The various Unix-like systems on the market, some of which have a long history and may show signs of archaic practices, differ in many important respects. All commercial variants were derived from either SVR4 or 4.4BSD; all of them tend to agree on some common standards like IEEE's POSIX (Portable Operating Systems based on Unix) and X/Open's CAE (Common Applications Environment).
The current standards specify only an application programming interface (API)—that is, a well-defined environment in which user programs should run. Therefore, the standards do not impose any restriction on internal design choices of a compliant kernel.
 As a matter of fact, several non-Unix operating systems like Windows NT are POSIX-compliant.
In order to define a common user interface, Unix-like kernels often share fundamental design ideas and features. In this respect, Linux is comparable with the other Unix-like operating systems. What you read in this book and see in the Linux kernel, therefore, may help you understand the other Unix variants too.
The 2.2 version of the Linux kernel aims to be compliant with the IEEE POSIX standard. This, of course, means that most existing Unix programs can be compiled and executed on a Linux system with very little effort or even without the need for patches to the source code. Moreover, Linux includes all the features of a modern Unix operating system, like virtual memory, a virtual filesystem, lightweight processes, ...