Merge pull request #129074 from bobby285271/pr11

nixos/doc: convert Chapter 57, 59, 60, 63 to CommonMark
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# Building Specific Parts of NixOS {#sec-building-parts}
With the command `nix-build`, you can build specific parts of your NixOS
configuration. This is done as follows:
```ShellSession
$ 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:
```ShellSession
$ 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:
```ShellSession
$ 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:
```ShellSession
$ 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`:
```ShellSession
$ 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.

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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>
<prompt>$ </prompt>cd <replaceable>/path/to/nixpkgs/nixos</replaceable>
<prompt>$ </prompt>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>
<prompt>$ </prompt>nix-build -A system</screen>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<varname>system.build.manual.manualHTML</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
QEMUs <option>-kernel</option> and <option>-initrd</option> options:
<screen>
<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
</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>
<prompt>$ </prompt>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>
<prompt>$ </prompt>cp $(nix-build -A 'config.systemd.units."httpd.service".unit')/httpd.service \
/run/systemd/system/tmp-httpd.service
<prompt># </prompt>systemctl daemon-reload
<prompt># </prompt>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>. Thats why the unit is
installed as <filename>tmp-httpd.service</filename> here.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</chapter>

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@ -9,11 +9,11 @@
This chapter describes how you can modify and extend NixOS.
</para>
</partintro>
<xi:include href="sources.xml" />
<xi:include href="../from_md/development/sources.chapter.xml" />
<xi:include href="writing-modules.xml" />
<xi:include href="building-parts.xml" />
<xi:include href="writing-documentation.xml" />
<xi:include href="../from_md/development/building-parts.chapter.xml" />
<xi:include href="../from_md/development/writing-documentation.chapter.xml" />
<xi:include href="../from_md/development/building-nixos.chapter.xml" />
<xi:include href="nixos-tests.xml" />
<xi:include href="testing-installer.xml" />
<xi:include href="../from_md/development/testing-installer.chapter.xml" />
</part>

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# Getting the Sources {#sec-getting-sources}
By default, NixOS's `nixos-rebuild` command uses the NixOS and Nixpkgs
sources provided by the `nixos` channel (kept in
`/nix/var/nix/profiles/per-user/root/channels/nixos`). To modify NixOS,
however, you should check out the latest sources from Git. This is as
follows:
```ShellSession
$ git clone https://github.com/NixOS/nixpkgs
$ cd nixpkgs
$ git remote update origin
```
This will check out the latest Nixpkgs sources to `./nixpkgs` the NixOS
sources to `./nixpkgs/nixos`. (The NixOS source tree lives in a
subdirectory of the Nixpkgs repository.) The `nixpkgs` repository has
branches that correspond to each Nixpkgs/NixOS channel (see
[](#sec-upgrading) for more information about channels). Thus, the
Git branch `origin/nixos-17.03` will contain the latest built and tested
version available in the `nixos-17.03` channel.
It's often inconvenient to develop directly on the master branch, since
if somebody has just committed (say) a change to GCC, then the binary
cache may not have caught up yet and you'll have to rebuild everything
from source. So you may want to create a local branch based on your
current NixOS version:
```ShellSession
$ nixos-version
17.09pre104379.6e0b727 (Hummingbird)
$ git checkout -b local 6e0b727
```
Or, to base your local branch on the latest version available in a NixOS
channel:
```ShellSession
$ git remote update origin
$ git checkout -b local origin/nixos-17.03
```
(Replace `nixos-17.03` with the name of the channel you want to use.)
You can use `git merge` or `git
rebase` to keep your local branch in sync with the channel, e.g.
```ShellSession
$ git remote update origin
$ git merge origin/nixos-17.03
```
You can use `git cherry-pick` to copy commits from your local branch to
the upstream branch.
