nixpkgs/nixos/doc/manual/development/sources.xml

<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-unstable</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 done using the following command:

<screen>
$ nixos-checkout <replaceable>/my/sources</replaceable>
</screen>

or

<screen>
$ mkdir -p <replaceable>/my/sources</replaceable>
$ cd <replaceable>/my/sources</replaceable>
$ nix-env -i git
$ git clone git://github.com/NixOS/nixpkgs.git
</screen>

This will check out the latest NixOS sources to
<filename><replaceable>/my/sources</replaceable>/nixpkgs/nixos</filename>
and the Nixpkgs sources to
<filename><replaceable>/my/sources</replaceable>/nixpkgs</filename>.
(The NixOS source tree lives in a subdirectory of the Nixpkgs
repository.)</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>
$ <replaceable>/my/sources</replaceable>/nixpkgs/maintainers/scripts/update-channel-branches.sh
Fetching channels from https://nixos.org/channels:
 * [new branch]      cbe467e           -> channels/remotes/nixos-unstable
Fetching channels from nixos-version:
 * [new branch]      9ff4738           -> channels/current-system
Fetching channels from ~/.nix-defexpr:
 * [new branch]      0d4acad           -> channels/root/nixos
$ git checkout -b local channels/current-system
</screen>

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

<screen>
$ <replaceable>/my/sources</replaceable>/nixpkgs/maintainers/scripts/update-channel-branches.sh
$ git checkout -b local channels/remotes/nixos-unstable
</screen>

You can then use <command>git rebase</command> to sync your local
branch with the upstream branch, and 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.</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>