1perlfaq8(3)           User Contributed Perl Documentation          perlfaq8(3)
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NAME

6       perlfaq8 - System Interaction
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VERSION

9       version 5.20210520
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DESCRIPTION

12       This section of the Perl FAQ covers questions involving operating
13       system interaction. Topics include interprocess communication (IPC),
14       control over the user-interface (keyboard, screen and pointing
15       devices), and most anything else not related to data manipulation.
16
17       Read the FAQs and documentation specific to the port of perl to your
18       operating system (eg, perlvms, perlplan9, ...). These should contain
19       more detailed information on the vagaries of your perl.
20
21   How do I find out which operating system I'm running under?
22       The $^O variable ($OSNAME if you use "English") contains an indication
23       of the name of the operating system (not its release number) that your
24       perl binary was built for.
25
26   How come exec() doesn't return?
27       (contributed by brian d foy)
28
29       The "exec" function's job is to turn your process into another command
30       and never to return. If that's not what you want to do, don't use
31       "exec". :)
32
33       If you want to run an external command and still keep your Perl process
34       going, look at a piped "open", "fork", or "system".
35
36   How do I do fancy stuff with the keyboard/screen/mouse?
37       How you access/control keyboards, screens, and pointing devices
38       ("mice") is system-dependent. Try the following modules:
39
40       Keyboard
41               Term::Cap               Standard perl distribution
42               Term::ReadKey           CPAN
43               Term::ReadLine::Gnu     CPAN
44               Term::ReadLine::Perl    CPAN
45               Term::Screen            CPAN
46
47       Screen
48               Term::Cap               Standard perl distribution
49               Curses                  CPAN
50               Term::ANSIColor         CPAN
51
52       Mouse
53               Tk                      CPAN
54               Wx                      CPAN
55               Gtk2                    CPAN
56               Qt4                     kdebindings4 package
57
58       Some of these specific cases are shown as examples in other answers in
59       this section of the perlfaq.
60
61   How do I print something out in color?
62       In general, you don't, because you don't know whether the recipient has
63       a color-aware display device. If you know that they have an ANSI
64       terminal that understands color, you can use the Term::ANSIColor module
65       from CPAN:
66
67           use Term::ANSIColor;
68           print color("red"), "Stop!\n", color("reset");
69           print color("green"), "Go!\n", color("reset");
70
71       Or like this:
72
73           use Term::ANSIColor qw(:constants);
74           print RED, "Stop!\n", RESET;
75           print GREEN, "Go!\n", RESET;
76
77   How do I read just one key without waiting for a return key?
78       Controlling input buffering is a remarkably system-dependent matter.
79       On many systems, you can just use the stty command as shown in "getc"
80       in perlfunc, but as you see, that's already getting you into
81       portability snags.
82
83           open(TTY, "+</dev/tty") or die "no tty: $!";
84           system "stty  cbreak </dev/tty >/dev/tty 2>&1";
85           $key = getc(TTY);        # perhaps this works
86           # OR ELSE
87           sysread(TTY, $key, 1);    # probably this does
88           system "stty -cbreak </dev/tty >/dev/tty 2>&1";
89
90       The Term::ReadKey module from CPAN offers an easy-to-use interface that
91       should be more efficient than shelling out to stty for each key.  It
92       even includes limited support for Windows.
93
94           use Term::ReadKey;
95           ReadMode('cbreak');
96           $key = ReadKey(0);
97           ReadMode('normal');
98
99       However, using the code requires that you have a working C compiler and
100       can use it to build and install a CPAN module. Here's a solution using
101       the standard POSIX module, which is already on your system (assuming
102       your system supports POSIX).
103
104           use HotKey;
105           $key = readkey();
106
107       And here's the "HotKey" module, which hides the somewhat mystifying
108       calls to manipulate the POSIX termios structures.
109
110           # HotKey.pm
111           package HotKey;
112
113           use strict;
114           use warnings;
115
116           use parent 'Exporter';
117           our @EXPORT = qw(cbreak cooked readkey);
118
119           use POSIX qw(:termios_h);
120           my ($term, $oterm, $echo, $noecho, $fd_stdin);
121
122           $fd_stdin = fileno(STDIN);
123           $term     = POSIX::Termios->new();
124           $term->getattr($fd_stdin);
125           $oterm     = $term->getlflag();
126
127           $echo     = ECHO | ECHOK | ICANON;
128           $noecho   = $oterm & ~$echo;
129
130           sub cbreak {
131               $term->setlflag($noecho);  # ok, so i don't want echo either
132               $term->setcc(VTIME, 1);
133               $term->setattr($fd_stdin, TCSANOW);
134           }
135
136           sub cooked {
137               $term->setlflag($oterm);
138               $term->setcc(VTIME, 0);
139               $term->setattr($fd_stdin, TCSANOW);
140           }
141
142           sub readkey {
143               my $key = '';
144               cbreak();
145               sysread(STDIN, $key, 1);
146               cooked();
147               return $key;
148           }
149
150           END { cooked() }
151
152           1;
153
154   How do I check whether input is ready on the keyboard?
155       The easiest way to do this is to read a key in nonblocking mode with
156       the Term::ReadKey module from CPAN, passing it an argument of -1 to
157       indicate not to block:
158
159           use Term::ReadKey;
160
161           ReadMode('cbreak');
162
163           if (defined (my $char = ReadKey(-1)) ) {
164               # input was waiting and it was $char
165           } else {
166               # no input was waiting
167           }
168
169           ReadMode('normal');                  # restore normal tty settings
170
171   How do I clear the screen?
172       (contributed by brian d foy)
173
174       To clear the screen, you just have to print the special sequence that
175       tells the terminal to clear the screen. Once you have that sequence,
176       output it when you want to clear the screen.
