The UNIX Time-Sharing System PS2:1-1
The UNIX Time-Sharing System*
D. M. Ritchie and K. Thompson
ABSTRACT
UNIX- is a general-purpose, multi-user, interac-
tive operating system for the larger Digital Equipment
Corporation PDP-11 and the Interdata 8/32 computers. It
offers a number of features seldom found even in larger
operating systems, including
i A hierarchical file system incorporating demount-
able volumes,
ii Compatible file, device, and inter-process I/O,
iii The ability to initiate asynchronous processes,
iv System command language selectable on a per-user
basis,
v Over 100 subsystems including a dozen languages,
vi High degree of portability.
This paper discusses the nature and implementation of
the file system and of the user command interface.
1. INTRODUCTION
There have been four versions of the UNIX time-sharing sys-
tem. The earliest (circa 1969-70) ran on the Digital Equipment
Corporation PDP-7 and -9 computers. The second version ran on the
_________________________
* Copyright 1974, Association for Computing Machinery,
Inc., reprinted by permission. This is a revised ver-
sion of an article that appeared in Communications of
the ACM, 17, No. 7 (July 1974), pp. 365-375. That arti-
cle was a revised version of a paper presented at the
Fourth ACM Symposium on Operating Systems Principles,
IBM Thomas J. Watson Research Center, Yorktown Heights,
New York, October 15-17, 1973.
- UNIX is a registered trademark of AT&T Bell Labora-
tories in the USA and other countries.
April 27, 2013
PS2:1-2 The UNIX Time-Sharing System
unprotected PDP-11/20 computer. The third incorporated multipro-
gramming and ran on the PDP-11/34, /40, /45, /60, and /70 comput-
ers; it is the one described in the previously published version
of this paper, and is also the most widely used today. This paper
describes only the fourth, current system that runs on the PDP-
11/70 and the Interdata 8/32 computers. In fact, the differences
among the various systems is rather small; most of the revisions
made to the originally published version of this paper, aside
from those concerned with style, had to do with details of the
implementation of the file system.
Since PDP-11 UNIX became operational in February, 1971, over
600 installations have been put into service. Most of them are
engaged in applications such as computer science education, the
preparation and formatting of documents and other textual
material, the collection and processing of trouble data from
various switching machines within the Bell System, and recording
and checking telephone service orders. Our own installation is
used mainly for research in operating systems, languages, com-
puter networks, and other topics in computer science, and also
for document preparation.
Perhaps the most important achievement of UNIX is to demon-
strate that a powerful operating system for interactive use need
not be expensive either in equipment or in human effort: it can
run on hardware costing as little as $40,000, and less than two
man-years were spent on the main system software. We hope, how-
ever, that users find that the most important characteristics of
the system are its simplicity, elegance, and ease of use.
Besides the operating system proper, some major programs
available under UNIX are
C compiler
Text editor based on QED
qed lampson
Assembler, linking loader, symbolic debugger
Phototypesetting and equation setting programs
cherry kernighan typesetting mathematics cacm
kernighan lesk ossanna document preparation bstj
%Q This issue
Dozens of languages including Fortran 77, Basic,
Snobol, APL, Algol 68, M6, TMG, Pascal
There is a host of maintenance, utility, recreation and novelty
programs, all written locally. The UNIX user community, which
numbers in the thousands, has contributed many more programs and
languages. It is worth noting that the system is totally self-
supporting. All UNIX software is maintained on the system; like-
wise, this paper and all other documents in this issue were gen-
erated and formatted by the UNIX editor and text formatting pro-
grams.
April 27, 2013
The UNIX Time-Sharing System PS2:1-3
II. HARDWARE AND SOFTWARE ENVIRONMENT
The PDP-11/70 on which the Research UNIX system is installed
is a 16-bit word (8-bit byte) computer with 768K bytes of core
memory; the system kernel occupies 90K bytes about equally
divided between code and data tables. This system, however,
includes a very large number of device drivers and enjoys a gen-
erous allotment of space for I/O buffers and system tables; a
minimal system capable of running the software mentioned above
can require as little as 96K bytes of core altogether. There are
even larger installations; see the description of the PWB/UNIX
systems, dolotta mashey workbench software engineering dolotta
haight mashey workbench bstj %Q This issue for example. There are
also much smaller, though somewhat restricted, versions of the
system. lycklama microprocessor bstj %Q This issue
Our own PDP-11 has two 200-Mb moving-head disks for file
system storage and swapping. There are 20 variable-speed communi-
cations interfaces attached to 300- and 1200-baud data sets, and
an additional 12 communication lines hard-wired to 9600-baud ter-
minals and satellite computers. There are also several 2400- and
4800-baud synchronous communication interfaces used for machine-
to-machine file transfer. Finally, there is a variety of miscel-
laneous devices including nine-track magnetic tape, a line
printer, a voice synthesizer, a phototypesetter, a digital
switching network, and a chess machine.
The preponderance of UNIX software is written in the above-
mentioned C language. c programming language kernighan ritchie
prentice-hall Early versions of the operating system were written
in assembly language, but during the summer of 1973, it was
rewritten in C. The size of the new system was about one-third
greater than that of the old. Since the new system not only
became much easier to understand and to modify but also included
many functional improvements, including multiprogramming and the
ability to share reentrant code among several user programs, we
consider this increase in size quite acceptable.
