Above: A Mini Computer that so many of us loved… including Dennis Ritchie at Bell Labs. A tribute to Ritchie
With the most recent passing of Steve Jobs, it was a little bit of a shock to learn that Dennis Ritchie had passed away so soon after Jobs. Here’s a Man any DIYer would salute, as he was all about ‘Building It Yourself’. He may not be as well known as Steve Jobs, but as I see it, he may have done just as much or more to change the world forever. Jobs knew the Apple Operating System Sucked! It’s the reason Apple worked so hard and spent the money to create their own version of UNIX. No doubt they did a first class job of it too 🙂
Back in the OLD days, Ritchie was looking for a better and quicker way to build tools for the Lab.. We need remember the time frame, acoustic coupled modems operating at speeds up to a blazing 300 baud at the time! This was far nearer the beginning of modern computing. I’m not sure the kids now days can even grasp how slow 300 baud is, would they believe that some of us in the mid 1970s would start loading a simple program off tape before dinner, so we’d have it in core by the time dinner was over.
Some of us will remember the first sort utilities we ever saw, and how close to magic it seemed… to submit a page of ASCII words, and redirect the output to another file with all in alphabetical order.
But back in the days, a lot of Researchers were still operating with fairly primitive tools, and doing such things by hand. There were a number of systems using punch cards for trouble analysis, maybe it was you goal to figure out what parts your company made had the highest failure rates. You might enter the trouble report on a punch card, and put holes in certain places on the card. Some systems even relied on rods to be inserted into the stack of cards and troubles of a certain kind could be pulled (sorted) counted by hand and then further analyzed. A lot of screwing around.
If you were a Bell Head, you may have used some better stuff for your routine work… adding to a program and producing a new copy on paper tape. Back in these days, we used rubber cement to glue several paper tapes together to order a major pile of relays to execute our test programs (x5xbar) in sequence.. that way you eliminated the labor intensive process of loading them one at a time. Failures even printed out on the ASR33, and you had your list of things to fix! Near magic and no processor involved!
Here’s a paper written by Ritchie, following it is more comment by me..
The Evolution of the Unix Time-sharing System*
- Dennis M. Ritchie
Bell Laboratories, Murray Hill, NJ, 07974
This paper presents a brief history of the early development of
the Unix operating system. It concentrates on the evolution of the file system,
the process-control mechanism, and the idea of pipelined commands. Some
attention is paid to social conditions during the development of the system.
- NOTE: *This paper was first presented at the Language Design and
Programming Methodology conference at Sydney, Australia, September 1979. The
conference proceedings were published as Lecture Notes in Computer Science
#79: Language Design and Programming Methodology, Springer-Verlag,
1980. This rendition is based on a reprinted version appearing in AT&T Bell
Laboratories Technical Journal 63 No. 6 Part 2, October 1984, pp.
During the past few years, the Unix operating system has come into wide use,
so wide that its very name has become a trademark of Bell Laboratories. Its
important characteristics have become known to many people. It has suffered much
rewriting and tinkering since the first publication describing it in 1974 ,
but few fundamental changes. However, Unix was born in 1969 not 1974, and the
account of its development makes a little-known and perhaps instructive story.
This paper presents a technical and social history of the evolution of the
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.
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. 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 time-sharing 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.
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. 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 KA-10 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!’
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
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. 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.
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. 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.
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. 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.
The PDP-7 Unix file system
Structurally, the file system of PDP-7 Unix was nearly identical to today’s.
- An i-list: a linear array of i-nodes each describing a file. An
i-node contained less than it does now, but the essential information was the
same: the protection mode of the file, its type and size, and the list of
physical blocks holding the contents.
- Directories: a special kind of file containing a sequence of names and the
- Special files describing devices. The device specification was not contained
explicitly in the i-node, but was instead encoded in the number: specific
i-numbers corresponded to specific files.
The important file system calls were also present from the start. Read,
write, open, creat (sic), close: with one very important exception, discussed
below, they were similar to what one finds now. A minor difference was that the
unit of I/O was the word, not the byte, because the PDP-7 was a word-addressed
machine. In practice this meant merely that all programs dealing with character
streams ignored null characters, because null was used to pad a file to an even
number of characters. Another minor, occasionally annoying difference was the
lack of erase and kill processing for terminals. Terminals, in effect, were
always in raw mode. Only a few programs (notably the shell and the editor)
bothered to implement erase-kill processing.
In spite of its considerable similarity to the current file system, the PDP-7
file system was in one way remarkably different: there were no path names, and
each file-name argument to the system was a simple name (without `/’) taken
relative to the current directory. Links, in the usual Unix sense, did exist.
Together with an elaborate set of conventions, they were the principal means by
which the lack of path names became acceptable.
The linkcall took the form
link(dir, file, newname)
where dir was a directory file in the current
directory, file an existing entry in that directory, and newname
the name of the link, which was added to the current directory. Because
dir needed to be in the current directory, it is evident that today’s
prohibition against links to directories was not enforced; the PDP-7 Unix file
system had the shape of a general directed graph.
