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Programming in Perl predefined variables

Programming in Perl predefined variables. Peter Verhás January 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

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Programming in Perl predefined variables

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  1. Programming in Perlpredefined variables Peter Verhás January 2002.

  2. Predefined Variables • Global, module independent variables • use English; defines English names • Only the most important variables are detailed here, consult the manual

  3. $_ $ARG • Default input and pattern matching variable • while(<>) reads into $_ • s/// m// tr// uses $_

  4. $n • Sub patterns of the previous m// or s/// operation "apple" =~ m/(.)(.)\2le/; print $1," ",$2; OUTPUT: a p

  5. $` $PREMATCH$& $MATCH $’ $POSTMATCH $_ = "apple"; /ppl/; print "pre $`\n"; print "mat $&\n"; print "pos $'\n"; OUTPUT: pre a mat ppl pos e Due to Perl implementation bugs there is performance penalty using any of these variables.

  6. $+ $LAST_PAREN_MATCH • The last bracket matched by the last search pattern. /Version: (.*)|Revision: (.*)/ && ($rev = $+);

  7. @+ @LAST_MATCH_END@- @LAST_MATCH_START $_ = "appleeeee"; # 012345678 /(.)\1(.)e/; print $-[0]," ",$+[0],"\n"; print $-[1]," ",$+[1],"\n"; print $-[2]," ",$+[2],"\n"; OUTPUT: 1 5 1 2 3 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.

  8. $. $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.)

  9. $/ $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

  10. $/ 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.

  11. $| $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

  12. $\ $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

  13. $? $CHILD_ERROR $! $ERRNO $OS_ERROR • $? is the error code of the last system() call, ` ` operator or popen/pclose • $! is the codeoferrnoafter a system call

  14. $@ $EVAL_ERROR • The Perl syntax error message from the lasteval() 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 EOF but we run fine 1 print "1\n";

  15. $$ $< $> $( $) • $$ $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

  16. $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

  17. $[ • This is 0 and indicates the first index of an array • Do not ever change it!!!

  18. $] • The version + patch level /1000 of the actual interpreter

  19. $^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

  20. @_ • Contains the arguments passed to the subroutine sub a { print $_[0],$_[1],$_[2]; } a 1,2,3; OUTPUT: 123

  21. %ENV • The environment variables • Changing this hash changes the environment for any sub process • This variable is heavily used in CGI programs

  22. %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

  23. Thank you for your kind attention.

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