@lanodan GNU has a fork of Mach known as GNUMach, so we do have a kernel.
The Hurd is also a collection of servers that I really reckon makes up a larger kernel.
Is it the case that Linux has never used code that has been forked from other places?
Going by that logic, I really reckon a number of BSD's don't have their own kernel, as their kernel was forked from elsewhere.
GNU cat is better than the example source you showed - just based off the license. GPLv3-or-later is better than GPLv2-only or GPLv3-only.
It's also faster and quite short for all the speed and functionality now that I read it.