perlvaro.txt

source of perl for linux application,

NAME
perlvar - Perl predefined variables
DESCRIPTION
Predefined Names
The following names have special meaning to Perl. Most punctuation names have reasonable mnemonics, or analogs in the shells. Nevertheless, if you wish to use long variable names, you need only say
    use English;
at the top of your program. This will alias all the short names to the long names in the current package. Some even have medium names, generally borrowed from awk.
If you don't mind the performance hit, variables that depend on the currently selected filehandle may instead be set by calling an appropriate object method on the IO::Handle object. (Summary lines below for this contain the word HANDLE.) First you must say
    use IO::Handle;
after which you may use either
    method HANDLE EXPR
or more safely,
    HANDLE->method(EXPR)
Each method returns the old value of the IO::Handle attribute. The methods each take an optional EXPR, which if supplied specifies the new value for the IO::Handle attribute in question. If not supplied, most methods do nothing to the current value--except for autoflush(), which will assume a 1 for you, just to be different. Because loading in the IO::Handle class is an expensive operation, you should learn how to use the regular built-in variables.
A few of these variables are considered "read-only". This means that if you try to assign to this variable, either directly or indirectly through a reference, you'll raise a run-time exception.
The following list is ordered by scalar variables first, then the arrays, then the hashes.
$ARG
$_
The default input and pattern-searching space. The following pairs are equivalent:
    while (<>) {...}    # equivalent only in while!
    while (defined($_ = <>)) {...}
    /^Subject:/
    $_ =~ /^Subject:/
    tr/a-z/A-Z/
    $_ =~ tr/a-z/A-Z/
    chomp
    chomp($_)
Here are the places where Perl will assume $_ even if you don't use it:
 Various unary functions, including functions like ord() and int(), as well as the all file tests (-f, -d) except for -t, which defaults to STDIN.
 Various list functions like print() and unlink().
 The pattern matching operations m//, s///, and tr/// when used without an =~ operator.
 The default iterator variable in a foreach loop if no other variable is supplied.
 The implicit iterator variable in the grep() and map() functions.
 The default place to put an input record when a <FH> operation's result is tested by itself as the sole criterion of a while test. Outside a while test, this will not happen.
(Mnemonic: underline is understood in certain operations.)
$<digits>
Contains the subpattern from the corresponding set of capturing parentheses from the last pattern match, not counting patterns matched in nested blocks that have been exited already. (Mnemonic: like \digits.) These variables are all read-only and dynamically scoped to the current BLOCK.
$MATCH
$&
The string matched by the last successful pattern match (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). (Mnemonic: like & in some editors.) This variable is read-only and dynamically scoped to the current BLOCK.
The use of this variable anywhere in a program imposes a considerable performance penalty on all regular expression matches. See BUGS.
$PREMATCH
$`
The string preceding whatever was matched by the last successful pattern match (not counting any matches hidden within a BLOCK or eval enclosed by the current BLOCK). (Mnemonic: ` often precedes a quoted string.) This variable is read-only.
The use of this variable anywhere in a program imposes a considerable performance penalty on all regular expression matches. See BUGS.
$POSTMATCH
$'
The string following whatever was matched by the last successful pattern match (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). (Mnemonic: ' often follows a quoted string.) Example:
    $_ = 'abcdefghi';
    /def/;
    print "$`:$&:$'\n";         # prints abc:def:ghi
This variable is read-only and dynamically scoped to the current BLOCK.
The use of this variable anywhere in a program imposes a considerable performance penalty on all regular expression matches. See BUGS.
$LAST_PAREN_MATCH
$+
The last bracket matched by the last search pattern. This is useful if you don't know which one of a set of alternative patterns matched. For example:
    /Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This variable is read-only and dynamically scoped to the current BLOCK.
@LAST_MATCH_END
@+
This array holds the offsets of the ends of the last successful submatches in the currently active dynamic scope. $+[0] is the offset into the string of the end of the entire match. This is the same value as what the pos function returns when called on the variable that was matched against. The nth element of this array holds the offset of the nth submatch, so $+[1] is the offset past where $1 ends, $+[2] the offset past where $2 ends, and so on. You can use $#+ to determine how many subgroups were in the last successful match. See the examples given for the @- variable.
$MULTILINE_MATCHING
$*
Set to a non-zero integer value to do multi-line matching within a string, 0 (or undefined) to tell Perl that it can assume that strings contain a single line, for the purpose of optimizing pattern matches. Pattern matches on strings containing multiple newlines can produce confusing results when $* is 0 or undefined. Default is undefined. (Mnemonic: * matches multiple things.) This variable influences the interpretation of only ^ and $. A literal newline can be searched for even when $* == 0.
Use of $* is deprecated in modern Perl, supplanted by the /s and /m modifiers on pattern matching.
Assigning a non-numerical value to $* triggers a warning (and makes $* act if $* == 0), while assigning a numerical value to $* makes that an implicit int is applied on the value.
input_line_number HANDLE EXPR
$INPUT_LINE_NUMBER
$NR
$.