III. THE FILE SYSTEM
The most important role of the system is to provide a file
system. From the point of view of the user, there are three kinds
of files: ordinary disk files, directories, and special files.
3.1 Ordinary files
A file contains whatever information the user places on it,
for example, symbolic or binary (object) programs. No particular
structuring is expected by the system. A file of text consists
simply of a string of characters, with lines demarcated by the
newline character. Binary programs are sequences of words as they
will appear in core memory when the program starts executing. A
few user programs manipulate files with more structure; for exam-
ple, the assembler generates, and the loader expects, an object
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file in a particular format. However, the structure of files is
controlled by the programs that use them, not by the system.
3.2 Directories
Directories provide the mapping between the names of files
and the files themselves, and thus induce a structure on the file
system as a whole. Each user has a directory of his own files; he
may also create subdirectories to contain groups of files con-
veniently treated together. A directory behaves exactly like an
ordinary file except that it cannot be written on by unprivileged
programs, so that the system controls the contents of direc-
tories. However, anyone with appropriate permission may read a
directory just like any other file.
The system maintains several directories for its own use.
One of these is the root directory. All files in the system can
be found by tracing a path through a chain of directories until
the desired file is reached. The starting point for such searches
is often the root. Other system directories contain all the pro-
grams provided for general use; that is, all the commands. As
will be seen, however, it is by no means necessary that a program
reside in one of these directories for it to be executed.
Files are named by sequences of 14 or fewer characters. When
the name of a file is specified to the system, it may be in the
form of a path name, which is a sequence of directory names
separated by slashes, ``/'', and ending in a file name. If the
sequence begins with a slash, the search begins in the root
directory. The name /alpha/beta/gamma causes the system to search
the root for directory alpha, then to search alpha for beta,
finally to find gamma in beta. gamma may be an ordinary file, a
directory, or a special file. As a limiting case, the name ``/''
refers to the root itself.
A path name not starting with ``/'' causes the system to
begin the search in the user's current directory. Thus, the name
alpha/beta specifies the file named beta in subdirectory alpha of
the current directory. The simplest kind of name, for example,
alpha, refers to a file that itself is found in the current
directory. As another limiting case, the null file name refers to
the current directory.
The same non-directory file may appear in several direc-
tories under possibly different names. This feature is called
linking; a directory entry for a file is sometimes called a link.
The UNIX system differs from other systems in which linking is
permitted in that all links to a file have equal status. That is,
a file does not exist within a particular directory; the direc-
tory entry for a file consists merely of its name and a pointer
to the information actually describing the file. Thus a file
exists independently of any directory entry, although in practice
a file is made to disappear along with the last link to it.
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The UNIX Time-Sharing System PS2:1-5
Each directory always has at least two entries. The name
``.'' in each directory refers to the directory itself. Thus a
program may read the current directory under the name ``.''
without knowing its complete path name. The name ``..'' by con-
vention refers to the parent of the directory in which it
appears, that is, to the directory in which it was created.
The directory structure is constrained to have the form of a
rooted tree. Except for the special entries ``.'' and ``..'',
each directory must appear as an entry in exactly one other
directory, which is its parent. The reason for this is to sim-
plify the writing of programs that visit subtrees of the direc-
tory structure, and more important, to avoid the separation of
portions of the hierarchy. If arbitrary links to directories were
permitted, it would be quite difficult to detect when the last
connection from the root to a directory was severed.
3.3 Special files
Special files constitute the most unusual feature of the
UNIX file system. Each supported I/O device is associated with at
least one such file. Special files are read and written just like
ordinary disk files, but requests to read or write result in
activation of the associated device. An entry for each special
file resides in directory /dev, although a link may be made to
one of these files just as it may to an ordinary file. Thus, for
example, to write on a magnetic tape one may write on the file
/dev/mt. Special files exist for each communication line, each
disk, each tape drive, and for physical main memory. Of course,
the active disks and the memory special file are protected from
indiscriminate access.
There is a threefold advantage in treating I/O devices this
way: file and device I/O are as similar as possible; file and
device names have the same syntax and meaning, so that a program
expecting a file name as a parameter can be passed a device name;
finally, special files are subject to the same protection mechan-
ism as regular files.
3.4 Removable file systems
Although the root of the file system is always stored on the
same device, it is not necessary that the entire file system
hierarchy reside on this device. There is a mount system request
with two arguments: the name of an existing ordinary file, and
the name of a special file whose associated storage volume (e.g.,
a disk pack) should have the structure of an independent file
system containing its own directory hierarchy. The effect of
mount is to cause references to the heretofore ordinary file to
refer instead to the root directory of the file system on the
removable volume. In effect, mount replaces a leaf of the hierar-
chy tree (the ordinary file) by a whole new subtree (the hierar-
chy stored on the removable volume). After the mount, there is
virtually no distinction between files on the removable volume
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and those in the permanent file system. In our installation, for
example, the root directory resides on a small partition of one
of our disk drives, while the other drive, which contains the
user's files, is mounted by the system initialization sequence. A
mountable file system is generated by writing on its correspond-
ing special file. A utility program is available to create an
empty file system, or one may simply copy an existing file sys-
tem.
There is only one exception to the rule of identical treat-
ment of files on different devices: no link may exist between one
file system hierarchy and another. This restriction is enforced
so as to avoid the elaborate bookkeeping that would otherwise be
required to assure removal of the links whenever the removable
volume is dismounted.
