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+<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-ad-hoc-packages">
+  <title>Ad-Hoc Package Management</title>
+  <para>
+    With the command <literal>nix-env</literal>, you can install and
+    uninstall packages from the command line. For instance, to install
+    Mozilla Thunderbird:
+  </para>
+  <programlisting>
+$ nix-env -iA nixos.thunderbird
+</programlisting>
+  <para>
+    If you invoke this as root, the package is installed in the Nix
+    profile <literal>/nix/var/nix/profiles/default</literal> and visible
+    to all users of the system; otherwise, the package ends up in
+    <literal>/nix/var/nix/profiles/per-user/username/profile</literal>
+    and is not visible to other users. The <literal>-A</literal> flag
+    specifies the package by its attribute name; without it, the package
+    is installed by matching against its package name (e.g.
+    <literal>thunderbird</literal>). The latter is slower because it
+    requires matching against all available Nix packages, and is
+    ambiguous if there are multiple matching packages.
+  </para>
+  <para>
+    Packages come from the NixOS channel. You typically upgrade a
+    package by updating to the latest version of the NixOS channel:
+  </para>
+  <programlisting>
+$ nix-channel --update nixos
+</programlisting>
+  <para>
+    and then running <literal>nix-env -i</literal> again. Other packages
+    in the profile are <emphasis>not</emphasis> affected; this is the
+    crucial difference with the declarative style of package management,
+    where running <literal>nixos-rebuild switch</literal> causes all
+    packages to be updated to their current versions in the NixOS
+    channel. You can however upgrade all packages for which there is a
+    newer version by doing:
+  </para>
+  <programlisting>
+$ nix-env -u '*'
+</programlisting>
+  <para>
+    A package can be uninstalled using the <literal>-e</literal> flag:
+  </para>
+  <programlisting>
+$ nix-env -e thunderbird
+</programlisting>
+  <para>
+    Finally, you can roll back an undesirable <literal>nix-env</literal>
+    action:
+  </para>
+  <programlisting>
+$ nix-env --rollback
+</programlisting>
+  <para>
+    <literal>nix-env</literal> has many more flags. For details, see the
+    nix-env(1) manpage or the Nix manual.
+  </para>
+</section>