<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>
$ git clone git://github.com/NixOS/nixpkgs.git
$ cd nixpkgs
$ git remote add channels git://github.com/NixOS/nixpkgs-channels.git
$ 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>
$ nixos-version
17.09pre104379.6e0b727 (Hummingbird)

$ git checkout -b local 6e0b727
</screen>

Or, to base your local branch on the latest version available in a
NixOS channel:

<screen>
$ git remote update channels
$ 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>
$ git remote update channels
$ 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>
# 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>
$ 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>
