nix-book/source/recipes/build/nixpkg/flake.nix

{
  description = "a very simple and friendly flake";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let
        pkgs = import nixpkgs { inherit system; };
      in
      {
        devShells = rec {
          default = pkgs.mkShell {
            packages = [ pkgs.cowsay ];
          };
        };

        packages = rec {
          hello = pkgs.writeShellApplication {
            name = "hello-cow";

            runtimeInputs = [
              pkgs.cowsay
            ];

            text = ''
              cowsay "hello";
            '';
          };
          default = hello;
        };

        apps = rec {
          hello = flake-utils.lib.mkApp { drv = self.packages.${system}.hello; };
          default = hello;
        };
      }
    );
}