AmigActive 2/AACD 2.iso/AACD/Programming/perlman/man/perlvar.txt

Text File | 1999-09-09 | 25.9 KB | 677 lines

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, 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($_)

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

       o          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.

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

       o          The pattern matching operations m//, s///, and
                  tr/// when used without an =~ operator.

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

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

       o          The default place to put an input record when a
                  <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.)

       $<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
               \digit.)  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.  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()).  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

       $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 "\n\n" 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 "\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

       $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.  This has 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 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

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

       $LIST_SEPARATOR

       $"      This is like "$," 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 "\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 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?)

       $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 $! only under VMS, where it provides
               the VMS status value from the last system error.
               The caveats 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, 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.

       $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().
               Because Perl doesn't currently use initgroups(),
               you can't set your group vector to multiple
               groups.
       $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.
               (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?)

       $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.)

       $OSNAME =item $^O
               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'}.

       $PERLDB

       $^P     The internal flag that the debugger clears so that
               it doesn't debug itself.  You could conceivably
               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.  If you need to
               modify this at runtime, you should use the use lib
               pragma in order to also get the machine-dependent
               library properly loaded:

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

               =item %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 the
               perlsubs 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__.