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108 lines
4.1 KiB
Markdown
108 lines
4.1 KiB
Markdown
# Overlays
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In the previous section, we learned about overriding derivations by `pkgs.xxx.override { ... }` or `pkgs.xxx.overrideAttrs (finalAttrs: previousAttrs: { ... });`.
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However, this approach will generate a new derivation and doesn't modify the original derivation in `pkgs` instance.
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If the derivation you want to override is also used by other Nix packages, they will still use the unmodified derivation.
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To globally modify derivations in the detault nixpkgs instance, Nix provides a feature called "overlays".
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In traditional Nix environments, overlays can be configured globally using the `~/.config/nixpkgs/overlays.nix` or `~/.config/nixpkgs/overlays/*.nix` files.
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However, with Flakes feature, to ensure system reproducibility, overlays cannot rely on configurations outside of the Git repository.
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When using `flake.nix` to configure NixOS, both Home Manager and NixOS provide the `nixpkgs.overlays` option to define overlays. You can refer to the following documentation for more details:
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- [Home Manager docs - `nixpkgs.overlays`](https://nix-community.github.io/home-manager/options.xhtml#opt-nixpkgs.overlays)
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- [Nixpkgs source code - `nixpkgs.overlays`](https://github.com/NixOS/nixpkgs/blob/30d7dd7e7f2cba9c105a6906ae2c9ed419e02f17/nixos/modules/misc/nixpkgs.nix#L169)
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Let's take a look at an example module that loads overlays. This module can be used as a Home Manager module or a NixOS module, as the definitions are the same:
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```nix
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# ./overlays/default.nix
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{ config, pkgs, lib, ... }:
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{
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nixpkgs.overlays = [
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# Overlay 1: Use `self` and `super` to express
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# the inheritance relationship
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(self: super: {
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google-chrome = super.google-chrome.override {
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commandLineArgs =
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"--proxy-server='https=127.0.0.1:3128;http=127.0.0.1:3128'";
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};
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})
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# Overlay 2: Use `final` and `prev` to express
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# the relationship between the new and the old
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(final: prev: {
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steam = prev.steam.override {
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extraPkgs = pkgs: with pkgs; [
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keyutils
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libkrb5
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libpng
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libpulseaudio
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libvorbis
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stdenv.cc.cc.lib
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xorg.libXcursor
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xorg.libXi
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xorg.libXinerama
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xorg.libXScrnSaver
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];
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extraProfile = "export GDK_SCALE=2";
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};
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})
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# Overlay 3: Define overlays in other files
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# The content of ./overlays/overlay3/default.nix is the same as above:
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# `(final: prev: { xxx = prev.xxx.override { ... }; })`
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(import ./overlay3)
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];
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}
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```
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In the above example, we define three overlays.
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1. Overlay 1 modifies the `google-chrome` derivation by adding a command-line argument for a proxy server.
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2. Overlay 2 modifies the `steam` derivation by adding extra packages and environment variables.
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3. Overlay 3 is defined in a separate file `./overlays/overlay3/default.nix`.
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One example of importing the above configuration as a NixOS module is as follows:
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```nix
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# ./flake.nix
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{
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inputs = {
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# ...
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};
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outputs = inputs@{ nixpkgs, ... }: {
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nixosConfigurations = {
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my-nixos = nixpkgs.lib.nixosSystem {
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system = "x86_64-linux";
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modules = [
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./configuration.nix
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# import the module that contains overlays
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(import ./overlays)
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];
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};
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};
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};
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}
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```
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This is just an example. Please write your own overlays according to your needs.
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## Multiple nixpkgs Instances with different Overlays
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The `nixpkgs.overlays = [...];` mentioned above directly modifies the global nixpkgs instance `pkgs`. If your overlays make changes to some low-level packages, it might impact other modules.
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One downside is an increase in local compilation (due to cache invalidation),
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and there might also be functionality issues with the affected packages.
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If you wish to utilize overlays only in a specific location without affecting the default nixpkgs instance, you can instantiate a new nixpkgs instance and apply your overlays to it.
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We will discuss how to do this in the next section [The Ingenious Uses of Multiple nixpkgs Instances](./multiple-nixpkgs.md).
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## References
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- [Chapter 3. Overlays - nixpkgs Manual](https://nixos.org/manual/nixpkgs/stable/#chap-overlays)
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