86 lines
4 KiB
XML
86 lines
4 KiB
XML
<section 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-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 <filename>configuration.nix</filename> as follows: <filename>
|
||
nixpkgs.config.firefox.enableGoogleTalkPlugin = true; </filename>
|
||
</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:
|
||
<programlisting>
|
||
<xref linkend="opt-environment.systemPackages"/> = [ (pkgs.emacs.override { gtk = pkgs.gtk3; }) ];
|
||
</programlisting>
|
||
The function <varname>override</varname> 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
|
||
<varname>gtk</varname> 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
|
||
<varname>overrideAttrs</varname>. While the <varname>override</varname>
|
||
mechanism above overrides the arguments of a package function,
|
||
<varname>overrideAttrs</varname> 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:
|
||
<programlisting>
|
||
<xref linkend="opt-environment.systemPackages"/> = [
|
||
(pkgs.emacs.overrideAttrs (oldAttrs: {
|
||
name = "emacs-25.0-pre";
|
||
src = /path/to/my/emacs/tree;
|
||
}))
|
||
];
|
||
</programlisting>
|
||
Here, <varname>overrideAttrs</varname> takes the Nix derivation specified by
|
||
<varname>pkgs.emacs</varname> 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
|
||
<varname>oldAttrs</varname>.
|
||
</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:
|
||
<screen>
|
||
nixpkgs.config.packageOverrides = pkgs:
|
||
{ emacs = pkgs.emacs.override { gtk = pkgs.gtk3; };
|
||
};
|
||
</screen>
|
||
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
|
||
<varname>nixpkgs.config.packageOverrides</varname> refers to the original
|
||
rather than overridden instance, to prevent an infinite recursion.)
|
||
</para>
|
||
</section>
|