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<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>
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