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 = {
      x86_64-linux.default =
        nixpkgs.legacyPackages.x86_64-linux.mkShell {
          packages = [
            nixpkgs.legacyPackages.x86_64-linux.cowsay
          ];
        }; # mkShell

      aarch64-linux.default =
        nixpkgs.legacyPackages.aarch64-linux.mkShell {
          packages = [
            nixpkgs.legacyPackages.aarch64-linux.cowsay
          ];
        }; # mkShell
    }; # devShells

    packages = {
      x86_64-linux.default =
        nixpkgs.legacyPackages.x86_64-linux.stdenv.mkDerivation {
          name = "cow-hello.sh";
          src = ./.;
          unpackPhase = "true";
          buildPhase = ":";
          installPhase =
            ''
              mkdir -p $out/bin
              cp $src/cow-hello.sh $out/bin
              chmod +x $out/bin/cow-hello.sh
            '';
          buildInputs = [
            nixpkgs.legacyPackages.x86_64-linux.cowsay
          ];
        }; # mkDerivation

      aarch64-linux.default =
        nixpkgs.legacyPackages.aarch64-linux.stdenv.mkDerivation {
          name = "cow-hello.sh";
          src = ./.;
          unpackPhase = "true";
          buildPhase = ":";
          installPhase =
            ''
              mkdir -p $out/bin
              cp $src/cow-hello.sh $out/bin
              chmod +x $out/bin/cow-hello.sh
            '';
          buildInputs = [
            nixpkgs.legacyPackages.aarch64-linux.cowsay
          ];
        }; # mkDerivation
    }; # packages

  }; # outputs
}