 |
Index for
Section 1 |
|
 |
Alphabetical
listing for P |
|
 |
Bottom of
page |
|
PERLVAR(1)
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 the BUGS manpage.
$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 the BUGS manpage.
$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 the BUGS manpage.
$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 the eof entry in
the perlfunc manpage). 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);
$_ = <FILE>;
will read a record of no more than 32768 bytes from FILE. If
you're not reading from a record-oriented file (or your OS doesn't
have record-oriented files), then you'll likely get a full chunk of
data with every read. If a record is larger than the record size
you've set, you'll get the record back in pieces.
On VMS, record reads are done with the equivalent of "sysread", so
it's best not to mix record and non-record reads on the same file.
(This is unlikely to be a problem, because any file you'd want to
read in record mode is probably unusable in line mode.) Non-VMS
systems do normal I/O, so it's safe to mix record and non-record
reads of a file.
See also the Newlines entry in the perlport manpage. Also see
"$.".
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 the getc entry in the perlfunc manpage 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 the
perllol manpage.
$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.
Thus after a match against $_, $& coincides with "substr $_, $-[0],
$+[0] - $-[0]". Similarly, "$"n coincides with "substr $_,
$-["n"], $+["n"] - $-["n"]" if "$-["n"]" is defined, and $+
coincides with "substr $_, $-[$#-], $+[$#-]". One can use "$#-" to
find the last matched subgroup in the last successful match.
Contrast with "$#+", the number of subgroups in the regular
expression. Compare with "@+".
This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
"$-[0]" is the offset into the string of the beginning of the
entire match. The nth element of this array holds the offset of
the nth submatch, so "$+[1]" is the offset where $1 begins, "$+[2]"
the offset where $2 begins, and so on. You can use "$#-" to
determine how many subgroups were in the last successful match.
Compare with the "@+" variable.
After a match against some variable $var:
"$`" is the same as "substr($var, 0, $-[0])"
"$&" is the same as "substr($var, $-[0], $+[0] - $-[0])"
"$'" is the same as "substr($var, $+[0])"
"$1" is the same as "substr($var, $-[1], $+[1] - $-[1])"
"$2" is the same as "substr($var, $-[2], $+[2] - $-[2])"
"$3" is the same as "substr $var, $-[3], $+[3] - $-[3])"
format_name HANDLE EXPR
$FORMAT_NAME
$~ The name of the current report format for the currently selected
output channel. Default is the name of the filehandle. (Mnemonic:
brother to "$^".)
format_top_name HANDLE EXPR
$FORMAT_TOP_NAME
$^ 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.)
format_line_break_characters HANDLE EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$: The current set of characters after which a string may be broken to
fill continuation fields (starting with ^) in a format. Default is
" \n-", to break on whitespace or hyphens. (Mnemonic: a "colon" in
poetry is a part of a line.)
format_formfeed HANDLE EXPR
$FORMAT_FORMFEED
$^L What formats output as a form feed. Default is \f.
$ACCUMULATOR
$^A The current value of the write() accumulator for format() lines. A
format contains formline() calls that put their result into "$^A".
After calling its format, write() prints out the contents of "$^A"
and empties. So you never really see the contents of "$^A" unless
you call formline() yourself and then look at it. See the perlform
manpage and the formline() entry in the perlfunc manpage.
$CHILD_ERROR
$? The status returned by the last pipe close, backtick ("``")
command, successful call to wait() or waitpid(), or from the
system() operator. This is just the 16-bit status word returned by
the wait() system call (or else is made up to look like it). Thus,
the exit value of the subprocess is really ("$? >> 8"), and "$? &
127" gives which signal, if any, the process died from, and "$? &
128" reports whether there was a core dump. (Mnemonic: similar to
sh and ksh.)
Additionally, if the "h_errno" variable is supported in C, its
value is returned via $? if any "gethost*()" function fails.
If you have installed a signal handler for "SIGCHLD", the value of
"$?" will usually be wrong outside that handler.
Inside an "END" subroutine "$?" contains the value that is going to
be given to "exit()". You can modify "$?" in an "END" subroutine
to change the exit status of your program. For example:
END {
$? = 1 if $? == 255; # die would make it 255
}
Under VMS, the pragma "use vmsish 'status'" makes "$?" reflect the
actual VMS exit status, instead of the default emulation of POSIX
status.
Also see the Error Indicators entry elsewhere in this document.
