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diff --git a/pod/perlvar.pod b/pod/perlvar.pod new file mode 100644 index 0000000000..bdf24f6c89 --- /dev/null +++ b/pod/perlvar.pod @@ -0,0 +1,608 @@ +=head1 NAME + +perlvar - Perl predefined variables + +=head1 DESCRIPTION + +=head2 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 B<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. + +=over 8 + +=item $ARG + +=item $_ + +The default input and pattern-searching space. The following pairs are +equivalent: + + while (<>) {...} # only equivalent in while! + while ($_ = <>) {...} + + /^Subject:/ + $_ =~ /^Subject:/ + + tr/a-z/A-Z/ + $_ =~ tr/a-z/A-Z/ + + chop + chop($_) + +(Mnemonic: underline is understood in certain operations.) + +=item $<I<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. + +=item $MATCH + +=item $& + +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. + +=item $PREMATCH + +=item $` + +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. + +=item $POSTMATCH + +=item $' + +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. + +=item $LAST_PAREN_MATCH + +=item $+ + +The last bracket matched by the last search pattern. This is useful if +you don't know which of a set of alternative patterns matched. For +example: + + /Version: (.*)|Revision: (.*)/ && ($rev = $+); + +(Mnemonic: be positive and forward looking.) +This variable is read-only. + +=item $MULTILINE_MATCHING + +=item $* + +Set to 1 to do multiline matching within a string, 0 to tell Perl +that it can assume that strings contain a single line, for the purpose +of optimizing pattern matches. Pattern matches on strings containing +multiple newlines can produce confusing results when "C<$*>" is 0. Default +is 0. (Mnemonic: * matches multiple things.) Note that this variable +only influences the interpretation of "C<^>" and "C<$>". A literal newline can +be searched for even when C<$* == 0>. + +Use of "C<$*>" is deprecated in Perl 5. + +=item input_line_number HANDLE EXPR + +=item $INPUT_LINE_NUMBER + +=item $NR + +=item $. + +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 "C<E<lt>E<gt>>" 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.) + +=item input_record_separator HANDLE EXPR + +=item $INPUT_RECORD_SEPARATOR + +=item $RS + +=item $/ + +The input record separator, newline by default. Works like B<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 C<"\n\n"> means +something slightly different than setting it to C<"">, if the file +contains consecutive blank lines. Setting it to C<""> will treat two or +more consecutive blank lines as a single blank line. Setting it to +C<"\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; + +=item autoflush HANDLE EXPR + +=item $OUTPUT_AUTOFLUSH + +=item $| + +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.) + +=item output_field_separator HANDLE EXPR + +=item $OUTPUT_FIELD_SEPARATOR + +=item $OFS + +=item $, + +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 B<awk>, set this variable +as you would set B<awk>'s OFS variable to specify what is printed +between fields. (Mnemonic: what is printed when there is a , in your +print statement.) + +=item output_record_separator HANDLE EXPR + +=item $OUTPUT_RECORD_SEPARATOR + +=item $ORS + +=item $\ + +The output record separator for the print operator. Ordinarily the +print operator simply prints out the comma separated fields you +specify, with no trailing newline or record separator assumed. In +order to get behavior more like B<awk>, set this variable as you would +set B<awk>'s ORS variable to specify what is printed at the end of the +print. (Mnemonic: you set "C<$\>" instead of adding \n at the end of the +print. Also, it's just like /, but it's what you get "back" from +Perl.) + +=item $LIST_SEPARATOR + +=item $" + +This is like "C<$,>" 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.) + +=item $SUBSCRIPT_SEPARATOR + +=item $SUBSEP + +=item $; + +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 B<awk>. Note that if your +keys contain binary data there might not be any safe value for "C<$;>". +(Mnemonic: comma (the syntactic subscript separator) is a +semi-semicolon. Yeah, I know, it's pretty lame, but "C<$,>" is already +taken for something more important.) + +Consider using "real" multi-dimensional arrays in Perl 5. + +=item $OFMT + +=item $# + +The output