If you want to rebuild your system using your (modified) sources, you
need to tell `nixos-rebuild` about them using the `-I` flag:
```ShellSession
# nixos-rebuild switch -I nixpkgs=/my/sources/nixpkgs
```
If you want `nix-env` to use the expressions in `/my/sources`, use
`nix-env -f
/my/sources/nixpkgs`, or change the default by adding a symlink in
`~/.nix-defexpr`:
```ShellSession
$ ln -s /my/sources/nixpkgs ~/.nix-defexpr/nixpkgs
```
You may want to delete the symlink `~/.nix-defexpr/channels_root` to
prevent root's NixOS channel from clashing with your own tree (this may
break the command-not-found utility though). If you want to go back to
the default state, you may just remove the `~/.nix-defexpr` directory
completely, log out and log in again and it should have been recreated
with a link to the root channels.

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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-getting-sources">
<title>Getting the Sources</title>
<para>
By default, NixOSs <command>nixos-rebuild</command> command uses the NixOS
and Nixpkgs sources provided by the <literal>nixos</literal> channel (kept in
<filename>/nix/var/nix/profiles/per-user/root/channels/nixos</filename>). To
modify NixOS, however, you should check out the latest sources from Git. This
is as follows:
<screen>
<prompt>$ </prompt>git clone https://github.com/NixOS/nixpkgs
<prompt>$ </prompt>cd nixpkgs
<prompt>$ </prompt>git remote update origin
</screen>
This will check out the latest Nixpkgs sources to
<filename>./nixpkgs</filename> the NixOS sources to
<filename>./nixpkgs/nixos</filename>. (The NixOS source tree lives in a
subdirectory of the Nixpkgs repository.) The
<literal>nixpkgs</literal> repository has branches that correspond
to each Nixpkgs/NixOS channel (see <xref linkend="sec-upgrading"/> for more
information about channels). Thus, the Git branch
<literal>origin/nixos-17.03</literal> will contain the latest built and
tested version available in the <literal>nixos-17.03</literal> channel.
</para>
<para>
Its often inconvenient to develop directly on the master branch, since if
somebody has just committed (say) a change to GCC, then the binary cache may
not have caught up yet and youll have to rebuild everything from source.
So you may want to create a local branch based on your current NixOS version:
<screen>
<prompt>$ </prompt>nixos-version
17.09pre104379.6e0b727 (Hummingbird)
<prompt>$ </prompt>git checkout -b local 6e0b727
</screen>
Or, to base your local branch on the latest version available in a NixOS
channel:
<screen>
<prompt>$ </prompt>git remote update origin
<prompt>$ </prompt>git checkout -b local origin/nixos-17.03
</screen>
(Replace <literal>nixos-17.03</literal> with the name of the channel you want
to use.) You can use <command>git merge</command> or <command>git
rebase</command> to keep your local branch in sync with the channel, e.g.
<screen>
<prompt>$ </prompt>git remote update origin
<prompt>$ </prompt>git merge origin/nixos-17.03
</screen>
You can use <command>git cherry-pick</command> to copy commits from your
local branch to the upstream branch.
</para>
<para>
If you want to rebuild your system using your (modified) sources, you need to
tell <command>nixos-rebuild</command> about them using the
<option>-I</option> flag:
<screen>
<prompt># </prompt>nixos-rebuild switch -I nixpkgs=<replaceable>/my/sources</replaceable>/nixpkgs
</screen>
</para>
<para>
If you want <command>nix-env</command> to use the expressions in
<replaceable>/my/sources</replaceable>, use <command>nix-env -f
<replaceable>/my/sources</replaceable>/nixpkgs</command>, or change the
default by adding a symlink in <filename>~/.nix-defexpr</filename>:
<screen>
<prompt>$ </prompt>ln -s <replaceable>/my/sources</replaceable>/nixpkgs ~/.nix-defexpr/nixpkgs
</screen>
You may want to delete the symlink
<filename>~/.nix-defexpr/channels_root</filename> to prevent roots NixOS
channel from clashing with your own tree (this may break the
command-not-found utility though). If you want to go back to the default
state, you may just remove the <filename>~/.nix-defexpr</filename> directory
completely, log out and log in again and it should have been recreated with a
link to the root channels.
</para>
<!-- FIXME: not sure what this means.
