PERLVAR(1) Perl Programmers Reference Guide PERLVAR(1)NAMEperlvar - 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 vari
able names, you need only say
use English;
at the top of your program. This will alias all the short
names to the long names in the current package. Some even
have medium names, generally borrowed from awk.
If you don't mind the performance hit, variables that
depend on the currently selected filehandle may instead be
set by calling an appropriate object method on the
IO::Handle object. (Summary lines below for this contain
the word HANDLE.) First you must say
use IO::Handle;
after which you may use either
method HANDLE EXPR
or more safely,
HANDLE->method(EXPR)
Each method returns the old value of the IO::Handle
attribute. The methods each take an optional EXPR, which
if supplied specifies the new value for the IO::Handle
attribute in question. If not supplied, most methods do
nothing to the current value--except for autoflush(),
which will assume a 1 for you, just to be different.
Because loading in the IO::Handle class is an expensive
operation, you should learn how to use the regular built-
in variables.
A few of these variables are considered "read-only". This
means that if you try to assign to this variable, either
directly or indirectly through a reference, you'll raise a
run-time exception.
The following list is ordered by scalar variables first,
then the arrays, then the hashes.
$ARG
$_ The default input and pattern-searching space.
The following pairs are equivalent:
while (<>) {...} # equivalent only in while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chomp
chomp($_)
Here are the places where Perl will assume $_ even
if you don't use it:
Various unary functions, including functions
like ord() and int(), as well as the all file
tests ("-f", "-d") except for "-t", which
defaults to STDIN.
Various list functions like print() and
unlink().
The pattern matching operations "m//", "s///",
and "tr///" when used without an "=~" operator.
The default iterator variable in a "foreach"
loop if no other variable is supplied.
The implicit iterator variable in the grep()
and map() functions.
The default place to put an input record when a
"<FH>" operation's result is tested by itself
as the sole criterion of a "while" test. Out
side a "while" test, this will not happen.
(Mnemonic: underline is understood in certain
operations.)
$<digits>
Contains the subpattern from the corresponding set
of capturing parentheses from the last pattern
match, not counting patterns matched in nested
blocks that have been exited already. (Mnemonic:
like \digits.) These variables are all read-only
and dynamically scoped to the current BLOCK.
$MATCH
$& The string matched by the last successful pattern
match (not counting any matches hidden within a
BLOCK or eval() enclosed by the current BLOCK).
(Mnemonic: like & in some editors.) This variable
is read-only and dynamically scoped to the current
BLOCK.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See the BUGS manpage.
$PREMATCH
$` The string preceding whatever was matched by the
last successful pattern match (not counting any
matches hidden within a BLOCK or eval enclosed by
the current BLOCK). (Mnemonic: "`" often precedes
a quoted string.) This variable is read-only.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See the BUGS manpage.
$POSTMATCH
$' The string following whatever was matched by the
last successful pattern match (not counting any
matches hidden within a BLOCK or eval() enclosed
by the current BLOCK). (Mnemonic: "'" often fol
lows a quoted string.) Example:
$_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
This variable is read-only and dynamically scoped
to the current BLOCK.
The use of this variable anywhere in a program
imposes a considerable performance penalty on all
regular expression matches. See the BUGS manpage.
$LAST_PAREN_MATCH
$+ The last bracket matched by the last search pat
tern. This is useful if you don't know which one
of a set of alternative patterns matched. For
example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This
variable is read-only and dynamically scoped to
the current BLOCK.
@LAST_MATCH_END
@+ This array holds the offsets of the ends of the
last successful submatches in the currently active
dynamic scope. "$+[0]" is the offset into the
string of the end of the entire match. This is
the same value as what the "pos" function returns
when called on the variable that was matched
against. The nth element of this array holds the
offset of the nth submatch, so "$+[1]" is the off
set 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 suc
cessful match. See the examples given for the
"@-" variable.
$MULTILINE_MATCHING
$* Set to a non-zero integer value to do multi-line
matching within a string, 0 (or undefined) to tell
Perl that it can assume that strings contain a
single line, for the purpose of optimizing pattern
matches. Pattern matches on strings containing
multiple newlines can produce confusing results
when "$*" is 0 or undefined. Default is undefined.
(Mnemonic: * matches multiple things.) This vari
able influences the interpretation of only "^" and
"$". A literal newline can be searched for even
when "$* == 0".