So that every user did not need to maintain a link to all directories of
interest, there existed a directory called dd that contained entries for
the directory of each user. Thus, to make a link to file x in directory
ken, I might do
ln dd ken ken ln ken x x rm ken
This scheme rendered subdirectories sufficiently hard to
use as to make them unused in practice. Another important barrier was that there
was no way to create a directory while the system was running; all were made
during recreation of the file system from paper tape, so that directories were
in effect a nonrenewable resource.
The dd convention made the chdir command relatively convenient.
It took multiple arguments, and switched the current directory to each named
directory in turn. Thus
chdir dd ken
would move to directory ken. (Incidentally,
chdir was spelled ch; why this was expanded when we went to the
PDP-11 I don’t remember.)
The most serious inconvenience of the implementation of the file system,
aside from the lack of path names, was the difficulty of changing its
configuration; as mentioned, directories and special files were both made only
when the disk was recreated. Installation of a new device was very painful,
because the code for devices was spread widely throughout the system; for
example there were several loops that visited each device in turn. Not
surprisingly, there was no notion of mounting a removable disk pack, because the
machine had only a single fixed-head disk.
The operating system code that implemented this file system was a drastically
simplified version of the present scheme. One important simplification followed
from the fact that the system was not multi-programmed; only one program was in
memory at a time, and control was passed between processes only when an explicit
swap took place. So, for example, there was an iget routine that made a
named i-node available, but it left the i-node in a constant, static location
rather than returning a pointer into a large table of active i-nodes. A
precursor of the current buffering mechanism was present (with about 4 buffers)
but there was essentially no overlap of disk I/O with computation. This was
avoided not merely for simplicity. The disk attached to the PDP-7 was fast for
its time; it transferred one 18-bit word every 2 microseconds. On the other
hand, the PDP-7 itself had a memory cycle time of 1 microsecond, and most
instructions took 2 cycles (one for the instruction itself, one for the
operand). However, indirectly addressed instructions required 3 cycles, and
indirection was quite common, because the machine had no index registers.
Finally, the DMA controller was unable to access memory during an instruction.
The upshot was that the disk would incur overrun errors if any
indirectly-addressed instructions were executed while it was transferring. Thus
control could not be returned to the user, nor in fact could general system code
be executed, with the disk running. The interrupt routines for the clock and
terminals, which needed to be runnable at all times, had to be coded in very
strange fashion to avoid indirection.
By `process control,’ I mean the mechanisms by which processes are created
and used; today the system calls fork, exec, wait, and
exit implement these mechanisms. Unlike the file system, which existed in
nearly its present form from the earliest days, the process control scheme
underwent considerable mutation after PDP-7 Unix was already in use. (The
introduction of path names in the PDP-11 system was certainly a considerable
notational advance, but not a change in fundamental structure.)
Today, the way in which commands are executed by the shell can be summarized
- The shell reads a command line from the terminal.
- It creates a child process by fork.
- The child process uses execto call in the command from a file.
- Meanwhile, the parent shell uses wait to wait for the child (command)
process to terminate by calling exit.
- The parent shell goes back to step 1).
Processes (independently executing entities) existed very early in PDP-7
Unix. There were in fact precisely two of them, one for each of the two
terminals attached to the machine. There was no fork, wait, or
exec. There was an exit, but its meaning was rather different, as
will be seen. The main loop of the shell went as follows.
- The shell closed all its open files, then opened the terminal special file
for standard input and output (file descriptors 0 and 1).
- It read a command line from the terminal.
- It linked to the file specifying the command, opened the file, and removed
the link. Then it copied a small bootstrap program to the top of memory and
jumped to it; this bootstrap program read in the file over the shell code, then
jumped to the first location of the command (in effect an exec).
- The command did its work, then terminated by calling exit. The
exit call caused the system to read in a fresh copy of the shell over the
terminated command, then to jump to its start (and thus in effect to go to step
The most interesting thing about this primitive implementation is the degree
to which it anticipated themes developed more fully later. True, it could
support neither background processes nor shell command files (let alone pipes
and filters); but IO redirection (via `<‘ and `>’) was soon there; it is
discussed below. The implementation of redirection was quite straightforward; in
step 3) above the shell just replaced its standard input or output with the
appropriate file. Crucial to subsequent development was the implementation of
the shell as a user-level program stored in a file, rather than a part of the
The structure of this process control scheme, with one process per terminal,
is similar to that of many interactive systems, for example CTSS, Multics,
Honeywell TSS, and IBM TSS and TSO. In general such systems require special
mechanisms to implement useful facilities such as detached computations and
command files; Unix at that stage didn’t bother to supply the special
mechanisms. It also exhibited some irritating, idiosyncratic problems. For
example, a newly recreated shell had to close all its open files both to get rid
of any open files left by the command just executed and to rescind previous IO
redirection. Then it had to reopen the special file corresponding to its
terminal, in order to read a new command line. There was no /dev
directory (because no path names); moreover, the shell could retain no memory
across commands, because it was reexecuted afresh after each command. Thus a
further file system convention was required: each directory had to contain an
entry tty for a special file that referred to the terminal of the process
that opened it. If by accident one changed into some directory that lacked this
entry, the shell would loop hopelessly; about the only remedy was to reboot.
