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Lions' Commentary on Unix : With Source Code


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Lions Book /n./ "Source Code and Commentary on Unix level 6", by John Lions. The two parts of this book contained (1) the entire source listing of the Unix Version 6 kernel, and (2) a commentary on the source discussing the algorithms. These were circulated internally at the University of New South Wales beginning 1976-77, and were, for years after, the *only* detailed kernel documentation available to anyone outside Bell Labs. Because Western Electric wished to maintain trade secret status on the kernel, the Lions Book was only supposed to be distributed to affiliates of source licensees. In spite of this, it soon spread by samizdat to a good many of the early Unix hackers.

Hackers Dictionary

"After 20 years, this is still the best exposition of the workings of a 'real' operating system."

--- Ken Thompson, 1996


Lions' Commentary on Unix : With Source Code ~ Usually ships in 24 hours

    John Lions / Paperback / Published 1996

    Amazon Price: $29.95

John Lions (New South Wales) was a Lecturer in Computer Science when an early version of UNIX arrived at the University of New South Wales in 1974. He wrote his commentary as an Operating Systems text for his students in 1977 but was never permitted to have it published commercially. Quality of his commentary and the fact of treating code the way he did inspired those who would later go on and start the Open Source movement.

The Lions Book has an interesting history -- it one of the few "prohibited books" on Unix. The author died only several weeks after the book was published (on Saturday the Dec 5th, 1998), being very seriously ill, but probably able to understand this fact before death. See additional information in USENIX In Memoriam: John Lions by Peter H. Salus

The author managed to prove that great code, like great literature should be read and enjoyed. That's why "The Lions book" is the bible of UNIX hackers and was widely circulated as a photocopied bootleg document since the late 1970's.  This legendary underground classic, reproduced without modification, is really two works in one:

Version of the book was published on the USENET alt.folklore.computers in May 1994. It's available in several forms:  Commentary on the Sixth Edition UNIX Operating System

Lions' marriage of source code with commentary was originally used as an operating systems textbook, a purpose for which it remains very well suited. As a self-study UNIX conceptual tutorial, it has informed and inspired computer scince students, programmers and system administrators for over twenty years.  Several prominent Unix developers, including Dennis Ritchie and Ken Thompson, have contributed essays about the role and the value of this outstanding book. As W. Richard Stevens noted in his FAQ:

I read lots of source code. In fact, the art of reading source code is something that most universities do not teach, but something that is easy to do on your own. (Remember Lions' fantastic A Commentary on the UNIX Operating System from 1977, which was a complete presentation and analysis of the UNIX Version 6 source code? I was fortunate to sit in on a graduate class taught by Dave Hanson that used this as the text.) Although the source code for most commercial versions of Unix is unavailable today, fortunately there are still lots of systems for which the source code is available: 4.4BSD-Lite, FreeBSD, Linux, Minix, GNU, etc.

Both complete source of  UNIX 6th Edition and binary version are now available; to use the binary version all you need is a PDP emulator. See for details. Various PDP-11 simulators can be downloaded from]

Again this is probably the most famous suppressed book in computer history. It was illegally pirated for many years after its publication, with fifth-generation photocopies being the most prized possessions of many Unix kernel hackers. Sadly enough it was published only in 1996 after the issues  of the ownership of the Unix sources settled down. Here is the press release:

Lions' Commentary: Press Release

For Immediate Release

The First-Ever UNIX Book Survives 18-Year Suppression

San Jose, California, September 15, 1996 -- After 18 years as computerdom's most famous underground manuscript, "The Lions book" is back in print. Lions' Commentary on UNIX 6th Edition, with Source Code, the first UNIX book ever written, contains the only UNIX source code ever publicly released by Bell Labs. Ken Thompson, the original architect of UNIX, says in his Foreword "After 20 years, this is still the best exposition of the workings of a 'real' operating system."

The book's long life is a story in itself. In 1977, John Lions, an instructor at Australia's University of New South Wales, based a course around AT&T Bell Labs' new UNIX operating system. Lions printed out all the kernel source code, then wrote a brilliant commentary detailing how the code worked. The UNIX community was thrilled, and this self-published book became the UNIX technical bible. As Dennis Ritchie (co-developer of UNIX with Thompson) notes in his Foreword, "The document reproduced here educated a generation." Unfortunately, AT&T's legal department saw the book as a major threat to AT&T's intellectual property rights to UNIX. AT&T secured "distribution" rights to the book but permitted each licensed UNIX *site* only a single copy! AT&T shut down distribution entirely within a few years. But "the Lions book" had a life of its own--- pirate photocopies (and photocopies of photocopies of photocopies) continued to circulate widely among UNIX gurus throughout the 1980s.

