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 as follows: $ 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 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-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.