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PERLVAR

Section: Perl Programmers Reference Guide (1)
Updated: perl 5.004, patch 04
Index 

NAME

perlvar - Perl predefined variables 

DESCRIPTION

 

Predefined Names

The following names have special meaning to Perl. Most of thepunctuation names have reasonable mnemonics, or analogues in one ofthe 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 thelong 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 currentlyselected filehandle may instead (and preferably) be set by calling anobject method on the FileHandle object. (Summary lines below for thiscontain 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 thenew value for the FileHandle attribute in question. If not supplied,most of the methods do nothing to the current value, except forautoflush(), which will assume a 1 for you, just to be different.

A few of these variables are considered ``read-only''. This means that ifyou try to assign to this variable, either directly or indirectly througha reference, you'll raise a run-time exception.

The following list is ordered by scalar variables first, then thearrays, 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 areequivalent:

    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 youdon't use it:

*

Various unary functions, including functions like ord() and int(), as wellas the all file tests (-f, -d) except for -t, which defaults toSTDIN.

*

Various list functions like print() and unlink().

*

The pattern matching operations m//, s///, and tr/// when usedwithout an =~ operator.

*

The default iterator variable in a foreach loop if no othervariable is supplied.

*

The implicit iterator variable in the grep() and map() functions.

*

The default place to put an input record when a <FH>operation's result is tested by itself as the sole criterion of a whiletest. 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 inthe last pattern matched, not counting patterns matched in nestedblocks 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 countingany matches hidden within a BLOCK or eval() enclosed by the currentBLOCK). (Mnemonic: like & in some editors.) This variable is read-only.

$PREMATCH

$`

The string preceding whatever was matched by the last successfulpattern match (not counting any matches hidden within a BLOCK or evalenclosed by the current BLOCK). (Mnemonic: ` often precedes a quotedstring.) This variable is read-only.

$POSTMATCH

$'

The string following whatever was matched by the last successfulpattern match (not counting any matches hidden within a BLOCK or eval()enclosed by the current BLOCK). (Mnemonic: ' often follows a quotedstring.) 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 ifyou don't know which of a set of alternative patterns matched. Forexample:

    /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 Perlthat it can assume that strings contain a single line, for the purposeof optimizing pattern matches. Pattern matches on strings containingmultiple newlines can produce confusing results when ``$*'' is 0. Defaultis 0. (Mnemonic: * matches multiple things.) Note that this variableinfluences the interpretation of only ``^'' and ``$''. A literal newline canbe 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 fromwhich you read (or performed a seek or tell on). Anexplicit close on a filehandle resets the line number. Because``<>'' never does an explicit close, line numbers increaseacross ARGV files (but see examples under eof()). Localizing $. hasthe effect of also localizing Perl's notion of ``the last readfilehandle''. (Mnemonic: many programs use ``.'' to mean the current linenumber.)

input_record_separator HANDLE EXPR

$INPUT_RECORD_SEPARATOR

$RS

$/

The input record separator, newline by default. Works like awk's RSvariable, including treating empty lines as delimiters if set to thenull string. (Note: An empty line cannot contain any spaces or tabs.)You may set it to a multi-character string to match a multi-characterdelimiter, or to undef to read to end of file. Note that setting itto "\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 emptyline. Setting it to "\n\n" will blindly assume that the next inputcharacter 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 bebetter for something :-)

autoflush HANDLE EXPR

$OUTPUT_AUTOFLUSH

$|

If set to nonzero, forces a flush right away and after every write or print on thecurrently selected output channel. Default is 0 (regardless of whetherthe channel is actually buffered by the system or not; $| tells youonly whether you've asked Perl explicitly to flush after each write).Note that STDOUT will typically be line buffered if output is to theterminal and block buffered otherwise. Setting this variable is usefulprimarily when you are outputting to a pipe, such as when you are runninga Perl script under rsh and want to see the output as it's happening. Thishas 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 theprint operator simply prints out the comma-separated fields youspecify. To get behavior more like awk, set this variableas you would set awk's OFS variable to specify what is printedbetween fields. (Mnemonic: what is printed when there is a , in yourprint statement.)

output_record_separator HANDLE EXPR

$OUTPUT_RECORD_SEPARATOR

$ORS

$\

The output record separator for the print operator. Ordinarily theprint operator simply prints out the comma-separated fields youspecify, with no trailing newline or record separator assumed.To get behavior more like awk, set this variable as you wouldset awk's ORS variable to specify what is printed at the end of theprint. (Mnemonic: you set ``$\'' instead of adding \n at the end of theprint. Also, it's just like $/, but it's what you get ``back'' fromPerl.)