<para>You should not pass the base directory
<filename><replaceable>/my/sources</replaceable></filename>
to <command>nix-env</command>, as it will break after interpreting expressions
in <filename>nixos/</filename> as packages.</para>
-->
</chapter>

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# Testing the Installer {#ch-testing-installer}
Building, burning, and booting from an installation CD is rather
tedious, so here is a quick way to see if the installer works properly:
```ShellSession
# mount -t tmpfs none /mnt
# nixos-generate-config --root /mnt
$ nix-build '<nixpkgs/nixos>' -A config.system.build.nixos-install
# ./result/bin/nixos-install
```
To start a login shell in the new NixOS installation in `/mnt`:
```ShellSession
$ nix-build '<nixpkgs/nixos>' -A config.system.build.nixos-enter
# ./result/bin/nixos-enter
```

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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="ch-testing-installer">
<title>Testing the Installer</title>
<para>
Building, burning, and booting from an installation CD is rather tedious, so
here is a quick way to see if the installer works properly:
<screen>
<prompt># </prompt>mount -t tmpfs none /mnt
<prompt># </prompt>nixos-generate-config --root /mnt
<prompt>$ </prompt>nix-build '&lt;nixpkgs/nixos>' -A config.system.build.nixos-install
<prompt># </prompt>./result/bin/nixos-install</screen>
To start a login shell in the new NixOS installation in
<filename>/mnt</filename>:
<screen>
<prompt>$ </prompt>nix-build '&lt;nixpkgs/nixos>' -A config.system.build.nixos-enter
<prompt># </prompt>./result/bin/nixos-enter
</screen>
</para>
</chapter>

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# Writing NixOS Documentation {#sec-writing-documentation}
As NixOS grows, so too does the need for a catalogue and explanation of
its extensive functionality. Collecting pertinent information from
disparate sources and presenting it in an accessible style would be a
worthy contribution to the project.
## Building the Manual {#sec-writing-docs-building-the-manual}
The DocBook sources of the [](#book-nixos-manual) are in the
[`nixos/doc/manual`](https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual)
subdirectory of the Nixpkgs repository.
You can quickly validate your edits with `make`:
```ShellSession
$ cd /path/to/nixpkgs/nixos/doc/manual
$ nix-shell
nix-shell$ make
```
Once you are done making modifications to the manual, it\'s important to
build it before committing. You can do that as follows:
```ShellSession
nix-build nixos/release.nix -A manual.x86_64-linux
```
When this command successfully finishes, it will tell you where the
manual got generated. The HTML will be accessible through the `result`
symlink at `./result/share/doc/nixos/index.html`.
## Editing DocBook XML {#sec-writing-docs-editing-docbook-xml}
For general information on how to write in DocBook, see [DocBook 5: The
Definitive Guide](http://www.docbook.org/tdg5/en/html/docbook.html).
Emacs nXML Mode is very helpful for editing DocBook XML because it
validates the document as you write, and precisely locates errors. To
use it, see [](#sec-emacs-docbook-xml).
[Pandoc](http://pandoc.org) can generate DocBook XML from a multitude of
formats, which makes a good starting point. Here is an example of Pandoc
invocation to convert GitHub-Flavoured MarkDown to DocBook 5 XML:
```ShellSession
pandoc -f markdown_github -t docbook5 docs.md -o my-section.md
```
Pandoc can also quickly convert a single `section.xml` to HTML, which is
helpful when drafting.
Sometimes writing valid DocBook is simply too difficult. In this case,
submit your documentation updates in a [GitHub
Issue](https://github.com/NixOS/nixpkgs/issues/new) and someone will
handle the conversion to XML for you.
## Creating a Topic {#sec-writing-docs-creating-a-topic}
You can use an existing topic as a basis for the new topic or create a
topic from scratch.
Keep the following guidelines in mind when you create and add a topic:
- The NixOS [`book`](http://www.docbook.org/tdg5/en/html/book.html)
element is in `nixos/doc/manual/manual.xml`. It includes several
[`parts`](http://www.docbook.org/tdg5/en/html/book.html) which are in
subdirectories.
- Store the topic file in the same directory as the `part` to which it
belongs. If your topic is about configuring a NixOS module, then the
XML file can be stored alongside the module definition `nix` file.
- If you include multiple words in the file name, separate the words
with a dash. For example: `ipv6-config.xml`.
