The Design of the Unix Operating System

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This is another book that is getting old, but at least it's focus is System V Unix, so it still has a lot of value.

Bach writes from a programmer's perspective, so anyone doing programming on modern Unix systems should have this as beginning material (I recommend Stevens "Advanced Programming in the Unix Environment" for the gritty details).

Bach ends each chapter with a dozen or more exercises that ask questions and pose scenarios designed to test your understanding of the material covered. Unfortunately, there is no appendix that discusses reasonable answers to those exercises, so you are on your own.