3.5 Protection
Although the access control scheme is quite simple, it has
some unusual features. Each user of the system is assigned a
unique user identification number. When a file is created, it is
marked with the user ID of its owner. Also given for new files is
a set of ten protection bits. Nine of these specify independently
read, write, and execute permission for the owner of the file,
for other members of his group, and for all remaining users.
If the tenth bit is on, the system will temporarily change
the user identification (hereafter, user ID) of the current user
to that of the creator of the file whenever the file is executed
as a program. This change in user ID is effective only during the
execution of the program that calls for it. The set-user-ID
feature provides for privileged programs that may use files inac-
cessible to other users. For example, a program may keep an
accounting file that should neither be read nor changed except by
the program itself. If the set-user-ID bit is on for the program,
it may access the file although this access might be forbidden to
other programs invoked by the given program's user. Since the
actual user ID of the invoker of any program is always available,
set-user-ID programs may take any measures desired to satisfy
themselves as to their invoker's credentials. This mechanism is
used to allow users to execute the carefully written commands
that call privileged system entries. For example, there is a sys-
tem entry invokable only by the ``super-user'' (below) that
creates an empty directory. As indicated above, directories are
expected to have entries for ``.'' and ``..''. The command which
creates a directory is owned by the super-user and has the set-
user-ID bit set. After it checks its invoker's authorization to
create the specified directory, it creates it and makes the
entries for ``.'' and ``..''.
Because anyone may set the set-user-ID bit on one of his own
files, this mechanism is generally available without administra-
tive intervention. For example, this protection scheme easily
solves the MOO accounting problem posed by ``Aleph-null.'' aleph
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null software practice
The system recognizes one particular user ID (that of the
``super-user'') as exempt from the usual constraints on file
access; thus (for example), programs may be written to dump and
reload the file system without unwanted interference from the
protection system.
3.6 I/O calls
The system calls to do I/O are designed to eliminate the
differences between the various devices and styles of access.
There is no distinction between ``random'' and ``sequential''
I/O, nor is any logical record size imposed by the system. The
size of an ordinary file is determined by the number of bytes
written on it; no predetermination of the size of a file is
necessary or possible.
To illustrate the essentials of I/O, some of the basic calls
are summarized below in an anonymous language that will indicate
the required parameters without getting into the underlying com-
plexities. Each call to the system may potentially result in an
error return, which for simplicity is not represented in the cal-
ling sequence.
To read or write a file assumed to exist already, it must be
opened by the following call:
filep = open(name, flag)
where name indicates the name of the file. An arbitrary path name
may be given. The flag argument indicates whether the file is to
be read, written, or ``updated,'' that is, read and written
simultaneously.
The returned value filep is called a file descriptor. It is
a small integer used to identify the file in subsequent calls to
read, write, or otherwise manipulate the file.
To create a new file or completely rewrite an old one, there
is a create system call that creates the given file if it does
not exist, or truncates it to zero length if it does exist;
create also opens the new file for writing and, like open,
returns a file descriptor.
The file system maintains no locks visible to the user, nor
is there any restriction on the number of users who may have a
file open for reading or writing. Although it is possible for the
contents of a file to become scrambled when two users write on it
simultaneously, in practice difficulties do not arise. We take
the view that locks are neither necessary nor sufficient, in our
environment, to prevent interference between users of the same
file. They are unnecessary because we are not faced with large,
single-file data bases maintained by independent processes. They
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are insufficient because locks in the ordinary sense, whereby one
user is prevented from writing on a file that another user is
reading, cannot prevent confusion when, for example, both users
are editing a file with an editor that makes a copy of the file
being edited.
There are, however, sufficient internal interlocks to main-
tain the logical consistency of the file system when two users
engage simultaneously in activities such as writing on the same
file, creating files in the same directory, or deleting each
other's open files.
Except as indicated below, reading and writing are sequen-
tial. This means that if a particular byte in the file was the
last byte written (or read), the next I/O call implicitly refers
to the immediately following byte. For each open file there is a
pointer, maintained inside the system, that indicates the next
byte to be read or written. If n bytes are read or written, the
pointer advances by n bytes.
Once a file is open, the following calls may be used:
n = read(filep, buffer, count)
n = write(filep, buffer, count)
Up to count bytes are transmitted between the file specified by
filep and the byte array specified by buffer. The returned value
n is the number of bytes actually transmitted. In the write case,
n is the same as count except under exceptional conditions, such
as I/O errors or end of physical medium on special files; in a
read, however, n may without error be less than count. If the
read pointer is so near the end of the file that reading count
characters would cause reading beyond the end, only sufficient
bytes are transmitted to reach the end of the file; also,
typewriter-like terminals never return more than one line of
input. When a read call returns with n equal to zero, the end of
the file has been reached. For disk files this occurs when the
read pointer becomes equal to the current size of the file. It is
possible to generate an end-of-file from a terminal by use of an
escape sequence that depends on the device used.
Bytes written affect only those parts of a file implied by
the position of the write pointer and the count; no other part of
the file is changed. If the last byte lies beyond the end of the
file, the file is made to grow as needed.
