PERLVAR(1) PERLVAR(1)
perlvar - Perl predefined variables
Predefined Names
The following names have special meaning to Perl. Most of the
punctuation names have reasonable mnemonics, or analogues in one of the
shells. Nevertheless, if you wish to use the long variable names, you
just need to 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 of them even have medium names,
generally borrowed from awk.
To go a step further, those variables that depend on the currently
selected filehandle may instead (and preferably) be set by calling an
object method on the FileHandle object. (Summary lines below for this
contain the word HANDLE.) First you must say
use FileHandle;
after which you may use either
method HANDLE EXPR
or
HANDLE->method(EXPR)
Each of the methods returns the old value of the FileHandle attribute.
The methods each take an optional EXPR, which if supplied specifies the
new value for the FileHandle attribute in question. If not supplied,
most of the methods do nothing to the current value, except for
autoflush(), which will assume a 1 for you, just to be different.
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 (except $^M was added in the wrong place). This is
somewhat obscured by the fact that %ENV and %SIG are listed as $ENV{expr}
and $SIG{expr}.
$ARG
$_ The default input and pattern-searching space. The following
pairs are equivalent:
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while (<>) {...} # equivalent in only while!
while (defined($_ = <>)) {...}
/^Subject:/
$_ =~ /^Subject:/
tr/a-z/A-Z/
$_ =~ tr/a-z/A-Z/
chop
chop($_)
Here are the places where Perl will assume $_ even if you don't
use it:
o 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.
o Various list functions like print() and unlink().
o The pattern matching operations m//, s///, and tr/// when used
without an =~ operator.
o The default iterator variable in a foreach loop if no other
variable is supplied.
o The implicit iterator variable in the grep() and map()
functions.
o 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. Note that outside of a while test, this will
not happen.
(Mnemonic: underline is understood in certain operations.)
$<digit>
Contains the subpattern from the corresponding set of parentheses
in the last pattern matched, not counting patterns matched in
nested blocks that have been exited already. (Mnemonic: like
\digit.) These variables are all read-only.
$MATCH
$& The string matched by the last successful pattern match (not
counting any matches hidden within a BLOCK or eval() enclosed by
the current BLOCK). (Mnemonic: like & in some editors.) This
variable is read-only.
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$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.
$POSTMATCH
$' The string following whatever was matched by the last successful
pattern match (not counting any matches hidden within a BLOCK or
eval() enclosed by the current BLOCK). (Mnemonic: ' often
follows a quoted string.) Example:
$_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
This variable is read-only.
$LAST_PAREN_MATCH
$+ The last bracket matched by the last search pattern. This is
useful if you don't know which of a set of alternative patterns
matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This variable is
read-only.
$MULTILINE_MATCHING
$* Set to 1 to do multi-line matching within a string, 0 to tell
Perl that it can assume that strings contain a single line, for
the purpose of optimizing pattern matches. Pattern matches on
strings containing multiple newlines can produce confusing
results when "$*" is 0. Default is 0. (Mnemonic: * matches
multiple things.) Note that this variable influences the
interpretation of only "^" and "$". A literal newline can be
searched for even when $* == 0.
Use of "$*" is deprecated in modern perls.
input_line_number HANDLE EXPR
$INPUT_LINE_NUMBER
$NR
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$. The current input line number for the last file handle from which
you read (or performed a seek or tell on). 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 under eof()). 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. Works like awk's
RS variable, including treating empty lines as delimiters if set
to the null string. (Note: An empty line cannot contain any
spaces or tabs.) You may set it to a multi-character string to
match a multi-character delimiter, or to undef to read to end of
file. Note that setting it to "\n\n" means something slightly
different than setting it to "", if the file contains consecutive
empty lines. Setting it to "" will treat two or more consecutive
empty lines as a single empty line. Setting it to "\n\n" will
blindly assume that the next input character belongs to the next
paragraph, even if it's a newline. (Mnemonic: / is used to
delimit line boundaries when quoting poetry.)
undef $/;
$_ = <FH>; # whole file now here
s/\n[ \t]+/ /g;
Remember: the value of $/ is a string, not a regexp. AWK has to
be better for something :-)
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 actually buffered by
the system or not; $| tells you only whether you've asked Perl
explicitly to flush after each write). Note that 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, such as when you are running a
Perl script under rsh and want to see the output as it's
happening. This has no effect on input buffering. (Mnemonic:
when you want your pipes to be piping hot.)