Use of "$*" is deprecated in modern Perl, sup
planted by the "/s" and "/m" modifiers on pattern
matching.
Assigning a non-numerical value to "$*" triggers a
warning (and makes "$*" act if "$* == 0"), while
assigning a numerical value to "$*" makes that an
implicit "int" is applied on the value.
input_line_number HANDLE EXPR
$INPUT_LINE_NUMBER
$NR
$. The current input record number for the last file
handle from which you just read() (or called a
"seek" or "tell" on). The value may be different
from the actual physical line number in the file,
depending on what notion of "line" is in
effect--see "$/" on how to change that. An
explicit close on a filehandle resets the line
number. Because "<>" never does an explicit
close, line numbers increase across ARGV files
(but see examples in the eof entry in the perlfunc
manpage). Consider this variable read-only: set
ting it does not reposition the seek pointer;
you'll have to do that on your own. Localizing
"$." has the effect of also localizing Perl's
notion of "the last read filehandle". (Mnemonic:
many programs use "." to mean the current line
number.)
input_record_separator HANDLE EXPR
$INPUT_RECORD_SEPARATOR
$RS
$/ The input record separator, newline by default.
This influences Perl's idea of what a "line" is.
Works like awk's RS variable, including treating
empty lines as a terminator if set to the null
string. (An empty line cannot contain any spaces
or tabs.) You may set it to a multi-character
string to match a multi-character terminator, or
to "undef" to read through the end of file. Set
ting it to ""\n\n"" means something slightly dif
ferent 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 new
line. (Mnemonic: / delimits line boundaries when
quoting poetry.)
undef $/; # enable "slurp" mode
$_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
Remember: the value of "$/" is a string, not a
regex. awk has to be better for something. :-)
Setting "$/" to a reference to an integer, scalar
containing an integer, or scalar that's convert
ible to an integer will attempt to read records
instead of lines, with the maximum record size
being the referenced integer. So this:
$/ = \32768; # or \"32768", or \$var_containing_32768
open(FILE, $myfile);
$_ = <FILE>;
will read a record of no more than 32768 bytes
from FILE. If you're not reading from a record-
oriented file (or your OS doesn't have record-ori
ented files), then you'll likely get a full chunk
of data with every read. If a record is larger
than the record size you've set, you'll get the
record back in pieces.
On VMS, record reads are done with the equivalent
of "sysread", so it's best not to mix record and
non-record reads on the same file. (This is
unlikely to be a problem, because any file you'd
want to read in record mode is probably unusable
in line mode.) Non-VMS systems do normal I/O, so
it's safe to mix record and non-record reads of a
file.
See also the Newlines entry in the perlport man
page. Also see "$.".
autoflush HANDLE EXPR
$OUTPUT_AUTOFLUSH
$| If set to nonzero, forces a flush right away and
after every write or print on the currently
selected output channel. Default is 0 (regardless
of whether the channel is really buffered by the
system or not; "$|" tells you only whether you've
asked Perl explicitly to flush after each write).
STDOUT will typically be line buffered if output
is to the terminal and block buffered otherwise.
Setting this variable is useful primarily when you
are outputting to a pipe or socket, such as when
you are running a Perl program under rsh and want
to see the output as it's happening. This has no
effect on input buffering. See the getc entry in
the perlfunc manpage for that. (Mnemonic: when
you want your pipes to be piping hot.)
output_field_separator HANDLE EXPR
$OUTPUT_FIELD_SEPARATOR
$OFS
$, The output field separator for the print operator.
Ordinarily the print operator simply prints out
its arguments without further adornment. To get
behavior more like awk, set this variable as you
would set awk's OFS variable to specify what is
printed between fields. (Mnemonic: what is
printed when there is a "," in your print state
ment.)
output_record_separator HANDLE EXPR
$OUTPUT_RECORD_SEPARATOR
$ORS
$\ The output record separator for the print opera
tor. Ordinarily the print operator simply prints
out its arguments as is, with no trailing newline
or other end-of-record string added. To get
behavior more like awk, set this variable as you
would set awk's ORS variable to specify what is
printed at the end of the print. (Mnemonic: you
set "$\" instead of adding "\n" at the end of the
print. Also, it's just like "$/", but it's what
you get "back" from Perl.)
