perlvar.pod

source of perl for linux application,

=item $FORMAT_LINES_LEFT

=item $-

The number of lines left on the page of the currently selected output
channel.  
Used with formats.
(Mnemonic: lines_on_page - lines_printed.)

=item @LAST_MATCH_START

=item @-

$-[0] is the offset of the start of the last successful match.
C<$-[>I<n>C<]> is the offset of the start of the substring matched by
I<n>-th subpattern, or undef if the subpattern did not match.

Thus after a match against $_, $& coincides with C<substr $_, $-[0],
$+[0] - $-[0]>.  Similarly, C<$>I<n> coincides with C<substr $_, $-[>I<n>C<],
$+[>I<n>C<] - $-[>I<n>C<]> if C<$-[>I<n>C<]> is defined, and $+ coincides with
C<substr $_, $-[$#-], $+[$#-]>.  One can use C<$#-> to find the last
matched subgroup in the last successful match.  Contrast with
C<$#+>, the number of subgroups in the regular expression.  Compare
with C<@+>.

This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
C<$-[0]> is the offset into the string of the beginning of the
entire match.  The I<n>th element of this array holds the offset
of the I<n>th submatch, so C<$+[1]> is the offset where $1
begins, C<$+[2]> the offset where $2 begins, and so on.
You can use C<$#-> to determine how many subgroups were in the
last successful match.  Compare with the C<@+> variable.

After a match against some variable $var:

=over 5

=item C<$`> is the same as C<substr($var, 0, $-[0])>

=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>

=item C<$'> is the same as C<substr($var, $+[0])>

=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>  

=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>

=item C<$3> is the same as C<substr $var, $-[3], $+[3] - $-[3])>

=back

=item format_name HANDLE EXPR

=item $FORMAT_NAME

=item $~

The name of the current report format for the currently selected output
channel.  Default is the name of the filehandle.  (Mnemonic: brother to
C<$^>.)

=item format_top_name HANDLE EXPR

=item $FORMAT_TOP_NAME

=item $^

The name of the current top-of-page format for the currently selected
output channel.  Default is the name of the filehandle with _TOP
appended.  (Mnemonic: points to top of page.)

=item format_line_break_characters HANDLE EXPR

=item $FORMAT_LINE_BREAK_CHARACTERS

=item $:

The current set of characters after which a string may be broken to
fill continuation fields (starting with ^) in a format.  Default is
S<" \n-">, to break on whitespace or hyphens.  (Mnemonic: a "colon" in
poetry is a part of a line.)

=item format_formfeed HANDLE EXPR

=item $FORMAT_FORMFEED

=item $^L

What formats output as a form feed.  Default is \f.

=item $ACCUMULATOR

=item $^A

The current value of the write() accumulator for format() lines.  A format
contains formline() calls that put their result into C<$^A>.  After
calling its format, write() prints out the contents of C<$^A> and empties.
So you never really see the contents of C<$^A> unless you call
formline() yourself and then look at it.  See L<perlform> and
L<perlfunc/formline()>.

=item $CHILD_ERROR

=item $?

The status returned by the last pipe close, backtick (C<``>) command,
successful call to wait() or waitpid(), or from the system()
operator.  This is just the 16-bit status word returned by the
wait() system call (or else is made up to look like it).  Thus, the
exit value of the subprocess is really (C<<< $? >> 8 >>>), and
C<$? & 127> gives which signal, if any, the process died from, and
C<$? & 128> reports whether there was a core dump.  (Mnemonic:
similar to B<sh> and B<ksh>.)

Additionally, if the C<h_errno> variable is supported in C, its value
is returned via $? if any C<gethost*()> function fails.

If you have installed a signal handler for C<SIGCHLD>, the
value of C<$?> will usually be wrong outside that handler.

Inside an C<END> subroutine C<$?> contains the value that is going to be
given to C<exit()>.  You can modify C<$?> in an C<END> subroutine to
change the exit status of your program.  For example:

    END {
	$? = 1 if $? == 255;  # die would make it 255
    } 

Under VMS, the pragma C<use vmsish 'status'> makes C<$?> reflect the
actual VMS exit status, instead of the default emulation of POSIX
status.

Also see L<Error Indicators>.

=item $OS_ERROR

=item $ERRNO

=item $!