177
178       You can use the Term::ANSIScreen module to get the special sequence.
179       Import the "cls" function (or the ":screen" tag):
180
181           use Term::ANSIScreen qw(cls);
182           my $clear_screen = cls();
183
184           print $clear_screen;
185
186       The Term::Cap module can also get the special sequence if you want to
187       deal with the low-level details of terminal control. The "Tputs" method
188       returns the string for the given capability:
189
190           use Term::Cap;
191
192           my $terminal = Term::Cap->Tgetent( { OSPEED => 9600 } );
193           my $clear_screen = $terminal->Tputs('cl');
194
195           print $clear_screen;
196
197       On Windows, you can use the Win32::Console module. After creating an
198       object for the output filehandle you want to affect, call the "Cls"
199       method:
200
201           Win32::Console;
202
203           my $OUT = Win32::Console->new(STD_OUTPUT_HANDLE);
204           my $clear_string = $OUT->Cls;
205
206           print $clear_screen;
207
208       If you have a command-line program that does the job, you can call it
209       in backticks to capture whatever it outputs so you can use it later:
210
211           my $clear_string = `clear`;
212
213           print $clear_string;
214
215   How do I get the screen size?
216       If you have Term::ReadKey module installed from CPAN, you can use it to
217       fetch the width and height in characters and in pixels:
218
219           use Term::ReadKey;
220           my ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
221
222       This is more portable than the raw "ioctl", but not as illustrative:
223
224           require './sys/ioctl.ph';
225           die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
226           open(my $tty_fh, "+</dev/tty")                     or die "No tty: $!";
227           unless (ioctl($tty_fh, &TIOCGWINSZ, $winsize='')) {
228               die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
229           }
230           my ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
231           print "(row,col) = ($row,$col)";
232           print "  (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
233           print "\n";
234
235   How do I ask the user for a password?
236       (This question has nothing to do with the web. See a different FAQ for
237       that.)
238
239       There's an example of this in "crypt" in perlfunc. First, you put the
240       terminal into "no echo" mode, then just read the password normally.
241       You may do this with an old-style "ioctl()" function, POSIX terminal
242       control (see POSIX or its documentation the Camel Book), or a call to
243       the stty program, with varying degrees of portability.
244
245       You can also do this for most systems using the Term::ReadKey module
246       from CPAN, which is easier to use and in theory more portable.
247
248           use Term::ReadKey;
249
250           ReadMode('noecho');
251           my $password = ReadLine(0);
252
253   How do I read and write the serial port?
254       This depends on which operating system your program is running on. In
255       the case of Unix, the serial ports will be accessible through files in
256       "/dev"; on other systems, device names will doubtless differ.  Several
257       problem areas common to all device interaction are the following:
258
259       lockfiles
260           Your system may use lockfiles to control multiple access. Make sure
261           you follow the correct protocol. Unpredictable behavior can result
262           from multiple processes reading from one device.
263
264       open mode
265           If you expect to use both read and write operations on the device,
266           you'll have to open it for update (see "open" in perlfunc for
267           details). You may wish to open it without running the risk of
268           blocking by using "sysopen()" and "O_RDWR|O_NDELAY|O_NOCTTY" from
269           the Fcntl module (part of the standard perl distribution). See
270           "sysopen" in perlfunc for more on this approach.
271
272       end of line
273           Some devices will be expecting a "\r" at the end of each line
274           rather than a "\n". In some ports of perl, "\r" and "\n" are
275           different from their usual (Unix) ASCII values of "\015" and
276           "\012". You may have to give the numeric values you want directly,
277           using octal ("\015"), hex ("0x0D"), or as a control-character
278           specification ("\cM").
279
280               print DEV "atv1\012";    # wrong, for some devices
281               print DEV "atv1\015";    # right, for some devices
282
283           Even though with normal text files a "\n" will do the trick, there
284           is still no unified scheme for terminating a line that is portable
285           between Unix, DOS/Win, and Macintosh, except to terminate ALL line
286           ends with "\015\012", and strip what you don't need from the
287           output.  This applies especially to socket I/O and autoflushing,
288           discussed next.
289
290       flushing output
291           If you expect characters to get to your device when you "print()"
292           them, you'll want to autoflush that filehandle. You can use
293           "select()" and the $| variable to control autoflushing (see "$|" in
294           perlvar and "select" in perlfunc, or perlfaq5, "How do I
295           flush/unbuffer an output filehandle? Why must I do this?"):
296
297               my $old_handle = select($dev_fh);
298               $| = 1;
299               select($old_handle);
300
301           You'll also see code that does this without a temporary variable,
302           as in
303
304               select((select($deb_handle), $| = 1)[0]);
305
306           Or if you don't mind pulling in a few thousand lines of code just
307           because you're afraid of a little $| variable:
308
309               use IO::Handle;
310               $dev_fh->autoflush(1);
311
312           As mentioned in the previous item, this still doesn't work when
313           using socket I/O between Unix and Macintosh. You'll need to hard
314           code your line terminators, in that case.
315
316       non-blocking input
317           If you are doing a blocking "read()" or "sysread()", you'll have to
318           arrange for an alarm handler to provide a timeout (see "alarm" in
319           perlfunc). If you have a non-blocking open, you'll likely have a
320           non-blocking read, which means you may have to use a 4-arg
321           "select()" to determine whether I/O is ready on that device (see
322           "select" in perlfunc.
323
324       While trying to read from his caller-id box, the notorious Jamie
325       Zawinski "<jwz@netscape.com>", after much gnashing of teeth and
326       fighting with "sysread", "sysopen", POSIX's "tcgetattr" business, and
327       various other functions that go bump in the night, finally came up with
328       this:
329
330           sub open_modem {
331               use IPC::Open2;
332               my $stty = `/bin/stty -g`;
333               open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
334               # starting cu hoses /dev/tty's stty settings, even when it has
335               # been opened on a pipe...
