Bash Reference Manual

2/25/2015 Bash Reference Manual

http://www.gnu.org/software/bash/manual/bash.html#Commands-For-History 1/158

BashReferenceManualTableofContents

1Introduction1.1WhatisBash?1.2Whatisashell?

2Definitions3BasicShellFeatures

3.1ShellSyntax3.1.1ShellOperation3.1.2Quoting

3.1.2.1EscapeCharacter3.1.2.2SingleQuotes3.1.2.3DoubleQuotes3.1.2.4ANSICQuoting3.1.2.5LocaleSpecificTranslation

3.1.3Comments3.2ShellCommands

3.2.1SimpleCommands3.2.2Pipelines3.2.3ListsofCommands3.2.4CompoundCommands

3.2.4.1LoopingConstructs3.2.4.2ConditionalConstructs3.2.4.3GroupingCommands

3.2.5Coprocesses3.2.6GNUParallel

3.3ShellFunctions3.4ShellParameters

3.4.1PositionalParameters3.4.2SpecialParameters

3.5ShellExpansions3.5.1BraceExpansion3.5.2TildeExpansion3.5.3ShellParameterExpansion3.5.4CommandSubstitution3.5.5ArithmeticExpansion3.5.6ProcessSubstitution3.5.7WordSplitting3.5.8FilenameExpansion

3.5.8.1PatternMatching3.5.9QuoteRemoval

3.6Redirections3.6.1RedirectingInput3.6.2RedirectingOutput3.6.3AppendingRedirectedOutput3.6.4RedirectingStandardOutputandStandardError3.6.5AppendingStandardOutputandStandardError3.6.6HereDocuments3.6.7HereStrings3.6.8DuplicatingFileDescriptors3.6.9MovingFileDescriptors3.6.10OpeningFileDescriptorsforReadingandWriting



3.7ExecutingCommands3.7.1SimpleCommandExpansion3.7.2CommandSearchandExecution3.7.3CommandExecutionEnvironment3.7.4Environment3.7.5ExitStatus3.7.6Signals

3.8ShellScripts4ShellBuiltinCommands

4.1BourneShellBuiltins4.2BashBuiltinCommands4.3ModifyingShellBehavior

4.3.1TheSetBuiltin4.3.2TheShoptBuiltin

4.4SpecialBuiltins5ShellVariables

5.1BourneShellVariables5.2BashVariables

6BashFeatures6.1InvokingBash6.2BashStartupFiles6.3InteractiveShells

6.3.1WhatisanInteractiveShell?6.3.2IsthisShellInteractive?6.3.3InteractiveShellBehavior

6.4BashConditionalExpressions6.5ShellArithmetic6.6Aliases6.7Arrays6.8TheDirectoryStack

6.8.1DirectoryStackBuiltins6.9ControllingthePrompt6.10TheRestrictedShell6.11BashPOSIXMode

7JobControl7.1JobControlBasics7.2JobControlBuiltins7.3JobControlVariables

8CommandLineEditing8.1IntroductiontoLineEditing8.2ReadlineInteraction

8.2.1ReadlineBareEssentials8.2.2ReadlineMovementCommands8.2.3ReadlineKillingCommands8.2.4ReadlineArguments8.2.5SearchingforCommandsintheHistory

8.3ReadlineInitFile8.3.1ReadlineInitFileSyntax8.3.2ConditionalInitConstructs8.3.3SampleInitFile

8.4BindableReadlineCommands8.4.1CommandsForMoving8.4.2CommandsForManipulatingTheHistory8.4.3CommandsForChangingText8.4.4KillingAndYanking8.4.5SpecifyingNumericArguments8.4.6LettingReadlineTypeForYou



8.4.7KeyboardMacros8.4.8SomeMiscellaneousCommands

8.5ReadlineviMode8.6ProgrammableCompletion8.7ProgrammableCompletionBuiltins8.8AProgrammableCompletionExample

9UsingHistoryInteractively9.1BashHistoryFacilities9.2BashHistoryBuiltins9.3HistoryExpansion

9.3.1EventDesignators9.3.2WordDesignators9.3.3Modifiers

10InstallingBash10.1BasicInstallation10.2CompilersandOptions10.3CompilingForMultipleArchitectures10.4InstallationNames10.5SpecifyingtheSystemType10.6SharingDefaults10.7OperationControls10.8OptionalFeatures

AppendixAReportingBugsAppendixBMajorDifferencesFromTheBourneShell

B.1ImplementationDifferencesFromTheSVR4.2ShellAppendixCGNUFreeDocumentationLicenseAppendixDIndexes

D.1IndexofShellBuiltinCommandsD.2IndexofShellReservedWordsD.3ParameterandVariableIndexD.4FunctionIndexD.5ConceptIndex

Next:Introduction,Previous:(dir),Up:(dir)[Contents][Index]

BashFeaturesThistextisabriefdescriptionofthefeaturesthatarepresentintheBashshell(version4.3,2February2014).TheBashhomepageishttp://www.gnu.org/software/bash/.

ThisisEdition4.3,lastupdated2February2014,ofTheGNUBashReferenceManual,forBash,Version4.3.

Bashcontainsfeaturesthatappearinotherpopularshells,andsomefeaturesthatonlyappearinBash.SomeoftheshellsthatBashhasborrowedconceptsfromaretheBourneShell(sh),theKornShell(ksh),andtheCshell(cshanditssuccessor,tcsh).Thefollowingmenubreaksthefeaturesupintocategories,notingwhichfeatureswereinspiredbyothershellsandwhicharespecifictoBash.

ThismanualismeantasabriefintroductiontofeaturesfoundinBash.TheBashmanualpageshouldbeusedasthedefinitivereferenceonshellbehavior.

Introduction: Anintroductiontotheshell.Definitions: Somedefinitionsusedintherestofthismanual.BasicShellFeatures: Theshell"buildingblocks".ShellBuiltinCommands: Commandsthatareapartoftheshell.ShellVariables: VariablesusedorsetbyBash.BashFeatures: FeaturesfoundonlyinBash.



JobControl: WhatjobcontrolisandhowBashallowsyoutouseit.CommandLineEditing: Chapterdescribingthecommandlineeditingfeatures.UsingHistoryInteractively: CommandHistoryExpansionInstallingBash: HowtobuildandinstallBashonyoursystem.ReportingBugs: HowtoreportbugsinBash.MajorDifferencesFromTheBourneShell:AterselistofthedifferencesbetweenBashandhistoricalversionsof/bin/sh.GNUFreeDocumentationLicense: Copyingandsharingthisdocumentation.Indexes: Variousindexesforthismanual.

Next:Definitions,Up:Top[Contents][Index]

1Introduction

WhatisBash?: AshortdescriptionofBash.Whatisashell?:Abriefintroductiontoshells.

Next:Whatisashell?,Up:Introduction[Contents][Index]

1.1WhatisBash?

Bashistheshell,orcommandlanguageinterpreter,fortheGNUoperatingsystem.ThenameisanacronymfortheBourne-Again SHell,apunonStephenBourne,theauthorofthedirectancestorofthecurrentUnixshellsh,whichappearedintheSeventhEditionBellLabsResearchversionofUnix.

BashislargelycompatiblewithshandincorporatesusefulfeaturesfromtheKornshellkshandtheCshellcsh.ItisintendedtobeaconformantimplementationoftheIEEEPOSIXShellandToolsportionoftheIEEEPOSIXspecification(IEEEStandard1003.1).Itoffersfunctionalimprovementsovershforbothinteractiveandprogramminguse.

WhiletheGNUoperatingsystemprovidesothershells,includingaversionofcsh,Bashisthedefaultshell.LikeotherGNUsoftware,Bashisquiteportable.ItcurrentlyrunsonnearlyeveryversionofUnixandafewotheroperatingsystemsindependentlysupportedportsexistforMSDOS,OS/2,andWindowsplatforms.

Previous:WhatisBash?,Up:Introduction[Contents][Index]

1.2Whatisashell?

Atitsbase,ashellissimplyamacroprocessorthatexecutescommands.Thetermmacroprocessormeansfunctionalitywheretextandsymbolsareexpandedtocreatelargerexpressions.

AUnixshellisbothacommandinterpreterandaprogramminglanguage.Asacommandinterpreter,theshellprovidestheuserinterfacetotherichsetofGNUutilities.Theprogramminglanguagefeaturesallowtheseutilitiestobecombined.Filescontainingcommandscanbecreated,andbecomecommandsthemselves.Thesenewcommandshavethesamestatusassystemcommandsindirectoriessuchas/bin,allowingusersorgroupstoestablishcustomenvironmentstoautomatetheircommontasks.

Shellsmaybeusedinteractivelyornoninteractively.Ininteractivemode,theyacceptinputtypedfromthekeyboard.Whenexecutingnoninteractively,shellsexecutecommandsreadfromafile.

AshellallowsexecutionofGNUcommands,bothsynchronouslyandasynchronously.Theshellwaitsforsynchronouscommandstocompletebeforeacceptingmoreinputasynchronouscommandscontinuetoexecuteinparallelwiththeshellwhileitreadsandexecutesadditionalcommands.Theredirectionconstructspermitfinegrainedcontroloftheinputandoutputofthosecommands.Moreover,theshellallowscontroloverthecontentsofcommandsenvironments.

Shellsalsoprovideasmallsetofbuiltincommands(builtins)implementingfunctionalityimpossibleorinconvenienttoobtainvia



separateutilities.Forexample,cd,break,continue,andexeccannotbeimplementedoutsideoftheshellbecausetheydirectlymanipulatetheshellitself.Thehistory,getopts,kill,orpwdbuiltins,amongothers,couldbeimplementedinseparateutilities,buttheyaremoreconvenienttouseasbuiltincommands.Alloftheshellbuiltinsaredescribedinsubsequentsections.

Whileexecutingcommandsisessential,mostofthepower(andcomplexity)ofshellsisduetotheirembeddedprogramminglanguages.Likeanyhighlevellanguage,theshellprovidesvariables,flowcontrolconstructs,quoting,andfunctions.

Shellsofferfeaturesgearedspecificallyforinteractiveuseratherthantoaugmenttheprogramminglanguage.Theseinteractivefeaturesincludejobcontrol,commandlineediting,commandhistoryandaliases.Eachofthesefeaturesisdescribedinthismanual.

Next:BasicShellFeatures,Previous:Introduction,Up:Top[Contents][Index]

2Definitions

Thesedefinitionsareusedthroughouttheremainderofthismanual.

POSIX

AfamilyofopensystemstandardsbasedonUnix.BashisprimarilyconcernedwiththeShellandUtilitiesportionofthePOSIX1003.1standard.

blank

Aspaceortabcharacter.

builtin

Acommandthatisimplementedinternallybytheshellitself,ratherthanbyanexecutableprogramsomewhereinthefilesystem.

control operator

Atokenthatperformsacontrolfunction.Itisanewlineoroneofthefollowing:||,&&,&,;,;;,|,|&,(,or).

exit status

Thevaluereturnedbyacommandtoitscaller.Thevalueisrestrictedtoeightbits,sothemaximumvalueis255.

field

Aunitoftextthatistheresultofoneoftheshellexpansions.Afterexpansion,whenexecutingacommand,theresultingfieldsareusedasthecommandnameandarguments.

filename

Astringofcharactersusedtoidentifyafile.

job

Asetofprocessescomprisingapipeline,andanyprocessesdescendedfromit,thatareallinthesameprocessgroup.

job control

Amechanismbywhichuserscanselectivelystop(suspend)andrestart(resume)executionofprocesses.

metacharacter



Acharacterthat,whenunquoted,separateswords.Ametacharacterisablankoroneofthefollowingcharacters:|,&,;,(,),.

name

Awordconsistingsolelyofletters,numbers,andunderscores,andbeginningwithaletterorunderscore.Namesareusedasshellvariableandfunctionnames.Alsoreferredtoasanidentifier.

operator

Acontrol operatororaredirection operator.SeeRedirections,foralistofredirectionoperators.Operatorscontainatleastoneunquotedmetacharacter.

process group

AcollectionofrelatedprocesseseachhavingthesameprocessgroupID.

process group ID

Auniqueidentifierthatrepresentsaprocess groupduringitslifetime.

reserved word

Awordthathasaspecialmeaningtotheshell.Mostreservedwordsintroduceshellflowcontrolconstructs,suchasforandwhile.

return status

Asynonymforexit status.

signal

Amechanismbywhichaprocessmaybenotifiedbythekernelofaneventoccurringinthesystem.

special builtin

AshellbuiltincommandthathasbeenclassifiedasspecialbythePOSIXstandard.

token

Asequenceofcharactersconsideredasingleunitbytheshell.Itiseitherawordoranoperator.

word

Asequenceofcharacterstreatedasaunitbytheshell.Wordsmaynotincludeunquotedmetacharacters.

