Overlays
Overlays provide a method to extend and change nixpkgs. They replace constructs like packageOverride and overridePackages.
Consider a simple example of setting the default proxy in Google Chrome:
let overlay1 = self: super:
{
google-chrome = super.google-chrome.override {
commandLineArgs =
"--proxy-server='https=127.0.0.1:3128;http=127.0.0.1:3128'";
};
};
Applying overlays manually
import <nixpkgs> { overlays = [ overlay1 overlay2 ]; }
Applying overlays automatically
On the user level
A list of overlays placed into ~/.config/nixpkgs/overlays.nix will be automatically loaded by all nix tools.
Alternatively, you can put each overlay in its own .nix file under your ~/.config/nixpkgs/overlays directory.
On the system level
If you want your overlays to be accessible by nix tools and also in the system-wide configuration, add nixpkgs-overlays to your NIX_PATH:
NIX_PATH="$NIX_PATH:nixpkgs-overlays=/cfg/overlays"
Currently nixos-rebuild only works with a <nixpkgs-overlays> path that is a directory.