<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         version="5.0"
         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:
  <itemizedlist>
   <listitem>
    <para>
     <emphasis>Stable channels</emphasis>, such as
     <literal
    xlink:href="https://nixos.org/channels/nixos-19.09">nixos-19.09</literal>.
     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.<replaceable>x</replaceable> to 4.20.<replaceable>x</replaceable> (a
     major change that has the potential to break things). Stable channels are
     generally maintained until the next stable branch is created.
    </para>
    <para></para>
   </listitem>
   <listitem>
    <para>
     The <emphasis>unstable channel</emphasis>,
     <literal
    xlink:href="https://nixos.org/channels/nixos-unstable">nixos-unstable</literal>.
     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
     <literal
    xlink:href="https://nixos.org/channels/nixos-19.09-small">nixos-19.09-small</literal>
     or
     <literal
    xlink:href="https://nixos.org/channels/nixos-unstable-small">nixos-unstable-small</literal>.
     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>
  To see what channels are available, go to
  <link xlink:href="https://nixos.org/channels"/>. (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"/> 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 19.09 ISO, you will be subscribed to the
  <literal>nixos-19.09</literal> channel. To see which NixOS channel you’re
  subscribed to, run the following as root:
<screen>
# nix-channel --list | grep nixos
nixos https://nixos.org/channels/nixos-unstable
</screen>
  To switch to a different NixOS channel, do
<screen>
# nix-channel --add https://nixos.org/channels/<replaceable>channel-name</replaceable> nixos
</screen>
  (Be sure to include the <literal>nixos</literal> parameter at the end.) For
  instance, to use the NixOS 19.09 stable channel:
<screen>
# nix-channel --add https://nixos.org/channels/nixos-19.09 nixos
</screen>
  If you have a server, you may want to use the “small” channel instead:
<screen>
# nix-channel --add https://nixos.org/channels/nixos-19.09-small nixos
</screen>
  And if you want to live on the bleeding edge:
<screen>
# nix-channel --add https://nixos.org/channels/nixos-unstable nixos
</screen>
 </para>
 <para>
  You can then upgrade NixOS to the latest version in your chosen channel by
  running
<screen>
# nixos-rebuild switch --upgrade
</screen>
  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 <filename>configuration.nix</filename>:
<programlisting>
<xref linkend="opt-system.autoUpgrade.enable"/> = true;
</programlisting>
   This enables a periodically executed systemd service named
   <literal>nixos-upgrade.service</literal>. It runs <command>nixos-rebuild
   switch --upgrade</command> to upgrade NixOS to the latest version in the
   current channel. (To see when the service runs, see <command>systemctl
   list-timers</command>.) You can also specify a channel explicitly, e.g.
<programlisting>
<xref linkend="opt-system.autoUpgrade.channel"/> = https://nixos.org/channels/nixos-19.09;
</programlisting>
  </para>
 </section>
</chapter>
