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If you're seeking a book that talks about how to build Mac software applications, this isn't your book (although it's quite helpful for developers to have a firm grasp of Unix essentials, because you never know when you're going to need them). If you're a complete beginner and are occasionally stymied by where the second mouse button went, this might be a better book to put on the shelf until you're more comfortable with your Macintosh.
Finally, if you live and breathe Unix every day, this book is probably too basic for you. I also don't cover either Unix system administration or Mac system administration from the command line. For example, if you already know what a PID is and how to killa program, this book is probably beneath your skill level. But if you don't know what that means, or if you're somewhere in between, you've found the right book!