How does xTerm communicate with the XWindows session that contains it?

Question

I want to script opening chromium in kiosk mode under Linux (openSUSE 12.3) and XWindows, while controlling which monitor chromium appear in. I also need to have this script be run outside of a standard xterm, e.g. via ssh into the machine where chromium AND the display will run (i.e. I want the application to appear on one of the displays of the machine I'm connecting to, not the machine I'm connecting from).

I can do everything I want by hand in xterm on the target machine, but I won't be running these scripts in xterm. Somehow, the xterm environment is special: it knows how to talk to the containing XWindows session, and in fact knows which actual monitor it is hosted on. For example:

• When I ssh into the machine and try to run an XWindows GUI on the target machine's display, it (reasonably) complains "Can't open display". However, even when I set the DISPLAY environment variable to the same value I see in xterm, I still get the error (although it now includes the current DISPLAY value). I've also tried setting DISPLAY to a variety of permutations on ":X.Y", where X and Y are small integers, but again to no avail.

• If I run chromium in --kiosk mode from xterm, it takes over the monitor that currently contains the xterm window. If I drag xterm to my other monitor then chromium will launch in that monitor. I want that control when I launch chromium from my script, but can't figure out the background magic.

For window manipulation, I've seen suggestions to use wmctrl or devilspie, but I'm a bit nervous about depending on tools that haven't been updated in years. I'd also prefer to have chromium launch in the right place right off the bat, rather than having it appear in the wrong place and then be moved. And neither app can help me launch the app remotely.

So, here are the questions. First, how does xterm tell chromium how to connect to the right XWindows session and monitor? Second, how do I simulate that from a completely separate ssh connection? (Again: it's more than just the DISPLAY variable.)

Thanks, Dan

Answer 1

Xterm is not special. Your mouse pointer is. Chromium will appear on the monitor your mouse pointer happens to be in. This is probably the default behaviour of your window manager.

Chromium does not respect the usual window position requests. You cannot tell Chromium where to appear, you must tell your window manager where to place the Chromium window. Methods of doing that, if they exist, totally depend on what window manager you have.

If you ssh to your machine as the user that has started the X session, you should have no problem connecting to that session.

Answer 2

You can't open X client on the target machine's display because of permissions: not anyone that can ssh into a machine may "take ownership" of it's screen. These are things controlled by xset -- read up on it.

As for the display: I assume that the specific monitor is controlled by the second part of the DISPLAY environment. At least that's what I remember from reading about X. I've never actually worked in a multi-monitor X environment.