336               system("/bin/stty $stty");
337               $_ = <MODEM_IN>;
338               chomp;
339               if ( !m/^Connected/ ) {
340                   print STDERR "$0: cu printed `$_' instead of `Connected'\n";
341               }
342           }
343
344   How do I decode encrypted password files?
345       You spend lots and lots of money on dedicated hardware, but this is
346       bound to get you talked about.
347
348       Seriously, you can't if they are Unix password files--the Unix password
349       system employs one-way encryption. It's more like hashing than
350       encryption. The best you can do is check whether something else hashes
351       to the same string. You can't turn a hash back into the original
352       string. Programs like Crack can forcibly (and intelligently) try to
353       guess passwords, but don't (can't) guarantee quick success.
354
355       If you're worried about users selecting bad passwords, you should
356       proactively check when they try to change their password (by modifying
357       passwd(1), for example).
358
359   How do I start a process in the background?
360       (contributed by brian d foy)
361
362       There's not a single way to run code in the background so you don't
363       have to wait for it to finish before your program moves on to other
364       tasks. Process management depends on your particular operating system,
365       and many of the techniques are covered in perlipc.
366
367       Several CPAN modules may be able to help, including IPC::Open2 or
368       IPC::Open3, IPC::Run, Parallel::Jobs, Parallel::ForkManager, POE,
369       Proc::Background, and Win32::Process. There are many other modules you
370       might use, so check those namespaces for other options too.
371
372       If you are on a Unix-like system, you might be able to get away with a
373       system call where you put an "&" on the end of the command:
374
375           system("cmd &")
376
377       You can also try using "fork", as described in perlfunc (although this
378       is the same thing that many of the modules will do for you).
379
380       STDIN, STDOUT, and STDERR are shared
381           Both the main process and the backgrounded one (the "child"
382           process) share the same STDIN, STDOUT and STDERR filehandles. If
383           both try to access them at once, strange things can happen. You may
384           want to close or reopen these for the child. You can get around
385           this with "open"ing a pipe (see "open" in perlfunc) but on some
386           systems this means that the child process cannot outlive the
387           parent.
388
389       Signals
390           You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
391           SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
392           sent when you write to a filehandle whose child process has closed
393           (an untrapped SIGPIPE can cause your program to silently die). This
394           is not an issue with "system("cmd&")".
395
396       Zombies
397           You have to be prepared to "reap" the child process when it
398           finishes.
399
400               $SIG{CHLD} = sub { wait };
401
402               $SIG{CHLD} = 'IGNORE';
403
404           You can also use a double fork. You immediately "wait()" for your
405           first child, and the init daemon will "wait()" for your grandchild
406           once it exits.
407
408               unless ($pid = fork) {
409                   unless (fork) {
410                       exec "what you really wanna do";
411                       die "exec failed!";
412                   }
413                   exit 0;
414               }
415               waitpid($pid, 0);
416
417           See "Signals" in perlipc for other examples of code to do this.
418           Zombies are not an issue with "system("prog &")".
419
420   How do I trap control characters/signals?
421       You don't actually "trap" a control character. Instead, that character
422       generates a signal which is sent to your terminal's currently
423       foregrounded process group, which you then trap in your process.
424       Signals are documented in "Signals" in perlipc and the section on
425       "Signals" in the Camel.
426
427       You can set the values of the %SIG hash to be the functions you want to
428       handle the signal. After perl catches the signal, it looks in %SIG for
429       a key with the same name as the signal, then calls the subroutine value
430       for that key.
431
432           # as an anonymous subroutine
433
434           $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
435
436           # or a reference to a function
437
438           $SIG{INT} = \&ouch;
439
440           # or the name of the function as a string
441
442           $SIG{INT} = "ouch";
443
444       Perl versions before 5.8 had in its C source code signal handlers which
445       would catch the signal and possibly run a Perl function that you had
446       set in %SIG. This violated the rules of signal handling at that level
447       causing perl to dump core. Since version 5.8.0, perl looks at %SIG
448       after the signal has been caught, rather than while it is being caught.
449       Previous versions of this answer were incorrect.
450
451   How do I modify the shadow password file on a Unix system?
452       If perl was installed correctly and your shadow library was written
453       properly, the "getpw*()" functions described in perlfunc should in
454       theory provide (read-only) access to entries in the shadow password
455       file. To change the file, make a new shadow password file (the format
456       varies from system to system--see passwd(1) for specifics) and use
457       pwd_mkdb(8) to install it (see pwd_mkdb(8) for more details).
458
459   How do I set the time and date?
460       Assuming you're running under sufficient permissions, you should be
461       able to set the system-wide date and time by running the date(1)
462       program. (There is no way to set the time and date on a per-process
463       basis.)  This mechanism will work for Unix, MS-DOS, Windows, and NT;
464       the VMS equivalent is "set time".
465
466       However, if all you want to do is change your time zone, you can
467       probably get away with setting an environment variable:
468
469           $ENV{TZ} = "MST7MDT";           # Unixish
470           $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
471           system('trn', 'comp.lang.perl.misc');
472
473   How can I sleep() or alarm() for under a second?
474       If you want finer granularity than the 1 second that the "sleep()"
475       function provides, the easiest way is to use the "select()" function as
476       documented in "select" in perlfunc. Try the Time::HiRes and the
477       BSD::Itimer modules (available from CPAN, and starting from Perl 5.8
478       Time::HiRes is part of the standard distribution).
479
480   How can I measure time under a second?
481       (contributed by brian d foy)
482
483       The Time::HiRes module (part of the standard distribution as of Perl
484       5.8) measures time with the "gettimeofday()" system call, which returns
485       the time in microseconds since the epoch. If you can't install
486       Time::HiRes for older Perls and you are on a Unixish system, you may be
487       able to call gettimeofday(2) directly. See "syscall" in perlfunc.