(Sometimes the missing link could be made from the other terminal.)
Process control in its modern form was designed and implemented within a
couple of days. It is astonishing how easily it fitted into the existing system;
at the same time it is easy to see how some of the slightly unusual features of
the design are present precisely because they represented small, easily-coded
changes to what existed. A good example is the separation of the fork and
exec functions. The most common model for the creation of new processes
involves specifying a program for the process to execute; in Unix, a forked
process continues to run the same program as its parent until it performs an
explicit exec. The separation of the functions is certainly not unique to
Unix, and in fact it was present in the Berkeley time-sharing system , which
was well-known to Thompson. Still, it seems reasonable to suppose that it exists
in Unix mainly because of the ease with which fork could be implemented
without changing much else. The system already handled multiple (i.e. two)
processes; there was a process table, and the processes were swapped between
main memory and the disk. The initial implementation of fork required
- Expansion of the process table
- Addition of a fork call that copied the current process to the disk swap
area, using the already existing swap IO primitives, and made some adjustments
to the process table.
In fact, the PDP-7’s fork call required precisely 27 lines of assembly
code. Of course, other changes in the operating system and user programs were
required, and some of them were rather interesting and unexpected. But a
combined fork-exec would have been considerably more complicated, if only
because exec as such did not exist; its function was already performed,
using explicit IO, by the shell.
The exit system call, which previously read in a new copy of the shell
(actually a sort of automatic exec but without arguments), simplified
considerably; in the new version a process only had to clean out its process
table entry, and give up control.
Curiously, the primitives that became wait were considerably more
general than the present scheme. A pair of primitives sent one-word messages
between named processes:
smes(pid, message) (pid, message) = rmes()
The target process of smes did not need to have any
ancestral relationship with the receiver, although the system provided no
explicit mechanism for communicating process IDs except that fork
returned to each of the parent and child the ID of its relative. Messages were
not queued; a sender delayed until the receiver read the message.
The message facility was used as follows: the parent shell, after creating a
process to execute a command, sent a message to the new process by smes;
when the command terminated (assuming it did not try to read any messages) the
shell’s blocked smes call returned an error indication that the target
process did not exist. Thus the shell’s smes became, in effect, the
equivalent of wait.
A different protocol, which took advantage of more of the generality offered
by messages, was used between the initialization program and the shells for each
terminal. The initialization process, whose ID was understood to be 1, created a
shell for each of the terminals, and then issued rmes; each shell, when
it read the end of its input file, used smes to send a conventional `I am
terminating’ message to the initialization process, which recreated a new shell
process for that terminal.
I can recall no other use of messages. This explains why the facility was
replaced by the wait call of the present system, which is less general,
but more directly applicable to the desired purpose. Possibly relevant also is
the evident bug in the mechanism: if a command process attempted to use messages
to communicate with other processes, it would disrupt the shell’s
synchronization. The shell depended on sending a message that was never
received; if a command executed rmes, it would receive the shell’s phony
message, and cause the shell to read another input line just as if the command
had terminated. If a need for general messages had manifested itself, the bug
would have been repaired.
At any rate, the new process control scheme instantly rendered some very
valuable features trivial to implement; for example detached processes (with
`&’) and recursive use of the shell as a command. Most systems have to
supply some sort of special `batch job submission’ facility and a special
command interpreter for files distinct from the one used interactively.
Although the multiple-process idea slipped in very easily indeed, there were
some aftereffects that weren’t anticipated. The most memorable of these became
evident soon after the new system came up and apparently worked. In the midst of
our jubilation, it was discovered that the chdir (change current
directory) command had stopped working. There was much reading of code and
anxious introspection about how the addition of fork could have broken
the chdir call. Finally the truth dawned: in the old system chdir
was an ordinary command; it adjusted the current directory of the (unique)
process attached to the terminal. Under the new system, the chdir command
correctly changed the current directory of the process created to execute it,
but this process promptly terminated and had no effect whatsoever on its parent
shell! It was necessary to make chdir a special command, executed
internally within the shell. It turns out that several command-like functions
have the same property, for example login.
Another mismatch between the system as it had been and the new process
control scheme took longer to become evident. Originally, the read/write pointer
associated with each open file was stored within the process that opened the
file. (This pointer indicates where in the file the next read or write will take
place.) The problem with this organization became evident only when we tried to
use command files. Suppose a simple command file contains
and it is executed as follows:
sh comfile >output
The sequence of events was
- The main shell creates a new process, which opens outfile to receive
the standard output and executes the shell recursively.
