The DISPLAY variable.

[peter@easthope.ca]

Hi,

https://wiki.debian.org/EnvironmentVariables has no mention 
of the DISPLAY variable.

https://www.debian.org/doc/manuals/debian-reference/ch07.en.html 
mentions DISPLAY only incidentally in the last section.

https://www.x.org/archive/X11R6.7.0/doc/X.7.html states, 
"... every X server has a display name of the form:
                     hostname:displaynumber.screennumber"

https://en.wikipedia.org/wiki/Environment_variable states,
"$DISPLAY
    Contains the identifier for the display that X11 programs 
      should use by default."

https://wiki.debian.org/Shell states 
"Within Debian, the default /bin/sh shell must be SUSv3/POSIX 
  compliant (see debian-policy)."

https://en.wikipedia.org/wiki/Environment_variable#Unix states,
"if used in front of a program to run, the variables will be 
  exported to the environment and thus appear as real environment 
  variables to the program:
    VARIABLE=value program_name [arguments]"

Here Debian jessie is running on a machine with two screens 
connected to one video card.  Therefore try these commands.
peter@dalton:~$ DISPLAY=:0.0 program
peter@dalton:~$ DISPLAY=:0.1 program
In both cases the program runs in a window opened on the left 
screen.  DISPLAY is ignored?

Also try program1.
peter@dalton:~$ DISPLAY=:0.0 program1
It runs in a window on the right screen.

peter@dalton:~$ DISPLAY=:0.1 program1
gives this error.
"program1: Kernel: Initialized and started.
Cannot open X11 display :0.1"

peter@dalton:~$ xrandr | grep Screen
Screen 0: minimum 320 x 200, current 2432 x 960, maximum 4096 x 4096

xrandr recognizes only one screen?  Why?  Any ideas to have 
the two screens recognized according to the documentation?

Thanks,             ... Peter E.

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