$OS_ERROR
$ERRNO
$! If used numerically, yields the current value of the C "errno"
variable, with all the usual caveats. (This means that you
shouldn't depend on the value of "$!" to be anything in particular
unless you've gotten a specific error return indicating a system
error.) If used an a string, yields the corresponding system error
string. You can assign a number to "$!" to set errno if, for
instance, you want ""$!"" to return the string for error n, or you
want to set the exit value for the die() operator. (Mnemonic: What
just went bang?)
Also see the Error Indicators entry elsewhere in this document.
$EXTENDED_OS_ERROR
$^E Error information specific to the current operating system. At the
moment, this differs from "$!" under only VMS, OS/2, and Win32 (and
for MacPerl). On all other platforms, "$^E" is always just the
same as "$!".
Under VMS, "$^E" provides the VMS status value from the last system
error. This is more specific information about the last system
error than that provided by "$!". This is particularly important
when "$!" is set to EVMSERR.
Under OS/2, "$^E" is set to the error code of the last call to OS/2
API either via CRT, or directly from perl.
Under Win32, "$^E" always returns the last error information
reported by the Win32 call "GetLastError()" which describes the
last error from within the Win32 API. Most Win32-specific code
will report errors via "$^E". ANSI C and Unix-like calls set
"errno" and so most portable Perl code will report errors via "$!".
Caveats mentioned in the description of "$!" generally apply to
"$^E", also. (Mnemonic: Extra error explanation.)
Also see the Error Indicators entry elsewhere in this document.
$EVAL_ERROR
$@ The Perl syntax error message from the last eval() operator. If
null, the last eval() parsed and executed correctly (although the
operations you invoked may have failed in the normal fashion).
(Mnemonic: Where was the syntax error "at"?)
Warning messages are not collected in this variable. You can,
however, set up a routine to process warnings by setting
"$SIG{__WARN__}" as described below.
Also see the Error Indicators entry elsewhere in this document.
$PROCESS_ID
$PID
$$ 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.)
$REAL_USER_ID
$UID
$< The real uid of this process. (Mnemonic: it's the uid you came
from, if you're running setuid.)
$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. Example:
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went to, if you're running setuid.)
"$<" and "$>" can be swapped only on machines supporting
setreuid().
$REAL_GROUP_ID
$GID
$( The real gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is the
one returned by getgid(), and the subsequent ones by getgroups(),
one of which may be the same as the first number.
However, a value assigned to "$(" must be a single number used to
set the real gid. So the value given by "$(" should not be
assigned back to "$(" without being forced numeric, such as by
adding zero.
(Mnemonic: parentheses are used to group things. The real gid is
the group you left, if you're running setgid.)
$EFFECTIVE_GROUP_ID
$EGID
$) The effective gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is the
one returned by getegid(), and the subsequent ones by getgroups(),
one of which may be the same as the first number.
Similarly, a value assigned to "$)" 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 " $) = "5 5" ".
(Mnemonic: parentheses are used to group things. The effective gid
is the group that's right for you, if you're running setgid.)
"$<", "$>", "$(" and "$)" can be set only on machines that support
the corresponding set[re][ug]id() routine. "$(" and "$)" can be
swapped only on machines supporting setregid().
$PROGRAM_NAME
$0 Contains the name of the program being executed. On some operating
systems assigning to "$0" modifies the argument area that the 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 sh and ksh.)
Note for BSD users: setting "$0" does not completely remove "perl"
from the ps(1) output. For example, setting "$0" to ""foobar""
will result in ""perl: foobar (perl)"". This is an operating
system feature.
$[ The index of the first element in an array, and of the first
character in a substring. Default is 0, but you could
theoretically set it to 1 to make Perl behave more like awk (or
Fortran) when subscripting and when evaluating the index() and
substr() functions. (Mnemonic: [ begins subscripts.)
As of release 5 of Perl, assignment to "$[" is treated as a
compiler directive, and cannot influence the behavior of any other
file. Its use is highly discouraged.
$] 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 $] < 3.019;
See also the documentation of "use VERSION" and "require VERSION"
for a convenient way to fail if the running Perl interpreter is too
old.
The use of this variable is deprecated. The floating point
representation can sometimes lead to inaccurate numeric
comparisons. See "$^V" for a more modern representation of the
Perl version that allows accurate string comparisons.
