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1、Project1SystemsProgrammingPractice:TheYalnixShellEducationalObjectives:Studentscometothiscoursewithvariousexperiences.Formanypeoplethisprojectwillbepracticeand/orawarmup.Forothers,itW川bealearningexercise.Regardlessofyourbackground,bytheendofthisproject,wehopethatyouwillcomfortablyandconfidentlybeabl
2、etodothefollowing: Developclear,readable,well-documentedandwell-designedprogramsintheCProgrammingLanguage. DevelopsoftwareintheUnix/Linuxusingtoolssuchasgcc,gdb,andmake. 1.ocateandinterpreting,manpagesapplicabletoapplication-levelsystemprogramming. UsethePOSIX/UnixAPItosystemfunctionstomanageprocess
3、andsessionsaswellasusesignalsandpipesforinter-processcommunication. Understandinghowsynchronizationmightbecomeproblematicinlightofconcurrency. Understandhowtocommunicateandcooperatewithaprojectpartner.ProjectOverviewInthisprojectyouareaskedtoimplementasimplecommand-interpreter,a.k,a.shell,forLinux.T
4、heshellthatyouwillimplement,knownasysh,shouldbesimilartopopularshellssuchasbash,csh1tcsh1zsh,&c,butitisnotrequiredtoimplementasmanyfeaturesasthesecommercial-gradeproducts.Althoughwedontrequireallofthe*,bellsandwhistlesthatareincorporatedintocommercial-gradeproducts,yshshouldhavemuchoftheimportantfun
5、ctionality: Allowtheusertoexecuteoneormoreprograms,fromexecutablefilesonthefile-system,asbackgroundorforegroundjobs. Providejob-control,includingajoblistandtoolsforchangingtheforeground/backgroundstatusofcurrentlyrunningjobsandjobsuspension/continuation/termination. Allowforthepipingofseveraltasksas
6、wellasinputandoutputredirection.Withrespecttotheotherprogrammingassignmentsthissemester,andmanyothersinyourexperience,thisislikelytobeasmallproject,butwewantyoutoapproachitasifitisabiggerandmorecomplexprojectsothatyouarepreparedfortheremainingprojects.Specifically,we,dlikeyoutodothefollowing: Usethe
7、makeutilitytobuildyourproject Useadebuggerinsteadofprint-ad-huntdebuggingwheneverpractical. Produceclean,well-documented,andwell-designedsolutions.SpecificationFormYoursolutionshouldbeanapplicationprograminvokedwithoutcommand-lineparametersorconfigurationfiles,&c.Ifyouwanttobefancyandsupportforareso
8、urcefilesimilartothoseusedwithcommercial-gradeshells,e.g.cshrc,you,reawelcometodothis.But,likecsh,yourshellshouldfunctioncorrectlyinabsenceofthisfile.1.ook-and-FeelThelookandfeelofyshshouldbeSimilartothatofotherUNIXshells,suchasbash,tcsh,csh,&c.Forexample,yourshellsworkloopshouldproduceaprompt,e.g.,
9、ysh,acceptinputfromtheuser,andthenproduceanotherprompt.Messagesshouldbewrittentothescreenasnecessary,andthepromptshouldbedelayedwhenuserinputshouldn,tbeaccepted,asnecessary.Needlesstosay,yourshellshouldtakeappropriateactioninresponsetotheuserSinput.InternalCommandsvs.ExternalProgramsInmostcases,theu
10、ser*sinputwillbeacommandtoexecuteprogramsstoredwithinafilesystem.We,Ilcalltheseexternalprograms.Yourshellshouldallowtheseprogramstoexecutewithstdinand/orstdoutreassignedtoafile.ItshouldallowprogramsI/Otobechainedtogetherusingpipes.Forourpurposes,acollectionofpipedprocessesorasingleprocessexecutedbyi
11、tselffromthecommandlineiscalledajob.Whenexecutingbackgroundsjobs,theshellshouldnotwaitforthejobtofinishbeforeprompting,reading,andprocessingthenextcommand.Whenabackgroundjobfinallyterminatesamessagetothateffectmustbeprinted,bytheshell,totheterminal.Thismessageshouldbeprintedassoonasthejobterminates.
