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 aliases all the short names to the
long names in the current package. Some even have medium names,
generally borrowed from awk. In general, it’s best to use the
use English '-no_match_vars';
invocation if you don’t need $PREMATCH, $MATCH, or $POSTMATCH, as it
avoids a certain performance hit with the use of regular expressions.
See English.
Variables that depend on the currently selected filehandle may be set
by calling an appropriate object method on the IO::Handle object,
although this is less efficient than using the regular built-in
variables. (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.
You should be very careful when modifying the default values of most
special variables described in this document. In most cases you want to
localize these variables before changing them, since if you don’t, the
change may affect other modules which rely on the default values of the
special variables that you have changed. This is one of the correct
ways to read the whole file at once:
open my $fh, "<", "foo" or die $!;
local $/; # enable localized slurp mode
my $content = <$fh>;
close $fh;
But the following code is quite bad:
open my $fh, "<", "foo" or die $!;
undef $/; # enable slurp mode
my $content = <$fh>;
close $fh;
since some other module, may want to read data from some file in the
default "line mode", so if the code we have just presented has been
executed, the global value of $/ is now changed for any other code
running inside the same Perl interpreter.
Usually when a variable is localized you want to make sure that this
change affects the shortest scope possible. So unless you are already
inside some short "{}" block, you should create one yourself. For
example:
my $content = '';
open my $fh, "<", "foo" or die $!;
{
local $/;
$content = <$fh>;
}
close $fh;
Here is an example of how your own code can go broken:
for (1..5){
nasty_break();
print "$_ ";
}
sub nasty_break {
$_ = 5;
# do something with $_
}
You probably expect this code to print:
1 2 3 4 5
but instead you get:
5 5 5 5 5
Why? Because nasty_break() modifies $_ without localizing it first. The
fix is to add local():
local $_ = 5;
It’s easy to notice the problem in such a short example, but in more
complicated code you are looking for trouble if you don’t localize
changes to the special variables.
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:
· The following functions:
abs, alarm, chomp, chop, chr, chroot, cos, defined, eval,
exp, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir,
oct, ord, pos, print, quotemeta, readlink, readpipe, ref,
require, reverse (in scalar context only), rmdir, sin, split
(on its second argument), sqrt, stat, study, uc, ucfirst,
unlink, unpack.
· All file tests ("-f", "-d") except for "-t", which defaults
to STDIN. See "-X" in perlfunc
· The pattern matching operations "m//", "s///" and "tr///"
(aka "y///") 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 implicit variable of given().
· 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.
As $_ is a global variable, this may lead in some cases to
unwanted side-effects. As of perl 5.9.1, you can now use a
lexical version of $_ by declaring it in a file or in a block
with "my". Moreover, declaring "our $_" restores the global $_
in the current scope.
(Mnemonic: underline is understood in certain operations.)
$a
$b Special package variables when using sort(), see "sort" in
perlfunc. Because of this specialness $a and $b don’t need to
be declared (using use vars, or our()) even when using the
"strict 'vars'" pragma. Don’t lexicalize them with "my $a" or
"my $b" if you want to be able to use them in the sort()
comparison block or function.
$<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".
See "@-" for a replacement.
${^MATCH}
This is similar to $& ($MATCH) except that it does not incur
the performance penalty associated with that variable, and is
only guaranteed to return a defined value when the pattern was
compiled or executed with the "/p" modifier.
$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".
See "@-" for a replacement.
${^PREMATCH}
This is similar to "$`" ($PREMATCH) except that it does not
incur the performance penalty associated with that variable,
and is only guaranteed to return a defined value when the
pattern was compiled or executed with the "/p" modifier.
$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:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'\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".
See "@-" for a replacement.
${^POSTMATCH}
This is similar to "$'" ($POSTMATCH) except that it does not
incur the performance penalty associated with that variable,
and is only guaranteed to return a defined value when the
pattern was compiled or executed with the "/p" modifier.
