Prentice hall UNIX shells by example 3rd edition oct 2001 ISBN 013066538x

UNIX®ShellsbyExample,ThirdEdition
ByEllieQuigley


Publisher :PrenticeHallPTR
PubDate :October01,2001
ISBN :0-13-066538-X
Pages :1040
FiveUNIXshells,threeessentialutilities,one
indispensableresource!
LearnUNIXshellprogrammingtheeasyway,
usinghands-onexamples
CoversallfiveleadingUNIXshells-C,Bourne,
Korn,bash,andtcsh
Bybest-sellingauthorEllieQuigley,Silicon
Valley'stopUNIXinstructor



Thebest-sellingUNIXShellsbyExamplecontinues
tobetheonlybookyouneedtolearnUNIXshell
programming.UNIXShellsbyExample,Third
Editionaddsthoroughcoverageofthenewbashand
tcshshellstothefullexplanationsinQuigley's
famoustreatmentoftheC,Bourne,andKornshells
andtheawk,sed,andgreputilities,makingthisthe

mostcompleteUNIXshellprogrammingbook
Tableof availableanywhere.Usingproventechniquesdrawn
•
Contents fromheracclaimedSiliconValleyUNIXclasses,

• Examples Quigleytransformsyouintoanexpert-levelshell
programmer.You'lllearnwhattheshellsare,what
theydo,andhowtoprogramthem,aswellashow
andwhentouseawk,sed,andgrep.Code
examples,completelyrevisedandclassroom-tested
forthisedition,explainconceptsfirst-handandcan
serveasthebasisforyourownprojects.
ExplainstheC,Bourne,Korn,bash,andtcshshells


inonecohesiveway-you'llunderstandwhichshellto
useandwhyDetailstheessentialawk,sed,and
grepprogrammingutilitiesOffersproventeaching
methodsfromatopUNIXshellinstructorProvides
sourcecodeanddatafilesforallexamplesonthe
CD-ROM,soyoucanexperimentwiththemonyour
ownsystemUNIXsystemadministrators,application
developers,andpoweruserswillturntothisbook
againandagain,bothasavitalclassroomlearning
toolandasafavoritereferencemanual.

777
Copyright



Preface


ACKNOWLEDGMENTS

Chapter1.IntroductiontoUNIXShells





Section1.1.DefinitionandFunction





Section1.2.SystemStartupandtheLoginShell





Section1.3.ProcessesandtheShell





Section1.4.TheEnvironmentandInheritance

Section1.5.ExecutingCommandsfromScripts




Chapter2.TheUNIXToolbox





Section2.1.RegularExpressions





Section2.2.CombiningRegularExpressionMetacharacters




Chapter3.ThegrepFamily

Section3.1.ThegrepCommand





Section3.2.grepExampleswithRegularExpressions





Section3.3.grepwithPipes





Section3.4.grepwithOptions





Section3.5.egrep(Extendedgrep)

Section3.6.FixedgreporFastgrep





UNIXTOOLSLABEXERCISE




Chapter4.sed,theStreamlinedEditor





Section4.1.WhatIssed?





Section4.2.HowDoessedWork?

Section4.3.Addressing





Section4.4.CommandsandOptions






Section4.5.ErrorMessagesandExitStatus





Section4.6.sedExamples





Section4.7.sedScripting

UNIXTOOLSLABEXERCISE




Chapter5.TheawkUtility:awkasaUNIXTool





Section5.1.WhatIsawk?





Section5.2.awk'sFormat





Section5.3.FormattingOutput

Section5.4.awkCommandsfromWithinaFile





Section5.5.RecordsandFields





Section5.6.PatternsandActions





Section5.7.RegularExpressions





Section5.8.awkCommandsinaScriptFile

Section5.9.Review





UNIXTOOLSLABEXERCISE




Chapter6.TheawkUtility:awkProgrammingConstructs





Section6.1.ComparisonExpressions





Section6.2.Review

UNIXTOOLSLABEXERCISE




Chapter7.TheawkUtility:awkProgramming





Section7.1.Variables





Section7.2.RedirectionandPipes





Section7.3.Pipes





Section7.4.ClosingFilesandPipes

Section7.5.Review





UNIXTOOLSLABEXERCISE





Section7.6.ConditionalStatements





Section7.7.Loops





Section7.8.ProgramControlStatements

Section7.9.Arrays





Section7.10.awkBuilt-InFunctions





Section7.11.Built-InArithmeticFunctions





Section7.12.User-DefinedFunctions(nawk)





