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+<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>