(* A word the shell passes on as it is written: no quote, expansion,
   assignment, glob, redirection or separator; any other is single-quoted. *)
let word s =
  let plain = function
    | 'a' .. 'z'
    | 'A' .. 'Z'
    | '0' .. '9'
    | '.' | '_' | '-' | '/' | '+' | '@' | ',' | ':' ->
        true
    | _ -> false
  in
  if s <> "" && String.for_all plain s then s else Filename.quote s