How to run an X program from outside the X session (e.g. from the console or SSH)

Answer 1

The short answer is that you have to set the DISPLAY environment variable, and then the app will run.

The long answer is that we've got Xauth, and unless you're running as the same user on the same machine that's probably not going to work unless you export the Xauth credential from the account running the X server to the account running the X client. ssh -X handles this for you, which is why it's awesome, but the manual procedure involves running xauth extract - $DISPLAY on the X server account and feeding that data into xauth merge - on the client account. (Warning: the data is binary.)

On modern Linux systems, there is one X session at :0 and the X11 authority data file is always $HOME/.Xauthority so you can most often set two environment variables, for example, in Bash:

export XAUTHORITY=/home/$your_username/.Xauthority
export DISPLAY=':0'

Answer 2

The upshot is that you have to know the X display (placed in the DISPLAY environment variable) and the magic cookie (placed in a file, with the filename in the XAUTHORITY environment variable).

The quick-and-dirty way

On the system running X, if you are root or you are the same user who logged in to X, just assume the most common display and cookie files (works on almost any standard desktop install of any distro).

env DISPLAY=:0 XAUTHORITY=/home/whoever/.Xauthority /path/to/my/X/program

The more surefire way

Find them from the environment of an already-running X program. Again, if you are root or the same user who is logged in, this will tell you (if the user is using GNOME):

cat /proc/`pgrep -f ^x-session-manager`/environ \
  | ruby -ne 'puts $_.split("\0").select { |e| e =~ /^(DISPLAY|XAUTHORITY)=/ }'