One of the more appealing benefits to Linux is that it isn’t a commercial operating system: its source code under the GNU General Public License (GPL) † is open and available to anyone to study (as we will in this book); if you download the code (the official site is http://www.kernel.org) or check the sources on a Linux CD, you will be able to explore, from top to bottom, one of the most successful modern operating systems. This book, in fact, assumes you have the source code on hand and can apply what we say to your own explorations.
相比于其他操作系统，Linux更具吸引力的地方是它不是一个商用操作系统：Linux的源代码在遵循GNU GPL的情况下对任何人都是开放和有效的（正如我们本书一样）；只要你在官网下载或通过光盘获得源码，你就可以从头至尾深入探究这个最成功的现代操作系统了。实际上，本书假定你手头上已经有了一份源代码并且能够把我们讲授的方法应用到你的探索之旅中。
Technically speaking, Linux is a true Unix kernel, although it is not a full Unix operating system because it does not include all the Unix applications, such as filesystem utilities, windowing systems and graphical desktops, system administrator commands, text editors, compilers, and so on. However, because most of these programs are freely available under the GPL, they can be installed in every Linux-based system.
Because the Linux kernel requires so much additional software to provide a useful environment, many Linux users prefer to rely on commercial distributions, available on CD-ROM, to get the code included in a standard Unix system. Alternatively, the code may be obtained from several different sites, for instance http://www.kernel.org. Several distributions put the Linux source code in the /usr/src/linux directory. In the rest of this book, all file pathnames will refer implicitly to the Linux source code directory.
从技术角度来说，Linux是一个纯粹的Unix内核，尽管它不是一个完整的Unix操作系统因为它并不包含全部的Unix应用程序，比如文件系统公用程序，视窗系统、图形桌面、系统管理员命令、文本编辑器、编译器等等。然而，由于以上大多数应用都可以在遵循GPL协议的情况下免费获得，在每一个基于Linux的系统中都可以安装。
由于Linux内核需要很多的附加软件才能为用户提供一个可用的环境，所以许多Linux用户倾向于通过光盘获得商用的发行版本，从而得到一个包含标准Unix系统的代码。或者你也可以从一些不同的网站获得这些代码，比如：http://www.kernel.org；有一些发行版将Linux源码放在/usr/src/linux目录下。在本书后续章节中，所有的文件路径都隐含指定在这一目录中。