$LAST_PAREN_MATCH
$+ The text matched by the last bracket of the last successful
search pattern. This is useful if you don’t know which one of
a set of alternative patterns matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This variable is
read-only and dynamically scoped to the current BLOCK.
$LAST_SUBMATCH_RESULT
$^N The text matched by the used group most-recently closed (i.e.
the group with the rightmost closing parenthesis) of the last
successful search pattern. (Mnemonic: the (possibly) Nested
parenthesis that most recently closed.)
This is primarily used inside "(?{...})" blocks for examining
text recently matched. For example, to effectively capture text
to a variable (in addition to $1, $2, etc.), replace "(...)"
with
(?:(...)(?{ $var = $^N }))
By setting and then using $var in this way relieves you from
having to worry about exactly which numbered set of parentheses
they are.
This variable is 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.
%LAST_PAREN_MATCH
%+ Similar to "@+", the "%+" hash allows access to the named
capture buffers, should they exist, in the last successful
match in the currently active dynamic scope.
For example, $+{foo} is equivalent to $1 after the following
match:
'foo' =~ /(?<foo>foo)/;
The keys of the "%+" hash list only the names of buffers that
have captured (and that are thus associated to defined values).
The underlying behaviour of "%+" is provided by the
Tie::Hash::NamedCapture module.
Note: "%-" and "%+" are tied views into a common internal hash
associated with the last successful regular expression.
Therefore mixing iterative access to them via "each" may have
unpredictable results. Likewise, if the last successful match
changes, then the results may be surprising.
HANDLE->input_line_number(EXPR)
$INPUT_LINE_NUMBER
$NR
$. Current line number for the last filehandle accessed.
Each filehandle in Perl counts the number of lines that have
been read from it. (Depending on the value of $/, Perl’s idea
of what constitutes a line may not match yours.) When a line
is read from a filehandle (via readline() or "<>"), or when
tell() or seek() is called on it, $. becomes an alias to the
line counter for that filehandle.
You can adjust the counter by assigning to $., but this will
not actually move the seek pointer. Localizing $. will not
localize the filehandles line count. Instead, it will
localize perl’s notion of which filehandle $. is currently
aliased to.
$. is reset when the filehandle is closed, but not when an open
filehandle is reopened without an intervening close(). For
more details, see "I/O Operators" in perlop. Because "<>"
never does an explicit close, line numbers increase across ARGV
files (but see examples in "eof" in perlfunc).
You can also use "HANDLE->input_line_number(EXPR)" to access
the line counter for a given filehandle without having to worry
about which handle you last accessed.
(Mnemonic: many programs use "." to mean the current line
number.)
IO::Handle->input_record_separator(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.)
local $/; # enable "slurp" mode
local $_ = <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:
local $/ = \32768; # or \"32768", or \$var_containing_32768
open my $fh, "<", $myfile or die $!;
local $_ = <$fh>;
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. Trying to set the record size to zero or less will
cause reading in the (rest of the) whole file.
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 $..
HANDLE->autoflush(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. See "select" in
perldoc on how to select the output channel. See also
IO::Handle. (Mnemonic: when you want your pipes to be piping
hot.)
IO::Handle->output_field_separator EXPR
$OUTPUT_FIELD_SEPARATOR
$OFS
$, The output field separator for the print operator. If defined,
this value is printed between each of print’s arguments.
Default is "undef". (Mnemonic: what is printed when there is a
"," in your print statement.)
IO::Handle->output_record_separator EXPR
$OUTPUT_RECORD_SEPARATOR
$ORS
$\ The output record separator for the print operator. If
defined, this value is printed after the last of print’s
arguments. Default is "undef". (Mnemonic: you set "$\"
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.
HANDLE->format_page_number(EXPR)
$FORMAT_PAGE_NUMBER
$% The current page number of the currently selected output
channel. Used with formats. (Mnemonic: % is page number in
nroff.)
HANDLE->format_lines_per_page(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.)
HANDLE->format_lines_left(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.