Section7.13.Review

UNIXTOOLSLABEXERCISE





Section7.14.OddsandEnds





Section7.15.Review





UNIXTOOLSLABEXERCISE




Chapter8.TheInteractiveBourneShell

Section8.1.Startup





Section8.2.ProgrammingwiththeBourneShell






BOURNESHELLLABEXERCISES




Chapter9.TheCShell





Section9.1.TheInteractiveCShell

Section9.2.ProgrammingwiththeCShell





CSHELLLABEXERCISES




Chapter10.TheKornShell





Section10.1.InteractiveKornShell





Section10.2.ProgrammingwiththeKornShell

KORNSHELLLABEXERCISES




Chapter11.TheInteractivebashShell





Section11.1.Introduction





Section11.2.CommandLineShortcuts





Section11.3.Variables

BASHSHELLLABEXERCISES




Chapter12.ProgrammingwiththebashShell





Section12.1.Introduction





Section12.2.ReadingUserInput





Section12.3.Arithmetic

Section12.4.PositionalParametersandCommandLineArguments





Section12.5.ConditionalConstructsandFlowControl





Section12.6.LoopingCommands





Section12.7.Functions





Section12.8.TrappingSignals

Section12.9.Debugging





Section12.10.ProcessingCommandLineOptionswithgetopts





Section12.11.TheevalCommandandParsingtheCommandLine





Section12.12.bashOptions





Section12.13.ShellBuilt-InCommands

BASHSHELLLABEXERCISES




Chapter13.TheInteractiveTCShell





Section13.1.Introduction





Section13.2.TheTCShellEnvironment





Section13.3.CommandLineShortcuts

Section13.4.JobControl





Section13.5.Metacharacters





Section13.6.RedirectionandPipes





Section13.7.Variables





Section13.8.Arrays

Section13.9.SpecialVariablesandModifiers





Section13.10.CommandSubstitution






Section13.11.Quoting





Section13.12.Built-InCommands





TCSHELLLABEXERCISES

AppendixA.UsefulUNIXUtilitiesforShellProgrammers





at—at,batch—executecommandsatalatertime





awk—patternscanningandprocessinglanguage





banner—makeposters





basename—withadirectorynamedeliversportionsofthepathname

bc—processesprecisionarithmetic





bdiff—comparestwobigfiles





cal—displaysacalendar





cat—concatenatesanddisplaysfiles





chmod—changethepermissionsmodeofafile

chown—changesowneroffile





clear—clearstheterminalscreen





cmp—comparestwofiles



compress—compress,uncompress,zcatcompress,uncompressfiles,ordisplay
expandedfiles





cp—copiesfiles





cpio—copyfilearchivesinandout

cron—theclockdaemon





crypt—encodesordecodesafile





cut—removesselectedfieldsorcharactersfromeachlineofafile





date—displaysthedateandtimeorsetsthedate





diff—comparestwofilesfordifferencesdiff[–bitw][–c|–Cn

du—summarizesdiskusage





echo—echoesarguments





egrep—searchesafileforapatternusingfullregularexpressions





expr—evaluatesargumentsasanexpression





fgrep—searchafileforacharacterstring

file—determinesthetypeofafilebylookingatitscontents





find—findsfiles





finger—displaysinformationaboutlocalandremoteusers





fmt—simpletextformatters





fold—foldslonglines

ftp—filetransferprogram





getopt(s)—parsescommandlineoptions





grep—searchesafileforapattern





groups—printsgroupmembershipofuser





id—printstheusername,userID,groupnameandgroupID

jsh—thestandard,jobcontrolshell





line—readsoneline





logname—getsthenameoftheuserrunningtheprocess





lp—sendsoutputtoaprinter(AT&T)

lpr—sendsoutputtoaprinter(UCB)





lpstat—printinformationaboutthestatusoftheLPprintservice(AT&T)





lpq—printinformationaboutthestatusoftheprinter(UCB)





ls—listscontentsofdirectory





mail—mail,rmail—readmailorsendmailtousers

mailx—interactivemessageprocessingsystem





make—maintains,updates,andregeneratesgroupsofrelatedprogramsandfiles





mesg—permitsordeniesmessagesresultingfromthewritecommand





mkdir—createsadirectory





more—browseorpagethroughatextfile

mv—moveorrenamefiles





nawk—patternscanningandprocessinglanguage





newgrp—logintoanewgroup





news—printsnewsitems





nice—runsacommandatlowpriority

nohup—makescommandsimmunetohangupsandquits





od—octaldump





pack—pack,pcat,unpack—compressesandexpandsfiles





passwd—changestheloginpasswordandpasswordattributes





paste—mergessamelinesofseveralfilesorsubsequentlinesofonefile

pcat—(seepack)





pg—displaysfilesapageatatime





pr—printsfiles





ps—reportsprocessstatus





pwd—displaysthepresentworkingdirectoryname

rcp—remotefilecopy





rlogin—remotelogin





rm—removesfilesfromdirectories





rmdir—removesadirectory





rsh—startsaremoteshell

ruptime—showsthehoststatusoflocalmachines





rwho—whoisloggedinonlocalmachines





script—createsatypescriptofaterminalsession





sed—streamlinededitor





size—printssectionsizesinbytesofobjectfiles

sleep—suspendsexecutionforsomenumberofseconds





sort—sortand/ormergefiles





spell—findsspellingerrors





split—splitsafileintopieces





strings—findsanyprintablestringsinanobjectorbinaryfile

stty—setstheoptionsforaterminal





su—becomesuperuseroranotheruser





sum—calculatesachecksumforafile





sync—updatesthesuperblockandsendschangedblockstodisk





tabs—settabstopsonaterminal

tail—displaysthetailendofafile.





