PERLVAR(1)
NNAAMMEE
perlvar - Perl predefined variables
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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 aawwkk.
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
$_ The default input and pattern-searching space.
The following pairs are equivalent:
while (<>) {...} # equivalent in only while!
while (defined($_ = <>)) {...}
/^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.
$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
$. 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 aawwkk'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.)
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 aawwkk, set this variable as you
would set aawwkk'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
$\ The output record separator for the print
operator. Ordinarily the print operator simply
prints out the comma-separated fields you specify,
with no trailing newline or record separator
assumed. To get behavior more like aawwkk, set this
variable as you would set aawwkk's ORS variable to
specify what is printed at the end of the print.
(Mnemonic: you set "$\" 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
($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in aawwkk.
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
$# The output format for printed numbers. This
variable is a half-hearted attempt to emulate
aawwkk's OFMT variable. There are times, however,
when aawwkk and Perl have differing notions of what
is in fact numeric. The initial value is %.ng,
where n is the value of the macro DBL_DIG from
your system's float.h. This is different from
aawwkk's default OFMT setting of %.6g, so you need to
set "$#" explicitly to get aawwkk'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
nnrrooffff.)
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
$~ The name of the current report format for the
currently selected output channel. Default is
name of the filehandle. (Mnemonic: brother to
"$^".)
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 sshh
and kksshh.)
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 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.)
$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
space-separated 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 "$)" can be set only on
machines that support the corresponding
set[re][ug]id() routine. "$(" 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 sshh and kksshh.)
$[ The index of the first element in an array, and of
the first character in a substring. Default is 0,
but you could set it to 1 to make Perl behave more
like aawwkk (or Fortran) when subscripting and when
evaluating the index() and substr() functions.
(Mnemonic: [ begins subscripts.)
As of Perl 5, assignment to "$[" 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 --DD 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.
$^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 --ii 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.
$^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 --MM, --AA, and --CC filetests
are based on this value.
$WARNING
$^W The current value of the warning switch, either
TRUE or FALSE. (Mnemonic: related to the --ww
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 --II 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.
%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 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.