$LIST_SEPARATOR

$ 8

This is like ``$,'' except that it applies to array values interpolatedinto a double-quoted string (or similar interpreted string). Defaultis a space. (Mnemonic: obvious, I think.)

$SUBSCRIPT_SEPARATOR

$SUBSEP

$;

The subscript separator for multidimensional array emulation. If yourefer to a hash element as

    $foo{$a,$b,$c}

it really means

    $foo{join($;, $a, $b, $c)}

But don't put

    @foo{$a,$b,$c}      # a slice--note the @

which means

    ($foo{$a},$foo{$b},$foo{$c})

Default is ``\034'', the same as SUBSEP in awk. Note that if yourkeys contain binary data there might not be any safe value for ``$;''.(Mnemonic: comma (the syntactic subscript separator) is asemi-semicolon. Yeah, I know, it's pretty lame, but ``$,'' is alreadytaken for something more important.)

Consider using ``real'' multidimensional arrays.

$OFMT

$#

The output format for printed numbers. This variable is a half-heartedattempt to emulate awk's OFMT variable. There are times, however,when awk and Perl have differing notions of what is in factnumeric. The initial value is %.ng, where n is the valueof the macro DBL_DIG from your system's float.h. This is different fromawk'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 selectedoutput 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 outputchannel. (Mnemonic: lines_on_page - lines_printed.)

format_name HANDLE EXPR

$FORMAT_NAME

$~

The name of the current report format for the currently selected outputchannel. 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 selectedoutput channel. Default is name of the filehandle with _TOPappended. (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 tofill continuation fields (starting with ^) in a format. Default is`` \n-'', to break on whitespace or hyphens. (Mnemonic: a ``colon'' inpoetry 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 formatcontains formline() commands that put their result into $^A. Aftercalling its format, write() prints out the contents of $^A and empties.So you never actually see the contents of $^A unless you callformline() yourself and then look at it. See the perlform manpage andthe 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 bythe 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, andwhether there was a core dump. (Mnemonic: similar to sh andksh.)

Note that if you have installed a signal handler for SIGCHLD, thevalue of $? will usually be wrong outside that handler.

Inside an END subroutine $? contains the value that is going to begiven to exit(). You can modify $? in an END subroutine tochange the exit status of the script.

Under VMS, the pragma use vmsish 'status' makes $? reflect theactual VMS exit status, instead of the default emulation of POSIXstatus.

$OS_ERROR

$ERRNO

$!

If used in a numeric context, yields the current value of errno, withall the usual caveats. (This means that you shouldn't depend on thevalue of ``$!'' to be anything in particular unless you've gotten aspecific error return indicating a system error.) If used in a stringcontext, yields the corresponding system error string. You can assignto ``$!'' to set errno if, for instance, you want ``$!'' to return thestring 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 itprovides the VMS status value from the last system error, and OS/2 errorcode of the last call to OS/2 API either via CRT, or directly from perl. Thecaveats 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, thelast eval() parsed and executed correctly (although the operations youinvoked may have failed in the normal fashion). (Mnemonic: Where wasthe 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: sameas 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 machinessupporting setreuid().

$REAL_GROUP_ID

$GID

$(

The real gid of this process. If you are on a machine that supportsmembership in multiple groups simultaneously, gives a space separatedlist of groups you are in. The first number is the one returned bygetgid(), and the subsequent ones by getgroups(), one of which may bethe same as the first number.

However, a value assigned to ``$('' must be a single number used toset the real gid. So the value given by ``$('' should not be assignedback to ``$('' without being forced numeric, such as by adding zero.

(Mnemonic: parentheses are used to GROUP things. The real gid is thegroup you LEFT, if you're running setgid.)

$EFFECTIVE_GROUP_ID

$EGID

$)

The effective gid of this process. If you are on a machine thatsupports membership in multiple groups simultaneously, gives a spaceseparated list of groups you are in. The first number is the onereturned by getegid(), and the subsequent ones by getgroups(), one ofwhich may be the same as the first number.

Similarly, a value assigned to ``$)'' must also be a space-separatedlist of numbers. The first number is used to set the effective gid, andthe rest (if any) are passed to setgroups(). To get the effect of anempty 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 gidis the group that's RIGHT for you, if you're running setgid.)

Note: ``$<'', ``$>'', ``$('' and ``$)'' can be set only onmachines 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 beingexecuted. On some operating systemsassigning to ``$0'' modifies the argument area that the ps(1)program sees. This is more useful as a way of indicating thecurrent 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 characterin a substring. Default is 0, but you could set it to 1 to makePerl behave more like awk (or Fortran) when subscripting and whenevaluating the index() and substr() functions. (Mnemonic: [ beginssubscripts.)