talk—allowsyoutotalktoanotheruser





tar—storesandretrievesfilesfromanarchivefile,normallyatapedevice





tee—replicatesthestandardoutput





telnet—communicateswitharemotehost

test—evaluatesanexpression





time—displaysasummaryoftimeusedbythisshellanditschildren





timex—timesacommand;reportsprocessdataandsystemactivity





touch—updatesaccesstimeand/ormodificationtimeofafile





tput—initializesaterminalorqueriestheterminfodatabase

tr—translatescharacters





true—providesuccessfulexitstatus





tsort—topologicalsort





tty—getsthenameoftheterminal





umask—setsfile-creationmodemaskforpermissions

uname—printsnameofcurrentmachine



uncompress—restoresfilestotheiroriginalstateaftertheyhavebeencompressed
usingthecompresscommand





uniq—reportsonduplicatelinesinafile





units—convertsquantitiesexpressedinstandardscalestootherscales





unpack—expandsfilescreatedbypack

uucp—copyfilestoanothersystem,UNIX-to-UNIXsystemcopy



uuencode—uuencode,uudecode—encodeabinaryfileintoASCIItextinordertosendit
throughe-mail,orconvertitbackintoitsoriginalform



wc—countslines,words,andcharacters



what—extractsSCCSversioninformationfromafilebyprintinginformationfoundafter
the@(#)pattern





which—locatesacommandanddisplaysitspathnameoralias(UCB)





whereis—locatesthebinary,source,andmanualpagefilesforacommand(UCB)

who—displayswhoisloggedonthesystem





write—writesamessagetoanotheruser





xargs—constructsanargumentlist(s)andexecutesacommand





zcat—uncompressacompressedfiletostandardoutput.Sameasuncompress–c

AppendixB.ComparisonoftheShells





SectionB.1.TheShellsCompared





SectionB.2.tcshversuscsh





SectionB.3.bashversussh




AppendixC.StepsforUsingQuotingCorrectly

SectionC.1.Backslash





SectionC.2.SingleQuotes





SectionC.3.DoubleQuotes





SectionC.4.CombiningQuotes





SectionC.5.SettingtheShellVariable


Copyright

LibraryofCongressCataloging-in-Publication
Data
Quigley,Ellie.
UNIXshellsbyexample/EllieQuigley.--3rded.
p.cm.
ISBN0-13-066538-X
1.UNIX(Computerfile)2.UNIXShells.I.Title.
QA76.76.O63Q542001
005.4'32--dc21
2001050075
©2002byPrenticeHallPTR
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Dedication
Thisbookisdedicatedtomypapa,ArchibaldMacNicholMain,Jr.,

thebestfatherintheworld.


Preface
Playingthe"shell"gameisalotoffun.Thisbookwaswrittentomake
yourlearningexperiencebothfunandprofitable.Sincethefirstedition
waspublished,Ihaveheardfrommanyofyouwhohavebeenhelpedby
mybooktorealizethatshellprogrammingdoesn'tneedtobedifficultat
all!Learningbyexamplemakesiteasyandfun.Infact,duetosuch
positivefeedback,IhavebeenaskedbyPrenticeHalltoproducethis
new,updatedversiontoincludetwoadditionalandpopularshells,the
BashandTCshells.AlthoughoftenassociatedwithLinuxsystems,the
BashandTCshellsarefreelyavailabletoanyoneusingUNIXaswell.In
fact,todaymanyUNIXusersprefertheseshellstothetraditionalUNIX
shellsbecausetheyofferanenhancedandflexibleinteractive
environment,aswellasimprovedprogrammingcapabilities.
WritingUNIXShellsbyExampleistheculminationof19yearsof
teachinganddevelopingclassesforthevariousshellsandthoseUNIX
utilitiesmostheavilyusedbyshellprogrammers.ThecoursenotesI
developedforteachingclasseshavebeenusedbytheUniversityof
CaliforniaSantaCruzandUniversityofCaliforniaDavisUNIXprograms,
SunMicrosystemsEducation,AppleComputer,DeAnzaCollege,and
numerousvendorsthroughouttheworld.Dependingontherequirements
ofmyclient,Inormallyteachoneshellatatimeratherthanallofthem.
Toaccommodatetheneedsofsomanyclients,Idevelopedseparate
materialsforeachoftherespectiveUNIXshellsandtools.
WhetherIamteaching"Grep,Sed,andAwk,""BourneShellforthe
SystemAdministrator,"or"TheInteractiveKornShell,"onestudent
alwaysasks,"WhatbookcanIgetthatcoversalltheshellsandthe
importantutilitiessuchasgrep,sed,andawk?ShouldIgettheawk