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])"
%- Similar to "%+", this variable allows access to the named
capture buffers in the last successful match in the currently
active dynamic scope. To each capture buffer name found in the
regular expression, it associates a reference to an array
containing the list of values captured by all buffers with that
name (should there be several of them), in the order where they
appear.
Here’s an example:
if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
foreach my $bufname (sort keys %-) {
my $ary = $-{$bufname};
foreach my $idx (0..$#$ary) {
print "\$-{$bufname}[$idx] : ",
(defined($ary->[$idx]) ? "'$ary->[$idx]'" : "undef"),
"\n";
}
}
}
would print out:
$-{A}[0] : '1'
$-{A}[1] : '3'
$-{B}[0] : '2'
$-{B}[1] : '4'
The keys of the "%-" hash correspond to all buffer names found
in the regular expression.
The behaviour of "%-" is implemented via the
Tie::Hash::NamedCapture module.
Note: "%-" and "%+" are tied views into a common internal hash
associated with the last successful regular expression.
Therefore mixing iterative access to them via "each" may have
unpredictable results. Likewise, if the last successful match
changes, then the results may be surprising.
HANDLE->format_name(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 $^.)
HANDLE->format_top_name(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.)
IO::Handle->format_line_break_characters 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.)
IO::Handle->format_formfeed 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 perlform and "formline()" in perlfunc.
$CHILD_ERROR
$? The status returned by the last pipe close, backtick ("``")
command, successful call to wait() or waitpid(), or from the
system() operator. This is just the 16-bit status word
returned by the traditional Unix wait() system call (or else is
made up to look like it). Thus, the exit value of the
subprocess is really ("$? >> 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 $? reflect
the actual VMS exit status, instead of the default emulation of
POSIX status; see "$?" in perlvms for details.
Also see "Error Indicators".
${^CHILD_ERROR_NATIVE}
The native status returned by the last pipe close, backtick
("``") command, successful call to wait() or waitpid(), or from
the system() operator. On POSIX-like systems this value can be
decoded with the WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG,
WIFSTOPPED, WSTOPSIG and WIFCONTINUED functions provided by the
POSIX module.
Under VMS this reflects the actual VMS exit status; i.e. it is
the same as $? when the pragma "use vmsish 'status'" is in
effect.
${^ENCODING}
The object reference to the Encode object that is used to
convert the source code to Unicode. Thanks to this variable
your perl script does not have to be written in UTF-8. Default
is undef. The direct manipulation of this variable is highly
discouraged.
$OS_ERROR
$ERRNO
$! If used numerically, yields the current value of the C "errno"
variable, or in other words, if a system or library call fails,
it sets this variable. This means that the value of $! is
meaningful only immediately after a failure:
if (open my $fh, "<", $filename) {
# Here $! is meaningless.
...
} else {
# ONLY here is $! meaningful.
...
# Already here $! might be meaningless.
}
# Since here we might have either success or failure,
# here $! is meaningless.
In the above meaningless stands for anything: zero, non-zero,
"undef". A successful system or library call does not set the
variable to zero.
If used as 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 "Error Indicators".
%OS_ERROR
%ERRNO
%! Each element of "%!" has a true value only if $! is set to that
value. For example, $!{ENOENT} is true if and only if the
current value of $! is "ENOENT"; that is, if the most recent
error was "No such file or directory" (or its moral equivalent:
not all operating systems give that exact error, and certainly
not all languages). To check if a particular key is meaningful
on your system, use "exists $!{the_key}"; for a list of legal
keys, use "keys %!". See Errno for more information, and also
see above for the validity of $!.
$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 "Error Indicators".
$EVAL_ERROR
$@ The Perl syntax error message from the last eval() operator.
If $@ is the null string, 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 "Error Indicators".
$PROCESS_ID
$PID
$$ The process number of the Perl running this script. You should
consider this variable read-only, although it will be altered
across fork() calls. (Mnemonic: same as shells.)
