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.
VariableContentsMnemonic
$|If set to nonzero, forces a flush after every write or printWhen 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 pagelines_on_page - lines_printed
$~Name of the current report formatClosely related to $^
$^Name of the current top-of-page formatPoints to top of page

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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.
VariableContentsMnemonic
$1..$9 Contains the subpattern from the corresponding set of parentheses in the last pattern matchedlike \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.
VariableContentsMnemonic
$_ 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?

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