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Man Page for PERLVAR



NAME
     perlvar - Perl predefined variables

DESCRIPTION
     Predefined Names

     The following names have special meaning to Perl.  Most of
     the punctuational 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

     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.  If you attempt
     to do so, you'll raise a run-time exception.

     $ARG

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

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



                 /^Subject:/
                 $_ =~ /^Subject:/

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

                 chop
                 chop($_)

             (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 igit.) 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

     $*      Set to 1 to do multiline matching within a string, 0
             to tell Perl that it can assume that strings contain
             a single line, for the purpose of optimizing pattern
             matches.  Pattern matches on strings containing
             multiple newlines can produce confusing results when
             "$*" is 0.  Default is 0.  (Mnemonic: * matches
             multiple things.)  Note that this variable only
             influences the interpretation of "^" and "$".  A
             literal newline can be searched for even when $* ==
             0.

             Use of "$*" is deprecated in Perl 5.

     input_line_number HANDLE EXPR

     $INPUT_LINE_NUMBER

     $NR

     $.      The current input line number of the last filehandle
             that was read.  This variable should be considered
             read-only. Remember that only an explicit close on
             the filehandle resets the line number.  Since "<>"
             never does an explicit close, line numbers increase
             across ARGV files (but see examples under eof()).
             (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 awk's RS variable, including treating
             blank lines as delimiters if set to the null string.
             You may set it to a multicharacter string to match a
             multi-character delimiter.  Note that setting it to
             "0 means something slightly different than
             setting it to "", if the file contains consecutive


             blank lines.  Setting it to "" will treat two or
             more consecutive blank lines as a single blank line.
             Setting it to "0 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/]+/ /g;


     autoflush HANDLE EXPR

     $OUTPUT_AUTOFLUSH

     $|      If set to nonzero, forces a flush after every write
             or print on the currently selected output channel.
             Default is 0.  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.
             (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.  In order 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

     $OUTPUT_RECORD_SEPARATOR

     $ORS

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

     $LIST_SEPARATOR
             " .nr )I 8"n

     $       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 multi-dimensional 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 " 34", 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 in
             Perl 5.

     $OFMT

     $#      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.  Also, the initial value is %.20g rather


             than %.6g, so you need to set "$#" explicitly to get
             awk's value.  (Mnemonic: # is the number sign.)

             Use of "$#" is deprecated in Perl 5.

     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 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 formfeed.  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

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

     $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 "$!" in order to set errno if, for instance, you
             want "$!" to return the string for error n, or you
             want to set the exit value for the die() operator.
             (Mnemonic: What just went bang?)

     $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"?)

     $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 only be
             swapped 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.  (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.  (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 only be set on
             machines that support the corresponding
             set[re][ug]id() routine.  "$(" and "$)" can only be
             swapped on machines supporting setregid().

     $PROGRAM_NAME

     $0      Contains the name of the file containing the Perl
             script being executed.  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 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.

     $PERL_VERSION

     $]      The string printed out when you say perl -v.  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) =
                          $] =~ /(+.+).*0atch level: (+)/;
                 print STDERR  "(No  filename  completion  available.)\n"
                          if  $version  *  1000  +  $patchlevel < 2016;

	     or, used numerically,

                 warn "No checksumming!0 if $] < 3.019;

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

     $DEBUGGING

     $^D     The current value of the debugging flags.
             (Mnemonic: value of -D 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.

     $INPLACE_EDIT

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

     $PERLDB

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

     $BASETIME

     $^T     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.

     $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 the script.  Note that $#ARGV is the
             generally number of arguments minus one, since
             $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 -I command line switches,
             followed by the default Perl library, probably
             "/usr/local/lib/perl", followed by ".", to represent
             the current directory.

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

     $SIG{expr}
             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";
                     close(LOG);
                     exit(0);
                 }

                 $SIG{'INT'} = 'handler';
                 $SIG{'QUIT'} = 'handler';
                 ...
                 $SIG{'INT'}  =  'DEFAULT';  # restore default action
                 $SIG{'QUIT'} = 'IGNORE';    # ignore SIGQUIT

             The %SIG array only contains values for 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 <perlsubs>.




 

 

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