<chapter xmlns="http://docbook.org/ns/docbook"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:xi="http://www.w3.org/2001/XInclude"
        version="5.0"
        xml:id="sec-building-parts">

<title>Building Specific Parts of NixOS</title>

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

<screen>
$ cd <replaceable>/path/to/nixpkgs/nixos</replaceable>
$ nix-build -A config.<replaceable>option</replaceable></screen>

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

<variablelist>

  <varlistentry>
    <term><varname>system.build.toplevel</varname></term>
    <listitem>
      <para>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</command>
      builds and what <filename>/run/current-system</filename> points
      to afterwards.</para>

      <para>A shortcut to build this is:

<screen>
$ nix-build -A system</screen>
      </para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>system.build.manual.manual</varname></term>
    <listitem><para>The NixOS manual.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>system.build.etc</varname></term>
    <listitem><para>A tree of symlinks that form the static parts of
    <filename>/etc</filename>.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>system.build.initialRamdisk</varname></term>
    <term><varname>system.build.kernel</varname></term>
    <listitem>
      <para>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 <option>-kernel</option> and
      <option>-initrd</option> options:

<screen>
$ 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
</screen>

      </para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>system.build.nixos-rebuild</varname></term>
    <term><varname>system.build.nixos-install</varname></term>
    <term><varname>system.build.nixos-generate-config</varname></term>
    <listitem>
      <para>These build the corresponding NixOS commands.</para>
    </listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>systemd.units.<replaceable>unit-name</replaceable>.unit</varname></term>
    <listitem>
      <para>This builds the unit with the specified name.  Note that
      since unit names contain dots
      (e.g. <literal>httpd.service</literal>), you need to put them
      between quotes, like this:

<screen>
$ nix-build -A 'config.systemd.units."httpd.service".unit'
</screen>

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

<screen>
$ 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
</screen>

      Note that the unit must not have the same name as any unit in
      <filename>/etc/systemd/system</filename> since those take
      precedence over <filename>/run/systemd/system</filename>.
      That’s why the unit is installed as
      <filename>tmp-httpd.service</filename> here.</para>
    </listitem>
  </varlistentry>

</variablelist>

</para>

</chapter>