# Getting the Sources

By default, NixOS’s `nixos-rebuild` command uses the NixOS and Nixpkgs sources provided by the `nixos-unstable` 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 done using the following command:

    $ nixos-checkout /my/sources

or

    $ mkdir -p /my/sources
    $ cd /my/sources
    $ nix-env -i git
    $ git clone git://github.com/NixOS/nixpkgs.git
    $ cd nixpkgs
    $ git remote add channels git://github.com/NixOS/nixpkgs-channels.git
    $ git remote update channels

This will check out the latest NixOS sources to `/my/sources/nixpkgs/nixos` and the Nixpkgs sources to `/my/sources/nixpkgs`. (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-14.12` will contain the latest built and tested version available in the `nixos-14.12` 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
    14.04.273.ea1952b (Baboon)

    $ git checkout -b local ea1952b

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-14.12

(Replace `nixos-14.12` 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-14.12

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.