in part of this history, I feel I have an obligation to set the record straight and correct some extremely serious
But first some background information. Ken Brown, President of the Alexis de Tocqueville Institution, contacted me in early March. He said he was
writing a book on the history of Unix and would like to interview me. Since I have written 15 books and have been
involved in the history of Unix in several ways, I said I was willing to help out. I have been interviewed by many
people for many reasons over the years, and have been on Dutch and US TV and radio and in various newspapers and
magazines, so I didn't think too much about it.
Brown flew over to Amsterdam to interview me on 23 March 2004. Apparently I was the only reason for his coming to
Europe. The interview got off to a shaky start, roughly paraphrased as follows:
AST: "What's the Alexis de Tocqueville Institution?"
KB: We do public policy work
AST: A think tank, like the Rand Corporation?
KB: Sort of
AST: What does it do?
KB: Issue reports and books
AST: Who funds it?
KB: We have multiple funding sources
AST: Is SCO one of them? Is this about the SCO lawsuit?
KB: We have multiple funding sources
AST: Is Microsoft one of them?
KB: We have multiple funding sources
He was extremely evasive about why he was there and who was funding him. He just kept saying he was just writing
a book about the history of Unix. I asked him what he thought of Peter Salus' book, A Quarter Century of Unix. He'd never heard of it! I mean, if you are writing a book on the
history of Unix and flying 3000 miles to interview some guy about the subject, wouldn't it make sense to at least
go to amazon.com and type "history Unix" in the search box, in which case Salus' book is the first hit? For $28
(and free shipping if you play your cards right) you could learn an awful lot about the material and not get any
jet lag. As I sooned learned, Brown is not the sharpest knife in the drawer, but I was already suspicious. As a
long-time author, I know it makes sense to at least be aware of what the competition is. He didn't bother.
Unix and me
I didn't think it odd that Brown would want to interview me about the history of Unix. There are worse people to
ask. In the late 1970s and early 1980s, I spent several summers in the Unix group (Dept. 1127) at Bell Labs. I knew
Ken Thompson, Dennis Ritchie, and the rest of the people involved in the development of Unix. I have stayed at Rob
Pike's house and Al Aho's house for extended periods of time. Dennis Ritchie, Steve Johnson, and Peter Weinberger,
among others have stayed at my house in Amsterdam. Three of my Ph.D. students have worked in the Unix group at Bell
Labs and one of them is a permanent staff member now.
Oddly enough, when I was at Bell Labs, my interest was not operating systems, although I had written one and
published a paper about it (see "Software - Practice & Experience," vol. 2, pp. 109-119, 1973). My interest then
was compilers, since I was the chief designer of the the Amsterdam Compiler Kit (see Commun. of the ACM, vol.
26, pp. 654-660, Sept. 1983.). I spent some time there discussing compilers with Steve Johnson, networking with Greg
Chesson, writing tools with Lorinda Cherry, and book authoring with Brian Kernighan, among many others. I also
became friends with the other "foreigner," there, Bjarne Stroustrup, who would later go on to design and implement
In short, although I had nothing to do with the development of the original Unix, I knew all the people involved
and much of the history quite well. Furthermore, my contact with the Unix group at Bell Labs was not a secret; I
even thanked them all for having me as a summer visitor in the preface to the first edition of my book Computer
Networks. Amazingly, Brown knew nothing about any of this. He didn't do his homework before embarking on his
MINIX and me
Years later, I was teaching a course on operating systems and using John Lions' book on Unix Version 6. When
AT&T decided to forbid the teaching of the Unix internals, I decided to write my own version of Unix, free of
all AT&T code and restrictions, so I could teach from it. My inspiration was not my time at Bell Labs,
although the knowledge that one person could write a Unix-like operating system (Ken Thompson wrote UNICS on a
PDP-7) told me it could be done. My real inspiration was an off-hand remark by Butler Lampson in an operating
systems course I took from him when I was a Ph.D. student at Berkeley. Lampson had just finished describing the
pioneering CTSS operating system and said, in his inimitable way: "Is there anybody here who couldn't write CTSS in
a month?" Nobody raised his hand. I concluded that you'd have to be real dumb not to be able to write an operating
system in a month. The paper cited above is an operating system I wrote at Berkeley with the help of Bill Benson. It
took a lot more than a month, but I am not as smart as Butler. Nobody is.
I set out to write a minimal Unix clone, MINIX,
and did it alone. The code was 100% free of AT&T's intellectual property. The full source code was published in
1987 as the appendix to a book, Operating Systems: Design and Implementation, which later went into a second edition
co-authored with Al Woodhull. MINIX 2.0 was even POSIX-conformant. Both editions contained hundreds of pages of
text describing the code in great detail. A box of 10 floppy disks containing all the binaries and source code
was available separately from Prentice Hall for $69.
