| $_ |
The default input and pattern-searching space. The following
pairs are equivalent:
while (<>) {... # only equivalent in while!
while ($_ = <>) {...
/^Subject:/
$_ =~ /^Subject:/
y/a-z/A-Z/
$_ =~ y/a-z/A-Z/
chop
chop($_)
(Mnemonic: underline is understood in certain operations.)
|
| $. |
The current input line number of the last filehandle that
was read. Readonly. 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.)
|
| $/ |
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.)
|
| $, |
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.)
|
| $"" |
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.)
|
| $\ |
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.)
|
| $# |
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.)
|
| $% |
The current page number of the currently selected output channel.
(Mnemonic: % is page number in nroff.)
|
| $= |
The current page length (printable lines) of the currently selected
output channel. Default is 60. (Mnemonic: = has horizontal lines.)
|
| $- |
The number of lines left on the page of the currently selected
output channel. (Mnemonic: lines_on_page - lines_printed.)
|
| $~ |
The name of the current report format for the currently selected
output channel. Default is name of the filehandle. (Mnemonic:
brother to $^.)
|
| $^ |
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.)
|
| $| |
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.)
|
| $$ |
The process number of the perl running this script. (Mnemonic:
same as shells.)
|
| $? |
The status returned by the last pipe
close, backtick (\`\`) command or system operator. Note that
this is the status word returned by the wait() system call, so
the exit value of the subprocess is actually ($? >> 8).
$? & 255 gives which signal, if any, the process died from,
and whether there was a core dump. (Mnemonic: similar to sh and ksh.)
|
| $& |
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.)
|
| $\` |
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.)
|
| $' |
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.) For example,
$_ = 'abcdefghi';
/def/;
print "$\`:$&:$'\n"; # prints abc:def:ghi
|
| $+ |
The last bracket matched by the last search pattern. This is
useful if you don't know which of a set of alternative patterns
matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.)
|
| $* |
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.
|
| $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. (Mnemonic: same as sh and ksh.)
|
| $<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.)
|
| $[ |
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.)
|
| $] |
The string printed out when you say "perl -v". It can be
used to determine at the beginning of a script whether the perl
interpreter executing the script is in the right range of versions.
If used in a numeric context, returns the version + patchlevel / 1000.
For example,
# see if getc is available
($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?)
|
| $; |
The subscript separator for multi-dimensional array emulation.
If you refer to an associative array 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.)
|
| $! |
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?)
|
| $@ |
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"?)
|
| $< |
The real uid of this process. (Mnemonic: it's the uid
you came FROM, if you're running setuid.)
|
| $> |
The effective uid of this process. For example,
$< = $>; # set real uid to the effective uid
($<,$>) = ($>,$<); # 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().
|
| $( |
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.)
|
| $) |
|
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().
|
| $: |
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.)
|
| $^D |
The current value of the debugging flags. (Mnemonic: value of
-D switch.)
|
| $^F |
The maximum system file descriptor, ordinarily 2. System file
descriptors are passed to subprocesses, while higher file
descriptors are not. During an open, system file descriptors
are preserved even if the open fails. Ordinary file descriptors
are closed before the open is
attempted.
|
| $^I |
The current value of the inplace-edit extension. Use
undef to disable
inplace editing. (Mnemonic: value of -i switch.)
|
| $^L |
What formats output to perform a formfeed. Default is \f.
|
| $^P |
The internal flag that the debugger clears so that it doesn't
debug itself. You could conceivable disable debugging yourself
by clearing it.
|
| $^T |
The time at which the script began running, in seconds since
the epoch. The values returned by the -M, -A
and -C filetests are based on this value.
|
| $^W |
The current value of the warning switch. (Mnemonic: related to the
-w switch.)
|
| $^X |
The name that Perl itself was executed as, from 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"
command or the "require" command.
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 associative array 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 associative array ENV contains your current environment. Setting
a value in ENV changes the environment for child processes.
|
| $SIG{expr} |
The associative array SIG is used to set signal handlers
for various signals. For example,
sub handler { # 1st argument is signal name
local($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.
|
