# 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

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.)

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 the NixOS channel:

    $ curl -sI http://nixos.org/channels/nixos-unstable/ | grep Location
    Location: http://releases.nixos.org/nixos/unstable/nixos-14.10pre43986.acaf4a6/

    $ git checkout -b local acaf4a6

You can then use `git rebase` to sync your local branch with the upstream branch, and 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.