$LIST_SEPARATOR
$"" This is like "$," except that it applies to array
and slice values interpolated into a double-quoted
string (or similar interpreted string). Default
is a space. (Mnemonic: obvious, I think.)
$SUBSCRIPT_SEPARATOR
$SUBSEP
$; The subscript separator for multidimensional array
emulation. If you refer to a hash element as
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
But don't put
@foo{$a,$b,$c} # a slice--note the @
which means
($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk. If
your keys contain binary data there might not be
any safe value for "$;". (Mnemonic: comma (the
syntactic subscript separator) is a semi-semi
colon. Yeah, I know, it's pretty lame, but "$,"
is already taken for something more important.)
Consider using "real" multidimensional arrays as
described in the perllol manpage.
$OFMT
$# The output format for printed numbers. This vari
able is a half-hearted attempt to emulate awk's
OFMT variable. There are times, however, when awk
and Perl have differing notions of what counts as
numeric. The initial value is "%.ng", where n is
the value of the macro DBL_DIG from your system's
float.h. This is different from awk's default
OFMT setting of "%.6g", so you need to set "$#"
explicitly to get awk's value. (Mnemonic: # is
the number sign.)
Use of "$#" is deprecated.
format_page_number HANDLE EXPR
$FORMAT_PAGE_NUMBER
$% The current page number of the currently selected
output channel. Used with formats. (Mnemonic: %
is page number in nroff.)
format_lines_per_page HANDLE EXPR
$FORMAT_LINES_PER_PAGE
$= The current page length (printable lines) of the
currently selected output channel. Default is 60.
Used with formats. (Mnemonic: = has horizontal
lines.)
format_lines_left HANDLE EXPR
$FORMAT_LINES_LEFT
$- The number of lines left on the page of the cur
rently selected output channel. Used with for
mats. (Mnemonic: lines_on_page - lines_printed.)
@LAST_MATCH_START
@- $-[0] is the offset of the start of the last suc
cessful match. "$-["n"]" is the offset of the
start of the substring matched by n-th subpattern,
or undef if the subpattern did not match.
Thus after a match against $_, $& coincides with
"substr $_, $-[0], $+[0] - $-[0]". Similarly,
"$"n coincides with "substr $_, $-["n"], $+["n"] -
$-["n"]" if "$-["n"]" is defined, and $+ coincides
with "substr $_, $-[$#-], $+[$#-]". One can use
"$#-" to find the last matched subgroup in the
last successful match. Contrast with "$#+", the
number of subgroups in the regular expression.
Compare with "@+".
This array holds the offsets of the beginnings of
the last successful submatches in the currently
active dynamic scope. "$-[0]" is the offset into
the string of the beginning of the entire match.
The nth element of this array holds the offset of
the nth submatch, so "$+[1]" is the offset where
$1 begins, "$+[2]" the offset where $2 begins, and
so on. You can use "$#-" to determine how many
subgroups were in the last successful match. Com
pare with the "@+" variable.
After a match against some variable $var:
""$`"" is the same as ""substr($var, 0, $-[0])""
""$&"" is the same as ""substr($var, $-[0], $+[0]
- $-[0])""
""$'"" is the same as ""substr($var, $+[0])""
""$1"" is the same as ""substr($var, $-[1], $+[1]
- $-[1])""
""$2"" is the same as ""substr($var, $-[2], $+[2]
- $-[2])""
""$3"" is the same as ""substr $var, $-[3], $+[3]
- $-[3])""
format_name HANDLE EXPR
$FORMAT_NAME
$~ The name of the current report format for the cur
rently selected output channel. Default is the
name of the filehandle. (Mnemonic: brother to
"$^".)
format_top_name HANDLE EXPR
$FORMAT_TOP_NAME
$^ The name of the current top-of-page format for the
currently selected output channel. Default is the
name of the filehandle with _TOP appended.
(Mnemonic: points to top of page.)
format_line_break_characters HANDLE EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$: The current set of characters after which a string
may be broken to fill continuation fields (start
ing with ^) in a format. Default is " \n-", to
break on whitespace or hyphens. (Mnemonic: a
"colon" in poetry is a part of a line.)
format_formfeed HANDLE EXPR
$FORMAT_FORMFEED
$^L What formats output as a form feed. Default is
\f.
