<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> <prompt>$ </prompt>git clone https://github.com/NixOS/nixpkgs <prompt>$ </prompt>cd nixpkgs <prompt>$ </prompt>git remote update origin </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 <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: <screen> <prompt>$ </prompt>nixos-version 17.09pre104379.6e0b727 (Hummingbird) <prompt>$ </prompt>git checkout -b local 6e0b727 </screen> Or, to base your local branch on the latest version available in a NixOS channel: <screen> <prompt>$ </prompt>git remote update origin <prompt>$ </prompt>git checkout -b local origin/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> <prompt>$ </prompt>git remote update origin <prompt>$ </prompt>git merge origin/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> <prompt># </prompt>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> <prompt>$ </prompt>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>