While this not free software in the sense of "free beer" it was free software in the sense of "free speech" since
all the source code was available for only slightly more than the manufacturing cost. But even "free speech" is not
completely "free"--think about slander, yelling "fire" in a crowded theater, etc. And this was before the Patriot
Act, which requires John Ashcroft's written permission before you can open your mouth. Also Remember (if you are old
enough) that by 1987, a university educational license for Unix cost $300, a commercial license for a university
cost $28,000, and a commercial license for a company cost a lot more. For the first time, MINIX brought the cost of
"Unix-like" source code down to something a student could afford. Prentice Hall wasn't really interested in selling
software. They were interested in selling books, so there was a fairly liberal policy on copying MINIX, but if a
company wanted to sell it to make big bucks, PH wanted a royalty. Hence the PH lawyers equipped MINIX with a lot of
boilerplate, but there was never any intention of really enforcing this against universities or students. Using the
Internet for distributing that much code was not feasible in 1987, even for people with a high-speed (i.e., 1200
bps) modem. When distribution via the Internet became feasible, I convinced Prentice Hall to drop its (extremely
modest) commercial ambitions and they gave me permission to put the source on my website for free downloading, where
it still is.
Within a couple of months of its release, MINIX became something of a cult item, with its own USENET newsgroup,
comp.os.minix, with 40,000 subscribers. Many people added new utility programs and improved the kernel in numerous
of ways, but the original kernel was just the work of one person--me. Many people started pestering me about
improving it. In addition to the many messages in the USENET newsgroup, I was getting 200 e-mails a day (at a time
when only the chosen few had e-mail at all) saying things like: "I need pseudoterminals and I need them by Friday."
My answer was generally quick and to the point: "No."
The reason for my frequent "no" was that everyone was trying to turn MINIX into a production-quality Unix system
and I didn't want it to get so complicated that it would become useless for my purpose, namely, teaching it to
students. I also expected that the niche for a free production-quality Unix system would be filled by either GNU or
Berkeley Unix shortly, so I wasn't really aiming at that. As it turned out, the GNU OS sort of went nowhere
(although many Unix utilities were written) and Berkeley Unix got tied up in a lawsuit when its designers formed a company,
BSDI, to sell it and they chose 1-800-ITS Unix as their phone number. AT&T felt this constituted copyright
infringement and sued them. It took a couple of years for this to get resolved. This delay in getting free BSD out
there gave Linux the breathing space it needed to catch on. If it hadn't been for the lawsuit, undoubtedly BSD would
have filled the niche for a powerful, free Unix clone as it was already a stable, mature system with a large
Ken Brown and me
Now Ken Brown shows up and begins asking questions. I quickly determined that he didn't
know a thing about the history of Unix, had never heard of the Salus book, and knew nothing about BSD and the
AT&T lawsuit. I started to tell him the history, but he stopped me and said he was more interested in the legal
aspects. I said: "Oh you mean about Dennis Ritchie's patent number 4135240 on the setuid bit?" Then I added:"That's
not a problem. Bell Labs dedicated the patent." That's when I discovered that (1) he had never heard of the patent,
(2) did not know what it meant to dedicate a patent (i.e., put it in the public domain), and (3) really did not know
a thing about intellectual property law. He was confused about patents, copyrights, and trademarks. Gratuitously, I
asked if he was a lawyer, but it was obvious he was not and he admitted it. At this point I was still thinking he
might be a spy from SCO, but if he was, SCO was not getting its money's worth.
He wanted to go on about the ownership issue, but he was also trying to avoid telling me what his real purpose
was, so he didn't phrase his questions very well. Finally he asked me if I thought Linus wrote Linux. I said that to
the best of my knowledge, Linus wrote the whole kernel himself, but after it was released, other people began
improving the kernel, which was very primitive initially, and adding new software to the system--essentially the
same development model as MINIX. Then he began to focus on this, with questions like: "Didn't he steal pieces of
MINIX without permission." I told him that MINIX had clearly had a huge influence on Linux in many ways, from the
layout of the file system to the names in the source tree, but I didn't think Linus had used any of my code. Linus
also used MINIX as his development platform initially, but there was nothing wrong with that. He asked if I objected
to that and I said no, I didn't, people were free to use it as they wished for noncommercial purposes. Later MINIX
was released under the Berkeley license, which freed it up for all purposes. It is still in surprisingly wide use,
both for education and in the Third World, where millions of people are happy as a clam to have an old castoff 1-MB
386, on which MINIX runs just fine. The MINIX home page cited above still gets more than 1000 hits a week.
