<section xmlns="http://docbook.org/ns/docbook"  xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xi="http://www.w3.org/2001/XInclude" xml:id="sec-declarative-package-mgmt">
  <title>Declarative Package Management</title>
  <para>
    With declarative package management, you specify which packages you
    want on your system by setting the option
    <xref linkend="opt-environment.systemPackages" />. For instance,
    adding the following line to <literal>configuration.nix</literal>
    enables the Mozilla Thunderbird email application:
  </para>
  <programlisting language="bash">
environment.systemPackages = [ pkgs.thunderbird ];
</programlisting>
  <para>
    The effect of this specification is that the Thunderbird package
    from Nixpkgs will be built or downloaded as part of the system when
    you run <literal>nixos-rebuild switch</literal>.
  </para>
  <note>
    <para>
      Some packages require additional global configuration such as
      D-Bus or systemd service registration so adding them to
      <xref linkend="opt-environment.systemPackages" /> might not be
      sufficient. You are advised to check the
      <link linkend="ch-options">list of options</link> whether a NixOS
      module for the package does not exist.
    </para>
  </note>
  <para>
    You can get a list of the available packages as follows:
  </para>
  <programlisting>
$ nix-env -qaP '*' --description
nixos.firefox   firefox-23.0   Mozilla Firefox - the browser, reloaded
...
</programlisting>
  <para>
    The first column in the output is the <emphasis>attribute
    name</emphasis>, such as <literal>nixos.thunderbird</literal>.
  </para>
  <para>
    Note: the <literal>nixos</literal> prefix tells us that we want to
    get the package from the <literal>nixos</literal> channel and works
    only in CLI tools. In declarative configuration use
    <literal>pkgs</literal> prefix (variable).
  </para>
  <para>
    To <quote>uninstall</quote> a package, simply remove it from
    <xref linkend="opt-environment.systemPackages" /> and run
    <literal>nixos-rebuild switch</literal>.
  </para>
  <xi:include href="customizing-packages.section.xml" />
  <xi:include href="adding-custom-packages.section.xml" />
</section>
