FreeBSD.orgThen Tcsh.
Unix is built around the idea that the kernel (code) have an api that is called from user software (syscall). The kernel have monopoly on hardware access and presents an restricted api to ensure no software run by a user can compromise the system, only the user "root" can do "everything". Several programs run in parallell (multitasking).
The kernel in turn use drivers to interact with hardware.
Also, unix has a philosophy of modularity and straight forward interfaces without clutter. And as little dependencies as possible.
This means that badly written software that uses the kernel api as normal user will be prevented from crashing the system. And that you can run without any graphical user interface. It's possible to not even have a console. The modularity means it's straightforward for programmers to change the way the system works.
