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1<chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-getting-sources">
2 <title>Getting the Sources</title>
3 <para>
4 By default, NixOS’s <literal>nixos-rebuild</literal> command uses
5 the NixOS and Nixpkgs sources provided by the
6 <literal>nixos</literal> channel (kept in
7 <literal>/nix/var/nix/profiles/per-user/root/channels/nixos</literal>).
8 To modify NixOS, however, you should check out the latest sources
9 from Git. This is as follows:
10 </para>
11 <programlisting>
12$ git clone https://github.com/NixOS/nixpkgs
13$ cd nixpkgs
14$ git remote update origin
15</programlisting>
16 <para>
17 This will check out the latest Nixpkgs sources to
18 <literal>./nixpkgs</literal> the NixOS sources to
19 <literal>./nixpkgs/nixos</literal>. (The NixOS source tree lives in
20 a subdirectory of the Nixpkgs repository.) The
21 <literal>nixpkgs</literal> repository has branches that correspond
22 to each Nixpkgs/NixOS channel (see <xref linkend="sec-upgrading" />
23 for more information about channels). Thus, the Git branch
24 <literal>origin/nixos-17.03</literal> will contain the latest built
25 and tested version available in the <literal>nixos-17.03</literal>
26 channel.
27 </para>
28 <para>
29 It’s often inconvenient to develop directly on the master branch,
30 since if somebody has just committed (say) a change to GCC, then the
31 binary cache may not have caught up yet and you’ll have to rebuild
32 everything from source. So you may want to create a local branch
33 based on your current NixOS version:
34 </para>
35 <programlisting>
36$ nixos-version
3717.09pre104379.6e0b727 (Hummingbird)
38
39$ git checkout -b local 6e0b727
40</programlisting>
41 <para>
42 Or, to base your local branch on the latest version available in a
43 NixOS channel:
44 </para>
45 <programlisting>
46$ git remote update origin
47$ git checkout -b local origin/nixos-17.03
48</programlisting>
49 <para>
50 (Replace <literal>nixos-17.03</literal> with the name of the channel
51 you want to use.) You can use <literal>git merge</literal> or
52 <literal>git rebase</literal> to keep your local branch in sync with
53 the channel, e.g.
54 </para>
55 <programlisting>
56$ git remote update origin
57$ git merge origin/nixos-17.03
58</programlisting>
59 <para>
60 You can use <literal>git cherry-pick</literal> to copy commits from
61 your local branch to the upstream branch.
62 </para>
63 <para>
64 If you want to rebuild your system using your (modified) sources,
65 you need to tell <literal>nixos-rebuild</literal> about them using
66 the <literal>-I</literal> flag:
67 </para>
68 <programlisting>
69# nixos-rebuild switch -I nixpkgs=/my/sources/nixpkgs
70</programlisting>
71 <para>
72 If you want <literal>nix-env</literal> to use the expressions in
73 <literal>/my/sources</literal>, use
74 <literal>nix-env -f /my/sources/nixpkgs</literal>, or change the
75 default by adding a symlink in <literal>~/.nix-defexpr</literal>:
76 </para>
77 <programlisting>
78$ ln -s /my/sources/nixpkgs ~/.nix-defexpr/nixpkgs
79</programlisting>
80 <para>
81 You may want to delete the symlink
82 <literal>~/.nix-defexpr/channels_root</literal> to prevent root’s
83 NixOS channel from clashing with your own tree (this may break the
84 command-not-found utility though). If you want to go back to the
85 default state, you may just remove the
86 <literal>~/.nix-defexpr</literal> directory completely, log out and
87 log in again and it should have been recreated with a link to the
88 root channels.
89 </para>
90</chapter>