Next:ShellBuiltinCommands,Previous:Definitions,Up:Top[Contents][Index]

3BasicShellFeatures

BashisanacronymforBourne-Again SHell.TheBourneshellisthetraditionalUnixshelloriginallywrittenbyStephenBourne.AlloftheBourneshellbuiltincommandsareavailableinBash,TherulesforevaluationandquotingaretakenfromthePOSIXspecificationforthestandardUnixshell.

Thischapterbrieflysummarizestheshellsbuildingblocks:commands,controlstructures,shellfunctions,shellparameters,shellexpansions,redirections,whichareawaytodirectinputandoutputfromandtonamedfiles,andhowtheshellexecutescommands.

ShellSyntax: Whatyourinputmeanstotheshell.ShellCommands: Thetypesofcommandsyoucanuse.



ShellFunctions: Groupingcommandsbyname.ShellParameters: Howtheshellstoresvalues.ShellExpansions: HowBashexpandsparametersandthevariousexpansionsavailable.Redirections: Awaytocontrolwhereinputandoutputgo.ExecutingCommands:Whathappenswhenyourunacommand.ShellScripts: Executingfilesofshellcommands.

Next:ShellCommands,Up:BasicShellFeatures[Contents][Index]

3.1ShellSyntax

ShellOperation:Thebasicoperationoftheshell.Quoting: Howtoremovethespecialmeaningfromcharacters.Comments: Howtospecifycomments.

Whentheshellreadsinput,itproceedsthroughasequenceofoperations.Iftheinputindicatesthebeginningofacomment,theshellignoresthecommentsymbol(#),andtherestofthatline.

Otherwise,roughlyspeaking,theshellreadsitsinputanddividestheinputintowordsandoperators,employingthequotingrulestoselectwhichmeaningstoassignvariouswordsandcharacters.

Theshellthenparsesthesetokensintocommandsandotherconstructs,removesthespecialmeaningofcertainwordsorcharacters,expandsothers,redirectsinputandoutputasneeded,executesthespecifiedcommand,waitsforthecommandsexitstatus,andmakesthatexitstatusavailableforfurtherinspectionorprocessing.

Next:Quoting,Up:ShellSyntax[Contents][Index]

3.1.1ShellOperation

Thefollowingisabriefdescriptionoftheshellsoperationwhenitreadsandexecutesacommand.Basically,theshelldoesthefollowing:

1. Readsitsinputfromafile(seeShellScripts),fromastringsuppliedasanargumenttothe-cinvocationoption(seeInvokingBash),orfromtheusersterminal.

2. Breakstheinputintowordsandoperators,obeyingthequotingrulesdescribedinQuoting.Thesetokensareseparatedbymetacharacters.Aliasexpansionisperformedbythisstep(seeAliases).

3. Parsesthetokensintosimpleandcompoundcommands(seeShellCommands).4. Performsthevariousshellexpansions(seeShellExpansions),breakingtheexpandedtokensintolistsoffilenames(see

FilenameExpansion)andcommandsandarguments.5. Performsanynecessaryredirections(seeRedirections)andremovestheredirectionoperatorsandtheiroperandsfrom

theargumentlist.6. Executesthecommand(seeExecutingCommands).7. Optionallywaitsforthecommandtocompleteandcollectsitsexitstatus(seeExitStatus).

Next:Comments,Previous:ShellOperation,Up:ShellSyntax[Contents][Index]

3.1.2Quoting

EscapeCharacter: Howtoremovethespecialmeaningfromasinglecharacter.

SingleQuotes: Howtoinhibitallinterpretationofasequenceofcharacters.DoubleQuotes: Howtosuppressmostoftheinterpretationofasequenceofcharacters.ANSICQuoting: HowtoexpandANSICsequencesinquotedstrings.



LocaleTranslation:Howtotranslatestringsintodifferentlanguages.

Quotingisusedtoremovethespecialmeaningofcertaincharactersorwordstotheshell.Quotingcanbeusedtodisablespecialtreatmentforspecialcharacters,topreventreservedwordsfrombeingrecognizedassuch,andtopreventparameterexpansion.

Eachoftheshellmetacharacters(seeDefinitions)hasspecialmeaningtotheshellandmustbequotedifitistorepresentitself.Whenthecommandhistoryexpansionfacilitiesarebeingused(seeHistoryInteraction),thehistoryexpansioncharacter,usually!,mustbequotedtopreventhistoryexpansion.SeeBashHistoryFacilities,formoredetailsconcerninghistoryexpansion.

Therearethreequotingmechanisms:theescapecharacter,singlequotes,anddoublequotes.

Next:SingleQuotes,Up:Quoting[Contents][Index]

3.1.2.1EscapeCharacter

Anonquotedbackslash\istheBashescapecharacter.Itpreservestheliteralvalueofthenextcharacterthatfollows,withtheexceptionofnewline.Ifa\newlinepairappears,andthebackslashitselfisnotquoted,the\newlineistreatedasalinecontinuation(thatis,itisremovedfromtheinputstreamandeffectivelyignored).

Next:DoubleQuotes,Previous:EscapeCharacter,Up:Quoting[Contents][Index]

3.1.2.2SingleQuotes

Enclosingcharactersinsinglequotes(')preservestheliteralvalueofeachcharacterwithinthequotes.Asinglequotemaynotoccurbetweensinglequotes,evenwhenprecededbyabackslash.

Next:ANSICQuoting,Previous:SingleQuotes,Up:Quoting[Contents][Index]

3.1.2.3DoubleQuotes

Enclosingcharactersindoublequotes(")preservestheliteralvalueofallcharacterswithinthequotes,withtheexceptionof$, ,\,and,whenhistoryexpansionisenabled,!.Thecharacters$and retaintheirspecialmeaningwithindoublequotes(seeShellExpansions).Thebackslashretainsitsspecialmeaningonlywhenfollowedbyoneofthefollowingcharacters:$, ,",\,ornewline.Withindoublequotes,backslashesthatarefollowedbyoneofthesecharactersareremoved.Backslashesprecedingcharacterswithoutaspecialmeaningareleftunmodified.Adoublequotemaybequotedwithindoublequotesbyprecedingitwithabackslash.Ifenabled,historyexpansionwillbeperformedunlessan!appearingindoublequotesisescapedusingabackslash.Thebackslashprecedingthe!isnotremoved.

Thespecialparameters*and@havespecialmeaningwhenindoublequotes(seeShellParameterExpansion).

Next:LocaleTranslation,Previous:DoubleQuotes,Up:Quoting[Contents][Index]

3.1.2.4ANSICQuoting

Wordsoftheform$'string'aretreatedspecially.Thewordexpandstostring,withbackslashescapedcharactersreplacedasspecifiedbytheANSICstandard.Backslashescapesequences,ifpresent,aredecodedasfollows:

\a

alert(bell)

\b



backspace

\e\E

anescapecharacter(notANSIC)

\f

formfeed

\n

newline

\r

carriagereturn

\t

horizontaltab

\v

verticaltab

\\

backslash

\'

singlequote

\"

doublequote

\nnn

theeightbitcharacterwhosevalueistheoctalvaluennn(onetothreedigits)

\xHH

theeightbitcharacterwhosevalueisthehexadecimalvalueHH(oneortwohexdigits)

\uHHHH

theUnicode(ISO/IEC10646)characterwhosevalueisthehexadecimalvalueHHHH(onetofourhexdigits)

\UHHHHHHHH

theUnicode(ISO/IEC10646)characterwhosevalueisthehexadecimalvalueHHHHHHHH(onetoeighthexdigits)

\cx

acontrolxcharacter

Theexpandedresultissinglequoted,asifthedollarsignhadnotbeenpresent.

Previous:ANSICQuoting,Up:Quoting[Contents][Index]



3.1.2.5LocaleSpecificTranslation

Adoublequotedstringprecededbyadollarsign($)willcausethestringtobetranslatedaccordingtothecurrentlocale.IfthecurrentlocaleisCorPOSIX,thedollarsignisignored.Ifthestringistranslatedandreplaced,thereplacementisdoublequoted.

SomesystemsusethemessagecatalogselectedbytheLC_MESSAGESshellvariable.OtherscreatethenameofthemessagecatalogfromthevalueoftheTEXTDOMAINshellvariable,possiblyaddingasuffixof.mo.IfyouusetheTEXTDOMAINvariable,youmayneedtosettheTEXTDOMAINDIRvariabletothelocationofthemessagecatalogfiles.Stillothersusebothvariablesinthisfashion:TEXTDOMAINDIR/LC_MESSAGES/LC_MESSAGES/TEXTDOMAIN.mo.

Previous:Quoting,Up:ShellSyntax[Contents][Index]

3.1.3Comments

Inanoninteractiveshell,oraninteractiveshellinwhichtheinteractive_commentsoptiontotheshoptbuiltinisenabled(seeTheShoptBuiltin),awordbeginningwith#causesthatwordandallremainingcharactersonthatlinetobeignored.Aninteractiveshellwithouttheinteractive_commentsoptionenableddoesnotallowcomments.Theinteractive_commentsoptionisonbydefaultininteractiveshells.SeeInteractiveShells,foradescriptionofwhatmakesashellinteractive.

Next:ShellFunctions,Previous:ShellSyntax,Up:BasicShellFeatures[Contents][Index]

3.2ShellCommands

Asimpleshellcommandsuchasecho a b cconsistsofthecommanditselffollowedbyarguments,separatedbyspaces.

Morecomplexshellcommandsarecomposedofsimplecommandsarrangedtogetherinavarietyofways:inapipelineinwhichtheoutputofonecommandbecomestheinputofasecond,inalooporconditionalconstruct,orinsomeothergrouping.

SimpleCommands: Themostcommontypeofcommand.Pipelines: Connectingtheinputandoutputofseveralcommands.Lists: Howtoexecutecommandssequentially.CompoundCommands:Shellcommandsforcontrolflow.Coprocesses: Twowaycommunicationbetweencommands.GNUParallel: Runningcommandsinparallel.

Next:Pipelines,Up:ShellCommands[Contents][Index]

3.2.1SimpleCommands

Asimplecommandisthekindofcommandencounteredmostoften.Itsjustasequenceofwordsseparatedbyblanks,terminatedbyoneoftheshellscontroloperators(seeDefinitions).Thefirstwordgenerallyspecifiesacommandtobeexecuted,withtherestofthewordsbeingthatcommandsarguments.

Thereturnstatus(seeExitStatus)ofasimplecommandisitsexitstatusasprovidedbythePOSIX1003.1waitpidfunction,or128+nifthecommandwasterminatedbysignaln.

Next:Lists,Previous:SimpleCommands,Up:ShellCommands[Contents][Index]

3.2.2Pipelines

Apipelineisasequenceofsimplecommandsseparatedbyoneofthecontroloperators|or|&.

Theformatforapipelineis



[time [-p]] [!] command1 [ | or |& command2 ]

Theoutputofeachcommandinthepipelineisconnectedviaapipetotheinputofthenextcommand.Thatis,eachcommandreadsthepreviouscommandsoutput.Thisconnectionisperformedbeforeanyredirectionsspecifiedbythecommand.

If|&isused,command1sstandarderror,inadditiontoitsstandardoutput,isconnectedtocommand2sstandardinputthroughthepipeitisshorthandfor2>&1 |.Thisimplicitredirectionofthestandarderrortothestandardoutputisperformedafteranyredirectionsspecifiedbythecommand.

Thereservedwordtimecausestimingstatisticstobeprintedforthepipelineonceitfinishes.Thestatisticscurrentlyconsistofelapsed(wallclock)timeanduserandsystemtimeconsumedbythecommandsexecution.The-poptionchangestheoutputformattothatspecifiedbyPOSIX.WhentheshellisinPOSIXmode(seeBashPOSIXMode),itdoesnotrecognizetimeasareservedwordifthenexttokenbeginswitha-.TheTIMEFORMATvariablemaybesettoaformatstringthatspecifieshowthetiminginformationshouldbedisplayed.SeeBashVariables,foradescriptionoftheavailableformats.Theuseoftimeasareservedwordpermitsthetimingofshellbuiltins,shellfunctions,andpipelines.Anexternaltimecommandcannottimetheseeasily.