- The new shell creates another process to execute ls, which correctly
writes on file outputand then terminates.
- Another process is created to execute the next command. However, the IO
pointer for the output is copied from that of the shell, and it is still 0,
because the shell has never written on its output, and IO pointers are
associated with processes. The effect is that the output of who
overwrites and destroys the output of the preceding ls command.
Solution of this problem required creation of a new system table to contain
the IO pointers of open files independently of the process in which they were
The very convenient notation for IO redirection, using the `>’ and `<‘
characters, was not present from the very beginning of the PDP-7 Unix system,
but it did appear quite early. Like much else in Unix, it was inspired by an
idea from Multics. Multics has a rather general IO redirection mechanism 
embodying named IO streams that can be dynamically redirected to various
devices, files, and even through special stream-processing modules. Even in the
version of Multics we were familiar with a decade ago, there existed a command
that switched subsequent output normally destined for the terminal to a file,
and another command to reattach output to the terminal. Where under Unix one
to get a listing of the names of one’s files in xx,
on Multics the notation was
iocall attach user_output file xx list iocall attach user_output syn user_i/o
Even though this very clumsy sequence was used often during
the Multics days, and would have been utterly straightforward to integrate into
the Multics shell, the idea did not occur to us or anyone else at the time. I
speculate that the reason it did not was the sheer size of the Multics project:
the implementors of the IO system were at Bell Labs in Murray Hill, while the
shell was done at MIT. We didn’t consider making changes to the shell (it was
their program); correspondingly, the keepers of the shell may not even
have known of the usefulness, albeit clumsiness, of iocall. (The 1969
Multics manual  lists iocall as an `author-maintained,’ that is
non-standard, command.) Because both the Unix IO system and its shell were under
the exclusive control of Thompson, when the right idea finally surfaced, it was
a matter of an hour or so to implement it.
The advent of the PDP-11
By the beginning of 1970, PDP-7 Unix was a going concern. Primitive by
today’s standards, it was still capable of providing a more congenial
programming environment than its alternatives. Nevertheless, it was clear that
the PDP-7, a machine we didn’t even own, was already obsolete, and its
successors in the same line offered little of interest. In early 1970 we
proposed acquisition of a PDP-11, which had just been introduced by Digital. In
some sense, this proposal was merely the latest in the series of attempts that
had been made throughout the preceding year. It differed in two important ways.
First, the amount of money (about $65,000) was an order of magnitude less than
what we had previously asked; second, the charter sought was not merely to write
some (unspecified) operating system, but instead to create a system specifically
designed for editing and formatting text, what might today be called a
`word-processing system.’ The impetus for the proposal came mainly from J. F.
Ossanna, who was then and until the end of his life interested in text
processing. If our early proposals were too vague, this one was perhaps too
specific; at first it too met with disfavor. Before long, however, funds were
obtained through the efforts of L. E. McMahon and an order for a PDP-11 was
placed in May.
The processor arrived at the end of the summer, but the PDP-11 was so new a
product that no disk was available until December. In the meantime, a
rudimentary, core-only version of Unix was written using a cross-assembler on
the PDP-7. Most of the time, the machine sat in a corner, enumerating all the
closed Knight’s tours on a 6×8 chess board—a three-month job.
The first PDP-11 system
Once the disk arrived, the system was quickly completed. In internal
structure, the first version of Unix for the PDP-11 represented a relatively
minor advance over the PDP-7 system; writing it was largely a matter of
transliteration. For example, there was no multi-programming; only one user
program was present in core at any moment. On the other hand, there were
important changes in the interface to the user: the present directory structure,
with full path names, was in place, along with the modern form of exec
and wait, and conveniences like character-erase and line-kill processing
for terminals. Perhaps the most interesting thing about the enterprise was its
small size: there were 24K bytes of core memory (16K for the system, 8K for user
programs), and a disk with 1K blocks (512K bytes). Files were limited to 64K
At the time of the placement of the order for the PDP-11, it had seemed
natural, or perhaps expedient, to promise a system dedicated to word processing.
During the protracted arrival of the hardware, the increasing usefulness of
PDP-7 Unix made it appropriate to justify creating PDP-11 Unix as a development
tool, to be used in writing the more special-purpose system. By the spring of
1971, it was generally agreed that no one had the slightest interest in
scrapping Unix. Therefore, we transliterated the roff text formatter into
PDP-11 assembler language, starting from the PDP-7 version that had been
transliterated from McIlroy’s BCPL version on Multics, which had in turn been
inspired by J. Saltzer’s runoff program on CTSS. In early summer, editor
and formatter in hand, we felt prepared to fulfill our charter by offering to
supply a text-processing service to the Patent department for preparing patent
applications. At the time, they were evaluating a commercial system for this
purpose; the main advantages we offered (besides the dubious one of taking part
in an in-house experiment) were two in number: first, we supported Teletype’s
model 37 terminals, which, with an extended type-box, could print most of the
math symbols they required; second, we quickly endowed roff with the
ability to produce line-numbered pages, which the Patent Office required and
which the other system could not handle.
