Custom NIX_PATH and Flake Registry
Introduction to NIX_PATH
The Nix search path is controlled by the environment variable NIX_PATH, which follows the same format as the Linux PATH environment variable, consisting of multiple paths separated by colons.
Paths in Nix expressions that look like <name> are resolved using the path named name from the NIX_PATH.
This usage pattern is no longer recommended under the Flakes feature because it results in Flake builds depending on a mutable environment variable NIX_PATH, compromising reproducibility.
However, in certain scenarios, we still need to use NIX_PATH, such as when we frequently use the command nix repl '<nixpkgs>', which utilizes the Nixpkgs found through NIX_PATH search.
Introduction to Flakes Registry
The Flakes Registry is a center for Flake registration that assists us in using shorter IDs instead of lengthy flake repository addresses when using commands like nix run, nix shell, and more.
By default, Nix looks up the corresponding GitHub repository address for this ID from https://github.com/NixOS/flake-registry/blob/master/flake-registry.json.
For instance, if we execute nix run nixpkgs#ponysay hello, Nix will automatically retrieve the GitHub repository address for nixpkgs from the aforementioned JSON file. It then downloads the repository, locates the flake.nix within, and runs the corresponding ponysay package.
Custom NIX_PATH and Flake Registry
The roles of NIX_PATH and the Flake Registry have been explained earlier. In daily use, we typically want the nixpkgs used in commands like nix repl '<nixpkgs>', nix run nixpkgs#ponysay hello to match the system's nixpkgs. This requires us to customize the NIX_PATH and Flake Registry.
In your NixOS configuration, adding the following module will achieve the mentioned requirements:
{lib, nixpkgs, ...}: {
# make `nix run nixpkgs#nixpkgs` use the same nixpkgs as the one used by this flake.
nix.registry.nixpkgs.flake = nixpkgs;
nix.channel.enable = false; # remove nix-channel related tools & configs, we use flakes instead.
# but NIX_PATH is still used by many useful tools, so we set it to the same value as the one used by this flake.
# Make `nix repl '<nixpkgs>'` use the same nixpkgs as the one used by this flake.
environment.etc."nix/inputs/nixpkgs".source = "${nixpkgs}";
# https://github.com/NixOS/nix/issues/9574
nix.settings.nix-path = lib.mkForce "nixpkgs=/etc/nix/inputs/nixpkgs";
}