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66 lines
2.9 KiB
Markdown
66 lines
2.9 KiB
Markdown
# Custom NIX_PATH and Flake Registry
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## Introduction to NIX_PATH {#nix-path-introduction}
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The Nix search path is controlled by the environment variable `NIX_PATH`, which follows
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the same format as the Linux `PATH` environment variable, consisting of multiple paths
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separated by colons.
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Paths in Nix expressions that look like `<name>` are resolved using the path named `name`
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from the `NIX_PATH`.
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This usage pattern is no longer recommended under the Flakes feature because it results in
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Flake builds depending on a mutable environment variable `NIX_PATH`, compromising
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reproducibility.
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However, in certain scenarios, we still need to use `NIX_PATH`, such as when we frequently
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use the command `nix repl '<nixpkgs>'`, which utilizes the Nixpkgs found through
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`NIX_PATH` search.
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## Introduction to Flakes Registry {#flakes-registry-introduction}
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The Flakes Registry is a center for Flake registration that assists us in using shorter
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IDs instead of lengthy flake repository addresses when using commands like `nix run`,
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`nix shell`, and more.
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By default, Nix looks up the corresponding GitHub repository address for this ID from
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<https://github.com/NixOS/flake-registry/blob/master/flake-registry.json>.
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For instance, if we execute `nix run nixpkgs#ponysay hello`, Nix will automatically
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retrieve the GitHub repository address for `nixpkgs` from the aforementioned JSON file. It
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then downloads the repository, locates the `flake.nix` within, and runs the corresponding
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`ponysay` package.
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## Custom NIX_PATH and Flake Registry {#custom-nix-path-and-flake-registry-1}
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> **NOTE: Newcomers should skip this section! Disabling `nix-channel` incorrectly may lead
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> to some headaches.**
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The roles of `NIX_PATH` and the Flake Registry have been explained earlier. In daily use,
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we typically want the `nixpkgs` used in commands like `nix repl '<nixpkgs>'`,
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`nix run nixpkgs#ponysay hello` to match the system's `nixpkgs`. This requires us to
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customize the `NIX_PATH` and Flake Registry. On the other hand, although `nix-channel` can
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coexist with the Flakes feature, in practice, Flakes can completely replace it, so we can
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also disable it.
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In your NixOS configuration, adding the following module will achieve the mentioned
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requirements:
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```nix
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{lib, nixpkgs, ...}: {
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# make `nix run nixpkgs#nixpkgs` use the same nixpkgs as the one used by this flake.
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nix.registry.nixpkgs.flake = nixpkgs;
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nix.channel.enable = false; # remove nix-channel related tools & configs, we use flakes instead.
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# 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.
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# Make `nix repl '<nixpkgs>'` use the same nixpkgs as the one used by this flake.
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environment.etc."nix/inputs/nixpkgs".source = "${nixpkgs}";
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# https://github.com/NixOS/nix/issues/9574
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nix.settings.nix-path = lib.mkForce "nixpkgs=/etc/nix/inputs/nixpkgs";
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}
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```
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## References
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- [Chapter 15. Nix Search Paths - Nix Pills](https://nixos.org/guides/nix-pills/nix-search-paths.html)
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