perlvar - Perl predefined variables
The following names have special meaning to Perl. Most punctuation names have reasonable mnemonics, or analogues in one of the shells. Nevertheless, if you wish to use 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 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 more safely,
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 $SIG{expr}.
while (<>) {...} # equivalent in only while!
while (defined($_ = <>)) {...}
/^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:
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.
print() and
unlink().
=~ 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. Note that outside of 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.
` often precedes a quoted string.) This variable is read-only.
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.
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This variable is read-only.
$*'' is 0. Default is 0. (Mnemonic: * matches multiple things.) Note that
this variable influences the interpretation of only ``^'' and ``$''.
A literal newline can be searched for even when $* == 0.
Use of ``$*'' is deprecated in modern Perls, supplanted by the /s and /m modifiers on pattern matching.
<>'' 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.)
"\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 :-)
Setting $/ to a reference to an integer, scalar containing an integer, or scalar that's convertable 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 likely not a problem, as any file you'd want to read in record mode is proably usable in line mode) Non-VMS systems perform normal I/O, so it's safe to mix record and non-record reads of a file.
$| 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.)
$\'' 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 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. 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.
$#'' explicitly to get awk's value. (Mnemonic: # is the number sign.)
Use of ``$#'' is deprecated.
$^''.)
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
formline().
``) 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 $? & 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 of the gethost*() functions fail.
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.
Also see Error Indicators.
$! 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?)
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() 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.
Also see Error Indicators.
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went TO, if you're running setuid.) Note: ``$<'' and ``$>'' can be swapped only on machines supporting
setreuid().
getgid(), and the subsequent ones by
getgroups(), one of which may be the same as the first number.
However, a value assigned to ``$('' 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.)
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
<PRE> $) = "5 5"
</PRE>
.
(Mnemonic: parentheses are used to GROUP things. The effective gid is the group that's RIGHT for you, if you're running setgid.)
Note: ``$<'', ``$>'', ``$('' 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().
$0'' modifies the argument area that the
ps(1) program sees. This is
more useful as a way of indicating the current program state than it is for
hiding the program you're running. (Mnemonic: same as sh and ksh.)
index() and
substr() functions. (Mnemonic: [ begins subscripts.)
As of Perl 5, assignment to ``$['' is treated as a compiler directive, and cannot influence the behavior of
any other file. Its use is discouraged.
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.
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.
use strict and other block scoped compiler hints. See the documentation of strict for more details.
$^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.
$Config{'osname'}.
Debug subroutine enter/exit.
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.
(?{ code })
regular expression assertion. (Excluding those used as switches.) May be
written to.
argv[0].
@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 ``$0'' for the command name.
@INC contains the list of places to look for Perl
scripts to be evaluated by the do EXPR, require, or use constructs. It initially consists of the arguments to any -I command line switches, followed by the default Perl library, probably /usr/local/lib/perl, followed by ``.'', to represent the current directory. If you need to
modify this at runtime, you should use the use lib pragma to get the machine-dependent library properly loaded also:
use lib '/mypath/libdir/';
use SomeMod;
@_ contains the parameters
passed to that subroutine. See the perlsub manpage.
%INC contains entries for each filename that has been
included via do or require. The key is the filename you specified, and the value is the location of
the file actually found. The require command uses this array to determine whether a given file has already been
included.
%ENV contains your current environment. Setting a
value in ENV changes the environment for child processes.
%SIG is used to set signal handlers for various
signals. Example:
sub handler { # 1st argument is signal name
my($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 die and $^S 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.
The variables $@, $!, $^E, and $? contain information about different types of error conditions that may appear during execution of Perl script. The variables are shown ordered by the ``distance'' between the subsystem which reported the error and the Perl process, and 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:
eval '
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 (either implicitly, say, if
open was imported from module the Fatal manpage, or the die after
close was triggered). In these cases the value of $@ is the compile error, or Fatal error (which will interpolate $!!), or the argument to die (which will interpolate $! and $?!).
When the above expression is executed,
open(), <PIPE>, and close
are translated to
C run-time library calls. $! is set if one of these calls fails. The value is a symbolic indicator chosen by the
C run-time library, say
No such file or directory.
On some systems the above
C library calls are further translated to calls to the kernel. The kernel may have set more verbose error indicator that one of the handful of standard
C errors. In such cases
$^E contains this verbose error indicator, which may be, say,
CDROM tray not
closed. On systems where
C library calls are identical to system calls
$^E is a duplicate of $!.
Finally, $? may be set to non-0 value if the external program
/cdrom/install fails. Upper bits of the particular value may reflect specific error conditions encountered by this program (this is program-dependent), lower-bits reflect mode of failure (segfault, completion, etc.). Note that 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.
For more details, see the individual descriptions at $@, $!, $^E, and $?.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.