To do random (direct-access) I/O it is only necessary to
move the read or write pointer to the appropriate location in the
file.
location = lseek(filep, offset, base)
The pointer associated with filep is moved to a position offset
bytes from the beginning of the file, from the current position
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of the pointer, or from the end of the file, depending on base.
offset may be negative. For some devices (e.g., paper tape and
terminals) seek calls are ignored. The actual offset from the
beginning of the file to which the pointer was moved is returned
in location.
There are several additional system entries having to do
with I/O and with the file system that will not be discussed. For
example: close a file, get the status of a file, change the pro-
tection mode or the owner of a file, create a directory, make a
link to an existing file, delete a file.
IV. IMPLEMENTATION OF THE FILE SYSTEM
As mentioned in Section 3.2 above, a directory entry con-
tains only a name for the associated file and a pointer to the
file itself. This pointer is an integer called the i-number (for
index number) of the file. When the file is accessed, its i-
number is used as an index into a system table (the i-list)
stored in a known part of the device on which the directory
resides. The entry found thereby (the file's i-node) contains the
description of the file:
i the user and group-ID of its owner
ii its protection bits
iii the physical disk or tape addresses for the file contents
iv its size
v time of creation, last use, and last modification
vi the number of links to the file, that is, the number of
times it appears in a directory
vii a code indicating whether the file is a directory, an ordi-
nary file, or a special file.
The purpose of an open or create system call is to turn the path
name given by the user into an i-number by searching the expli-
citly or implicitly named directories. Once a file is open, its
device, i-number, and read/write pointer are stored in a system
table indexed by the file descriptor returned by the open or
create. Thus, during a subsequent call to read or write the file,
the descriptor may be easily related to the information necessary
to access the file.
When a new file is created, an i-node is allocated for it
and a directory entry is made that contains the name of the file
and the i-node number. Making a link to an existing file involves
creating a directory entry with the new name, copying the i-
number from the original file entry, and incrementing the link-
count field of the i-node. Removing (deleting) a file is done by
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decrementing the link-count of the i-node specified by its direc-
tory entry and erasing the directory entry. If the link-count
drops to 0, any disk blocks in the file are freed and the i-node
is de-allocated.
The space on all disks that contain a file system is divided
into a number of 512-byte blocks logically addressed from 0 up to
a limit that depends on the device. There is space in the i-node
of each file for 13 device addresses. For nonspecial files, the
first 10 device addresses point at the first 10 blocks of the
file. If the file is larger than 10 blocks, the 11 device address
points to an indirect block containing up to 128 addresses of
additional blocks in the file. Still larger files use the twelfth
device address of the i-node to point to a double-indirect block
naming 128 indirect blocks, each pointing to 128 blocks of the
file. If required, the thirteenth device address is a triple-
indirect block. Thus files may conceptually grow to
[(10+128+1282+1283).512] bytes. Once opened, bytes numbered below
5120 can be read with a single disk access; bytes in the range
5120 to 70,656 require two accesses; bytes in the range 70,656 to
8,459,264 require three accesses; bytes from there to the largest
file (1,082,201,088) require four accesses. In practice, a device
cache mechanism (see below) proves effective in eliminating most
of the indirect fetches.
The foregoing discussion applies to ordinary files. When an
I/O request is made to a file whose i-node indicates that it is
special, the last 12 device address words are immaterial, and the
first specifies an internal device name, which is interpreted as
a pair of numbers representing, respectively, a device type and
subdevice number. The device type indicates which system routine
will deal with I/O on that device; the subdevice number selects,
for example, a disk drive attached to a particular controller or
one of several similar terminal interfaces.
In this environment, the implementation of the mount system
call (Section 3.4) is quite straightforward. mount maintains a
system table whose argument is the i-number and device name of
the ordinary file specified during the mount, and whose
corresponding value is the device name of the indicated special
file. This table is searched for each i-number/device pair that
turns up while a path name is being scanned during an open or
create; if a match is found, the i-number is replaced by the i-
number of the root directory and the device name is replaced by
the table value.
To the user, both reading and writing of files appear to be
synchronous and unbuffered. That is, immediately after return
from a read call the data are available; conversely, after a
write the user's workspace may be reused. In fact, the system
maintains a rather complicated buffering mechanism that reduces
greatly the number of I/O operations required to access a file.
Suppose a write call is made specifying transmission of a single
byte. The system will search its buffers to see whether the
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affected disk block currently resides in main memory; if not, it
will be read in from the device. Then the affected byte is
replaced in the buffer and an entry is made in a list of blocks
to be written. The return from the write call may then take
place, although the actual I/O may not be completed until a later
time. Conversely, if a single byte is read, the system determines
whether the secondary storage block in which the byte is located
is already in one of the system's buffers; if so, the byte can be
returned immediately. If not, the block is read into a buffer and
the byte picked out.
The system recognizes when a program has made accesses to
sequential blocks of a file, and asynchronously pre-reads the
next block. This significantly reduces the running time of most
programs while adding little to system overhead.
A program that reads or writes files in units of 512 bytes
has an advantage over a program that reads or writes a single
byte at a time, but the gain is not immense; it comes mainly from
the avoidance of system overhead. If a program is used rarely or
does no great volume of I/O, it may quite reasonably read and
write in units as small as it wishes.
