<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-upgrading">
  <title>Upgrading NixOS</title>
  <para>
    The best way to keep your NixOS installation up to date is to use
    one of the NixOS <emphasis>channels</emphasis>. A channel is a Nix
    mechanism for distributing Nix expressions and associated binaries.
    The NixOS channels are updated automatically from NixOS’s Git
    repository after certain tests have passed and all packages have
    been built. These channels are:
  </para>
  <itemizedlist>
    <listitem>
      <para>
        <emphasis>Stable channels</emphasis>, such as
        <link xlink:href="https://nixos.org/channels/nixos-21.11"><literal>nixos-21.11</literal></link>.
        These only get conservative bug fixes and package upgrades. For
        instance, a channel update may cause the Linux kernel on your
        system to be upgraded from 4.19.34 to 4.19.38 (a minor bug fix),
        but not from 4.19.x to 4.20.x (a major change that has the
        potential to break things). Stable channels are generally
        maintained until the next stable branch is created.
      </para>
    </listitem>
    <listitem>
      <para>
        The <emphasis>unstable channel</emphasis>,
        <link xlink:href="https://nixos.org/channels/nixos-unstable"><literal>nixos-unstable</literal></link>.
        This corresponds to NixOS’s main development branch, and may
        thus see radical changes between channel updates. It’s not
        recommended for production systems.
      </para>
    </listitem>
    <listitem>
      <para>
        <emphasis>Small channels</emphasis>, such as
        <link xlink:href="https://nixos.org/channels/nixos-21.11-small"><literal>nixos-21.11-small</literal></link>
        or
        <link xlink:href="https://nixos.org/channels/nixos-unstable-small"><literal>nixos-unstable-small</literal></link>.
        These are identical to the stable and unstable channels
        described above, except that they contain fewer binary packages.
        This means they get updated faster than the regular channels
        (for instance, when a critical security patch is committed to
        NixOS’s source tree), but may require more packages to be built
        from source than usual. They’re mostly intended for server
        environments and as such contain few GUI applications.
      </para>
    </listitem>
  </itemizedlist>
  <para>
    To see what channels are available, go to
    <link xlink:href="https://nixos.org/channels">https://nixos.org/channels</link>.
    (Note that the URIs of the various channels redirect to a directory
    that contains the channel’s latest version and includes ISO images
    and VirtualBox appliances.) Please note that during the release
    process, channels that are not yet released will be present here as
    well. See the Getting NixOS page
    <link xlink:href="https://nixos.org/nixos/download.html">https://nixos.org/nixos/download.html</link>
    to find the newest supported stable release.
  </para>
  <para>
    When you first install NixOS, you’re automatically subscribed to the
    NixOS channel that corresponds to your installation source. For
    instance, if you installed from a 21.11 ISO, you will be subscribed
    to the <literal>nixos-21.11</literal> channel. To see which NixOS
    channel you’re subscribed to, run the following as root:
  </para>
  <programlisting>
# nix-channel --list | grep nixos
nixos https://nixos.org/channels/nixos-unstable
</programlisting>
  <para>
    To switch to a different NixOS channel, do
  </para>
  <programlisting>
# nix-channel --add https://nixos.org/channels/channel-name nixos
</programlisting>
  <para>
    (Be sure to include the <literal>nixos</literal> parameter at the
    end.) For instance, to use the NixOS 21.11 stable channel:
  </para>
  <programlisting>
# nix-channel --add https://nixos.org/channels/nixos-21.11 nixos
</programlisting>
  <para>
    If you have a server, you may want to use the <quote>small</quote>
    channel instead:
  </para>
  <programlisting>
# nix-channel --add https://nixos.org/channels/nixos-21.11-small nixos
</programlisting>
  <para>
    And if you want to live on the bleeding edge:
  </para>
  <programlisting>
# nix-channel --add https://nixos.org/channels/nixos-unstable nixos
</programlisting>
  <para>
    You can then upgrade NixOS to the latest version in your chosen
    channel by running
  </para>
  <programlisting>
# nixos-rebuild switch --upgrade
</programlisting>
  <para>
    which is equivalent to the more verbose
    <literal>nix-channel --update nixos; nixos-rebuild switch</literal>.
  </para>
  <note>
    <para>
      Channels are set per user. This means that running
      <literal>nix-channel --add</literal> as a non root user (or
      without sudo) will not affect configuration in
      <literal>/etc/nixos/configuration.nix</literal>
    </para>
  </note>
  <warning>
    <para>
      It is generally safe to switch back and forth between channels.
      The only exception is that a newer NixOS may also have a newer Nix
      version, which may involve an upgrade of Nix’s database schema.
      This cannot be undone easily, so in that case you will not be able
      to go back to your original channel.
    </para>
  </warning>
  <section xml:id="sec-upgrading-automatic">
    <title>Automatic Upgrades</title>
    <para>
      You can keep a NixOS system up-to-date automatically by adding the
      following to <literal>configuration.nix</literal>:
    </para>
    <programlisting language="bash">
system.autoUpgrade.enable = true;
system.autoUpgrade.allowReboot = true;
</programlisting>
    <para>
      This enables a periodically executed systemd service named
      <literal>nixos-upgrade.service</literal>. If the
      <literal>allowReboot</literal> option is <literal>false</literal>,
      it runs <literal>nixos-rebuild switch --upgrade</literal> to
      upgrade NixOS to the latest version in the current channel. (To
      see when the service runs, see
      <literal>systemctl list-timers</literal>.) If
      <literal>allowReboot</literal> is <literal>true</literal>, then
      the system will automatically reboot if the new generation
      contains a different kernel, initrd or kernel modules. You can
      also specify a channel explicitly, e.g.
    </para>
    <programlisting language="bash">
system.autoUpgrade.channel = https://nixos.org/channels/nixos-21.11;
</programlisting>
  </section>
</chapter>
