Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!sahayman
From: sahayman@iuvax.cs.indiana.edu (Steve Hayman)
Newsgroups: comp.windows.x
Subject: Figuring out the right DISPLAY variable when you rlogin.
Message-ID: <19688@iuvax.cs.indiana.edu>
Date: 15 Apr 89 00:44:58 GMT
Reply-To: sahayman@iuvax.cs.indiana.edu (Steve Hayman)
Organization: Computer Science Department, Indiana University
Lines: 60


Many X users that I've seen seem to have something like this in, for instance,
a .login file on a big Vax, to figure out what to set their DISPLAY
environment variable to whenever they rlogin from a workstation:

	if ( $TERM =~ xterm* ) then
		echo -n "What workstation are you using ? "

		# oops, I got this line wrong the first time I posted this...
		set host=$<
		setenv DISPLAY ${host}:0
	endif

or if they really want to get fancy and try to figure it out
automatically,

	if ( $TERM =~ $xterm* ) then
		# "who am i" shows something like
		# iuvax!sahayman ttyrd   Apr 14 12:25	(cssun1)
		# so cut out the remote host part

		set rhost=`who am i | sed -e 's/.*(//' -e 's/.)*$//'`
		setenv DISPLAY ${host}:0
	endif

Unforunately the latter technique is a kludge at best, and fails
if you do a chain of rlogins, since the "who am i" has no way of telling
you the original machine that you started on.

[Please stop me right here if there's some better way to do this that
I don't know about.  Thanks]

A suggestion.  xterm knows what the right DISPLAY is, since xterm is
actively using that information.  How about we extend the xterm protocol
a bit so that there is some escape sequence you can send it that will
case xterm to echo back what its idea of the DISPLAY variable is.
That way, your .login file could simply echo a few bytes and
read back a string and set the DISPLAY according to that string
The same idea as  eval `resize`.

This would work across chains of rlogins, since the escape sequence
would be interpreted by the original xterm and not by anything on
any of the intervening hosts.

As an experiment, I made this particular modification to our own xterm here,
having more or less chosen a suitable escape sequence at random.  In fact
I made it a bit more general,  <esc> ] 999 ; STRING <bel> makes
xterm echo back the value of its environment variable STRING.
(If one was gung ho, one could also make it echo back other xterm private
variables such as the current window name, etc etc, but DISPLAY
is about the only thing I can think of a use for at the moment.)

Perhaps something like this would be a worthwhile addition to the
standard xterm.  Anybody else got a better way to set your DISPLAY?

..Steve

-- 
Steve Hayman    Workstation Manager    Computer Science Department   Indiana U.
sahayman@iuvax.cs.indiana.edu                                   (812) 855-6984