Is there a way to prevent Emacs from deleting an auto-saved file?

Question

I frequently run shell sessions under Emacs (via C-x shell). My practice is to turn auto-saving on these shell-interaction buffers, so that the resulting (auto-saved) files serve as a persistent record of what I've done (including all the generated output).

This works pretty well most of the time, but occasionally, Emacs deletes the auto-saved file.

The conditions that lead to such deletions are not entirely clear to me, but in any case, I would like to prevent them altogether.

Is there a way to tell Emacs to never delete an auto-saved file?

I realize that here I am probably using Emacs' auto-save feature in a way that is very far from the standard use-case for this feature, and therefore, it may be unreasonable to try to do what I am asking about here (i.e. keeping Emacs from deleting an auto-saved file). If so, is there some other way to have Emacs periodically save a shell-interaction buffer to disk, and never deleting the target file?

Answer 1

Have you tried customizing option delete-auto-save-files to nil?

delete-auto-save-files is a variable defined in files.el.

Its value is t

Documentation:

Non-nil means delete auto-save file when a buffer is saved or killed.

Note that the auto-save file will not be deleted if the buffer is killed when it has unsaved changes.

You can customize this variable.

See also:

• Elisp manual node Auto-Saving

• Emacs manual node Auto-Save Files

Answer 2

At least as far as my installation of Emacs (v. 28.0.50) goes, below is the function responsible for deleting auto-saved files:

(defun delete-auto-save-file-if-necessary (&optional force)
  "Delete auto-save file for current buffer if `delete-auto-save-files' is t.
Normally delete only if the file was written by this Emacs since
the last real save, but optional arg FORCE non-nil means delete anyway."
  (and buffer-auto-save-file-name delete-auto-save-files
       (not (string= buffer-file-name buffer-auto-save-file-name))
       (or force (recent-auto-save-p))
       (progn
     (condition-case ()
         (delete-file buffer-auto-save-file-name)
       (file-error nil))
     (set-buffer-auto-saved))))

Therefore, it appears that it suffices to make the value of the buffer's buffer-file-name variable match that of its buffer-auto-save-file-name variable to keep Emacs from deleting the buffer's auto-saved file.

Note that while setting delete-auto-save-files to nil would cause all auto-save'd files to be preserved, making a buffer's buffer-file-name and buffer-auto-save-file-name variables match disables the deletion of only that buffer's auto-save'd file.