book,orshouldIgetabookongrepandsed?Isthereonebookthat
reallycoversitall?Idon'twanttobuythreeorfourbooksinorderto
becomeashellprogrammer."
Inresponse,Icanrecommendanumberofexcellentbookscovering
thesetopicsseparately,andsomeUNIXbooksthatattempttodoitall,
butthestudentswantonebookwitheverythingandnotjustaquick


survey.TheywanttheUNIXtools,regularexpressions,allthreeshells,
quotingrules,acomparisonoftheshells,exercises,andsoforth,allin
onebook.Thisisthatbook.AsIwroteit,IthoughtabouthowIteachthe
classesandorganizedthechaptersinthesameformat.Intheshell
programmingclasses,thefirsttopicisalwaysanintroductiontowhatthe
shellisandhowitworks.ThenwetalkabouttheUNIXutilitiessuchas
grep,sed,andawk,themostimportanttoolsintheshellprogrammer's
toolbox.Whenlearningabouttheshell,itispresentedfirstasan
interactiveprogramwhereeverythingcanbeaccomplishedatthe
commandline,andthenasaprogramminglanguagewherethe
programmingconstructsaredescribedanddemonstratedinshellscripts.
(SincetheCandTCshellsarealmostidenticalasprogramming
languages,thereareseparatechaptersdescribinginteractiveuse,but
onlyonechapterdiscussingprogrammingconstructs.)Whenshell
programmingclassesareover,whethertheylasttwodaysoraweekor
evenasemester,thestudentsareproficientandexcitedaboutwriting
scripts.Theyhavelearnedhowtoplaytheshellgame.Thisbookwill
teachhowtoplaythesamegamewhetheryoutakeaclassorjustplay
byyourself.
Havingalwaysfoundthatsimpleexamplesareeasierforquick
comprehension,eachconceptiscapturedinasmallexamplefollowedby
theoutputandanexplanationofeachlineoftheprogram.Thismethod

hasproventobeverypopularwiththosewholearnedPerlprogramming
frommyfirstbook,PerlbyExample,andUNIXShellsbyExamplenow
hasbeenwell-receivedforthosewhoneededtowrite,read,andmaintain
shellprograms.
Thefiveshellsarepresentedinparallelsothatif,forexample,youwant
toknowhowredirectionisperformedinoneshell,thereisaparallel
discussionofthattopicineachoftheothershellchapters.Foraquick
comparisonchart,seeAppendixBofthisbook.
ItisanuisancetohavetogotoanotherbookortheUNIXmanpages
whenallyouwantisenoughinformationaboutaparticularcommandto
jogyourmemoryonhowthecommandworks.Tosaveyoutime,
AppendixAcontainsalistofusefulcommands,theirsyntaxand
definitions.Examplesandexplanationsareprovidedforthemorerobust
andoften-usedcommands.


ThecomparisonchartinAppendixBwillhelpyoukeepthedifferent
shellsstraight,especiallywhenyouportscriptsfromoneshelltoanother,
andserveasaquicksyntaxcheckwhenallyouneedisareminderof
howtheconstructworks.
Oneofthebiggesthurdlesforshellprogrammersisusingquotes
properly.ThesectiononquotingrulesinAppendixCpresentsastep-bystepprocessforsuccessfulquotinginsomeofthemostcomplex
commandlines.Thisprocedurehasdramaticallyreducedtheamountof
timeprogrammerswastewhendebuggingscriptswithfutileattemptsat
matchingquotesproperly.
Ithinkyou'llfindthisbookavaluabletutorialandreference.Theobjective
istoexplainthroughexampleandkeepthingssimplesothatyouhave
funlearningandsavetime.SincethebookreplicateswhatIsayinmy
classes,Iamconfidentthatyouwillbeaproductiveshellprogrammerin
ashortamountoftime.Everythingyouneedisrighthereatyour

fingertips.Playingtheshellgameisfun.You'llsee!
EllieQuigley()

ACKNOWLEDGMENTS
Iwouldliketothankandacknowledgethefollowingpeople,without
whosehelpthisbookwouldnothavebeenpublished:
MarkTaub,myacquisitionseditor,andVanessaMoore,myproduction
editoratPrenticeHall;BethGerra,RobertaHarvey,andGaryWilsonfor
reviewingtheoriginalmaterial;SteveHansenforhardwareandsoftware
support.Finally,IwouldliketothankallmystudentsatUCSantaCruz,
UCDavis,andSunMicrosystemsfortheirfeedback.