WhentheshellisinPOSIXmode(seeBashPOSIXMode),timemaybefollowedbyanewline.Inthiscase,theshelldisplaysthetotaluserandsystemtimeconsumedbytheshellanditschildren.TheTIMEFORMATvariablemaybeusedtospecifytheformatofthetimeinformation.

Ifthepipelineisnotexecutedasynchronously(seeLists),theshellwaitsforallcommandsinthepipelinetocomplete.

Eachcommandinapipelineisexecutedinitsownsubshell(seeCommandExecutionEnvironment).Theexitstatusofapipelineistheexitstatusofthelastcommandinthepipeline,unlessthepipefailoptionisenabled(seeTheSetBuiltin).Ifpipefailisenabled,thepipelinesreturnstatusisthevalueofthelast(rightmost)commandtoexitwithanonzerostatus,orzeroifallcommandsexitsuccessfully.Ifthereservedword!precedesthepipeline,theexitstatusisthelogicalnegationoftheexitstatusasdescribedabove.Theshellwaitsforallcommandsinthepipelinetoterminatebeforereturningavalue.

Next:CompoundCommands,Previous:Pipelines,Up:ShellCommands[Contents][Index]

3.2.3ListsofCommands

Alistisasequenceofoneormorepipelinesseparatedbyoneoftheoperators;,&,&&,or||,andoptionallyterminatedbyoneof;,&,oranewline.

Oftheselistoperators,&&and||haveequalprecedence,followedby;and&,whichhaveequalprecedence.

Asequenceofoneormorenewlinesmayappearinalisttodelimitcommands,equivalenttoasemicolon.

Ifacommandisterminatedbythecontroloperator&,theshellexecutesthecommandasynchronouslyinasubshell.Thisisknownasexecutingthecommandinthebackground.Theshelldoesnotwaitforthecommandtofinish,andthereturnstatusis0(true).Whenjobcontrolisnotactive(seeJobControl),thestandardinputforasynchronouscommands,intheabsenceofanyexplicitredirections,isredirectedfrom/dev/null.

Commandsseparatedbya;areexecutedsequentiallytheshellwaitsforeachcommandtoterminateinturn.Thereturnstatusistheexitstatusofthelastcommandexecuted.

ANDandORlistsaresequencesofoneormorepipelinesseparatedbythecontroloperators&&and||,respectively.ANDandORlistsareexecutedwithleftassociativity.

AnANDlisthastheform

command1 && command2

command2isexecutedif,andonlyif,command1returnsanexitstatusofzero.

AnORlisthastheform



command1 || command2

command2isexecutedif,andonlyif,command1returnsanonzeroexitstatus.

ThereturnstatusofANDandORlistsistheexitstatusofthelastcommandexecutedinthelist.

Next:Coprocesses,Previous:Lists,Up:ShellCommands[Contents][Index]

3.2.4CompoundCommands

LoopingConstructs: Shellcommandsforiterativeaction.ConditionalConstructs: Shellcommandsforconditionalexecution.CommandGrouping: Waystogroupcommands.

Compoundcommandsaretheshellprogrammingconstructs.Eachconstructbeginswithareservedwordorcontroloperatorandisterminatedbyacorrespondingreservedwordoroperator.Anyredirections(seeRedirections)associatedwithacompoundcommandapplytoallcommandswithinthatcompoundcommandunlessexplicitlyoverridden.

Inmostcasesalistofcommandsinacompoundcommandsdescriptionmaybeseparatedfromtherestofthecommandbyoneormorenewlines,andmaybefollowedbyanewlineinplaceofasemicolon.

Bashprovidesloopingconstructs,conditionalcommands,andmechanismstogroupcommandsandexecutethemasaunit.

Next:ConditionalConstructs,Up:CompoundCommands[Contents][Index]

3.2.4.1LoopingConstructs

Bashsupportsthefollowingloopingconstructs.

Notethatwherevera;appearsinthedescriptionofacommandssyntax,itmaybereplacedwithoneormorenewlines.

until

Thesyntaxoftheuntilcommandis:

until test-commands; do consequent-commands; done

Executeconsequentcommandsaslongastestcommandshasanexitstatuswhichisnotzero.Thereturnstatusistheexitstatusofthelastcommandexecutedinconsequentcommands,orzeroifnonewasexecuted.

while

Thesyntaxofthewhilecommandis:

while test-commands; do consequent-commands; done

Executeconsequentcommandsaslongastestcommandshasanexitstatusofzero.Thereturnstatusistheexitstatusofthelastcommandexecutedinconsequentcommands,orzeroifnonewasexecuted.

for

Thesyntaxoftheforcommandis:

for name [ [in [words ] ] ; ] do commands; done

Expandwords,andexecutecommandsonceforeachmemberintheresultantlist,withnameboundtothecurrentmember.Ifin wordsisnotpresent,theforcommandexecutesthecommandsonceforeachpositionalparameterthatisset,asifin "$@"hadbeenspecified(seeSpecialParameters).Thereturnstatusistheexitstatusofthelast



commandthatexecutes.Iftherearenoitemsintheexpansionofwords,nocommandsareexecuted,andthereturnstatusiszero.

Analternateformoftheforcommandisalsosupported:

for (( expr1 ; expr2 ; expr3 )) ; do commands ; done

First,thearithmeticexpressionexpr1isevaluatedaccordingtotherulesdescribedbelow(seeShellArithmetic).Thearithmeticexpressionexpr2isthenevaluatedrepeatedlyuntilitevaluatestozero.Eachtimeexpr2evaluatestoanonzerovalue,commandsareexecutedandthearithmeticexpressionexpr3isevaluated.Ifanyexpressionisomitted,itbehavesasifitevaluatesto1.Thereturnvalueistheexitstatusofthelastcommandincommandsthatisexecuted,orfalseifanyoftheexpressionsisinvalid.

Thebreakandcontinuebuiltins(seeBourneShellBuiltins)maybeusedtocontrolloopexecution.

Next:CommandGrouping,Previous:LoopingConstructs,Up:CompoundCommands[Contents][Index]

3.2.4.2ConditionalConstructs

if

Thesyntaxoftheifcommandis:

if test-commands; then consequent-commands;[elif more-test-commands; then more-consequents;][else alternate-consequents;]fi

Thetestcommandslistisexecuted,andifitsreturnstatusiszero,theconsequentcommandslistisexecuted.Iftestcommandsreturnsanonzerostatus,eacheliflistisexecutedinturn,andifitsexitstatusiszero,thecorrespondingmoreconsequentsisexecutedandthecommandcompletes.Ifelse alternate-consequentsispresent,andthefinalcommandinthefinaliforelifclausehasanonzeroexitstatus,thenalternateconsequentsisexecuted.Thereturnstatusistheexitstatusofthelastcommandexecuted,orzeroifnoconditiontestedtrue.

case

Thesyntaxofthecasecommandis:

case word in [ [(] pattern [| pattern]) command-list ;;] esac

casewillselectivelyexecutethecommandlistcorrespondingtothefirstpatternthatmatchesword.Iftheshelloptionnocasematch(seethedescriptionofshoptinTheShoptBuiltin)isenabled,thematchisperformedwithoutregardtothecaseofalphabeticcharacters.The|isusedtoseparatemultiplepatterns,andthe)operatorterminatesapatternlist.Alistofpatternsandanassociatedcommandlistisknownasaclause.

Eachclausemustbeterminatedwith;;,;&,or;;&.Thewordundergoestildeexpansion,parameterexpansion,commandsubstitution,arithmeticexpansion,andquoteremovalbeforematchingisattempted.Eachpatternundergoestildeexpansion,parameterexpansion,commandsubstitution,andarithmeticexpansion.

Theremaybeanarbitrarynumberofcaseclauses,eachterminatedbya;;,;&,or;;&.Thefirstpatternthatmatchesdeterminesthecommandlistthatisexecuted.Itsacommonidiomtouse*asthefinalpatterntodefinethedefaultcase,sincethatpatternwillalwaysmatch.

Hereisanexampleusingcaseinascriptthatcouldbeusedtodescribeoneinterestingfeatureofananimal:

echo -n "Enter the name of an animal: "read ANIMALecho -n "The $ANIMAL has "



case $ANIMAL in horse | dog | cat) echo -n "four";; man | kangaroo ) echo -n "two";; *) echo -n "an unknown number of";;esacecho " legs."

Ifthe;;operatorisused,nosubsequentmatchesareattemptedafterthefirstpatternmatch.Using;&inplaceof;;causesexecutiontocontinuewiththecommandlistassociatedwiththenextclause,ifany.Using;;&inplaceof;;causestheshelltotestthepatternsinthenextclause,ifany,andexecuteanyassociatedcommandlistonasuccessfulmatch.

Thereturnstatusiszeroifnopatternismatched.Otherwise,thereturnstatusistheexitstatusofthecommandlistexecuted.

select

Theselectconstructallowstheeasygenerationofmenus.Ithasalmostthesamesyntaxastheforcommand:

select name [in words ]; do commands; done

Thelistofwordsfollowinginisexpanded,generatingalistofitems.Thesetofexpandedwordsisprintedonthestandarderroroutputstream,eachprecededbyanumber.Ifthein wordsisomitted,thepositionalparametersareprinted,asifin "$@"hadbeenspecified.ThePS3promptisthendisplayedandalineisreadfromthestandardinput.Ifthelineconsistsofanumbercorrespondingtooneofthedisplayedwords,thenthevalueofnameissettothatword.Ifthelineisempty,thewordsandpromptaredisplayedagain.IfEOFisread,theselectcommandcompletes.Anyothervaluereadcausesnametobesettonull.ThelinereadissavedinthevariableREPLY.

Thecommandsareexecutedaftereachselectionuntilabreakcommandisexecuted,atwhichpointtheselectcommandcompletes.

Hereisanexamplethatallowstheusertopickafilenamefromthecurrentdirectory,anddisplaysthenameandindexofthefileselected.

select fname in *;do echo you picked $fname $$REPLY$ break;done

(())

(( expression ))

Thearithmeticexpressionisevaluatedaccordingtotherulesdescribedbelow(seeShellArithmetic).Ifthevalueoftheexpressionisnonzero,thereturnstatusis0otherwisethereturnstatusis1.Thisisexactlyequivalentto

let "expression"

SeeBashBuiltins,forafulldescriptionoftheletbuiltin.

[[]]

[[ expression ]]

Returnastatusof0or1dependingontheevaluationoftheconditionalexpressionexpression.ExpressionsarecomposedoftheprimariesdescribedbelowinBashConditionalExpressions.Wordsplittingandfilenameexpansionarenotperformedonthewordsbetweenthe[[and]]tildeexpansion,parameterandvariableexpansion,arithmeticexpansion,commandsubstitution,processsubstitution,andquoteremovalareperformed.Conditionaloperatorssuchas-fmustbeunquotedtoberecognizedasprimaries.

Whenusedwith[[,theoperatorssortlexicographicallyusingthecurrentlocale.



Whenthe==and!=operatorsareused,thestringtotherightoftheoperatorisconsideredapatternandmatchedaccordingtotherulesdescribedbelowinPatternMatching,asiftheextglobshelloptionwereenabled.The=operatorisidenticalto==.Iftheshelloptionnocasematch(seethedescriptionofshoptinTheShoptBuiltin)isenabled,thematchisperformedwithoutregardtothecaseofalphabeticcharacters.Thereturnvalueis0ifthestringmatches(==)ordoesnotmatch(!=)thepattern,and1otherwise.Anypartofthepatternmaybequotedtoforcethequotedportiontobematchedasastring.

