<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 = with stdenv.lib; {
  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 = licenses.gpl3Plus;
  maintainers = [ maintainers.eelco ];
  platforms = platforms.all;
};
</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 be 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 --json
{
    "hello": {
        "meta": {
            "description": "A program that produces a familiar, friendly greeting",
            "homepage": "http://www.gnu.org/software/hello/manual/",
            "license": {
                "fullName": "GNU General Public License version 3 or later",
                "shortName": "GPLv3+",
                "url": "http://www.fsf.org/licensing/licenses/gpl.html"
            },
            "longDescription": "GNU Hello is a program that prints \"Hello, world!\" when you run it.\nIt is fully customizable.\n",
            "maintainers": [
                "Ludovic Court\u00e8s &lt;ludo@gnu.org>"
            ],
            "platforms": [
                "i686-linux",
                "x86_64-linux",
                "armv5tel-linux",
                "armv7l-linux",
                "mips32-linux",
                "x86_64-darwin",
                "i686-cygwin",
                "i686-freebsd",
                "x86_64-freebsd",
                "i686-openbsd",
                "x86_64-openbsd"
            ],
            "position": "/home/user/dev/nixpkgs/pkgs/applications/misc/hello/default.nix:14"
        },
        "name": "hello-2.9",
        "system": "x86_64-linux"
    }
}


</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 xml:id="sec-standard-meta-attributes">
  <title>Standard meta-attributes</title>

  <para>
   It is expected that each meta-attribute is one of the following:
  </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>branch</varname>
    </term>
    <listitem>
     <para>
      Release branch. Used to specify that a package is not going to receive
      updates that are not in this branch; for example, Linux kernel 3.0 is
      supposed to be updated to 3.0.X, not 3.1.
     </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>downloadPage</varname>
    </term>
    <listitem>
     <para>
      The page where a link to the current version can be found. Example:
      <literal>http://ftp.gnu.org/gnu/hello/</literal>
     </para>
    </listitem>
   </varlistentry>
   <varlistentry>
    <term>
     <varname>license</varname>
    </term>
    <listitem>
     <para>
      The license, or licenses, for the package. One from the attribute set
      defined in
      <link
          xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix">
      <filename>nixpkgs/lib/licenses.nix</filename></link>. At this moment
      using both a list of licenses and a single license is valid. If the
      license field is in the form of a list representation, then it means that
      parts of the package are licensed differently. Each license should
      preferably be referenced by their attribute. The non-list attribute value
      can also be a space delimited string representation of the contained
      attribute shortNames or spdxIds. The following are all valid examples:
      <itemizedlist>
       <listitem>
        <para>
         Single license referenced by attribute (preferred)
         <literal>stdenv.lib.licenses.gpl3</literal>.
        </para>
       </listitem>
       <listitem>
        <para>
         Single license referenced by its attribute shortName (frowned upon)
         <literal>"gpl3"</literal>.
        </para>
       </listitem>
       <listitem>
        <para>
         Single license referenced by its attribute spdxId (frowned upon)
         <literal>"GPL-3.0"</literal>.
        </para>
       </listitem>
       <listitem>
        <para>
         Multiple licenses referenced by attribute (preferred) <literal>with
         stdenv.lib.licenses; [ asl20 free ofl ]</literal>.
        </para>
       </listitem>
       <listitem>
        <para>
         Multiple licenses referenced as a space delimited string of attribute
         shortNames (frowned upon) <literal>"asl20 free ofl"</literal>.
        </para>
       </listitem>
      </itemizedlist>
      For details, see <xref linkend='sec-meta-license'/>.
     </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/maintainers/maintainer-list.nix"><filename>nixpkgs/maintainers/maintainer-list.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. Hydra
      builds packages according to the platform specified. If no platform is
      specified, the package does not have prebuilt binaries. An example is:
<programlisting>
meta.platforms = stdenv.lib.platforms.linux;
</programlisting>
      Attribute Set <varname>stdenv.lib.platforms</varname> defines
      <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/systems/doubles.nix">
      various common lists</link> 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 won’t 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>
   <varlistentry>
    <term>
     <varname>updateWalker</varname>
    </term>
    <listitem>
     <para>
      If set to <literal>true</literal>, the package is tested to be updated
      correctly by the <literal>update-walker.sh</literal> script without
      additional settings. Such packages have <varname>meta.version</varname>
      set and their homepage (or the page specified by
      <varname>meta.downloadPage</varname>) contains a direct link to the
      package tarball.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </section>
 <section xml:id="sec-meta-license">
  <title>Licenses</title>

  <para>
   The <varname>meta.license</varname> attribute should preferrably contain a
   value from <varname>stdenv.lib.licenses</varname> defined in
   <link xlink:href="https://github.com/NixOS/nixpkgs/blob/master/lib/licenses.nix">
   <filename>nixpkgs/lib/licenses.nix</filename></link>, or in-place license
   description of the same format if the license is unlikely to be useful in
   another expression.
  </para>

  <para>
   Although it's typically better to indicate the specific license, a few
   generic options are available:
   <variablelist>
    <varlistentry>
     <term>
      <varname>stdenv.lib.licenses.free</varname>, <varname>"free"</varname>
     </term>
     <listitem>
      <para>
       Catch-all for free software licenses not listed above.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      <varname>stdenv.lib.licenses.unfreeRedistributable</varname>, <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>stdenv.lib.licenses.unfree</varname>, <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>stdenv.lib.licenses.unfreeRedistributableFirmware</varname>, <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>
