Perfect, that did what I wanted more precisely:

-----------------------
(require 'help-fns)
(require 'seq)

(defun aa/show-interactive-commands ()
  (interactive)
  (seq-sort-by #'length #'<
	       (all-completions ""
				#'help--symbol-completion-table  
				(lambda (f)
				  (commandp f)))))
-----------------------

Returns a list (sorted by size) of available interactive commands. I can
now make them known to i3 to integrate them on demand. Great!

Thanks,

        Alex


On Do, Nov 04 2021, 10:44:17, Robert Pluim wrote:

>>>>>> On Thu, 04 Nov 2021 09:08:26 +0100, Alexander Asteroth <alexander.asteroth@h-brs.de> said:
>
>     Alexander> Dear all,
>     Alexander> I don't know if I'm right here but I don't find a group that is devoted
>     Alexander> to elisp programming. I'm trying to write a little funtion that I can
>     Alexander> call from emacsclient to get a list of interactive functions
>     Alexander> available. The result shoud be a list of strings or just one string,
>     Alexander> more or less what the *Completions* buffer displays when pressing
>     Alexander> M-x followed by <TAB>. I'm sure there must be a very simple solution to
>     Alexander> this but somehow I get lost in the code in simple.el on my search for
>     Alexander> it.
>
> You mean something like this?
>
> (all-completions ""
>                #'help--symbol-completion-table  
>                (lambda (f)
>                  (commandp f)))
>
> (that returns quite a few things)
>
> Robert