Note for Linux users: on Linux, the C functions "getpid()" and
"getppid()" return different values from different threads. In
order to be portable, this behavior is not reflected by $$,
whose value remains consistent across threads. If you want to
call the underlying "getpid()", you may use the CPAN module
"Linux::Pid".
$REAL_USER_ID
$UID
$< The real uid of this process. (Mnemonic: it’s the uid you came
from, if you’re running setuid.) You can change both the real
uid and the effective uid at the same time by using
POSIX::setuid(). Since changes to $< require a system call,
check $! after a change attempt to detect any possible errors.
$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. Example:
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
You can change both the effective uid and the real uid at the
same time by using POSIX::setuid(). Changes to $> require a
check to $! to detect any possible errors after an attempted
change.
(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 $( without being forced numeric, such as by
adding zero. Note that this is different to the effective gid
($)) which does take a list.
You can change both the real gid and the effective gid at the
same time by using POSIX::setgid(). Changes to $( require a
check to $! to detect any possible errors after an attempted
change.
(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" ".
You can change both the effective gid and the real gid at the
same time by using POSIX::setgid() (use only a single numeric
argument). Changes to $) require a check to $! to detect any
possible errors after an attempted change.
(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 (read: not all) operating systems assigning to $0
modifies the argument area that the "ps" program sees. On some
platforms you may have to use special "ps" options or a
different "ps" to see the changes. Modifying the $0 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 that there are platform specific limitations on the
maximum length of $0. In the most extreme case it may be
limited to the space occupied by the original $0.
In some platforms there may be arbitrary amount of padding, for
example space characters, after the modified name as shown by
"ps". In some platforms this padding may extend all the way to
the original length of the argument area, no matter what you do
(this is the case for example with Linux 2.2).
Note for BSD users: setting $0 does not completely remove
"perl" from the ps(1) output. For example, setting $0 to
"foobar" may result in "perl: foobar (perl)" (whether both the
"perl: " prefix and the " (perl)" suffix are shown depends on
your exact BSD variant and version). This is an operating
system feature, Perl cannot help it.
In multithreaded scripts Perl coordinates the threads so that
any thread may modify its copy of the $0 and the change becomes
visible to ps(1) (assuming the operating system plays along).
Note that the view of $0 the other threads have will not change
since they have their own copies of it.
If the program has been given to perl via the switches "-e" or
"-E", $0 will contain the string "-e".
$[ 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. (That’s why you can only assign compile-time
constants to it.) Its use is highly discouraged.
Note that, unlike other compile-time directives (such as
strict), assignment to $[ can be seen from outer lexical scopes
in the same file. However, you can use local() on it to
strictly bind its value to a lexical block.
$] 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 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
$^C The current value of the flag associated with the -c switch.
Mainly of use with -MO=... to allow code to alter its behavior
when being compiled, such as for example to AUTOLOAD at compile
time rather than normal, deferred loading. Setting "$^C = 1"
is similar to calling "B::minus_c".
$DEBUGGING
$^D The current value of the debugging flags. (Mnemonic: value of
-D switch.) May be read or set. Like its command-line
equivalent, you can use numeric or symbolic values, eg "$^D =
10" or "$^D = "st"".
${^RE_DEBUG_FLAGS}
The current value of the regex debugging flags. Set to 0 for no
debug output even when the re ’debug’ module is loaded. See re
for details.
${^RE_TRIE_MAXBUF}
Controls how certain regex optimisations are applied and how
much memory they utilize. This value by default is 65536 which
corresponds to a 512kB temporary cache. Set this to a higher
value to trade memory for speed when matching large
alternations. Set it to a lower value if you want the
optimisations to be as conservative of memory as possible but
still occur, and set it to a negative value to prevent the
optimisation and conserve the most memory. Under normal
situations this variable should be of no interest to you.
$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 The %^H hash provides the same scoping semantic as $^H. This
makes it useful for implementation of lexically scoped pragmas.
See perlpragma.
$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
add custom C compilation flags when compiling perl. 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 Config and
the -V command-line switch documented in perlrun.
