* lisp/dired-aux.el (shell-command-guess-open): New defcustom (bug#18132).

(shell-command-guess-open): New function.
(shell-command-guess-functions): Add 'shell-command-guess-open' to choice.

etc/NEWS

@@ -505,9 +505,11 @@ default is nil.
 
 *** New user option 'shell-command-guess-functions'.
 It defines how to populate a list of commands available
-for 'M-!', 'M-&', '!', '&' based on marked files in Dired.
-Possible backends are 'dired-guess-default', MIME types,
-XDG configuration.
+for 'M-!', 'M-&', '!', '&' and the context menu "Open With"
+based on marked files in Dired.  Possible backends are
+'dired-guess-default', MIME types, XDG configuration
+and a universal command such as "open" or "start"
+that delegates to the OS.
 
 ** Ediff
 

lisp/dired-aux.el

@@ -1329,6 +1329,7 @@ such as added new commands."
           (choice (function-item shell-command-guess-dired)
                   (function-item shell-command-guess-mailcap)
                   (function-item shell-command-guess-xdg)
+                  (function-item shell-command-guess-open)
                   (function :tag "Custom function")))
   :group 'dired
   :version "30.1")
@@ -1380,6 +1381,27 @@ after adding own commands to the composite list."
                   xdg-mime-apps)))
     (append xdg-commands commands)))
 
+(defcustom shell-command-guess-open
+  (cond
+   ((executable-find "xdg-open")
+    "xdg-open")
+   ((memq system-type '(gnu/linux darwin))
+    "open")
+   ((memq system-type '(windows-nt ms-dos))
+    "start")
+   ((eq system-type 'cygwin)
+    "cygstart")
+   ((executable-find "run-mailcap")
+    "run-mailcap"))
+  "A shell command to open a file externally."
+  :type 'string
+  :group 'dired
+  :version "30.1")
+
+(defun shell-command-guess-open (commands _files)
+  "Populate COMMANDS by the `open' command."
+  (append (ensure-list shell-command-guess-open) commands))
+
 
 ;;; Commands that delete or redisplay part of the dired buffer