12、ThesyntaxfordoingthiswillbedescribedinthesectionofthisdocumentdescribingtheshellSparser.Yourparsershouldalsosupportseveralinterna!commandsthesecommands,ifissuedbytheuser,shoulddirecttheshelltotakeaparticularactionitselfinsteadofdirectingittoexecuteotherprograms.Thedetailsofthisarediscussedinthesecti
13、ondescribinginternalcommands.Foregroundvs.BackgroundJobsYourshellshouldbecapableofexecutingbothforegroundandbackgroundjobs.Whereasyourshellshouldwaitforforegroundjobstocompletebeforecontinuing,itshouldimmediatelycontinue,prompttheuser,&c,afterplacingajobintothebackground.CommandlinesWhentheuserrespo
14、ndstoaprompt,whattheytypecomposesacommandHnestring.Yourshellshouldstoreeachcommand-linestring,untilthejobisfinishedexecuting.Thisincludesbothbackgroundandsuspendedjobs.Theshellshouldassigneachcommand-linestringanon-negativeintegeridentifier.Thedatastructureusedtostorethejobsshouldallowaccesstoeachel
15、ementusingthisidentifier.Oncetheactionsdirectedbyacommand-linestringarecompleted,yourshellshouldremoveitfromthedatastructure.Identifierscanberecycledifyouchoose.Pleasenotethatthisdatastructureshouldkeeptrackofwholecommandlinestrings,notjustthenamesoftheindividualtasksthatmaycomposethem.Youshouldnotk
16、eeptrackofcommandlinestringsthatcontaininternalcommands,since,bytheirnature,theywillcompletebeforethisinformationcouldbecomeuseful.InternalCommandsThefollowingaretheinternalcommands.Ifaninternalcommandissubmittedbytheuser,theshellshouldtakethedescribedactionsitself.exit：Killallchildprocessesandexity
17、shwithameaningfulreturncode.jobs：Printoutthecommandlinestringsforjobsthatarecurrentlyexecutinginthebackgroundandjobsthatarecurrentlysuspended,aswellastheidentifierassociatedwitheachcommandlinestring.Youmayformattheoutputofthiscommandinwaythatisconvenienttotheuser.Pleaserememberthatjobsitselfisaninte
18、rnalcommandandconsequentlyshouldnotappearintheoutput. echo$status:Printstheexitstatusofthemostrecentforegroundchildprocesstohaveexited.Return0ifnosuchchildhasexisted. fg%:Bringsthejobidentifiedbyintotheforeground.Ifthisjobwaspreviouslystopped,itshouldnowberunning.Yourshellshouldwaitforaforegoundchil
19、dtoterminatebeforereturningacommandpromptortakinganyotheraction. bg%:Executethesuspendedjobidentifiedbyinthebackground. InternalcommandscantakeadvantageofpipedI/O,executeinthebackground,&c,asappropriate.SpecialKeystrokesThroughaninteractionwiththeterminaldriver,certaincombinationsofkeystrokeswillgen
20、eratesignalstoyourshellinsteadofappearingwithinstdin.Yourshellshouldrespondappropriatelytothesesignals. Control-ZgeneratesaSIGSTOP.Thisshouldnotcauseyourshelltobesuspended.Instead,itshouldcauseyourshelltosuspendtheprocessesinthecurrentforegroundjob.Ifthereisnoforegroundjob,itshouldhavenoeffect. Cont
21、rol-CgeneratesaSIGINT.Thisshouldnotkillyourshell.Insteaditshouldcauseyourshelltokilltheprocessesinthecurrentforegroundjob.Ifthereisnoforegroundjob,itshouldhavenoeffect.ParsingUserInput-Overview,DelimitersandSpecialCharactersYourparserforuserinputshouldbecapableofacceptinginputfromtheuserasdescribedi
22、nthissection.Itshouldalsodetectimproperinputfromtheuser.Iftheuserenterssomethingimproper,yourshellshouldproduceameaningfulerrormessage.Justlikecommercial-gradeshells,yourshellshouldacceptinputfromtheuseronelineatatime.Youshouldbeginparsingtheusersinputwhenhe/shehitsenter.Emptycommandlinesshouldbetre
23、atedasno-opsandyieldanewprompt.Blank-spacecharactersshouldbetreatedasdelimiters,butyourshellshouldbeinsensitivetorepeatedblankspaces.Itshouldalsobeinsensitivetoblankspacesatthebeginningorendofthecommandline.Certaincharacters,knownasmeta-characters,havespecialmeaningswithinthecontextofuserinput.These
