nix-book/source/new-flake/bash-flake/flake-3.nix

{
  description = "what does the cow say";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs";
  };

  outputs = { self, nixpkgs }: {

    devShells =
      let
        system = "x86_64-linux";
        pkgs = nixpkgs.legacyPackages.${system};
      in {
        ${system}.default =
            pkgs.mkShell {
              packages = [
                pkgs.cowsay
              ];
            }; # mkShell
    }; # devShells

  };
}