$ACCUMULATOR
$^A The current value of the write() accumulator for
format() lines. A format contains formline()
calls that put their result into "$^A". After
calling its format, write() prints out the con
tents of "$^A" and empties. So you never really
see the contents of "$^A" unless you call form_
line() yourself and then look at it. See the
perlform manpage and the formline() entry in the
perlfunc manpage.
$CHILD_ERROR
$? The status returned by the last pipe close, back
tick ("``") command, successful call to wait() or
waitpid(), or from the system() operator. This is
just the 16-bit status word returned by the wait()
system call (or else is made up to look like it).
Thus, the exit value of the subprocess is really
("$? >> 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 sup
ported in C, its value is returned via $? if any
"gethost*()" function fails.
If you have installed a signal handler for
"SIGCHLD", the value of "$?" will usually be wrong
outside that handler.
Inside an "END" subroutine "$?" contains the value
that is going to be given to "exit()". You can
modify "$?" in an "END" subroutine to change the
exit status of your program. For example:
END {
$? = 1 if $? == 255; # die would make it 255
}
Under VMS, the pragma "use vmsish 'status'" makes
"$?" reflect the actual VMS exit status, instead
of the default emulation of POSIX status.
Also see the Error Indicators entry elsewhere in
this document.
$OS_ERROR
$ERRNO
$! If used numerically, yields the current value of
the C "errno" variable, with all the usual
caveats. (This means that you shouldn't depend on
the value of "$!" to be anything in particular
unless you've gotten a specific error return indi
cating a system error.) If used an a string,
yields the corresponding system error string. You
can assign a number to "$!" to set errno if, for
instance, you want ""$!"" to return the string for
error n, or you want to set the exit value for the
die() operator. (Mnemonic: What just went bang?)
Also see the Error Indicators entry elsewhere in
this document.
$EXTENDED_OS_ERROR
$^E Error information specific to the current operat
ing system. At the moment, this differs from "$!"
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, "$^E" always returns the last error
information reported by the Win32 call "GetLastEr
ror()" which describes the last error from within
the Win32 API. Most Win32-specific code will
report errors via "$^E". ANSI C and Unix-like
calls set "errno" and so most portable Perl code
will report errors via "$!".
Caveats mentioned in the description of "$!" gen
erally apply to "$^E", also. (Mnemonic: Extra
error explanation.)
Also see the Error Indicators entry elsewhere in
this document.
$EVAL_ERROR
$@ The Perl syntax error message from the last eval()
operator. If null, the last eval() parsed and
executed correctly (although the operations you
invoked may have failed in the normal fashion).
(Mnemonic: Where was the syntax error "at"?)
Warning messages are not collected in this vari
able. You can, however, set up a routine to pro
cess warnings by setting "$SIG{__WARN__}" as
described below.
Also see the Error Indicators entry elsewhere in
this document.
$PROCESS_ID
$PID
$$ The process number of the Perl running this
script. You should consider this variable read-
only, although it will be altered across fork()
calls. (Mnemonic: same as shells.)
$REAL_USER_ID
$UID
$< The real uid of this process. (Mnemonic: it's the
uid you came from, if you're running setuid.)
$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. Example:
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went to, if you're
running setuid.) "$<" and "$>" can be swapped
only on machines supporting setreuid().
$REAL_GROUP_ID
$GID
$( The real gid of this process. If you are on a
machine that supports membership in multiple
groups simultaneously, gives a space separated
list of groups you are in. The first number is
the one returned by getgid(), and the subsequent
ones by getgroups(), one of which may be the same
as the first number.
However, a value assigned to "$(" must be a single
number used to set the real gid. So the value
given by "$(" should not be assigned back to "$("
without being forced numeric, such as by adding
zero.
(Mnemonic: parentheses are used to group things.
The real gid is the group you left, if you're run
ning setgid.)
$EFFECTIVE_GROUP_ID
$EGID
$) The effective gid of this process. If you are on
a machine that supports membership in multiple
groups simultaneously, gives a space separated
list of groups you are in. The first number is
the one returned by getegid(), and the subsequent
ones by getgroups(), one of which may be the same
as the first number.
Similarly, a value assigned to "$)" must also be a
space-separated list of numbers. The first number
sets the effective gid, and the rest (if any) are
passed to setgroups(). To get the effect of an
empty list for setgroups(), just repeat the new
effective gid; that is, to force an effective gid
of 5 and an effectively empty setgroups() list,
say " $) = "5 5" ".