During the last half of 1971, we supported three typists from the Patent
department, who spent the day busily typing, editing, and formatting patent
applications, and meanwhile tried to carry on our own work. Unix has a
reputation for supplying interesting services on modest hardware, and this
period may mark a high point in the benefit/equipment ratio; on a machine with
no memory protection and a single .5 MB disk, every test of a new program
required care and boldness, because it could easily crash the system, and every
few hours’ work by the typists meant pushing out more information onto DECtape,
because of the very small disk.
The experiment was trying but successful. Not only did the Patent department
adopt Unix, and thus become the first of many groups at the Laboratories to
ratify our work, but we achieved sufficient credibility to convince our own
management to acquire one of the first PDP 11/45 systems made. We have
accumulated much hardware since then, and labored continuously on the software,
but because most of the interesting work has already been published, (e.g. on
the system itself [1, 5, 6, 7, 8, 9]) it seems unnecessary to repeat it here.
One of the most widely admired contributions of Unix to the culture of
operating systems and command languages is the pipe, as used in a
pipeline of commands. Of course, the fundamental idea was by no means new; the
pipeline is merely a specific form of coroutine. Even the implementation was not
unprecedented, although we didn’t know it at the time; the `communication files’
of the Dartmouth Time-Sharing System  did very nearly what Unix pipes do,
though they seem not to have been exploited so fully.
Pipes appeared in Unix in 1972, well after the PDP-11 version of the system
was in operation, at the suggestion (or perhaps insistence) of M. D. McIlroy, a
long-time advocate of the non-hierarchical control flow that characterizes
coroutines. Some years before pipes were implemented, he suggested that commands
should be thought of as binary operators, whose left and right operand specified
the input and output files. Thus a `copy’ utility would be commanded by
inputfile copy outputfile
To make a pipeline, command operators could be stacked up.
Thus, to sort input, paginate it neatly, and print the result off-line,
one would write
input sort paginate offprint
In today’s system, this would correspond to
sort input | pr | opr
The idea, explained one afternoon on a blackboard,
intrigued us but failed to ignite any immediate action. There were several
objections to the idea as put: the infix notation seemed too radical (we were
too accustomed to typing `cp x y’ to copy x to y); and we were
unable to see how to distinguish command parameters from the input or output
files. Also, the one-input one-output model of command execution seemed too
confining. What a failure of imagination!
Some time later, thanks to McIlroy’s persistence, pipes were finally
installed in the operating system (a relatively simple job), and a new notation
was introduced. It used the same characters as for I/O redirection. For example,
the pipeline above might have been written
sort input >pr>opr>
The idea is that following a `>’ may be either a file,
to specify redirection of output to that file, or a command into which the
output of the preceding command is directed as input. The trailing `>’ was
needed in the example to specify that the (nonexistent) output of opr
should be directed to the console; otherwise the command opr would not
have been executed at all; instead a file oprwould have been created.
The new facility was enthusiastically received, and the term `filter’ was
soon coined. Many commands were changed to make them usable in pipelines. For
example, no one had imagined that anyone would want the sort or pr
utility to sort or print its standard input if given no explicit arguments.
Soon some problems with the notation became evident. Most annoying was a
silly lexical problem: the string after `>’ was delimited by blanks, so, to
give a parameter to prin the example, one had to quote:
sort input >"pr -2">opr>
Second, in attempt to give generality, the pipe notation
accepted `<‘ as an input redirection in a way corresponding to `>’; this
meant that the notation was not unique. One could also write, for example,
opr <pr<"sort input"<
pr <"sort input"< >opr>
The pipe notation using `<‘ and `>’ survived only a
couple of months; it was replaced by the present one that uses a unique operator
to separate components of a pipeline. Although the old notation had a certain
charm and inner consistency, the new one is certainly superior. Of course, it
too has limitations. It is unabashedly linear, though there are situations in
which multiple redirected inputs and outputs are called for. For example, what
is the best way to compare the outputs of two programs? What is the appropriate
notation for invoking a program with two parallel output streams?
I mentioned above in the section on IO redirection that Multics provided a
mechanism by which IO streams could be directed through processing modules on
the way to (or from) the device or file serving as source or sink. Thus it might
seem that stream-splicing in Multics was the direct precursor of Unix pipes, as
Multics IO redirection certainly was for its Unix version. In fact I do not
think this is true, or is true only in a weak sense. Not only were coroutines
well-known already, but their embodiment as Multics spliceable IO modules
required that the modules be specially coded in such a way that they could be
used for no other purpose. The genius of the Unix pipeline is precisely that it
is constructed from the very same commands used constantly in simplex fashion.
The mental leap needed to see this possibility and to invent the notation is
Every program for the original PDP-7 Unix system was written in assembly
language, and bare assembly language it was—for example, there were no macros.