CONTENTS


Chapter1.IntroductiontoUNIXShells
1.1DefinitionandFunction
1.2SystemStartupandtheLoginShell
1.3ProcessesandtheShell
1.4TheEnvironmentandInheritance
1.5ExecutingCommandsfromScripts

1.1DefinitionandFunction
Theshellisaspecialprogramusedasaninterfacebetweentheuserand
theheartoftheUNIXoperatingsystem,aprogramcalledthekernel,as
showninFigure1.1.Thekernelisloadedintomemoryatboot-uptime
andmanagesthesystemuntilshutdown.Itcreatesandcontrols
processes,andmanagesmemory,filesystems,communications,andso
forth.Allotherprograms,includingshellprograms,resideoutonthedisk.

Thekernelloadsthoseprogramsintomemory,executesthem,and
cleansupthesystemwhentheyterminate.Theshellisautilityprogram
thatstartsupwhenyoulogon.Itallowsuserstointeractwiththekernel


byinterpretingcommandsthataretypedeitheratthecommandlineorin
ascriptfile.
Figure1.1.Thekernel,theshell,andyou.

Whenyoulogon,aninteractiveshellstartsupandpromptsyouforinput.
Afteryoutypeacommand,itistheresponsibilityoftheshellto(a)parse
thecommandline;(b)handlewildcards,redirection,pipes,andjob
control;and(c)searchforthecommand,andiffound,executethat
command.WhenyoufirstlearnUNIX,youspendmostofyourtime
executingcommandsfromtheprompt.Youusetheshellinteractively.
Ifyoutypethesamesetofcommandsonaregularbasis,youmaywant
toautomatethosetasks.Thiscanbedonebyputtingthecommandsina
file,calledascriptfile,andthenexecutingthefile.Ashellscriptismuch
likeabatchfile:ItisalistofUNIXcommandstypedintoafile,andthen
thefileisexecuted.Moresophisticatedscriptscontainprogramming
constructsformakingdecisions,looping,filetesting,andsoforth.Writing
scriptsnotonlyrequireslearningprogrammingconstructsand
techniques,butassumesthatyouhaveagoodunderstandingofUNIX
utilitiesandhowtheywork.Therearesomeutilities,suchasgrep,sed,
andawk,thatareextremelypowerfultoolsusedinscriptsforthe
manipulationofcommandoutputandfiles.Afteryouhavebecome
familiarwiththesetoolsandtheprogrammingconstructsforyour


particularshell,youwillbereadytostartwritingusefulscripts.When

executingcommandsfromwithinascript,youareusingtheshellasa
programminglanguage.

1.1.1TheThreeMajorUNIXShells
ThethreeprominentandsupportedshellsonmostUNIXsystemsarethe
Bourneshell(AT&Tshell),theCshell(Berkeleyshell),andtheKornshell
(supersetoftheBourneshell).Allthreeofthesebehaveprettymuchthe
samewaywhenrunninginteractively,buthavesomedifferencesin
syntaxandefficiencywhenusedasscriptinglanguages.
TheBourneshellisthestandardUNIXshell,andisusedtoadminister
thesystem.Mostofthesystemadministrationscripts,suchasthercstart
andstopscriptsandshutdownareBourneshellscripts,andwhenin
singleusermode,thisistheshellcommonlyusedbytheadministrator
whenrunningasroot.ThisshellwaswrittenatAT&Tandisknownfor
beingconcise,compact,andfast.ThedefaultBourneshellpromptisthe
dollarsign($).
TheCshellwasdevelopedatBerkeleyandaddedanumberoffeatures,
suchascommandlinehistory,aliasing,built-inarithmetic,filename
completion,andjobcontrol.TheCshellhasbeenfavoredoverthe
Bourneshellbyusersrunningtheshellinteractively,butadministrators
prefertheBourneshellforscripting,becauseBourneshellscriptsare
simplerandfasterthanthesamescriptswritteninCshell.ThedefaultC
shellpromptisthepercentsign(%).
TheKornshellisasupersetoftheBourneshellwrittenbyDavidKornat
AT&T.Anumberoffeatureswereaddedtothisshellaboveandbeyond
theenhancementsoftheCshell.Kornshellfeaturesincludeaneditable
history,aliases,functions,regularexpressionwildcards,built-in
arithmetic,jobcontrol,coprocessing,andspecialdebuggingfeatures.
TheBourneshellisalmostcompletelyupward-compatiblewiththeKorn
shell,soolderBourneshellprogramswillrunfineinthisshell.The

defaultKornshellpromptisthedollarsign($).