Anadditionalbinaryoperator,=~,isavailable,withthesameprecedenceas==and!=.Whenitisused,thestringtotherightoftheoperatorisconsideredanextendedregularexpressionandmatchedaccordingly(asinregex3)).Thereturnvalueis0ifthestringmatchesthepattern,and1otherwise.Iftheregularexpressionissyntacticallyincorrect,theconditionalexpressionsreturnvalueis2.Iftheshelloptionnocasematch(seethedescriptionofshoptinTheShoptBuiltin)isenabled,thematchisperformedwithoutregardtothecaseofalphabeticcharacters.Anypartofthepatternmaybequotedtoforcethequotedportiontobematchedasastring.Bracketexpressionsinregularexpressionsmustbetreatedcarefully,sincenormalquotingcharacterslosetheirmeaningsbetweenbrackets.Ifthepatternisstoredinashellvariable,quotingthevariableexpansionforcestheentirepatterntobematchedasastring.SubstringsmatchedbyparenthesizedsubexpressionswithintheregularexpressionaresavedinthearrayvariableBASH_REMATCH.TheelementofBASH_REMATCHwithindex0istheportionofthestringmatchingtheentireregularexpression.TheelementofBASH_REMATCHwithindexnistheportionofthestringmatchingthenthparenthesizedsubexpression.

Forexample,thefollowingwillmatchaline(storedintheshellvariableline)ifthereisasequenceofcharactersinthevalueconsistingofanynumber,includingzero,ofspacecharacters,zerooroneinstancesofa,thenab:

[[ $line =~ [[:space:]]*(a)?b ]]

Thatmeansvalueslikeaaband aaaaaabwillmatch,aswillalinecontainingabanywhereinitsvalue.

Storingtheregularexpressioninashellvariableisoftenausefulwaytoavoidproblemswithquotingcharactersthatarespecialtotheshell.Itissometimesdifficulttospecifyaregularexpressionliterallywithoutusingquotes,ortokeeptrackofthequotingusedbyregularexpressionswhilepayingattentiontotheshellsquoteremoval.Usingashellvariabletostorethepatterndecreasestheseproblems.Forexample,thefollowingisequivalenttotheabove:

pattern='[[:space:]]*(a)?b'[[ $line =~ $pattern ]]

Ifyouwanttomatchacharacterthatsspecialtotheregularexpressiongrammar,ithastobequotedtoremoveitsspecialmeaning.Thismeansthatinthepatternxxx.txt,the.matchesanycharacterinthestring(itsusualregularexpressionmeaning),butinthepattern"xxx.txt"itcanonlymatchaliteral..Shellprogrammersshouldtakespecialcarewithbackslashes,sincebackslashesareusedbothbytheshellandregularexpressionstoremovethespecialmeaningfromthefollowingcharacter.Thefollowingtwosetsofcommandsarenotequivalent:

pattern='\.'

[[ . =~ $pattern ]][[ . =~ \. ]]

[[ . =~ "$pattern" ]][[ . =~ '\.' ]]

Thefirsttwomatcheswillsucceed,butthesecondtwowillnot,becauseinthesecondtwothebackslashwillbepartofthepatterntobematched.Inthefirsttwoexamples,thebackslashremovesthespecialmeaningfrom.,sotheliteral.matches.Ifthestringinthefirstexampleswereanythingotherthan.,saya,thepatternwouldnotmatch,becausethequoted.inthepatternlosesitsspecialmeaningofmatchinganysinglecharacter.

Expressionsmaybecombinedusingthefollowingoperators,listedindecreasingorderofprecedence:

( expression )

Returnsthevalueofexpression.Thismaybeusedtooverridethenormalprecedenceofoperators.

! expression



Trueifexpressionisfalse.

expression1 && expression2

Trueifbothexpression1andexpression2aretrue.

expression1 || expression2

Trueifeitherexpression1orexpression2istrue.

The&&and||operatorsdonotevaluateexpression2ifthevalueofexpression1issufficienttodeterminethereturnvalueoftheentireconditionalexpression.

Previous:ConditionalConstructs,Up:CompoundCommands[Contents][Index]

3.2.4.3GroupingCommands

Bashprovidestwowaystogroupalistofcommandstobeexecutedasaunit.Whencommandsaregrouped,redirectionsmaybeappliedtotheentirecommandlist.Forexample,theoutputofallthecommandsinthelistmayberedirectedtoasinglestream.

()

( list )

Placingalistofcommandsbetweenparenthesescausesasubshellenvironmenttobecreated(seeCommandExecutionEnvironment),andeachofthecommandsinlisttobeexecutedinthatsubshell.Sincethelistisexecutedinasubshell,variableassignmentsdonotremainineffectafterthesubshellcompletes.

{}

{ list; }

Placingalistofcommandsbetweencurlybracescausesthelisttobeexecutedinthecurrentshellcontext.Nosubshelliscreated.Thesemicolon(ornewline)followinglistisrequired.

Inadditiontothecreationofasubshell,thereisasubtledifferencebetweenthesetwoconstructsduetohistoricalreasons.Thebracesarereserved words,sotheymustbeseparatedfromthelistbyblanksorothershellmetacharacters.Theparenthesesareoperators,andarerecognizedasseparatetokensbytheshelleveniftheyarenotseparatedfromthelistbywhitespace.

Theexitstatusofbothoftheseconstructsistheexitstatusoflist.

Next:GNUParallel,Previous:CompoundCommands,Up:ShellCommands[Contents][Index]

3.2.5Coprocesses

Acoprocessisashellcommandprecededbythecoprocreservedword.Acoprocessisexecutedasynchronouslyinasubshell,asifthecommandhadbeenterminatedwiththe&controloperator,withatwowaypipeestablishedbetweentheexecutingshellandthecoprocess.

Theformatforacoprocessis:

coproc [NAME] command [redirections]

ThiscreatesacoprocessnamedNAME.IfNAMEisnotsupplied,thedefaultnameisCOPROC.NAMEmustnotbesuppliedifcommandisasimplecommand(seeSimpleCommands)otherwise,itisinterpretedasthefirstwordofthesimplecommand.



Whenthecoprocessisexecuted,theshellcreatesanarrayvariable(seeArrays)namedNAMEinthecontextoftheexecutingshell.Thestandardoutputofcommandisconnectedviaapipetoafiledescriptorintheexecutingshell,andthatfiledescriptorisassignedtoNAME[0].Thestandardinputofcommandisconnectedviaapipetoafiledescriptorintheexecutingshell,andthatfiledescriptorisassignedtoNAME[1].Thispipeisestablishedbeforeanyredirectionsspecifiedbythecommand(seeRedirections).Thefiledescriptorscanbeutilizedasargumentstoshellcommandsandredirectionsusingstandardwordexpansions.Thefiledescriptorsarenotavailableinsubshells.

TheprocessIDoftheshellspawnedtoexecutethecoprocessisavailableasthevalueofthevariableNAME_PID.Thewaitbuiltincommandmaybeusedtowaitforthecoprocesstoterminate.

Sincethecoprocessiscreatedasanasynchronouscommand,thecoproccommandalwaysreturnssuccess.Thereturnstatusofacoprocessistheexitstatusofcommand.

Previous:Coprocesses,Up:ShellCommands[Contents][Index]

3.2.6GNUParallel

TherearewaystoruncommandsinparallelthatarenotbuiltintoBash.GNUParallelisatooltodojustthat.

GNUParallel,asitsnamesuggests,canbeusedtobuildandruncommandsinparallel.Youmayrunthesamecommandwithdifferentarguments,whethertheyarefilenames,usernames,hostnames,orlinesreadfromfiles.GNUParallelprovidesshorthandreferencestomanyofthemostcommonoperations(inputlines,variousportionsoftheinputline,differentwaystospecifytheinputsource,andsoon).ParallelcanreplacexargsorfeedcommandsfromitsinputsourcestoseveraldifferentinstancesofBash.

Foracompletedescription,refertotheGNUParalleldocumentation.Afewexamplesshouldprovideabriefintroductiontoitsuse.

Forexample,itiseasytoreplacexargstogzipallhtmlfilesinthecurrentdirectoryanditssubdirectories:

find . -type f -name '*.html' -print | parallel gzip

Ifyouneedtoprotectspecialcharacterssuchasnewlinesinfilenames,usefinds-print0optionandparallels-0option.

YoucanuseParalleltomovefilesfromthecurrentdirectorywhenthenumberoffilesistoolargetoprocesswithonemvinvocation:

ls | parallel mv {} destdir

Asyoucansee,the{}isreplacedwitheachlinereadfromstandardinput.Whileusinglswillworkinmostinstances,itisnotsufficienttodealwithallfilenames.Ifyouneedtoaccommodatespecialcharactersinfilenames,youcanuse

find . -depth 1 \! -name '.*' -print0 | parallel -0 mv {} destdir

asalludedtoabove.

Thiswillrunasmanymvcommandsastherearefilesinthecurrentdirectory.Youcanemulateaparallelxargsbyaddingthe-Xoption:

find . -depth 1 \! -name '.*' -print0 | parallel -0 -X mv {} destdir

GNUParallelcanreplacecertaincommonidiomsthatoperateonlinesreadfromafile(inthiscase,filenameslistedoneperline):

while IFS= read -r x; do do-something1 "$x" "config-$x" do-something2 < "$x" done < file | process-output



withamorecompactsyntaxreminiscentoflambdas:

cat list | parallel "do-something1 {} config-{} ; do-something2 < {}" | process-output

Parallelprovidesabuiltinmechanismtoremovefilenameextensions,whichlendsitselftobatchfiletransformationsorrenaming:

ls *.gz | parallel -j+0 "zcat {} | bzip2 >{.}.bz2 && rm {}"

Thiswillrecompressallfilesinthecurrentdirectorywithnamesendingin.gzusingbzip2,runningonejobperCPU(j+0)inparallel.(Weuselsforbrevityhereusingfindasaboveismorerobustinthefaceoffilenamescontainingunexpectedcharacters.)Parallelcantakeargumentsfromthecommandlinetheabovecanalsobewrittenas

parallel "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz

Ifacommandgeneratesoutput,youmaywanttopreservetheinputorderintheoutput.Forinstance,thefollowingcommand

{ echo foss.org.my ; echo debian.org; echo freenetproject.org; } | parallel traceroute

willdisplayasoutputthetracerouteinvocationthatfinishesfirst.Addingthe-koption

{ echo foss.org.my ; echo debian.org; echo freenetproject.org; } | parallel -k traceroute

willensurethattheoutputoftraceroute foss.org.myisdisplayedfirst.

Finally,Parallelcanbeusedtorunasequenceofshellcommandsinparallel,similartocat file | bash.Itisnotuncommontotakealistoffilenames,createaseriesofshellcommandstooperateonthem,andfeedthatlistofcommndstoashell.Parallelcanspeedthisup.Assumingthatfilecontainsalistofshellcommands,oneperline,

parallel -j 10 < file

willevaluatethecommandsusingtheshell(sincenoexplicitcommandissuppliedasanargument),inblocksoftenshelljobsatatime.

Next:ShellParameters,Previous:ShellCommands,Up:BasicShellFeatures[Contents][Index]

3.3ShellFunctions

Shellfunctionsareawaytogroupcommandsforlaterexecutionusingasinglenameforthegroup.Theyareexecutedjustlikea"regular"command.Whenthenameofashellfunctionisusedasasimplecommandname,thelistofcommandsassociatedwiththatfunctionnameisexecuted.Shellfunctionsareexecutedinthecurrentshellcontextnonewprocessiscreatedtointerpretthem.

Functionsaredeclaredusingthissyntax:

name () compound-command [ redirections ]

or

function name [()] compound-command [ redirections ]

Thisdefinesashellfunctionnamedname.Thereservedwordfunctionisoptional.Ifthefunctionreservedwordissupplied,theparenthesesareoptional.Thebodyofthefunctionisthecompoundcommandcompoundcommand(seeCompoundCommands).Thatcommandisusuallyalistenclosedbetween{and},butmaybeanycompoundcommandlistedabove.compoundcommandisexecutedwhenevernameisspecifiedasthenameofacommand.WhentheshellisinPOSIXmode(seeBashPOSIXMode),namemaynotbethesameasoneofthespecialbuiltins(seeSpecialBuiltins).Anyredirections(seeRedirections)associatedwiththeshellfunctionareperformedwhenthefunctionisexecuted.

Afunctiondefinitionmaybedeletedusingthe-foptiontotheunsetbuiltin(seeBourneShellBuiltins).

Theexitstatusofafunctiondefinitioniszerounlessasyntaxerroroccursorareadonlyfunctionwiththesamenamealready



exists.Whenexecuted,theexitstatusofafunctionistheexitstatusofthelastcommandexecutedinthebody.

Notethatforhistoricalreasons,inthemostcommonusagethecurlybracesthatsurroundthebodyofthefunctionmustbeseparatedfromthebodybyblanksornewlines.Thisisbecausethebracesarereservedwordsandareonlyrecognizedassuchwhentheyareseparatedfromthecommandlistbywhitespaceoranothershellmetacharacter.Also,whenusingthebraces,thelistmustbeterminatedbyasemicolon,a&,oranewline.

