Merge branch 'master' of github.com:thatdocslady/nix

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	doc/manual/release-notes.xml
	doc/manual/writing-nix-expressions.xml

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diff --git a/doc/manual/packages/channels.xml b/doc/manual/packages/channels.xml
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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+stability, but right now is just outdated.)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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