Finally, Brown began to focus sharply. He kept asking, in different forms, how one person could write an
operating system all by himself. He simply didn't believe that was possible. So I had to give him more history,
sigh. To start with, Ken Thompson wrote UNICS for the PDP-7 all by himself. When it was later moved to the PDP-11
and rewritten in C, Dennis Ritchie joined the team, but primarily focused on designing the C language, writing the C
compiler, and writing the I/O system and device drivers. Ken wrote nearly all of the kernel himself.
In 1983, a now-defunct company named the Mark Williams company produced and sold a very good Unix clone called
Coherent. Most of the work was done by Bob Swartz. I used this system for a while and it was very solid.
In 1983, Rick Holt published a book, now out of print, on the TUNIS system, a Unix-like system. This was
certainly a rewrite since TUNIS was written in a completely new language, concurrent Euclid.
Then Doug Comer wrote XINU. While also not a Unix clone, it was a comparable system.
By the time Linus started, five people had independently implemented the Unix kernel or something approximating
it, namely, Thompson, Swartz, Holt, Comer, and me. All of this was perfectly legal and nobody stole anything. Given
this history, it is pretty hard to make a case that one person can't implement a system of the complexity of Linux,
whose original size was about the same as V1.0 of MINIX.
Of course it is always true in science that people build upon the work of their predecessors. Even Ken Thompson
wasn't the first. Before writing Unix, Ken had worked on the MIT MULTICS (MULTiplexed Information and Computing
Service) system. In fact, the original name of Unix was UNICS, a joke made by Brian Kernighan standing for the
UNIplexed Information and Computing Service, since the PDP-7 version could support only one user--Ken. After too
many bad puns about EUNUCHS being a castrated MULTICS, the name was changed to Unix. But even MULTICS wasn't first.
Before it was the above-mentioned CTSS, designed by the same team at MIT.
Thus, of course, Linus didn't sit down in a vacuum and suddenly type in the Linux source code. He had my book,
was running MINIX, and undoubtedly knew the history (since it is in my book). But the code was his. The proof of
this is that he messed the design up. MINIX is a nice, modular microkernel system, with the memory manager and file
system running as user-space processes. This makes the system cleaner and more reliable than a big monolithic kernel
and easier to debug and maintain, at a small price in performance, although even on a 4.77 MHz 8088 it booted in
maybe 5 seconds (vs. a minute for Windows on hardware 500 times faster). Instead of writing a new file system and a
new memory manager, which would have been easy, he rewrote the whole thing as a big monolithic kernel, complete with
inline assembly code :-( . The first version of Linux was like a time machine. It went back to a system worse than
what he already had on his desk. Of course, he was just a kid and didn't know better (although if he had paid better
attention in class he should have), but producing a system that was fundamentally different from the base he started
with seems pretty good proof that it was a redesign. I don't think he could have copied Unix because he didn't have
access to the Unix source code, except maybe John Lions' book, which is about an earlier version of Unix that does
not resemble Linux so much.
My conclusion is the Ken Brown doesn't have a clue what he is talking about. I also have grave questions about
his methodology. After he talked to me, he prowled the university halls buttonholing random students and asking them
questions. Not exactly primary sources.
The six people I know of who (re)wrote Unix all did it independently and nobody stole anything from anyone.
Brown's remark that people have tried and failed for 30 years to build Unix-like systems is patent nonsense. Six
different people did it independently of one another. In science it is considered important to credit people for
their ideas, and I think Linus has done this far less than he should have. Ken and Dennis are the real heros here.
But Linus sloppiness about attribution is no reason to assert that Linus didn't write Linux. He didn't write CTSS
and he didn't write MULTICS and didn't write Unix and he didn't write MINIX, but he did write Linux. I think Brown
owes a number of us an apology.
Linus and me
Some of you may find it odd that I am defending Linus here. After all, he and I had a fairly public "debate" some
years back. My primary concern here is getting trying to get the truth out and not blame everything on some teenage
girl from the back hills of West Virginia. Also, Linus and I are not "enemies" or anything like that. I met him once
and he seemed like a nice friendly, smart guy. My only regret is that he didn't develop Linux based on the
microkernel technology of MINIX. With all the security problems Windows has now, it is increasingly obvious to
everyone that tiny microkernels, like that of MINIX, are a better base for operating systems than huge monolithic
systems. Linux has been the victim of fewer attacks than Windows because (1) it actually is more secure, but also
(2) most attackers think hitting Windows offers a bigger bang for the buck so Windows simply gets attacked more. As
I did 20 years ago, I still fervently believe that the only way to make software secure, reliable, and fast is to
make it small. Fight Features.
Andrew S. Tanenbaum is a Professor of Computer Science at Vrije Universiteit in Amsterdam, The Netherlands. This article originally appeared on his own web site and is republished here with his permission.