1.1.2TheLinuxShells


Althoughoftencalled"Linux"shells,BashandTCshellsarefreely
availableandcanbecompiledonanyUNIXsystem;infact,theshells
arenowbundledwithSolaris8andSun'sUNIXoperatingsystem.But
whenyouinstallLinux,youwillhaveaccesstotheGNUshellsandtools,
andnotthestandardUNIXshellsandtools.AlthoughLinuxsupportsa
numberofshells,theBourneAgainshell(bash)andtheTCshell(tcsh)
arebyfarthemostpopular.TheZshellisanotherLinuxshellthat
incorporatesanumberoffeaturesfromtheBourneAgainshell,theTC
shell,andtheKornshell.ThePublicDomainKornshell(pdksh)aKorn
shellclone,isalsoavailable,andforafeeyoucangetAT&T'sKornshell,
nottomentionahostofotherunknownsmallershells.
ToseewhatshellsareavailableunderyourversionofLinux,lookinthe
file,/etc/shell.
Tochangetooneoftheshellslistedin/etc/shell,typethechshcommand
andthenameoftheshell.Forexample,tochangepermanentlytotheTC
shell,usethechshcommand.Attheprompt,type:

chsh/bin/tcsh
1.1.3HistoryoftheShell
Thefirstsignificant,standardUNIXshellwasintroducedinV7(seventh
editionofAT&T)UNIXinlate1979,andwasnamedafteritscreator,
StephenBourne.TheBourneshellasaprogramminglanguageisbased
onalanguagecalledAlgol,andwasprimarilyusedtoautomatesystem
administrationtasks.Althoughpopularforitssimplicityandspeed,it
lacksmanyofthefeaturesforinteractiveuse,suchashistory,aliasing,

andjobcontrol.Enterbash,theBourneAgainshell,whichwas
developedbyBrianFoxoftheFreeSoftwareFoundationundertheGNU
copyrightlicenseandisthedefaultshellfortheverypopularLinux
operatingsystem.ItwasintendedtoconformtotheIEEEPOSIX
P1003.2/ISO9945.2ShellandToolsstandard.Bashalsooffersanumber
ofnewfeatures(bothattheinteractiveandprogramminglevel)missingin
theoriginalBourneshell(yetBourneshellscriptswillstillrun
unmodified).ItalsoincorporatesthemostusefulfeaturesofboththeC
shellandKornshell.It'sbig.TheimprovementsoverBourneshellare:


commandlinehistoryandediting,directorystacks,jobcontrol,functions,
aliases,arrays,integerarithmetic(inanybasefrom2to64),andKorn
shellfeatures,suchasextendedmetacharacters,selectloopsfor
creatingmenus,theletcommand,etc.
TheCshell,developedattheUniversityofCaliforniaatBerkeleyinthe
late1970s,wasreleasedaspartof2BSDUNIX.Theshell,written
primarilybyBillJoy,offeredanumberofadditionalfeaturesnotprovided
inthestandardBourneshell.TheCshellisbasedontheCprogramming
language,andwhenusedasaprogramminglanguage,itsharesasimilar
syntax.Italsooffersenhancementsforinteractiveuse,suchas
commandlinehistory,aliases,andjobcontrol.Becausetheshellwas
designedonalargemachineandanumberofadditionalfeatureswere
added,theCshellhasatendencytobeslowonsmallmachinesand
sluggishevenonlargemachineswhencomparedtotheBourneshell.
TheTCshellisanexpandedversionoftheCshell.Someofthenew
featuresare:commandlineediting(emacsandvi),scrollingthehistory
list,advancedfilename,variable,andcommandcompletion,spelling
correction,schedulingjobs,automaticlockingandlogout,timestampsin
thehistorylist,etc.It'salsobig.

WithboththeBourneshellandtheCshellavailable,theUNIXusernow
hadachoice,andconflictsaroseoverwhichwasthebettershell.David
Korn,fromAT&T,inventedtheKornshellinthemid-1980s.Itwas
releasedin1986andofficiallybecamepartoftheSVR4distributionof
UNIXin1988.TheKornshell,reallyasupersetoftheBourneshell,runs
notonlyonUNIXsystems,butalsoonOS/2,VMS,andDOS.Itprovides
upward-compatibilitywiththeBourneshell,addsmanyofthepopular
featuresoftheCshell,andisfastandefficient.TheKornshellhasgone
throughanumberofrevisions.ThemostwidelyusedversionoftheKorn
shellisthe1988version,althoughthe1993versionisgainingpopularity.
LinuxusersmayfindtheyarerunningthefreeversionoftheKornshell,
calledThePublicDomainKornshell,orsimplypdksh,acloneofDavid
Korn's1988shell.Itisfreeandportableandcurrentlyworkisunderway
tomakeitfullycompatiblewithitsnamesake,Kornshell,andtomakeit
POSIXcompliant.AlsoavailableistheZshell(zsh),anotherKornshell
clonewithTCshellfeatures,writtenbyPaulFalsted,andfreelyavailable
atanumberofWebsites.


