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Thisproblemiseasilysolvedbyoverridingtheenvironmentvariable

PATH

onaper-executionbasis.Consider

thefollowingscript:

#!/bin/sh

echo $MYVAR

Thisscriptprintsthevalueofthevariable

MYVAR

.Normally,thisvariableisempty,sothisscriptjustprintsa

blankline.Savethescriptas

printmyvar.sh

,thentypethefollowingcommands:

chmod a+x printmyvar.sh

# makes the script executable

MYVAR=7 ./printmyvar.sh

# runs the script

echo "MYVAR IS $MYVAR"

# prints the variable

Noticethattheassignmentstatement

MYVAR=7

applies

only

tothecommandthatfollowsit.Thevalueof

MYVAR

isalteredintheenvironmentofthecommand

./printmyvar.sh

,sothescriptprintsthenumber7.

However,theoriginal(empty)valueisrestoredafterexecutingthatcommand,sotheechostatementafterwards

printsanemptystringforthevalueof

MYVAR

Thus,tomodifythe

PATH

variablelocallybutexecuteacommandwiththeoriginal

PATH

value,youcanwrite

ascriptlikethis:

#!/bin/sh

GLOBAL_PATH="$PATH"

PATH=/usr/local/bin

PATH="$GLOBAL_PATH" /bin/ls

UsingthesetenvBuiltin(Cshell)

IntheCshell,variablesareexportedifyousetthemwith

setenv

,butnotifyousetthemwith

set

.Thus,if

youwantyourshellvariablemodificationstobeseenbyanytoolorscriptthatyoucall,youshouldusethe

setenv

builtin.ThisbuiltinistheCshellequivalenttoissuinganassignmentstatementwiththe

export

builtinintheBourneshell.

setenv VALUE "Four"

echo "VALUE is '$VALUE'."

ShellScriptBasics

ExportingShellVariables

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Ifyouwantyourshellvariablestoonlybeavailabletoyourscript,youshouldusethe

set

builtin(described

inShellVariablesandPrinting (page 23)).The

set

builtinisequivalenttoasimpleassignmentstatementin

theBourneshell.

set VALUE = "Four"

echo "VALUE is '$VALUE'."

Noticethatthelocalvariableversionrequiresanequalssign(

),buttheexportedenvironmentversiondoes

not(andproducesanerrorifyouputonein).

ToremovevariablesintheCshell,youcanusethe

unsetenv

unset

builtin.Forexample:

setenv VALUE "Four"

unsetenv VALUE

set VALUE = "Four"

unset VALUE

echo "VALUE is '$VALUE'."

Thiswillgenerateanerrormessage.IntheCshell,itisnotpossibletoprintthevalueofanundefinedvariable,

soifyouthinkyoumayneedtoprintthevaluelater,youshouldsetittoanemptystringratherthanusing

unset

unsetenv

Ifyouneedtotestanenvironmentvariable(notashell-localvariable)thatmayormaynotbepartofyour

environment(avariablesetbywhateverprocesscalledyourscript),youcanusethe

printenv

builtin.This

printsthevalueofavariableifset,butprintsnothingifthevariableisnotset,andthusbehavesjustlikethe

variablebehavesintheBourneshell.

Forexample:

set X = `printenv VALUE`

echo "X is "\"$X\"

Thisprints

X is ""

ifthevariableiseitheremptyorundefined.Otherwise,itprintsthevalueofthevariable

betweenthequotationmarks.

Ifyouneedtofindoutifavariableissimplyemptyorisactuallynotset,youcanalsouse

printenv

toobtain

acompletelistofdefinedvariablesanduse

grep

toseeifitisinthelist.Forexample:

ShellScriptBasics

ExportingShellVariables

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set DEFINED = `printenv | grep -c '^VARIABLE='`

Theresultingvariablewillcontain1ifthevariableisdefinedintheenvironmentor0ifitisnot.

OverridingEnvironmentVariablesforChildProcesses(CShell)

UnliketheBourneshell,theCshelldoesnotprovideabuilt-insyntaxforoverridingenvironmentvariables

whenexecutingexternalcommands.However,itispossibletosimulatethiseitherbyusingthe

env

command.