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output_field_separator HANDLE EXPR
$OUTPUT_FIELD_SEPARATOR
$OFS
$, The output field separator for the print operator. Ordinarily
the print operator simply prints out the comma-separated fields
you specify. To get behavior more like awk, set this variable as
you would set awk's OFS variable to specify what is printed
between fields. (Mnemonic: what is printed when there is a , in
your print statement.)
output_record_separator HANDLE EXPR
$OUTPUT_RECORD_SEPARATOR
$ORS
$\ The output record separator for the print operator. Ordinarily
the print operator simply prints out the comma-separated fields
you specify, with no trailing newline or record separator
assumed. To get behavior more like awk, set this variable as you
would set awk's ORS variable to specify what is printed at the
end of the print. (Mnemonic: you set "$\" 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 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
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($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk. Note that 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.
$OFMT
$# The output format for printed numbers. This variable is a halfhearted
attempt to emulate awk's OFMT variable. There are times,
however, when awk and Perl have differing notions of what is in
fact numeric. The initial value is %.ng, where n is the value of
the macro DBL_DIG from your system's float.h. This is different
from awk's default OFMT setting of %.6g, so you need to set "$#"
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.
(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. (Mnemonic: = has
horizontal lines.)
format_lines_left HANDLE EXPR
$FORMAT_LINES_LEFT
$- The number of lines left on the page of the currently selected
output channel. (Mnemonic: lines_on_page - lines_printed.)
format_name HANDLE EXPR
$FORMAT_NAME
$~ The name of the current report format for the currently selected
output channel. Default is name of the filehandle. (Mnemonic:
brother to "$^".)
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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 name of the filehandle with
_TOP appended. (Mnemonic: points to top of page.)
format_line_break_characters HANDLE EXPR
$FORMAT_LINE_BREAK_CHARACTERS
$: The current set of characters after which a string may be broken
to fill continuation fields (starting with ^) in a format.
Default is " \n-", to break on whitespace or hyphens. (Mnemonic:
a "colon" in poetry is a part of a line.)
format_formfeed HANDLE EXPR
$FORMAT_FORMFEED
$^L What formats output to perform a form feed. Default is \f.
$ACCUMULATOR
$^A The current value of the write() accumulator for format() lines.
A format contains formline() commands that put their result into
$^A. After calling its format, write() prints out the contents
of $^A and empties. So you never actually see the contents of
$^A unless you call formline() yourself and then look at it. See
the perlform manpage and the formline() entry in the perlfunc
manpage.
$CHILD_ERROR
$? The status returned by the last pipe close, backtick (``)
command, or system() operator. Note that this is the status word
returned by the wait() system call (or else is made up to look
like it). Thus, the exit value of the subprocess is actually ($?
>> 8), and $? & 255 gives which signal, if any, the process died
from, and whether there was a core dump. (Mnemonic: similar to
sh and ksh.)
Note that 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 the script.
Under VMS, the pragma use vmsish 'status' makes $? reflect the
actual VMS exit status, instead of the default emulation of POSIX
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status.
$OS_ERROR
$ERRNO
$! If used in a numeric context, yields the current value of errno,
with all the usual caveats. (This means that you shouldn't
depend on the value of "$!" to be anything in particular unless
you've gotten a specific error return indicating a system error.)
If used in a string context, yields the corresponding system
error string. You can assign 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?)
$EXTENDED_OS_ERROR
$^E More specific information about the last system error than that
provided by $!, if available. (If not, it's just $! again.) At
the moment, this differs from $! under only VMS and OS/2, where
it provides the VMS status value from the last system error, and
OS/2 error code of the last call to OS/2 API either via CRT, or
directly from perl. The caveats mentioned in the description of
$! apply here, too. (Mnemonic: Extra error explanation.)
$EVAL_ERROR
$@ The Perl syntax error message from the last eval() command. 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"?)
Note that 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.
$PROCESS_ID
$PID
$$ The process number of the Perl running this script. (Mnemonic:
same as shells.)
$REAL_USER_ID
$UID
$< The real uid of this process. (Mnemonic: it's the uid you came
FROM, if you're running setuid.)
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$EFFECTIVE_USER_ID
$EUID
$> The effective uid of this process. Example:
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went TO, if you're running setuid.)
Note: "$<" and "$>" can be swapped only on machines supporting
setreuid().
$REAL_GROUP_ID
$GID
$( The real gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is
the one returned by getgid(), and the subsequent ones by
getgroups(), one of which may be the same as the first number.
However, a value assigned to "$(" must be a single number used to
set the real gid. So the value given by "$(" should not be
assigned back to "$(" without being forced numeric, such as by
adding zero.
