PERL - Special Variables

Special Variables

Table of Contents

Please bear with me while I try to complete these pages. I'll get to them when I can.
Cheers...

Per-filehandle Special Variables
These variables never need to be mentioned in a local()because they always refer to some value pertaining to the currently selected output filehandle - each filehandle keeps its own set of values.
Variable	Contents	Mnemonic
$\|	If set to nonzero, forces a flush after every write or print	When you want your pipes to be piping hot
$%	Current page number	% is page number in nroff
$=	Current page length	= has horizontal lines
$-	Number of lines left on the page	lines_on_page - lines_printed
$~	Name of the current report format	Closely related to $^
$^	Name of the current top-of-page format	Points to top of page

Local Special Variables
These variables that are always local to the current block, so you never need to mention them in a local(). All of them are associated with the last successful pattern match.
Variable	Contents	Mnemonic
$1..$9	Contains the subpattern from the corresponding set of parentheses in the last pattern matched	like \1..\9
$&	Contains the string matched by the last pattern match	like & in some editors
$`	The string preceding whatever was matched by the last pattern match, not counting patterns matched in nested blocks that have been exited already.	` often precedes a quoted string in normal text
$'	The string following whatever was matched by the last pattern match, not counting patterns matched in nested blockes that have been exited already. For example: $_ = 'abcdefghi'; /def/; print "$`:$&:$'\n"; # prints abc:def:ghi	' often follows a quoted string in normal text
$+	the last bracket matched by the last search pattern. This is useful if you don't know which of a set of alternative patterns matched. For example: */Version: (.)\|Revision: (.)/ && ($rev = $+);*	be positive and forward looking

Global Special Variables
There are quite a few variables that are global in the fullest sense -- they mean the same thing in every package. If you want a private copy of one of them, you must localize it in the current block.
Variable	Contents	Mnemonic
$_	The default input and pattern-searching space. The following pairs are equivalent: while (<>) {... # equivalent only in while! while ($_ =<>) {... /^Subject:/ $_ =~ /^Subject:/ y/a-z/A-Z/ $_ =~ y/a-z/A-Z/ chop chop($_)	underline is understood to be underlying certain undertakings
$.	The current input line number of the last filehandle that was read. Rember that only an explicit close on the filehandle resets the line number.	many programs use . to mean the current line number
$/	The input record separator, newline by default. $/ may be set to a value longer than one character in order to match a multi-character delimiter. If $/ is undefined, no record separator is matched, and <FILEHANDLE> will read everything to the end of the current file.	/ is used to delimit line boundries when quoting poetry. Or, if you prefer, think of mad slashers cutting things to ribbons.
$\	The output record separator for the print operator.	You set $\ instead of adding \n at the end of the print.
$,	The output field separator for the print operator.	What is printed when there is a , in your print statement
$"	This is similar to $, except that it applies to array values interpolated into a double-quoted string (or similar interpreted string). Default is space.	Obvious, I think
$#	The output format for numbers display via the print operator	# is the number sign
$$	The process number of the Perl running this script	Same as shells
$?	The status returned by the last pipe close, backtick(``) command or system operator. Note that this is the status word returned by the wait() system call, so the exit value of the subprocess is actually *($? >>). $? & 255** gives which signal, if any, the process died from, and whether there was a core dump.	Similar to sh and ksh
$*	Set to 1 to do multi-line matching within a string, 0 to tell Perl that it can assume that strings contain a single line, for the purpose of optimizing pattern matches. Default is 0	* matches multiple things
$0	Contains the name of the file containing the Perl script being executed. Depending on your OS, it may or may not include the full pathname.	Same as sh and ksh
$[	The index of the first element in an array, and of the first character in a substring.	[ begins subscripts
$]	The first part of the string printed out when you say perl -v. It can be used to determine at the beginning of a script whether the Perl interpreter executing the script is in the right range of versions. If used in a numeric context, $] returns version + patchlevel /1000.	Is this version of Perl in the "rightbracket"?
$;	The subscript separator for multi-dimensional array emulation. If you refer to an associative array element as: $foo{$a,$b,$c} it really means: $foo{join($;, $a, $b, $c)} but don't put @foo{$a,$b,$c} which means ($foo{$a},$foo{$b},$foo{$c})	Comma (the syntactic subscript separator) is a semi-semicolon. Yeah, it's pretty lame, but $, is already taken for something more important.
$!	If used in a numeric context, yields the current value of errno, with all the usual caveats. (This means that you shouldn't depend on the value of $! to be anything in particular unless you've gotten a specific error return indicating a system error.) If used in a string context, yields the corresponding sysem error string.	What just went bang?
$@	The Perl syntax error or routine error message from the last eval, do-FILE, or require command. If set, either the compilation failed, or the die function was executed within the code of the eval.	Where was the syntax error at?