xdg-open and start-process-shell-command: Unable to open files in external apps

[Chandan Babu R <chandanrlinux@gmail.com>]

Hi all,

I have been using the following Emacs lisp code snippet to open files from
dired buffer on Emacs 26.1 which is shipped with Debian 10.

(defun sjihs-open-in-external-app ()
  (interactive)
  (let ((do-it nil)
	(file-path nil)
	(file-list
	 (cond
	  ((string-equal major-mode "dired-mode") (dired-get-marked-files))
	  (t (list (buffer-file-name))))))
    (setq do-it (if (<= (length file-list) 5)
		    t
		  (y-or-n-p "Open more than 5 files?")))
    (when do-it
      (mapc (lambda (file-path)
	      (start-process-shell-command
	       (concat "xdg-open " file-path) "sjihs async processes"
	       (format "xdg-open %s" (shell-quote-argument file-path))))
	    file-list))))
(define-key dired-mode-map (kbd "<f12>") 'sjihs-open-in-external-app)

The above code was originally obtained from
http://ergoemacs.org/emacs/emacs_dired_open_file_in_ext_apps.html.

After moving to (via a fresh install) Debian 11 (which ships Emacs 27.1) the
code snippet has stopped working.

However, the command "xdg-open /home/chandan/2016-2017.pdf" when invoked from the
shell command line opens the pdf application viewer (i.e. Okular)
correctly.

The failure occurs only when start-process-shell-command procedure is invoked
from within Emacs. The failure can also be recreated when
start-process-shell-command is invoked from ielm buffer as shown below,

(start-process-shell-command
	"Opening file"
        "sjihs async processes"
        (concat "xdg-open "
        	(shell-quote-argument "/home/chandan/2016-2017.pdf")))

I wrote the following trivial script ...

	#!/usr/bin/zsh -f

	echo "\$0 = $0; \$1 = $1" > /tmp/debug.log

and associated the script with application/pdf MIME type.

debug.log file contains the following string,
	$0 = /home/chandan/bin/open-pdf.sh; $1 = /home/chandan/2016-2017.pdf
... when xdg-open is invoked either from the command line or through Emacs'
start-process-shell-command procedure. However, Okular does not get spawned
only when trying to open the pdf file from within Emacs.

Any solutions or pointers to debugging this problem is very much appreciated.

-- 
chandan