Whenafunctionisexecuted,theargumentstothefunctionbecomethepositionalparametersduringitsexecution(seePositionalParameters).Thespecialparameter#thatexpandstothenumberofpositionalparametersisupdatedtoreflectthechange.Specialparameter0isunchanged.ThefirstelementoftheFUNCNAMEvariableissettothenameofthefunctionwhilethefunctionisexecuting.

Allotheraspectsoftheshellexecutionenvironmentareidenticalbetweenafunctionanditscallerwiththeseexceptions:theDEBUGandRETURNtrapsarenotinheritedunlessthefunctionhasbeengiventhetraceattributeusingthedeclarebuiltinorthe-o functraceoptionhasbeenenabledwiththesetbuiltin,(inwhichcaseallfunctionsinherittheDEBUGandRETURNtraps),andtheERRtrapisnotinheritedunlessthe-o errtraceshelloptionhasbeenenabled.SeeBourneShellBuiltins,forthedescriptionofthetrapbuiltin.

TheFUNCNESTvariable,ifsettoanumericvaluegreaterthan0,definesamaximumfunctionnestinglevel.Functioninvocationsthatexceedthelimitcausetheentirecommandtoabort.

Ifthebuiltincommandreturnisexecutedinafunction,thefunctioncompletesandexecutionresumeswiththenextcommandafterthefunctioncall.AnycommandassociatedwiththeRETURNtrapisexecutedbeforeexecutionresumes.Whenafunctioncompletes,thevaluesofthepositionalparametersandthespecialparameter#arerestoredtothevaluestheyhadpriortothefunctionsexecution.Ifanumericargumentisgiventoreturn,thatisthefunctionsreturnstatusotherwisethefunctionsreturnstatusistheexitstatusofthelastcommandexecutedbeforethereturn.

Variableslocaltothefunctionmaybedeclaredwiththelocalbuiltin.Thesevariablesarevisibleonlytothefunctionandthecommandsitinvokes.

Functionnamesanddefinitionsmaybelistedwiththe-foptiontothedeclare(typeset)builtincommand(seeBashBuiltins).The-Foptiontodeclareortypesetwilllistthefunctionnamesonly(andoptionallythesourcefileandlinenumber,iftheextdebugshelloptionisenabled).Functionsmaybeexportedsothatsubshellsautomaticallyhavethemdefinedwiththe-foptiontotheexportbuiltin(seeBourneShellBuiltins).Notethatshellfunctionsandvariableswiththesamenamemayresultinmultipleidenticallynamedentriesintheenvironmentpassedtotheshellschildren.Careshouldbetakenincaseswherethismaycauseaproblem.

Functionsmayberecursive.TheFUNCNESTvariablemaybeusedtolimitthedepthofthefunctioncallstackandrestrictthenumberoffunctioninvocations.Bydefault,nolimitisplacedonthenumberofrecursivecalls.

Next:ShellExpansions,Previous:ShellFunctions,Up:BasicShellFeatures[Contents][Index]

3.4ShellParameters

PositionalParameters:Theshellscommandlinearguments.SpecialParameters: Parametersdenotedbyspecialcharacters.

Aparameterisanentitythatstoresvalues.Itcanbeaname,anumber,oroneofthespecialcharacterslistedbelow.Avariableisaparameterdenotedbyaname.Avariablehasavalueandzeroormoreattributes.Attributesareassignedusingthedeclarebuiltincommand(seethedescriptionofthedeclarebuiltininBashBuiltins).

Aparameterissetifithasbeenassignedavalue.Thenullstringisavalidvalue.Onceavariableisset,itmaybeunsetonlybyusingtheunsetbuiltincommand.

Avariablemaybeassignedtobyastatementoftheform

name=[value]



Ifvalueisnotgiven,thevariableisassignedthenullstring.Allvaluesundergotildeexpansion,parameterandvariableexpansion,commandsubstitution,arithmeticexpansion,andquoteremoval(detailedbelow).Ifthevariablehasitsintegerattributeset,thenvalueisevaluatedasanarithmeticexpressionevenifthe$(())expansionisnotused(seeArithmeticExpansion).Wordsplittingisnotperformed,withtheexceptionof"$@"asexplainedbelow.Filenameexpansionisnotperformed.Assignmentstatementsmayalsoappearasargumentstothealias,declare,typeset,export,readonly,andlocalbuiltincommands.WheninPOSIXmode(seeBashPOSIXMode),thesebuiltinsmayappearinacommandafteroneormoreinstancesofthecommandbuiltinandretaintheseassignmentstatementproperties.

Inthecontextwhereanassignmentstatementisassigningavaluetoashellvariableorarrayindex(seeArrays),the+=operatorcanbeusedtoappendtooraddtothevariablespreviousvalue.When+=isappliedtoavariableforwhichtheintegerattributehasbeenset,valueisevaluatedasanarithmeticexpressionandaddedtothevariablescurrentvalue,whichisalsoevaluated.When+=isappliedtoanarrayvariableusingcompoundassignment(seeArrays),thevariablesvalueisnotunset(asitiswhenusing=),andnewvaluesareappendedtothearraybeginningatonegreaterthanthearraysmaximumindex(forindexedarrays),oraddedasadditionalkeyvaluepairsinanassociativearray.Whenappliedtoastringvaluedvariable,valueisexpandedandappendedtothevariablesvalue.

Avariablecanbeassignedthenamerefattributeusingthe-noptiontothe\fBdeclare\fPor\fBlocal\fPbuiltincommands(seeBashBuiltins)tocreateanameref,orareferencetoanothervariable.Thisallowsvariablestobemanipulatedindirectly.Wheneverthenamerefvariableisreferencedorassignedto,theoperationisactuallyperformedonthevariablespecifiedbythenamerefvariablesvalue.Anamerefiscommonlyusedwithinshellfunctionstorefertoavariablewhosenameispassedasanargumenttothefunction.Forinstance,ifavariablenameispassedtoashellfunctionasitsfirstargument,running

declare -n ref=$1

insidethefunctioncreatesanamerefvariablerefwhosevalueisthevariablenamepassedasthefirstargument.Referencesandassignmentstorefaretreatedasreferencesandassignmentstothevariablewhosenamewaspassedas$1.

Ifthecontrolvariableinaforloophasthenamerefattribute,thelistofwordscanbealistofshellvariables,andanamereferencewillbeestablishedforeachwordinthelist,inturn,whentheloopisexecuted.Arrayvariablescannotbegiventhe-nattribute.However,namerefvariablescanreferencearrayvariablesandsubscriptedarrayvariables.Namerefscanbeunsetusingthe-noptiontotheunsetbuiltin(seeBourneShellBuiltins).Otherwise,ifunsetisexecutedwiththenameofanamerefvariableasanargument,thevariablereferencedbythenamerefvariablewillbeunset.

Next:SpecialParameters,Up:ShellParameters[Contents][Index]

3.4.1PositionalParameters

Apositionalparameterisaparameterdenotedbyoneormoredigits,otherthanthesingledigit0.Positionalparametersareassignedfromtheshellsargumentswhenitisinvoked,andmaybereassignedusingthesetbuiltincommand.PositionalparameterNmaybereferencedas${N},oras$NwhenNconsistsofasingledigit.Positionalparametersmaynotbeassignedtowithassignmentstatements.Thesetandshiftbuiltinsareusedtosetandunsetthem(seeShellBuiltinCommands).Thepositionalparametersaretemporarilyreplacedwhenashellfunctionisexecuted(seeShellFunctions).

Whenapositionalparameterconsistingofmorethanasingledigitisexpanded,itmustbeenclosedinbraces.

Previous:PositionalParameters,Up:ShellParameters[Contents][Index]

3.4.2SpecialParameters

Theshelltreatsseveralparametersspecially.Theseparametersmayonlybereferencedassignmenttothemisnotallowed.

*

($*)Expandstothepositionalparameters,startingfromone.Whentheexpansionisnotwithindoublequotes,eachpositionalparameterexpandstoaseparateword.Incontextswhereitisperformed,thosewordsaresubjecttofurtherwordsplittingandpathnameexpansion.Whentheexpansionoccurswithindoublequotes,itexpandstoasingleword



withthevalueofeachparameterseparatedbythefirstcharacteroftheIFSspecialvariable.Thatis,"$*"isequivalentto"$1c$2c",wherecisthefirstcharacterofthevalueoftheIFSvariable.IfIFSisunset,theparametersareseparatedbyspaces.IfIFSisnull,theparametersarejoinedwithoutinterveningseparators.

@

($@)Expandstothepositionalparameters,startingfromone.Whentheexpansionoccurswithindoublequotes,eachparameterexpandstoaseparateword.Thatis,"$@"isequivalentto"$1" "$2" .Ifthedoublequotedexpansionoccurswithinaword,theexpansionofthefirstparameterisjoinedwiththebeginningpartoftheoriginalword,andtheexpansionofthelastparameterisjoinedwiththelastpartoftheoriginalword.Whentherearenopositionalparameters,"$@"and$@expandtonothing(i.e.,theyareremoved).

#

($#)Expandstothenumberofpositionalparametersindecimal.

?

($?)Expandstotheexitstatusofthemostrecentlyexecutedforegroundpipeline.

-

($,ahyphen.)Expandstothecurrentoptionflagsasspecifieduponinvocation,bythesetbuiltincommand,orthosesetbytheshellitself(suchasthe-ioption).

$

($$)ExpandstotheprocessIDoftheshell.Ina()subshell,itexpandstotheprocessIDoftheinvokingshell,notthesubshell.

!

($!)ExpandstotheprocessIDofthejobmostrecentlyplacedintothebackground,whetherexecutedasanasynchronouscommandorusingthebgbuiltin(seeJobControlBuiltins).

0

($0)Expandstothenameoftheshellorshellscript.Thisissetatshellinitialization.IfBashisinvokedwithafileofcommands(seeShellScripts),$0issettothenameofthatfile.IfBashisstartedwiththe-coption(seeInvokingBash),then$0issettothefirstargumentafterthestringtobeexecuted,ifoneispresent.Otherwise,itissettothefilenameusedtoinvokeBash,asgivenbyargumentzero.

_

($_,anunderscore.)Atshellstartup,settotheabsolutepathnameusedtoinvoketheshellorshellscriptbeingexecutedaspassedintheenvironmentorargumentlist.Subsequently,expandstothelastargumenttothepreviouscommand,afterexpansion.Alsosettothefullpathnameusedtoinvokeeachcommandexecutedandplacedintheenvironmentexportedtothatcommand.Whencheckingmail,thisparameterholdsthenameofthemailfile.

Next:Redirections,Previous:ShellParameters,Up:BasicShellFeatures[Contents][Index]

3.5ShellExpansions

Expansionisperformedonthecommandlineafterithasbeensplitintotokens.Therearesevenkindsofexpansionperformed:

braceexpansiontildeexpansionparameterandvariableexpansioncommandsubstitution



arithmeticexpansionwordsplittingfilenameexpansion

BraceExpansion: Expansionofexpressionswithinbraces.TildeExpansion: Expansionofthe~character.ShellParameterExpansion:HowBashexpandsvariablestotheirvalues.CommandSubstitution: Usingtheoutputofacommandasanargument.ArithmeticExpansion: Howtousearithmeticinshellexpansions.ProcessSubstitution: Awaytowriteandreadtoandfromacommand.WordSplitting: Howtheresultsofexpansionaresplitintoseparatearguments.FilenameExpansion: Ashorthandforspecifyingfilenamesmatchingpatterns.QuoteRemoval: Howandwhenquotecharactersareremovedfromwords.

Theorderofexpansionsis:braceexpansiontildeexpansion,parameterandvariableexpansion,arithmeticexpansion,andcommandsubstitution(doneinalefttorightfashion)wordsplittingandfilenameexpansion.

Onsystemsthatcansupportit,thereisanadditionalexpansionavailable:processsubstitution.Thisisperformedatthesametimeastilde,parameter,variable,andarithmeticexpansionandcommandsubstitution.

Onlybraceexpansion,wordsplitting,andfilenameexpansioncanchangethenumberofwordsoftheexpansionotherexpansionsexpandasinglewordtoasingleword.Theonlyexceptionstothisaretheexpansionsof"$@"(seeSpecialParameters)and"${name[@]}"(seeArrays).

