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The Lions book is a tech slang term for the volume "Source Code and Commentary on UNIX Level 6" written by John Lions in 1976. During the late 1970s, this volume was an exclusive type of reference for analysis of UNIX.
As UNIX has developed over several decades, many students have researched the design of this kernel-based operating system that delves into key ideas about computing, including the use of program threads and interactivity between programs. UNIX is a high-level technology that is mainly the province of knowledgeable developers or engineers, and not generally used by end-user consumers.
From the time that UNIX was developed by AT&T Bell Labs, the "Lions book" was the sole resource for some of the code and functionality documentation around UNIX. That means it was extensively studied by those trying to learn this operating system and its use. Since then, the book has been reprinted as a guide to UNIX source code.