Added new function in this changeset (http://hg.savannah.gnu.org/hgweb/octave/rev/5ff843d739fc).  Closing report.

Please find attached a new version of desktop.m reflecting recent discussion.
Is there a way in Octave to start the GUI from the command line?

(file #28849)

I think we're all in agreement the only thing this function should do in Octave is tell whether the GUI is running for compatibility.

The proposed script that Guillaume attached treats zero args as an error, regardless of if the GUI is running. I'd even say treat more than one arg as an error.

if (nargin != 1)
  print_usage ();
elseif (! strcmpi (varargin{1}, "-inuse"))
  error ("desktop: invalid argument");
endif

I was not sure where the file should be (scripts/general or scripts/ui?) so the attachment is not a changeset but a straight file.
You might also have changed your mind after the recent discussion on the maintainer list regarding on whether to call isguirunning() or not.

(file #28840)

Thanks. I was not aware of isguirunning(). Is the conclusion that we could add a desktop("-inuse") function that would call isguirunning() in Octave?

I think 'desktop ("-inuse") is closer in equivalency to isguirunning() for Octave since we won't use a Java desktop even if the GUI is running.

I couldn't find it on the MathWorks' website as well; that's why I copied the help text.
There is no mention of this function being deprecated though. It is the matlab/general folder and is mentioned in the Contents.m there:

% Controlling the command window.
%   echo          - Display statements during function execution.
%   more          - Control paged output in command window.
%   diary         - Save text of MATLAB session.
%   format        - Set output format.
%   beep          - Produce beep sound.
%   desktop       - Start and query the MATLAB Desktop.
%   preferences   - Bring up MATLAB user settable preferences dialog.

An example of use is the following:

if desktop('-inuse')
  fprintf('<a href="matlab:doSomethingOnClick">message</a>\n');
else
  fprintf('message\n');
end

When not using the desktop, you don't want the HTML code being displayed in your console.

In MATLAB (at least since 7.4 (R2007a)), there is a desktop() function:

 DESKTOP Start and query the MATLAB Desktop.
    DESKTOP starts the MATLAB Desktop using the configuration
    stored the last time the Desktop was run, or the default
    configuration if no configuration file is present.

    DESKTOP -NORESTORE doesn't restore the configuration from the last
    time the desktop was run.

    USED = DESKTOP('-INUSE') returns whether or not the Desktop is
    currently in use.  It does not start the Desktop.

It could be added to the list of Octave's unimplemented function; but having at least the option desktop('-inuse') to return false would help compatibility, in case the other options are not relevant in the Octave GUI framework.