488
489   How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
490       You can use the "END" block to simulate "atexit()". Each package's
491       "END" block is called when the program or thread ends. See the perlmod
492       manpage for more details about "END" blocks.
493
494       For example, you can use this to make sure your filter program managed
495       to finish its output without filling up the disk:
496
497           END {
498               close(STDOUT) || die "stdout close failed: $!";
499           }
500
501       The "END" block isn't called when untrapped signals kill the program,
502       though, so if you use "END" blocks you should also use
503
504           use sigtrap qw(die normal-signals);
505
506       Perl's exception-handling mechanism is its "eval()" operator. You can
507       use "eval()" as "setjmp" and "die()" as "longjmp". For details of this,
508       see the section on signals, especially the time-out handler for a
509       blocking "flock()" in "Signals" in perlipc or the section on "Signals"
510       in Programming Perl.
511
512       If exception handling is all you're interested in, use one of the many
513       CPAN modules that handle exceptions, such as Try::Tiny.
514
515       If you want the "atexit()" syntax (and an "rmexit()" as well), try the
516       "AtExit" module available from CPAN.
517
518   Why doesn't my sockets program work under System V (Solaris)? What does the
519       error message "Protocol not supported" mean?
520       Some Sys-V based systems, notably Solaris 2.X, redefined some of the
521       standard socket constants. Since these were constant across all
522       architectures, they were often hardwired into perl code. The proper way
523       to deal with this is to "use Socket" to get the correct values.
524
525       Note that even though SunOS and Solaris are binary compatible, these
526       values are different. Go figure.
527
528   How can I call my system's unique C functions from Perl?
529       In most cases, you write an external module to do it--see the answer to
530       "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
531       However, if the function is a system call, and your system supports
532       "syscall()", you can use the "syscall" function (documented in
533       perlfunc).
534
535       Remember to check the modules that came with your distribution, and
536       CPAN as well--someone may already have written a module to do it. On
537       Windows, try Win32::API. On Macs, try Mac::Carbon. If no module has an
538       interface to the C function, you can inline a bit of C in your Perl
539       source with Inline::C.
540
541   Where do I get the include files to do ioctl() or syscall()?
542       Historically, these would be generated by the h2ph tool, part of the
543       standard perl distribution. This program converts cpp(1) directives in
544       C header files to files containing subroutine definitions, like
545       "SYS_getitimer()", which you can use as arguments to your functions.
546       It doesn't work perfectly, but it usually gets most of the job done.
547       Simple files like errno.h, syscall.h, and socket.h were fine, but the
548       hard ones like ioctl.h nearly always need to be hand-edited.  Here's
549       how to install the *.ph files:
550
551           1. Become the super-user
552           2. cd /usr/include
553           3. h2ph *.h */*.h
554
555       If your system supports dynamic loading, for reasons of portability and
556       sanity you probably ought to use h2xs (also part of the standard perl
557       distribution). This tool converts C header files to Perl extensions.
558       See perlxstut for how to get started with h2xs.
559
560       If your system doesn't support dynamic loading, you still probably
561       ought to use h2xs. See perlxstut and ExtUtils::MakeMaker for more
562       information (in brief, just use make perl instead of a plain make to
563       rebuild perl with a new static extension).
564
565   Why do setuid perl scripts complain about kernel problems?
566       Some operating systems have bugs in the kernel that make setuid scripts
567       inherently insecure. Perl gives you a number of options (described in
568       perlsec) to work around such systems.
569
570   How can I open a pipe both to and from a command?
571       The IPC::Open2 module (part of the standard perl distribution) is an
572       easy-to-use approach that internally uses "pipe()", "fork()", and
573       "exec()" to do the job. Make sure you read the deadlock warnings in its
574       documentation, though (see IPC::Open2). See "Bidirectional
575       Communication with Another Process" in perlipc and "Bidirectional
576       Communication with Yourself" in perlipc
577
578       You may also use the IPC::Open3 module (part of the standard perl
579       distribution), but be warned that it has a different order of arguments
580       from IPC::Open2 (see IPC::Open3).
581
582   Why can't I get the output of a command with system()?
583       You're confusing the purpose of "system()" and backticks (``).
584       "system()" runs a command and returns exit status information (as a 16
585       bit value: the low 7 bits are the signal the process died from, if any,
586       and the high 8 bits are the actual exit value). Backticks (``) run a
587       command and return what it sent to STDOUT.
588
589           my $exit_status   = system("mail-users");
590           my $output_string = `ls`;
591
592   How can I capture STDERR from an external command?
593       There are three basic ways of running external commands:
594
595           system $cmd;        # using system()
596           my $output = `$cmd`;        # using backticks (``)
597           open (my $pipe_fh, "$cmd |");    # using open()
598
599       With "system()", both STDOUT and STDERR will go the same place as the
600       script's STDOUT and STDERR, unless the "system()" command redirects
601       them.  Backticks and "open()" read only the STDOUT of your command.
