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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-quick-start">
<title>Quick Start to Adding a Package</title>
<para>To add a package to Nixpkgs:
<orderedlist>
<listitem>
<para>Checkout the Nixpkgs source tree:
<screen>
$ git clone git://github.com/NixOS/nixpkgs.git
$ cd nixpkgs</screen>
</para>
</listitem>
<listitem>
<para>Find a good place in the Nixpkgs tree to add the Nix
expression for your package. For instance, a library package
typically goes into
<filename>pkgs/development/libraries/<replaceable>pkgname</replaceable></filename>,
while a web browser goes into
<filename>pkgs/applications/networking/browsers/<replaceable>pkgname</replaceable></filename>.
See <xref linkend="sec-organisation" /> for some hints on the tree
organisation. Create a directory for your package, e.g.
<screen>
$ mkdir pkgs/development/libraries/libfoo</screen>
</para>
</listitem>
<listitem>
<para>In the package directory, create a Nix expression — a piece
of code that describes how to build the package. In this case, it
should be a <emphasis>function</emphasis> that is called with the
package dependencies as arguments, and returns a build of the
package in the Nix store. The expression should usually be called
<filename>default.nix</filename>.
<screen>
$ emacs pkgs/development/libraries/libfoo/default.nix
$ git add pkgs/development/libraries/libfoo/default.nix</screen>
</para>
<para>You can have a look at the existing Nix expressions under
<filename>pkgs/</filename> to see how it’s done. Here are some
good ones:
<itemizedlist>
<listitem>
<para>GNU Hello: <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/misc/hello/default.nix"><filename>pkgs/applications/misc/hello/default.nix</filename></link>.
Trivial package, which specifies some <varname>meta</varname>
attributes which is good practice.</para>
</listitem>
<listitem>
<para>GNU cpio: <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/archivers/cpio/default.nix"><filename>pkgs/tools/archivers/cpio/default.nix</filename></link>.
Also a simple package. The generic builder in
<varname>stdenv</varname> does everything for you. It has
no dependencies beyond <varname>stdenv</varname>.</para>
</listitem>
<listitem>
<para>GNU Multiple Precision arithmetic library (GMP): <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/gmp/5.1.x.nix"><filename>pkgs/development/libraries/gmp/5.1.x.nix</filename></link>.
Also done by the generic builder, but has a dependency on
<varname>m4</varname>.</para>
</listitem>
<listitem>
<para>Pan, a GTK-based newsreader: <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/newsreaders/pan/default.nix"><filename>pkgs/applications/networking/newsreaders/pan/default.nix</filename></link>.
Has an optional dependency on <varname>gtkspell</varname>,
which is only built if <varname>spellCheck</varname> is
<literal>true</literal>.</para>
</listitem>
<listitem>
<para>Apache HTTPD: <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/servers/http/apache-httpd/2.4.nix"><filename>pkgs/servers/http/apache-httpd/2.4.nix</filename></link>.
A bunch of optional features, variable substitutions in the
configure flags, a post-install hook, and miscellaneous
hackery.</para>
</listitem>
<listitem>
<para>Thunderbird: <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/networking/mailreaders/thunderbird/default.nix"><filename>pkgs/applications/networking/mailreaders/thunderbird/default.nix</filename></link>.
Lots of dependencies.</para>
</listitem>
<listitem>
<para>JDiskReport, a Java utility: <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/misc/jdiskreport/default.nix"><filename>pkgs/tools/misc/jdiskreport/default.nix</filename></link>
(and the <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/tools/misc/jdiskreport/builder.sh">builder</link>).
Nixpkgs doesn’t have a decent <varname>stdenv</varname> for
Java yet so this is pretty ad-hoc.</para>
</listitem>
<listitem>
<para>XML::Simple, a Perl module: <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/perl-packages.nix"><filename>pkgs/top-level/perl-packages.nix</filename></link>
(search for the <varname>XMLSimple</varname> attribute).
Most Perl modules are so simple to build that they are
defined directly in <filename>perl-packages.nix</filename>;
no need to make a separate file for them.</para>
</listitem>
<listitem>
<para>Adobe Reader: <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/misc/adobe-reader/default.nix"><filename>pkgs/applications/misc/adobe-reader/default.nix</filename></link>.
Shows how binary-only packages can be supported. In
particular the <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/misc/adobe-reader/builder.sh">builder</link>
uses <command>patchelf</command> to set the RUNPATH and ELF
interpreter of the executables so that the right libraries
are found at runtime.</para>
</listitem>
</itemizedlist>
</para>
<para>Some notes:
<itemizedlist>
<listitem>
<para>All <varname linkend="chap-meta">meta</varname>
attributes are optional, but it’s still a good idea to
provide at least the <varname>description</varname>,
<varname>homepage</varname> and <varname
linkend="sec-meta-license">license</varname>.</para>
</listitem>
<listitem>
<para>You can use <command>nix-prefetch-url</command> (or similar nix-prefetch-git, etc)
<replaceable>url</replaceable> to get the SHA-256 hash of
source distributions. There are similar commands as <command>nix-prefetch-git</command> and
<command>nix-prefetch-hg</command> available in <literal>nix-prefetch-scripts</literal> package.</para>
</listitem>
<listitem>
<para>A list of schemes for <literal>mirror://</literal>
URLs can be found in <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/build-support/fetchurl/mirrors.nix"><filename>pkgs/build-support/fetchurl/mirrors.nix</filename></link>.</para>
</listitem>
</itemizedlist>
</para>
<para>The exact syntax and semantics of the Nix expression
language, including the built-in function, are described in the
Nix manual in the <link
xlink:href="http://hydra.nixos.org/job/nix/trunk/tarball/latest/download-by-type/doc/manual/#chap-writing-nix-expressions">chapter
on writing Nix expressions</link>.</para>
</listitem>
<listitem>
<para>Add a call to the function defined in the previous step to
<link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/top-level/all-packages.nix"><filename>pkgs/top-level/all-packages.nix</filename></link>
with some descriptive name for the variable,
e.g. <varname>libfoo</varname>.
<screen>
$ emacs pkgs/top-level/all-packages.nix</screen>
</para>
<para>The attributes in that file are sorted by category (like
“Development / Libraries”) that more-or-less correspond to the
directory structure of Nixpkgs, and then by attribute name.</para>
</listitem>
<listitem>
<para>To test whether the package builds, run the following command
from the root of the nixpkgs source tree:
<screen>
$ nix-build -A libfoo</screen>
where <varname>libfoo</varname> should be the variable name
defined in the previous step. You may want to add the flag
<option>-K</option> to keep the temporary build directory in case
something fails. If the build succeeds, a symlink
<filename>./result</filename> to the package in the Nix store is
created.</para>
</listitem>
<listitem>
<para>If you want to install the package into your profile
(optional), do
<screen>
$ nix-env -f . -iA libfoo</screen>
</para>
</listitem>
<listitem>
<para>Optionally commit the new package and open a pull request, or send a patch to
<literal>https://groups.google.com/forum/#!forum/nix-devel</literal>.</para>
</listitem>
</orderedlist>
</para>
</chapter>
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