<refentry 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-nix-channel">

<refmeta>
  <refentrytitle>nix-channel</refentrytitle>
  <manvolnum>1</manvolnum>
  <refmiscinfo class="source">Nix</refmiscinfo>
  <refmiscinfo class="version"><xi:include href="../version.txt" parse="text"/></refmiscinfo>
</refmeta>

<refnamediv>
  <refname>nix-channel</refname>
  <refpurpose>manage Nix channels</refpurpose>
</refnamediv>

<refsynopsisdiv>
  <cmdsynopsis>
    <command>nix-channel</command>
    <group choice='req'>
      <arg choice='plain'><option>--add</option> <replaceable>url</replaceable> <arg choice='opt'><replaceable>name</replaceable></arg></arg>
      <arg choice='plain'><option>--remove</option> <replaceable>name</replaceable></arg>
      <arg choice='plain'><option>--list</option></arg>
      <arg choice='plain'><option>--update</option> <arg rep='repeat'><replaceable>names</replaceable></arg></arg>
      <arg choice='plain'><option>--rollback</option> <arg choice='opt'><replaceable>generation</replaceable></arg></arg>
    </group>
  </cmdsynopsis>
</refsynopsisdiv>

<refsection><title>Description</title>

<para>A Nix channel is a mechanism that allows you to automatically
stay up-to-date with a set of pre-built Nix expressions.  A Nix
channel is just a URL that points to a place containing a set of Nix
expressions.  <phrase condition="manual">See also <xref
linkend="sec-channels" />.</phrase></para>

<para>This command has the following operations:

<variablelist>

  <varlistentry><term><option>--add</option> <replaceable>url</replaceable> [<replaceable>name</replaceable>]</term>

    <listitem><para>Adds a channel named
    <replaceable>name</replaceable> with URL
    <replaceable>url</replaceable> to the list of subscribed channels.
    If <replaceable>name</replaceable> is omitted, it defaults to the
    last component of <replaceable>url</replaceable>, with the
    suffixes <literal>-stable</literal> or
    <literal>-unstable</literal> removed.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--remove</option> <replaceable>name</replaceable></term>

    <listitem><para>Removes the channel named
    <replaceable>name</replaceable> from the list of subscribed
    channels.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--list</option></term>

    <listitem><para>Prints the names and URLs of all subscribed
    channels on standard output.</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--update</option> [<replaceable>names</replaceable>…]</term>

    <listitem><para>Downloads the Nix expressions of all subscribed
    channels (or only those included in
    <replaceable>names</replaceable> if specified) and makes them the
    default for <command>nix-env</command> operations (by symlinking
    them from the directory
    <filename>~/.nix-defexpr</filename>).</para></listitem>

  </varlistentry>

  <varlistentry><term><option>--rollback</option> [<replaceable>generation</replaceable>]</term>

    <listitem><para>Reverts the previous call to <command>nix-channel
    --update</command>. Optionally, you can specify a specific channel
    generation number to restore.</para></listitem>

  </varlistentry>

</variablelist>

</para>

<para>Note that <option>--add</option> does not automatically perform
an update.</para>

<para>The list of subscribed channels is stored in
<filename>~/.nix-channels</filename>.</para>

</refsection>

<refsection><title>Examples</title>

<para>To subscribe to the Nixpkgs channel and install the GNU Hello package:</para>

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

<para>You can revert channel updates using <option>--rollback</option>:</para>

<screen>
$ nix-instantiate --eval -E '(import &lt;nixpkgs> {}).lib.nixpkgsVersion'
"14.04.527.0e935f1"

$ nix-channel --rollback
switching from generation 483 to 482

$ nix-instantiate --eval -E '(import &lt;nixpkgs> {}).lib.nixpkgsVersion'
"14.04.526.dbadfad"
</screen>

</refsection>

<refsection><title>Files</title>

<variablelist>

  <varlistentry><term><filename>/nix/var/nix/profiles/per-user/<replaceable>username</replaceable>/channels</filename></term>

    <listitem><para><command>nix-channel</command> uses a
    <command>nix-env</command> profile to keep track of previous
    versions of the subscribed channels. Every time you run
    <command>nix-channel --update</command>, a new channel generation
    (that is, a symlink to the channel Nix expressions in the Nix store)
    is created. This enables <command>nix-channel --rollback</command>
    to revert to previous versions.</para></listitem>

  </varlistentry>

  <varlistentry><term><filename>~/.nix-defexpr/channels</filename></term>

    <listitem><para>This is a symlink to
    <filename>/nix/var/nix/profiles/per-user/<replaceable>username</replaceable>/channels</filename>. It
    ensures that <command>nix-env</command> can find your channels. In
    a multi-user installation, you may also have
    <filename>~/.nix-defexpr/channels_root</filename>, which links to
    the channels of the root user.</para></listitem>

  </varlistentry>

</variablelist>

</refsection>

<refsection><title>Channel format</title>

<para>A channel URL should point to a directory containing the
following files:</para>

<variablelist>

  <varlistentry><term><filename>nixexprs.tar.xz</filename></term>

    <listitem><para>A tarball containing Nix expressions and files
    referenced by them (such as build scripts and patches). At the
    top level, the tarball should contain a single directory. That
    directory must contain a file <filename>default.nix</filename>
    that serves as the channel’s “entry point”.</para></listitem>

  </varlistentry>

</variablelist>

</refsection>

</refentry>
