Some variables have a predefined and special meaning in Perl. They are the variables that use punctuation characters after the usual variable indicator ( $ ...
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SpecialVariables
Somevariableshaveapredefined
andspecialmeaninginPerl.Theyarethevariables
thatusepunctuationcharacters
aftertheusualvariableindicator($,@,or
%),suchas$_.
Theexplicit,long-formnamesshownarethevariables’equivalents
whenyouusetheEnglishmodulebyincluding
"useEnglish;"atthetopofyourprogram.GlobalSpecialVariables
Themostcommonlyusedspecialvariableis$_,whichcontains
thedefaultinputandpattern-searchingstring.
Forexample,inthefollowinglines:
foreach('hickory','dickory','doc'){
print;
}
Thefirsttimetheloopisexecuted,“hickory”isprinted.
Thesecondtimearound,“dickory”isprinted,andthethirdtime,“doc”
isprinted.That’sbecauseineachiterationoftheloop,
thecurrentstringisplaced
in$_,andisusedbydefaultbyprint.
HerearetheplaceswherePerlwillassume$_evenifyoudon’t
specifyit:Variousunaryfunctions,includingfunctionslikeordand
int,aswellastheallfiletests(-f,-d)exceptfor
-t,whichdefaultstoSTDIN.Variouslistfunctionslikeprintandunlink.Thepattern-matchingoperationsm//,s///,andtr///
whenusedwithoutan=~operator.Thedefaultiteratorvariableinaforeachloopifnoother
variableissupplied.Theimplicititeratorvariableinthegrepandmap
functions.Thedefaultplacetoputaninputrecordwhenaline-inputoperation’s
resultistestedbyitselfasthesolecriterionofa
whiletest(i.e.,).
Notethatoutsideofawhiletest,this
willnothappen.Thefollowingisacompletelistingofglobalspecialvariables:
$_
,
$ARG
Thedefaultinputandpattern-searchingspace.
$.
,
$INPUT_LINE_NUMBER
$NR
Thecurrentinputlinenumberofthelastfilehandlethatwasread.An
explicitcloseonthefilehandleresetsthelinenumber.
$/
,
$INPUT_RECORD_SEPARATOR
$RS
Theinputrecordseparator;newlinebydefault.
Ifsettothenullstring,ittreatsblanklinesas
delimiters.
$,
,
$OUTPUT_FIELD_SEPARATOR
$OFS
Theoutputfieldseparatorfortheprintoperator.
$\
,
$OUTPUT_RECORD_SEPARATOR
,
$ORS
Theoutputrecordseparatorfortheprintoperator.
$
,
$LIST_SEPARATOR
Like"$,"exceptthatitappliestolistvaluesinterpolated
intoadouble-quotedstring(orsimilarinterpretedstring).Default
isaspace.
$;
,
$SUBSCRIPT_SEPARATOR
,
$SUBSEP
Thesubscriptseparatorformultidimensionalarrayemulation.
Defaultis"\034".
$^L
,
$FORMAT_FORMFEED
Whataformatoutputstoperformaformfeed.Defaultis"\f".
$:
,
$FORMAT_LINE_BREAK_CHARACTERS
Thecurrentsetofcharactersafterwhichastringmaybebrokentofill
continuationfields(startingwith^)inaformat.
Defaultis"\n"".
$^A
,
$ACCUMULATOR
Thecurrentvalueofthewrite
accumulatorforformatlines.
$#
,
$OFMT
Containstheoutputformatforprintednumbers
(deprecated).
$?
,
$CHILD_ERROR
Thestatusreturnedbythelastpipeclose,backtick(``)command,
orsystemoperator.
$!
,
$OS_ERROR
,
$ERRNO
Ifusedinanumericcontext,yieldsthecurrentvalueofthe
errnovariable,identifyingthelastsystemcallerror.
Ifusedinastringcontext,
yieldsthecorrespondingsystemerrorstring.
$@
,
$EVAL_ERROR
ThePerlsyntaxerrormessagefromthelastevalcommand.
$$
,
$PROCESS_ID
,
$PID
ThepidofthePerlprocessrunningthisscript.
$<
,
$REAL_USER_ID
,
$UID
TherealuserID(uid)ofthisprocess.
$>
,
$EFFECTIVE_USER_ID
,
$EUID
Theeffectiveuidofthisprocess.
$(
,
$REAL_GROUP_ID
,
$GID
TherealgroupID(gid)ofthisprocess.
$)
,
$EFFECTIVE_GROUP_ID
,
$EGID
Theeffectivegidofthisprocess.
$0
,
$PROGRAM_NAME
ContainsthenameofthefilecontainingthePerlscriptbeingexecuted.
$[
Theindexofthefirstelementinanarrayandofthefirstcharacterin
asubstring.Defaultis0.
$]
,
$PERL_VERSION
Returnstheversionpluspatchleveldividedby1000.
$^D
,
$DEBUGGING
Thecurrentvalueofthedebuggingflags.
$^E
,
$EXTENDED_OS_ERROR
Extendederrormessageonsomeplatforms.
$^F
,
$SYSTEM_FD_MAX
Themaximumsystemfiledescriptor,ordinarily2.
