perlvar - Perl predefined variables
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 builtin
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:
* 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.)
$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 igits.) These variables are all read-only
and dynamically scoped to the current BLOCK.
$MATCH
$& The string matched by the last successful pattern
match (not counting any matches hidden within a
BLOCK or eval() enclosed by the current BLOCK).
(Mnemonic: like & in some editors.) This variable
is read-only and dynamically scoped to the current
BLOCK.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See "BUGS".
$PREMATCH
$` The string preceding whatever was matched by the
last successful pattern match (not counting any
matches hidden within a BLOCK or eval enclosed by
the current BLOCK). (Mnemonic: "`" often precedes
a quoted string.) This variable is read-only.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See "BUGS".
$POSTMATCH
$' The string following whatever was matched by the
last successful pattern match (not counting any
matches hidden within a BLOCK or eval() enclosed
by the current BLOCK). (Mnemonic: "'" often follows
a quoted string.) Example:
local $_ = 'abcdefghi';
/def/;
print "$`:$&:$'0; # prints abc:def:ghi
This variable is read-only and dynamically scoped
to the current BLOCK.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See "BUGS".
$LAST_PAREN_MATCH
$+ The text matched by the last bracket of the last
successful search pattern. This is useful if you
don't know which one of a set of alternative patterns
matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This
variable is read-only and dynamically scoped to
the current BLOCK.
$^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.
$* 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.
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 filehandle's
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 "0 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 "0 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/]+/ /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
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 recordoriented
file (or your OS doesn't have record-oriented
files), then you'll likely get a full chunk
of data with every read. If a record is larger
than the record size you've set, you'll get the
record back in pieces.
On VMS, record reads are done with the equivalent
of "sysread", so it's best not to mix record and
non-record reads on the same file. (This is
unlikely to be a problem, because any file you'd
want to read in record mode is probably unusable
in line mode.) Non-VMS systems do normal I/O, so
it's safe to mix record and non-record reads of a
file.
See also "Newlines" in perlport. Also see $..
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. (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.
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.)
IO::Handle->output_record_separator 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 "$
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 " 34", 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.
$# 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.
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])"
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 " 0, 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
$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 "form-
line()" in perlfunc.
$CHILD_ERROR
$? The status returned by the last pipe close, backtick
(``) command, successful call to wait() or
waitpid(), or from the system() operator. This is
just the 16-bit status word returned by the wait()
system call (or else is made up to look like it).
Thus, the exit value of the subprocess is really
("$? >> 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".
${^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. See encoding for more details.
$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(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".
%! 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().
$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().
(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.
You can change both the real gid and the effective
gid at the same time by using POSIX::setgid().
(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).
(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 the view of $0 the other threads have
will not change since they have their own copies
of it.
$[ 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 sub-
str() 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 bound
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!0 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.
When testing the variable, to steer clear of
floating point inaccuracies you might want to prefer
the inequality tests "<" and ">" to the tests
containing equivalence: "<=", "==", and ">=".
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
$^C The current value of the flag associated with the
-c switch. Mainly of use with -MO=... to allow
code to alter its behavior when being compiled,
such as for example to AUTOLOAD at compile time
rather than normal, deferred loading. See perlcc.
Setting "$^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"".
$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 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 " " 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.
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 Debug assertion subroutines enter/exit.
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
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).
${^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.
$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
To convert $^V into its string representation use
sprintf()'s "%vd" conversion:
printf "version is v%vd0, $^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.
$EXECUTABLE_NAME
$^X The name used to execute the current copy of Perl,
from C's "argv[0]".
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 commandline
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 "RGV" 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.
@_ 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 down0;
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;
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 [Toc] [Back]
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
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