Linux Versus Other Unix-Like Kernels
The various Unix-like systems on the market, some of which have a long history and show signs of archaic practices, differ in many important respects. All commercial variants were derived from either SVR4 or 4.4BSD, and all tend to agree on some common standards like IEEE’s Portable Operating Systems based on Unix (POSIX) and X/Open’s Common Applications Environment (CAE).
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. *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. Reading this book and studying the Linux kernel, therefore, may help you understand the other Unix variants, too.
Linux和其他类Unix系统的比较
在市面上众多类Unix操作系统中，有一些已经年代久远显得有些过时了，在许多重要的方面有所不同；所有类Unix商业变种都趋向于遵循一些共同的标准，比如IEEE的POSIX和X/Open的CAE；
现有的标准仅仅制定了一个应用程序接口——即明确定义一个用户程序能够运行所需的环境，但是这些标准并没有对内核内部的实现方式的选择上做出强制规定；为了定义一个通用的用户接口，类Unix内核经常会共享底层的设计理念和特性，在这方面，Linux可以兼容其他的类Unix操作系统。因此通过阅读本书学习Linux内核也有助于你理解其他的Unix变种。
The 2.6 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, such as virtual memory, a virtual filesystem, lightweight processes, Unix signals, SVR4 interprocess communications, support for Symmetric Multiprocessor (SMP) systems, and so on.
Linux2.6内核试图与IEEE的POSIX标准兼容，这当然就意味着那些现存的大多数Unix程序可以只需稍微的改动源码甚至不需要补丁包就可以在Linux中编译和执行。此外，Linux包含了所有现代Unix操作系统的特性，诸如虚拟内存、虚拟文件系统、轻量级进程管理、Unix信号、SVR4进程间通信以及支持对称多处理器等。