nixpkgs/nixos/doc/manual/configuration/ad-hoc-packages.xml

<section xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xmlns:xi="http://www.w3.org/2001/XInclude"
         version="5.0"
         xml:id="sec-ad-hoc-packages">
 <title>Ad-Hoc Package Management</title>

 <para>
  With the command <command>nix-env</command>, you can install and uninstall
  packages from the command line. For instance, to install Mozilla Thunderbird:
<screen>
<prompt>$ </prompt>nix-env -iA nixos.thunderbird</screen>
  If you invoke this as root, the package is installed in the Nix profile
  <filename>/nix/var/nix/profiles/default</filename> and visible to all users
  of the system; otherwise, the package ends up in
  <filename>/nix/var/nix/profiles/per-user/<replaceable>username</replaceable>/profile</filename>
  and is not visible to other users. The <option>-A</option> 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:
<screen>
<prompt>$ </prompt>nix-channel --update nixos
</screen>
  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
  <command>nixos-rebuild switch</command> 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:
<screen>
<prompt>$ </prompt>nix-env -u '*'
</screen>
 </para>

 <para>
  A package can be uninstalled using the <option>-e</option> flag:
<screen>
<prompt>$ </prompt>nix-env -e thunderbird
</screen>
 </para>

 <para>
  Finally, you can roll back an undesirable <command>nix-env</command> action:
<screen>
<prompt>$ </prompt>nix-env --rollback
</screen>
 </para>

 <para>
  <command>nix-env</command> has many more flags. For details, see the
  <citerefentry>
  <refentrytitle>nix-env</refentrytitle>
  <manvolnum>1</manvolnum></citerefentry> manpage or the Nix manual.
 </para>
</section>