The notion of the i-list is an unusual feature of UNIX. In
practice, this method of organizing the file system has proved
quite reliable and easy to deal with. To the system itself, one
of its strengths is the fact that each file has a short, unambi-
guous name related in a simple way to the protection, addressing,
and other information needed to access the file. It also permits
a quite simple and rapid algorithm for checking the consistency
of a file system, for example, verification that the portions of
each device containing useful information and those free to be
allocated are disjoint and together exhaust the space on the dev-
ice. This algorithm is independent of the directory hierarchy,
because it need only scan the linearly organized i-list. At the
same time the notion of the i-list induces certain peculiarities
not found in other file system organizations. For example, there
is the question of who is to be charged for the space a file
occupies, because all directory entries for a file have equal
status. Charging the owner of a file is unfair in general, for
one user may create a file, another may link to it, and the first
user may delete the file. The first user is still the owner of
the file, but it should be charged to the second user. The sim-
plest reasonably fair algorithm seems to be to spread the charges
equally among users who have links to a file. Many installations
avoid the issue by not charging any fees at all.
V. PROCESSES AND IMAGES
An image is a computer execution environment. It includes a
memory image, general register values, status of open files,
current directory and the like. An image is the current state of
a pseudo-computer.
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A process is the execution of an image. While the processor
is executing on behalf of a process, the image must reside in
main memory; during the execution of other processes it remains
in main memory unless the appearance of an active, higher-
priority process forces it to be swapped out to the disk.
The user-memory part of an image is divided into three logi-
cal segments. The program text segment begins at location 0 in
the virtual address space. During execution, this segment is
write-protected and a single copy of it is shared among all
processes executing the same program. At the first hardware pro-
tection byte boundary above the program text segment in the vir-
tual address space begins a non-shared, writable data segment,
the size of which may be extended by a system call. Starting at
the highest address in the virtual address space is a stack seg-
ment, which automatically grows downward as the stack pointer
fluctuates.
5.1 Processes
Except while the system is bootstrapping itself into opera-
tion, a new process can come into existence only by use of the
fork system call:
processid = fork()
When fork is executed, the process splits into two independently
executing processes. The two processes have independent copies of
the original memory image, and share all open files. The new
processes differ only in that one is considered the parent pro-
cess: in the parent, the returned processid actually identifies
the child process and is never 0, while in the child, the
returned value is always 0.
Because the values returned by fork in the parent and child
process are distinguishable, each process may determine whether
it is the parent or child.
5.2 Pipes
Processes may communicate with related processes using the
same system read and write calls that are used for file-system
I/O. The call:
filep = pipe()
returns a file descriptor filep and creates an inter-process
channel called a pipe. This channel, like other open files, is
passed from parent to child process in the image by the fork
call. A read using a pipe file descriptor waits until another
process writes using the file descriptor for the same pipe. At
this point, data are passed between the images of the two
processes. Neither process need know that a pipe, rather than an
ordinary file, is involved.
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Although inter-process communication via pipes is a quite
valuable tool (see Section 6.2), it is not a completely general
mechanism, because the pipe must be set up by a common ancestor
of the processes involved.
5.3 Execution of programs
Another major system primitive is invoked by
execute(file, arg1, arg2, ... , argn)
which requests the system to read in and execute the program
named by file, passing it string arguments arg1, arg2, ..., argn.
All the code and data in the process invoking execute is replaced
from the file, but open files, current directory, and inter-
process relationships are unaltered. Only if the call fails, for
example because file could not be found or because its execute-
permission bit was not set, does a return take place from the
execute primitive; it resembles a ``jump'' machine instruction
rather than a subroutine call.
5.4 Process synchronization
Another process control system call:
processid = wait(status)
causes its caller to suspend execution until one of its children
has completed execution. Then wait returns the processid of the
terminated process. An error return is taken if the calling pro-
cess has no descendants. Certain status from the child process is
also available.
5.5 Termination
Lastly:
exit(status)
terminates a process, destroys its image, closes its open files,
and generally obliterates it. The parent is notified through the
wait primitive, and status is made available to it. Processes may
also terminate as a result of various illegal actions or user-
generated signals (Section VII below).
VI. THE SHELL
For most users, communication with the system is carried on
with the aid of a program called the shell. The shell is a
command-line interpreter: it reads lines typed by the user and
interprets them as requests to execute other programs. (The shell
is described fully elsewhere, bourne shell bstj %Q This issue so
this section will discuss only the theory of its operation.) In
simplest form, a command line consists of the command name
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followed by arguments to the command, all separated by spaces:
command arg1 arg2 ... argn
The shell splits up the command name and the arguments into
separate strings. Then a file with name command is sought;
command may be a path name including the ``/'' character to
specify any file in the system. If command is found, it is
brought into memory and executed. The arguments collected by the
shell are accessible to the command. When the command is fin-
ished, the shell resumes its own execution, and indicates its
readiness to accept another command by typing a prompt character.
If file command cannot be found, the shell generally pre-
fixes a string such as /bin/ to command and attempts again to
find the file. Directory /bin contains commands intended to be
generally used. (The sequence of directories to be searched may
be changed by user request.)
6.1 Standard I/O
The discussion of I/O in Section III above seems to imply
that every file used by a program must be opened or created by
the program in order to get a file descriptor for the file. Pro-
grams executed by the shell, however, start off with three open
files with file descriptors 0, 1, and 2. As such a program begins
execution, file 1 is open for writing, and is best understood as
the standard output file. Except under circumstances indicated
below, this file is the user's terminal. Thus programs that wish
to write informative information ordinarily use file descriptor
1. Conversely, file 0 starts off open for reading, and programs
that wish to read messages typed by the user read this file.
