The UNIX TimeSharing System.docx

The UNIX TimeSharing System.docx

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TheUNIXTimeSharingSystem

TheUNIXTime-SharingSystem*

D.M.RitchieandK.Thompson

ABSTRACT

Unixisageneral-purpose,multi-user,interactiveoperatingsystemforthelargerDigitalEquipmentCorporationPDP-11andtheInterdata8/32computers.Itoffersanumberoffeaturesseldomfoundeveninlargeroperatingsystems,including

●Ahierarchicalfilesystemincorporatingdemountablevolumes,

●Compatiblefile,device,andinter-processI/O,

●Theabilitytoinitiateasynchronousprocesses,

●Systemcommandlanguageselectableonaper-userbasis,

●Over100subsystemsincludingadozenlanguages,

●Highdegreeofportability.

Thispaperdiscussesthenatureandimplementationofthefilesystemandoftheusercommandinterface.

NOTE:

*Copyright1974,AssociationforComputingMachinery,Inc.,reprintedbypermission.ThiselectroniceditionofthispaperisareprintoftheversionappearinginTheBellSystemTechnicalJournal57no.6,part2（July-August1978）.Inturn,thatwasarevisedversionofanarticlethatappearedinCommunicationsoftheACM,17,No.7（July1974）,pp.365-375.ThatarticlewasarevisedversionofapaperpresentedattheFourthACMSymposiumonOperatingSystemsPrinciples,IBMThomasJ.WatsonResearchCenter,YorktownHeights,NewYork,October15-17,1973.MostofthedifferencesbetweenversionsoccurbetweentheC.ACMversionandtheBSTJprinting;weincorporatedupdatednumbersandmaterialonportability.

I.INTRODUCTION

TherehavebeenfourversionsoftheUnixtime-sharingsystem.12Theearliest（circa1969-70）ranontheDigitalEquipmentCorporationPDP-7and-9computers.ThesecondversionranontheunprotectedPDP-11/20computer.ThethirdincorporatedmultiprogrammingandranonthePDP-11/34,/40,/45,/60,and/70computers;itistheonedescribedinthepreviouslypublishedversionofthispaper,andisalsothemostwidelyusedtoday.Thispaperdescribesonlythefourth,currentsystemthatrunsonthePDP-11/70andtheInterdata8/32computers.Infact,thedifferencesamongthevarioussystemsisrathersmall;mostoftherevisionsmadetotheoriginallypublishedversionofthispaper,asidefromthoseconcernedwithstyle,hadtodowithdetailsoftheimplementationofthefilesystem.

SincePDP-11UnixbecameoperationalinFebruary,1971,over600installationshavebeenputintoservice.Mostofthemareengagedinapplicationssuchascomputerscienceeducation,thepreparationandformattingofdocumentsandothertextualmaterial,thecollectionandprocessingoftroubledatafromvariousswitchingmachineswithintheBellSystem,andrecordingandcheckingtelephoneserviceorders.Ourowninstallationisusedmainlyforresearchinoperatingsystems,languages,computernetworks,andothertopicsincomputerscience,andalsofordocumentpreparation.

PerhapsthemostimportantachievementofUnixistodemonstratethatapowerfuloperatingsystemforinteractiveuseneednotbeexpensiveeitherinequipmentorinhumaneffort:

itcanrunonhardwarecostingaslittleas$40,000,andlessthantwoman-yearswerespentonthemainsystemsoftware.Wehope,however,thatusersfindthatthemostimportantcharacteristicsofthesystemareitssimplicity,elegance,andeaseofuse.

Besidestheoperatingsystemproper,somemajorprogramsavailableunderUnixare

Ccompiler

TexteditorbasedonQED[1];

Assembler,linkingloader,symbolicdebugger

Phototypesettingandequationsettingprograms[2,3]

DozensoflanguagesincludingFortran77,Basic,Snobol,APL,Algol68,M6,TMG,PascalThereisahostofmaintenance,utility,recreationandnoveltyprograms,allwrittenlocally.TheUnixusercommunity,whichnumbersinthethousands,hascontributedmanymoreprogramsandlanguages.Itisworthnotingthatthesystemistotallyself-supporting.AllUnixsoftwareismaintainedonthesystem;likewise,thispaperandallotherdocumentsinthisissueweregeneratedandformattedbytheUnixeditorandtextformattingprograms.

II.HARDWAREANDSOFTWAREENVIRONMENT

ThePDP-11/70onwhichtheResearchUnixsystemisinstalledisa16-bitword（8-bitbyte）computerwith768Kbytesofcorememory;thesystemkerneloccupies90Kbytesaboutequallydividedbetweencodeanddatatables.Thissystem,however,includesaverylargenumberofdevicedriversandenjoysagenerousallotmentofspaceforI/Obuffersandsystemtables;aminimalsystemcapableofrunningthesoftwarementionedabovecanrequireaslittleas96Kbytesofcorealtogether.Thereareevenlargerinstallations;seethedescriptionofthePWB/UNIXsystems[4,5],forexample.Therearealsomuchsmaller,thoughsomewhatrestricted,versionsofthesystem[6].