If used numerically, yields the current value of the C C<errno>
variable, with all the usual caveats.  (This means that you shouldn't
depend on the value of C<$!> to be anything in particular unless
you've gotten a specific error return indicating a system error.)
If used an a string, yields the corresponding system error string.
You can assign a number to C<$!> to set I<errno> if, for instance,
you want C<"$!"> to return the string for error I<n>, or you want
to set the exit value for the die() operator.  (Mnemonic: What just
went bang?)

Also see L<Error Indicators>.

=item $EXTENDED_OS_ERROR

=item $^E

Error information specific to the current operating system.  At
the moment, this differs from C<$!> under only VMS, OS/2, and Win32
(and for MacPerl).  On all other platforms, C<$^E> is always just
the same as C<$!>.

Under VMS, C<$^E> provides the VMS status value from the last
system error.  This is more specific information about the last
system error than that provided by C<$!>.  This is particularly
important when C<$!> is set to B<EVMSERR>.

Under OS/2, C<$^E> is set to the error code of the last call to
OS/2 API either via CRT, or directly from perl.

Under Win32, C<$^E> always returns the last error information
reported by the Win32 call C<GetLastError()> which describes
the last error from within the Win32 API.  Most Win32-specific
code will report errors via C<$^E>.  ANSI C and Unix-like calls
set C<errno> and so most portable Perl code will report errors
via C<$!>. 

Caveats mentioned in the description of C<$!> generally apply to
C<$^E>, also.  (Mnemonic: Extra error explanation.)

Also see L<Error Indicators>.

=item $EVAL_ERROR

=item $@

The Perl syntax error message from the last eval() operator.  If null, the
last eval() parsed and executed correctly (although the operations you
invoked may have failed in the normal fashion).  (Mnemonic: Where was
the syntax error "at"?)

Warning messages are not collected in this variable.  You can,
however, set up a routine to process warnings by setting C<$SIG{__WARN__}>
as described below.

Also see L<Error Indicators>.

=item $PROCESS_ID

=item $PID

=item $$

The process number of the Perl running this script.  You should
consider this variable read-only, although it will be altered
across fork() calls.  (Mnemonic: same as shells.)

=item $REAL_USER_ID

=item $UID

=item $<

The real uid of this process.  (Mnemonic: it's the uid you came I<from>,
if you're running setuid.)

=item $EFFECTIVE_USER_ID

=item $EUID

=item $>

The effective uid of this process.  Example:

    $< = $>;		# set real to effective uid
    ($<,$>) = ($>,$<);	# swap real and effective uid

(Mnemonic: it's the uid you went I<to>, if you're running setuid.)
C<< $< >> and C<< $> >> can be swapped only on machines
supporting setreuid().

=item $REAL_GROUP_ID

=item $GID

=item $(

The real gid of this process.  If you are on a machine that supports
membership in multiple groups simultaneously, gives a space separated
list of groups you are in.  The first number is the one returned by
getgid(), and the subsequent ones by getgroups(), one of which may be
the same as the first number.

However, a value assigned to C<$(> must be a single number used to
set the real gid.  So the value given by C<$(> should I<not> be assigned
back to C<$(> without being forced numeric, such as by adding zero.

(Mnemonic: parentheses are used to I<group> things.  The real gid is the
group you I<left>, if you're running setgid.)

=item $EFFECTIVE_GROUP_ID

=item $EGID

=item $)

The effective gid of this process.  If you are on a machine that
supports membership in multiple groups simultaneously, gives a space
separated list of groups you are in.  The first number is the one
returned by getegid(), and the subsequent ones by getgroups(), one of
which may be the same as the first number.

Similarly, a value assigned to C<$)> must also be a space-separated
list of numbers.  The first number sets the effective gid, and
the rest (if any) are passed to setgroups().  To get the effect of an
empty list for setgroups(), just repeat the new effective gid; that is,
to force an effective gid of 5 and an effectively empty setgroups()
list, say C< $) = "5 5" >.

(Mnemonic: parentheses are used to I<group> things.  The effective gid
is the group that's I<right> for you, if you're running setgid.)

C<< $< >>, C<< $> >>, C<$(> and C<$)> can be set only on
machines that support the corresponding I<set[re][ug]id()> routine.  C<$(>
and C<$)> can be swapped only on machines supporting setregid().