(Mnemonic: parentheses are used to group things.
The effective gid is the group that's right for
you, if you're running setgid.)
"$<", "$>", "$(" and "$)" can be set only on
machines that support the corresponding
set[re][ug]id() routine. "$(" and "$)" can be
swapped only on machines supporting setregid().
$PROGRAM_NAME
$0 Contains the name of the program being executed.
On some operating systems assigning to "$0" modi
fies the argument area that the ps program sees.
This is more useful as a way of indicating the
current program state than it is for hiding the
program you're running. (Mnemonic: same as sh and
ksh.)
Note for BSD users: setting "$0" does not com
pletely remove "perl" from the ps(1) output. For
example, setting "$0" to ""foobar"" will result in
""perl: foobar (perl)"". This is an operating
system feature.
$[ The index of the first element in an array, and of
the first character in a substring. Default is 0,
but you could theoretically set it to 1 to make
Perl behave more like awk (or Fortran) when sub
scripting and when evaluating the index() and sub_
str() functions. (Mnemonic: [ begins subscripts.)
As of release 5 of Perl, assignment to "$[" is
treated as a compiler directive, and cannot influ
ence the behavior of any other file. Its use is
highly discouraged.
$] The version + patchlevel / 1000 of the Perl inter
preter. 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?) Exam
ple:
warn "No checksumming!\n" if $] < 3.019;
See also the documentation of "use VERSION" and
"require VERSION" for a convenient way to fail if
the running Perl interpreter is too old.
The use of this variable is deprecated. The
floating point representation can sometimes lead
to inaccurate numeric comparisons. See "$^V" for
a more modern representation of the Perl version
that allows accurate string comparisons.
$COMPILING
$^C The current value of the flag associated with the
-c switch. Mainly of use with -MO=... to allow
code to alter its behavior when being compiled,
such as for example to AUTOLOAD at compile time
rather than normal, deferred loading. See the
perlcc manpage. Setting "$^C = 1" is similar to
calling "B::minus_c".
$DEBUGGING
$^D The current value of the debugging flags.
(Mnemonic: value of -D switch.)
$SYSTEM_FD_MAX
$^F The maximum system file descriptor, ordinarily 2.
System file descriptors are passed to exec()ed
processes, while higher file descriptors are not.
Also, during an open(), system file descriptors
are preserved even if the open() fails. (Ordinary
file descriptors are closed before the open() is
attempted.) The close-on-exec status of a file
descriptor will be decided according to the value
of "$^F" when the corresponding file, pipe, or
socket was opened, not the time of the exec().
$^H WARNING: This variable is strictly for internal
use only. Its availability, behavior, and con
tents 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 con
ditional 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 imple
mented. 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 con
tents 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.)
$^M By default, running out of memory is an untrap
pable, fatal error. However, if suitably built,
Perl can use the contents of "$^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 emer
gency. See the INSTALL file in the Perl distribu
tion for information on how to enable this option.
To discourage casual use of this advanced feature,
there is no English long name for this variable.
$OSNAME
$^O The name of the operating system under which this
copy of Perl was built, as determined during the
configuration process. The value is identical to
"$Config{'osname'}". See also the Config manpage
and the -V command-line switch documented in the
perlrun manpage.
$PERLDB
$^P The internal variable for debugging support. The
meanings of the various bits are subject to
change, but currently indicate:
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging.
0x04 Switch off optimizations.
0x08 Preserve more data for future interactive
inspections.
0x10 Keep info about source lines on which a sub
routine 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 sub
routines based on the place they were com
piled.
Some bits may be relevant at compile-time only,
some at run-time only. This is a new mechanism
and the details may change.
$LAST_REGEXP_CODE_RESULT
$^R The result of evaluation of the last successful
"(?{ code })" regular expression assertion (see
the perlre manpage). May be written to.
$EXCEPTIONS_BEING_CAUGHT
$^S Current state of the interpreter. Undefined if
parsing of the current module/eval is not finished
(may happen in $SIG{__DIE__} and $SIG{__WARN__}
handlers). True if inside an eval(), otherwise
false.
$BASETIME
$^T The time at which the program began running, in
seconds since the epoch (beginning of 1970). The
values returned by the -M, -A, and -C filetests
are based on this value.
