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A Short History of Unix
Stephen W. Nuchia
Center for Information Security
The University of Tulsa
Chapter 1: Prehistory
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General Electric, the people who brought you the
electric power industry, manufactured large
computers in the '60s.
AT&T, the people who brought you the telephone
system, had a network.
Bell Labs, the people who brought you the
transistor, Information Theory, optical
communications, and military radar.
Were collaborating on a plan ...
Multics
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The concept was that computing would become a
utility in the future. Much like cable TV has.
Ordinary people would subscribe and gain access
(over AT&T's network) to a user-friendly
environment running on the next generation of
GE mainframes.
Secure cooperative use of the machine was an
explicit design goal of Multics.
The Bell Labs Group
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Bell labs was, for most of the 20th century, a very
special place.
Ritchie, in his 1983 Turing award lecture,
presciently foreshadowed its downfall:
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On the other hand, in smaller companies, even the
most vigorous research support is highly dependent
on market conditions. The New York Times, in an
article describing Alan Kay's passage from Atari to
Apple, notes the problem [...] ``When I left last month
it was clear that they would be putting their efforts
into the short term. [...] I guess the tree of research
must from time to time be watered with the blood of
bean counters.''
Digital Equipment Company
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DEC made a line of small(ish) computers as well
as supplying sub assemblies to IBM. When IBM
made them an offer they couldn't refuse, they
responded by ceasing to call their machines
computers. Thus was born the PDP-N series of
Peripheral Data Processors.
Most scientific and technical organizations had
them laying around. They made the machines of
choice for most early Unix sites.
Much later, DEC became an important direct
force in the Unix marketplace.
Dennis Ritchie
Quite possibly the
nicest person I've ever
met, and a truly great
programmer. He was
the scribe throughout
the AT&T phase of
Unix history as well
as one of its most
prolific visionaries.
Ch. 2: From The Evolution of the
Unix Time-sharing System (1979)
For computer science at Bell Laboratories, the period
1968-1969 was somewhat unsettled. The main reason
for this was the slow, though clearly inevitable,
withdrawal of the Labs from the Multics project. To the
Labs computing community as a whole, the problem
was the increasing obviousness of the failure of Multics
to deliver promptly any sort of usable system, let alone
the panacea envisioned earlier. For much of this time,
the Murray Hill Computer Center was also running a
costly GE 645 machine that inadequately simulated the
GE 635. Another shake-up that occurred during this
period was the organizational separation of computing
services and computing research.
Ritchie, contd.
From the point of view of the group that was to be most
involved in the beginnings of Unix (K. Thompson,
Ritchie, M. D. McIlroy, J. F. Ossanna), the decline and
fall of Multics had a directly felt effect. We were among
the last Bell Laboratories holdouts actually working on
Multics, so we still felt some sort of stake in its success.
More important, the convenient interactive computing
service that Multics had promised to the entire
community was in fact available to our limited group, at
first under the CTSS system used to develop Multics,
and later under Multics itself.
Ritchie, contd.
Even though Multics could not then support many
users, it could support us, albeit at exorbitant cost. We
didn't want to lose the pleasant niche we occupied,
because no similar ones were available; even the timesharing service that would later be offered under GE's
operating system did not exist. What we wanted to
preserve was not just a good environment in which to
do programming, but a system around which a
fellowship could form. We knew from experience that
the essence of communal computing, as supplied by
remote-access, time-shared machines, is not just to type
programs into a terminal instead of a keypunch, but to
encourage close communication.
Ritchie, contd.
Thus, during 1969, we began trying to find an
alternative to Multics. The search took several forms.
Throughout 1969 we (mainly Ossanna, Thompson,
Ritchie) lobbied intensively for the purchase of a
medium-scale machine for which we promised to write
an operating system; the machines we suggested were
the DEC PDP-10 and the SDS (later Xerox) Sigma 7.
The effort was frustrating, because our proposals were
never clearly and finally turned down, but yet were
certainly never accepted. Several times it seemed we
were very near success.
Ritchie, contd.
The final blow to this effort came when we presented an
exquisitely complicated proposal, designed to minimize
financial outlay, that involved some outright purchase,
some third-party lease, and a plan to turn in a DEC KA10 processor on the soon-to-be-announced and more
capable KI-10. The proposal was rejected, and rumor
soon had it that W. O. Baker (then vice-president of
Research) had reacted to it with the comment `Bell
Laboratories just doesn't do business this way!'
Ritchie, contd.