By the early 1990s, however, UNIX had gone through many revisions, and any danger to AT&T of competitors copying the source code from the book was moot. Peter Salus, former Executive Director of USENIX, and Ritchie spent years futilely pressing AT&T's legal bureaucracy, and later Novell's, to release this classic computer science work.

The Santa Cruz Operation's (SCO) December 1995 acquisition of UNIX from Novell rescued the book from limbo. Michael Tilson, SCO's Chief Information Officer and President of Uniforum, understood first-hand what a treasure the book was; he helped Salus and Ritchie obtain quick permission from SCO to reprint the book.

Tilson and fellow UNIX luminaries Peter Salus, Berny Goodheart (Australia), Mike O'Dell, Peter Collinson (UK), Greg Rose (Australia), and Peter Reintjes also contributed retrospective essays on the book's enduring technical, educational, and historical value.

Lions' Commentary on UNIX 6th Edition, with Source Code ($29.95 US, ISBN# 1-57398-013-7) is distributed through bookstores world-wide by International Thomson Publishing, or can be purchased directly from the publisher, Peer-to-Peer Communications Inc. (

This book became classic because John Lions believed that good software is kind of literature and as in literature, the ability to read and analyze great works can be enhanced by a skillful commentary. This was one of first books that really teach you read and criticize working code written by top professionals and here lies its lasting value.  He chose Unix, 6th edition, running on the PDP-11 which was a pretty compact in comparison with current Unixes (~12K lines) and his book is a subset of the kernel sources, with commentary.

Actually this book became classic largely because the code itself is, in general, became a classic. It includes the classic section in sched.c that included the legendary comment "You are not expected to understand this."  It's amazing that so much of modern Unix functionality already existed in the mid-70s and ran in only 32kbytes of RAM, the size of the memory that is not enough to run hello world program in modern OSes. Please note that DOS run in 128K and was much more primitive OS.  

The third reason is that this is an important historical document that describes a real operating system, that greatly influenced the whole history of operating systems design.

It's a difficult read and the effort worth mainly for programmers well versed in C, but if you are such a programmer you can greatly benefit from this effort.

Here is an interesting paper in Salon Technology Code critic

The genesis of Lions' books is, of course, tied to the Unix tradition that began 25 years ago when the magazine Communications of the Association for Computing Machinery -- technology's equivalent to Nature -- published a paper by Ken Thompson and Dennis Ritchie, the two Bell Labs employees who created Unix and the C programming language. The paper was called "The Unix Time-Sharing System" and included a description of the operating system, a justification of its design and a few notes on why it was built in the first place. The article captured the imagination of many a programmer, including Ken Robinson, a teacher at the University of New South Wales (UNSW), who wrote away for a copy of the new operating system. When it arrived, Lions, his colleague, read the source.

Source code is the blueprint for software, like a set of spells that humans can read and use to control machines. Given source code, a programmer can modify an application by fixing bugs or adding features. But most commercial software is sold only in machine-readable form. Access to source code is power.

Lions' career had followed the classic path for an Australian academic of his generation. In 1959, he took a first-class honors degree from Sydney University and promptly left the country. He earned his doctorate at Cambridge in 1963 and spent the next decade working for Burroughs Corp. in Canada and Los Angeles. By 1972 he was married and had a young family. He moved back to Australia and took a position as senior lecturer in UNSW's department of computing. He would teach there for the rest of his working life.

The Unix code base enchanted Lions -- so much so that he decided to make significant changes to two of the courses he taught. Until then, most teachers of operating systems loftily imparted general principles about programs their students had probably never seen, or encouraged students to build toy operating systems of their own. Unix offered a third approach. It ran on a comparatively affordable computer system, a Digital Equipment Corp. PDP 11 -- a machine that UNSW already owned. Unix was compact and accessible but offered a remarkable set of features. To top it off, in Lions' words, it was "intrinsically interesting." Unix could be read and understood with far less effort than IBM's bloated OS/360 and TSS/360 (which, funnily enough, are pygmies by modern standards), but it had the industrial-strength functionality to which homemade toys could never aspire. One student, Greg Rose, remembers, "John expressed this by saying, 'The only other big programs they see were written by them; at least this one is written well.'"

...The first editions of Lions' books were slender computer printouts covered with red cardboard and stamped with the UNSW crest; they bore the titles "Unix Operating System Source Code Level 6" and "A Commentary on the Unix Operating System." Lions had originally prepared them for his students, who were astonished at the availability of the source code. Here was an entire operating system you could hold in your hand. "The whole documentation is not unreasonably transportable in a student's briefcase," noted Lions. Later he would joke that in subsequent editions of Unix, this had been fixed.

The books worked. "In general, students seem to find the new courses more onerous, but much more satisfying than the previous courses," Lions dryly observed. His students describe the classes as a revelation. "He enjoyed seeing if any of us were alert enough to catch any of the few bugs or inexplicable oddities in the code as we read through it. I don't think we managed it often," remembers Lucy Chubb, now president of the Australian Unix and Open Systems Users Group (AUUG). "Serious reading of someone else's code was something that no other subject gave us."

