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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">
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<title > Building Specific Parts of NixOS</title>
<para >
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With the command <command > nix-build</command> , you can build specific parts of your NixOS configuration. This is done as follows:
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<screen >
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<prompt > $ </prompt> cd <replaceable > /path/to/nixpkgs/nixos</replaceable>
<prompt > $ </prompt> nix-build -A config.<replaceable > option</replaceable> </screen>
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where <replaceable > option</replaceable> is a NixOS option with type “derivation” (i.e. something that can be built). Attributes of interest include:
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<variablelist >
<varlistentry >
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<term >
<varname > system.build.toplevel</varname>
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</term>
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<listitem >
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<para >
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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.
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</para>
<para >
A shortcut to build this is:
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<screen >
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<prompt > $ </prompt> nix-build -A system</screen>
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</para>
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</listitem>
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</varlistentry>
<varlistentry >
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<term >
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<varname > system.build.manual.manualHTML</varname>
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</term>
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<listitem >
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<para >
The NixOS manual.
</para>
</listitem>
</varlistentry>
<varlistentry >
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<term >
<varname > system.build.etc</varname>
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</term>
<listitem >
<para >
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A tree of symlinks that form the static parts of <filename > /etc</filename> .
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</para>
</listitem>
</varlistentry>
<varlistentry >
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<term >
<varname > system.build.initialRamdisk</varname>
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</term>
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<term >
<varname > system.build.kernel</varname>
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</term>
<listitem >
<para >
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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:
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<screen >
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<prompt > $ </prompt> nix-build -A config.system.build.initialRamdisk -o initrd
<prompt > $ </prompt> nix-build -A config.system.build.kernel -o kernel
<prompt > $ </prompt> qemu-system-x86_64 -kernel ./kernel/bzImage -initrd ./initrd/initrd -hda /dev/null
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</screen>
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</para>
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</listitem>
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</varlistentry>
<varlistentry >
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<term >
<varname > system.build.nixos-rebuild</varname>
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</term>
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<term >
<varname > system.build.nixos-install</varname>
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</term>
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<term >
<varname > system.build.nixos-generate-config</varname>
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</term>
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<listitem >
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<para >
These build the corresponding NixOS commands.
</para>
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</listitem>
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</varlistentry>
<varlistentry >
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<term >
<varname > systemd.units.<replaceable > unit-name</replaceable> .unit</varname>
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</term>
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<listitem >
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<para >
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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:
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<screen >
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<prompt > $ </prompt> nix-build -A 'config.systemd.units."httpd.service".unit'
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</screen>
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You can also test individual units, without rebuilding the whole system, by putting them in <filename > /run/systemd/system</filename> :
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<screen >
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<prompt > $ </prompt> cp $(nix-build -A 'config.systemd.units."httpd.service".unit')/httpd.service \
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/run/systemd/system/tmp-httpd.service
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<prompt > # </prompt> systemctl daemon-reload
<prompt > # </prompt> systemctl start tmp-httpd.service
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</screen>
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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.
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</para>
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</listitem>
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</varlistentry>
</variablelist>
</para>
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</chapter>