602
603       You can also use the "open3()" function from IPC::Open3. Benjamin
604       Goldberg provides some sample code:
605
606       To capture a program's STDOUT, but discard its STDERR:
607
608           use IPC::Open3;
609           use File::Spec;
610           my $in = '';
611           open(NULL, ">", File::Spec->devnull);
612           my $pid = open3($in, \*PH, ">&NULL", "cmd");
613           while( <PH> ) { }
614           waitpid($pid, 0);
615
616       To capture a program's STDERR, but discard its STDOUT:
617
618           use IPC::Open3;
619           use File::Spec;
620           my $in = '';
621           open(NULL, ">", File::Spec->devnull);
622           my $pid = open3($in, ">&NULL", \*PH, "cmd");
623           while( <PH> ) { }
624           waitpid($pid, 0);
625
626       To capture a program's STDERR, and let its STDOUT go to our own STDERR:
627
628           use IPC::Open3;
629           my $in = '';
630           my $pid = open3($in, ">&STDERR", \*PH, "cmd");
631           while( <PH> ) { }
632           waitpid($pid, 0);
633
634       To read both a command's STDOUT and its STDERR separately, you can
635       redirect them to temp files, let the command run, then read the temp
636       files:
637
638           use IPC::Open3;
639           use IO::File;
640           my $in = '';
641           local *CATCHOUT = IO::File->new_tmpfile;
642           local *CATCHERR = IO::File->new_tmpfile;
643           my $pid = open3($in, ">&CATCHOUT", ">&CATCHERR", "cmd");
644           waitpid($pid, 0);
645           seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
646           while( <CATCHOUT> ) {}
647           while( <CATCHERR> ) {}
648
649       But there's no real need for both to be tempfiles... the following
650       should work just as well, without deadlocking:
651
652           use IPC::Open3;
653           my $in = '';
654           use IO::File;
655           local *CATCHERR = IO::File->new_tmpfile;
656           my $pid = open3($in, \*CATCHOUT, ">&CATCHERR", "cmd");
657           while( <CATCHOUT> ) {}
658           waitpid($pid, 0);
659           seek CATCHERR, 0, 0;
660           while( <CATCHERR> ) {}
661
662       And it'll be faster, too, since we can begin processing the program's
663       stdout immediately, rather than waiting for the program to finish.
664
665       With any of these, you can change file descriptors before the call:
666
667           open(STDOUT, ">logfile");
668           system("ls");
669
670       or you can use Bourne shell file-descriptor redirection:
671
672           $output = `$cmd 2>some_file`;
673           open (PIPE, "cmd 2>some_file |");
674
675       You can also use file-descriptor redirection to make STDERR a duplicate
676       of STDOUT:
677
678           $output = `$cmd 2>&1`;
679           open (PIPE, "cmd 2>&1 |");
680
681       Note that you cannot simply open STDERR to be a dup of STDOUT in your
682       Perl program and avoid calling the shell to do the redirection.  This
683       doesn't work:
684
685           open(STDERR, ">&STDOUT");
686           $alloutput = `cmd args`;  # stderr still escapes
687
688       This fails because the "open()" makes STDERR go to where STDOUT was
689       going at the time of the "open()". The backticks then make STDOUT go to
690       a string, but don't change STDERR (which still goes to the old STDOUT).
691
692       Note that you must use Bourne shell (sh(1)) redirection syntax in
693       backticks, not csh(1)!  Details on why Perl's "system()" and backtick
694       and pipe opens all use the Bourne shell are in the versus/csh.whynot
695       article in the "Far More Than You Ever Wanted To Know" collection in
696       <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz> . To capture a
697       command's STDERR and STDOUT together:
698
699           $output = `cmd 2>&1`;                       # either with backticks
700           $pid = open(PH, "cmd 2>&1 |");              # or with an open pipe
701           while (<PH>) { }                            #    plus a read
702
703       To capture a command's STDOUT but discard its STDERR:
704
705           $output = `cmd 2>/dev/null`;                # either with backticks
706           $pid = open(PH, "cmd 2>/dev/null |");       # or with an open pipe
707           while (<PH>) { }                            #    plus a read
708
709       To capture a command's STDERR but discard its STDOUT:
710
711           $output = `cmd 2>&1 1>/dev/null`;           # either with backticks
712           $pid = open(PH, "cmd 2>&1 1>/dev/null |");  # or with an open pipe
713           while (<PH>) { }                            #    plus a read
714
715       To exchange a command's STDOUT and STDERR in order to capture the
716       STDERR but leave its STDOUT to come out our old STDERR:
717
718           $output = `cmd 3>&1 1>&2 2>&3 3>&-`;        # either with backticks
719           $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
720           while (<PH>) { }                            #    plus a read
721
722       To read both a command's STDOUT and its STDERR separately, it's easiest
723       to redirect them separately to files, and then read from those files
724       when the program is done:
725
726           system("program args 1>program.stdout 2>program.stderr");
727
728       Ordering is important in all these examples. That's because the shell
729       processes file descriptor redirections in strictly left to right order.
730
731           system("prog args 1>tmpfile 2>&1");
732           system("prog args 2>&1 1>tmpfile");
733
734       The first command sends both standard out and standard error to the
735       temporary file. The second command sends only the old standard output
736       there, and the old standard error shows up on the old standard out.
737
738   Why doesn't open() return an error when a pipe open fails?
739       If the second argument to a piped "open()" contains shell
740       metacharacters, perl "fork()"s, then "exec()"s a shell to decode the
741       metacharacters and eventually run the desired program. If the program
742       couldn't be run, it's the shell that gets the message, not Perl. All
743       your Perl program can find out is whether the shell itself could be
744       successfully started. You can still capture the shell's STDERR and
745       check it for error messages. See "How can I capture STDERR from an
746       external command?" elsewhere in this document, or use the IPC::Open3
747       module.
748
749       If there are no shell metacharacters in the argument of "open()", Perl
750       runs the command directly, without using the shell, and can correctly
751       report whether the command started.
752
753   What's wrong with using backticks in a void context?
754       Strictly speaking, nothing. Stylistically speaking, it's not a good way
755       to write maintainable code. Perl has several operators for running
756       external commands. Backticks are one; they collect the output from the
757       command for use in your program. The "system" function is another; it
758       doesn't do this.
