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How to identify if $param is a null value or empty value, same as [ -z from ksh? #!/usr/bin/perl my $param = $ARGV[0]; if ($param = ...
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perl+identifyifparamisemptyvaluefromARG
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whenIrunthefollowingscript.plscriptwithnoarguments:
./script.pl
IdonotgetthemessageNoarg.Why?Howtoidentifyif$paramisanullvalueoremptyvalue,sameas[-zfromksh?
#!/usr/bin/perl
my$param=$ARGV[0];
if($param=""){
printNoarg;
}else{
printarg:$param;
}
perl
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editedJul27,2010at8:13
daxim
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askedJul27,2010at8:07
lidialidia
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1
$#ARGV==-1istheeasiest.perldoc.perl.org/perlvar.html#%40ARGV
– msw
Jul27,2010at8:17
butifARGV[1];isnullorARGV[2];howtocompareparamwith..?
– lidia
Jul27,2010at8:22
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5Answers
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11
Becauseit'snotPerl.Wheredidyoulearnthatsyntax?Somuchiswrongwithit.
$param=""assignsanemptystringto$param,that'snotwhatyouwant.
nullisspelledundefinPerl.
Tocomparestrings,usetheeqoperator.
Youmustquotestrings:print"Noarg"
Mucheasier:
#!/usr/bin/perl
if(@ARGV){
print'haveparameters';
}else{
printq{don'thaveparameters};
}
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answeredJul27,2010at8:30
daximdaxim
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butifIhaveARG1andnotARG2Igetthedon'thaveparameters?
– lidia
Jul27,2010at8:39
Inearlymadethatsamesuggestion,butI'mguessingtheOPwantstoextendtohandledifferentnumbersofarguments,andthentestingthelengthof@ARGVisn'tquitesotidy.
– AndyMortimer
Jul27,2010at8:43
Justtryitandsee.Whenyoupassanyparametersonthecommand-line,@ARGVisfilledwithvaluesandtheprogramgoesintothefirstbranch.
– daxim
Jul27,2010at8:46
canyougivemeexampleforthatTHX
– lidia
Jul27,2010at8:52
whyneedtoprintq{don'thaveparameters};andnotprint"don'thaveparameters"??
– lidia
Jul27,2010at8:54
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5
Hereisanexampletoillustrateitabitbetter.
if($#ARGV==-1){
print"Noargumentspassed.\n";
}elseif($#ARGV==0){
print"Oneargumentpassed.\n";
}else{
print$#ARGV+1."argumentspassed.\n";
}
Ilikeusing"scalar@ARGV"asthisrepresentsnumberofelementsinanarray.$#ARGV,instead,istheindexoflastelementinthearray.Thisiswhyitisoffbyone.If$[(thisisaspecialvariablethatsetsthestartingindexofPerlarrays.)wassetto1andnot0(thedefault),then$#ARGVwouldnotbeoffbyoneforourpurposes.Iwouldnotmesswith$[asitisaglobal.Changingmaybreakalotmodules.
my$argument_count=scalar@ARGV;
if($argument_count==0){
print"Noargumentspassed.\n";
}elseif($argument_count==1){
print"Oneargumentpassed.\n";
}else{
print"$argument_countargumentspassed.\n";
}
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editedJul29,2010at13:52
answeredJul29,2010at13:40
gdeygdey
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OK,itseemspeoplefoundmypreviousanswermis-leading,sohere'sanothertry,moredirectlythistime.
Thetestyou'relookingforisdefined:
my$param=$ARGV[0];
if(defined$param){
print"arg:$param\n";
}else{
print"Noarg\n";
}
Ifthereweren'tenoughparameterstofillin$ARGV[0](orotherlaterelements)thevalueinthatpositionwillbeundef,sothedefinedtestwillevaluatetofalse.
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answeredJul28,2010at8:37
AndyMortimerAndyMortimer
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ifparamisemptyyoucanautomakeaparamval:
$ARGV[0]||'yourParam';
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answeredJun17,2013at8:47
GankGank
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Here'sconvenientsnippettomakeascriptabortwithanerrorifnoargumentsarepassed:
if(notdefined$ARGV[0]){
printSTDERR"Error:missingparameter.Usage:perl$0
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