nixpkgs/doc/meta.xml

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<chapter xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xml:id="chap-meta">
<title>Meta-attributes</title>
<para>Nix packages can declare <emphasis>meta-attributes</emphasis>
that contain information about a package such as a description, its
homepage, its license, and so on. For instance, the GNU Hello package
has a <varname>meta</varname> declaration like this:
<programlisting>
meta = {
description = "A program that produces a familiar, friendly greeting";
longDescription = ''
GNU Hello is a program that prints "Hello, world!" when you run it.
It is fully customizable.
'';
homepage = http://www.gnu.org/software/hello/manual/;
license = stdenv.lib.licenses.gpl3Plus;
maintainers = [ stdenv.lib.maintainers.eelco ];
platforms = stdenv.lib.platforms.all;
};
</programlisting>
</para>
<para>Meta-attributes are not passed to the builder of the package.
Thus, a change to a meta-attribute doesnt trigger a recompilation of
the package. The value of a meta-attribute must a string.</para>
<para>The meta-attributes of a package can be queried from the
command-line using <command>nix-env</command>:
<screen>
$ nix-env -qa hello --meta --xml
&lt;?xml version=&apos;1.0&apos; encoding=&apos;utf-8&apos;?&gt;
&lt;items&gt;
&lt;item attrPath=&quot;nixos.pkgs.hello&quot; name=&quot;hello-2.9&quot; system=&quot;x86_64-linux&quot;&gt;
&lt;meta name=&quot;description&quot; type=&quot;string&quot; value=&quot;A program that produces a familiar, friendly greeting&quot; /&gt;
&lt;meta name=&quot;homepage&quot; type=&quot;string&quot; value=&quot;http://www.gnu.org/software/hello/manual/&quot; /&gt;
&lt;meta name=&quot;license&quot; type=&quot;string&quot; value=&quot;GPLv3+&quot; /&gt;
&lt;meta name=&quot;longDescription&quot; type=&quot;string&quot; value=&quot;GNU Hello is a program that prints &amp;quot;Hello, world!&amp;quot; when you run it.&amp;#xA;It is fully customizable.&amp;#xA;&quot; /&gt;
&lt;meta name=&quot;maintainers&quot; type=&quot;strings&quot;&gt;
&lt;string value=&quot;Ludovic Courtès &amp;lt;ludo@gnu.org&amp;gt;&quot; /&gt;
&lt;/meta&gt;
&lt;meta name=&quot;platforms&quot; type=&quot;strings&quot;&gt;
&lt;string value=&quot;i686-linux&quot; /&gt;
&lt;string value=&quot;x86_64-linux&quot; /&gt;
&lt;string value=&quot;armv5tel-linux&quot; /&gt;
&lt;string value=&quot;armv7l-linux&quot; /&gt;
&lt;string value=&quot;mips64el-linux&quot; /&gt;
&lt;string value=&quot;x86_64-darwin&quot; /&gt;
&lt;string value=&quot;i686-cygwin&quot; /&gt;
&lt;string value=&quot;i686-freebsd&quot; /&gt;
&lt;string value=&quot;x86_64-freebsd&quot; /&gt;
&lt;string value=&quot;i686-openbsd&quot; /&gt;
&lt;string value=&quot;x86_64-openbsd&quot; /&gt;
&lt;/meta&gt;
&lt;meta name=&quot;position&quot; type=&quot;string&quot; value=&quot;/nix/store/cn8zjjdd9kvmp1p5d21h7ya0cr1jhkk3-nixos-14.10pre44264.12f06b3/nixos/nixpkgs/pkgs/applications/misc/hello/ex-2/default.nix:14&quot; /&gt;
&lt;/item&gt;
&lt;/items&gt;
</screen>
<command>nix-env</command> knows about the
<varname>description</varname> field specifically:
<screen>
$ nix-env -qa hello --description
hello-2.3 A program that produces a familiar, friendly greeting
</screen>
</para>
<section><title>Standard meta-attributes</title>
<para>The following meta-attributes have a standard
interpretation:</para>
<variablelist>
<varlistentry>
<term><varname>description</varname></term>
<listitem><para>A short (one-line) description of the package.
