module Users = Set.Make(String)

let just_users line =
  let i = List.hd @@ String.split line ' ' in
  match i with
  | Some x -> "User was: " ^ x
  | None -> ""

let print_set set =
  Users.iter set (fun item -> print_endline @@ "Users on system includes: " ^ item)

let () =
  let names = Users.empty in
  let readme = Unix.open_process_in "ps aux" in
  let rec loop line =
    let names = Users.add names @@ just_users line in
    loop (Pervasives.input_line readme) in
  try
    loop (Pervasives.input_line readme)
  with
    End_of_file -> ignore (Unix.close_process_in readme);
		   print_set names