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<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, use <command>nix-pull</command> to register
pre-built binaries (if available), 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>You can “subscribe” to a channel using
<command>nix-channel --add</command>, e.g.,
<screen>
$ nix-channel --add http://nixos.org/channels/nixpkgs-unstable</screen>
subscribes you to a channel that always contains that latest version
of the Nix Packages collection. (Instead of
<literal>nixpkgs-unstable</literal> you could also subscribe to
<literal>nixpkgs-stable</literal>, which should have a higher level of
stability, but right now is just outdated.) Subscribing really just
means that the URL is added to the file
<filename>~/.nix-channels</filename>. Right now there is no command
to “unsubscribe”; you should just edit that file manually
and delete the offending URL.</para>
<para>To obtain the latest Nix expressions available in a channel, do
<screen>
$ nix-channel --update</screen>
This downloads the Nix expressions in every channel (downloaded from
<literal><replaceable>url</replaceable>/nixexprs.tar.bz2</literal>)
and registers any available pre-built binaries in every channel
(by <command>nix-pull</command>ing
<literal><replaceable>url</replaceable>/MANIFEST</literal>). It also
makes the union of each channel’s Nix expressions the default for
<command>nix-env</command> operations. 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>
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