The current input record number for the last file handle from which you just read() (or called a seek or tell on). The value may be different from the actual physical line number in the file, depending on what notion of "line" is in effect--see $/ on how to change that. An explicit close on a filehandle resets the line number. Because <> never does an explicit close, line numbers increase across ARGV files (but see examples in "eof" in perlfunc). Consider this variable read-only: setting it does not reposition the seek pointer; you'll have to do that on your own. Localizing $. has the effect of also localizing Perl's notion of "the last read filehandle". (Mnemonic: many programs use "." to mean the current line number.)
input_record_separator HANDLE EXPR
$INPUT_RECORD_SEPARATOR
$RS
$/
The input record separator, newline by default. This influences Perl's idea of what a "line" is. Works like awk's RS variable, including treating empty lines as a terminator if set to the null string. (An empty line cannot contain any spaces or tabs.) You may set it to a multi-character string to match a multi-character terminator, or to undef to read through the end of file. Setting it to "\n\n" means something slightly different than setting to "", if the file contains consecutive empty lines. Setting to "" will treat two or more consecutive empty lines as a single empty line. Setting to "\n\n" will blindly assume that the next input character belongs to the next paragraph, even if it's a newline. (Mnemonic: / delimits line boundaries when quoting poetry.)
    undef $/;           # enable "slurp" mode
    $_ = <FH>;          # whole file now here
    s/\n[ \t]+/ /g;
Remember: the value of $/ is a string, not a regex. awk has to be better for something. :-)
Setting $/ to a reference to an integer, scalar containing an integer, or scalar that's convertible to an integer will attempt to read records instead of lines, with the maximum record size being the referenced integer. So this:
    $/ = \32768; # or \"32768", or \$var_containing_32768
    open(FILE, $myfile);
    $_ = <FILE>;
will read a record of no more than 32768 bytes from FILE. If you're not reading from a record-oriented file (or your OS doesn't have record-oriented files), then you'll likely get a full chunk of data with every read. If a record is larger than the record size you've set, you'll get the record back in pieces.
On VMS, record reads are done with the equivalent of sysread, so it's best not to mix record and non-record reads on the same file. (This is unlikely to be a problem, because any file you'd want to read in record mode is probably unusable in line mode.) Non-VMS systems do normal I/O, so it's safe to mix record and non-record reads of a file.
See also "Newlines" in perlport. Also see $..
autoflush HANDLE EXPR
$OUTPUT_AUTOFLUSH
$|
If set to nonzero, forces a flush right away and after every write or print on the currently selected output channel. Default is 0 (regardless of whether the channel is really buffered by the system or not; $| tells you only whether you've asked Perl explicitly to flush after each write). STDOUT will typically be line buffered if output is to the terminal and block buffered otherwise. Setting this variable is useful primarily when you are outputting to a pipe or socket, such as when you are running a Perl program under rsh and want to see the output as it's happening. This has no effect on input buffering. See "getc" in perlfunc for that. (Mnemonic: when you want your pipes to be piping hot.)
output_field_separator HANDLE EXPR
$OUTPUT_FIELD_SEPARATOR
$OFS
$,
The output field separator for the print operator. Ordinarily the print operator simply prints out its arguments without further adornment. To get behavior more like awk, set this variable as you would set awk's OFS variable to specify what is printed between fields. (Mnemonic: what is printed when there is a "," in your print statement.)
output_record_separator HANDLE EXPR
$OUTPUT_RECORD_SEPARATOR
$ORS
$\
The output record separator for the print operator. Ordinarily the print operator simply prints out its arguments as is, with no trailing newline or other end-of-record string added. To get behavior more like awk, set this variable as you would set awk's ORS variable to specify what is printed at the end of the print. (Mnemonic: you set $\ instead of adding "\n" at the end of the print. Also, it's just like $/, but it's what you get "back" from Perl.)
$LIST_SEPARATOR
$"
This is like $, except that it applies to array and slice values interpolated into a double-quoted string (or similar interpreted string). Default is a space. (Mnemonic: obvious, I think.)
$SUBSCRIPT_SEPARATOR
$SUBSEP
$;
The subscript separator for multidimensional array emulation. If you refer to a hash element as
    $foo{$a,$b,$c}
it really means
    $foo{join($;, $a, $b, $c)}
But don't put
    @foo{$a,$b,$c}      # a slice--note the @
which means
    ($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk. If your keys contain binary data there might not be any safe value for $;. (Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon. Yeah, I know, it's pretty lame, but $, is already taken for something more important.)
Consider using "real" multidimensional arrays as described in perllol.
$OFMT
$#
The output format for printed numbers. This variable is a half-hearted attempt to emulate awk's OFMT variable. There are times, however, when awk and Perl have differing notions of what counts as numeric. The initial value is "%.ng", where n is the value of the macro DBL_DIG from your system's float.h. This is different from awk's default OFMT setting of "%.6g", so you need to set $# explicitly to get awk's value. (Mnemonic: # is the number sign.)
Use of $# is deprecated.
format_page_number HANDLE EXPR
$FORMAT_PAGE_NUMBER
$%
The current page number of the currently selected output channel. Used with formats. (Mnemonic: % is page number in nroff.)
format_lines_per_page HANDLE EXPR
$FORMAT_LINES_PER_PAGE
$=
The current page length (printable lines) of the currently selected output channel. Default is 60. Used with formats. (Mnemonic: = has horizontal lines.)
format_lines_left HANDLE EXPR
$FORMAT_LINES_LEFT
$-
The number of lines left on the page of the currently selected output channel. Used with formats. (Mnemonic: lines_on_page - lines_printed.)
@LAST_MATCH_START
@-
$-[0] is the offset of the start of the last successful match. $-[n] is the offset of the start of the substring matched by n-th subpattern, or undef if the subpattern did not match.