How to suppress/avoid *Async Shell Command* buffer?

[Marius Hofert]

Hi,

I use

--8<---------------cut here---------------start------------->8---
(setq dired-guess-shell-alist-user
      '(("\\.\\(?:pdf\\|djvu\\|jp?g\\)\\'" "okular ? &")
        ("\\.html\\'" "google-chrome ? &")))
--8<---------------cut here---------------end--------------->8---

to open pdf files in dired-mode via "!". Once I do this, my dired buffer
is vertically divided into two pieces by opening a buffer named
*Async Shell Command*. The buffer does not show anything. It is quite
useless and *very* tedious to close (not only for pdf files). How can I
suppress this buffer / avoid this buffer being opened?

Here is what I found on this topic:
1)
http://stackoverflow.com/questions/3679930/how-to-automatically-remove-or-prevent-popping-up-async-shell-command-in-ema
2)
http://stackoverflow.com/questions/6915314/emacs-automatically-close-async-output-buffers-on-command-completion3)
http://stackoverflow.com/questions/12083228/how-do-i-speed-up-emacs-output-from-an-asynchronous-shell-command
4) 
http://stackoverflow.com/questions/13348230/create-buffer-and-immediately-hide-it
5) 
http://stackoverflow.com/questions/7298372/run-a-program-from-emacs-and-dont-wait-for-output
6) https://groups.google.com/forum/?fromgroups#!topic/gnu.emacs.help/TNqE7R7mULk

1), 2), 3): idea with save-window-excursion
4), 5): idea with start-process
6): call-process-shell-command

The problem is that I don't know how to *use* (one of) these approaches,
especially not with dired-mode's "!". Ideally, the buffer *Async Shell
Command* is not opened at all or at least is opened in the background
but does not 'destroy' (take up space from) the current buffer.

Cheers,

Marius