In Windows platforms, $^O is not very helpful: since it is
always "MSWin32", it doesn’t tell the difference between
95/98/ME/NT/2000/XP/CE/.NET. Use Win32::GetOSName() or
Win32::GetOSVersion() (see Win32 and perlport) to distinguish
between the variants.
${^OPEN}
An internal variable used by PerlIO. A string in two parts,
separated by a "\0" byte, the first part describes the input
layers, the second part describes the output layers.
$PERLDB
$^P The internal variable for debugging support. The meanings of
the various bits are subject to change, but currently indicate:
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging. Causes DB::DB() subroutine to be
called for each statement executed. Also causes saving
source code lines (like 0x400).
0x04 Switch off optimizations.
0x08 Preserve more data for future interactive inspections.
0x10 Keep info about source lines on which a subroutine is
defined.
0x20 Start with single-step on.
0x40 Use subroutine address instead of name when reporting.
0x80 Report "goto &subroutine" as well.
0x100 Provide informative "file" names for evals based on the
place they were compiled.
0x200 Provide informative names to anonymous subroutines based
on the place they were compiled.
0x400 Save source code lines into "@{"_<$filename"}".
Some bits may be relevant at compile-time only, some at run-
time only. This is a new mechanism and the details may change.
See also perldebguts.
$LAST_REGEXP_CODE_RESULT
$^R The result of evaluation of the last successful "(?{ code })"
regular expression assertion (see perlre). May be written to.
$EXCEPTIONS_BEING_CAUGHT
$^S Current state of the interpreter.
$^S State
--------- -------------------
undef Parsing module/eval
true (1) Executing an eval
false (0) Otherwise
The first state may happen in $SIG{__DIE__} and $SIG{__WARN__}
handlers.
$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.
${^TAINT}
Reflects if taint mode is on or off. 1 for on (the program was
run with -T), 0 for off, -1 when only taint warnings are
enabled (i.e. with -t or -TU). This variable is read-only.
${^UNICODE}
Reflects certain Unicode settings of Perl. See perlrun
documentation for the "-C" switch for more information about
the possible values. This variable is set during Perl startup
and is thereafter read-only.
${^UTF8CACHE}
This variable controls the state of the internal UTF-8 offset
caching code. 1 for on (the default), 0 for off, -1 to debug
the caching code by checking all its results against linear
scans, and panicking on any discrepancy.
${^UTF8LOCALE}
This variable indicates whether an UTF-8 locale was detected by
perl at startup. This information is used by perl when it’s in
adjust-utf8ness-to-locale mode (as when run with the "-CL"
command-line switch); see perlrun for more info on this.
$PERL_VERSION
$^V The revision, version, and subversion of the Perl interpreter,
represented as a "version" object.
This variable first appeared in perl 5.6.0; earlier versions of
perl will see an undefined value. Before perl 5.10.0 $^V was
represented as a v-string.
$^V can be used to determine whether the Perl interpreter
executing a script is in the right range of versions.
(Mnemonic: use ^V for Version Control.) Example:
warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1
To convert $^V into its string representation use sprintf()’s
"%vd" conversion:
printf "version is v%vd\n", $^V; # Perl's version
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 warnings.
${^WARNING_BITS}
The current set of warning checks enabled by the "use warnings"
pragma. See the documentation of "warnings" for more details.
${^WIN32_SLOPPY_STAT}
If this variable is set to a true value, then stat() on Windows
will not try to open the file. This means that the link count
cannot be determined and file attributes may be out of date if
additional hardlinks to the file exist. On the other hand, not
opening the file is considerably faster, especially for files
on network drives.
This variable could be set in the sitecustomize.pl file to
configure the local Perl installation to use "sloppy" stat() by
default. See perlrun for more information about site
customization.
$EXECUTABLE_NAME
$^X The name used to execute the current copy of Perl, from C’s
"argv[0]" or (where supported) /proc/self/exe.
