# Getting the Sources

By default, NixOS’s `nixos-rebuild` command uses the NixOS and Nixpkgs sources provided by the `nixos` channel (kept in `/nix/var/nix/profiles/per-user/root/channels/nixos`). To modify NixOS, however, you should check out the latest sources from Git. This is as follows:

    $ git clone https://github.com/NixOS/nixpkgs
    $ cd nixpkgs
    $ git remote add channels https://github.com/NixOS/nixpkgs-channels
    $ git remote update channels

This will check out the latest Nixpkgs sources to `./nixpkgs` the NixOS sources to `./nixpkgs/nixos`. (The NixOS source tree lives in a subdirectory of the Nixpkgs repository.) The remote `channels` refers to a read-only repository that tracks the Nixpkgs/NixOS channels (see for more information about channels). Thus, the Git branch `channels/nixos-17.03` will contain the latest built and tested version available in the `nixos-17.03` channel.

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:

    $ nixos-version
    17.09pre104379.6e0b727 (Hummingbird)

    $ git checkout -b local 6e0b727

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

    $ git remote update channels
    $ git checkout -b local channels/nixos-17.03

(Replace `nixos-17.03` with the name of the channel you want to use.) You can use `git merge` or `git rebase` to keep your local branch in sync with the channel, e.g.

    $ git remote update channels
    $ git merge channels/nixos-17.03

You can use `git cherry-pick` to copy commits from your local branch to the upstream branch.

If you want to rebuild your system using your (modified) sources, you need to tell `nixos-rebuild` about them using the `-I` flag:

    # nixos-rebuild switch -I nixpkgs=/my/sources/nixpkgs

If you want `nix-env` to use the expressions in /my/sources, use `nix-env -f /my/sources/nixpkgs`, or change the default by adding a symlink in `~/.nix-defexpr`:

    $ ln -s /my/sources/nixpkgs ~/.nix-defexpr/nixpkgs

You may want to delete the symlink `~/.nix-defexpr/channels_root` 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 `~/.nix-defexpr` directory completely, log out and log in again and it should have been recreated with a link to the root channels.