The shell is able to change the standard assignments of
these file descriptors from the user's terminal printer and key-
board. If one of the arguments to a command is prefixed by ``>'',
file descriptor 1 will, for the duration of the command, refer to
the file named after the ``>''. For example:
ls
ordinarily lists, on the typewriter, the names of the files in
the current directory. The command:
ls >there
creates a file called there and places the listing there. Thus
the argument >there means ``place output on there.'' On the other
hand:
ed
ordinarily enters the editor, which takes requests from the user
via his keyboard. The command
April 27, 2013
The UNIX Time-Sharing System PS2:1-15
ed <script
interprets script as a file of editor commands; thus <script
means ``take input from script.''
Although the file name following ``<'' or ``>'' appears to
be an argument to the command, in fact it is interpreted com-
pletely by the shell and is not passed to the command at all.
Thus no special coding to handle I/O redirection is needed within
each command; the command need merely use the standard file
descriptors 0 and 1 where appropriate.
File descriptor 2 is, like file 1, ordinarily associated
with the terminal output stream. When an output-diversion request
with ``>'' is specified, file 2 remains attached to the terminal,
so that commands may produce diagnostic messages that do not
silently end up in the output file.
6.2 Filters
An extension of the standard I/O notion is used to direct
output from one command to the input of another. A sequence of
commands separated by vertical bars causes the shell to execute
all the commands simultaneously and to arrange that the standard
output of each command be delivered to the standard input of the
next command in the sequence. Thus in the command line:
ls | pr -2 | opr
ls lists the names of the files in the current directory; its
output is passed to pr, which paginates its input with dated
headings. (The argument ``-2'' requests double-column output.)
Likewise, the output from pr is input to opr; this command spools
its input onto a file for off-line printing.
This procedure could have been carried out more clumsily by:
ls >temp1
pr -2 <temp1 >temp2
opr <temp2
followed by removal of the temporary files. In the absence of the
ability to redirect output and input, a still clumsier method
would have been to require the ls command to accept user requests
to paginate its output, to print in multi-column format, and to
arrange that its output be delivered off-line. Actually it would
be surprising, and in fact unwise for efficiency reasons, to
expect authors of commands such as ls to provide such a wide
variety of output options.
A program such as pr which copies its standard input to its
standard output (with processing) is called a filter. Some
filters that we have found useful perform character
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transliteration, selection of lines according to a pattern, sort-
ing of the input, and encryption and decryption.
6.3 Command separators; multitasking
Another feature provided by the shell is relatively
straightforward. Commands need not be on different lines; instead
they may be separated by semicolons:
ls; ed
will first list the contents of the current directory, then enter
the editor.
A related feature is more interesting. If a command is fol-
lowed by ``&,'' the shell will not wait for the command to finish
before prompting again; instead, it is ready immediately to
accept a new command. For example:
as source >output &
causes source to be assembled, with diagnostic output going to
output; no matter how long the assembly takes, the shell returns
immediately. When the shell does not wait for the completion of a
command, the identification number of the process running that
command is printed. This identification may be used to wait for
the completion of the command or to terminate it. The ``&'' may
be used several times in a line:
as source >output & ls >files &
does both the assembly and the listing in the background. In
these examples, an output file other than the terminal was pro-
vided; if this had not been done, the outputs of the various com-
mands would have been intermingled.
The shell also allows parentheses in the above operations.
For example:
(date; ls) >x &
writes the current date and time followed by a list of the
current directory onto the file x. The shell also returns immedi-
ately for another request.
6.4 The shell as a command; command files
The shell is itself a command, and may be called recur-
sively. Suppose file tryout contains the lines:
as source
mv a.out testprog
testprog
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The UNIX Time-Sharing System PS2:1-17
The mv command causes the file a.out to be renamed testprog.
a.out is the (binary) output of the assembler, ready to be exe-
cuted. Thus if the three lines above were typed on the keyboard,
source would be assembled, the resulting program renamed
testprog, and testprog executed. When the lines are in tryout,
the command:
sh <tryout
would cause the shell sh to execute the commands sequentially.
The shell has further capabilities, including the ability to
substitute parameters and to construct argument lists from a
specified subset of the file names in a directory. It also pro-
vides general conditional and looping constructions.
6.5 Implementation of the shell
The outline of the operation of the shell can now be under-
stood. Most of the time, the shell is waiting for the user to
type a command. When the newline character ending the line is
typed, the shell's read call returns. The shell analyzes the com-
mand line, putting the arguments in a form appropriate for
execute. Then fork is called. The child process, whose code of
course is still that of the shell, attempts to perform an execute
with the appropriate arguments. If successful, this will bring in
and start execution of the program whose name was given.
Meanwhile, the other process resulting from the fork, which is
the parent process, waits for the child process to die. When this
happens, the shell knows the command is finished, so it types its
prompt and reads the keyboard to obtain another command.
Given this framework, the implementation of background
processes is trivial; whenever a command line contains ``&,'' the
shell merely refrains from waiting for the process that it
created to execute the command.
