The client program (for instance, your graphics application) knows
which display to connect to by inspecting the DISPLAY environment
variable. This setting can be overridden, though, by giving the client
the command line argument -display hostname:0 when it's started. Some
examples may clarify things.
Our computer is known to the outside as light, and we're in domain
uni.verse. If we're running a normal X server, the display is known as
light.uni.verse:0. We want to run the drawing program xfig on a remote
computer, called dark.matt.er, and display its output here on light.
Suppose you have already telnetted into the remote computer,
dark.matt.er.
If you have csh running on the remote computer:
dark% setenv DISPLAY light.uni.verse:0
dark% xfig &
or alternatively:
dark% xfig -display light.uni.verse:0 &
If you have sh running on the remote computer:
dark$ DISPLAY=light.uni.verse:0
dark$ export DISPLAY
dark$ xfig &
or, alternatively:
dark$ DISPLAY=light.uni.verse:0 xfig &
or, of course, also:
dark$ xfig -display light.uni.verse:0 &
It seems that some versions of telnet automatically transport the
DISPLAY variable to the remote host. If you have one of those,
you're lucky, and you don't have to set it by hand. If not, most versions
of telnet do transport the TERM environment variable; with some
judicious hacking it is possible to piggyback the DISPLAY variable
on to the TERM variable.
The idea with piggybacking is that you do some scripting to achieve
the following: before telnetting, attach the value of DISPLAY
to TERM. Then telnet out. At the remote end, in the applicable
.*shrc file, read the value of DISPLAY from TERM.