Thebestandsimplestwaytodothisiswiththe

env

command.Forexample:

env PATH="/usr/local/bin" /bin/ls

Asanalternative,youcanusethe

set

builtintomakeatemporarycopyofanyvariableyouneedtooverride,

changethevalue,executethecommand,andrestorethevaluefromthetemporarycopy.

Youshouldnotice,however,thatwhetheryouusethe

env

commandormanuallymakeacopy,the

PATH

variableisalteredpriortosearchingforthecommand.Becausethe

PATH

variablecontrolswheretheshell

looksforprogramstoexecute,youmustthereforeexplicitlyprovideacompletepathtothe

commandor

itwillnotbefound(unlessyouhaveacopyin

/usr/local/bin

,ofcourse).The

PATH

environmentvariable

isexplainedinSpecialShellVariables (page 265).

Asaworkaround,youcandeterminethepathoftheexecutableusingthe

which

commandpriortoaltering

the

PATH

environmentvariable.

set GLOBAL_PATH = "$PATH"

set LS = `which ls`

setenv PATH "/usr/local/bin"

$LS

setenv PATH "$GLOBAL_PATH"

unset GLOBAL_PATH

Or,using

env

set LS = `which ls`

env PATH='/usr/local/bin' $LS

Theuseofthebacktick(`)operatorinthisfashionisdescribedinInlineExecution (page 68).

ShellScriptBasics

ExportingShellVariables

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Security Note: Ifyourpurposeforoverridinganenvironmentvariableistopreventdisclosureof

sensitiveinformationtoapotentiallyuntrustedprocess,youshouldbeawarethatifyouuse

setenv

forthecopy,thecalledprocesshasaccesstothattemporarycopyjustasithadaccesstotheoriginal

variable.Toavoidthis,besuretocreatethetemporarycopyusingthe

set

builtininsteadof

setenv

DeletingShellVariables

Forthemostpart,inBourneshellscripts,whenyouneedtogetridofavariable,settingittoanemptystring

issufficient.However,inlong-runningscriptsthatmightencountermemorypressure,itcanbemarginally

usefultodeletethevariableentirely.Todothis,usethe

unset

builtin.

Forexample:

MYVAR="this is a test"

unset MYVAR

echo "MYVAR IS \"$MYVAR\""

The

unset

builtincanalsobeusedtodeleteenvironmentvariables.

C Shell Note: TheCshell

unset

builtinisidenticalexceptthatitcannotbeusedtodelete

environmentvariables.Use

unsetenv

instead,asshowninOverridingEnvironmentVariablesfor

ChildProcesses(CShell) (page 33).

Also,inCshell,ifyoutrytouseadeletedvariable,itisconsideredanerror.(InBourneshell,anunset

variableistreatedlikeanemptystring.)

ShellScriptBasics

DeletingShellVariables

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TheBourneshellprovidesanumberofwaystoreadandwritefiles,displaytext,andgetinformationfromthe

user,including

echo

(describedpreviouslyinShellScriptBasics (page 21)),

printf

read

cat

,pipes,and

redirection.Thischapterdescribesthesemechanisms.

ShellScriptInputandOutputUsingprintfandread

TheBourneshellsyntaxprovidesbasicinputwithverylittleeffort.

#!/bin/sh

printf "What is your name?

-> "

read NAME

echo "Hello, $NAME.

Nice to meet you."

Youwillnoticetwothingsaboutthisscript.Thefirstisthatitintroducesthe

printf

command.Thiscommand

isusedbecause,unlike

echo

,the

printf

commanddoesnotautomaticallyaddanewlinetotheendofthe

lineofoutput.Thisbehaviorisusefulwhenyouneedtousemultiplelinesofcodetooutputasinglelineof

text.Italsojusthappenstobehandyforprompts.

Note: Inmostoperatingsystems,youcantell

echo

tosuppressthenewline.However,thesyntax

fordoingsovaries.Thus,

printf

isrecommendedforprintingprompts.SeeDesigningScriptsfor

Cross-PlatformDeployment (page 146)formoreinformationandotheralternatives.

Thesecondthingyou'llnoticeisthe

read

command.Thiscommandtakesalineofinputandseparatesitinto

aseriesofarguments.Eachoftheseargumentsisassignedtothevariablesinthe

read

statementintheorder

ofappearance.Anyadditionalinputfieldsareappendedtothelastentry.

