A Commentary on the UNIX Operating System by John Lions (later reissued as Lions' Commentary on UNIX 6th Edition) is a highly influential 1976 publication containing analytical commentary on the source code of the 6th Edition Unix computer operating system "resident nucleus" (i.e., kernel) software, plus copy formatted and indexed by Lions, of said source code obtained from the authors at AT&T Bell Labs. It is also commonly referred to as the Lions Book. Itself an exemplar of the early success of UNIX as portable code for a publishing platform, Lions's work was typeset using UNIX tools, on systems running code ported at the University, similar to that which it documented, see page vi. It was commonly held to be the most copied book in computer science. Despite its age, Lions's book is still considered an excellent commentary on simple, high quality code. Lions's work was most recently reprinted in 1996 by Peer-To-Peer Communications, and has been circulated, recreated or reconstructed variously in a number of media by other parties, e.g. see webpage of Greg Lehey. Synopsis UNIX Operating System Source Code Level Six is the kernel source code, lightly edited by Lions to better separate the functionality — system initialization and process management, interrupts and system calls, basic I/O, file systems and pipes and character devices. All procedures and symbols are listed alphabetically with a cross reference. The code as presented will run on a PDP-11/40 with RK-05 disk drive, LP-11 line printer interface, PCL-11 paper tape writer and KL-11 terminal interface, or a suitable PDP-11 emulator, such as SIMH. A Commentary on the UNIX Operating System starts with notes on UNIX and other useful documentation (the UNIX manual pages, DEC hardware manuals and so on), a section on the architecture of the PDP-11 and a chapter on how to read C programs. The source commentary follows, divided into the same sections as the code. The book ends with suggested exercises for the student. As Lions explains, this commentary supplements the comments in the source. It is possible to understand the code without the extra commentary, and the reader is advised to do so and only read the notes as needed. The commentary also remarks on how the code might be improved. History The source code and commentary were printed in book form in 1977, after first being assembled in May 1976, as a set of lecture notes for