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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
NOTE: Newcomers should skip this section! Disabling
nix-channelincorrectly may lead to some headaches.
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. On the other hand, although nix-channel can
coexist with the Flakes feature, in practice, Flakes can completely replace it, so we can
also disable it.
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";
}