$COMPILING
$^C The current value of the flag associated with the -c switch.
Mainly of use with -MO=... to allow code to alter its behavior when
being compiled, such as for example to AUTOLOAD at compile time
rather than normal, deferred loading. See the perlcc manpage.
Setting "$^C = 1" is similar to calling "B::minus_c".
$DEBUGGING
$^D The current value of the debugging flags. (Mnemonic: value of -D
switch.)
$SYSTEM_FD_MAX
$^F The maximum system file descriptor, ordinarily 2. System file
descriptors are passed to exec()ed processes, while higher file
descriptors are not. Also, during an open(), system file
descriptors are preserved even if the open() fails. (Ordinary file
descriptors are closed before the open() is attempted.) The
close-on-exec status of a file descriptor will be decided according
to the value of "$^F" when the corresponding file, pipe, or socket
was opened, not the time of the exec().
$^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 "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 "use strict 'vars'" is implemented. Here's a
conditional version of the same lexical pragma:
BEGIN { require strict; strict->import('vars') if $condition }
%^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.
$INPLACE_EDIT
$^I The current value of the inplace-edit extension. Use "undef" to
disable inplace editing. (Mnemonic: value of -i switch.)
$^M By default, running out of memory is an untrappable, fatal error.
However, if suitably built, Perl can use the contents of "$^M" as
an emergency memory pool after die()ing. Suppose that your Perl
were compiled with -DPERL_EMERGENCY_SBRK and used Perl's malloc.
Then
$^M = 'a' x (1 << 16);
would allocate a 64K buffer for use in an emergency. See the
INSTALL file in the Perl distribution for information on how to
enable this option. To discourage casual use of this advanced
feature, there is no English long name for this variable.
$OSNAME
$^O The name of the operating system under which this copy of Perl was
built, as determined during the configuration process. The value
is identical to "$Config{'osname'}". See also the Config manpage
and the -V command-line switch documented in the perlrun manpage.
$PERLDB
$^P The internal variable for debugging support. The meanings of the
various bits are subject to change, but currently indicate:
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging.
0x04 Switch off optimizations.
0x08 Preserve more data for future interactive inspections.
0x10 Keep info about source lines on which a subroutine is
defined.
0x20 Start with single-step on.
0x40 Use subroutine address instead of name when reporting.
0x80 Report "goto &subroutine" as well.
0x100 Provide informative "file" names for evals based on the place
they were compiled.
0x200 Provide informative names to anonymous subroutines based on
the place they were compiled.
Some bits may be relevant at compile-time only, some at run-time
only. This is a new mechanism and the details may change.
$LAST_REGEXP_CODE_RESULT
$^R The result of evaluation of the last successful "(?{ code })"
regular expression assertion (see the perlre manpage). May be
written to.
$EXCEPTIONS_BEING_CAUGHT
$^S Current state of the interpreter. Undefined if parsing of the
current module/eval is not finished (may happen in $SIG{__DIE__}
and $SIG{__WARN__} handlers). True if inside an eval(), otherwise
false.
$BASETIME
$^T The time at which the program began running, in seconds since the
epoch (beginning of 1970). The values returned by the -M, -A, and
-C filetests are based on this value.
$PERL_VERSION
$^V The revision, version, and subversion of the Perl interpreter,
represented as a string composed of characters with those ordinals.
Thus in Perl v5.6.0 it equals "chr(5) . chr(6) . chr(0)" and will
return true for "$^V eq v5.6.0". Note that the characters in this
string value can potentially be in Unicode range.
This can be used to determine whether the Perl interpreter
executing a script is in the right range of versions. (Mnemonic:
use ^V for Version Control.) Example:
warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
See the documentation of "use VERSION" and "require VERSION" for a
convenient way to fail if the running Perl interpreter is too old.
See also "$]" for an older representation of the Perl version.
$WARNING
$^W The current value of the warning switch, initially true if -w was
used, false otherwise, but directly modifiable. (Mnemonic: related
to the -w switch.) See also the warnings manpage.
${^WARNING_BITS}
The current set of warning checks enabled by the "use warnings"
pragma. See the documentation of "warnings" for more details.
${^WIDE_SYSTEM_CALLS}
Global flag that enables system calls made by Perl to use wide
character APIs native to the system, if available. This is
currently only implemented on the Windows platform.
This can also be enabled from the command line using the "-C"
switch.
