How do I get emacs to write to read-only files automatically?

Question

I am finding myself editing a lot of files that are read-only. I usually hit C-x C-q to call toggle-read-only. Then I hit C-x C-s to save and get,

File foo.txt is write-protected; try to save anyway? (y or n)

After hitting y, the file is saved and the permissions on the file remain read-only.

Is there a way to shorten this process and make it so that simply saving a file with C-x C-s does the whole thing without prompting? Should I look into inserting chmod in before-save-hook and after-save-hook or is there a better way?

Answer 1

Adding a call to chmod in before-save-hook would be clean way to accomplish this. There isn't any setting you can change to avoid the permissions check.

Based on the follow-up question, it sounds like you'd like the files to be changed to writable by you automatically upon opening. This code does the trick:

(defun change-file-permissions-to-writable ()
  "to be run from find-file-hook, change write permissions"
  (when (not (file-writable-p buffer-file-name))
    (chmod buffer-file-name (file-modes-symbolic-to-number "u+w" (nth 8 (file-attributes buffer-file-name))))
    (if (not (file-writable-p buffer-file-name))
        (message "Unable to make file writable."))))

(add-hook 'find-file-hook 'change-file-permissions-to-writable)

Note: When I tested it on my Windows machine, the file permissions didn't show up until I tried to save the buffer, but it worked as expected. I personally feel uneasy about this customization, but it's your Emacs. :)

Answer 2

I agree with Trey that universally doing a chmod on write is risky -- read-only files are read-only for a reason, IMHO. Here's a way to specifically override things on a per-buffer basis. It's not ideal in that it overrides file-writable-p for the life of the buffer (or at least until you toggle my-override-mode-on-save back to nil), but it makes you make a conscious decision on a file-by-file basis (sort-of; it's really a buffer-by-buffer basis, which is fairly similar). Of course since you're looking to automatically toggle the read-only flag when the file is visited, you might not be interested in this distinction. Still, here it is; enjoy it or ignore it as you will.

(make-variable-buffer-local
 (defvar my-override-mode-on-save nil
   "Can be set to automatically ignore read-only mode of a file when saving."))

(defadvice file-writable-p (around my-overide-file-writeable-p act)
  "override file-writable-p if `my-override-mode-on-save' is set."
  (setq ad-return-value (or
                         my-override-mode-on-save
                         ad-do-it)))

(defun my-override-toggle-read-only ()
  "Toggle buffer's read-only status, keeping `my-override-mode-on-save' in sync."
  (interactive)
  (setq my-override-mode-on-save (not my-override-mode-on-save))
  (toggle-read-only))

P.S. Thanks to Trey for the ad-return-value pointer in the other SO question.