Afterallexpansions,quote removal(seeQuoteRemoval)isperformed.

Next:TildeExpansion,Up:ShellExpansions[Contents][Index]

3.5.1BraceExpansion

Braceexpansionisamechanismbywhicharbitrarystringsmaybegenerated.Thismechanismissimilartofilenameexpansion(seeFilenameExpansion),butthefilenamesgeneratedneednotexist.Patternstobebraceexpandedtaketheformofanoptionalpreamble,followedbyeitheraseriesofcommaseparatedstringsorasequenceexpressionbetweenapairofbraces,followedbyanoptionalpostscript.Thepreambleisprefixedtoeachstringcontainedwithinthebraces,andthepostscriptisthenappendedtoeachresultingstring,expandinglefttoright.

Braceexpansionsmaybenested.Theresultsofeachexpandedstringarenotsortedlefttorightorderispreserved.Forexample,

bash$ echo a{d,c,b}eade ace abe

Asequenceexpressiontakestheform{x..y[..incr]},wherexandyareeitherintegersorsinglecharacters,andincr,anoptionalincrement,isaninteger.Whenintegersaresupplied,theexpressionexpandstoeachnumberbetweenxandy,inclusive.Suppliedintegersmaybeprefixedwith0toforceeachtermtohavethesamewidth.Wheneitherxorybeginswithazero,theshellattemptstoforceallgeneratedtermstocontainthesamenumberofdigits,zeropaddingwherenecessary.Whencharactersaresupplied,theexpressionexpandstoeachcharacterlexicographicallybetweenxandy,inclusive,usingthedefaultClocale.Notethatbothxandymustbeofthesametype.Whentheincrementissupplied,itisusedasthedifferencebetweeneachterm.Thedefaultincrementis1or1asappropriate.

Braceexpansionisperformedbeforeanyotherexpansions,andanycharactersspecialtootherexpansionsarepreservedintheresult.Itisstrictlytextual.Bashdoesnotapplyanysyntacticinterpretationtothecontextoftheexpansionorthetextbetweenthebraces.Toavoidconflictswithparameterexpansion,thestring${isnotconsideredeligibleforbraceexpansion.

Acorrectlyformedbraceexpansionmustcontainunquotedopeningandclosingbraces,andatleastoneunquotedcommaoravalidsequenceexpression.Anyincorrectlyformedbraceexpansionisleftunchanged.



A{or,maybequotedwithabackslashtopreventitsbeingconsideredpartofabraceexpression.Toavoidconflictswithparameterexpansion,thestring${isnotconsideredeligibleforbraceexpansion.

Thisconstructistypicallyusedasshorthandwhenthecommonprefixofthestringstobegeneratedislongerthanintheaboveexample:

mkdir /usr/local/src/bash/{old,new,dist,bugs}

or

chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

Next:ShellParameterExpansion,Previous:BraceExpansion,Up:ShellExpansions[Contents][Index]

3.5.2TildeExpansion

Ifawordbeginswithanunquotedtildecharacter(~),allofthecharactersuptothefirstunquotedslash(orallcharacters,ifthereisnounquotedslash)areconsideredatildeprefix.Ifnoneofthecharactersinthetildeprefixarequoted,thecharactersinthetildeprefixfollowingthetildearetreatedasapossibleloginname.Ifthisloginnameisthenullstring,thetildeisreplacedwiththevalueoftheHOMEshellvariable.IfHOMEisunset,thehomedirectoryoftheuserexecutingtheshellissubstitutedinstead.Otherwise,thetildeprefixisreplacedwiththehomedirectoryassociatedwiththespecifiedloginname.

Ifthetildeprefixis~+,thevalueoftheshellvariablePWDreplacesthetildeprefix.Ifthetildeprefixis~-,thevalueoftheshellvariableOLDPWD,ifitisset,issubstituted.

IfthecharactersfollowingthetildeinthetildeprefixconsistofanumberN,optionallyprefixedbya+ora-,thetildeprefixisreplacedwiththecorrespondingelementfromthedirectorystack,asitwouldbedisplayedbythedirsbuiltininvokedwiththecharactersfollowingtildeinthetildeprefixasanargument(seeTheDirectoryStack).Ifthetildeprefix,sansthetilde,consistsofanumberwithoutaleading+or-,+isassumed.

Iftheloginnameisinvalid,orthetildeexpansionfails,thewordisleftunchanged.

Eachvariableassignmentischeckedforunquotedtildeprefixesimmediatelyfollowinga:orthefirst=.Inthesecases,tildeexpansionisalsoperformed.Consequently,onemayusefilenameswithtildesinassignmentstoPATH,MAILPATH,andCDPATH,andtheshellassignstheexpandedvalue.

ThefollowingtableshowshowBashtreatsunquotedtildeprefixes:

~

Thevalueof$HOME

~/foo

$HOME/foo

~fred/foo

Thesubdirectoryfooofthehomedirectoryoftheuserfred

~+/foo

$PWD/foo

~-/foo

${OLDPWD-'~-'}/foo

~N



Thestringthatwouldbedisplayedbydirs +N

~+N

Thestringthatwouldbedisplayedbydirs +N

~-N

Thestringthatwouldbedisplayedbydirs -N

Next:CommandSubstitution,Previous:TildeExpansion,Up:ShellExpansions[Contents][Index]

3.5.3ShellParameterExpansion

The$characterintroducesparameterexpansion,commandsubstitution,orarithmeticexpansion.Theparameternameorsymboltobeexpandedmaybeenclosedinbraces,whichareoptionalbutservetoprotectthevariabletobeexpandedfromcharactersimmediatelyfollowingitwhichcouldbeinterpretedaspartofthename.

Whenbracesareused,thematchingendingbraceisthefirst}notescapedbyabackslashorwithinaquotedstring,andnotwithinanembeddedarithmeticexpansion,commandsubstitution,orparameterexpansion.

Thebasicformofparameterexpansionis${parameter}.Thevalueofparameterissubstituted.Theparameterisashellparameterasdescribedabove(seeShellParameters)oranarrayreference(seeArrays).Thebracesarerequiredwhenparameterisapositionalparameterwithmorethanonedigit,orwhenparameterisfollowedbyacharacterthatisnottobeinterpretedaspartofitsname.

Ifthefirstcharacterofparameterisanexclamationpoint(!),itintroducesalevelofvariableindirection.Bashusesthevalueofthevariableformedfromtherestofparameterasthenameofthevariablethisvariableisthenexpandedandthatvalueisusedintherestofthesubstitution,ratherthanthevalueofparameteritself.Thisisknownasindirect expansion.Theexceptionstothisaretheexpansionsof${!prefix*}and${!name[@]}describedbelow.Theexclamationpointmustimmediatelyfollowtheleftbraceinordertointroduceindirection.

Ineachofthecasesbelow,wordissubjecttotildeexpansion,parameterexpansion,commandsubstitution,andarithmeticexpansion.

Whennotperformingsubstringexpansion,usingtheformdescribedbelow(e.g.,:-),Bashtestsforaparameterthatisunsetornull.Omittingthecolonresultsinatestonlyforaparameterthatisunset.Putanotherway,ifthecolonisincluded,theoperatortestsforbothparametersexistenceandthatitsvalueisnotnullifthecolonisomitted,theoperatortestsonlyforexistence.

${parameter:-word}

Ifparameterisunsetornull,theexpansionofwordissubstituted.Otherwise,thevalueofparameterissubstituted.

${parameter:=word}

Ifparameterisunsetornull,theexpansionofwordisassignedtoparameter.Thevalueofparameteristhensubstituted.Positionalparametersandspecialparametersmaynotbeassignedtointhisway.

${parameter:?word}

Ifparameterisnullorunset,theexpansionofword(oramessagetothateffectifwordisnotpresent)iswrittentothestandarderrorandtheshell,ifitisnotinteractive,exits.Otherwise,thevalueofparameterissubstituted.

${parameter:+word}

Ifparameterisnullorunset,nothingissubstituted,otherwisetheexpansionofwordissubstituted.

${parameter:offset}



${parameter:offset:length}

ThisisreferredtoasSubstringExpansion.Itexpandstouptolengthcharactersofthevalueofparameterstartingatthecharacterspecifiedbyoffset.Ifparameteris@,anindexedarraysubscriptedby@or*,oranassociativearrayname,theresultsdifferasdescribedbelow.Iflengthisomitted,itexpandstothesubstringofthevalueofparameterstartingatthecharacterspecifiedbyoffsetandextendingtotheendofthevalue.lengthandoffsetarearithmeticexpressions(seeShellArithmetic).

Ifoffsetevaluatestoanumberlessthanzero,thevalueisusedasanoffsetincharactersfromtheendofthevalueofparameter.Iflengthevaluatestoanumberlessthanzero,itisinterpretedasanoffsetincharactersfromtheendofthevalueofparameterratherthananumberofcharacters,andtheexpansionisthecharactersbetweenoffsetandthatresult.Notethatanegativeoffsetmustbeseparatedfromthecolonbyatleastonespacetoavoidbeingconfusedwiththe:-expansion.

Herearesomeexamplesillustratingsubstringexpansiononparametersandsubscriptedarrays:

$ string=01234567890abcdefgh$ echo ${string:7}7890abcdefgh$ echo ${string:7:0}

$ echo ${string:7:2}78$ echo ${string:7:-2}7890abcdef$ echo ${string: -7}bcdefgh$ echo ${string: -7:0}

$ echo ${string: -7:2}bc$ echo ${string: -7:-2}bcdef$ set -- 01234567890abcdefgh$ echo ${1:7}7890abcdefgh$ echo ${1:7:0}

$ echo ${1:7:2}78$ echo ${1:7:-2}7890abcdef$ echo ${1: -7}bcdefgh$ echo ${1: -7:0}

$ echo ${1: -7:2}bc$ echo ${1: -7:-2}bcdef$ array[0]=01234567890abcdefgh$ echo ${array[0]:7}7890abcdefgh$ echo ${array[0]:7:0}

$ echo ${array[0]:7:2}78$ echo ${array[0]:7:-2}7890abcdef$ echo ${array[0]: -7}bcdefgh$ echo ${array[0]: -7:0}

$ echo ${array[0]: -7:2}bc$ echo ${array[0]: -7:-2}bcdef



Ifparameteris@,theresultislengthpositionalparametersbeginningatoffset.Anegativeoffsetistakenrelativetoonegreaterthanthegreatestpositionalparameter,soanoffsetof1evaluatestothelastpositionalparameter.Itisanexpansionerroriflengthevaluatestoanumberlessthanzero.

Thefollowingexamplesillustratesubstringexpansionusingpositionalparameters:

$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h$ echo ${@:7}7 8 9 0 a b c d e f g h$ echo ${@:7:0}

$ echo ${@:7:2}7 8$ echo ${@:7:-2}bash: -2: substring expression < 0$ echo ${@: -7:2}b c$ echo ${@:0}./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h$ echo ${@:0:2}./bash 1$ echo ${@: -7:0}

Ifparameterisanindexedarraynamesubscriptedby@or*,theresultisthelengthmembersofthearraybeginningwith${parameter[offset]}.Anegativeoffsetistakenrelativetoonegreaterthanthemaximumindexofthespecifiedarray.Itisanexpansionerroriflengthevaluatestoanumberlessthanzero.

Theseexamplesshowhowyoucanusesubstringexpansionwithindexedarrays:

$ array=(0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h)$ echo ${array[@]:7}7 8 9 0 a b c d e f g h$ echo ${array[@]:7:2}7 8$ echo ${array[@]: -7:2}b c$ echo ${array[@]: -7:-2}bash: -2: substring expression < 0$ echo ${array[@]:0}0 1 2 3 4 5 6 7 8 9 0 a b c d e f g h$ echo ${array[@]:0:2}0 1$ echo ${array[@]: -7:0}

Substringexpansionappliedtoanassociativearrayproducesundefinedresults.

Substringindexingiszerobasedunlessthepositionalparametersareused,inwhichcasetheindexingstartsat1bydefault.Ifoffsetis0,andthepositionalparametersareused,$@isprefixedtothelist.

${!prefix*}${!prefix@}

Expandstothenamesofvariableswhosenamesbeginwithprefix,separatedbythefirstcharacteroftheIFSspecialvariable.When@isusedandtheexpansionappearswithindoublequotes,eachvariablenameexpandstoaseparateword.