- Make sure that the `xml:id` value is unique. You can use abbreviations
if the ID is too long. For example: `nixos-config`.
- Determine whether your topic is a chapter or a section. If you are
unsure, open an existing topic file and check whether the main
element is chapter or section.
## Adding a Topic to the Book {#sec-writing-docs-adding-a-topic}
Open the parent XML file and add an `xi:include` element to the list of
chapters with the file name of the topic that you created. If you
created a `section`, you add the file to the `chapter` file. If you created
a `chapter`, you add the file to the `part` file.
If the topic is about configuring a NixOS module, it can be
automatically included in the manual by using the `meta.doc` attribute.
See [](#sec-meta-attributes) for an explanation.

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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-writing-documentation">
<title>Writing NixOS Documentation</title>
<para>
As NixOS grows, so too does the need for a catalogue and explanation of its
extensive functionality. Collecting pertinent information from disparate
sources and presenting it in an accessible style would be a worthy
contribution to the project.
</para>
<section xml:id="sec-writing-docs-building-the-manual">
<title>Building the Manual</title>
<para>
The DocBook sources of the <xref linkend="book-nixos-manual"/> are in the
<link xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual"><filename>nixos/doc/manual</filename></link>
subdirectory of the Nixpkgs repository.
</para>
<para>
You can quickly validate your edits with <command>make</command>:
</para>
<screen>
<prompt>$ </prompt>cd /path/to/nixpkgs/nixos/doc/manual
<prompt>$ </prompt>nix-shell
<prompt>nix-shell$ </prompt>make
</screen>
<para>
Once you are done making modifications to the manual, it's important to
build it before committing. You can do that as follows:
</para>
<screen>nix-build nixos/release.nix -A manual.x86_64-linux</screen>
<para>
When this command successfully finishes, it will tell you where the manual
got generated. The HTML will be accessible through the
<filename>result</filename> symlink at
<filename>./result/share/doc/nixos/index.html</filename>.
</para>
</section>
<section xml:id="sec-writing-docs-editing-docbook-xml">
<title>Editing DocBook XML</title>
<para>
For general information on how to write in DocBook, see
<link xlink:href="http://www.docbook.org/tdg5/en/html/docbook.html"> DocBook
5: The Definitive Guide</link>.
</para>
<para>
Emacs nXML Mode is very helpful for editing DocBook XML because it validates
the document as you write, and precisely locates errors. To use it, see
<xref linkend="sec-emacs-docbook-xml"/>.
</para>
<para>
<link xlink:href="http://pandoc.org">Pandoc</link> can generate DocBook XML
from a multitude of formats, which makes a good starting point.
<example xml:id="ex-pandoc-xml-conv">
<title>Pandoc invocation to convert GitHub-Flavoured MarkDown to DocBook 5 XML</title>
<screen>pandoc -f markdown_github -t docbook5 docs.md -o my-section.md</screen>
</example>
Pandoc can also quickly convert a single <filename>section.xml</filename> to
HTML, which is helpful when drafting.
</para>
<para>
Sometimes writing valid DocBook is simply too difficult. In this case,
submit your documentation updates in a
<link
xlink:href="https://github.com/NixOS/nixpkgs/issues/new">GitHub
Issue</link> and someone will handle the conversion to XML for you.
</para>
</section>
<section xml:id="sec-writing-docs-creating-a-topic">
<title>Creating a Topic</title>
<para>
You can use an existing topic as a basis for the new topic or create a topic
from scratch.
</para>
<para>
Keep the following guidelines in mind when you create and add a topic:
<itemizedlist>
<listitem>
<para>
The NixOS
<link xlink:href="http://www.docbook.org/tdg5/en/html/book.html"><tag>book</tag></link>
element is in <filename>nixos/doc/manual/manual.xml</filename>. It
includes several
<link xlink:href="http://www.docbook.org/tdg5/en/html/book.html"><tag>part</tag>s</link>
which are in subdirectories.
</para>
</listitem>
<listitem>
<para>
Store the topic file in the same directory as the <tag>part</tag> to
which it belongs. If your topic is about configuring a NixOS module, then
the XML file can be stored alongside the module definition
<filename>nix</filename> file.
