Unix is an operating system. It's been developed over the past 30 years by several different vendors. This book can't hope to be a tutorial on the use of Unix. I assume that you know enough to log into your Unix system, get a command prompt, and issue commands. Even so, there are some important things to review before you get started with this book.
The history of Unix goes back to 1969, when the first versions of Unix were developed by AT&T's Bell Labs. The operating system had a certain elegance, was freely available, and quickly caught on with vendors of mini-computer systems who needed an operating system for the hardware that they were selling.
As different vendors adopted Unix, they each began to create their own, slightly unique versions of the operating system. Today, you have HP-UX, Sun Solaris, IBM AIX, and a number of other variants to deal with.
Linux is a Unix-like operating system first put together by Linus Torvalds in 1991 because he needed an operating system for his PC and could not afford any of the commercial Unix variants of that day. Linux has gone on to achieve phenomenal growth and is widely used today as a server operating system on x86 machines. Linux is also available for the PowerPC, Sparc, IBM S/390, and Amiga.
Most commonly used Unix commands work more or less identically on all Unix and Linux platforms. The ls command, which lists files in a directory, is an example of such a command. I've never seen a Unix or Linux version ...