Youcanmodifythebehaviorofthe

read

commandbymodifyingtheshellvariable

IFS

(shortforinternal

fieldseparators).Thedefaultbehavioristosplitinputseverywherethereisaspace,tab,ornewline.Bychanging

thisvariable,youcanmaketheshellsplittheinputfieldsbytabs,newlines,semicolons,oreventheletter'q'.

Thischangeinbehaviorisdemonstratedinthefollowingexample:

Shell Input and Output

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#!/bin/sh

printf "Type three numbers separated by 'q'. -> "

IFS="q"

read NUMBER1 NUMBER2 NUMBER3

echo "You said: $NUMBER1, $NUMBER2, $NUMBER3"

If,forexample,yourunthisscriptandenter

1q3q57q65

,thescriptreplieswith

You said: 1, 3, 57q65

Thethirdvaluecontains

57q65

becauseonlythreevaluesarerequestedinthereadstatement.

Note: The

read

statementalwaysstopsreadingatthefirstnewlineencountered.Thus,ifyouset

IFS

toanewline,youcannotreadmultipleentrieswithasingle

read

statement.

Warning: Changing

IFS

maycauseunexpectedconsequencesforvariableexpansion.Formore

information,seeVariableExpansionandFieldSeparators (page 62).

Butwhatifyoudon’tknowhowmanyparameterstheuserwillspecify?Obviously,asinglereadstatement

cannotsplittheinputupintoanarbitrarynumberofvariables,andtheBourneshelldoesnotcontaintrue

arrays.Fortunately,the

eval

builtincanbeusedtosimulateanarrayusingmultipleshellvariables.This

techniqueisdescribedinUsingtheevalBuiltinforDataStructures,Arrays,andIndirection (page 168).

Alternatively,youcanusethe

for

statement,whichsplitsasinglevariableintomultiplepiecesbasedonthe

internalfieldseparators.ThisstatementisdescribedinTheforStatement (page 52).

ShellInputandOutput

ShellScriptInputandOutputUsingprintfandread

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C Shell Note: IntheCshell,thesyntaxforreadingiscompletelydifferent.Thefollowingscriptis

theCshellequivalentofthescriptearlierinthissection:

printf "What is your name? -> "

set NAME = "$<"

echo "Hello, $NAME.

Nice to meet you."

TheCshelldoesnotprovideawaytoreadmultiplevaluesinasinglecommand,thoughyoucan

approximatethiswithcarefuluseof

sed

asdescribedinRegularExpressionsUnfettered (page 100)

cut

.Forexample:

#!/bin/csh

printf "Type three numbers separated by 'q'. -> "

set LINE = "$<"

set NUMBER1 = `echo "$LINE" | cut -f 1 -d 'q'`

set NUMBER2 = `echo "$LINE" | cut -f 2 -d 'q'`

set NUMBER3 = `echo "$LINE" | cut -f 3 -d 'q'`

echo "You said: $NUMBER1, $NUMBER2, $NUMBER3"

BulkI/OUsingthecatCommand

ForsmallI/O,the

echo

commandiswellsuited.However,whenyouneedtocreatelargeamountsofdata,it

maybeconvenienttosendmultiplelinestoafilesimultaneously.Forthesepurposes,the

cat

commandcan

beparticularlyuseful.

Byitself,the

cat

commandreallydoesn’tdoanythingthatcan’tbedoneusingredirectoperators(exceptfor

printingthecontentsofafiletotheuser’sscreen).However,bycombiningitwiththespecialoperator

,you

canuseittosendalargequantityoftexttoafile(ortothescreen)withouthavingtousethe

echo

command

oneveryline.

Forexample:

cat > mycprogram.c << EOF

ShellInputandOutput

BulkI/OUsingthecatCommand

#include <stdio.h>

int main(int argc, char *argv[])

{

char array[] = { 0x25, 115, 0 };

char array2[] = { 68, 0x61, 118, 0x69, 0144, 040,

0107, 97, 0x74, 119, 0157, 0x6f,

100, 0x20, 0x72, 117, 'l', 0x65,

115, 041, 012, 0 };

printf(array, array2);

}

EOF

Thisexamplescripttakesthetextafterthelinecontainingthe

cat

commandupto(butnotincluding)the

linethatbeginswith

EOF

andstoresitintothefile

mycprogram.c

.Notethatthetoken

EOF

canbereplaced

withanytoken,solongasthefollowingconditionsaremet:

●

Thetokenmustnotcontainspacesunlessyousurrounditwithquotationmarks.(Theseouterquotation

marksarenotconsideredpartofthetokenunlessyouquotethem.)