Actually, it is perfectly obvious in retrospect (and
should have been at the time) that we were asking the
Labs to spend too much money on too few people with
too vague a plan. Moreover, I am quite sure that at that
time operating systems were not, for our management,
an attractive area in which to support work. They were
in the process of extricating themselves not only from
an operating system development effort that had failed,
but from running the local Computation Center. Thus it
may have seemed that buying a machine such as we
suggested might lead on the one hand to yet another
Multics, or on the other, if we produced something
useful, to yet another Comp Center for them to be
responsible for.
Ritchie, contd.
Besides the financial agitations that took place in 1969,
there was technical work also. Thompson, R. H.
Canaday, and Ritchie developed, on blackboards and
scribbled notes, the basic design of a file system that
was later to become the heart of Unix. Most of the
design was Thompson's, as was the impulse to think
about file systems at all, but I believe I contributed the
idea of device files. Thompson's itch for creation of an
operating system took several forms during this period;
he also wrote (on Multics) a fairly detailed simulation
of the performance of the proposed file system design
and of paging behavior of programs.
Ritchie, contd.
In addition, he started work on a new operating system
for the GE-645, going as far as writing an assembler for
the machine and a rudimentary operating system kernel
whose greatest achievement, so far as I remember, was
to type a greeting message. The complexity of the
machine was such that a mere message was already a
fairly notable accomplishment, but when it became
clear that the lifetime of the 645 at the Labs was
measured in months, the work was dropped.
Ritchie, contd.
Also during 1969, Thompson developed the game of
`Space Travel.' First written on Multics, then
transliterated into Fortran for GECOS (the operating
system for the GE, later Honeywell, 635), it was
nothing less than a simulation of the movement of the
major bodies of the Solar System, with the player
guiding a ship here and there, observing the scenery,
and attempting to land on the various planets and
moons. The GECOS version was unsatisfactory in two
important respects: first, the display of the state of the
game was jerky and hard to control because one had to
type commands at it, and second, a game cost about $75
for CPU time on the big computer.
Ritchie, contd.
It did not take long, therefore, for Thompson to find a
little-used PDP-7 computer with an excellent display
processor; the whole system was used as a Graphic-II
terminal. He and I rewrote Space Travel to run on this
machine. The undertaking was more ambitious than it
might seem; because we disdained all existing software,
we had to write a floating-point arithmetic package, the
pointwise specification of the graphic characters for the
display, and a debugging subsystem that continuously
displayed the contents of typed-in locations in a corner
of the screen. All this was written in assembly language
for a cross-assembler that ran under GECOS and
produced paper tapes to be carried to the PDP-7.
Ritchie, contd.
Space Travel, though it made a very attractive game,
served mainly as an introduction to the clumsy
technology of preparing programs for the PDP-7. Soon
Thompson began implementing the paper file system
(perhaps `chalk file system' would be more accurate)
that had been designed earlier. A file system without a
way to exercise it is a sterile proposition, so he
proceeded to flesh it out with the other requirements for
a working operating system, in particular the notion of
processes.
Ritchie, contd.
Then came a small set of user-level utilities: the means
to copy, print, delete, and edit files, and of course a
simple command interpreter (shell). Up to this time all
the programs were written using GECOS and files were
transferred to the PDP-7 on paper tape; but once an
assembler was completed the system was able to
support itself. Although it was not until well into 1970
that Brian Kernighan suggested the name `Unix,' in a
somewhat treacherous pun on `Multics,' the operating
system we know today was born.
Chapter 2a: Berkeley
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It has been reported that Ken Thompson hacked
out the original Unix file system in one weekend,
while his wife was visiting relatives at Berkeley.
Thompson later took a sabbatical at UCB, and
introduced Unix to their CS people.
From Leffler et al: ``The second important
distinction of Unix was its early release from Bell
Laboratories to other research environments in
source form. By providing source, the system's
founders ensured that other organizations would
be able not only to use the system, but also to
tinker with its inner workings. …
Leffler, cont'd.
... The ease with which new ideas could be adopted
into the system always has been the key to
changes that have been made to it. Whenever a
new system that tried to upstage Unix came
along, somebody would dissect the newcomer
and clone its central ideas into Unix. The unique
ability to use a small, comprehensible system,
written in a high-level language, in an
environment swimming in new ideas led to a
Unix system that evolved far beyond its humble
beginnings.’’
Chapter 3: Commercial Evolution
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Companies seeking to exploit the demand for
Unix-based systems chose either AT&T or BSD
as their starting point. Since the Unix Support
Group at AT&T did not incorporate
``Berkeleyisms'' into their source tree, this led to
The Great Schism
Trivia Question: What does SUN (as in Sun
Microsystems) stand for?