OurownPDP-11hastwo200-Mbmoving-headdisksforfilesystemstorageandswapping.Thereare20variable-speedcommunicationsinterfacesattachedto300-and1200-bauddatasets,andanadditional12communicationlineshard-wiredto9600-baudterminalsandsatellitecomputers.Therearealsoseveral2400-and4800-baudsynchronouscommunicationinterfacesusedformachine-to-machinefiletransfer.Finally,thereisavarietyofmiscellaneousdevicesincludingnine-trackmagnetictape,alineprinter,avoicesynthesizer,aphototypesetter,adigitalswitchingnetwork,andachessmachine.

ThepreponderanceofUnixsoftwareiswrittenintheabovementionedClanguage[7].Earlyversionsoftheoperatingsystemwerewritteninassemblylanguage,butduringthesummerof1973,itwasrewritteninC.Thesizeofthenewsystemwasaboutone-thirdgreaterthanthatoftheold.Sincethenewsystemnotonlybecamemucheasiertounderstandandtomodifybutalsoincludedmanyfunctionalimprovements,includingmultiprogrammingandtheabilitytosharereentrantcodeamongseveraluserprograms,weconsiderthisincreaseinsizequiteacceptable.

III.THEFILESYSTEM

Themostimportantroleofthesystemistoprovideafilesystem.Fromthepointofviewoftheuser,therearethreekindsoffiles:

ordinarydiskfiles,directories,andspecialfiles.

3.1Ordinaryfiles

Afilecontainswhateverinformationtheuserplacesonit,forexample,symbolicorbinary（object）programs.Noparticularstructuringisexpectedbythesystem.Afileoftextconsistssimplyofastringofcharacters,withlinesdemarcatedbythenewlinecharacter.Binaryprogramsaresequencesofwordsastheywillappearincorememorywhentheprogramstartsexecuting.Afewuserprogramsmanipulatefileswithmorestructure;forexample,theassemblergenerates,andtheloaderexpects,anobjectfileinaparticularformat.However,thestructureoffilesiscontrolledbytheprogramsthatusethem,notbythesystem.

3.2Directories

Directoriesprovidethemappingbetweenthenamesoffilesandthefilesthemselves,andthusinduceastructureonthefilesystemasawhole.Eachuserhasadirectoryofhisownfiles;hemayalsocreatesubdirectoriestocontaingroupsoffilesconvenientlytreatedtogether.Adirectorybehavesexactlylikeanordinaryfileexceptthatitcannotbewrittenonbyunprivilegedprograms,sothatthesystemcontrolsthecontentsofdirectories.However,anyonewithappropriatepermissionmayreadadirectoryjustlikeanyotherfile.

Thesystemmaintainsseveraldirectoriesforitsownuse.Oneoftheseistherootdirectory.Allfilesinthesystemcanbefoundbytracingapaththroughachainofdirectoriesuntilthedesiredfileisreached.Thestartingpointforsuchsearchesisoftentheroot.Othersystemdirectoriescontainalltheprogramsprovidedforgeneraluse;thatis,allthecommands.Aswillbeseen,however,itisbynomeansnecessarythataprogramresideinoneofthesedirectoriesforittobeexecuted.

Filesarenamedbysequencesof14orfewercharacters.Whenthenameofafileisspecifiedtothesystem,itmaybeintheformofapathname,whichisasequenceofdirectorynamesseparatedbyslashes,``/'',andendinginafilename.Ifthesequencebeginswithaslash,thesearchbeginsintherootdirectory.Thename/alpha/beta/gammacausesthesystemtosearchtherootfordirectoryalpha,thentosearchalphaforbeta,finallytofindgammainbeta.gammamaybeanordinaryfile,adirectory,oraspecialfile.Asalimitingcase,thename``/''referstotherootitself.

Apathnamenotstartingwith``/''causesthesystemtobeginthesearchintheuser'scurrentdirectory.Thus,thenamealpha/betaspecifiesthefilenamedbetainsubdirectoryalphaofthecurrentdirectory.Thesimplestkindofname,forexample,alpha,referstoafilethatitselfisfoundinthecurrentdirectory.Asanotherlimitingcase,thenullfilenamereferstothecurrentdirectory.

Thesamenon-directoryfilemayappearinseveraldirectoriesunderpossiblydifferentnames.Thisfeatureiscalledlinking;adirectoryentryforafileissometimescalledalink.TheUnixsystemdiffersfromothersystemsinwhichlinkingispermittedinthatalllinkstoafilehaveequalstatus.Thatis,afiledoesnotexistwithinaparticulardirectory;thedirectoryentryforafileconsistsmerelyofitsnameandapointertotheinformationactuallydescribingthefile.Thusafileexistsindependentlyofanydirectoryentry,althoughinpracticeafileismadetodisappearalongwiththelastlinktoit.

Eachdirectoryalwayshasatleasttwoentries.Thename``.''ineachdirectoryreferstothedirectoryitself.Thusaprogrammayreadthecurrentdirectoryunderthename``.''withoutknowingitscompletepathname.Thename``..''byconventionreferstotheparentofthedirectoryinwhichitappears,thatis,tothedirectoryinwhichitwascreated.

Thedirectorystructureisconstrainedtohavetheformofarootedtree.Except