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The UNIX Time-Sharing System 1. Introduction ACM Symposium on Operating Systems Principles There have been three versions of UNIX. The earliest Yorktown Heights, N.Y.: October, 1963 version (circa 1969-70) ran on the Digital Equipment Corporation PDP-7 and -9 computers. The second ver. sion ran on the unprotected PDP.~~'20 computer. hi^ paper describes only the PDP-ll 40 and .'45 [I] system System The UNIX Time- since it is more modern and many of the diferences The UNMO Time-Sharing between it and older UNlx systems result from redesign Dennis M. Ritchie and Ken Thompson Sharing System of features found to be deficient or lacking. Since PDP-l l UNIX became operational in February July, 1974 Dennis M. Ritchie and Ken Thompson 1971, about 40 installations have been put into service; Volume 17, Number 7 Bell Laboratories they are generally smaller than the system described here. Most of them are engaged in applications such as pp. 365-375 the preparation and formatting of patent applications and other textual material, the collection and processing of trouble data from vartous switching machines within the Bell System, and recording and checking telephone service orders. Our own installation is used mainly machines, most notably the PDP-I1 family. for research in operating systems, languages, com- UNIPhas become one of the most widely puter networks, and other topics in computer scienc', known and imitated operating systems of all The genius of Ritchie and Thompson is in UNIX is a general-purpose, multi-user, interactive and also for document preparation. time. Ken Thompson, in working on a pro- their selection of a subset of the most pow- operating system for the Digital Equipment Corporation Perhaps the most important achievement of L.NIX PDP-II/~Oand 11/45 computers. It offers a number of is to demonstrate that a powerful operating system gram for simulating the movements of celes- e@lfunctions of MUUICS (especially pro- features seldom found even in larger operating systems, for interactive use need not be expensive either in cesses, directory hierarchies, stream- including: (I) a hierarchical file system incorporating equipment or in human eRort: u~ixcan run on hardware tial bodies in the solar system, became dis- demountable volumes; (2) compatible file, device. and costlng as little as $40,000, and less than two man- satisjied with the excessively slow rate by oriented IIO, file-like devices, and the shell) inter-process 1/O; (3) the ability to initiate asynchronous years were spent on the main system software. Yet which he developed programs within MUL- together with their own invention of the pipe. processes; (4) system command language selectable on a UNlX contains a number of features seldom ofered even per-user basis; and (5) over LOO subsystems including a in much larger systems. It is hoped, however, the users TICS as then constituted. (MUUICS was These elements made their operating system dozen languages. This paper discusses the nature and of UNIX will find that the most important characteristics the advanced time-sharing system con- a very powe@l programming environment. implementation of the file system and of the user of the system are its simplicity, elegance, and ease of use. command interface. Besides the system proper, the mior programs structed jointly by Bell Labs and MIT) The name, UNIX, was a tra&omtion of Key Words and Phrases: time-sharing, operating available under u~ixare: assembler, text editor based Thompson wrote an operating systemfor the MUUICS, with MUL.TI- becoming UNI- system, file system, command language, PDP-11 on QED (21, linking loader, symbolic debugger, compiler CR Categories: 4.30,4.32 for a language resembling BCPL [3] with types and PDP-7 minicomputer and worked on his and -CS becoming -X. These tramfom- structures (C), interpreter for a dialect of BASIC, text "space program" there. Dennis Ritchie , tions connoted the simplijicatiom in their formatting program, Fortran compiler, Snobol inter- preter, top-down compiler-comp~ler (TMC) 141, bot- who wrote the C programming language, operating system, its orientation toward tom-up compiler-compiler (YAK), form letter generator, became a collaborator and rewrote single-user systems, and its ejiciency. This macro processor (M6) [S], and permuted index program. There is also a host of maintenance, utility, recred- Thompson's operating system in C.Because paper received the ACM award for best tion, and novelty programs. All of these programs were the operating system was small enough tofit paper in programming languages and sys- written locally. It is worth noting that the system is totally self-supporting. AII UNIX software is maintained in a minicomputer,the C language allowed it tems in 1974. under UNIX; likewise, UNIX documents are generated to be