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:
- 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.
- Various list functions like
print()
and
unlink()
.
- The pattern matching operations
m//
,
s///
, and
tr///
when used
without an
=~
operator.
- *
The default iterator variable in a
foreach
loop if no other
variable is supplied.
- *
The implicit iterator variable in the
grep()
and
map()
functions.
- *
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 $/;
$_ = ; # 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
formline
.
$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(). 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.)
$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. 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;
%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__.
.