(Mnemonic: parentheses are used to GROUP things. The real gid is
the group you LEFT, if you're running setgid.)
$EFFECTIVE_GROUP_ID
$EGID
$) The effective gid of this process. If you are on a machine that
supports membership in multiple groups simultaneously, gives a
space separated list of groups you are in. The first number is
the one returned by getegid(), and the subsequent ones by
getgroups(), one of which may be the same as the first number.
Similarly, a value assigned to "$)" must also be a spaceseparated
list of numbers. The first number is used to set 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 $) = "5 5" .
(Mnemonic: parentheses are used to GROUP things. The effective
gid is the group that's RIGHT for you, if you're running setgid.)
Note: "$<", "$>", "$(" and "$)" can be set only on machines that
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support the corresponding set[re][ug]id() routine. "$(" and "$)"
can be swapped only on machines supporting setregid().
$PROGRAM_NAME
$0 Contains the name of the file containing the Perl script being
executed. On some operating systems assigning to "$0" modifies
the argument area that the ps(1) 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.)
$[ The index of the first element in an array, and of the first
character in a substring. Default is 0, but you could set it to
1 to make Perl behave more like awk (or Fortran) when
subscripting and when evaluating the index() and substr()
functions. (Mnemonic: [ begins subscripts.)
As of Perl 5, assignment to "$[" is treated as a compiler
directive, and cannot influence the behavior of any other file.
Its use is discouraged.
$PERL_VERSION
$] 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 Perl interpreter is too old.
$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.)
Note that the close-on-exec status of a file descriptor will be
decided according to the value of $^F at the time of the open,
not the time of the exec.
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$^H The current set of syntax checks enabled by use strict and other
block scoped compiler hints. See the documentation of strict for
more details.
$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 it is not trappable. However,
if compiled for this, Perl may use the contents of $^M as an
emergency pool after die()ing with this message. 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 when in emergency. See the
INSTALL file for information on how to enable this option. As a
disincentive to casual use of this advanced feature, there is no
the English manpage 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'}.
$PERLDB
$^P The internal variable for debugging support. Different bits mean
the following (subject to change):
0x01 Debug subroutine enter/exit.
0x02 Line-by-line debugging.
0x04 Switch off optimizations.
0x08 Preserve more data for future interactive inspections.
0x10 Keep info about source lines on which a subroutine is
defined.
0x20 Start with single-step on.
Note that some bits may be relevent at compile-time only,
some at run-time only. This is a new mechanism and the
details may change.
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$^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, othewise
false.
$BASETIME
$^T The time at which the script 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.
$WARNING
$^W The current value of the warning switch, either TRUE or FALSE.
(Mnemonic: related to the -w switch.)
$EXECUTABLE_NAME
$^X The name that the Perl binary itself was executed as, from C's
argv[0].
$ARGV contains the name of the current file when reading from <>.
@ARGV The array @ARGV contains the command line arguments intended for
the script. Note that $#ARGV is the generally number of
arguments minus one, because $ARGV[0] is the first argument, NOT
the command name. See "$0" for the command name.
@INC The array @INC contains the list of places to look for Perl
scripts to be evaluated by the do EXPR, require, or use
constructs. 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 that has been
included via do or require. The key is the filename you
specified, and the value is the location of the file actually
found. The require command uses this array to determine whether
a given file has already been included.
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%ENV $ENV{expr}
The hash %ENV contains your current environment. Setting a value
in ENV changes the environment for child processes.
%SIG $SIG{expr}
The hash %SIG is used to set signal handlers for various signals.
Example:
sub handler { # 1st argument is signal name
my($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
The %SIG array contains values for only the signals actually set
within the Perl script. Here are some other examples:
$SIG{"PIPE"} = Plumber; # SCARY!!
$SIG{"PIPE"} = "Plumber"; # assumes main::Plumber (not recommended)
$SIG{"PIPE"} = \&Plumber; # just fine; assume current Plumber
$SIG{"PIPE"} = Plumber(); # oops, what did Plumber() return??
The one marked scary is problematic because it's a bareword,
which means sometimes it's a string representing the function,
and sometimes it's going to call the subroutine call right then
and there! Best to be sure and quote it or take a reference to
it. *Plumber works too. See the perlsub manpage.
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 it 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
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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__.
Note that the $SIG{__DIE__} hook is called even inside eval()ed
blocks/strings. See the die entry in the perlfunc manpage, the
section on $^S in the perlvar manpage for how to circumvent this.
Note that __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 calls which
result/may-result in parsing Perl should be used with extreme
causion, 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 and the eval entry in the perlfunc manpage for
additional info.
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