</para>
</listitem>
<listitem>
<para>
If you include multiple words in the file name, separate the words with a
dash. For example: <filename>ipv6-config.xml</filename>.
</para>
</listitem>
<listitem>
<para>
Make sure that the <tag>xml:id</tag> value is unique. You can use
abbreviations if the ID is too long. For example:
<varname>nixos-config</varname>.
</para>
</listitem>
<listitem>
<para>
Determine whether your topic is a chapter or a section. If you are
unsure, open an existing topic file and check whether the main element is
chapter or section.
</para>
</listitem>
</itemizedlist>
</para>
</section>
<section xml:id="sec-writing-docs-adding-a-topic">
<title>Adding a Topic to the Book</title>
<para>
Open the parent XML file and add an <varname>xi:include</varname> element to
the list of chapters with the file name of the topic that you created. If
you created a <tag>section</tag>, you add the file to the <tag>chapter</tag>
file. If you created a <tag>chapter</tag>, you add the file to the
<tag>part</tag> file.
</para>
<para>
If the topic is about configuring a NixOS module, it can be automatically
included in the manual by using the <varname>meta.doc</varname> attribute.
See <xref
linkend="sec-meta-attributes"/> for an explanation.
</para>
</section>
</chapter>

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<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-building-parts">
<title>Building Specific Parts of NixOS</title>
<para>
With the command <literal>nix-build</literal>, you can build
specific parts of your NixOS configuration. This is done as follows:
</para>
<programlisting>
$ cd /path/to/nixpkgs/nixos
$ nix-build -A config.option
</programlisting>
<para>
where <literal>option</literal> is a NixOS option with type
<quote>derivation</quote> (i.e. something that can be built).
Attributes of interest include:
</para>
<variablelist>
<varlistentry>
<term>
<literal>system.build.toplevel</literal>
</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 <literal>nixos-rebuild</literal>
builds and what <literal>/run/current-system</literal> points
to afterwards.
</para>
<para>
A shortcut to build this is:
</para>
<programlisting>
$ nix-build -A system
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>system.build.manual.manualHTML</literal>
</term>
<listitem>
<para>
The NixOS manual.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>system.build.etc</literal>
</term>
<listitem>
<para>
A tree of symlinks that form the static parts of
<literal>/etc</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>system.build.initialRamdisk</literal> ,
<literal>system.build.kernel</literal>
</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 QEMUs <literal>-kernel</literal> and
<literal>-initrd</literal> options:
</para>
<programlisting>
$ 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
</programlisting>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>system.build.nixos-rebuild</literal> ,
<literal>system.build.nixos-install</literal> ,
<literal>system.build.nixos-generate-config</literal>
</term>
<listitem>
<para>
These build the corresponding NixOS commands.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<literal>systemd.units.unit-name.unit</literal>
</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:
</para>
<programlisting>
$ nix-build -A 'config.systemd.units.&quot;httpd.service&quot;.unit'
</programlisting>
<para>
You can also test individual units, without rebuilding the
whole system, by putting them in
<literal>/run/systemd/system</literal>:
</para>
<programlisting>
$ cp $(nix-build -A 'config.systemd.units.&quot;httpd.service&quot;.unit')/httpd.service \
/run/systemd/system/tmp-httpd.service
# systemctl daemon-reload
# systemctl start tmp-httpd.service
</programlisting>
<para>
Note that the unit must not have the same name as any unit in
<literal>/etc/systemd/system</literal> since those take
precedence over <literal>/run/systemd/system</literal>. Thats
why the unit is installed as
<literal>tmp-httpd.service</literal> here.
</para>
</listitem>
</varlistentry>
</variablelist>
</chapter>

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<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-getting-sources">
<title>Getting the Sources</title>
<para>
By default, NixOSs <literal>nixos-rebuild</literal> command uses
the NixOS and Nixpkgs sources provided by the
<literal>nixos</literal> channel (kept in
<literal>/nix/var/nix/profiles/per-user/root/channels/nixos</literal>).