$PERL_VERSION
$^V The revision, version, and subversion of the Perl
interpreter, represented as a string composed of
characters with those ordinals. Thus in Perl
v5.6.0 it equals "chr(5) . chr(6) . chr(0)" and
will return true for "$^V eq v5.6.0". Note that
the characters in this string value can poten
tially be in Unicode range.
This can be used to determine whether the Perl
interpreter executing a script is in the right
range of versions. (Mnemonic: use ^V for Version
Control.) Example:
warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;
See the documentation of "use VERSION" and
"require VERSION" for a convenient way to fail if
the running Perl interpreter is too old.
See also "$]" for an older representation of the
Perl version.
$WARNING
$^W The current value of the warning switch, initially
true if -w was used, false otherwise, but directly
modifiable. (Mnemonic: related to the -w switch.)
See also the warnings manpage.
${^WARNING_BITS}
The current set of warning checks enabled by the
"use warnings" pragma. See the documentation of
"warnings" for more details.
${^WIDE_SYSTEM_CALLS}
Global flag that enables system calls made by Perl
to use wide character APIs native to the system,
if available. This is currently only implemented
on the Windows platform.
This can also be enabled from the command line
using the "-C" switch.
The initial value is typically "0" for compatibil
ity with Perl versions earlier than 5.6, but may
be automatically set to "1" by Perl if the system
provides a user-settable default (e.g.,
"$ENV{LC_CTYPE}").
The "bytes" pragma always overrides the effect of
this flag in the current lexical scope. See the
bytes manpage.
$EXECUTABLE_NAME
$^X The name that the Perl binary itself was executed
as, from C's "argv[0]". This may not be a full
pathname, nor even necessarily in your path.
$ARGV contains the name of the current file when reading
from <>.
@ARGV The array @ARGV contains the command-line argu
ments intended for the script. "$#ARGV" is gener
ally the number of arguments minus one, because
"$ARGV[0]" is the first argument, not the pro
gram's command name itself. See "$0" for the com
mand name.
@INC The array @INC contains the list of places that
the "do EXPR", "require", or "use" constructs look
for their library files. It initially consists of
the arguments to any -I command-line switches,
followed by the default Perl library, probably
/usr/local/lib/perl, followed by ".", to represent
the current directory. If you need to modify this
at runtime, you should use the "use lib" pragma to
get the machine-dependent library properly loaded
also:
use lib '/mypath/libdir/';
use SomeMod;
@_ Within a subroutine the array @_ contains the
parameters passed to that subroutine. See the
perlsub manpage.
%INC The hash %INC contains entries for each filename
included via the "do", "require", or "use" opera
tors. The key is the filename you specified (with
module names converted to pathnames), and the
value is the location of the file found. The
"require" operator uses this hash to determine
whether a particular file has already been
included.
%ENV
$ENV{expr}
The hash %ENV contains your current environment.
Setting a value in "ENV" changes the environment
for any child processes you subsequently fork()
off.
%SIG
$SIG{expr}
The hash %SIG contains signal handlers for sig
nals. For example:
sub handler { # 1st argument is signal name
my($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" sig
nal. See the perlipc manpage for more about this
special case.
Here are some other examples:
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
$SIG{"PIPE"} = *Plumber; # somewhat esoteric
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
Be sure not to use a bareword as the name of a
signal handler, lest you inadvertently call it.
If your system has the sigaction() function then
signal handlers are installed using it. This
means you get reliable signal handling. If your
system has the SA_RESTART flag it is used when
signals handlers are installed. This means that
system calls for which restarting is supported
continue rather than returning when a signal
arrives. If you want your system calls to be
interrupted by signal delivery then do something
like this:
use POSIX ':signal_h';
my $alarm = 0;
sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
or die "Error setting SIGALRM handler: $!\n";
See the POSIX manpage.
Certain internal hooks can be also set using the
%SIG hash. The routine indicated by
"$SIG{__WARN__}" is called when a warning message
is about to be printed. The warning message is
passed as the first argument. The presence of a
__WARN__ hook causes the ordinary printing of
warnings to STDERR to be suppressed. You can use
this to save warnings in a variable, or turn warn
ings 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 dis
abled during the call, so that you can die from a
"__DIE__" handler. Similarly for "__WARN__".
