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perlvar - Perl predefined variables |
perlvar - Perl predefined variables
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 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.
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:
ord() and int(), as well
as the all file tests (-f, -d) except for -t, which defaults to
STDIN.
print() and unlink().
m//, s///, and tr/// when used
without an =~ operator.
foreach loop if no other
variable is supplied.
grep() and map() functions.
<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.)
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.
` 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.
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 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.
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This variable is read-only and dynamically scoped to the current BLOCK.
$+[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.
$* 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.
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 eof in the perlfunc manpage).
Consider this variable read-only: setting 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.)
undef to read through the end
of file. Setting it to "\n\n" means something slightly
different than setting to "", if the file contains consecutive
empty lines. Setting to "" will treat two or more consecutive
empty lines as a single empty line. Setting to "\n\n" will
blindly assume that the next input character belongs to the next
paragraph, even if it's a newline. (Mnemonic: / delimits
line boundaries when quoting poetry.)
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 convertible 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-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 the perlport manpage. Also see $..
$| 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 the perlfunc manpage
for that. (Mnemonic: when you want your pipes to be piping hot.)
$\ instead of adding ``\n'' at the
end of the print. Also, it's just like $/, but it's what you
get ``back'' from Perl.)
$, 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.)
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
But don't put
@foo{$a,$b,$c} # a slice--note the @
which means
($foo{$a},$foo{$b},$foo{$c})
Default is ``\034'', the same as SUBSEP in awk. If your
keys contain binary data there might not be any safe value for $;.
(Mnemonic: comma (the syntactic subscript separator) is a
semi-semicolon. Yeah, I know, it's pretty lame, but $, is already
taken for something more important.)
Consider using ``real'' multidimensional arrays as described in the perllol manpage.
$#
explicitly to get awk's value. (Mnemonic: # is the number sign.)
Use of $# is deprecated.
$-[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. Compare 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] - $-[3])$^.)
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 $^A unless you call
formline() yourself and then look at it. See the perlform manpage and
formline() in the perlfunc manpage.
``) command,
successful call to wait() or waitpid(), or from the system()
operator. This is just the 16-bit status word returned by the
wait() system call (or else is made up to look like it). Thus, the
exit value of the subprocess is really ($? >> 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.
Also see Error Indicators.
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 indicating 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 Error Indicators.
$! 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() 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 variable. You can,
however, set up a routine to process warnings by setting $SIG{__WARN__}
as described below.
Also see Error Indicators.
fork() calls. (Mnemonic: same as shells.)
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # 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().
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.)
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().
$0 modifies 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 completely 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.
index() and substr() functions.
(Mnemonic: [ begins subscripts.)
As of release 5 of Perl, assignment to $[ is treated as a compiler
directive, and cannot influence the behavior of any other file.
Its use is highly discouraged.
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.
$^C = 1 is similar to calling B::minus_c.
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().
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 }
The %^H hash provides the same scoping semantic as $^H. This makes it useful for implementation of lexically scoped pragmas.
undef to disable
inplace editing. (Mnemonic: value of -i switch.)
$^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.
$Config{'osname'}. See also the Config manpage and the
-V command-line switch documented in the perlrun manpage.
goto &subroutine as well.
Some bits may be relevant at compile-time only, some at run-time only. This is a new mechanism and the details may change.
(?{ code })
regular expression assertion (see the perlre manpage). May be written to.
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 \"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.
use warnings pragma.
See the documentation of warnings for more details.
This can also be enabled from the command line using the -C switch.
The initial value is typically 0 for compatibility 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.
argv[0].
This may not be a full pathname, nor even necessarily in your path.
$#ARGV is generally the number of arguments minus
one, because $ARGV[0] is the first argument, not the program's
command name itself. See $0 for the command name.
do EXPR,
require, or use constructs look for their library files. It
initially consists of the arguments to any -I command-line
switches, followed by the default Perl library, probably
/usr/local/lib/perl, followed by ``.'', to represent the current
directory. If you need to modify this at runtime, you should use
the use lib pragma to get the machine-dependent library properly
loaded also:
use lib '/mypath/libdir/';
use SomeMod;
do, require, or use operators. The key is the filename
you specified (with module names converted to pathnames), and the
value is the location of the file found. The require
operator uses this hash to determine whether a particular file has
already been included.
ENV changes the environment for any child processes
you subsequently fork() off.
sub handler { # 1st argument is signal name
my($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
close(LOG);
exit(0);
}
$SIG{'INT'} = \&handler;
$SIG{'QUIT'} = \&handler;
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
Using a value of 'IGNORE' usually has the effect of ignoring the
signal, except for the CHLD signal. See 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 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 the perlfunc manpage, warn in the perlfunc manpage, eval in the perlfunc manpage, and the warnings manpage for additional information.
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 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 evaluation die()d . In these cases
the value of $@ is the compile error, or the argument to die
(which will interpolate $! 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 kernel. $! is set to the C library's
errno if one of these calls fails.
Under a few operating systems, $^E may contain a more verbose
error indicator, such as in this case, ``CDROM tray not closed.''
Systems that do not support extended error messages leave $^E
the same as $!.
Finally, $? may be set to non-0 value if the external program
/cdrom/install fails. The upper eight bits reflect specific
error conditions encountered by the program (the program's exit()
value). The lower eight bits reflect mode of failure, like signal
death and core dump information See wait(2) for details. In
contrast to $! and $^E, which are set only if error condition
is detected, the variable $? is set on each wait or pipe
close, overwriting the old value. This is more like $@, which
on every eval() is always set on failure and cleared on success.
For more details, see the individual descriptions at $@, $!, $^E,
and $?.
Variable names in Perl can have several formats. Usually, they
must begin with a letter or underscore, in which case they can be
arbitrarily long (up to an internal limit of 251 characters) and
may contain letters, digits, underscores, or the special sequence
:: or '. In this case, the part before the last :: or
' is taken to be a package qualifier; see 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 backreferences after a regular expression
match. Perl has a special syntax for the single-control-character
names: It understands ^X (caret X) to mean the control-X
character. For example, the notation $^W (dollar-sign caret
W) is the scalar variable whose name is the single character
control-W. This is better than typing a literal control-W
into your program.
Finally, new in Perl 5.6, Perl variable names may be alphanumeric
strings that begin with control characters (or better yet, a caret).
These variables must be written in the form ${^Foo}; the braces
are not optional. ${^Foo} denotes the scalar variable whose
name is a control-F followed by two o's. These variables are
reserved for future special uses by Perl, except for the ones that
begin with ^_ (control-underscore or caret-underscore). No
control-character name that begins with ^_ will acquire a special
meaning in any future version of Perl; such names may therefore be
used safely in programs. $^_ itself, however, is reserved.
Perl identifiers that begin with digits, control characters, or
punctuation characters are exempt from the effects of the package
declaration and are always forced to be in package main. 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.
Due to an unfortunate accident of Perl's implementation, use
English imposes a considerable performance penalty on all regular
expression matches in a program, regardless of whether they occur
in the scope of use English. For that reason, saying use
English in libraries is strongly discouraged. See the
Devel::SawAmpersand module documentation from CPAN
(http://www.perl.com/CPAN/modules/by-module/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.
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perlvar - Perl predefined variables |