Moreover, there was no loader or link-editor, so every program had to be
complete in itself. The first interesting language to appear was a version of
McClure’s TMG  that was implemented by McIlroy. Soon after TMG became
available, Thompson decided that we could not pretend to offer a real computing
service without Fortran, so he sat down to write a Fortran in TMG. As I recall,
the intent to handle Fortran lasted about a week. What he produced instead was a
definition of and a compiler for the new language B . B was much influenced
by the BCPL language ; other influences were Thompson’s taste for spartan
syntax, and the very small space into which the compiler had to fit. The
compiler produced simple interpretive code; although it and the programs it
produced were rather slow, it made life much more pleasant. Once interfaces to
the regular system calls were made available, we began once again to enjoy the
benefits of using a reasonable language to write what are usually called
`systems programs:’ compilers, assemblers, and the like. (Although some might
consider the PL/I we used under Multics unreasonable, it was much better than
assembly language.) Among other programs, the PDP-7 B cross-compiler for the
PDP-11 was written in B, and in the course of time, the B compiler for the PDP-7
itself was transliterated from TMG into B.
When the PDP-11 arrived, B was moved to it almost immediately. In fact, a
version of the multi-precision `desk calculator’ program dc was one of
the earliest programs to run on the PDP-11, well before the disk arrived.
However, B did not take over instantly. Only passing thought was given to
rewriting the operating system in B rather than assembler, and the same was true
of most of the utilities. Even the assembler was rewritten in assembler. This
approach was taken mainly because of the slowness of the interpretive code. Of
smaller but still real importance was the mismatch of the word-oriented B
language with the byte-addressed PDP-11.
Thus, in 1971, work began on what was to become the C language . The
story of the language developments from BCPL through B to C is told elsewhere
, and need not be repeated here. Perhaps the most important watershed
occurred during 1973, when the operating system kernel was rewritten in C. It
was at this point that the system assumed its modern form; the most far-reaching
change was the introduction of multi-programming. There were few
externally-visible changes, but the internal structure of the system became much
more rational and general. The success of this effort convinced us that C was
useful as a nearly universal tool for systems programming, instead of just a toy
for simple applications.
Today, the only important Unix program still written in assembler is the
assembler itself; virtually all the utility programs are in C, and so are most
of the applications programs, although there are sites with many in Fortran,
Pascal, and Algol 68 as well. It seems certain that much of the success of Unix
follows from the readability, modifiability, and portability of its software
that in turn follows from its expression in high-level languages.
One of the comforting things about old memories is their tendency to take on
a rosy glow. The programming environment provided by the early versions of Unix
seems, when described here, to be extremely harsh and primitive. I am sure that
if forced back to the PDP-7 I would find it intolerably limiting and lacking in
conveniences. Nevertheless, it did not seem so at the time; the memory fixes on
what was good and what lasted, and on the joy of helping to create the
improvements that made life better. In ten years, I hope we can look back with
the same mixed impression of progress combined with continuity.
I am grateful to S. P. Morgan, K. Thompson, and M. D. McIlroy for providing
early documents and digging up recollections.
Because I am most interested in describing the evolution of ideas, this paper
attributes ideas and work to individuals only where it seems most important. The
reader will not, on the average, go far wrong if he reads each occurrence of
`we’ with unclear antecedent as `Thompson, with some assistance from me.’
- D. M. Ritchie and K. Thompson, `The Unix Time-sharing System, C. ACM
17No. 7 (July 1974), pp 365-37.
- L. P. Deutch and B. W. Lampson, `SDS 930 Time-sharing System Preliminary
Reference Manual,’ Doc. 30.10.10, Project Genie, Univ. Cal. at Berkeley (April
- R. J. Feiertag and E. I. Organick, `The Multics input-output system,’ Proc.
Third Symposium on Operating Systems Principles, October 18-20, 1971, pp. 35-41.
- The Multiplexed Information and Computing Service: Programmers’
Manual,Mass. Inst. of Technology, Project MAC, Cambridge MA, (1969).
- K. Thompson, `Unix Implementation,’ Bell System Tech J. 57 No. 6,
(July-August 1978), pp. 1931-46.
- S. C. Johnson and D. M. Ritchie, Portability of C Programs and the Unix
System,’ Bell System Tech J. 57No. 6, (July-August 1978), pp. 2021-48.
- B. W. Kernighan, M. E. Lesk, and J. F. Ossanna. `Document Preparation,’ Bell
Sys. Tech. J., 57No. 6, pp. 2115-2135.
- B. W. Kernighan and L. L. Cherry, `A System for Typesetting Mathematics,’ J.
Comm. Assoc. Comp. Mach. 18,pp. 151-157 (March 1975).
- M. E. Lesk and B. W. Kernighan, `Computer Typesetting of Technical Journals
on Unix,’ Proc. AFIPS NCC 46(1977), pp. 879-88.
