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.OH 'The UNIX Time-Sharing System''PSD:1-%'
.EH 'PSD:1-%''The UNIX Time-Sharing System'
.ds n \s+2
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The UNIX
Time-Sharing System\f1\s10\v'-.2n'*\v'.2n'\s0\fP
.AU
D. M. Ritchie and K. Thompson
.AB
.FS
* Copyright 1974,
Association for Computing Machinery, Inc.,
reprinted by permission.
This is a revised version of an article
that appeared in Communications of the \*sACM\*n,
.IT 17 ,
No. 7 (July 1974), pp. 365-375.
That article was a
revised version of a paper presented
at the Fourth \*sACM\*n Symposium on Operating
Systems Principles,
\*sIBM\*n Thomas J. Watson Research Center,
Yorktown Heights,
New York,
October 15-17, 1973.
.FE
.UX
is a general-purpose, multi-user, interactive
operating system for the larger Digital Equipment Corporation
\*sPDP\*n-11 and
the Interdata 8/32 computers.
It offers a number of features
seldom found even in larger operating
systems, including
.IP i
A hierarchical file system incorporating
demountable volumes,
.IP ii
Compatible file, device, and inter-process I/O,
.IP iii
The ability to initiate asynchronous processes,
.IP iv
System command language selectable on a per-user basis,
.IP v
Over 100 subsystems including a dozen languages,
.IP vi
High degree of portability.
.LP
This paper discusses the nature
and implementation of the file system
and of the user command interface.
.AE
.NH
INTRODUCTION
.PP
There have been four versions of
the
.UX
time-sharing system.
.hy 12
The earliest (circa 1969-70) ran on
the Digital Equipment Corporation \*sPDP\*n-7 and -9 computers.
The second version ran on the unprotected
\*sPDP\*n-11/20 computer.
The third incorporated multiprogramming and ran
on the \*sPDP\*n-11/34, /40, /45, /60, and /70 computers;
it is the one described in the previously published version
of this paper, and is also the most widely used today.
.hy 14
This paper describes only the
fourth, current
system that runs on the \*sPDP\*n-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.
.PP
Since
\*sPDP\*n-11
.UX
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,
computer networks,
and other topics in computer science, and also for
document preparation.
.PP
Perhaps the most important achievement of
.UX
is to demonstrate
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, however, that users find
that the
most important characteristics of the system
are its simplicity, elegance, and ease of use.
.PP
Besides the operating system proper, some major programs
available under
.UX
are
.DS
.nf
C compiler
Text editor based on \*sQED\*n
.[
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
.]
.fi
.in +3n
.ll -5n
.ti -3n
Dozens of languages including
Fortran 77, Basic, Snobol, \*sAPL\*n, Algol 68, M6, \*sTMG\*n, Pascal
.in
.ll
.DE
There is a host of maintenance, utility, recreation and novelty programs,
all written locally.
The
.UX
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
.UX
software is maintained on
the
system;
likewise, this paper and all other
documents
in this issue
were generated and formatted by the
.UX
editor and text formatting
programs.
.SH
II. HARDWARE AND SOFTWARE ENVIRONMENT
.PP
The \*sPDP\*n-11/70 on which the Research
.UX
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 generous 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
\*sPWB/UNIX\*n 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
.]
.PP
Our own \*sPDP\*n-11 has two
200-Mb moving-head disks
for file system storage and swapping.
There are 20 variable-speed
communications interfaces
attached to 300- and 1200-baud data sets,
and an additional 12 communication lines
hard-wired to 9600-baud terminals 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 miscellaneous
devices including
nine-track magnetic tape,
a line printer,
a voice synthesizer,
a phototypesetter,
a digital switching network,
and a chess machine.
.PP
The preponderance of
.UX
software is written in the
abovementioned 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.
.SH
III. THE FILE SYSTEM
.PP
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.
.SH
3.1 Ordinary files
.PP
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 example, the assembler generates, and the loader
expects, an object file in a particular format.
