nix-book/source/new-flake/bash-flake/flake-4.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

    packages =
      let
        system = "x86_64-linux";
        pkgs = nixpkgs.legacyPackages.${system};
      in rec {
        ${system}.hello = pkgs.stdenv.mkDerivation {
            name = "cow-hello";

            src = ./.;

            unpackPhase = "true";

            buildPhase = ":";

            installPhase =
              ''
                mkdir -p $out/bin
                cp $src/cow-hello.sh $out/bin
                chmod +x $out/bin/cow-hello.sh
              '';

            buildInputs = [ pkgs.cowsay ];
          };
          ${system}.default = hello;

    }; # packages
  };
}