No single published standard describes all Unix architecture computer operating systems — this is in part a legacy of the Unix wars.
The distinctions between Unix and Unix-like systems have been the subject of heated legal battles, and the holders of the UNIX brand, The Open Group, object to "Unix-like" and similar terms.
Each of the subsystems has some features: The kernel provides these and other basic services: interrupt and trap handling, separation between user and system space, system calls, scheduling, timer and clock handling, file descriptor management.
However, although some information is quite dated and cannot be applied to modern Unixes or Unix-like operating systems such as Linux, Eric S. Raymond discovered that several issues are still prevailing, while others were resolved.
Raymond concludes that not all concepts behind Unix can be deemed as non-functional even though the book's intention may have been to portray Unix as inferior without encouraging discussions with developers to actually fix the issues.