in your configuration is indirectly pulled in by this option. This is
    what :command:`nixos-rebuild` builds and what
    :file:`/run/current-system` points to afterwards.

    A shortcut to build this is:
    ::

        $ nix-build -A system

``system.build.manual.manualHTML``

    The NixOS manual.

``system.build.etc``

    A tree of symlinks that form the static parts of
    :file:`/etc`.

``system.build.initialRamdisk``, ``system.build.kernel``

    The initial ramdisk and kernel of the system. This allows a quick way to
    test whether the kernel and the initial ramdisk boot correctly, by using
    QEMU’s and options:
    ::

        $ nix-build -A config.system.build.initialRamdisk -o initrd$ nix-build -A config.system.build.kernel -o kernel$ qemu-system-x86_64 -kernel ./kernel/bzImage -initrd ./initrd/initrd -hda /dev/null

``system.build.nixos-rebuild``, ``system.build.nixos-install``, ``system.build.nixos-generate-config``

    These build the corresponding NixOS commands.

``systemd.units.``

    This builds the unit with the specified name. Note that since unit names
    contain dots (e.g. ``httpd.service``), you need to put
    them between quotes, like this:
    ::

        $ nix-build -A 'config.systemd.units."httpd.service".unit'

    You can also test individual units, without rebuilding the whole system,
    by putting them in :file:`/run/systemd/system`:
    ::

        $ cp $(nix-build -A 'config.systemd.units."httpd.service".unit')/httpd.service \\
        /run/systemd/system/tmp-httpd.service# systemctl daemon-reload# systemctl start tmp-httpd.service

    Note that the unit must not have the same name as any unit in
    :file:`/etc/systemd/system` since those take precedence over
    :file:`/run/systemd/system`. That’s why the unit is
    installed as :file:`tmp-httpd.service` here.