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.
while (<>) {...} # equivalent in only 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:
ord and
int, as well as the all file tests (-f, -d) except for -t, which defaults to
STDIN.
print and unlink.
=~ operator.
foreach loop if no other variable is supplied.
grep and
map functions.
<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.
eval enclosed by the current
BLOCK). (Mnemonic: like & in some editors.) This variable is read-only.
` often precedes a quoted string.) This variable is read-only.
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.
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.) This variable is read-only.
$*'' is 0. Default is 0. (Mnemonic: * matches multiple things.) Note that
this variable influences the interpretation of only ``^'' and ``$''.
A literal newline can be searched for even when $* == 0.
Use of ``$*'' is deprecated in modern perls.
<>'' 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.)
"\n\n" means something slightly different than setting it to "", if the file contains consecutive empty lines. Setting it to "" will treat two or more consecutive empty lines as a single empty 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;
$| tells you only whether you've asked Perl explicitly to flush after each write). 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.)
$\'' instead of adding \n at the end of the print. Also, it's just like $/, but it's what you get ``back'' from Perl.)
$,'' 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.)
$foo{$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.
$#'' explicitly to get awk's value. (Mnemonic: # is the number sign.)
Use of ``$#'' is deprecated.
$^''.)
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 $^A unless you call formline yourself and then look at it. See the perlform manpage and
formline().
``) command, or system operator. Note that this is the status
word returned by the wait system call (or else is made up to
look like it). Thus, the exit value of the subprocess is actually ($? >> 8), and
$? & 255 gives which signal, if any, the process died from, and whether there was a
core dump. (Mnemonic: similar to sh and
ksh.)
Note that if you have installed a signal handler for SIGCHLD, the value of $? will usually be wrong outside that handler.
Inside an END subroutine $? contains the value that is going to be given to exit. You can modify $? in an END subroutine to change the exit status of the script.
Under
VMS, the pragma use vmsish 'status' makes $? reflect the actual
VMS exit status, instead of the default emulation of
POSIX status.
$!'' to be anything in particular unless you've gotten a specific error
return indicating a system error.) If used in a string context, yields the
corresponding system error string. You can assign to ``$!'' 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?)
$!, if available. (If not, it's just $! again, except under
OS/2.) At the moment, this differs from $! under only
VMS and
OS/2, where it provides the
VMS status value from the last system error, and
OS/2 error code of the last call to
OS/2
API which was not directed via
CRT. The caveats mentioned in the description of
$! apply here, too. (Mnemonic: Extra error explanation.)
Note that under
OS/2 $! and $^E do not track each other, so if an OS/2-specific call is performed, you may
need to check both.
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.
$< = $>; # set real to effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went TO, if you're running setuid.) Note: ``$<'' and ``$>'' can be swapped on only machines supporting setreuid.
getgid, and the subsequent ones by getgroups, one
of which may be the same as the first number. (Mnemonic: parentheses are
used to GROUP
things. The real gid is the group you LEFT, if you're running setgid.)
getegid, and the subsequent ones by getgroups,
one of which may be the same as the first number. (Mnemonic: parentheses
are used to GROUP things. The effective gid is the group that's RIGHT for you, if you're running setgid.)
Note: ``$<'', ``$>'', ``$('' and ``$)'' can be set only on machines that support the corresponding set[re][ug]id() routine. ``$('' and ``$)'' can be swapped on only machines supporting setregid.
Because Perl doesn't currently use initgroups, you can't set
your group vector to multiple groups.
$0'' modifies the argument area that the ps program sees. This
is more useful as a way of indicating the current program state than it is
for hiding the program you're running. (Mnemonic: same as sh and ksh.)
index
and substr functions. (Mnemonic: [ begins subscripts.)
As of Perl 5, assignment to ``$['' is treated as a compiler directive, and cannot influence the behavior of
any other file. Its use is discouraged.
perl -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?)
execed 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.
use strict. See the documentation of strict for more details.
$Config{'osname'}.
argv[0].
@ARGV contains the command line arguments intended
for the script. Note that $#ARGV is the generally number of arguments minus one, because $ARGV[0] is the first argument, NOT the command name. See ``$0'' for the command name.
@INC contains the list of places to look for Perl
scripts to be evaluated by the do EXPR, require, or use constructs. It initially consists of the arguments to any -I command line switches, followed by the default Perl library, probably /usr/local/lib/perl, followed by ``.'', to represent the current directory. If you need to
modify this at runtime, you should use the use lib pragma to get the machine-dependent library properly loaded also:
use lib '/mypath/libdir/';
use SomeMod;
%INC contains entries for each filename that has been
included via do or require. The key is the filename you specified, and the value is the location of
the file actually found. The require command uses this array to determine whether a given file has already been
included.
%ENV contains your current environment. Setting a
value in ENV changes the environment for child processes.
%SIG is used to set signal handlers for various
signals. Example:
sub handler { # 1st argument is signal name
local($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 contains values for only 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 perlsub manpage.
If your system has the sigaction function then signal handlers are installed using it. This means you get reliable signal handling. If your system has the
SA_RESTART flag it is used when signals handlers are installed. This means that system calls for which it is supported continue rather than returning when a signal arrives. If you want your system calls to be interrupted by signal delivery then do something like this:
use POSIX ':signal_h';
my $alarm = 0;
sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
or die "Error setting SIGALRM handler: $!\n";
See the POSIX 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__. See
die, warn and eval.