Eric S. Raymond and Rob Landley have suggested that there are three kinds of Unix-like systems: "genetic UNIX" derived from the original Bell Labs UNIX, "trademark UNIX" (or "branded UNIX") for which the
Open Group have licensed the UNIX trademark, and a third category of operating systems that are not in either of those categories but that are patterned after the original Bell Labs UNIX.
Genetic UNIX Those systems with a historical connection to the
AT&T codebase. Most commercial UNIX systems fall into this category. So do the
BSD systems, which are descendants of work done at the
University of California, Berkeley in the late 1970s and early 1980s. Some of these systems have no original AT&T code but can still trace their ancestry to AT&T designs.
Trademark or branded UNIX These systemslargely commercial in naturehave been determined by the
Open Group to meet the
Single UNIX Specification and are allowed to carry the UNIX name. Most such systems are commercial derivatives of the System V code base in one form or another, although Apple macOS 10.5 and later is a BSD variant that has been certified, and
EulerOS and
Inspur K-UX are Linux distributions that have been certified. A few other systems (such as IBM z/OS) earned the trademark through a POSIX compatibility layer and are not otherwise inherently Unix systems. Many
ancient UNIX systems no longer meet this definition.
Other UNIXes Broadly, any Unix-like system that behaves in a manner roughly consistent with the UNIX specification, including having a "
program which manages your login and
command line sessions"; more specifically, this can refer to systems such as
Linux or
Minix that behave similarly to a UNIX system but have no genetic or trademark connection to the AT&T code base. Most free/open-source implementations of the UNIX design, whether genetic UNIX or not, fall into the restricted definition of this third category due to the expense of obtaining Open Group certification, which costs thousands of dollars.
Dennis Ritchie, one of the original creators of Unix, mentioned Linux as one of the healthiest of the direct Unix derivatives due to its strong adherence to Unix principles. Around 2001 Linux was given the opportunity to get a certification including free help from the POSIX chair Andrew Josey for the symbolic price of one dollar. There have been some activities to make Linux POSIX-compliant, with Josey having prepared a list of differences between the POSIX standard and the
Linux Standard Base specification, but in August 2005, this project was shut down because of missing interest at the LSB work group. As per , some Linux distributions have been certified. ==Compatibility layers==