24、charactersinclude&,Yourshellcanassumethatthesemeta-characterscannotoccurinsidestringsrepresentingprograms,arguments,orfiles.Insteadtheyarereservedforusebytheshell.Thepurposeofmeta-charactersisdiscussedlaterinthissection.ParsingUserInput-InternalCommandsIfthecommandlinematchestheformatofaninternalcom
25、mandasdescribedearlierinthisdocument,itshouldbeacceptedasaninternalcommand.Ifnot,itshouldbeconsideredtospecifytheexecutionofexternalprograms,oranerror,asappropriate.ParsingUserInput-ExecutingASingleProgramTheexecutionofaprogramisspecifiedbyasequenceofdelimitedstrings.Thefirstoftheseisthenameoftheexe
26、cutablefilethatcontainsthedesiredprogram(moduloasearchpathasexplainedintheeecvpmanpage,seeman-s2execvp)andtheothersareargumentspassedtotheprogram.Thecommandisanerroriftheexecutablefilenamedbythefirststringdoesnotexist,orisnotanexecutable.ParingUserInput-I/ORedirectionAprogramsexecutionspecifiedasabo
27、vemaybefollowedbythemeta-characterwhichisinturnfollowedbyafilename.Inthecaseof,theoutputoftheprogramwillberedirectedtothespecifiedfilename.Iftheoutputfiledoesnotexist,itshouldbecreated.Iftheinputfiledoesnotexist,thisisanerror.ParsingUserInput-PipesandCooperatingProgramsSeveralprograminvocationscanbe
28、presentinasinglecommandline,whenseparatedbytheshellmetacharacterT.Inthiscase,theshellshouldforkallofthem,chainingtheiroutputsandinputsusingpipesappropriately.Forinstance,thecommandlineprogAargAlargA2outfileshouldforkprogAandprogB,maketheinputforprogAcomefromfileinfile,theoutputfromprogAgototheinputo
29、fprogB,andtheoutputofprogBgotothefileoutfile.ThisshouldbeaccomplishedusingapipeIPCprimative.Acommandlinewithoneormorepipesisanerrorifanyofitscomponentprograminvocationsisanerror.Acommandlinewithpipesisanerroriftheinputofanybutthefirstcommandisredirected,oriftheoutputofanybutthelastcommandisredirecte
30、d.Ajobconsistingofpipedprocessesisnotconsideredtohavecompleteduntilallofitscomponentprocesseshavecompleted.ParsingUserInput-BackgroundJobsTheusercanspecifythatajobshouldbeexecutedinthebackgroundbyendingthecommandlinewiththemeta-character&.Ifthisisthecase,allprograminvocationsrequiredbythecommandline
31、aretobecarriedoutinthebackground.ParsingUserInput-AGrammarThegrammarbelowprovidesamoreformaldescriptionofthesyntaxgoverninguserinput.Ifyouareusinglex/yacctoimplementyourparser,you*IlhavetomassagethegrammarslightlytomakeitLALR(I).Thisgrammardoesn,tincludethespecialkeystrokes,becausetheywontshowupinst
32、dinasuserinputandshouldbehandledseparately.ACommandLineislegalinputprovidedbytheuser,asadirectiontotheshell,inresponsetotheprompt.Thegrammarassumesthattheexistenceofalexicalanalyzerthatconsidersblank-spacetobeadelimiter,recognizesthemeta-charactersastokens,&c.CommandLine= NULLFgCommandLineFgCommandL
33、ine &FgCommandLine:= SimpIeCommandFirstCommand MidCommand LastCommandSimpIeCommand:= Proglnvocation InputRedirect OutputRedirectFirstCommand=Proglnvocation InputRedirectMidCommand := NULL I Proglnvocation MidCommandLastCommand= I Proglnvocation OutputRedirectProglnvocation :=ExecFiIe ArgsInputRedire
34、ct :=NULL STRINGExecFiIe :=STRINGA SuggestedPlan Of Attack1) Readthemanpagesforfork,exec,waitandexit.2) Writeafewsmallprogramstoexperimentwiththesecommands.3) Readthemanpagesfortcsetgrp()andsetpgid()4) Writesomecodetoexperimentwithprocessgroups,&c.PayattentiontoSIGTTIN&S1GTTOU.5) Designyourparser.6)
35、 Implementyourparser.7) Usingyourparser,writeasimpleshellthatcanexecutesinglecommands.8) Addsupportforrunningprogramsinthebackground,butdontworryaboutprintingthemessagewhenabackgroundjobterminates(asynchronousnotification).Addthejobscommandwhileyouaredoingthis-itmayprovehelpfulfordebugging.9) Addinp
36、utandoutputredirection10) Addcodetoprintamessagewhenabackgroundjobterminates.11) Addjobcontrolfeatures-implementthebehaviorofControl-Z(and,ifapplicable,CONTROL-C)tfgandbg.12) Addsupportforpipes.13) Finishupallofthedetails14) Test,testtest.15) CelebrateEnvironmentWhereasyoucandothisassignmentonanyUNI
