Allow async command output buffer to be shown only on output

* lisp/simple.el (async-shell-command-display-buffer): Add
defcustom.
(shell-command): Use the new defcustom to determine whether to show
the buffer immediately, or add a process filter that shows it only
when there is some output.
* etc/NEWS: Document the new variable.
* doc/emacs/misc.texi: Likewise.

Thanks to Juri Linkov and Eli Zaretskii for advice and guidance.

doc/emacs/misc.texi

@@ -742,6 +742,11 @@ this; e.g., whether to rename the pre-existing output buffer, or to
 use a different buffer for the new command.  Consult the variable's
 documentation for more possibilities.
 
+@vindex async-shell-command-display-buffer
+  If you want the output buffer for asynchronous shell commands to be
+displayed only when the command generates output, set
+@code{async-shell-command-display-buffer} to @code{nil}.
+
 @kindex M-|
 @findex shell-command-on-region
   @kbd{M-|} (@code{shell-command-on-region}) is like @kbd{M-!}, but

etc/NEWS

@@ -205,6 +205,11 @@ When 'shell-command-dont-erase-buffer' is nil, the default value,
 the behavior of 'shell-command', 'shell-command-on-region' and
 'async-shell-command' is as usual.
 
++++
+** The new user option 'async-shell-command-display-buffer' controls
+whether the output buffer of an asynchronous command is shown
+immediately, or only when there is output.
+
 +++
 ** The new user option 'mouse-select-region-move-to-beginning'
 controls the position of point when double-clicking mouse-1 on the end

lisp/simple.el

@@ -3279,6 +3279,17 @@ output buffer and running a new command in the default buffer,
   :group 'shell
   :version "24.3")
 
+(defcustom async-shell-command-display-buffer t
+  "Whether to display the command buffer immediately.
+If t, display the buffer immediately; if nil, wait until there
+is output."
+  :type '(choice (const :tag "Display buffer immediately"
+			t)
+		 (const :tag "Display buffer on output"
+			nil))
+  :group 'shell
+  :version "26.1")
+
 (defun shell-command--save-pos-or-erase ()
   "Store a buffer position or erase the buffer.
 See `shell-command-dont-erase-buffer'."
@@ -3525,7 +3536,6 @@ the use of a shell (with its need to quote arguments)."
 		    (setq buffer (get-buffer-create
 				  (or output-buffer "*Async Shell Command*"))))))
 		(with-current-buffer buffer
-		  (display-buffer buffer '(nil (allow-no-window . t)))
                   (shell-command--save-pos-or-erase)
 		  (setq default-directory directory)
 		  (setq proc (start-process "Shell" buffer shell-file-name
@@ -3536,7 +3546,16 @@ the use of a shell (with its need to quote arguments)."
 		  ;; Use the comint filter for proper handling of carriage motion
 		  ;; (see `comint-inhibit-carriage-motion'),.
 		  (set-process-filter proc 'comint-output-filter)
-		  ))
+                  (if async-shell-command-display-buffer
+                      (display-buffer buffer '(nil (allow-no-window . t)))
+                    (add-function :before (process-filter proc)
+                                  `(lambda (process string)
+                                     (when (and (= 0 (buffer-size (process-buffer process)))
+                                                (string= (buffer-name (process-buffer process))
+                                                    ,(or output-buffer "*Async Shell Command*")))
+                                       (display-buffer (process-buffer process))))
+                                  ))
+                  ))
 	    ;; Otherwise, command is executed synchronously.
 	    (shell-command-on-region (point) (point) command
 				     output-buffer nil error-buffer)))))))