transported to a wide variety of other -I? J D. and formatted by the UNIX editqr and text formatting program. Copyright @ 1974, Association for Computing Mach~nery,Inc. General permission to republish, but not for profit, all or part 2. Hardware and Software Environment of this material is granted provided that ACM's copyr~ghtnotice is given and that reference is made to the publication, to its date The PDP-II/~Son which our UNIX installation is of issue, and to the fact that reprrnting privileges were granted by permission of the Association for Computing Machinery. implemented is a 16-bit word (8-bit byte) computer with This is a revised venion of a paper presented at the Fourth 144K bytes of core memory; UNIX occupies 42K bytes. ACM Symposium on Operating Systems Principles, IBM Thomas This system, however, includes a very large number of I. Watson Rerearch Center, Yorktown Heights, New York, October 15-17, 1973. Authors' address: Bell Laboratories, Murray Hill, device drivers and enjoys a generous allotment of space NJ 07974. for I/o buffers and system tables; a minimal system Communications 25th Anniversary Issue January, 1983 of Volume 26 the ACM Number 1 capable of running the software mentioned above can directory of his own files; he may also create subdirec- visit subtrees of the directory structure, and more im- keeping which would otherwise be required to assure require as little as 50K bytes of core altogether. tories to contain groups of files conveniently treated portant, to avoid the separation of portions of the removal of the links when the removable volume is The PDP-ll has a 1M byte fixed-head disk, used for together. A directory behaves exactly like an ordinary hierarchy. If arbitrary links to directories were per- finally dismounted. In particular, in the root directories file system storage and swapping, four moving-head file except that it cannot be written on by unprivileged mitted, it would be quite difficult to detect when the of all file systems, removable or not, the name ".." disk drives which each provide 2.5M bytes on removable programs, so that the system controls the contents last connection from the root to a directory was severed. refers to the directory itself instead of to its parent. disk cartridges, and a single moving-head disk drive of directories. However, anyone with appropriate per- which uses removable 40M byte disk packs. There are mission may read a directory just like any other file. 3.3 Special Files 3.5 Protection also a high-speed paper tape reader-punch, nine-track The system maintains several directories for its own Special files constitute the most unusual feature of Although the access control scheme in UNlx is quite magnetic tape, and DEctape (a variety of magnetic use. One of these is the root directory. All files in the the UNIX file system. Each I/O device supported by simple, it has some unusual features. Each user of the tape facility in which individual records may be ad- system can be found by tracing a path through a chain UNIX is associated with at least one such file. Special system is assigned a unique user identification number. dressed and rewritten). Besides the console typewriter, of directories until the desired file is reached. The files are read and written just like ordinary disk files, When a file is created, it is marked with the user ID of there are 14 variable-speed communications interfaces startingpolnt forsuch searches isoften the root.Another but requests to read or write result in activation of the its owner. Also given for new files is a set of seven attached to 100-series datasets and a 201 dataset in- system directory contains all the programs provided for associated device. An entry for each special file resides in protection bits. Six of these specrfy independently read, terface used primarily for spooling printout to a com- general use; that is, all the commands. As will be seen, directory ,,dev, although a link may be made to one of write, and execute permission for the owner of the munal line printer. There are also several one-of-a-kind however, it is by no means necessary that a program these files just like an ordinary file. Thus, for example, file and for all other users. devices including a Picturephonea interface, a voice reside in this directory for it to be executed. to punch paper tape, one may write on the file !dev/ppt. If the seventh bit is on, the system will temporarily response unit, a voice synthesizer, a phototypesetter, a Files are named by sequences of 14 or fewer Special files exist for each communication line, each change the user identification of the current user to digital switching network, and a satellite PDP-II~~Ocharacters. When the name of a file is specified to the dlsk, each tape drive, and for physical core memory.
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