To modify NixOS, however, you should check out the latest sources
from Git. This is as follows:
</para>
<programlisting>
$ git clone https://github.com/NixOS/nixpkgs
$ cd nixpkgs
$ git remote update origin
</programlisting>
<para>
This will check out the latest Nixpkgs sources to
<literal>./nixpkgs</literal> the NixOS sources to
<literal>./nixpkgs/nixos</literal>. (The NixOS source tree lives in
a subdirectory of the Nixpkgs repository.) The
<literal>nixpkgs</literal> repository has branches that correspond
to each Nixpkgs/NixOS channel (see <xref linkend="sec-upgrading" />
for more information about channels). Thus, the Git branch
<literal>origin/nixos-17.03</literal> will contain the latest built
and tested version available in the <literal>nixos-17.03</literal>
channel.
</para>
<para>
Its often inconvenient to develop directly on the master branch,
since if somebody has just committed (say) a change to GCC, then the
binary cache may not have caught up yet and youll have to rebuild
everything from source. So you may want to create a local branch
based on your current NixOS version:
</para>
<programlisting>
$ nixos-version
17.09pre104379.6e0b727 (Hummingbird)
$ git checkout -b local 6e0b727
</programlisting>
<para>
Or, to base your local branch on the latest version available in a
NixOS channel:
</para>
<programlisting>
$ git remote update origin
$ git checkout -b local origin/nixos-17.03
</programlisting>
<para>
(Replace <literal>nixos-17.03</literal> with the name of the channel
you want to use.) You can use <literal>git merge</literal> or
<literal>git rebase</literal> to keep your local branch in sync with
the channel, e.g.
</para>
<programlisting>
$ git remote update origin
$ git merge origin/nixos-17.03
</programlisting>
<para>
You can use <literal>git cherry-pick</literal> to copy commits from
your local branch to the upstream branch.
</para>
<para>
If you want to rebuild your system using your (modified) sources,
you need to tell <literal>nixos-rebuild</literal> about them using
the <literal>-I</literal> flag:
</para>
<programlisting>
# nixos-rebuild switch -I nixpkgs=/my/sources/nixpkgs
</programlisting>
<para>
If you want <literal>nix-env</literal> to use the expressions in
<literal>/my/sources</literal>, use
<literal>nix-env -f /my/sources/nixpkgs</literal>, or change the
default by adding a symlink in <literal>~/.nix-defexpr</literal>:
</para>
<programlisting>
$ ln -s /my/sources/nixpkgs ~/.nix-defexpr/nixpkgs
</programlisting>
<para>
You may want to delete the symlink
<literal>~/.nix-defexpr/channels_root</literal> to prevent roots
NixOS channel from clashing with your own tree (this may break the
command-not-found utility though). If you want to go back to the
default state, you may just remove the
<literal>~/.nix-defexpr</literal> directory completely, log out and
log in again and it should have been recreated with a link to the
root channels.
</para>
</chapter>

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<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="ch-testing-installer">
<title>Testing the Installer</title>
<para>
Building, burning, and booting from an installation CD is rather
tedious, so here is a quick way to see if the installer works
properly:
</para>
<programlisting>
# mount -t tmpfs none /mnt
# nixos-generate-config --root /mnt
$ nix-build '&lt;nixpkgs/nixos&gt;' -A config.system.build.nixos-install
# ./result/bin/nixos-install
</programlisting>
<para>
To start a login shell in the new NixOS installation in
<literal>/mnt</literal>:
</para>
<programlisting>
$ nix-build '&lt;nixpkgs/nixos&gt;' -A config.system.build.nixos-enter
# ./result/bin/nixos-enter
</programlisting>
</chapter>

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<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-writing-documentation">
<title>Writing NixOS Documentation</title>
<para>
As NixOS grows, so too does the need for a catalogue and explanation
of its extensive functionality. Collecting pertinent information
from disparate sources and presenting it in an accessible style
would be a worthy contribution to the project.
</para>
<section xml:id="sec-writing-docs-building-the-manual">
<title>Building the Manual</title>
<para>
The DocBook sources of the <xref linkend="book-nixos-manual" />
are in the
<link xlink:href="https://github.com/NixOS/nixpkgs/tree/master/nixos/doc/manual"><literal>nixos/doc/manual</literal></link>
subdirectory of the Nixpkgs repository.
