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<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>
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