As of Perl 5, assignment to ``$['' is treated as a compiler directive,and cannot influence the behavior of any other file. Its use isdiscouraged.

$PERL_VERSION

$]

The version + patchlevel / 1000 of the Perl interpreter. This variablecan be used to determine whether the Perl interpreter executing ascript is in the right range of versions. (Mnemonic: Is this versionof perl in the right bracket?) Example:

    warn "No checksumming!\n" if $] < 3.019;

See also the documentation of use VERSION and require VERSIONfor a convenient way to fail if the Perl interpreter is too old.

$DEBUGGING

$^D

The current value of the debugging flags. (Mnemonic: value of -Dswitch.)

$SYSTEM_FD_MAX

$^F

The maximum system file descriptor, ordinarily 2. System filedescriptors are passed to exec()ed processes, while higher filedescriptors are not. Also, during an open(), system file descriptors arepreserved even if the open() fails. (Ordinary file descriptors areclosed before the open() is attempted.) Note that the close-on-execstatus 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 blockscoped compiler hints. See the documentation of strict for more details.

$INPLACE_EDIT

$^I

The current value of the inplace-edit extension. Use undef to disableinplace editing. (Mnemonic: value of -i switch.)

$^M

By default, running out of memory it is not trappable. However, ifcompiled for this, Perl may use the contents of $^M as an emergencypool after die()ing with this message. Suppose that your Perl werecompiled 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 INSTALLfile for information on how to enable this option. As a disincentive tocasual use of this advanced feature, there is no the English manpage long name forthis variable.

$OSNAME

$^O

The name of the operating system under which this copy of Perl wasbuilt, as determined during the configuration process. The valueis identical to $Config{'osname'}.

$PERLDB

$^P

The internal variable for debugging support. Different bits mean thefollowing (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 atrun-time only. This is a new mechanism and the details may change.

$^S

Current state of the interpreter. Undefined if parsing of the currentmodule/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 theepoch (beginning of 1970). The values returned by the -M, -A,and -C filetests arebased 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 thescript. Note that $#ARGV is the generally number of arguments minusone, 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 tobe evaluated by the do EXPR, require, or use constructs. Itinitially 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 tomodify this at runtime, you should use the use lib pragmato get the machine-dependent library properly loaded also:

    use lib '/mypath/libdir/';    use SomeMod;

@_

Within a subroutine the array @_ contains the parameters passed to thatsubroutine. See the perlsub manpage.

%INC

The hash %INC contains entries for each filename that hasbeen included via do or require. The key is the filename youspecified, and the value is the location of the file actually found.The require command uses this array to determine whether a given filehas already been included.

%ENV $ENV{expr}

The hash %ENV contains your current environment. Setting avalue in ENV changes the environment for child processes.

%SIG $SIG{expr}

The hash %SIG is used to set signal handlers for varioussignals. 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 withinthe 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 meanssometimes it's a string representing the function, and sometimes it'sgoing to call the subroutine call right then and there! Best to be sureand 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 areinstalled using it. This means you get reliable signal handling. Ifyour system has the SA_RESTART flag it is used when signals handlers areinstalled. This means that system calls for which it is supportedcontinue rather than returning when a signal arrives. If you want yoursystem calls to be interrupted by signal delivery then do something likethis:

    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. Theroutine indicated by $SIG{__WARN__} is called when a warning message isabout to be printed. The warning message is passed as the firstargument. The presence of a __WARN__ hook causes the ordinary printingof warnings to STDERR to be suppressed. You can use this to save warningsin 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 exceptionis about to be thrown. The error message is passed as the firstargument. When a __DIE__ hook routine returns, the exceptionprocessing 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 youcan die from a __DIE__ handler. Similarly for __WARN__.

Note that the $SIG{__DIE__} hook is called even inside eval()edblocks/strings. See the die entry in the perlfunc manpage, the section on $^S in the perlvar manpage for how tocircumvent this.

Note that __DIE__/__WARN__ handlers are very special in onerespect: they may be called to report (probable) errors found by theparser. In such a case the parser may be in inconsistent state, soany attempt to evaluate Perl code from such a handler will probablyresult in a segfault. This means that calls which result/may-resultin 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 whocalled the handler. The second line will print backtrace and die ifCarp was available. The third line will be executed only if Carp wasnot available.

See the die entry in the perlfunc manpage, the warn entry in the perlfunc manpage and the eval entry in the perlfunc manpage foradditional info.

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Index

NAME

DESCRIPTION

Predefined Names

---

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