<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-getting-sources">
 <title>Getting the Sources</title>
 <para>
  By default, NixOS’s <command>nixos-rebuild</command> command uses the NixOS
  and Nixpkgs sources provided by the <literal>nixos</literal> channel (kept in
  <filename>/nix/var/nix/profiles/per-user/root/channels/nixos</filename>). To
  modify NixOS, however, you should check out the latest sources from Git. This
  is as follows:
<screen>
<prompt>$ </prompt>git clone https://github.com/NixOS/nixpkgs
<prompt>$ </prompt>cd nixpkgs
<prompt>$ </prompt>git remote add channels https://github.com/NixOS/nixpkgs-channels
<prompt>$ </prompt>git remote update channels
</screen>
  This will check out the latest Nixpkgs sources to
  <filename>./nixpkgs</filename> the NixOS sources to
  <filename>./nixpkgs/nixos</filename>. (The NixOS source tree lives in a
  subdirectory of the Nixpkgs repository.) The remote
  <literal>channels</literal> refers to a read-only repository that tracks the
  Nixpkgs/NixOS channels (see <xref linkend="sec-upgrading"/> for more
  information about channels). Thus, the Git branch
  <literal>channels/nixos-17.03</literal> will contain the latest built and
  tested version available in the <literal>nixos-17.03</literal> channel.
 </para>
 <para>
  It’s often inconvenient to develop directly on the master branch, since if
  somebody has just committed (say) a change to GCC, then the binary cache may
  not have caught up yet and you’ll have to rebuild everything from source.
  So you may want to create a local branch based on your current NixOS version:
<screen>
<prompt>$ </prompt>nixos-version
17.09pre104379.6e0b727 (Hummingbird)

<prompt>$ </prompt>git checkout -b local 6e0b727
</screen>
  Or, to base your local branch on the latest version available in a NixOS
  channel:
<screen>
<prompt>$ </prompt>git remote update channels
<prompt>$ </prompt>git checkout -b local channels/nixos-17.03
</screen>
  (Replace <literal>nixos-17.03</literal> with the name of the channel you want
  to use.) You can use <command>git merge</command> or <command>git
  rebase</command> to keep your local branch in sync with the channel, e.g.
<screen>
<prompt>$ </prompt>git remote update channels
<prompt>$ </prompt>git merge channels/nixos-17.03
</screen>
  You can use <command>git cherry-pick</command> to copy commits from your
  local branch to the upstream branch.
 </para>
 <para>
  If you want to rebuild your system using your (modified) sources, you need to
  tell <command>nixos-rebuild</command> about them using the
  <option>-I</option> flag:
<screen>
<prompt># </prompt>nixos-rebuild switch -I nixpkgs=<replaceable>/my/sources</replaceable>/nixpkgs
</screen>
 </para>
 <para>
  If you want <command>nix-env</command> to use the expressions in
  <replaceable>/my/sources</replaceable>, use <command>nix-env -f
  <replaceable>/my/sources</replaceable>/nixpkgs</command>, or change the
  default by adding a symlink in <filename>~/.nix-defexpr</filename>:
<screen>
<prompt>$ </prompt>ln -s <replaceable>/my/sources</replaceable>/nixpkgs ~/.nix-defexpr/nixpkgs
</screen>
  You may want to delete the symlink
  <filename>~/.nix-defexpr/channels_root</filename> to prevent root’s NixOS
  channel from clashing with your own tree (this may break the
  command-not-found utility though). If you want to go back to the default
  state, you may just remove the <filename>~/.nix-defexpr</filename> directory
  completely, log out and log in again and it should have been recreated with a
  link to the root channels.
 </para>
<!-- FIXME: not sure what this means.
<para>You should not pass the base directory
<filename><replaceable>/my/sources</replaceable></filename>
to <command>nix-env</command>, as it will break after interpreting expressions
in <filename>nixos/</filename> as packages.</para>
-->
</chapter>
