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 <xreflinkend="sec-organisation"/> for some hints on the tree organisation. Create a directory for your package, e.g.
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>.
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:
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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>. Nixpkgs doesn’t have a decent <varname>stdenv</varname> for Java yet so this is pretty ad-hoc.
</para>
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<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>
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<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>
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</itemizedlist>
</para>
<para>
Some notes:
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<listitem>
<para>
All <varnamelinkend="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>
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<listitem>
<para>
You can use <command>nix-prefetch-url</command><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>
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<para>
A list of schemes for <literal>mirror://</literal> URLs can be found in <link
xlink:href="https://hydra.nixos.org/job/nix/trunk/tarball/latest/download-by-type/doc/manual/#chap-writing-nix-expressions">chapter on writing Nix expressions</link>.
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>.
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>
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<para>
To test whether the package builds, run the following command from the root of the nixpkgs source tree:
<screen>
<prompt>$ </prompt>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>
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<para>
If you want to install the package into your profile (optional), do
Optionally commit the new package and open a pull request <link
xlink:href="https://github.com/NixOS/nixpkgs/pulls">to nixpkgs</link>, or use <link
xlink:href="https://discourse.nixos.org/t/about-the-patches-category/477"> the Patches category</link> on Discourse for sending a patch without a GitHub account.