Interlude: Features & Characteristics
or, What Makes Unix so Great?
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#1: Great Documentation
Written by top-shelf research programmers, for
programmers.
Contrast with Microsoft fighting disclosure of the
Windows API.
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It's fun to read.
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Table of contents of v7 manual follows point 4.
What Makes Unix Great
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#2: Profoundly simple API
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Everything is a file (except processes)
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A very manageable set of system calls
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An easy-to-live-with process model
Seventh Edition API
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File System
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Access
Chdir
Chmod
Chown
Chroot
Close
Dup
Ioctl
Link
Lseek
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Mknod
Mount
Open
Pipe
Read
Stat
Sync
Umask
Unlink
Utimes
Write
Seventh Edition API
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Process Control
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Brk
Execve
Exit
Fork
Kill
Profil
Ptrace
Signal
Wait
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Security/Misc
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Access
Acct
Getgid/getegid
Getuid/geteuid
Setgid
Setuid
Time
What Makes Unix Great
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#3: Portability
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Written in C, arguably a high-level language.
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Licensable for finite money in source form.
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Consent decree to MFJ, AT&T not competing with
licencees. Unique in this respect = startup appeal.
Led to wide commercialization
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Bill Gates' Mom and Xenix
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Amdahl and Unix
What Makes Unix Great
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#4: Accretion
Attractive environment for programmers => lots
of good programmers use it => many good
programs (at least from a programmer's
perspective!) written (portably!) for Unix.
Many of these programs became part of the base
Unix system.
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Trivia question: what criteria were used to partition
commands between /bin and /usr/bin?
What Makes Unix Great
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#5: The Shell
Steve Bourne's experiment in functional
programming leads to the pipes-and-filters
paradigm, the surround-and-tinker programming
methodology, and a very programmer-friendly
user environment.
The idea of a command line processor that is a
full-featured and conceptually well-founded
programming language interpreter was novel and
terribly important. Most people still don't get it.
Chapter 4: Opening the Source
and The Lesser Schism
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``Big'' sites had source. Mystique creates a desire
for source in mere mortals.
The advent of PC machine with 80286 processors
put virtual memory-capable machines
(comparable to PDP-11 architecture) in their
hands for a tiny fraction of the cost of a source
license. => Demand not economically satisfiable.
Xenix available, but: it is AT&T flavored, not
source, and not cool.
Use the Source, Luke
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Most of the good stuff was contributed software,
much of it more-or-less open source already.
Stallman began the Open Source movement
(Software should be free. As in Free Speech, not
as in Free Beer. -approximate quote). His
articulation of the philosophy and economics of
free software gave the demand form and voice.
UCB's mission & commercial interests led to
schizophrenic paralysis, but the CSRG began a
review process.
Let My Bits Go!
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They concluded that only a tiny fraction of BSD
Unix was ``contaminated'' with AT&T-licensed
code and released the rest.
Unfortunately, some of the contaminated source
was in the kernel's hindbrain. It was not possible
to build a runnable system from the freed source.
A widespread effort was begun to replace the
missing modules.
The effort focused on making a kernel that was
bootable on off-the-shelf PC hardware.
The Jolix War
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This was a seminal event in the cultural evolution
of the free software movement, but it was
crippled by
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The stupidity of the PC architecture and
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The personalities involved.
More time was spent arguing about which camp
had the true Unix nature than on writing code.
A Bolt from the Blue
• Meanwhile, writing operating
systems had gotten easy. At many
schools, a basic kernel is written as
a standard undergraduate exercise.
So, Linus Torvald (Helsinki
University) writes a kernel in his
spare time. Linux becomes usable
and unambiguously uncontaminated
before FreeBSD happens.
The rest is history.
Linux Takes Over. Long Live Unix
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The army of salivating developers adopts Linux
and begin ``accreting'' good stuff for it. Soon
they had essentially all the classic Unix stuff
ported or replaced, and Linux became the feature
leader of the Unix family.
Linux is Unix, mostly of the AT&T flavor. Same
philosophy (except on documentation), same
herd-of-cats development culture. Different
kernel architecture, so no common drivers.
Linux Innovations
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Linux has innovated in administrative as well as
user-oriented features, but its culture lacks the
heavy industrial component that the mainstream
(or old-school) Unix community had (remember
Multics). Thus, security is not always a wellthought-out part of their innovations.
Questions?