Diff two buffers

diff-buffers.el

(defun diff-buffers (buffer-A buffer-B)
  "Diff two buffers."
  (interactive
   (let* ((only-two? (eq 2 (count-windows)))
          (wins (sort (window-list)
                      (lambda (a b) (< (window-use-time a)
                                       (window-use-time b)))))
          (b1 (if only-two?
                  (window-buffer (first wins))
                (read-buffer "Buffer A to compare")))
          (b2 (if only-two?
                  (window-buffer (second wins))
                (read-buffer "Buffer B to compare"))))
     (list b1 b2)))
  (let ((old "/tmp/old-diff")
        (new "/tmp/new-diff"))
    (with-temp-file new
      (insert-buffer-substring buffer-A))
    (with-temp-file old
      (insert-buffer-substring buffer-B))
    (diff old new "-u" t)))