${!name[@]}${!name[*]}

Ifnameisanarrayvariable,expandstothelistofarrayindices(keys)assignedinname.Ifnameisnotanarray,expandsto0ifnameissetandnullotherwise.When@isusedandtheexpansionappearswithindoublequotes,eachkeyexpandstoaseparateword.



${#parameter}

Thelengthincharactersoftheexpandedvalueofparameterissubstituted.Ifparameteris*or@,thevaluesubstitutedisthenumberofpositionalparameters.Ifparameterisanarraynamesubscriptedby*or@,thevaluesubstitutedisthenumberofelementsinthearray.Ifparameterisanindexedarraynamesubscriptedbyanegativenumber,thatnumberisinterpretedasrelativetoonegreaterthanthemaximumindexofparameter,sonegativeindicescountbackfromtheendofthearray,andanindexof1referencesthelastelement.

${parameter#word}${parameter##word}

Thewordisexpandedtoproduceapatternjustasinfilenameexpansion(seeFilenameExpansion).Ifthepatternmatchesthebeginningoftheexpandedvalueofparameter,thentheresultoftheexpansionistheexpandedvalueofparameterwiththeshortestmatchingpattern(the#case)orthelongestmatchingpattern(the##case)deleted.Ifparameteris@or*,thepatternremovaloperationisappliedtoeachpositionalparameterinturn,andtheexpansionistheresultantlist.Ifparameterisanarrayvariablesubscriptedwith@or*,thepatternremovaloperationisappliedtoeachmemberofthearrayinturn,andtheexpansionistheresultantlist.

${parameter%word}${parameter%%word}

Thewordisexpandedtoproduceapatternjustasinfilenameexpansion.Ifthepatternmatchesatrailingportionoftheexpandedvalueofparameter,thentheresultoftheexpansionisthevalueofparameterwiththeshortestmatchingpattern(the%case)orthelongestmatchingpattern(the%%case)deleted.Ifparameteris@or*,thepatternremovaloperationisappliedtoeachpositionalparameterinturn,andtheexpansionistheresultantlist.Ifparameterisanarrayvariablesubscriptedwith@or*,thepatternremovaloperationisappliedtoeachmemberofthearrayinturn,andtheexpansionistheresultantlist.

${parameter/pattern/string}

Thepatternisexpandedtoproduceapatternjustasinfilenameexpansion.Parameterisexpandedandthelongestmatchofpatternagainstitsvalueisreplacedwithstring.Ifpatternbeginswith/,allmatchesofpatternarereplacedwithstring.Normallyonlythefirstmatchisreplaced.Ifpatternbeginswith#,itmustmatchatthebeginningoftheexpandedvalueofparameter.Ifpatternbeginswith%,itmustmatchattheendoftheexpandedvalueofparameter.Ifstringisnull,matchesofpatternaredeletedandthe/followingpatternmaybeomitted.Ifparameteris@or*,thesubstitutionoperationisappliedtoeachpositionalparameterinturn,andtheexpansionistheresultantlist.Ifparameterisanarrayvariablesubscriptedwith@or*,thesubstitutionoperationisappliedtoeachmemberofthearrayinturn,andtheexpansionistheresultantlist.

${parameterpattern}${parameter pattern}${parameter,pattern}${parameter,,pattern}

Thisexpansionmodifiesthecaseofalphabeticcharactersinparameter.Thepatternisexpandedtoproduceapatternjustasinfilenameexpansion.Eachcharacterintheexpandedvalueofparameteristestedagainstpattern,and,ifitmatchesthepattern,itscaseisconverted.Thepatternshouldnotattempttomatchmorethanonecharacter.The operatorconvertslowercaselettersmatchingpatterntouppercasethe,operatorconvertsmatchinguppercaseletterstolowercase.The^ and,,expansionsconverteachmatchedcharacterintheexpandedvaluethe and,expansionsmatchandconvertonlythefirstcharacterintheexpandedvalue.Ifpatternisomitted,itistreatedlikea?,whichmatcheseverycharacter.Ifparameteris@or*,thecasemodificationoperationisappliedtoeachpositionalparameterinturn,andtheexpansionistheresultantlist.Ifparameterisanarrayvariablesubscriptedwith@or*,thecasemodificationoperationisappliedtoeachmemberofthearrayinturn,andtheexpansionistheresultantlist.

Next:ArithmeticExpansion,Previous:ShellParameterExpansion,Up:ShellExpansions[Contents][Index]

3.5.4CommandSubstitution

Commandsubstitutionallowstheoutputofacommandtoreplacethecommanditself.Commandsubstitutionoccurswhena



commandisenclosedasfollows:

$(command)

or

command

Bashperformstheexpansionbyexecutingcommandandreplacingthecommandsubstitutionwiththestandardoutputofthecommand,withanytrailingnewlinesdeleted.Embeddednewlinesarenotdeleted,buttheymayberemovedduringwordsplitting.Thecommandsubstitution$(cat file)canbereplacedbytheequivalentbutfaster$(< file).

Whentheoldstylebackquoteformofsubstitutionisused,backslashretainsitsliteralmeaningexceptwhenfollowedby$, ,or\.Thefirstbackquotenotprecededbyabackslashterminatesthecommandsubstitution.Whenusingthe$(command)form,allcharactersbetweentheparenthesesmakeupthecommandnonearetreatedspecially.

Commandsubstitutionsmaybenested.Tonestwhenusingthebackquotedform,escapetheinnerbackquoteswithbackslashes.

Ifthesubstitutionappearswithindoublequotes,wordsplittingandfilenameexpansionarenotperformedontheresults.

Next:ProcessSubstitution,Previous:CommandSubstitution,Up:ShellExpansions[Contents][Index]

3.5.5ArithmeticExpansion

Arithmeticexpansionallowstheevaluationofanarithmeticexpressionandthesubstitutionoftheresult.Theformatforarithmeticexpansionis:

$(( expression ))

Theexpressionistreatedasifitwerewithindoublequotes,butadoublequoteinsidetheparenthesesisnottreatedspecially.Alltokensintheexpressionundergoparameterandvariableexpansion,commandsubstitution,andquoteremoval.Theresultistreatedasthearithmeticexpressiontobeevaluated.Arithmeticexpansionsmaybenested.

Theevaluationisperformedaccordingtotheruleslistedbelow(seeShellArithmetic).Iftheexpressionisinvalid,Bashprintsamessageindicatingfailuretothestandarderrorandnosubstitutionoccurs.

Next:WordSplitting,Previous:ArithmeticExpansion,Up:ShellExpansions[Contents][Index]

3.5.6ProcessSubstitution

Processsubstitutionissupportedonsystemsthatsupportnamedpipes(FIFOs)orthe/dev/fdmethodofnamingopenfiles.Ittakestheformof

(list)

TheprocesslistisrunwithitsinputoroutputconnectedtoaFIFOorsomefilein/dev/fd.Thenameofthisfileispassedasanargumenttothecurrentcommandastheresultoftheexpansion.Ifthe>(list)formisused,writingtothefilewillprovideinputforlist.Ifthe



Next:FilenameExpansion,Previous:ProcessSubstitution,Up:ShellExpansions[Contents][Index]

3.5.7WordSplitting

Theshellscanstheresultsofparameterexpansion,commandsubstitution,andarithmeticexpansionthatdidnotoccurwithindoublequotesforwordsplitting.

Theshelltreatseachcharacterof$IFSasadelimiter,andsplitstheresultsoftheotherexpansionsintowordsusingthesecharactersasfieldterminators.IfIFSisunset,oritsvalueisexactly,thedefault,thensequencesof,,andatthebeginningandendoftheresultsofthepreviousexpansionsareignored,andanysequenceofIFScharactersnotatthebeginningorendservestodelimitwords.IfIFShasavalueotherthanthedefault,thensequencesofthewhitespacecharactersspaceandtabareignoredatthebeginningandendoftheword,aslongasthewhitespacecharacterisinthevalueofIFS(anIFSwhitespacecharacter).AnycharacterinIFSthatisnotIFSwhitespace,alongwithanyadjacentIFSwhitespacecharacters,delimitsafield.AsequenceofIFSwhitespacecharactersisalsotreatedasadelimiter.IfthevalueofIFSisnull,nowordsplittingoccurs.

Explicitnullarguments(""or'')areretained.Unquotedimplicitnullarguments,resultingfromtheexpansionofparametersthathavenovalues,areremoved.Ifaparameterwithnovalueisexpandedwithindoublequotes,anullargumentresultsandisretained.

Notethatifnoexpansionoccurs,nosplittingisperformed.

Next:QuoteRemoval,Previous:WordSplitting,Up:ShellExpansions[Contents][Index]

3.5.8FilenameExpansion

PatternMatching:Howtheshellmatchespatterns.

Afterwordsplitting,unlessthe-foptionhasbeenset(seeTheSetBuiltin),Bashscanseachwordforthecharacters*,?,and[.Ifoneofthesecharactersappears,thenthewordisregardedasapattern,andreplacedwithanalphabeticallysortedlistoffilenamesmatchingthepattern(seePatternMatching).Ifnomatchingfilenamesarefound,andtheshelloptionnullglobisdisabled,thewordisleftunchanged.Ifthenullgloboptionisset,andnomatchesarefound,thewordisremoved.Ifthefailglobshelloptionisset,andnomatchesarefound,anerrormessageisprintedandthecommandisnotexecuted.Iftheshelloptionnocaseglobisenabled,thematchisperformedwithoutregardtothecaseofalphabeticcharacters.

Whenapatternisusedforfilenameexpansion,thecharacter.atthestartofafilenameorimmediatelyfollowingaslashmustbematchedexplicitly,unlesstheshelloptiondotglobisset.Whenmatchingafilename,theslashcharactermustalwaysbematchedexplicitly.Inothercases,the.characterisnottreatedspecially.

SeethedescriptionofshoptinTheShoptBuiltin,foradescriptionofthenocaseglob,nullglob,failglob,anddotgloboptions.

TheGLOBIGNOREshellvariablemaybeusedtorestrictthesetoffilenamesmatchingapattern.IfGLOBIGNOREisset,eachmatchingfilenamethatalsomatchesoneofthepatternsinGLOBIGNOREisremovedfromthelistofmatches.Thefilenames.and..arealwaysignoredwhenGLOBIGNOREissetandnotnull.However,settingGLOBIGNOREtoanonnullvaluehastheeffectofenablingthedotglobshelloption,soallotherfilenamesbeginningwitha.willmatch.Togettheoldbehaviorofignoringfilenamesbeginningwitha.,make.*oneofthepatternsinGLOBIGNORE.ThedotgloboptionisdisabledwhenGLOBIGNOREisunset.

Up:FilenameExpansion[Contents][Index]

3.5.8.1PatternMatching

Anycharacterthatappearsinapattern,otherthanthespecialpatterncharactersdescribedbelow,matchesitself.TheNUL



charactermaynotoccurinapattern.Abackslashescapesthefollowingcharactertheescapingbackslashisdiscardedwhenmatching.Thespecialpatterncharactersmustbequotediftheyaretobematchedliterally.

Thespecialpatterncharactershavethefollowingmeanings:

*

Matchesanystring,includingthenullstring.Whentheglobstarshelloptionisenabled,and*isusedinafilenameexpansioncontext,twoadjacent*susedasasinglepatternwillmatchallfilesandzeroormoredirectoriesandsubdirectories.Iffollowedbya/,twoadjacent*swillmatchonlydirectoriesandsubdirectories.

?

Matchesanysinglecharacter.

[]

Matchesanyoneoftheenclosedcharacters.Apairofcharactersseparatedbyahyphendenotesarangeexpressionanycharacterthatfallsbetweenthosetwocharacters,inclusive,usingthecurrentlocalescollatingsequenceandcharacterset,ismatched.Ifthefirstcharacterfollowingthe[isa!ora thenanycharacternotenclosedismatched.A-maybematchedbyincludingitasthefirstorlastcharacterintheset.A]maybematchedbyincludingitasthefirstcharacterintheset.ThesortingorderofcharactersinrangeexpressionsisdeterminedbythecurrentlocaleandthevaluesoftheLC_COLLATEandLC_ALLshellvariables,ifset.

