Flakes simple shell

flake.nix

{
  description = "Multishell test";
  inputs = {
    nixpkgs.url = "nixpkgs/nixos-22.11";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = nixpkgs.legacyPackages.${system};
      in
      rec {
        devShells.default = pkgs.mkShell {
          packages = with pkgs; [
            ruby
          ];
        };
        devShells.buildShell = pkgs.mkShell {
          packages = with pkgs; [
            python38
          ];
        };
        # Not sure how to avoid duplication yet
        packages.default = pkgs.mkShell {
          packages = with pkgs; [ ruby ];
        };
        packages.buildShell = pkgs.mkShell {
          packages = with pkgs; [ python38 ];
        };
      });
}

nix develop - build shell

nix develop '.#buildShell' --command python --version
Python 3.8.16

nix develop - default shell

nix develop  --command ruby --version
ruby 2.7.7p221 (2022-11-24 revision 168ec2b1e5) [x86_64-linux]

nix shell - default

nix shell -c ruby
error: unable to execute 'ruby': No such file or directory