- Systems Programmers Manual for the Dartmouth Time Sharing System for the
GE 635 Computer,Dartmouth College, Hanover, New Hampshire, 1971.
- R. M. McClure, `TMG–A Syntax-Directed Compiler,’ Proc 20th ACM National
Conf. (1968), pp. 262-74.
- S. C. Johnson and B. W. Kernighan, `The Programming Language B,’ Comp. Sci.
Tech. Rep. #8, Bell Laboratories, Murray Hill NJ (1973).
- M. Richards, `BCPL: A Tool for Compiler Writing and Systems Programming,’
Proc. AFIPS SJCC 34(1969), pp. 557-66.
- B. W. Kernighan and D. M. Ritchie, The C Programming Language,
Prentice-Hall, Englewood Cliffs NJ, 1978. Second Edition, 1979.
- D. M. Ritchie, S. C. Johnson, and M. E. Lesk, `The C Programming Language,’
Bell Sys. Tech. J. 57 No. 6 (July-August 1978) pp. 1991-2019.
1996 Lucent Technologies Inc. All rights reserved.
Is Lucent even a company anymore???
If you ever met some of these guys in the labs, you’d suspect Ritchie likely wrote most of his code after hours, a lot of these guys got so involved in their persuit, and didn’t put down their pencils at 1630 hours. And thank God researchers in the Labs weren’t unionized, so they weren’t forced to quit thinking at quitting time.
Dennis Ritchie had a PDP7 and later a PDP11 in the Labs to play with, add to them the C language and Unix, and you might trace all that is popular today back to the folks at the lab who developed them because there wasn’t anything really worth a shit at the time.
Other Bell Telephone History..
We might note that the Bell System had designed their first electronic switching system by 1948! This switch (a large >computer< often the size of a building) were still being installed in the mid 1970s! One of the First ESS offices in the NW went into the city of Bellevue in Washington State, and was known as the Bellevue Glencourt Exchange. It was a show place and during business hours, you could walk into the main entrance and look at what you thought was futuristic, (about 1970) some failed to realize this system was designed well before they were born!
I noted early on, the Master Control Console of the #1ESS machine and later the 1AESS made the command center on the USS Enterprise look stark in comparison! If you wanted to leave an impression, just show a guy a bunch of lighted buttons, knobs, switches and whirly Jigs!
In later offices (#1A, above), we see more modern tape drives to the left.
Thanks to Dennis Ritchie and others, we had surveillance Computers by then, (PDP117os running Unix). The way it worked, is the ESS would dump out an error of some kind….. it might have been a call processing error were the computer attempted to access data in a Program Store and saw a parity error, the computer having a high degree of redundancy likely cycled onto a backup program store and completed a call just fine, but now it needed to run a diagnostic, maybe quarantine a bad part of the computer, record the error, and report it to you in a most cryptic way.
With Unix, you could build any kind of tool you needed, it was a tool set that built tools, and unlike other environments, the lowly user had access to these tools. tools to build tools.
That Cryptic message…dumped by switches of the day…a bunch of octal words with an error number header.. you might lay out the data by hand and decipher the bits and their meaning. What a time-consuming pain in the butt.., but some people liked it. I’d bet Ritchie wasn’t one of those people, perhaps busy work is best appreciated by those paid hourly?
At some point in the development of shooting trouble in the “Stored Program Switches” modems became fast enough where one could move all that ascii data the switch was dumping to a minicomputer room and tie it to an I/O port of a PDP11 via a private circuit and modem. The 1170 would basically string match on the error headers, and break down the data below it, and even bin failures of the same kind and keep track of the number.
We had mini programs that monitored connections between switches and between switches and business. An example of how effective the mini surveillance programs were; might be revealed by sharing a problem I saw between a major switch in downtown seattle and a big PBX (company).
All of a sudden, the switch would roll TN08s! This message was short hand for transmitter time outs, and looking up the trunk group, it was a large PBX. Back in those days, being part of a monopoly ment you had all kinds of regulators imposing rules and even giving you a grade on your efforts to provide service. I cared about these TNO8s, because we were graded on the amount of these failures each month, and it was part of an ‘index’ or report card. It didn’t matter that the Private Company was causing the problem, our transmitters were timing out because they failed to put a friggen receiver on at the other end… didn’t matter, it lowered our grade.
So just what the heck are these people up to anyway? I think they have more traffic into and out of that place than the city of Seattle has..
So I call and ask for their Telecom guy. ” I dunno, seems the PBX is working just fine by the time I get in there to check it out.
Yelp, sure enough, we’ll either help them fix the problem, or we’ll live with it.. I was convinced of that..
Asking the mini for some help on this problem, he lays the data out and clearly notes that the bulk of the failures (99.9x percent of them) happen on thursdays, not only thursdays but the second thursday of each month. Ohhh… but there’s more.. they start at 1:45PM, and normally stop by 2:05PM..
