Open a file with su/sudo inside Emacs

Suppose I want to open a file in an existing Emacs session using su or sudo, without dropping down to a shell and doing sudoedit or sudo emacs. One way to do this is

C-x C-f /sudo::/path/to/file

but this requires an expensive round-trip through SSH. Is there a more direct way?

[EDIT] @JBB is right. I want to be able to invoke su/sudo to save as well as open. It would be OK (but not ideal) to re-authorize when saving. What I'm looking for is variations of find-file and save-buffer that can be "piped" through su/sudo.

The nice thing about Tramp is that you only pay for that round-trip to SSH when you open the first file. Sudo then caches your credentials, and Emacs saves a handle, so that subsequent sudo-opened files take much less time.

I haven't found the extra time it takes to save burdening, either. It's fast enough, IMO.

Tramp does not round-trip sudo via SSH, it uses a subshell. See the manual: https://www.gnu.org/software/tramp/#Inline-methods

Therefore, I recommend that you stick with TRAMP.

If you use helm, helm-find-files supports opening a file as root with C-c r.

Not really an answer to the original question, but here's a helper function to make doing the tramp/sudo route a bit easier:

(defun sudo-find-file (file-name)
"Like find file, but opens the file as root."
(interactive "FSudo Find File: ")
(let ((tramp-file-name (concat "/sudo::" (expand-file-name file-name))))
(find-file tramp-file-name)))

Your example doesn't start ssh at all, at least not with my version of TRAMP ("2.1.13-pre"). Both find-file and save-buffer work great.

At least for saving, a sudo-save package was written exactly for that kind of problem.

I recommend you to use advising commands. Put this function in your ~/.emacs

(defadvice ido-find-file (after find-file-sudo activate)
  "Find file as root if necessary."
  (unless (and buffer-file-name
               (file-writable-p buffer-file-name))
    (find-alternate-file (concat "/sudo:root@localhost:" buffer-file-name))))

(works only locally. Need to be updated to work correctly via tramp)

A little bit extended Burton's answer:

(defun sudo-find-file (file-name)
"Like find file, but opens the file as root."
(interactive "FSudo Find File: ")
(let ((tramp-file-name (concat "/sudo::" (expand-file-name file-name))))
(find-file tramp-file-name)))


(add-hook 'dired-mode-hook
    (lambda ()
      ;; open current file as sudo 
      (local-set-key (kbd "C-x <M-S-return>") (lambda()
        (interactive)
        (message "!!! SUDO opening %s" (dired-file-name-at-point))
        (sudo-find-file (dired-file-name-at-point))
      ))
    )
)

Ugh. Perhaps you could open a shell in Emacs and exec sudo emacs.

The problem is that you presumably don't just want to open the file. You want to be able to save it later. Thus you need your root privs to persist, not just exist for opening the file.

Sounds like you want Emacs to become your window manager. It's bloated enough without that. :)

I find sudo edit function very useful for that. After opening a file, press s-e to have sudo access to edit/save the file.