One of Lions' greatest insights was that what Thompson and Ritchie had written deserved to be studied in this way. "You will find that most of the code in Unix is of a very high standard," he wrote in an introduction to the books. "Many sections which initially seem complex and obscure, appear in the light of further investigation and reflection, to be perfectly obvious and 'the only way to fly.'"

Unix was beautiful and useful and it rewarded close attention. As programmer Peter Reintjes observed, "We had acquired what amounted to a literary criticism of computer software."

The implications were huge. Thompson and Ritchie's succinct and elegant code lent itself to investigation and play. Now a cadre of well-trained students was equipped to do just that. "Enhancements from institutions around the world began to be exchanged, and contributed in no small way to the growth of Unix," Rose recalls. "These two volumes made it far easier to get started with this kind of experimentation, and contributed greatly to the success of Unix during the late 1970s and early 1980s."

News of the revolution trickled back to its instigators. "The first Ken and I heard of John was when the original of the 'Commentary' book arrived, I think, out of the blue," says Ritchie. "We were much impressed by the quality of the work and highly flattered by its very existence." Lions had worked hard to understand what Thompson and Ritchie had been trying to achieve. In their view, he succeeded. "After 20 years, this is still the best exposition of the workings of a 'real' operating system," Thompson has said.

As much as he admires the course notes, Ritchie praises Lions' teaching still more. "Probably the most important contribution John made was to start, at UNSW and indirectly at Sydney Uni, a very strong group of Unix people, many of whom have visited or stayed here, and whom we have often visited," he says. Lions also founded the AUUG, and it is at least partly to his credit that Unix thrives in Australia to this day. (The group recently celebrated Lions' contribution with a John Lions Award for Research Work in Open Systems.)

A visionary teacher, a wonderful teaching tool, a commentary drenched with wit and insight, an environment in which talented students thrived -- this is all building up to a happy ending, right? Yes and no. Even when they were first published, Lions' books were technically only available to licensees of sixth edition Unix. The operating system's new owner, Western Electric, didn't want just anyone learning the inner workings of the Unix kernel.

There was also Slashdot discussion that along with usual amount of noise has some interesting comments:

I think that one of the most significant things about Lions's work is that it's a commentary on a complete kernel. Works like Bach's seminal Design of the Unix Operating System notwithstanding, there are no follow-the-code examples of a real OS out there today; modern Unices like Linux and BSD are far too complex to just sit down with and start understanding the deep mysteries of OS construction. Of those two, I'd prefer BSD for its more cohesive design, but even then, you're looking at twenty or a hundred times the code of the original Version [67] kernels. A 1990s Unix contains deep kernel hacks that make sense only in implementing advanced networking, scheduling, and virtual memory contexts; the study of these should be postponed until after the fundamentals are mastered. In contrast, there are wristwatches that could run V6; on a Palm Pilot or an embedded 386SX/20 system, V6 would scream compared to the now incredibly bloated 2.2.x Linuces.

Another interesting post:

Lessee, yes, I still do have my nth-generation photocopy. The front page of the Commentary (and the source listing) says,

"The UNIX Software System was written by K. Thompson and D. Ritchie of Bell Telephone Laboratories, Murray Hill, NJ. It has been made available to the University of New South Wales under a licence from the Western Electric Company.

This document may contain information covered by one or more licences, copyrights and non-disclosure agreements. Circulation of this document is restricted to holders of a licence for the UNIX Software System from Western Electric. All other circulation or reproduction is prohibited.

J. Lions
Department of Computer Science
The University of New South Wales
C Copyright 1977 J. Lions"

Here is an interesting post from Phil Karn that suggest that Cornell University was using the book in 1977-1978:

I actually took a course on the UNIX kernel based on Lions' books during my senior year at Cornell (1977-78), apparently before the Bell lawyers clamped down on this stuff. It was my first introduction to UNIX, and I was immediately hooked.

Five years later, when I was working at Bell Labs at Murray Hill, I had the honor of working with John Lions, who had joined my department for his sabbatical. He was probably the first Australian I got to know personally, and I took a liking to his witty Aussie irreverence even if it cut a little sharply at times.

I had heard a while ago that John was sick, but I didn't know until I read the Salon article that he had died a year ago. How sad. He really did contribute so much to the open, academic study and development of real, live computer operating systems. Yes, I do think he deserves much of the credit for inspiring those who would later go on and start the Open Source movement.

And yes, I still have my copies of Lion's original books...guess they're collectors' items now!

Phil Karn

Recommended Links

Lions' Commentary on UNIX 6th Edition, with Source Code - Wikipedia, the free encyclopedia

Commentary on the Sixth Edition UNIX Operating System

Lions' Commentary on UNIX


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