<chapter 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-channels">

<title>Channels</title>

<para>If you want to stay up to date with a set of packages, it’s not
very convenient to manually download the latest set of Nix expressions
for those packages and upgrade using <command>nix-env</command>.
Fortunately, there’s a better way: <emphasis>Nix
channels</emphasis>.</para>

<para>A Nix channel is just a URL that points to a place that contains
a set of Nix expressions and a manifest.  Using the command <link
linkend="sec-nix-channel"><command>nix-channel</command></link> you
can automatically stay up to date with whatever is available at that
URL.</para>
      
<para>To see the list of official NixOS channels, visit <link
xlink:href="https://nixos.org/channels" />.</para>

<para>You can “subscribe” to a channel using
<command>nix-channel --add</command>, e.g.,

<screen>
$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable</screen>

subscribes you to a channel that always contains that latest version
of the Nix Packages collection.  (Subscribing really just means that
the URL is added to the file <filename>~/.nix-channels</filename>,
where it is read by subsequent calls to <command>nix-channel
--update</command>.) You can “unsubscribe” using <command>nix-channel
--remove</command>:

<screen>
$ nix-channel --remove nixpkgs
</screen>
</para>

<para>To obtain the latest Nix expressions available in a channel, do

<screen>
$ nix-channel --update</screen>

This downloads and unpacks the Nix expressions in every channel
(downloaded from <literal><replaceable>url</replaceable>/nixexprs.tar.bz2</literal>).
It also makes the union of each channel’s Nix expressions available by
default to <command>nix-env</command> operations (via the symlink
<filename>~/.nix-defexpr/channels</filename>).  Consequently, you can
then say

<screen>
$ nix-env -u</screen>

to upgrade all packages in your profile to the latest versions
available in the subscribed channels.</para>

</chapter>