759
760       Writing backticks in your program sends a clear message to the readers
761       of your code that you wanted to collect the output of the command.  Why
762       send a clear message that isn't true?
763
764       Consider this line:
765
766           `cat /etc/termcap`;
767
768       You forgot to check $? to see whether the program even ran correctly.
769       Even if you wrote
770
771           print `cat /etc/termcap`;
772
773       this code could and probably should be written as
774
775           system("cat /etc/termcap") == 0
776           or die "cat program failed!";
777
778       which will echo the cat command's output as it is generated, instead of
779       waiting until the program has completed to print it out. It also checks
780       the return value.
781
782       "system" also provides direct control over whether shell wildcard
783       processing may take place, whereas backticks do not.
784
785   How can I call backticks without shell processing?
786       This is a bit tricky. You can't simply write the command like this:
787
788           @ok = `grep @opts '$search_string' @filenames`;
789
790       As of Perl 5.8.0, you can use "open()" with multiple arguments.  Just
791       like the list forms of "system()" and "exec()", no shell escapes
792       happen.
793
794           open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
795           chomp(@ok = <GREP>);
796           close GREP;
797
798       You can also:
799
800           my @ok = ();
801           if (open(GREP, "-|")) {
802               while (<GREP>) {
803                   chomp;
804                   push(@ok, $_);
805               }
806               close GREP;
807           } else {
808               exec 'grep', @opts, $search_string, @filenames;
809           }
810
811       Just as with "system()", no shell escapes happen when you "exec()" a
812       list. Further examples of this can be found in "Safe Pipe Opens" in
813       perlipc.
814
815       Note that if you're using Windows, no solution to this vexing issue is
816       even possible. Even though Perl emulates "fork()", you'll still be
817       stuck, because Windows does not have an argc/argv-style API.
818
819   Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on
820       MS-DOS)?
821       This happens only if your perl is compiled to use stdio instead of
822       perlio, which is the default. Some (maybe all?) stdios set error and
823       eof flags that you may need to clear. The POSIX module defines
824       "clearerr()" that you can use. That is the technically correct way to
825       do it. Here are some less reliable workarounds:
826
827       1.  Try keeping around the seekpointer and go there, like this:
828
829               my $where = tell($log_fh);
830               seek($log_fh, $where, 0);
831
832       2.  If that doesn't work, try seeking to a different part of the file
833           and then back.
834
835       3.  If that doesn't work, try seeking to a different part of the file,
836           reading something, and then seeking back.
837
838       4.  If that doesn't work, give up on your stdio package and use
839           sysread.
840
841   How can I convert my shell script to perl?
842       Learn Perl and rewrite it. Seriously, there's no simple converter.
843       Things that are awkward to do in the shell are easy to do in Perl, and
844       this very awkwardness is what would make a shell->perl converter nigh-
845       on impossible to write. By rewriting it, you'll think about what you're
846       really trying to do, and hopefully will escape the shell's pipeline
847       datastream paradigm, which while convenient for some matters, causes
848       many inefficiencies.
849
850   Can I use perl to run a telnet or ftp session?
851       Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
852       CPAN).  <http://www.cpan.org/scripts/netstuff/telnet.emul.shar> will
853       also help for emulating the telnet protocol, but Net::Telnet is quite
854       probably easier to use.
855
856       If all you want to do is pretend to be telnet but don't need the
857       initial telnet handshaking, then the standard dual-process approach
858       will suffice:
859
860           use IO::Socket;             # new in 5.004
861           my $handle = IO::Socket::INET->new('www.perl.com:80')
862               or die "can't connect to port 80 on www.perl.com $!";
863           $handle->autoflush(1);
864           if (fork()) {               # XXX: undef means failure
865               select($handle);
866               print while <STDIN>;    # everything from stdin to socket
867           } else {
868               print while <$handle>;  # everything from socket to stdout
869           }
870           close $handle;
871           exit;
872
873   How can I write expect in Perl?
874       Once upon a time, there was a library called chat2.pl (part of the
875       standard perl distribution), which never really got finished. If you
876       find it somewhere, don't use it. These days, your best bet is to look
877       at the Expect module available from CPAN, which also requires two other
878       modules from CPAN, IO::Pty and IO::Stty.
879
880   Is there a way to hide perl's command line from programs such as "ps"?
881       First of all note that if you're doing this for security reasons (to
882       avoid people seeing passwords, for example) then you should rewrite
883       your program so that critical information is never given as an
884       argument. Hiding the arguments won't make your program completely
885       secure.
886
887       To actually alter the visible command line, you can assign to the
888       variable $0 as documented in perlvar. This won't work on all operating
889       systems, though. Daemon programs like sendmail place their state there,
890       as in:
891
892           $0 = "orcus [accepting connections]";
893
894   I {changed directory, modified my environment} in a perl script. How come
895       the change disappeared when I exited the script? How do I get my
896       changes to be visible?
897       Unix
898           In the strictest sense, it can't be done--the script executes as a
899           different process from the shell it was started from. Changes to a
900           process are not reflected in its parent--only in any children
901           created after the change. There is shell magic that may allow you
902           to fake it by "eval()"ing the script's output in your shell; check
903           out the comp.unix.questions FAQ for details.
904
905   How do I close a process's filehandle without waiting for it to complete?
906       Assuming your system supports such things, just send an appropriate
907       signal to the process (see "kill" in perlfunc). It's common to first
908       send a TERM signal, wait a little bit, and then send a KILL signal to
909       finish it off.
