yes they are.

Do they all login using CDE or Gnome, or are they running the program by some other means to get it on their screen?

Basically, how are you going to know who to send the messages to and which screens?

Are you then going to require a confirmation from each person or are you going to shutdown the server in the time period anyway?

If you want to open an x-window on a remote machine uninvited, it's generally non-trivial.

If the users are not using Xauth security (eg MIT magic cookies etc) then it's not _too_ hard to determine the display to send to and fire a window at it.

This 'military software'; does it run with the display name (ie <hostname>:<display number>) in the commandline or do they rely on a $DISPLAY environment variable?
For that matter, does it even use X or is it run within a terminal session via X?

It's totally doable, but there are a lot of factors that will determine how to go about it.

High-level approach:
Produce a list of users to warn (probably from 'ps' or 'who' output)
Identify the X server they are connecting from
Obtain a copy of appropriate authorisation to send something to their X server
Send the message, taking care not to present them with an app that has elevated privilages.
Log the response to the message (if you need to).

CDE to login. yes we have a built in program that can run locally on the machine to show the pop up box. the dialog box no matter if they click ok or not does not dictate the reboot, it is more of a warning of what i am about to do.

this is what i have used in the past but i think i am missing some variables.

rsh test004 `cd /h/test/bin`; DISPLAY=0.0; export DISPLAY; ./Msg "message here";&

but it keeps outputting the pop up box on my local box not the remote machines.

hostname is in the terminal window when you open it

now the thing that is different is that the host name will be a variable from a list so it would be more like rsh test0${x}
list="01 02 03 04 05 06"
for x in $list ; do
echo $x
then the command will be here

When the user logins in you want to kick off a program that sits silently waiting for a message to be displayed.

This will effectively capture the user's DISPLAY and authority. It needs to wait on some IPC mechanism. When it gets triggered, it can then run your "display a dialog message" program with the correct context and environment.

When the user logs out this process needs to be cleaned up.

One way of knowing when client has gone is to have a program create a hidden window as the child of the root. When the X session dies you could capture the X error back. This would require some Xlib programming.

see the process for that program is launched already it is a child to other process that are running on the system for other functions. All i need to do is run the program and a dialog box will show up. i guess what i really need is a rsh to execute that program on every client and export the display locally to each machine with the message