<chapter xmlns="http://docbook.org/ns/docbook"
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<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 = "GPLv3+";
};
</programlisting>

</para>

<para>Meta-attributes are not passed to the builder of the package.
Thus, a change to a meta-attribute doesn’t 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='1.0' encoding='utf-8'?>
&lt;items>
  &lt;item attrPath="hello" name="hello-2.3" system="i686-linux">
    &lt;meta name="description" value="A program that produces a familiar, friendly greeting" />
    &lt;meta name="homepage" value="http://www.gnu.org/software/hello/manual/" />
    &lt;meta name="license" value="GPLv3+" />
    &lt;meta name="longDescription" value="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;" />
  &lt;/item>
&lt;/items>
</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>Don’t include a period at the end.  Don’t include newline
    characters.  Capitalise the first character.  For brevity, don’t
    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 package’s 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.  See below for the
    allowed values.</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>

</variablelist>


</section>


<section><title>Licenses</title>

<note><para>This is just a first attempt at standardising the license
attribute.</para></note>

<para>The <varname>meta.license</varname> attribute must be one of the
following:

<variablelist>

  <varlistentry>
    <term><varname>GPL</varname></term>
    <listitem><para>GNU General Public License; version not
    specified.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>GPLv2</varname></term>
    <listitem><para>GNU General Public License, version
    2.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>GPLv2+</varname></term>
    <listitem><para>GNU General Public License, version
    2 or higher.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>GPLv3</varname></term>
    <listitem><para>GNU General Public License, version
    3.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>GPLv3+</varname></term>
    <listitem><para>GNU General Public License, version
    3 or higher.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>free</varname></term>
    <listitem><para>Catch-all for free software licenses not listed
    above.</para></listitem>
  </varlistentry>

  <varlistentry>
    <term><varname>free-copyleft</varname></term>
    <listitem><para>Catch-all for free, copyleft 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, it’s 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 doesn’t actually modify the
    original binaries; otherwise we’re 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>
