This is something I frequently want to do: open a named file, work on it programmatically using Lisp code, and save it back – all without user intervention. Like a lot of things in Emacs, it’s easy once you know how.

The trick is to create a new, named, buffer for the file to get its contents. This is done with find-file-noselect as opposed to the more usual find-file that’s usually bound to C-x C-f, and as its name suggests finds (opens) the file without bringing it to the user’s attention. For example,

    ;; open the file in its own buffer
    (with-current-buffer (find-file-noselect fn)

      ;; work on it as required, as the current buffer
      (goto-char (point-min))
      (search-forward "#+END_COMMENT" nil t)
      (beginning-of-line 2)
      (delete-region (point) (point-max))
      (newline 2)

      ;; save the results back
      (save-buffer))

(This example comes from my Emacs interface to the Nikola static site builder used to maintain this site.) The code fragment leaves the current buffer unchanged as far as the user (and the rest of the code) is concerned, and so doesn’t need to be protected by save-excursion or the like.