program names, file names
variable names
prompts, commands, program code

If you ever need to run IDL without graphics (for example, if you need to use IDL to precess coordinates while you're at an observatory, and you only have a text interface), run

UNIXprompt> ~/idlpro/nodisp.sh

(on the licensed IDL machine, of course). This will start up a text-only IDL session. Everything else should be the same as a graphical IDL session. This text-only session will be configured by the ~/idlpro/startup.nodisp.pro file, as opposed to the ~/idlpro/startup.pro file which is usually used.

Make sure that any changes in one startup file are (if relevant) made to BOTH startup files.