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<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-customising-packages">
<title>Customising Packages</title>
<para>
Some packages in Nixpkgs have options to enable or disable optional
functionality or change other aspects of the package. For instance,
the Firefox wrapper package (which provides Firefox with a set of
plugins such as the Adobe Flash player) has an option to enable the
Google Talk plugin. It can be set in
<literal>configuration.nix</literal> as follows:
<literal>nixpkgs.config.firefox.enableGoogleTalkPlugin = true;</literal>
</para>
<warning>
<para>
Unfortunately, Nixpkgs currently lacks a way to query available
configuration options.
</para>
</warning>
<para>
Apart from high-level options, it’s possible to tweak a package in
almost arbitrary ways, such as changing or disabling dependencies of
a package. For instance, the Emacs package in Nixpkgs by default has
a dependency on GTK 2. If you want to build it against GTK 3, you
can specify that as follows:
</para>
<programlisting language="bash">
environment.systemPackages = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ];
</programlisting>
<para>
The function <literal>override</literal> performs the call to the
Nix function that produces Emacs, with the original arguments
amended by the set of arguments specified by you. So here the
function argument <literal>gtk</literal> gets the value
<literal>pkgs.gtk3</literal>, causing Emacs to depend on GTK 3. (The
parentheses are necessary because in Nix, function application binds
more weakly than list construction, so without them,
<xref linkend="opt-environment.systemPackages" /> would be a list
with two elements.)
</para>
<para>
Even greater customisation is possible using the function
<literal>overrideAttrs</literal>. While the
<literal>override</literal> mechanism above overrides the arguments
of a package function, <literal>overrideAttrs</literal> allows
changing the <emphasis>attributes</emphasis> passed to
<literal>mkDerivation</literal>. This permits changing any aspect of
the package, such as the source code. For instance, if you want to
override the source code of Emacs, you can say:
</para>
<programlisting language="bash">
environment.systemPackages = [
(pkgs.emacs.overrideAttrs (oldAttrs: {
name = "emacs-25.0-pre";
src = /path/to/my/emacs/tree;
}))
];
</programlisting>
<para>
Here, <literal>overrideAttrs</literal> takes the Nix derivation
specified by <literal>pkgs.emacs</literal> and produces a new
derivation in which the original’s <literal>name</literal> and
<literal>src</literal> attribute have been replaced by the given
values by re-calling <literal>stdenv.mkDerivation</literal>. The
original attributes are accessible via the function argument, which
is conventionally named <literal>oldAttrs</literal>.
</para>
<para>
The overrides shown above are not global. They do not affect the
original package; other packages in Nixpkgs continue to depend on
the original rather than the customised package. This means that if
another package in your system depends on the original package, you
end up with two instances of the package. If you want to have
everything depend on your customised instance, you can apply a
<emphasis>global</emphasis> override as follows:
</para>
<programlisting language="bash">
nixpkgs.config.packageOverrides = pkgs:
{ emacs = pkgs.emacs.override { gtk = pkgs.gtk3; };
};
</programlisting>
<para>
The effect of this definition is essentially equivalent to modifying
the <literal>emacs</literal> attribute in the Nixpkgs source tree.
Any package in Nixpkgs that depends on <literal>emacs</literal> will
be passed your customised instance. (However, the value
<literal>pkgs.emacs</literal> in
<literal>nixpkgs.config.packageOverrides</literal> refers to the
original rather than overridden instance, to prevent an infinite
recursion.)
</para>
</section>
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