format for printed numbers. This variable is a half-hearted +attempt to emulate B<awk>'s OFMT variable. There are times, however, +when B<awk> and Perl have differing notions of what is in fact +numeric. Also, the initial value is %.20g rather than %.6g, so you +need to set "C<$#>" explicitly to get B<awk>'s value. (Mnemonic: # is the +number sign.) + +Use of "C<$#>" is deprecated in Perl 5. + +=item format_page_number HANDLE EXPR + +=item $FORMAT_PAGE_NUMBER + +=item $% + +The current page number of the currently selected output channel. +(Mnemonic: % is page number in B<nroff>.) + +=item format_lines_per_page HANDLE EXPR + +=item $FORMAT_LINES_PER_PAGE + +=item $= + +The current page length (printable lines) of the currently selected +output channel. Default is 60. (Mnemonic: = has horizontal lines.) + +=item format_lines_left HANDLE EXPR + +=item $FORMAT_LINES_LEFT + +=item $- + +The number of lines left on the page of the currently selected output +channel. (Mnemonic: lines_on_page - lines_printed.) + +=item format_name HANDLE EXPR + +=item $FORMAT_NAME + +=item $~ + +The name of the current report format for the currently selected output +channel. Default is name of the filehandle. (Mnemonic: brother to +"C<$^>".) + +=item format_top_name HANDLE EXPR + +=item $FORMAT_TOP_NAME + +=item $^ + +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.) + +=item format_line_break_characters HANDLE EXPR + +=item $FORMAT_LINE_BREAK_CHARACTERS + +=item $: + +The current set of characters after which a string may be broken to +fill continuation fields (starting with ^) in a format. Default is +S<" \n-">, to break on whitespace or hyphens. (Mnemonic: a "colon" in +poetry is a part of a line.) + +=item format_formfeed HANDLE EXPR + +=item $FORMAT_FORMFEED + +=item $^L + +What formats output to perform a formfeed. Default is \f. + +=item $ACCUMULATOR + +=item $^A + +The current value of the write() accumulator for format() lines. A format +contains formline() commands that put their result into C<$^A>. After +calling its format, write() prints out the contents of C<$^A> and empties. +So you never actually see the contents of C<$^A> unless you call +formline() yourself and then look at it. See L<perlform> and +L<perlfunc/formline()>. + +=item $CHILD_ERROR + +=item $? + +The status returned by the last pipe close, backtick (C<``>) command, +or system() operator. Note that this is the status word returned by +the wait() system call, so the exit value of the subprocess is actually +(C<$? E<gt>E<gt> 8>). Thus on many systems, C<$? & 255> gives which signal, +if any, the process died from, and whether there was a core dump. +(Mnemonic: similar to B<sh> and B<ksh>.) + +=item $OS_ERROR + +=item $ERRNO + +=item $! + +If used in a numeric context, yields the current value of errno, with +all the usual caveats. (This means that you shouldn't depend on the +value of "C<$!>" 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 "C<$!>" in order to set I<errno> if, for instance, you want "C<$!>" to return the +string for error I<n>, or you want to set the exit value for the die() +operator. (Mnemonic: What just went bang?) + +=item $EVAL_ERROR + +=item $@ + +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"?) + +=item $PROCESS_ID + +=item $PID + +=item $$ + +The process number of the Perl running this script. (Mnemonic: same +as shells.) + +=item $REAL_USER_ID + +=item $UID + +=item $< + +The real uid of this process. (Mnemonic: it's the uid you came I<FROM>, +if you're running setuid.) + +=item $EFFECTIVE_USER_ID + +=item $EUID + +=item $> + +The effective uid of this process. Example: + + $< = $>; # set real to effective uid + ($<,$>) = ($>,$<); # swap real and effective uid + +(Mnemonic: it's the uid you went I<TO>, if you're running setuid.) Note: +"C<$E<lt>>" and "C<$E<gt>>" can only be swapped on machines supporting setreuid(). + +=item $REAL_GROUP_ID + +=item $GID + +=item $( + +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 I<GROUP> +things. The real gid is the group you I<LEFT>, if you're running setgid.) + +=item $EFFECTIVE_GROUP_ID + +=item $EGID + +=item $) + +The effective gid of this process. If you are on a machine that +supports membership in multiple groups simultaneously, gives a space +separated list of groups you are in. The first number is the one +returned by getegid(), and