This is shown by <command>nix-env -q --description</command> and
also on the Nixpkgs release pages.</para>
<para>Dont include a period at the end. Dont include newline
characters. Capitalise the first character. For brevity, dont
repeat the name of package — just describe what it does.</para>
<para>Wrong: <literal>"libpng is a library that allows you to decode PNG images."</literal></para>
<para>Right: <literal>"A library for decoding PNG images"</literal></para>
</listitem>
</varlistentry>
<varlistentry>
<term><varname>longDescription</varname></term>
<listitem><para>An arbitrarily long description of the
package.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>homepage</varname></term>
<listitem><para>The packages homepage. Example:
<literal>http://www.gnu.org/software/hello/manual/</literal></para></listitem>
</varlistentry>
<varlistentry>
<term><varname>license</varname></term>
<listitem><para>The license for the package. One from attribute set defined in
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/lib/licenses.nix">
<filename>nixpkgs/lib/licenses.nix</filename></link>.
Example:
<literal>stdenv.lib.licenses.gpl3</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>maintainers</varname></term>
<listitem><para>A list of names and e-mail addresses of the
maintainers of this Nix expression. If
you would like to be a maintainer of a package, you may want to add
yourself to <link
xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/lib/maintainers.nix"><filename>nixpkgs/lib/maintainers.nix</filename></link>
and write something like <literal>[ stdenv.lib.maintainers.alice
stdenv.lib.maintainers.bob ]</literal>.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>priority</varname></term>
<listitem><para>The <emphasis>priority</emphasis> of the package,
used by <command>nix-env</command> to resolve file name conflicts
between packages. See the Nix manual page for
<command>nix-env</command> for details. Example:
<literal>"10"</literal> (a low-priority
package).</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>platforms</varname></term>
<listitem><para>The list of Nix platform types on which the
package is supported. An example is:
<programlisting>
meta.platforms = stdenv.lib.platforms.linux;
</programlisting>
Attribute Set <varname>stdenv.lib.platforms</varname> in
<link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/pkgs/lib/platforms.nix">
<filename>nixpkgs/lib/platforms.nix</filename></link> defines various common
lists of platforms types.
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>hydraPlatforms</varname></term>
<listitem><para>The list of Nix platform types for which the Hydra
instance at <literal>hydra.nixos.org</literal> will build the
package. (Hydra is the Nix-based continuous build system.) It
defaults to the value of <varname>meta.platforms</varname>. Thus,
the only reason to set <varname>meta.hydraPlatforms</varname> is
if you want <literal>hydra.nixos.org</literal> to build the
package on a subset of <varname>meta.platforms</varname>, or not
at all, e.g.
<programlisting>
meta.platforms = stdenv.lib.platforms.linux;
meta.hydraPlatforms = [];
</programlisting>
</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>broken</varname></term>
<listitem><para>If set to <literal>true</literal>, the package is
marked as “broken”, meaning that it wont show up in
<literal>nix-env -qa</literal>, and cannot be built or installed.
Such packages should be removed from Nixpkgs eventually unless
they are fixed.</para></listitem>
</varlistentry>
</variablelist>
</section>
<section xml:id="sec-meta-license"><title>Licenses</title>
<para>The <varname>meta.license</varname> attribute could be one of the
following:
<variablelist>
<varlistentry>
<term><varname>free</varname></term>
<listitem><para>Catch-all for free software licenses not listed
above.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>unfree-redistributable</varname></term>
<listitem><para>Unfree package that can be redistributed in binary
form. That is, its legal to redistribute the
<emphasis>output</emphasis> of the derivation. This means that
the package can be included in the Nixpkgs
channel.</para>
<para>Sometimes proprietary software can only be redistributed
unmodified. Make sure the builder doesnt actually modify the
original binaries; otherwise were breaking the license. For
instance, the NVIDIA X11 drivers can be redistributed unmodified,
but our builder applies <command>patchelf</command> to make them
work. Thus, its license is <varname>unfree</varname> and it
cannot be included in the Nixpkgs channel.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>unfree</varname></term>
<listitem><para>Unfree package that cannot be redistributed. You
can build it yourself, but you cannot redistribute the output of
the derivation. Thus it cannot be included in the Nixpkgs
channel.</para></listitem>
</varlistentry>
<varlistentry>
<term><varname>unfree-redistributable-firmware</varname></term>
<listitem><para>This package supplies unfree, redistributable
firmware. This is a separate value from
<varname>unfree-redistributable</varname> because not everybody
cares whether firmware is free.</para></listitem>
</varlistentry>
</variablelist>
</para>
</section>
</chapter>