PERLVAR(1)PERLVAR(1)NAMEperlvar - 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/
$_ =~ tr/a-z/A-Z/
chop
chop($_)
(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. 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 "\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. (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?)
$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().
$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) =
$] =~ /(\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.)
$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 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.
3rd Berkeley DistributionPERLVAR(1)