Depending on the host operating system, the value of $^X may be
a relative or absolute pathname of the perl program file, or
may be the string used to invoke perl but not the pathname of
the perl program file. Also, most operating systems permit
invoking programs that are not in the PATH environment
variable, so there is no guarantee that the value of $^X is in
PATH. For VMS, the value may or may not include a version
number.
You usually can use the value of $^X to re-invoke an
independent copy of the same perl that is currently running,
e.g.,
@first_run = `$^X -le "print int rand 100 for 1..100"`;
But recall that not all operating systems support forking or
capturing of the output of commands, so this complex statement
may not be portable.
It is not safe to use the value of $^X as a path name of a
file, as some operating systems that have a mandatory suffix on
executable files do not require use of the suffix when invoking
a command. To convert the value of $^X to a path name, use the
following statements:
# Build up a set of file names (not command names).
use Config;
$this_perl = $^X;
if ($^O ne 'VMS')
{$this_perl .= $Config{_exe}
unless $this_perl =~ m/$Config{_exe}$/i;}
Because many operating systems permit anyone with read access
to the Perl program file to make a copy of it, patch the copy,
and then execute the copy, the security-conscious Perl
programmer should take care to invoke the installed copy of
perl, not the copy referenced by $^X. The following statements
accomplish this goal, and produce a pathname that can be
invoked as a command or referenced as a file.
use Config;
$secure_perl_path = $Config{perlpath};
if ($^O ne 'VMS')
{$secure_perl_path .= $Config{_exe}
unless $secure_perl_path =~ m/$Config{_exe}$/i;}
ARGV The special filehandle that iterates over command-line
filenames in @ARGV. Usually written as the null filehandle in
the angle operator "<>". Note that currently "ARGV" only has
its magical effect within the "<>" operator; elsewhere it is
just a plain filehandle corresponding to the last file opened
by "<>". In particular, passing "\*ARGV" as a parameter to a
function that expects a filehandle may not cause your function
to automatically read the contents of all the files in @ARGV.
$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.
ARGVOUT The special filehandle that points to the currently open output
file when doing edit-in-place processing with -i. Useful when
you have to do a lot of inserting and don’t want to keep
modifying $_. See perlrun for the -i switch.
@F The array @F contains the fields of each line read in when
autosplit mode is turned on. See perlrun for the -a switch.
This array is package-specific, and must be declared or given a
full package name if not in package main when running under
"strict 'vars'".
@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. ("." will not be appended if taint checks are
enabled, either by "-T" or by "-t".) 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;
You can also insert hooks into the file inclusion system by
putting Perl code directly into @INC. Those hooks may be
subroutine references, array references or blessed objects.
See "require" in perlfunc for details.
@ARG
@_ Within a subroutine the array @_ contains the parameters passed
to that subroutine. See perlsub.
%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.
If the file was loaded via a hook (e.g. a subroutine reference,
see "require" in perlfunc for a description of these hooks),
this hook is by default inserted into %INC in place of a
filename. Note, however, that the hook may have set the %INC
entry by itself to provide some more specific info.
%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 perlipc 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.
The default delivery policy of signals changed in Perl 5.8.0
from immediate (also known as "unsafe") to deferred, also known
as "safe signals". See perlipc for more information.
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;
As the 'IGNORE' hook is not supported by "__WARN__", you can
disable warnings using the empty subroutine:
local $SIG{__WARN__} = sub {};
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::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 "die" in perlfunc, "warn" in perlfunc, "eval" in perlfunc,
and warnings 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 my $pipe, "/cdrom/install |" or die $!;
my @res = <$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 Fatal, 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 perlmod.
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.
$^_ itself, however, is reserved.
Perl identifiers that begin with digits, control characters, or
punctuation characters are exempt from the effects of the "package"
declaration and are always forced to be in package "main"; they are
also exempt from "strict 'vars'" errors. A few other names are also
exempt in these ways:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT _
SIG
In particular, the new special "${^_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.cpan.org/modules/by-module/Devel/ ) for more
information. Writing "use English '-no_match_vars';" avoids the
performance penalty.
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.