910
911   How do I fork a daemon process?
912       If by daemon process you mean one that's detached (disassociated from
913       its tty), then the following process is reported to work on most
914       Unixish systems. Non-Unix users should check their Your_OS::Process
915       module for other solutions.
916
917       •   Open /dev/tty and use the TIOCNOTTY ioctl on it. See tty(1) for
918           details. Or better yet, you can just use the "POSIX::setsid()"
919           function, so you don't have to worry about process groups.
920
921       •   Change directory to /
922
923       •   Reopen STDIN, STDOUT, and STDERR so they're not connected to the
924           old tty.
925
926       •   Background yourself like this:
927
928               fork && exit;
929
930       The Proc::Daemon module, available from CPAN, provides a function to
931       perform these actions for you.
932
933   How do I find out if I'm running interactively or not?
934       (contributed by brian d foy)
935
936       This is a difficult question to answer, and the best answer is only a
937       guess.
938
939       What do you really want to know? If you merely want to know if one of
940       your filehandles is connected to a terminal, you can try the "-t" file
941       test:
942
943           if( -t STDOUT ) {
944               print "I'm connected to a terminal!\n";
945           }
946
947       However, you might be out of luck if you expect that means there is a
948       real person on the other side. With the Expect module, another program
949       can pretend to be a person. The program might even come close to
950       passing the Turing test.
951
952       The IO::Interactive module does the best it can to give you an answer.
953       Its "is_interactive" function returns an output filehandle; that
954       filehandle points to standard output if the module thinks the session
955       is interactive. Otherwise, the filehandle is a null handle that simply
956       discards the output:
957
958           use IO::Interactive;
959
960           print { is_interactive } "I might go to standard output!\n";
961
962       This still doesn't guarantee that a real person is answering your
963       prompts or reading your output.
964
965       If you want to know how to handle automated testing for your
966       distribution, you can check the environment. The CPAN Testers, for
967       instance, set the value of "AUTOMATED_TESTING":
968
969           unless( $ENV{AUTOMATED_TESTING} ) {
970               print "Hello interactive tester!\n";
971           }
972
973   How do I timeout a slow event?
974       Use the "alarm()" function, probably in conjunction with a signal
975       handler, as documented in "Signals" in perlipc and the section on
976       "Signals" in the Camel. You may instead use the more flexible
977       Sys::AlarmCall module available from CPAN.
978
979       The "alarm()" function is not implemented on all versions of Windows.
980       Check the documentation for your specific version of Perl.
981
982   How do I set CPU limits?
983       (contributed by Xho)
984
985       Use the BSD::Resource module from CPAN. As an example:
986
987           use BSD::Resource;
988           setrlimit(RLIMIT_CPU,10,20) or die $!;
989
990       This sets the soft and hard limits to 10 and 20 seconds, respectively.
991       After 10 seconds of time spent running on the CPU (not "wall" time),
992       the process will be sent a signal (XCPU on some systems) which, if not
993       trapped, will cause the process to terminate. If that signal is
994       trapped, then after 10 more seconds (20 seconds in total) the process
995       will be killed with a non-trappable signal.
996
997       See the BSD::Resource and your systems documentation for the gory
998       details.
999
1000   How do I avoid zombies on a Unix system?
1001       Use the reaper code from "Signals" in perlipc to call "wait()" when a
1002       SIGCHLD is received, or else use the double-fork technique described in
1003       "How do I start a process in the background?" in perlfaq8.
1004
1005   How do I use an SQL database?
1006       The DBI module provides an abstract interface to most database servers
1007       and types, including Oracle, DB2, Sybase, mysql, Postgresql, ODBC, and
1008       flat files. The DBI module accesses each database type through a
1009       database driver, or DBD. You can see a complete list of available
1010       drivers on CPAN: <http://www.cpan.org/modules/by-module/DBD/> .  You
1011       can read more about DBI on <http://dbi.perl.org/> .
1012
1013       Other modules provide more specific access: Win32::ODBC, Alzabo,
1014       "iodbc", and others found on CPAN Search: <https://metacpan.org/> .
1015
1016   How do I make a system() exit on control-C?
1017       You can't. You need to imitate the "system()" call (see perlipc for
1018       sample code) and then have a signal handler for the INT signal that
1019       passes the signal on to the subprocess. Or you can check for it:
1020
1021           $rc = system($cmd);
1022           if ($rc & 127) { die "signal death" }
1023
1024   How do I open a file without blocking?
1025       If you're lucky enough to be using a system that supports non-blocking
1026       reads (most Unixish systems do), you need only to use the "O_NDELAY" or
1027       "O_NONBLOCK" flag from the "Fcntl" module in conjunction with
1028       "sysopen()":
1029
1030           use Fcntl;
1031           sysopen(my $fh, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
1032               or die "can't open /foo/somefile: $!":
1033
1034   How do I tell the difference between errors from the shell and perl?
1035       (answer contributed by brian d foy)
1036
1037       When you run a Perl script, something else is running the script for
1038       you, and that something else may output error messages. The script
1039       might emit its own warnings and error messages. Most of the time you
1040       cannot tell who said what.
1041
1042       You probably cannot fix the thing that runs perl, but you can change
1043       how perl outputs its warnings by defining a custom warning and die
1044       functions.
1045
1046       Consider this script, which has an error you may not notice
1047       immediately.
1048
1049           #!/usr/locl/bin/perl
1050
1051           print "Hello World\n";
1052
1053       I get an error when I run this from my shell (which happens to be
1054       bash). That may look like perl forgot it has a "print()" function, but
1055       my shebang line is not the path to perl, so the shell runs the script,
1056       and I get the error.
1057
1058           $ ./test
1059           ./test: line 3: print: command not found
1060
1061       A quick and dirty fix involves a little bit of code, but this may be
1062       all you need to figure out the problem.
