; dirname path: all of a path but its last part, as POSIX's: "." when
; there is no slash, "/" for what hangs from the root.
(def trim-slashes
  (fn (s)
    (let ((n (str-len s)))
      (if (and (> n 1) (= (str-sub s (- n 1)) "/"))
        (trim-slashes (str-sub s 0 (- n 1)))
        s))))

(if (= (length ARGS) 0)
  (echo "usage: dirname <path>")
  (let ((p (trim-slashes (nth ARGS 0)))
        (parts (str-split "/" p)))
    (cond
      ((= p "/") (echo "/"))
      ((= (length parts) 1) (echo "."))
      (else
        (let ((dir (trim-slashes (str-join "/" (reverse (tail (reverse parts)))))))
          (echo (if (= dir "") "/" dir)))))))