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+<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-20.09">nixos-20.09</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-20.09-small">nixos-20.09-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 20.09 ISO, you will be subscribed to the
+  <literal>nixos-20.09</literal> channel. To see which NixOS channel you’re
+  subscribed to, run the following as root:
+<screen>
+<prompt># </prompt>nix-channel --list | grep nixos
+nixos https://nixos.org/channels/nixos-unstable
+</screen>
+  To switch to a different NixOS channel, do
+<screen>
+<prompt># </prompt>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 20.09 stable channel:
+<screen>
+<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-20.09 nixos
+</screen>
+  If you have a server, you may want to use the “small” channel instead:
+<screen>
+<prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-20.09-small nixos
+</screen>
+  And if you want to live on the bleeding edge:
+<screen>
+<prompt># </prompt>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>
+<prompt># </prompt>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;
+<xref linkend="opt-system.autoUpgrade.allowReboot"/> = true;
+</programlisting>
+   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 <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>.)
+   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.
+<programlisting>
+<xref linkend="opt-system.autoUpgrade.channel"/> = https://nixos.org/channels/nixos-20.09;
+</programlisting>
+  </para>
+ </section>
+</chapter>