Assumptions Assumptions, well at least we have a computer to consult for facts, as we’re getting nothing from the other end! In the old days, when you had trouble at the distant end going into another telephone company, and you didn’t get help clearing it, maybe you broke something important to them and reminded them you needed help at their end when they called 🙂 But you can’t do that to your Customer!
One of the folks analyzed the mini output further and noted their whole trunk group went stone dead for a period of time on this certain day.. but if that was the case, why weren’t we getting any reports from customers trying to call them, or from them trying to call other people??
We came to learn that they had a fire drill once a month. To make it real as possible someone would pull the Fire Alarm, and make all the noise and racket of the real thing. As per design of Seattle regulations and the HVAC Engineer, certain air handlers (fans and blowers) were shut off as a result of the fire alarm being pulled.
Here’s where Murphy steps in 🙂 Some of their Telecommunications gear was powered off that panel designed to go dead WHEN the fire alarm was pulled.
All the folks in the building were told there was no excuse not to participate in the fire drill, and perhaps the managers appreciated the fact that the phones went dead, as this assured people would get off the phone and participate.
Their phone system out there might have remained that way for years had it not been for one question we asked them….
If you have a real fire, wouldn’t you want your phone system to work so you could call the fire department?
That one question prompted them to take the situation serious and actually power the communications gear off a panel that would stay powered up during a fire 🙂 End of problem!
Things like this were a lot harder to pattern prior to the 1170s and tools the boys at the Labs had developed.
Remember Kramer? Thanks to Dennis Ritchie and others, I can find near anything with a unix tool like ‘grep’ or one designed to emulate grep. I can find it on a page, or search the whole internet. It took me 10 seconds to find this reference to a favorite Seinfeld episode, then cut and paste it below. It reminds me of what Unix and the mini FIRST allowed us to do, and how we added user-friendly features just like Kramer envisioned long ago ‘movie phone’ 🙂
KRAMER: Hewwo and welcome to Movie phone. If you know the name of the
movie you’d like to see, press one.
GEORGE: Come on. Come on.
KRAMER: Using your touch-tone keypad, please enter the first three
letters of the movie title, now.
(George presses 3 keys)
KRAMER: You’ve selected … Agent Zero? If that’s correct, press one.
KRAMER: Ah, you’ve selected … Brown-Eyed Girl? If this is correct,
(George looks baffled)
KRAMER: Why don’t you just tell me the name of the movie you’ve
KRAMER: To find the theater nearest you, please enter your five digit
(George enters his zip-code)
KRAMER: Why don’t you just tell me where you want to see the movie?
GEORGE: Lowes Paragon, 84th and Broadway.
KRAMER: (picks up paper) Chunnel, is playing at the Paragon 84th Street
cinema in the main theater at 9:30 PM.
GEORGE: Yeah, now I gotcha! (hangs up the phone and rushes out the
KRAMER: It’s also playing in theater number two at 9:00.
Some of the most elegant tools I’ve ever seen were found on PDP1170 surveillance Computers, and folks who worked in the switching control centers (SCCs) perfomed surveillance functions for a Metro Area or Several States used them. Many of these folks got really good at using the minis, and wrote programs to do a lot of their work, the mini may have been most valuable in executing a lot of changes that had to be done in a compressed time frame, and it seemed there were new tools created every month.
As in most environments, the Techs often learn a bunch by pulling pranks on other techs, and the most excitable are often the ones who are targeted for this treatment. One Tech went back through history to find some of the larger and more disturbing switch failures, and strung them all together, he then used some script to simulate real troubles in real offices all displayed across large panels in the SCC.. everybody was warned this was going to happen… except for the victim… of course he went crazy as he watched all the people around him totally ignore what equated to the world coming to an end in down town Seattle, and all these dumb asses around him were ignoring it!!! He was animated to say the least..
Unix and the Minis made just about anything you could think of possible thanks to Dennis R. and other Lab guys.
Ritchie gives us a glimpse of what it was like to submit a proposal for funding inside a private company, and how they failed to get a few necessary toys in the day. Perhaps there’s a number of ways to interpet his account, I’d suggest that none of them had any marketing experience, or knew a thing about grant writing. I’d bet none of the guys that worked in the labs had any appreciation for bull shit, nor did they know that it would eventually be the key to funding beyond their wildest dreams!
Ritchie and Gang were excited when their proposal was accepted and their $65,000 mini arrived! Their proposal was about building a real product with a real purpose, and there were no known physical barriers that stoood between them and their goals. Compare that with today 10/15/11 when we know our Government handed over Millions in grants (our money) to Companies that had business plans that had already failed!
And… some still don’t understand why we’re broke! Meanwhile, Al Gore enjoys that $500 million in Government grants awarded his Company? No wonder there’s so much Bull Shit all around it works!
Here’s an example I found the other day at the store:
Tell me, who watches a Free Range Chicken and makes sure he’s eating organic stuff?
Bull Shit Rules…or so it seems.