perlvar - Perl predefined variables


Predefined Names

The following names have special meaning to Perl. Most of the punctuation names have reasonable mnemonics, or analogues in one of the shells. Nevertheless, if you wish to use the long variable names, you just need to say

    use English;

at the top of your program. This will alias all the short names to the long names in the current package. Some of them even have medium names, generally borrowed from awk.

To go a step further, those variables that depend on the currently selected filehandle may instead 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



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 default input and pattern-searching space. The following pairs are equivalent:

    while (<>) {...}	# equivalent in only while!
    while ($_ = <>) {...}

    $_ =~ /^Subject:/

    $_ =~ tr/a-z/A-Z/


Here are the places where Perl will assume $_ even if you don't use it:

(Mnemonic: underline is understood in certain operations.)

Contains the sub-pattern 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.

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.

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.

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:

    $_ = 'abcdefghi';
    print "$`:$&:$'\n";  	# prints abc:def:ghi

This variable is read-only.

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.

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
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
The input record separator, newline by default. Works like awk's RS variable, including treating empty lines as delimiters if set to the null string. (Note: An empty line cannot contain any spaces or tabs.) You may set it to a multicharacter string to match a multi-character delimiter. 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;

autoflush HANDLE EXPR
If set to nonzero, forces a flush 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
The output field separator for the print operator. Ordinarily the print operator simply prints out the comma-separated fields you specify. To get behavior more like awk, set this variable as you would set awk's OFS variable to specify what is printed between fields. (Mnemonic: what is printed when there is a , in your print statement.)

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

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.)

The subscript separator for multi-dimensional array emulation. If you refer to a hash element as


it really means

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

But don't put

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

which means


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'' multi-dimensional arrays.

The output format for printed numbers. This variable is a half-hearted attempt to emulate awk's OFMT variable. There are times, however, when awk and Perl have differing notions of what is in fact numeric. The initial value is, where n is the value of the macro DBL_DIG from your system's float.h. This is different from awk's default OFMT setting of %.6g, so you need to set ``$#'' explicitly to get awk's value. (Mnemonic: # is the number sign.)

Use of ``$#'' is deprecated.

format_page_number HANDLE EXPR
The current page number of the currently selected output channel. (Mnemonic: % is page number in nroff.)

format_lines_per_page HANDLE EXPR
The current page length (printable lines) of the currently selected output channel. Default is 60. (Mnemonic: = has horizontal lines.)

format_lines_left HANDLE EXPR
The number of lines left on the page of the currently selected output channel. (Mnemonic: lines_on_page - lines_printed.)

format_name HANDLE EXPR
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
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
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
What formats output to perform a form feed. Default is \f.

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 formline().

The status returned by the last pipe close, back-tick (``) command, or system operator. Note that this is the status word returned by the wait system call (or else is made up to look like it). Thus, the exit value of the subprocess is actually ($? >> 8), and $? & 255 gives which signal, if any, the process died from, and whether there was a core dump. (Mnemonic: similar to sh and ksh.)

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

Inside an END subroutine $? contains the value that is going to be given to exit. You can modify $? in an END subroutine to change the exit status of the script.

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

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?)

More specific information about the last system error than that provided by $!, if available. (If not, it's just $! again, except under OS/2.) 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 which was not directed via CRT. The caveats mentioned in the description of $! apply here, too. (Mnemonic: Extra error explanation.)

Note that under OS/2 $! and $^E do not track each other, so if an OS/2-specific call is performed, you may need to check both.

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__} below.

The process number of the Perl running this script. (Mnemonic: same as shells.)

The real uid of this process. (Mnemonic: it's the uid you came FROM, if you're running setuid.)

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 on only machines supporting setreuid.

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

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. (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 on only machines supporting setregid. Because Perl doesn't currently use initgroups, you can't set your group vector to multiple groups.

Contains the name of the file containing the Perl script being executed. Assigning to ``$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.)

The index of the first element in an array, and of the first character in a substring. Default is 0, but you could set it to 1 to make Perl behave more like awk (or Fortran) when subscripting and when evaluating the index and substr functions. (Mnemonic: [ begins subscripts.)

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

The string printed out when you say perl -v. (This is currently BROKEN). It can be used to determine at the beginning of a script whether the perl interpreter executing the script is in the right range of versions. If used in a numeric context, returns the version + patchlevel / 1000. Example:

    # see if getc is available
    ($version,$patchlevel) =
	     $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
    print STDERR "(No filename completion available.)\n"
	     if $version * 1000 + $patchlevel < 2016;

or, used numerically,

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

(Mnemonic: Is this version of perl in the right bracket?)

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

The maximum system file descriptor, ordinarily 2. System file descriptors are passed to execed 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.

The current set of syntax checks enabled by use strict. See the documentation of strict for more details.

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

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'}.

The internal flag that the debugger clears so that it doesn't debug itself. You could conceivably disable debugging yourself by clearing it.

The time at which the script began running, in seconds since the epoch (beginning of 1970). The values returned by the -M, -A, and -C filetests are based on this value.

The current value of the warning switch, either TRUE or FALSE. (Mnemonic: related to the -w switch.)

The name that the Perl binary itself was executed as, from C's argv[0].

contains the name of the current file when reading from <>.

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.

The array @INC contains the list of places to look for Perl scripts to be evaluated by the do EXPR, require, or use constructs. It initially consists of the arguments to any -I command line switches, followed by the default Perl library, probably /usr/local/lib/perl, followed by ``.'', to represent the current directory. If you need to modify this at runtime, you should use the use lib pragma to get the machine-dependent library properly loaded also:

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

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.

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

The hash %SIG is used to set signal handlers for various signals. Example:

    sub handler {	# 1st argument is signal name
	local($sig) = @_;
	print "Caught a SIG$sig--shutting down\n";

    $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";   # just fine, assumes main::Plumber
    $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__. See die, warn and eval.