use English;at the top of your program. This will alias all the short names to the long names in the current package. Some even have medium names, generally borrowed from awk.
To go a step further, those variables that depend on the currently selected filehandle may instead (and preferably) 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 EXPRor more safely,
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.
The following list is ordered by scalar variables first, then the
arrays, then the hashes (except $^M was added in the wrong place).
This is somewhat obscured by the fact that %ENV and %SIG are listed as
$ENV{expr} and $SIG{expr}.
while (<>) {...} # equivalent in only while! while (defined($_ = <>)) {...}
/^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:
(Mnemonic: underline is understood in certain operations.)
$_ = 'abcdefghi'; /def/; print "$`:$&:$'\n"; # prints abc:def:ghiThis variable is read-only.
/Version: (.*)|Revision: (.*)/ && ($rev = $+);(Mnemonic: be positive and forward looking.) This variable is read-only.
Use of ``$*'' is deprecated in modern Perls, supplanted by
the /s and /m modifiers on pattern matching.
undef $/; $_ = <FH>; # whole file now here s/\n[ \t]+/ /g;Remember: the value of $/ is a string, not a regexp. AWK has to be better for something :-)
Setting $/ to a reference to an integer, scalar containing an integer, or scalar that's convertable to an integer will attempt to read records instead of lines, with the maximum record size being the referenced integer. So this:
$/ = \32768; # or \"32768", or \$var_containing_32768 open(FILE, $myfile); $_ = <FILE>;will read a record of no more than 32768 bytes from FILE. If you're not reading from a record-oriented file (or your OS doesn't have record-oriented files), then you'll likely get a full chunk of data with every read. If a record is larger than the record size you've set, you'll get the record back in pieces.
On VMS, record reads are done with the equivalent of sysread, so it's
best not to mix record and non-record reads on the same file. (This is
likely not a problem, as any file you'd want to read in record mode is
proably usable in line mode) Non-VMS systems perform normal I/O, so
it's safe to mix record and non-record reads of a file.
$foo{$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'' multidimensional arrays.
Use of ``$#'' is deprecated.
Additionally, if the h_errno variable is supported in C, its value is returned via $? if any of the gethost*() functions fail.
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.
Also see the section on Error Indicators.
Also see the section on Error Indicators.
Under VMS, $^E provides the VMS status value from the last system error. This is more specific information about the last system error than that provided by $!. This is particularly important when $! is set to EVMSERR.
Under OS/2, $^E is set to the error code of the last call to OS/2 API either via CRT, or directly from perl.
Under Win32, $^E always returns the last error information reported by the Win32 call GetLastError() which describes the last error from within the Win32 API. Most Win32-specific code will report errors via $^E. ANSI C and UNIX-like calls set errno and so most portable Perl code will report errors via $!.
Caveats mentioned in the description of $! generally apply to $^E, also. (Mnemonic: Extra error explanation.)
Also see the section on Error Indicators.
Note that warning messages are not collected in this variable. You can, however, set up a routine to process warnings by setting $SIG{__WARN__} as described below.
Also see the section on Error Indicators.
$< = $>; # set real to effective uid ($<,$>) = ($>,$<); # swap real and effective uid(Mnemonic: it's the uid you went TO, if you're running setuid.) Note: ``$<'' and ``$>'' can be swapped only on machines supporting setreuid().
However, a value assigned to ``$('' must be a single number used to set the real gid. So the value given by ``$('' should not be assigned back to ``$('' without being forced numeric, such as by adding zero.
(Mnemonic: parentheses are used to GROUP things. The real gid is the
group you LEFT, if you're running setgid.)
Similarly, a value assigned to ``$)'' must also be a space-separated list of numbers. The first number is used to set the effective gid, and the rest (if any) are passed to setgroups(). To get the effect of an empty list for setgroups(), just repeat the new effective gid; that is, to force an effective gid of 5 and an effectively empty setgroups() list, say $) = "5 5" .
(Mnemonic: parentheses are used to GROUP things. The effective gid is the group that's RIGHT for you, if you're running setgid.)
Note: ``$<'', ``$>'', ``$('' and ``$)'' can be set only on
machines that support the corresponding set[re][ug]id() routine. ``$(''
and ``$)'' can be swapped only on machines supporting setregid().
As of Perl 5, assignment to ``$['' is treated as a compiler directive,
and cannot influence the behavior of any other file. Its use is
discouraged.
warn "No checksumming!\n" if $] < 3.019;See also the documentation of use VERSION and require VERSION for a convenient way to fail if the Perl interpreter is too old.
$^M = 'a' x (1<<16);would allocate a 64K buffer for use when in emergency. See the INSTALL file for information on how to enable this option. As a disincentive to casual use of this advanced feature, there is no the English manpage long name for this variable.
Note that some bits may be relevent at compile-time only, some at
run-time only. This is a new mechanism and the details may change.
use lib '/mypath/libdir/'; use SomeMod;
sub handler { # 1st argument is signal name my($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 SIGQUITThe %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"; # assumes main::Plumber (not recommended) $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__.
Note that the $SIG{__DIE__} hook is called even inside eval()ed blocks/strings. See the die entry in the perlfunc manpage and the section on $^S in the perlvar manpage for how to circumvent this.
Note that __DIE__/__WARN__ handlers are very special in one respect: they may be called to report (probable) errors found by the parser. In such a case the parser may be in inconsistent state, so any attempt to evaluate Perl code from such a handler will probably result in a segfault. This means that calls which result/may-result in parsing Perl should be used with extreme causion, like this:
require Carp if defined $^S; Carp::confess("Something wrong") if defined &Carp::confess; die "Something wrong, but could not load Carp to give backtrace... To see backtrace try starting Perl with -MCarp switch";Here the first line will load Carp unless it is the parser who called the handler. The second line will print backtrace and die if Carp was available. The third line will be executed only if Carp was not available.
See the die entry in the perlfunc manpage, the warn entry in the perlfunc manpage and the eval entry in the perlfunc manpage for additional info.
To illustrate the differences between these variables, consider the following Perl expression:
eval ' open PIPE, "/cdrom/install |"; @res = <PIPE>; close PIPE or die "bad pipe: $?, $!"; ';After execution of this statement all 4 variables may have been set.
$@ is set if the string to be eval-ed did not compile (this may happen if open or close were imported with bad prototypes), or if Perl code executed during evaluation die()d (either implicitly, say, if open was imported from module the Fatal manpage, or the die after close was triggered). In these cases the value of $@ is the compile error, or Fatal error (which will interpolate $!!), or the argument to die (which will interpolate $! and $?!).
When the above expression is executed, open(), <PIPE>, and close are translated to C run-time library calls. $! is set if one of these calls fails. The value is a symbolic indicator chosen by the C run-time library, say No such file or directory.
On some systems the above C library calls are further translated to calls to the kernel. The kernel may have set more verbose error indicator that one of the handful of standard C errors. In such cases $^E contains this verbose error indicator, which may be, say, CDROM tray not closed. On systems where C library calls are identical to system calls $^E is a duplicate of $!.
Finally, $? may be set to non-0 value if the external program /cdrom/install fails. Upper bits of the particular value may reflect specific error conditions encountered by this program (this is program-dependent), lower-bits reflect mode of failure (segfault, completion, etc.). Note that in contrast to $@, $!, and $^E, which are set only if error condition is detected, the variable $? is set on each wait or pipe close, overwriting the old value.
For more details, see the individual descriptions at the section on $@, the section on $!, the section on $^E, and the section on $?.