The initial value is typically "0" for compatibility with Perl
versions earlier than 5.6, but may be automatically set to "1" by
Perl if the system provides a user-settable default (e.g.,
"$ENV{LC_CTYPE}").
The "bytes" pragma always overrides the effect of this flag in the
current lexical scope. See the bytes manpage.
$EXECUTABLE_NAME
$^X The name that the Perl binary itself was executed as, from C's
"argv[0]". This may not be a full pathname, nor even necessarily
in your path.
$ARGV contains the name of the current file when reading from <>.
@ARGV The array @ARGV contains the command-line arguments intended for
the script. "$#ARGV" is generally the number of arguments minus
one, because "$ARGV[0]" is the first argument, not the program's
command name itself. See "$0" for the command name.
@INC The array @INC contains the list of places that the "do EXPR",
"require", or "use" constructs look for their library files. It
initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl, followed by ".", to represent the current
directory. If you need to modify this at runtime, you should use
the "use lib" pragma to get the machine-dependent library properly
loaded also:
use lib '/mypath/libdir/';
use SomeMod;
@_ Within a subroutine the array @_ contains the parameters passed to
that subroutine. See the perlsub manpage.
%INC The hash %INC contains entries for each filename included via the
"do", "require", or "use" operators. The key is the filename you
specified (with module names converted to pathnames), and the value
is the location of the file found. The "require" operator uses
this hash to determine whether a particular file has already been
included.
%ENV
$ENV{expr}
The hash %ENV contains your current environment. Setting a value
in "ENV" changes the environment for any child processes you
subsequently fork() off.
%SIG
$SIG{expr}
The hash %SIG contains signal handlers for signals. For example:
sub handler { # 1st argument is signal name
my($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
Using a value of "'IGNORE'" usually has the effect of ignoring the
signal, except for the "CHLD" signal. See the perlipc manpage for
more about this special case.
Here are some other examples:
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = <!>Plumber; # just fine; assume current Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
Be sure not to use a bareword as the name of a signal handler, lest
you inadvertently call it.
If your system has the sigaction() function then signal handlers
are installed using it. This means you get reliable signal
handling. If your system has the SA_RESTART flag it is used when
signals handlers are installed. This means that system calls for
which restarting is supported continue rather than returning when a
signal arrives. If you want your system calls to be interrupted by
signal delivery then do something like this:
use POSIX ':signal_h';
my $alarm = 0;
sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
or die "Error setting SIGALRM handler: $!\n";
See the POSIX manpage.
Certain internal hooks can be also set using the %SIG hash. The
routine indicated by "$SIG{__WARN__}" is called when a warning
message is about to be printed. The warning message is passed as
the first argument. The presence of a __WARN__ hook causes the
ordinary printing of warnings to STDERR to be suppressed. You can
use this to save warnings in a variable, or turn warnings into
fatal errors, like this:
local $SIG{__WARN__} = sub { die $_[0] };
eval $proggie;
The routine indicated by "$SIG{__DIE__}" is called when a fatal
exception is about to be thrown. The error message is passed as
the first argument. When a __DIE__ hook routine returns, the
exception processing continues as it would have in the absence of
the hook, unless the hook routine itself exits via a "goto", a loop
exit, or a die(). The "__DIE__" handler is explicitly disabled
during the call, so that you can die from a "__DIE__" handler.
Similarly for "__WARN__".
Due to an implementation glitch, the "$SIG{__DIE__}" hook is called
even inside an eval(). Do not use this to rewrite a pending
exception in "$@", or as a bizarre substitute for overriding
CORE::GLOBAL:\fIs0:die(). This strange action at a distance may be
fixed in a future release so that "$SIG{__DIE__}" is only called if
your program is about to exit, as was the original intent. Any
other use is deprecated.
"__DIE__"/"__WARN__" handlers are very special in one respect: they
may be called to report (probable) errors found by the parser. In
such a case the parser may be in inconsistent state, so any attempt
to evaluate Perl code from such a handler will probably result in a
segfault. This means that warnings or errors that result from
parsing Perl should be used with extreme caution, like this:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give backtrace...
To see backtrace try starting Perl with -MCarp switch";
Here the first line will load Carp unless it is the parser who
called the handler. The second line will print backtrace and die
if Carp was available. The third line will be executed only if
Carp was not available.
See the die entry in the perlfunc manpage, the warn entry in the
perlfunc manpage, the eval entry in the perlfunc manpage, and the
warnings manpage for additional information.