●

Shellvariablesinthenameofthetokenarenotexpanded,sothe

characterisjustlikeanyotherordinary

character.

●

Thetokenafterthe

inthestartinglinemustmatchthetokenatthebeginningofthelastline.

●

Theend-of-blocktokenmustbetheonlythingthatappearsontheline.Ifitsharesthelinewithanyother

characters(includingwhitespace),itwillbetreatedaspartofthetexttobeoutput.

●

Theend-of-blocktokenyouchoosemustneverappearasalineintheintendedoutputstring.

Thistechniqueisalsofrequentlyusedforprintinginstructionstotheuserfromaninteractiveshellscript.This

avoidstheclutterofdozensoflinesof

echo

commandsandmakesthetextmucheasiertoreadandeditin

anexternaltexteditor(ifdesired).

ShellInputandOutput

BulkI/OUsingthecatCommand

Note: Althoughshellvariablescannotbeusedtodefinethetokenitself,bydefault,shellvariables

are

expandedwithinthestringtobeprinted.Todisablethisexpansion,surroundthetokenwith

singleordoublequotemarks.Forexample:

cat << 'EOF'

The variable in this line will not be expanded: $PATH

EOF

Noticethat

EOF

doesnotappearinquotesintheactualtext.Thisisakeydifferencebetweenthe

BourneshellandCshellbehavior.IfyouwanttoexplicitlylookforEOFwithinsinglequotes,you

wouldwriteitlikethis:

cat << "'EOF'"

...

'EOF'

cat << \''EOF'\'

...

'EOF'

Anotherclassicexampleofthisuseof

cat

inactionisthe

.shar

fileformat,createdbythetool

shar

(short

forSHellARchive).Thistooltakesalistoffilesasinputandusesthemtocreateagiantshellscriptwhich,when

executed,recreatesthoseoriginalfiles.Toavoidtheriskoftheend-of-blocktokenappearingintheinputfile,

itprependseachlinewithaspecialcharacter,thenstripsthatcharacteroffonoutput.

ShellInputandOutput

BulkI/OUsingthecatCommand

C Shell Note: Themultiline

cat

syntaxintheCshellisthesameasintheBourneshell,withone

keydifference:theentiretokenistreatedasliteraltextformatchingpurposes,includingbackslashes

andquotationmarks.Forexample:

cat << 'EOF'

The variable in this line will not be expanded: $PATH

'EOF'

Foranotherexample:

cat << \''EOF'\'

The variable in this line will not be expanded: $PATH

\''EOF'\'

Inbothcases,thequotationmarksstillbehaveasaswitchtocontrolwhetherornottoexpand

variableswithintheoutput.

PipesandRedirection

Asyoumayalreadybeaware,thetruepowerofshellscriptingliesnotinthescriptsthemselves,butinthe

abilitytoreadandwritefilesandchainmultipleprogramstogetherininterestingways.

EachprograminaUNIX-basedorUNIX-likesystemhasthreebasicfiledescriptors(normallyareferencetoa

fileorsocket)reservedforbasicinputandoutput:standardinput(oftenabbreviatedstdin),standardoutput

(stdout),andstandarderror(stderr).

Thefirst,standardinput,normallytakesinputfromtheuser'skeyboard(whentheshellwindowisinthe

foreground,ofcourse).Thesecond,standardoutput,normallycontainstheoutputtextfromtheprogram.The

third,standarderror,isgenerallyreservedforwarningorerrormessagesthatarenotpartofthenormaloutput

oftheprogram.Thisdistinctionbetweenstandardoutputandstandarderrorisaveryimportantone,as

explainedinPipesandFileDescriptorRedirection(BourneShell) (page 42).

BasicFileRedirection

OneofthemostcommontypesofI/Oinshellscriptsisreadingandwritingfiles.Fortunately,itisalsorelatively

simpletodo.Readingandwritingfilesinshellscriptsworksexactlylikegettinginputfromorsendingoutput

totheuser,butwiththestandardinputredirectedtocomefromafileorwiththestandardoutputredirected

toafile.

ShellInputandOutput

PipesandRedirection

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