37、X,itmustrunontheLinuxmachinesforyourdemo.Weencourageyoutoworkinthatenvironment,becausethelibrariesforfutureprojectsareonlyavailableintheAndrewenvironment.Althoughyoucansolvethisassignmentinyourchoiceoflanguages,itwouldprobablybemoredifficultinanythingotherthanC(orperhapsC+).Westronglyencourageyoutou
38、seC.ForfutureprojectsyoullalmostcertainlyhavetouseC.C+maywork,butweoffernoguarantees.Thereisadifferenceinthelinkingconventionbetweenthetwolanguagesthatcausesincompatibilities.Wevetriedtocorrectthis,butwemakenopromisesaboutC+.SomeUsefulInformation(Someofwhichisareview)SystemCallsYouhaveprobablyalread
39、yheardthetermSystemCall,Doyouknowwhatitmeans?Asitsnameimplies,asystemcallisa,cal*,thatis,atransferofcontrolfromoneinstructiontoadistantinstruction.Asystemcallisdifferentfromaregularprocedurecallinthatthecalleeisexecutedinaprivilegedstate,i.e,thatthecalleeiswithintheoperatingsystem.Because,forsecurit
40、yandsanity,callsintotheoperatingsystemmustbecarefullycontrolled,thereisawell-definedandlimitedsetofsystemcalls.Thisrestrictionisenforcedbythehardwarethroughtrapvectors:onlythoseOSaddressesentered,atboottime,intothetrap(interrupt)vectorarevaliddestinationsofasystemcall.Thus,asystemcallisacallthattres
41、passesaprotectionboundaryinacontrolledmanner.SincetheprocessabstractionismaintainedbytheOS,yshwillneedtomakecallsintotheOSinordertocontrolitschildprocesses.Thesecallsaresystemcalls.InUNIX,youcandistinguishsystemcallsfromuser-levellibrary(programmersAPI)callsbecausesystemcallsappearinsection2oftheman
42、ual,whereasuser-levelcallsappearinsection3ofthemanual.Themanualis,inUNIX,whatyougetwhenyouusethemancommand.Forexample,manforkwillgetyouthemanpageinsection2ofthemanualthatdescribesthefork()syscall1andman-s2execwillgetyouthemanpagethatdescribesthefamilyofexecsyscalls(asyscall,hence-s2.)ThefollowingUNI
43、Xsyscallsmayprovetobeespeciallyusefulinyoursolutiontothisproject.Thereareplentyofothers,soyoumayfind*,maandgoodreferencebooksuseful,especiallyifyouarenewtosystemprogramming. pid_tfork(void):ItcreatesaprocessthatisanaImost-exactcopyofthecallingprocess;inparticular,afterasuccessfulreturnfromfork(),bot
44、hparentandchildprocessesareexecutingthesameprogram.Thetwoprocessescanbedistinguishedbythereturnvaluefromfork(). intexecvp(constchar*file,char*constargv):Loadstheexecutablefilepath,orafilefoundthroughasearchpath,intothememoryassociatedwiththecallingprocess,andstartsexecutingtheprogramtherein.Ifsucces
45、sful,itobliterateswhateverprogramiscurrentlyrunninginthecallingprocess.Thereareseveralother,similarformsofexec. voidexit(intstatus):Exitsthecallingprogram,destroyingthecallingprocess.Itreturnsstatusastheexitvaluetotheparent,shouldtheparentbeinterested.Theparentreceivesthisexitvaluethroughthewaitsysc
46、all,below.Notethatthelinkerintroducesanexit()callattheendofeveryprogram,forinstance,attheendofacmainprocedure,eveniftheccodedoesntexplicitlyhaveone. pid_twait(int*stat_loc):Returnstheexitstatusofanexitedchild,ifany.Returnserroriftherearenochildrenrunning.Blocksthecallingprocessuntilachildexitsifther
47、earechildrenbuttheyareallcurrentlyrunning. pid_twaitpid(pid_tpid,int*stat_loc,intoptions):Similartowait()butallowsyoutowaitforaspecificprocessofgroupofprocesses,andallowsthespecificationofflagssuchaswnohang. wait3(.),wait4(.):Similartowait()butallowdifferentcombinationsofparametersandflags. inttcset
48、pgrp(intfildes,pid_tpgidjd):Setstheforegroundprocessgroupidtobetheforegroundgroupassociatedwiththecontrollingterminal.Thecontrollingterminalisusuallyassociatedwithstdin,stdout,andstderr(filedescriptorsO11,and2) intsetpgid(pid_tpid,pid_tpgid):SetstheprocessgroupIDoftheprocesswithIDpidtopgid. intdup2(intfiledes,intfiledes2):Causesthefiledescriptorfiledes2torefertothesamefileasfiledes.
