lief try building this

flake.nix

[nix-shell:~/src/cowsay]$ cat flake.nix 

{
  description = "A simple script that runs cowsay";
 
  outputs = { self, nixpkgs }: {
    defaultPackage.x86_64-linux = self.packages.x86_64-linux.my-script;
 
    packages.x86_64-linux.my-script =
      let
        pkgs = import nixpkgs { system = "x86_64-linux"; };
      in
      pkgs.writeShellScriptBin "my-script" ''
        DATE="$(\${pkgs.coreutils}/bin/date +'the %d of %m, %Y')"
        \${pkgs.cowsay}/bin/cowsay Hello, world! Today is $DATE.
      '';
  };
}


copy this to a flake.nix
and do 
nix build 
and you'll get a new script in result/bin/my-script ... 
enjoy!
it will work! it's impossible for it not to!

I can help with that, if you got questions or get stuck, let me know.