</para>
<para>
You can quickly validate your edits with <literal>make</literal>:
</para>
<programlisting>
$ cd /path/to/nixpkgs/nixos/doc/manual
$ nix-shell
nix-shell$ make
</programlisting>
<para>
Once you are done making modifications to the manual, it's
important to build it before committing. You can do that as
follows:
</para>
<programlisting>
nix-build nixos/release.nix -A manual.x86_64-linux
</programlisting>
<para>
When this command successfully finishes, it will tell you where
the manual got generated. The HTML will be accessible through the
<literal>result</literal> symlink at
<literal>./result/share/doc/nixos/index.html</literal>.
</para>
</section>
<section xml:id="sec-writing-docs-editing-docbook-xml">
<title>Editing DocBook XML</title>
<para>
For general information on how to write in DocBook, see
<link xlink:href="http://www.docbook.org/tdg5/en/html/docbook.html">DocBook
5: The Definitive Guide</link>.
</para>
<para>
Emacs nXML Mode is very helpful for editing DocBook XML because it
validates the document as you write, and precisely locates errors.
To use it, see <xref linkend="sec-emacs-docbook-xml" />.
</para>
<para>
<link xlink:href="http://pandoc.org">Pandoc</link> can generate
DocBook XML from a multitude of formats, which makes a good
starting point. Here is an example of Pandoc invocation to convert
GitHub-Flavoured MarkDown to DocBook 5 XML:
</para>
<programlisting>
pandoc -f markdown_github -t docbook5 docs.md -o my-section.md
</programlisting>
<para>
Pandoc can also quickly convert a single
<literal>section.xml</literal> to HTML, which is helpful when
drafting.
</para>
<para>
Sometimes writing valid DocBook is simply too difficult. In this
case, submit your documentation updates in a
<link xlink:href="https://github.com/NixOS/nixpkgs/issues/new">GitHub
Issue</link> and someone will handle the conversion to XML for
you.
</para>
</section>
<section xml:id="sec-writing-docs-creating-a-topic">
<title>Creating a Topic</title>
<para>
You can use an existing topic as a basis for the new topic or
create a topic from scratch.
</para>
<para>
Keep the following guidelines in mind when you create and add a
topic:
</para>
<itemizedlist>
<listitem>
<para>
The NixOS
<link xlink:href="http://www.docbook.org/tdg5/en/html/book.html"><literal>book</literal></link>
element is in <literal>nixos/doc/manual/manual.xml</literal>.
It includes several
<link xlink:href="http://www.docbook.org/tdg5/en/html/book.html"><literal>parts</literal></link>
which are in subdirectories.
</para>
</listitem>
<listitem>
<para>
Store the topic file in the same directory as the
<literal>part</literal> to which it belongs. If your topic is
about configuring a NixOS module, then the XML file can be
stored alongside the module definition <literal>nix</literal>
file.
</para>
</listitem>
<listitem>
<para>
If you include multiple words in the file name, separate the
words with a dash. For example:
<literal>ipv6-config.xml</literal>.
</para>
</listitem>
<listitem>
<para>
Make sure that the <literal>xml:id</literal> value is unique.
You can use abbreviations if the ID is too long. For example:
<literal>nixos-config</literal>.
</para>
</listitem>
<listitem>
<para>
Determine whether your topic is a chapter or a section. If you
are unsure, open an existing topic file and check whether the
main element is chapter or section.
</para>
</listitem>
</itemizedlist>
</section>
<section xml:id="sec-writing-docs-adding-a-topic">
<title>Adding a Topic to the Book</title>
<para>
Open the parent XML file and add an <literal>xi:include</literal>
element to the list of chapters with the file name of the topic
that you created. If you created a <literal>section</literal>, you
add the file to the <literal>chapter</literal> file. If you
created a <literal>chapter</literal>, you add the file to the
<literal>part</literal> file.
</para>
<para>
If the topic is about configuring a NixOS module, it can be
automatically included in the manual by using the
<literal>meta.doc</literal> attribute. See
<xref linkend="sec-meta-attributes" /> for an explanation.
</para>
</section>
</chapter>