1.1.4UsesoftheShell
Oneofthemajorfunctionsofashellistointerpretcommandsenteredat
thecommandlinepromptwhenrunninginteractively.Theshellparses
thecommandline,breakingitintowords(calledtokens),separatedby
whitespace,whichconsistsoftabs,spaces,oranewline.Ifthewords
containspecialmetacharacters,theshellevaluatesthem.Theshell
handlesfileI/Oandbackgroundprocessing.Afterthecommandlinehas
beenprocessed,theshellsearchesforthecommandandstartsits
execution.
Anotherimportantfunctionoftheshellistocustomizetheuser's
environment,normallydoneinshellinitializationfiles.Thesefilescontain

definitionsforsettingterminalkeysandwindowcharacteristics;setting
variablesthatdefinethesearchpath,permissions,prompts,andthe
terminaltype;andsettingvariablesthatarerequiredforspecific
applicationssuchaswindows,text-processingprograms,andlibrariesfor
programminglanguages.TheKornshellandCshellalsoprovidefurther
customizationwiththeadditionofhistoryandaliases,built-invariables
settoprotecttheuserfromclobberingfilesorinadvertentlyloggingout,
andtonotifytheuserwhenajobhascompleted.
Theshellcanalsobeusedasaninterpretedprogramminglanguage.
Shellprograms,alsocalledscripts,consistofcommandslistedinafile.
Theprogramsarecreatedinaneditor(althoughon-linescriptingis
permitted).TheyconsistofUNIXcommandsinterspersedwith
fundamentalprogrammingconstructssuchasvariableassignment,
conditionaltests,andloops.Youdonothavetocompileshellscripts.The
shellinterpretseachlineofthescriptasifithadbeenenteredfromthe
keyboard.Becausetheshellisresponsibleforinterpretingcommands,it
isnecessaryfortheusertohaveanunderstandingofwhatthose
commandsare.SeeAppendixAforalistofusefulcommands.

1.1.5ResponsibilitiesoftheShell
Theshellisultimatelyresponsibleformakingsurethatanycommands
typedatthepromptgetproperlyexecuted.Includedinthose
responsibilitiesare:


1. Readinginputandparsingthecommandline.
Evaluatingspecialcharacters.
Settinguppipes,redirection,andbackgroundprocessing.
Handlingsignals.
Settingupprogramsforexecution.

Eachofthesetopicsisdiscussedindetailasitpertainstoaparticular
shell.

1.2SystemStartupandtheLoginShell
Whenyoustartupyoursystem,thefirstprocessiscalledinit.Each
processhasaprocessidentificationnumberassociatedwithit,calledthe
PID.Sinceinitisthefirstprocess,itsPIDis1.Theinitprocessinitializes
thesystemandthenstartsanotherprocesstoopenterminallinesandset
upthestandardinput(stdin),standardoutput(stdout),andstandarderror
(stderr),whichareallassociatedwiththeterminal.Thestandardinput
normallycomesfromthekeyboard;thestandardoutputandstandard
errorgotothescreen.Atthispoint,aloginpromptwouldappearonyour
terminal.
Afteryoutypeyourloginname,youwillbepromptedforapassword.The
/bin/loginprogramthenverifiesyouridentitybycheckingthefirstfieldin
thepasswdfile.Ifyourusernameisthere,thenextstepistorunthe
passwordyoutypedthroughanencryptionprogramtodetermineifitis
indeedthecorrectpassword.Onceyourpasswordisverified,thelogin
programsetsupaninitialenvironmentconsistingofvariablesthatdefine
theworkingenvironmentthatwillbepassedontotheshell.TheHOME,
SHELL,USER,andLOGNAMEvariablesareassignedvaluesextracted
frominformationinthepasswdfile.TheHOMEvariableisassignedyour
homedirectory;theSHELLvariableisassignedthenameofthelogin
shell,whichisthelastentryinthepasswdfile.TheUSERand/or
LOGNAMEvariablesareassignedyourloginname.Asearchpath
variableissetsothatcommonlyusedutilitiesmaybefoundinspecified


directories.Whenloginhasfinished,itwillexecutetheprogramfoundin
thelastentryofthepasswdfile.Normally,thisprogramisashell.Ifthe

lastentryinthepasswdfileis/bin/csh,theCshellprogramisexecuted.If
thelastentryinthepasswdfileis/bin/shorisnull,theBourneshellstarts
up.Ifthelastentryis/bin/ksh,theKornshellisexecuted.Thisshellis
calledtheloginshell.
Aftertheshellstartsup,itchecksforanysystemwideinitializationfiles
setupbythesystemadministratorandthenchecksyourhomedirectory
toseeifthereareanyshell-specificinitializationfilesthere.Ifanyof
thesefilesexist,theyareexecuted.Theinitializationfilesareusedto
furthercustomizetheuserenvironment.Afterthecommandsinthose
fileshavebeenexecuted,apromptappearsonthescreen.Theshellis
nowwaitingforyourinput.

