Running without a Display

To run Processing on a server, in “headless” mode, it's necessary to install what's known as a virtual frame buffer.

On Ubuntu 14.04, these are the things you need to install first:

sudo apt-get install xvfb libxrender1 libxtst6 libxi6 

Then you create a fake "display" that Processing can use:

sudo Xvfb :1 -screen 0 1024x768x24
export DISPLAY=":1"

Finally, run your sketch from an exported .jar file or as a Linux application that was exported from the PDE.

It's necessary to use the virtual frame buffer because it would require a truly ridiculous amount of work to make Processing so general that it could be done without a display. This would involve a great deal of code and abstraction that simply doesn't make sense when 99.9999% of the code created with Processing will be run with a display.