Happily, all of this mechanism meshes very nicely with the
notion of standard input and output files. When a process is
created by the fork primitive, it inherits not only the memory
image of its parent but also all the files currently open in its
parent, including those with file descriptors 0, 1, and 2. The
shell, of course, uses these files to read command lines and to
write its prompts and diagnostics, and in the ordinary case its
children-the command programs-inherit them automatically. When an
argument with ``<'' or ``>'' is given, however, the offspring
process, just before it performs execute, makes the standard I/O
file descriptor (0 or 1, respectively) refer to the named file.
This is easy because, by agreement, the smallest unused file
descriptor is assigned when a new file is opened (or created); it
is only necessary to close file 0 (or 1) and open the named file.
Because the process in which the command program runs simply ter-
minates when it is through, the association between a file speci-
fied after ``<'' or ``>'' and file descriptor 0 or 1 is ended
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PS2:1-18 The UNIX Time-Sharing System
automatically when the process dies. Therefore the shell need not
know the actual names of the files that are its own standard
input and output, because it need never reopen them.
Filters are straightforward extensions of standard I/O
redirection with pipes used instead of files.
In ordinary circumstances, the main loop of the shell never
terminates. (The main loop includes the branch of the return from
fork belonging to the parent process; that is, the branch that
does a wait, then reads another command line.) The one thing that
causes the shell to terminate is discovering an end-of-file con-
dition on its input file. Thus, when the shell is executed as a
command with a given input file, as in:
sh <comfile
the commands in comfile will be executed until the end of comfile
is reached; then the instance of the shell invoked by sh will
terminate. Because this shell process is the child of another
instance of the shell, the wait executed in the latter will
return, and another command may then be processed.
6.6 Initialization
The instances of the shell to which users type commands are
themselves children of another process. The last step in the ini-
tialization of the system is the creation of a single process and
the invocation (via execute) of a program called init. The role
of init is to create one process for each terminal channel. The
various subinstances of init open the appropriate terminals for
input and output on files 0, 1, and 2, waiting, if necessary, for
carrier to be established on dial-up lines. Then a message is
typed out requesting that the user log in. When the user types a
name or other identification, the appropriate instance of init
wakes up, receives the log-in line, and reads a password file. If
the user's name is found, and if he is able to supply the correct
password, init changes to the user's default current directory,
sets the process's user ID to that of the person logging in, and
performs an execute of the shell. At this point, the shell is
ready to receive commands and the logging-in protocol is com-
plete.
Meanwhile, the mainstream path of init (the parent of all
the subinstances of itself that will later become shells) does a
wait. If one of the child processes terminates, either because a
shell found an end of file or because a user typed an incorrect
name or password, this path of init simply recreates the defunct
process, which in turn reopens the appropriate input and output
files and types another log-in message. Thus a user may log out
simply by typing the end-of-file sequence to the shell.
April 27, 2013
The UNIX Time-Sharing System PS2:1-19
6.7 Other programs as shell
The shell as described above is designed to allow users full
access to the facilities of the system, because it will invoke
the execution of any program with appropriate protection mode.
Sometimes, however, a different interface to the system is desir-
able, and this feature is easily arranged for.
Recall that after a user has successfully logged in by sup-
plying a name and password, init ordinarily invokes the shell to
interpret command lines. The user's entry in the password file
may contain the name of a program to be invoked after log-in
instead of the shell. This program is free to interpret the
user's messages in any way it wishes.
For example, the password file entries for users of a secre-
tarial editing system might specify that the editor ed is to be
used instead of the shell. Thus when users of the editing system
log in, they are inside the editor and can begin work immedi-
ately; also, they can be prevented from invoking programs not
intended for their use. In practice, it has proved desirable to
allow a temporary escape from the editor to execute the format-
ting program and other utilities.
Several of the games (e.g., chess, blackjack, 3D tic-tac-
toe) available on the system illustrate a much more severely res-
tricted environment. For each of these, an entry exists in the
password file specifying that the appropriate game-playing pro-
gram is to be invoked instead of the shell. People who log in as
a player of one of these games find themselves limited to the
game and unable to investigate the (presumably more interesting)
offerings of the UNIX system as a whole.
VII. TRAPS
The PDP-11 hardware detects a number of program faults, such
as references to non-existent memory, unimplemented instructions,
and odd addresses used where an even address is required. Such
faults cause the processor to trap to a system routine. Unless
other arrangements have been made, an illegal action causes the
system to terminate the process and to write its image on file
core in the current directory. A debugger can be used to deter-
mine the state of the program at the time of the fault.
Programs that are looping, that produce unwanted output, or
about which the user has second thoughts may be halted by the use
of the interrupt signal, which is generated by typing the
``delete'' character. Unless special action has been taken, this
signal simply causes the program to cease execution without pro-
ducing a core file. There is also a quit signal used to force an
image file to be produced. Thus programs that loop unexpectedly
may be halted and the remains inspected without prearrangement.
The hardware-generated faults and the interrupt and quit
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PS2:1-20 The UNIX Time-Sharing System
signals can, by request, be either ignored or caught by a pro-
cess. For example, the shell ignores quits to prevent a quit from
logging the user out. The editor catches interrupts and returns
to its command level. This is useful for stopping long printouts
without losing work in progress (the editor manipulates a copy of
the file it is editing). In systems without floating-point
hardware, unimplemented instructions are caught and floating-
point instructions are interpreted.