Forexample,inthedefaultClocale,[a-dx-z]isequivalentto[abcdxyz].Manylocalessortcharactersindictionaryorder,andintheselocales[a-dx-z]istypicallynotequivalentto[abcdxyz]itmightbeequivalentto[aBbCcDdxXyYz],forexample.Toobtainthetraditionalinterpretationofrangesinbracketexpressions,youcanforcetheuseoftheClocalebysettingtheLC_COLLATEorLC_ALLenvironmentvariabletothevalueC,orenabletheglobasciirangesshelloption.

Within[and],characterclassescanbespecifiedusingthesyntax[:class:],whereclassisoneofthefollowingclassesdefinedinthePOSIXstandard:

alnum alpha ascii blank cntrl digit graph lowerprint punct space upper word xdigit

Acharacterclassmatchesanycharacterbelongingtothatclass.Thewordcharacterclassmatchesletters,digits,andthecharacter_.

Within[and],anequivalenceclasscanbespecifiedusingthesyntax[=c=],whichmatchesallcharacterswiththesamecollationweight(asdefinedbythecurrentlocale)asthecharacterc.

Within[and],thesyntax[.symbol.]matchesthecollatingsymbolsymbol.

Iftheextglobshelloptionisenabledusingtheshoptbuiltin,severalextendedpatternmatchingoperatorsarerecognized.Inthefollowingdescription,apatternlistisalistofoneormorepatternsseparatedbya|.Compositepatternsmaybeformedusingoneormoreofthefollowingsubpatterns:

?(pattern-list)

Matcheszerooroneoccurrenceofthegivenpatterns.

*(pattern-list)

Matcheszeroormoreoccurrencesofthegivenpatterns.

+(pattern-list)

Matchesoneormoreoccurrencesofthegivenpatterns.

@(pattern-list)



Matchesoneofthegivenpatterns.

!(pattern-list)

Matchesanythingexceptoneofthegivenpatterns.

Previous:FilenameExpansion,Up:ShellExpansions[Contents][Index]

3.5.9QuoteRemoval

Aftertheprecedingexpansions,allunquotedoccurrencesofthecharacters\,',and"thatdidnotresultfromoneoftheaboveexpansionsareremoved.

Next:ExecutingCommands,Previous:ShellExpansions,Up:BasicShellFeatures[Contents][Index]

3.6Redirections

Beforeacommandisexecuted,itsinputandoutputmayberedirectedusingaspecialnotationinterpretedbytheshell.Redirectionallowscommandsfilehandlestobeduplicated,opened,closed,madetorefertodifferentfiles,andcanchangethefilesthecommandreadsfromandwritesto.Redirectionmayalsobeusedtomodifyfilehandlesinthecurrentshellexecutionenvironment.Thefollowingredirectionoperatorsmayprecedeorappearanywherewithinasimplecommandormayfollowacommand.Redirectionsareprocessedintheordertheyappear,fromlefttoright.

Eachredirectionthatmaybeprecededbyafiledescriptornumbermayinsteadbeprecededbyawordoftheform{varname}.Inthiscase,foreachredirectionoperatorexcept>&and&or dirlist 2>&1

directsbothstandardoutput(filedescriptor1)andstandarderror(filedescriptor2)tothefiledirlist,whilethecommand

ls 2>&1 > dirlist

directsonlythestandardoutputtofiledirlist,becausethestandarderrorwasmadeacopyofthestandardoutputbeforethestandardoutputwasredirectedtodirlist.

Bashhandlesseveralfilenamesspeciallywhentheyareusedinredirections,asdescribedinthefollowingtable:

/dev/fd/fd

Iffdisavalidinteger,filedescriptorfdisduplicated.

/dev/stdin

Filedescriptor0isduplicated.

/dev/stdout




/dev/stderr


/dev/tcp/host/port

IfhostisavalidhostnameorInternetaddress,andportisanintegerportnumberorservicename,BashattemptstoopenthecorrespondingTCPsocket.

/dev/udp/host/port

IfhostisavalidhostnameorInternetaddress,andportisanintegerportnumberorservicename,BashattemptstoopenthecorrespondingUDPsocket.

Afailuretoopenorcreateafilecausestheredirectiontofail.

Redirectionsusingfiledescriptorsgreaterthan9shouldbeusedwithcare,astheymayconflictwithfiledescriptorstheshellusesinternally.

3.6.1RedirectingInput

Redirectionofinputcausesthefilewhosenameresultsfromtheexpansionofwordtobeopenedforreadingonfiledescriptorn,orthestandardinput(filedescriptor0)ifnisnotspecified.

Thegeneralformatforredirectinginputis:

[n][|]word

Iftheredirectionoperatoris>,andthenoclobberoptiontothesetbuiltinhasbeenenabled,theredirectionwillfailifthefilewhosenameresultsfromtheexpansionofwordexistsandisaregularfile.Iftheredirectionoperatoris>|,ortheredirectionoperatoris>andthenoclobberoptionisnotenabled,theredirectionisattemptedevenifthefilenamedbywordexists.

3.6.3AppendingRedirectedOutput

Redirectionofoutputinthisfashioncausesthefilewhosenameresultsfromtheexpansionofwordtobeopenedforappendingonfiledescriptorn,orthestandardoutput(filedescriptor1)ifnisnotspecified.Ifthefiledoesnotexistitiscreated.

Thegeneralformatforappendingoutputis:

[n]>>word

3.6.4RedirectingStandardOutputandStandardError

Thisconstructallowsboththestandardoutput(filedescriptor1)andthestandarderroroutput(filedescriptor2)toberedirectedtothefilewhosenameistheexpansionofword.

Therearetwoformatsforredirectingstandardoutputandstandarderror:



&>word

and

>&word

Ofthetwoforms,thefirstispreferred.Thisissemanticallyequivalentto

>word 2>&1

Whenusingthesecondform,wordmaynotexpandtoanumberor-.Ifitdoes,otherredirectionoperatorsapply(seeDuplicatingFileDescriptorsbelow)forcompatibilityreasons.

3.6.5AppendingStandardOutputandStandardError

Thisconstructallowsboththestandardoutput(filedescriptor1)andthestandarderroroutput(filedescriptor2)tobeappendedtothefilewhosenameistheexpansionofword.

Theformatforappendingstandardoutputandstandarderroris:

&>>word

Thisissemanticallyequivalentto

>>word 2>&1

(seeDuplicatingFileDescriptorsbelow).

3.6.6HereDocuments

Thistypeofredirectioninstructstheshelltoreadinputfromthecurrentsourceuntilalinecontainingonlyword(withnotrailingblanks)isseen.Allofthelinesreaduptothatpointarethenusedasthestandardinputforacommand.

Theformatofheredocumentsis:



Theredirectionoperator

[n]&word

isusedsimilarlytoduplicateoutputfiledescriptors.Ifnisnotspecified,thestandardoutput(filedescriptor1)isused.Ifthedigitsinworddonotspecifyafiledescriptoropenforoutput,aredirectionerroroccurs.Ifwordevaluatesto-,filedescriptornisclosed.Asaspecialcase,ifnisomitted,andworddoesnotexpandtooneormoredigitsor-,thestandardoutputandstandarderrorareredirectedasdescribedpreviously.

3.6.9MovingFileDescriptors


[n]&digit-

movesthefiledescriptordigittofiledescriptorn,orthestandardoutput(filedescriptor1)ifnisnotspecified.

3.6.10OpeningFileDescriptorsforReadingandWriting


[n]word

causesthefilewhosenameistheexpansionofwordtobeopenedforbothreadingandwritingonfiledescriptorn,oronfiledescriptor0ifnisnotspecified.Ifthefiledoesnotexist,itiscreated.

Next:ShellScripts,Previous:Redirections,Up:BasicShellFeatures[Contents][Index]

3.7ExecutingCommands

SimpleCommandExpansion: HowBashexpandssimplecommandsbeforeexecutingthem.CommandSearchandExecution: HowBashfindscommandsandrunsthem.CommandExecutionEnvironment:TheenvironmentinwhichBashexecutescommandsthatarenotshellbuiltins.Environment: Theenvironmentgiventoacommand.ExitStatus: ThestatusreturnedbycommandsandhowBashinterpretsit.Signals: WhathappenswhenBashoracommanditrunsreceivesasignal.

Next:CommandSearchandExecution,Up:ExecutingCommands[Contents][Index]

3.7.1SimpleCommandExpansion



Whenasimplecommandisexecuted,theshellperformsthefollowingexpansions,assignments,andredirections,fromlefttoright.

1. Thewordsthattheparserhasmarkedasvariableassignments(thoseprecedingthecommandname)andredirectionsaresavedforlaterprocessing.

2. Thewordsthatarenotvariableassignmentsorredirectionsareexpanded(seeShellExpansions).Ifanywordsremainafterexpansion,thefirstwordistakentobethenameofthecommandandtheremainingwordsarethearguments.

3. Redirectionsareperformedasdescribedabove(seeRedirections).4. Thetextafterthe=ineachvariableassignmentundergoestildeexpansion,parameterexpansion,commandsubstitution,

arithmeticexpansion,andquoteremovalbeforebeingassignedtothevariable.

Ifnocommandnameresults,thevariableassignmentsaffectthecurrentshellenvironment.Otherwise,thevariablesareaddedtotheenvironmentoftheexecutedcommandanddonotaffectthecurrentshellenvironment.Ifanyoftheassignmentsattemptstoassignavaluetoareadonlyvariable,anerroroccurs,andthecommandexitswithanonzerostatus.

Ifnocommandnameresults,redirectionsareperformed,butdonotaffectthecurrentshellenvironment.Aredirectionerrorcausesthecommandtoexitwithanonzerostatus.

Ifthereisacommandnameleftafterexpansion,executionproceedsasdescribedbelow.Otherwise,thecommandexits.Ifoneoftheexpansionscontainedacommandsubstitution,theexitstatusofthecommandistheexitstatusofthelastcommandsubstitutionperformed.Iftherewerenocommandsubstitutions,thecommandexitswithastatusofzero.

Next:CommandExecutionEnvironment,Previous:SimpleCommandExpansion,Up:ExecutingCommands[Contents][Index]

3.7.2CommandSearchandExecution

Afteracommandhasbeensplitintowords,ifitresultsinasimplecommandandanoptionallistofarguments,thefollowingactionsaretaken.

1. Ifthecommandnamecontainsnoslashes,theshellattemptstolocateit.Ifthereexistsashellfunctionbythatname,thatfunctionisinvokedasdescribedinShellFunctions.

2. Ifthenamedoesnotmatchafunction,theshellsearchesforitinthelistofshellbuiltins.Ifamatchisfound,thatbuiltinisinvoked.

3. Ifthenameisneitherashellfunctionnorabuiltin,andcontainsnoslashes,Bashsearcheseachelementof$PATHforadirectorycontaininganexecutablefilebythatname.BashusesahashtabletorememberthefullpathnamesofexecutablefilestoavoidmultiplePATHsearches(seethedescriptionofhashinBourneShellBuiltins).Afullsearchofthedirectoriesin$PATHisperformedonlyifthecommandisnotfoundinthehashtable.Ifthesearchisunsuccessful,theshellsearchesforadefinedshellfunctionnamedcommand_not_found_handle.Ifthatfunctionexists,itisinvokedwiththeoriginalcommandandtheoriginalcommandsargumentsasitsarguments,andthefunctionsexitstatusbecomestheexitstatusoftheshell.Ifthatfunctionisnotdefined,theshellprintsanerrormessageandreturnsanexitstatusof127.

4. Ifthesearchissuccessful,orifthecommandnamecontainsoneormoreslashes,theshellexecutesthenamedprograminaseparateexecutionenvironment.Argument0issettothenamegiven,andtheremainingargumentstothecommandaresettotheargumentssupplied,ifany.

5. Ifthisexecutionfailsbecausethefileisnotinexecutableformat,andthefileisnotadirectory,itisassumedtobeashellscriptandtheshellexecutesitasdescribedinShellScripts.

6. Ifthecommandwasnotbegunasynchronously,theshellwaitsforthecommandtocompleteandcollectsitsexitstatus.

Next:Environment,Previous:CommandSearchandExecution,Up:ExecutingCommands[Contents][Index]

3.7.3CommandExecutionEnvironment

Theshellhasanexecutionenvironment,whichconsistsofthefollowing:

openfilesinheritedbytheshellatinvocation,asmodifiedbyredirectionssuppliedtotheexecbuiltinthecurrentworkingdirectoryassetbycd,pushd,orpopd,orinheritedbytheshellatinvocationthefilecreationmodemaskassetbyumaskorinheritedfromtheshellsparent