$^H
Containsinternalcompilerhintsenabledbycertain
pragmaticmodules.
$^I
,
$INPLACE_EDIT
Thecurrentvalueoftheinplace-editextension.Useundeftodisable
inplaceediting.
$^M
Thecontentsof$Mcanbeusedasanemergencymemorypoolin
casePerldieswithanout-of-memoryerror.Useof$M
requiresaspecialcompilationofPerl.SeetheINSTALLdocumentfor
moreinformation.
$^O
,
$OSNAME
Containsthenameoftheoperatingsystemthatthecurrent
Perlbinarywascompiledfor.
$^P
,
$PERLDB
Theinternalflagthatthedebuggerclearssothatitdoesn’tdebug
itself.
$^T
,
$BASETIME
Thetimeatwhichthescriptbeganrunning,insecondssincetheepoch.
$^W
,
$WARNING
Thecurrentvalueofthewarningswitch,eithertrueor
false.
$^X
,
$EXECUTABLE_NAME
ThenamethatthePerlbinaryitselfwasexecutedas.
$ARGV
Containsthenameofthecurrentfilewhenreadingfrom.GlobalSpecialArraysandHashes
@ARGV
Thearraycontainingthecommand-lineargumentsintendedforthe
script.
@INC
ThearraycontainingthelistofplacestolookforPerlscripts
tobeevaluatedbythedo,require,oruse
constructs.
@F
Thearrayintowhichtheinputlinesaresplitwhenthe-a
command-lineswitchisgiven.
%INC
Thehashcontainingentriesforthefilenameofeachfilethathasbeen
includedviadoorrequire.
%ENV
Thehashcontainingyourcurrentenvironment.
%SIG
Thehashusedtosetsignalhandlersforvarioussignals.GlobalSpecialFilehandles
ARGV
Thespecialfilehandlethatiteratesovercommandlinefilenamesin
@ARGV.Usuallywrittenasthenullfilehandlein<>.
STDERR
Thespecialfilehandleforstandarderrorinanypackage.
STDIN
Thespecialfilehandleforstandardinputinanypackage.
STDOUT
Thespecialfilehandleforstandardoutputinanypackage.
DATA
Thespecialfilehandlethatreferstoanythingfollowingthe
__END__tokeninthefile
containingthescript.Or,thespecialfilehandleforanything
followingthe__DATA__tokeninarequiredfile,aslongas
you’rereadingdatainthesamepackage__DATA__was
foundin.
_(underscore)
Thespecialfilehandleusedtocachetheinformationfromthelaststat,
lstat,orfiletestoperator.GlobalSpecialConstants
__END__
Indicatesthelogicalendofyourprogram.Anyfollowingtextisignored,
butmaybereadviatheDATAfilehandle.
__FILE__
Representsthefilenameatthepointinyourprogramwhereit’sused.Not
interpolatedintostrings.
__LINE__
Representsthecurrentlinenumber.Notinterpolatedintostrings.
__PACKAGE__
Representsthecurrentpackagenameatcompiletime,orundefinedifthere
isnocurrentpackage.Notinterpolatedintostrings.RegularExpressionSpecialVariables
Formoreinformationonregularexpressions,
seeSection4.6laterinthischapter.
$
digit
Containsthetextmatchedbythecorrespondingsetofparenthesesin
thelastpatternmatched.Forexample,$1matcheswhateverwascontained
inthefirstsetofparenthesesinthepreviousregularexpression.
$&
,
$MATCH
Thestringmatchedbythelastsuccessfulpatternmatch.
$`
,
$PREMATCH
Thestringprecedingwhateverwasmatchedbythelastsuccessfulpattern
match.
$'
,
$POSTMATCH
Thestringfollowingwhateverwasmatchedbythelastsuccessfulpattern
match.
$+
,
$LAST_PAREN_MATCH
Thelastbracketmatchedbythelastsearchpattern.Thisisusefulifyou
don’tknowwhichofasetofalternativepatternswasmatched.Forexample:
/Version:(.*)|Revision:(.*)/&&($rev=$+);
FilehandleSpecialVariables
Mostofthesevariablesonlyapplywhenusingformats.
SeeSection4.10laterinthischapter.
$|
$OUTPUT_AUTOFLUSH
Ifsettononzero,forcesanfflush(3)aftereverywriteor
printonthecurrentlyselectedoutputchannel.
$%
$FORMAT_PAGE_NUMBER
Thecurrentpagenumberofthecurrentlyselectedoutputchannel.
$=
$FORMAT_LINES_PER_PAGE
Thecurrentpagelength(printablelines)ofthecurrentlyselectedoutput
channel.Defaultis60.
$-
$FORMAT_LINES_LEFT
Thenumberoflinesleftonthepageofthecurrentlyselectedoutput
channel.
$~
$FORMAT_NAME
Thenameofthecurrentreportformatforthecurrentlyselectedoutput
channel.Defaultisthenameofthefilehandle.
$^
$FORMAT_TOP_NAME
Thenameofthecurrenttop-of-pageformatforthecurrentlyselected
outputchannel.Defaultisthenameofthefilehandle
with_TOPappended.
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