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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>
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