However,
the structure of files is controlled by
the programs that use them, not by the system.
.SH
3.2 Directories
.PP
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 conveniently 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 directories.
However, anyone with
appropriate permission may read a directory just like any other file.
.PP
The system maintains several directories
for its own use.
One of these is the
.UL 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
.UL root .
Other system directories contain all the programs provided
for general use; that is, all the
.IT 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.
.PP
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
.IT path
.IT 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
.UL /alpha/beta/gamma
causes the system to search
the root for directory
.UL alpha ,
then to search
.UL alpha
for
.UL beta ,
finally to find
.UL gamma
in
.UL beta .
.UL \&gamma
may be an ordinary file, a directory, or a special
file.
As a limiting case, the name ``/\^'' refers to the root itself.
.PP
A path name not starting with ``/\^'' causes the system to begin the
search in the user's current directory.
Thus, the name
.UL alpha/beta
specifies the file named
.UL beta
in
subdirectory
.UL alpha
of the current
directory.
The simplest kind of name, for example,
.UL 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.
.PP
The same non-directory file may appear in several directories under
possibly different names.
This feature is called
.IT linking ;
a directory entry for a file is sometimes called a link.
The
.UX
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 directory 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.
.PP
Each directory always has at least two entries.
The name
``\|\fB.\|\fP'' in each directory refers to the directory itself.
Thus a program
may read the current directory under the name ``\fB\|.\|\fP'' without knowing
its complete path name.
The name ``\fB\|.\|.\|\fP'' by convention refers to the parent of the
directory in which it appears, that is, to the directory in which
it was created.
.PP
The directory structure is constrained to have the form
of a rooted tree.
Except for the special entries ``\|\fB\|.\|\fP'' and ``\fB\|.\|.\|\fP'', each directory
must appear as an entry in exactly one other directory, which is its
parent.
The reason for this is to simplify the writing of programs
that visit subtrees of the directory 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.
.SH
3.3 Special files
.PP
Special files constitute the most unusual feature of the
.UX
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
.UL /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
.UL /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.
.PP
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 mechanism as regular files.
.SH
3.4 Removable file systems
.PP
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
.UL 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
.UL 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,
.UL mount
replaces a leaf of the hierarchy tree (the ordinary file)
by a whole new subtree (the hierarchy stored on the
removable volume).
After the
.UL mount ,
there is virtually no distinction
between files on the removable volume 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 corresponding special file.
A utility program is available to create
an empty file system,
or one may simply copy an existing file system.
.PP
There is only one exception to the rule of identical
treatment 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.
.SH
3.5 Protection
.PP
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 \*sID\*n 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.
.PP
If the tenth bit is on, the system
will temporarily change the user identification
(hereafter, user \*sID\*n)
of the current user to that of the creator of the file whenever
the file is executed as a program.
This change in user \*sID\*n is effective only
during the execution of the program that calls for it.
The set-user-\*sID\*n feature provides
for privileged programs that may use files
inaccessible 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-\*sID\*n 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 \*sID\*n
of the invoker of any program
is always available,
set-user-\*sID\*n 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 system entry
invokable only by the ``super-user'' (below)
that creates
an empty directory.
As indicated above, directories are expected to
have entries for ``\fB\|.\|\fP'' and ``\fB\|.\|.\|\fP''.
The command which creates a directory
is owned by the super-user
and has the set-user-\*sID\*n bit set.
After it checks its invoker's authorization to
create the specified directory,
it creates it and makes the entries
for ``\fB\|.\|\fP'' and ``\fB\|.\|.\|\fP''.
.PP
Because anyone may set the set-user-\*sID\*n
bit on one of his own files,
this mechanism is generally
available without administrative intervention.
For example,
this protection scheme easily solves the \*sMOO\*n
accounting problem posed by ``Aleph-null.''
.[
aleph null software practice
.]
.PP
The system recognizes one particular user \*sID\*n (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.