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4 version="5.0"
5 xml:id="sec-building-parts">
6
7<title>Building Specific Parts of NixOS</title>
8
9<para>With the command <command>nix-build</command>, you can build
10specific parts of your NixOS configuration. This is done as follows:
11
12<screen>
13$ cd <replaceable>/path/to/nixpkgs/nixos</replaceable>
14$ nix-build -A config.<replaceable>option</replaceable></screen>
15
16where <replaceable>option</replaceable> is a NixOS option with type
17“derivation” (i.e. something that can be built). Attributes of
18interest include:
19
20<variablelist>
21
22 <varlistentry>
23 <term><varname>system.build.toplevel</varname></term>
24 <listitem>
25 <para>The top-level option that builds the entire NixOS system.
26 Everything else in your configuration is indirectly pulled in by
27 this option. This is what <command>nixos-rebuild</command>
28 builds and what <filename>/run/current-system</filename> points
29 to afterwards.</para>
30
31 <para>A shortcut to build this is:
32
33<screen>
34$ nix-build -A system</screen>
35 </para>
36 </listitem>
37 </varlistentry>
38
39 <varlistentry>
40 <term><varname>system.build.manual.manual</varname></term>
41 <listitem><para>The NixOS manual.</para></listitem>
42 </varlistentry>
43
44 <varlistentry>
45 <term><varname>system.build.etc</varname></term>
46 <listitem><para>A tree of symlinks that form the static parts of
47 <filename>/etc</filename>.</para></listitem>
48 </varlistentry>
49
50 <varlistentry>
51 <term><varname>system.build.initialRamdisk</varname></term>
52 <term><varname>system.build.kernel</varname></term>
53 <listitem>
54 <para>The initial ramdisk and kernel of the system. This allows
55 a quick way to test whether the kernel and the initial ramdisk
56 boot correctly, by using QEMU’s <option>-kernel</option> and
57 <option>-initrd</option> options:
58
59<screen>
60$ nix-build -A config.system.build.initialRamdisk -o initrd
61$ nix-build -A config.system.build.kernel -o kernel
62$ qemu-system-x86_64 -kernel ./kernel/bzImage -initrd ./initrd/initrd -hda /dev/null
63</screen>
64
65 </para>
66 </listitem>
67 </varlistentry>
68
69 <varlistentry>
70 <term><varname>system.build.nixos-rebuild</varname></term>
71 <term><varname>system.build.nixos-install</varname></term>
72 <term><varname>system.build.nixos-generate-config</varname></term>
73 <listitem>
74 <para>These build the corresponding NixOS commands.</para>
75 </listitem>
76 </varlistentry>
77
78 <varlistentry>
79 <term><varname>systemd.units.<replaceable>unit-name</replaceable>.unit</varname></term>
80 <listitem>
81 <para>This builds the unit with the specified name. Note that
82 since unit names contain dots
83 (e.g. <literal>httpd.service</literal>), you need to put them
84 between quotes, like this:
85
86<screen>
87$ nix-build -A 'config.systemd.units."httpd.service".unit'
88</screen>
89
90 You can also test individual units, without rebuilding the whole
91 system, by putting them in
92 <filename>/run/systemd/system</filename>:
93
94<screen>
95$ cp $(nix-build -A 'config.systemd.units."httpd.service".unit')/httpd.service \
96 /run/systemd/system/tmp-httpd.service
97# systemctl daemon-reload
98# systemctl start tmp-httpd.service
99</screen>
100
101 Note that the unit must not have the same name as any unit in
102 <filename>/etc/systemd/system</filename> since those take
103 precedence over <filename>/run/systemd/system</filename>.
104 That’s why the unit is installed as
105 <filename>tmp-httpd.service</filename> here.</para>
106 </listitem>
107 </varlistentry>
108
109</variablelist>
110
111</para>
112
113</chapter>