Question

Without being the person logged in at the console, how do I run an X application and have it display on that X session? Assume I am either root, or I am the same user who logged in, so in principle I have persmission to do this. But how do I convince X of this?

Some examples of situations like this:

• Log in with SSH and run a program that displays on the remote computer’s screen (not tunneled through SSH—that is totally different)
• A cron job to take a screenshot of the X session via ImageMagick’s import command
• Running a keystroke logger for audit purposes

This is a simpler version of Launch OpenGL app straight from a windowless Linux Terminal

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'