Actually, yes. It is so well designed and documented with a public interface that it has long since been defined away fron “whatever runs on Ken Thompson’s desktop.”
The most important fact about Unix (as versus Plan 9) is that Unix was originally distributed as a precursor of Open Source. I had access to Unix source code in 1981 in college.
Just because it is defined doesn’t mean that I agree with the underlying design-philosophies... the super-user idea, for example, is a GIANT security-risk as all one has to do to compromise the entire system is gain that root-access.
The star-expansion requirement for shells is, IMO, a pain in the ass. For example, you can’t see if the user typed in “*.pdf” on the command-line vs “Doctorate.pdf Thesis.pdf ugh.pdf”. This means that the primitive file-management command “rename” cannot be used sanely on groups of files; rename *.c *.cpp does NOT rename all .c files to .cpp
Another thing that I personally don’t like is the unhelpfulness of that OS-family; it shows great similarity to the C/C++ tradition of unhelpful errors and ‘surprises’... and given the relation between *nix and C/C++ it is unsurprising.