1.2.1ParsingtheCommandLine
Whenyoutypeacommandattheprompt,theshellreadsalineofinput
andparsesthecommandline,breakingthelineintowords,calledtokens.
Tokensareseparatedbyspacesandtabsandthecommandlineis
terminatedbyanewline.[1]Theshellthencheckstoseewhetherthefirst
wordisabuilt-incommandoranexecutableprogramlocatedsomewhere
outondisk.Ifitisbuilt-in,theshellwillexecutethecommandinternally.
Otherwise,theshellwillsearchthedirectorieslistedinthepathvariable
tofindoutwheretheprogramresides.Ifthecommandisfound,theshell
willforkanewprocessandthenexecutetheprogram.Theshellwill
sleep(orwait)untiltheprogramfinishesexecutionandthen,if
necessary,willreportthestatusoftheexitingprogram.Apromptwill
appearandthewholeprocesswillstartagain.Theorderofprocessing
thecommandlineisasfollows:
1. Historysubstitutionisperformed(ifapplicable).
Commandlineisbrokenupintotokens,orwords.
Historyisupdated(ifapplicable).
Quotesareprocessed.

Aliassubstitutionandfunctionsaredefined(ifapplicable).
Redirection,background,andpipesaresetup.
Variablesubstitution($user,$name,etc.)isperformed.
Commandsubstitution(echofortodayis'date')isperformed.
Filenamesubstitution,calledglobbing(catabc.??,rm*.c,etc.)is
performed.
Programexecution.

1.2.2TypesofCommands
Whenacommandisexecuted,itisanalias,afunction,abuilt-in
command,oranexecutableprogramondisk.Aliasesareabbreviations
(nicknames)forexistingcommandsandapplytotheC,TC,Bash,and
Kornshells.FunctionsapplytotheBourne(introducedwithAT&TSystem
V,Release2.0),Bash,andKornshells.Theyaregroupsofcommands
organizedasseparateroutines.Aliasesandfunctionsaredefinedwithin
theshell'smemory.Built-incommandsareinternalroutinesintheshell,
andexecutableprogramsresideondisk.Theshellusesthepathvariable
tolocatetheexecutableprogramsondiskandforksachildprocess
beforethecommandcanbeexecuted.Thistakestime.Whentheshellis
readytoexecutethecommand,itevaluatescommandtypesinthe
followingorder:[2]
1. Aliases
Keywords
Functions(bash)
Built-incommands
Executableprograms
If,forexample,thecommandisxyztheshellwillchecktoseeifxyzisan

alias.Ifnot,isitabuilt-incommandorafunction?Ifneitherofthose,it
mustbeanexecutablecommandresidingonthedisk.Theshellthen
mustsearchthepathforthecommand.

1.3ProcessesandtheShell
Aprocessisaprograminexecutionandcanbeidentifiedbyitsunique
PID(processidentification)number.Thekernelcontrolsandmanages
processes.Aprocessconsistsoftheexecutableprogram,itsdataand
stack,programandstackpointer,registers,andalltheinformation
neededfortheprogramtorun.Whenyoustarttheshell,itisaprocess.
Theshellbelongstoaprocessgroupidentifiedbythegroup'sPID.Only
oneprocessgrouphascontroloftheterminalatatimeandissaidtobe
runningintheforeground.Whenyoulogon,yourshellisincontrolofthe
terminalandwaitsforyoutotypeacommandattheprompt.
Theshellcanspawnotherprocesses.Infact,whenyouentera
commandatthepromptorfromashellscript,theshellhasthe
responsibilityoffindingthecommandeitherinitsinternalcode(built-in)
oroutonthediskandthenarrangingforthecommandtobeexecuted.
Thisisdonewithcallstothekernel,calledsystemcalls.Asystemcallis
arequestforkernelservicesandistheonlywayaprocesscanaccess
thesystem'shardware.Thereareanumberofsystemcallsthatallow
processestobecreated,executed,andterminated.(Theshellprovides
otherservicesfromthekernelwhenitperformsredirectionandpiping,
commandsubstitution,andtheexecutionofusercommands.)
Thesystemcallsusedbytheshelltocausenewprocessestorunare
discussedinthefollowingsections.SeeFigure1.2.
Figure1.2.Theshellandcommandexecution.

1.3.1WhatProcessesAreRunning?
ThepsCommand.Thepscommandwithitsmanyoptionsdisplaysalist
oftheprocessescurrentlyrunninginanumberofformats.Example1.1
showsallprocessesthatarerunningbyusersonaLinuxsystem.(See
AppendixAforpsanditsoptions.)