Using Your Flake for nix run

Currently when you run nix run nixpkgs#python3, nix will checkout the latest nixpkgs and grab python3.

This poses three potential problems

1. Having to redownload nixpkgs is a bit much, especially when we already have it.
2. If your package is marked unfree or insecure and you allowed it explicitly in your nix config previously, that exception won’t apply here.
3. Overlays will not be applied.

Setting up a Flake Alias

To fix this, we can expose the instance of nixpkgs that we make in flake.nix to the nix CLI.

1. Add a new output in flake.nix for legacyPackages

   flake.nix

   outputs = inputs @ {
       ...
   }: ... {
       legacyPackages.${system} = pkgs;
   }

2. In your nix configuration, add an entry to the flake registry

   nix.nix

   {pkgs, inputs, ...}: {
       nix = {
           # ...
           registry."p".flake = inputs.self;
       };
   }

   I use p here as that’s what I use in my config, you can change this to any string (I recommend a short one).
3. Rebuild and switch

Now, whenever you want to run/shell a package, use p (or whatever alias you set up) instead of nixpkgs.

nix run p#python3

This will use the instance of nixpkgs that your flake exports, with all of your config and overlays applied!