I sometimes find it useful to start an extra X server to run a particular application. Full-screen games are a good example. Ordinarily, a full-screen game grabs the display and there is no way to switch back to the desktop without exiting the game completely. When run in its own X server though, it is easy to switch between servers. The following shell script shows how to do this:

#!/bin/bash

display=:1
authfile=$HOME/.Xauthority

if ! xauth list "$display" | grep "$display " >/dev/null 2>&1; then
    xauth add $display . `mcookie`
    removelist="$display"
fi

xinit /usr/bin/xterm -- $display -auth $authfile -nolisten tcp

if [ -n "$removelist" ] ; then
    xauth remove $removelist
fi

This script will start a new X server on the next available virtual terminal (normally vt8) using display :1 and launch an xterm. Toggle between the servers using ctrl-alt-F7 and ctrl-alt-F8. When the xterm exits, so does the X server.

If you try to run this code on a Debian-based system, though, you will get the following error:

X: user not authorized to run the X server, aborting.
xinit:  Server error.

I found this out the hard way when I started using ubuntu. This is because Debian uses a wrapper to start the X server, which enforces tighter security constraints by default. To get this to work, you will need to edit the /etc/X11/Xwrapper.config file, and change this line:

allowed_users=console

to this:

allowed_users=anybody

Check the man page for Xwrapper.config for more options; there aren’t very many. Personally, I don’t consider this a security risk for a single-user home system.