VIII. PERSPECTIVE
Perhaps paradoxically, the success of the UNIX system is
largely due to the fact that it was not designed to meet any
predefined objectives. The first version was written when one of
us (Thompson), dissatisfied with the available computer facili-
ties, discovered a little-used PDP-7 and set out to create a more
hospitable environment. This (essentially personal) effort was
sufficiently successful to gain the interest of the other author
and several colleagues, and later to justify the acquisition of
the PDP-11/20, specifically to support a text editing and format-
ting system. When in turn the 11/20 was outgrown, the system had
proved useful enough to persuade management to invest in the
PDP-11/45, and later in the PDP-11/70 and Interdata 8/32
machines, upon which it developed to its present form. Our goals
throughout the effort, when articulated at all, have always been
to build a comfortable relationship with the machine and to
explore ideas and inventions in operating systems and other
software. We have not been faced with the need to satisfy someone
else's requirements, and for this freedom we are grateful.
Three considerations that influenced the design of UNIX are
visible in retrospect.
First: because we are programmers, we naturally designed the
system to make it easy to write, test, and run programs. The most
important expression of our desire for programming convenience
was that the system was arranged for interactive use, even though
the original version only supported one user. We believe that a
properly designed interactive system is much more productive and
satisfying to use than a ``batch'' system. Moreover, such a sys-
tem is rather easily adaptable to noninteractive use, while the
converse is not true.
Second: there have always been fairly severe size con-
straints on the system and its software. Given the partially
antagonistic desires for reasonable efficiency and expressive
power, the size constraint has encouraged not only economy, but
also a certain elegance of design. This may be a thinly disguised
version of the ``salvation through suffering'' philosophy, but in
our case it worked.
Third: nearly from the start, the system was able to, and
did, maintain itself. This fact is more important than it might
seem. If designers of a system are forced to use that system,
April 27, 2013
The UNIX Time-Sharing System PS2:1-21
they quickly become aware of its functional and superficial defi-
ciencies and are strongly motivated to correct them before it is
too late. Because all source programs were always available and
easily modified on-line, we were willing to revise and rewrite
the system and its software when new ideas were invented,
discovered, or suggested by others.
The aspects of UNIX discussed in this paper exhibit clearly
at least the first two of these design considerations. The inter-
face to the file system, for example, is extremely convenient
from a programming standpoint. The lowest possible interface
level is designed to eliminate distinctions between the various
devices and files and between direct and sequential access. No
large ``access method'' routines are required to insulate the
programmer from the system calls; in fact, all user programs
either call the system directly or use a small library program,
less than a page long, that buffers a number of characters and
reads or writes them all at once.
Another important aspect of programming convenience is that
there are no ``control blocks'' with a complicated structure par-
tially maintained by and depended on by the file system or other
system calls. Generally speaking, the contents of a program's
address space are the property of the program, and we have tried
to avoid placing restrictions on the data structures within that
address space.
Given the requirement that all programs should be usable
with any file or device as input or output, it is also desirable
to push device-dependent considerations into the operating system
itself. The only alternatives seem to be to load, with all pro-
grams, routines for dealing with each device, which is expensive
in space, or to depend on some means of dynamically linking to
the routine appropriate to each device when it is actually
needed, which is expensive either in overhead or in hardware.
Likewise, the process-control scheme and the command inter-
face have proved both convenient and efficient. Because the shell
operates as an ordinary, swappable user program, it consumes no
``wired-down'' space in the system proper, and it may be made as
powerful as desired at little cost. In particular, given the
framework in which the shell executes as a process that spawns
other processes to perform commands, the notions of I/O redirec-
tion, background processes, command files, and user-selectable
system interfaces all become essentially trivial to implement.
Influences
The success of UNIX lies not so much in new inventions but
rather in the full exploitation of a carefully selected set of
fertile ideas, and especially in showing that they can be keys to
the implementation of a small yet powerful operating system.
The fork operation, essentially as we implemented it, was
April 27, 2013
PS2:1-22 The UNIX Time-Sharing System
present in the GENIE time-sharing system. lampson deutsch 930
manual 1965 system preliminary On a number of points we were
influenced by Multics, which suggested the particular form of the
I/O system calls multics input output feiertag organick and both
the name of the shell and its general functions. The notion that
the shell should create a process for each command was also sug-
gested to us by the early design of Multics, although in that
system it was later dropped for efficiency reasons. A similar
scheme is used by TENEX. bobrow burchfiel tenex
IX. STATISTICS
The following numbers are presented to suggest the scale of
the Research UNIX operation. Those of our users not involved in
document preparation tend to use the system for program develop-
ment, especially language work. There are few important ``appli-
cations'' programs.
Overall, we have today:
125 user population
33 maximum simultaneous users
1,630 directories
28,300 files
301,700 512-byte secondary storage blocks used
There is a ``background'' process that runs at the lowest possi-
ble priority; it is used to soak up any idle CPU time. It has
been used to produce a million-digit approximation to the con-
stant e, and other semi-infinite problems. Not counting this
background work, we average daily:
13,500 commands
9.6 CPU hours
230 connect hours
62 different users
240 log-ins
X. ACKNOWLEDGMENTS
The contributors to UNIX are, in the traditional but here
especially apposite phrase, too numerous to mention. Certainly,
collective salutes are due to our colleagues in the Computing
Science Research Center. R. H. Canaday contributed much to the
basic design of the file system. We are particularly appreciative
of the inventiveness, thoughtful criticism, and constant support
of R. Morris, M. D. McIlroy, and J. F. Ossanna. $LIST$
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