Due to an implementation glitch, the
"$SIG{__DIE__}" hook is called even inside an
eval(). Do not use this to rewrite a pending
exception in "$@", or as a bizarre substitute for
overriding CORE::GLOBAL:\fIs0:die(). This strange
action at a distance may be fixed in a future
release so that "$SIG{__DIE__}" is only called if
your program is about to exit, as was the original
intent. Any other use is deprecated.
"__DIE__"/"__WARN__" handlers are very special in
one respect: they may be called to report (proba
ble) errors found by the parser. In such a case
the parser may be in inconsistent state, so any
attempt to evaluate Perl code from such a handler
will probably result in a segfault. This means
that warnings or errors that result from parsing
Perl should be used with extreme caution, like
this:
require Carp if defined $^S;
Carp::confess("Something wrong") if defined &Carp::confess;
die "Something wrong, but could not load Carp to give backtrace...
To see backtrace try starting Perl with -MCarp switch";
Here the first line will load Carp unless it is
the parser who called the handler. The second
line will print backtrace and die if Carp was
available. The third line will be executed only
if Carp was not available.
See the die entry in the perlfunc manpage, the
warn entry in the perlfunc manpage, the eval entry
in the perlfunc manpage, and the warnings manpage
for additional information.
Error Indicators
The variables "$@", "$!", "$^E", and "$?" contain informa
tion 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 cor
respond to errors detected by the Perl interpreter, C
library, operating system, or an external program, respec
tively.
To illustrate the differences between these variables,
consider the following Perl expression, which uses a sin
gle-quoted string:
eval q{
open PIPE, "/cdrom/install |";
@res = <PIPE>;
close PIPE or die "bad pipe: $?, $!";
};
After execution of this statement all 4 variables may have
been set.
"$@" is set if the string to be "eval"-ed did not compile
(this may happen if "open" or "close" were imported with
bad prototypes), or if Perl code executed during evalua
tion die()d . In these cases the value of $@ is the com
pile error, or the argument to "die" (which will interpo
late "$!" and "$?"!). (See also the Fatal manpage,
though.)
When the eval() expression above is executed, open(),
"<PIPE>", and "close" are translated to calls in the C
run-time library and thence to the operating system ker
nel. "$!" 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 pro
gram (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, under
scores, or the special sequence "::" or "'". In this
case, the part before the last "::" or "'" is taken to be
a package qualifier; see the perlmod manpage.
Perl variable names may also be a sequence of digits or a
single punctuation or control character. These names are
all reserved for special uses by Perl; for example, the
all-digits names are used to hold data captured by back
references 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" char
acter. For example, the notation "$^W" (dollar-sign caret
"W") is the scalar variable whose name is the single char
acter control-"W". This is better than typing a literal
control-"W" into your program.
Finally, new in Perl 5.6, Perl variable names may be
alphanumeric strings that begin with control characters
(or better yet, a caret). These variables must be written
in the form "${^Foo}"; the braces are not optional.
"${^Foo}" denotes the scalar variable whose name is a con
trol-"F" followed by two "o"'s. These variables are
reserved for future special uses by Perl, except for the
ones that begin with "^_" (control-underscore or caret-
underscore). No control-character name that begins with
"^_" will acquire a special meaning in any future version
of Perl; such names may therefore be used safely in pro
grams. "$^_" itself, however, is reserved.
Perl identifiers that begin with digits, control charac
ters, or punctuation characters are exempt from the
effects of the "package" declaration and are always forced
to be in package "main". A few other names are also
exempt:
ENV STDIN
INC STDOUT
ARGV STDERR
ARGVOUT
SIG
In particular, the new special "${^_XYZ}" variables are
always taken to be in package "main", regardless of any
"package" declarations presently in scope.
BUGS
Due to an unfortunate accident of Perl's implementation,
"use English" imposes a considerable performance penalty
on all regular expression matches in a program, regardless
of whether they occur in the scope of "use English". For
that reason, saying "use English" in libraries is strongly
discouraged. See the Devel::SawAmpersand module documen
tation from CPAN (http://www.perl.com/CPAN/modules/by-mod
ule/Devel/) for more information.
Having to even think about the "$^S" variable in your
exception handlers is simply wrong. "$SIG{__DIE__}" as
currently implemented invites grievous and difficult to
track down errors. Avoid it and use an "END{}" or
CORE::GLOBAL::die override instead.
2001-03-18 perl v5.6.1 PERLVAR(1)
