We also have a homemade documentation system, again outdated but certified. I don't remember the system but it is definitely not a modern Linux. I think there is also a Fortran tool used to compute the lifespan of parts on the same system.

A bit more recent, we have test benches made of several computers, mostly Linux desktops, each one deal with different things like real or simulated avionic hardware, they can be remote controlled but some apps run locally and have their own displays.

We also have certified build chains running on specific systems, and while a GUI is typically not needed. It may be useful from time to time, mostly for debugging.

We are in a process of modernizing all that stuff, but because of certification and the average lifespan of a program, it is a very slow process...