the subsequent ones by getgroups(), one of +which may be the same as the first number. (Mnemonic: parentheses are +used to I<GROUP> things. The effective gid is the group that's I<RIGHT> for +you, if you're running setgid.) + +Note: "C<$E<lt>>", "C<$E<gt>>", "C<$(>" and "C<$)>" can only be set on machines +that support the corresponding I<set[re][ug]id()> routine. "C<$(>" and "C<$)>" +can only be swapped on machines supporting setregid(). + +=item $PROGRAM_NAME + +=item $0 + +Contains the name of the file containing the Perl script being +executed. Assigning to "C<$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 B<sh> and B<ksh>.) + +=item $[ + +The index of the first element in an array, and of the first character +in a substring. Default is 0, but you could set it to 1 to make +Perl behave more like B<awk> (or Fortran) when subscripting and when +evaluating the index() and substr() functions. (Mnemonic: [ begins +subscripts.) + +As of Perl 5, assignment to "C<$[>" is treated as a compiler directive, +and cannot influence the behavior of any other file. Its use is +discouraged. + +=item $PERL_VERSION + +=item $] + +The string printed out when you say C<perl -v>. It can be used to +determine at the beginning of a script whether the perl interpreter +executing the script is in the right range of versions. If used in a +numeric context, returns the version + patchlevel / 1000. Example: + + # see if getc is available + ($version,$patchlevel) = + $] =~ /(\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?) + +=item $DEBUGGING + +=item $^D + +The current value of the debugging flags. (Mnemonic: value of B<-D> +switch.) + +=item $SYSTEM_FD_MAX + +=item $^F + +The maximum system file descriptor, ordinarily 2. System file +descriptors are passed to exec()ed processes, while higher file +descriptors are not. Also, during an open(), system file descriptors are +preserved even if the open() fails. (Ordinary file descriptors are +closed before the open() is attempted.) Note that the close-on-exec +status of a file descriptor will be decided according to the value of +C<$^F> at the time of the open, not the time of the exec. + +=item $INPLACE_EDIT + +=item $^I + +The current value of the inplace-edit extension. Use C<undef> to disable +inplace editing. (Mnemonic: value of B<-i> switch.) + +=item $PERLDB + +=item $^P + +The internal flag that the debugger clears so that it doesn't debug +itself. You could conceivable disable debugging yourself by clearing +it. + +=item $BASETIME + +=item $^T + +The time at which the script began running, in seconds since the +epoch (beginning of 1970). The values returned by the B<-M>, B<-A> +and B<-C> filetests are +based on this value. + +=item $WARNING + +=item $^W + +The current value of the warning switch, either TRUE or FALSE. (Mnemonic: related to the +B<-w> switch.) + +=item $EXECUTABLE_NAME + +=item $^X + +The name that the Perl binary itself was executed as, from C's C<argv[0]>. + +=item $ARGV + +contains the name of the current file when reading from <>. + +=item @ARGV + +The array @ARGV contains the command line arguments intended for the +script. Note that C<$#ARGV> is the generally number of arguments minus +one, since C<$ARGV[0]> is the first argument, I<NOT> the command name. See +"C<$0>" for the command name. + +=item @INC + +The array @INC contains the list of places to look for Perl scripts to +be evaluated by the C<do EXPR>, C<require>, or C<use> constructs. It +initially consists of the arguments to any B<-I> command line switches, +followed by the default Perl library, probably "/usr/local/lib/perl", +followed by ".", to represent the current directory. + +=item %INC + +The hash %INC contains entries for each filename that has +been included via C<do> or C<require>. The key is the filename you +specified, and the value is the location of the file actually found. +The C<require> command uses this array to determine whether a given file +has already been included. + +=item $ENV{expr} + +The hash %ENV contains your current environment. Setting a +value in C<ENV> changes the environment for child processes. + +=item $SIG{expr} + +The hash %SIG is used to set signal handlers for various +signals. Example: + + sub handler { # 1st argument is signal name + local($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>. + +=back + |