Dennis Ritchie, one of the original creators of Unix, expressed his opinion that Unix-like systems such as Linux are de facto Unix systems. Eric S. Raymond and Rob Landley have suggested that there are three kinds of Unix-like systems:
Those systems with a historical connection to the AT&T codebase. Most (but not all) 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 systems틀:Mdashblargely commercial in nature틀:Mdashbhave 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 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.
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 dollars틀:Citation needed for commercial closed source systems.
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.틀:Citation needed 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.틀:Citation needed