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+<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-getting-sources">
+  <title>Getting the Sources</title>
+  <para>
+    By default, NixOS’s <literal>nixos-rebuild</literal> command uses
+    the NixOS and Nixpkgs sources provided by the
+    <literal>nixos</literal> channel (kept in
+    <literal>/nix/var/nix/profiles/per-user/root/channels/nixos</literal>).
+    To modify NixOS, however, you should check out the latest sources
+    from Git. This is as follows:
+  </para>
+  <programlisting>
+$ git clone https://github.com/NixOS/nixpkgs
+$ cd nixpkgs
+$ git remote update origin
+</programlisting>
+  <para>
+    This will check out the latest Nixpkgs sources to
+    <literal>./nixpkgs</literal> the NixOS sources to
+    <literal>./nixpkgs/nixos</literal>. (The NixOS source tree lives in
+    a subdirectory of the Nixpkgs repository.) The
+    <literal>nixpkgs</literal> repository has branches that correspond
+    to each Nixpkgs/NixOS channel (see <xref linkend="sec-upgrading" />
+    for more information about channels). Thus, the Git branch
+    <literal>origin/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:
+  </para>
+  <programlisting>
+$ nixos-version
+17.09pre104379.6e0b727 (Hummingbird)
+
+$ git checkout -b local 6e0b727
+</programlisting>
+  <para>
+    Or, to base your local branch on the latest version available in a
+    NixOS channel:
+  </para>
+  <programlisting>
+$ git remote update origin
+$ git checkout -b local origin/nixos-17.03
+</programlisting>
+  <para>
+    (Replace <literal>nixos-17.03</literal> with the name of the channel
+    you want to use.) You can use <literal>git merge</literal> or
+    <literal>git rebase</literal> to keep your local branch in sync with
+    the channel, e.g.
+  </para>
+  <programlisting>
+$ git remote update origin
+$ git merge origin/nixos-17.03
+</programlisting>
+  <para>
+    You can use <literal>git cherry-pick</literal> 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 <literal>nixos-rebuild</literal> about them using
+    the <literal>-I</literal> flag:
+  </para>
+  <programlisting>
+# nixos-rebuild switch -I nixpkgs=/my/sources/nixpkgs
+</programlisting>
+  <para>
+    If you want <literal>nix-env</literal> to use the expressions in
+    <literal>/my/sources</literal>, use
+    <literal>nix-env -f /my/sources/nixpkgs</literal>, or change the
+    default by adding a symlink in <literal>~/.nix-defexpr</literal>:
+  </para>
+  <programlisting>
+$ ln -s /my/sources/nixpkgs ~/.nix-defexpr/nixpkgs
+</programlisting>
+  <para>
+    You may want to delete the symlink
+    <literal>~/.nix-defexpr/channels_root</literal> 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
+    <literal>~/.nix-defexpr</literal> directory completely, log out and
+    log in again and it should have been recreated with a link to the
+    root channels.
+  </para>
+</chapter>