Programming in Perlpredefined variables

Peter VerhásJanuary 2002.

Predefined Variables

• Global, module independent variables

• use English; defines English names

• Only the most important variables are detailed here, consult the manual

$_ $ARG

• Default input and pattern matching variable

• while(<>) reads into $_• s/// m// tr// uses $_

$n

• Sub patterns of the previous m// or s/// operation

"apple" =~ m/(.)(.)\2le/;

print $1," ",$2;

OUTPUT:a p

$` $PREMATCH$& $MATCH

$’ $POSTMATCH

$_ = "apple";/ppl/;print "pre $`\n";print "mat $&\n";print "pos $'\n";OUTPUT:pre amat pplpos e

Due to Perl implementation bugs there is performance penalty using any of these

variables.

$+ $LAST_PAREN_MATCH

• The last bracket matched by the last search pattern.

/Version: (.*)|Revision: (.*)/ && ($rev = $+);

@+ @LAST_MATCH_END@- @LAST_MATCH_START

$_ = "appleeeee";# 012345678/(.)\1(.)e/;print $-[0]," ",$+[0],"\n";print $-[1]," ",$+[1],"\n";print $-[2]," ",$+[2],"\n";

OUTPUT:1 51 23 4

$+[0] is the position after, $-[0] is the position start the last match, $+[n] is the position after, $-[n] is the position start the nth sub match.

$. $NR $INPUT_LINE_NUMBER

open(F,"test.pl");

$l = <F>;

print $.;

$l = <F>;

print $.;

close F;

OUTPUT:12

Actual value depends on what the $/ record separator is.

(See next slide.)

$/ $RS $INPUT_RECORD_SEPARATOR

• A string (not a regexp and not only a single character!) that separates records in the input

• undef $/; makes slurp mode (read the whole file in a single read as a big string

$/ referencing an integer

$/ = \3;open(F,"test.pl");while( <F> ){ print "$_|"; }close F;OUTPUT:$/ |= \|3;|ope|n(F|,"t|est|.pl|");|wh|ile|( <|F> |){| p|rin|t $|_,"||";| |}c|los|e F|;|

Reads at most the referenced number of bytes from the file.

On VMS or other systems where records are supported reads a record but at most

that number of bytes.

$| $OUTPUT_AUTOFLUSH

• $| = 1; to get automatic flush of output after each print statement on the selected channel

• Useful when used on sockets or• STDERR and STDOUT in debug

environment

$\ $ORS $OUTPUT_RECORD_SEPARATOR$, $OFS $OUTPUT_FIELD_SEPARATOR

• $, is printed between two items on he print list

• $\ is printed after each print statement

• Both are null string by default

$? $CHILD_ERROR $! $ERRNO $OS_ERROR

• $? is the error code of the last system() call, ` ` operator or popen/pclose

• $! is the code of errno after a system call

$@ $EVAL_ERROR

• The Perl syntax error message from the last eval() operator.$a = "print \"1\\n\";\nwhat is this?";eval $a;print $a,"\n",$@;print "but we run fine\n";$a = "print \"1\\n\";";eval $a;print $a,"\n",$@;OUTPUT:print "1\n";what is this?syntax error at (eval 1) line 3, at EOFbut we run fine1print "1\n";

$$ $< $> $( $)

• $$ $PID $PROCESS_ID– Process ID (read only)

• $< $UID $REAL_USER_ID– Real user ID of the process

• $> $EUID $EFFECTIVE_USER_ID– Effective user id

• $( $GID $REAL_GROUP_ID– The real group id of the process

• $) $EGID $EFFECTIVE_GROUP_ID– The effective group id of the process

$0 $PROGRAM_NAME

• The name of the program• On some system if you assign

value to this variable that name may be seen on the output of the program ps

$[

• This is 0 and indicates the first index of an array

• Do not ever change it!!!

$]

• The version + patch level /1000 of the actual interpreter

$^O $OSNAME

• The name of the operating system the program runs on

• On my test NT it prints:

– MSWin32 using ActivePerl– cygwin using Cygwin Perl

@_

• Contains the arguments passed to the subroutine

sub a {

print $_[0],$_[1],$_[2];

}

a 1,2,3;

OUTPUT:123

%ENV

• The environment variables• Changing this hash changes the

environment for any sub process

• This variable is heavily used in CGI programs

%SIG

sub handler { # 1st argument is signal name my($sig) = @_; print "Caught a SIG$sig--shutting down\n"; close(LOG); exit(0); } $SIG{'INT'} = \&handler; $SIG{'QUIT'} = \&handler; ... $SIG{'INT'} = 'DEFAULT'; # restore default action $SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT

INT is the signal for CONTROL-C

Thank you for your kind attention.