The function run-with-idle-timer has a million and one uses. The following use has vastly improved my life using NTEmacsWithCygwin, for example.

Here’s the problem it solves: I frequently want to switch to a buffer named “Shell”; I’ve formed the habit of typing merely * S h e RET, and letting completion complete the buffer name. However, sometimes there’s a buffer lying around named “Shell Command Output”. Since that buffer’s name shares a common prefix with the name of the buffer to which I want to switch, completion doesn’t completely complete :-) the name “Shell”; instead it leaves me with “*Shell”. Thus, when I then hit RET, I find myself looking at a new, empty, useless buffer instead of the one I wanted. Rather than telling completion to honor case (which is annoying on Win32, since the file system is itself case-insensitive), I simply tell Emacs to periodically kill any buffers named “Shell Command Output” that it finds lying around (if they’re not currently displayed in a window). The “periodically” part is what run-with-idle-timer does for us.

      (if completion-ignore-case
          (defvar reaper
            (run-with-idle-timer 5 t (lambda ()
                                       (let ((victim (get-buffer "*Shell Command Output*")))
                                         (when (and victim (not (get-buffer-window victim t)))
                                           (message "Killing buffer %s" (buffer-name victim))
                                           (kill-buffer victim)))))
            "idle-timer (see \\[run-with-idle-timer]) that deletes the
      *Shell Command Output* buffer -- that buffer's name is so similar to
      *shell* that completion makes the latter hard to type.  Use
      \\[cancel-timer] to cancel it."))

Icicles, IswitchBuffers, and BSBufferSelection all solve this problem in simpler ways. – DrewAdams

Yeah, I use IswitchBuffers now. – EricHanchrow


There are a few steps to set up a function to be repeated every now and again..

;; variable for the timer object
(defvar idle-timer-cookbook-timer nil)

;; callback function 
(defun idle-timer-cookbook-callback ()
  (message "I have been called (%s)" (current-time-string)))

;; start functions
(defun idle-timer-cookbook-run-once ()
  (when (timerp idle-timer-cookbook-timer)
    (cancel-timer idle-timer-cookbook-timer))
  (setq idle-timer-cookbook-timer
          (run-with-idle-timer 1 nil #'idle-timer-cookbook-callback)))

(defun idle-timer-cookbook-start ()
  (when (timerp idle-timer-cookbook-timer)
    (cancel-timer idle-timer-cookbook-timer))
  (setq idle-timer-cookbook-timer
          (run-with-timer 1 1 #'idle-timer-cookbook-callback)))

;; stop function
(defun idle-timer-cookbook-stop ()
  (when (timerp idle-timer-cookbook-timer)
    (cancel-timer idle-timer-cookbook-timer))
  (setq idle-timer-cookbook-timer nil))

run-with-timer vs run-with-idle-timer

These functions are quite similar, the main difference being that run-with-timer will call the callback function repeatedly whereas run-with-idle-timer does so only once (unless you specify the REPEAT parameter, that is)

Use with Anything

Anything is a candidate selection framework.

Start with M-x anything-timers, narrow the list by typing some patterns(multiple patterns are space-delimited string), select with up/down/pgup/pgdown/C-p/C-n/C-v/M-v, choose with enter to cancel,


See EmacsNiftyTricks