Org-mode LaTeX export set AUCTeX local variables

Question

I have an org-mode document, say

#+TITLE: My Title

* Section

My first section

and I want to export it to LaTeX with org-latex-export-to-latex. This generates a .tex file that I can now compile.

Question: Is there a way to append something like

%%% Local Variables:                                             
%%% coding: utf-8
%%% mode: latex
%%% TeX-engine: xetex
%%% End:

To the end of the .tex file, so that when I compile it with AUCTex, it will know to use the XeTeX engine to compile it instead of using the default one I have set? I tried the following

Using the org-export-before-parsing-functions hook

I defined the functions

(defun insert-at-end (text)
 "Insert the specified TEXT at the end of the current buffer."
   (save-excursion
      (goto-char (point-max))
      (insert text)))

(defun add-postamble ()
    (insert-at-end "%%% Local Variables: \n")
    (insert-at-end "%%% coding: utf-8\n")
    (insert-at-end "%%% mode: latex\n")
    (insert-at-end "%%% TeX-engine: xetex\n")
    (insert-at-end "%%% End: \n"))

Then tried adding it to the org-export-before-parsing-functions hook via

(add-hook 'org-export-before-parsing-functions
              'add-postamble)

What I got was the LaTeX file where the last few lines of the file looked like:

\%\%\% Local Variables: 
\%\%\% coding: utf-8
\%\%\% mode: latex
\%\%\% \TeX{}-engine: xetex
\%\%\% End: 
\end{document}

Using org-export-filter-final-output-functions

Defined a function

(defun my/org-export-add-lines (output backend info)
 "Add custom lines to the exported output."
 (when (and (eq backend 'latex)
                 (string= "add-lines-to-export" (plist-get info :post-export-output)))
   (concat output
           "%%% Local Variables: \n"
           "%%% coding: utf-8\n"
           "%%% mode: latex\n"
           "%%% TeX-engine: xetex\n"
           "%%% End: \n")))

and added them to org-export-filter-final-output-functions via

(add-to-list 'org-export-filter-final-output-functions 'my/org-export-add-lines)

This did nothing to the final output of the LaTeX file

Augmenting org-latex-classes

Suggested by ChatGPT I also tried to supplement org-latex-classes with a custom class with a line

("\\end{document}" . "\\end{document}\n%%%% Local Variables:\n%%%% coding: utf-8\n%%%% mode: latex\n%%%% TeX-engine: xetex\n%%%% End:"))

but that didn't work.

"Local variables"

I also tried adding "local-variables"?

# Local Variables:                                             
# org-coding: utf-8
# org-mode: latex
# org-TeX-engine: xetex
# End:

to my org buffer according to Using File Local Variables in org-mode, but this didn't seem to do anything. Perhaps I misunderstood what they were saying?

Answer 1

Here's a slight modification of your final-output filter function that works. It primarily involves getting rid of a test that AFAICT will never evaluate to t:

(defun my/org-export-add-lines (output backend info)
  "Add custom lines to the exported output."
  (when (org-export-derived-backend-p backend 'latex)
    (concat output "\n\n"
            "%%% Local Variables: \n"
            "%%% coding: utf-8\n"
            "%%% mode: latex\n"
            "%%% TeX-engine: xetex\n"
            "%%% End: \n")))

(add-to-list 'org-export-filter-final-output-functions 'my/org-export-add-lines)

I'm not sure how you expect the test (string= "add-lines-to-export" (plist-get info :post-export-output)) to ever succeed, so I've eliminated it.

Two smaller changes: a) do not use eq to test the backend; instead use org-export-derived-backend-p as the documentation shows. That way, the function will apply if the backend is a derived backend of the indicated backend, not only when it is equal to it; and b) added a couple of newlines to make the output a bit neater.