Error Indicators
The variables "$@", "$!", "$^E", and "$?" contain information about
different types of error conditions that may appear during execution of a
Perl program. The variables are shown ordered by the "distance" between
the subsystem which reported the error and the Perl process. They
correspond to errors detected by the Perl interpreter, C library, operating
system, or an external program, respectively.
To illustrate the differences between these variables, consider the
following Perl expression, which uses a single-quoted string:
eval q{
open PIPE, "/cdrom/install |";
@res = <PIPE>;
close PIPE or die "bad pipe: $?, $!";
};
After execution of this statement all 4 variables may have been set.
"$@" is set if the string to be "eval"-ed did not compile (this may happen
if "open" or "close" were imported with bad prototypes), or if Perl code
executed during evaluation die()d . In these cases the value of $@ is the
compile error, or the argument to "die" (which will interpolate "$!" and
"$?"!). (See also the Fatal manpage, though.)
When the eval() expression above is executed, open(), "<PIPE>", and "close"
are translated to calls in the C run-time library and thence to the
operating system kernel. "$!" is set to the C library's "errno" if one of
these calls fails.
Under a few operating systems, "$^E" may contain a more verbose error
indicator, such as in this case, "CDROM tray not closed." Systems that do
not support extended error messages leave "$^E" the same as "$!".
Finally, "$?" may be set to non-0 value if the external program
/cdrom/install fails. The upper eight bits reflect specific error
conditions encountered by the program (the program's exit() value). The
lower eight bits reflect mode of failure, like signal death and core dump
information See wait(2) for details. In contrast to "$!" and "$^E", which
are set only if error condition is detected, the variable "$?" is set on
each "wait" or pipe "close", overwriting the old value. This is more like
"$@", which on every eval() is always set on failure and cleared on
success.
For more details, see the individual descriptions at "$@", "$!", "$^E", and
"$?".
Technical Note on the Syntax of Variable Names
Variable names in Perl can have several formats. Usually, they must begin
with a letter or underscore, in which case they can be arbitrarily long (up
to an internal limit of 251 characters) and may contain letters, digits,
underscores, or the special sequence "::" or "'". In this case, the part
before the last "::" or "'" is taken to be a package qualifier; see the
perlmod manpage.
Perl variable names may also be a sequence of digits or a single
punctuation or control character. These names are all reserved for special
uses by Perl; for example, the all-digits names are used to hold data
captured by backreferences after a regular expression match. Perl has a
special syntax for the single-control-character names: It understands "^X"
(caret "X") to mean the control-"X" character. For example, the notation
"$^W" (dollar-sign caret "W") is the scalar variable whose name is the
single character control-"W". This is better than typing a literal
control-"W" into your program.
Finally, new in Perl 5.6, Perl variable names may be alphanumeric strings
that begin with control characters (or better yet, a caret). These
variables must be written in the form "${^Foo}"; the braces are not
optional. "${^Foo}" denotes the scalar variable whose name is a control-
"F" followed by two "o"'s. These variables are reserved for future special
uses by Perl, except for the ones that begin with "^_" (control-underscore
or caret-underscore). No control-character name that begins with "^_" will
acquire a special meaning in any future version of Perl; such names may
therefore be used safely in programs. "$^_" itself, however, is reserved.
Perl identifiers that begin with digits, control characters, or punctuation
characters are exempt from the effects of the "package" declaration and are
always forced to be in package "main". A few other names are also exempt:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT
SIG
In particular, the new special "${^_XYZ}" variables are always taken to be
in package "main", regardless of any "package" declarations presently in
scope.
BUGS
Due to an unfortunate accident of Perl's implementation, "use English"
imposes a considerable performance penalty on all regular expression
matches in a program, regardless of whether they occur in the scope of "use
English". For that reason, saying "use English" in libraries is strongly
discouraged. See the Devel::SawAmpersand module documentation from CPAN
(http://www.perl.com/CPAN/modules/by-module/Devel/) for more information.
Having to even think about the "$^S" variable in your exception handlers is
simply wrong. "$SIG{__DIE__}" as currently implemented invites grievous
and difficult to track down errors. Avoid it and use an "END{}" or
CORE::GLOBAL::die override instead.
|
Index for
Section 1 |
|
|
Alphabetical
listing for P |
|
 |
Top of
page |
|