How to indicate in the mode-line that current-buffer's file has changed on disk?

Question

How can I show if a file has changed on disk, in the mode-line?

(I'm trying to troubleshoot why auto-revert doesn't work for me, and such an indication would help.)

Answer 1

Your question are actually two questions asked:

• How can I show if a file has changed on disk?
• How can I show a notification in the mode-line?

From what you provide as additional information:

I'm trying to troubleshoot why auto-revert doesn't work for me, and such an indication would help.

I conclude that the more important one and your actual question is:

How can I show if a file has changed on disk

One part required to arrive at a solution was already given in the comment to your question by Phil Hudson mentioning verify-visited-file-modtime . The missing part was how to run this function periodically and give a visual feedback if a file was modified by an external application.

The code below if placed in the initialization file does what you ask for:

(defun check-external-file-modification ()
  (if (verify-visited-file-modtime)
      nil
      (message "%s" "Info: BUFFER changed by external application")
  )
  (run-with-timer 5 nil 'check-external-file-modification)
)
(check-external-file-modification)

Notice that the value 5 in the code above means that the check is done each 5 seconds, but you can change it also to any (also a floating point) other value.

The notification about changed file status is shown each 5 seconds in the minibuffer and mirrored in *Messages*.

Your other question:

• How can I show notifications in the mode-line?

is worth another question if the code you have written yourself for this purpose and for your specific Mode Line layout should for some reason not work as expected.

Answer 2

This is my different implementation of auto-revert just wrapping revert-buffer with change detection using file-attributes that is provided only to retrieve file information, instead of verify-visited-file-modtime used by the original auto-revert. I haven't tested this much, but if you don't mind, try using this to find out if the cause is auto-revert or other environmental factors, HTH.

(setq-default my-file-attributes-table (make-hash-table)) ;; global hash table to store file attributes at visiting and saving.

;; to store file attributes.
(defun my-update-file-attributes (&optional buffer)
  (let ((buf (or buffer (current-buffer))))
    (puthash buf
             (file-attributes (buffer-file-name buf)) ;; using `file-attributes` to get file info more directly than `verify-visited-file-modtime`
             my-file-attributes-table)))

(add-hook 'find-file-hook 'my-update-file-attributes) ;; update at visiting.
(add-hook 'after-save-hook 'my-update-file-attributes) ;; update at saving.

;; to read modified date field in `file-attributes`
(defun my-read-modified-time (attributes)
  (current-time-string (nth 5 attributes))) ;; the 5th element is "The time of last modification as a Lisp timestamp (file-attribute-modification-time). This is the last time when the file’s contents were modified." (ref: 26.6.4 File Attributes)

;; to check file difference and prompt `revert-file`
(defun my-check-files-diff ()
  (maphash
   (lambda (buf attrs)
     (unless (string= (my-read-modified-time attrs) (my-read-modified-time (file-attributes (buffer-file-name buf))))
       (switch-to-buffer buf)
       (revert-buffer) ;; prompt
       (my-update-file-attributes buf))) ;; update to prevent from asking again after canceling revert.
   my-file-attributes-table))

(setq-default my-check-files-diff-interval 5) ;; check interval seconds.
(run-with-timer my-check-files-diff-interval my-check-files-diff-interval 'my-check-files-diff)