# Building Specific Parts of NixOS

With the 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 `nixos-rebuild` builds and what `/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 `/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 `-kernel` and `-initrd` 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.unit-name.unit`

  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 `/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 `/etc/systemd/system` since those take precedence over `/run/systemd/system`. That’s why the unit is installed as `tmp-httpd.service` here.