=item $PROGRAM_NAME

=item $0

Contains the name of the program being executed.  On some operating
systems assigning to C<$0> modifies the argument area that the B<ps>
program sees.  This is more useful as a way of indicating the current
program state than it is for hiding the program you're running.
(Mnemonic: same as B<sh> and B<ksh>.)

Note for BSD users: setting C<$0> does not completely remove "perl"
from the ps(1) output.  For example, setting C<$0> to C<"foobar"> will
result in C<"perl: foobar (perl)">.  This is an operating system
feature.

=item $[

The index of the first element in an array, and of the first character
in a substring.  Default is 0, but you could theoretically set it
to 1 to make Perl behave more like B<awk> (or Fortran) when
subscripting and when evaluating the index() and substr() functions.
(Mnemonic: [ begins subscripts.)

As of release 5 of Perl, assignment to C<$[> is treated as a compiler
directive, and cannot influence the behavior of any other file.
Its use is highly discouraged.

=item $]

The version + patchlevel / 1000 of the Perl interpreter.  This variable
can be used to determine whether the Perl interpreter executing a
script is in the right range of versions.  (Mnemonic: Is this version
of perl in the right bracket?)  Example:

    warn "No checksumming!\n" if $] < 3.019;

See also the documentation of C<use VERSION> and C<require VERSION>
for a convenient way to fail if the running Perl interpreter is too old.

The use of this variable is deprecated.  The floating point representation
can sometimes lead to inaccurate numeric comparisons.  See C<$^V> for a
more modern representation of the Perl version that allows accurate string
comparisons.

=item $COMPILING

=item $^C

The current value of the flag associated with the B<-c> switch.
Mainly of use with B<-MO=...> to allow code to alter its behavior
when being compiled, such as for example to AUTOLOAD at compile
time rather than normal, deferred loading.  See L<perlcc>.  Setting
C<$^C = 1> is similar to calling C<B::minus_c>.

=item $DEBUGGING

=item $^D

The current value of the debugging flags.  (Mnemonic: value of B<-D>
switch.)

=item $SYSTEM_FD_MAX

=item $^F

The maximum system file descriptor, ordinarily 2.  System file
descriptors are passed to exec()ed processes, while higher file
descriptors are not.  Also, during an open(), system file descriptors are
preserved even if the open() fails.  (Ordinary file descriptors are
closed before the open() is attempted.)  The close-on-exec
status of a file descriptor will be decided according to the value of
C<$^F> when the corresponding file, pipe, or socket was opened, not the
time of the exec().

=item $^H

WARNING: This variable is strictly for internal use only.  Its availability,
behavior, and contents are subject to change without notice.

This variable contains compile-time hints for the Perl interpreter.  At the
end of compilation of a BLOCK the value of this variable is restored to the
value when the interpreter started to compile the BLOCK.

When perl begins to parse any block construct that provides a lexical scope
(e.g., eval body, required file, subroutine body, loop body, or conditional
block), the existing value of $^H is saved, but its value is left unchanged.
When the compilation of the block is completed, it regains the saved value.
Between the points where its value is saved and restored, code that
executes within BEGIN blocks is free to change the value of $^H.

This behavior provides the semantic of lexical scoping, and is used in,
for instance, the C<use strict> pragma.

The contents should be an integer; different bits of it are used for
different pragmatic flags.  Here's an example:

    sub add_100 { $^H |= 0x100 }

    sub foo {
	BEGIN { add_100() }
	bar->baz($boon);
    }

Consider what happens during execution of the BEGIN block.  At this point
the BEGIN block has already been compiled, but the body of foo() is still
being compiled.  The new value of $^H will therefore be visible only while
the body of foo() is being compiled.

Substitution of the above BEGIN block with:

    BEGIN { require strict; strict->import('vars') }

demonstrates how C<use strict 'vars'> is implemented.  Here's a conditional
version of the same lexical pragma:

    BEGIN { require strict; strict->import('vars') if $condition }

=item %^H

WARNING: This variable is strictly for internal use only.  Its availability,
behavior, and contents are subject to change without notice.

The %^H hash provides the same scoping semantic as $^H.  This makes it
useful for implementation of lexically scoped pragmas.

=item $INPLACE_EDIT

=item $^I

The current value of the inplace-edit extension.  Use C<undef> to disable
inplace editing.  (Mnemonic: value of B<-i> switch.)

=item $^M

By default, running out of memory is an untrappable, fatal error.
However, if suitably built, Perl can use the contents of C<$^M>