This book portends to be a universal command guide, and in all fairness they managed to get a good number of commands in the text. Unfortunately the ones the missed are significant (e.g. UNIX tar, which is universal in UNIX and Linux). I would not recommend this book for someone looking for help in distinguishing the difference in Linux and BSD commands.
While initially quite skeptical of a book attempting to cover such disparate operating systems, the reality is that I have found this volume to be indespensible. Since I work with Red Hat Linux primarily and Solaris (v8) also rather frequently, it’s easy use the UCG to get the syntax differences of those two environments. Sure, I could just scan the man pages — but the already well-worn out copy of my UCG has enough bookmarks and paperclip (and yellow highlights) to make turning to it easier. It has also helped me better understand that there are quite a few ways to perform networking tasks using the Windows command line (as opposed to becoming dependent on the GUI). The charts showing the equivalency of commands across the diverse operating environments is a real time-saver. As a partisan of the O’Reilly books, I’ve come to the general conclusion that the value of a book is inversely proportional to its size. A big, thick book is generally fluff. But, this one is the exception.