1063
1064           #!/usr/bin/perl -w
1065
1066           BEGIN {
1067               $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
1068               $SIG{__DIE__}  = sub{ print STDERR "Perl: ", @_; exit 1};
1069           }
1070
1071           $a = 1 + undef;
1072           $x / 0;
1073           __END__
1074
1075       The perl message comes out with "Perl" in front. The "BEGIN" block
1076       works at compile time so all of the compilation errors and warnings get
1077       the "Perl:" prefix too.
1078
1079           Perl: Useless use of division (/) in void context at ./test line 9.
1080           Perl: Name "main::a" used only once: possible typo at ./test line 8.
1081           Perl: Name "main::x" used only once: possible typo at ./test line 9.
1082           Perl: Use of uninitialized value in addition (+) at ./test line 8.
1083           Perl: Use of uninitialized value in division (/) at ./test line 9.
1084           Perl: Illegal division by zero at ./test line 9.
1085           Perl: Illegal division by zero at -e line 3.
1086
1087       If I don't see that "Perl:", it's not from perl.
1088
1089       You could also just know all the perl errors, and although there are
1090       some people who may know all of them, you probably don't. However, they
1091       all should be in the perldiag manpage. If you don't find the error in
1092       there, it probably isn't a perl error.
1093
1094       Looking up every message is not the easiest way, so let perl to do it
1095       for you. Use the diagnostics pragma with turns perl's normal messages
1096       into longer discussions on the topic.
1097
1098           use diagnostics;
1099
1100       If you don't get a paragraph or two of expanded discussion, it might
1101       not be perl's message.
1102
1103   How do I install a module from CPAN?
1104       (contributed by brian d foy)
1105
1106       The easiest way is to have a module also named CPAN do it for you by
1107       using the "cpan" command that comes with Perl. You can give it a list
1108       of modules to install:
1109
1110           $ cpan IO::Interactive Getopt::Whatever
1111
1112       If you prefer "CPANPLUS", it's just as easy:
1113
1114           $ cpanp i IO::Interactive Getopt::Whatever
1115
1116       If you want to install a distribution from the current directory, you
1117       can tell "CPAN.pm" to install "." (the full stop):
1118
1119           $ cpan .
1120
1121       See the documentation for either of those commands to see what else you
1122       can do.
1123
1124       If you want to try to install a distribution by yourself, resolving all
1125       dependencies on your own, you follow one of two possible build paths.
1126
1127       For distributions that use Makefile.PL:
1128
1129           $ perl Makefile.PL
1130           $ make test install
1131
1132       For distributions that use Build.PL:
1133
1134           $ perl Build.PL
1135           $ ./Build test
1136           $ ./Build install
1137
1138       Some distributions may need to link to libraries or other third-party
1139       code and their build and installation sequences may be more
1140       complicated.  Check any README or INSTALL files that you may find.
1141
1142   What's the difference between require and use?
1143       (contributed by brian d foy)
1144
1145       Perl runs "require" statement at run-time. Once Perl loads, compiles,
1146       and runs the file, it doesn't do anything else. The "use" statement is
1147       the same as a "require" run at compile-time, but Perl also calls the
1148       "import" method for the loaded package. These two are the same:
1149
1150           use MODULE qw(import list);
1151
1152           BEGIN {
1153               require MODULE;
1154               MODULE->import(import list);
1155           }
1156
1157       However, you can suppress the "import" by using an explicit, empty
1158       import list. Both of these still happen at compile-time:
1159
1160           use MODULE ();
1161
1162           BEGIN {
1163               require MODULE;
1164           }
1165
1166       Since "use" will also call the "import" method, the actual value for
1167       "MODULE" must be a bareword. That is, "use" cannot load files by name,
1168       although "require" can:
1169
1170           require "$ENV{HOME}/lib/Foo.pm"; # no @INC searching!
1171
1172       See the entry for "use" in perlfunc for more details.
1173
1174   How do I keep my own module/library directory?
1175       When you build modules, tell Perl where to install the modules.
1176
1177       If you want to install modules for your own use, the easiest way might
1178       be local::lib, which you can download from CPAN. It sets various
1179       installation settings for you, and uses those same settings within your
1180       programs.
1181
1182       If you want more flexibility, you need to configure your CPAN client
1183       for your particular situation.
1184
1185       For "Makefile.PL"-based distributions, use the INSTALL_BASE option when
1186       generating Makefiles:
1187
1188           perl Makefile.PL INSTALL_BASE=/mydir/perl
1189
1190       You can set this in your "CPAN.pm" configuration so modules
1191       automatically install in your private library directory when you use
1192       the CPAN.pm shell:
1193
1194           % cpan
1195           cpan> o conf makepl_arg INSTALL_BASE=/mydir/perl
1196           cpan> o conf commit
1197
1198       For "Build.PL"-based distributions, use the --install_base option:
1199
1200           perl Build.PL --install_base /mydir/perl
1201
1202       You can configure "CPAN.pm" to automatically use this option too:
1203
1204           % cpan
1205           cpan> o conf mbuild_arg "--install_base /mydir/perl"
1206           cpan> o conf commit
1207
1208       INSTALL_BASE tells these tools to put your modules into
1209       /mydir/perl/lib/perl5. See "How do I add a directory to my include path
1210       (@INC) at runtime?" for details on how to run your newly installed
1211       modules.
1212
1213       There is one caveat with INSTALL_BASE, though, since it acts
1214       differently from the PREFIX and LIB settings that older versions of
1215       ExtUtils::MakeMaker advocated. INSTALL_BASE does not support installing
1216       modules for multiple versions of Perl or different architectures under
1217       the same directory. You should consider whether you really want that
1218       and, if you do, use the older PREFIX and LIB settings. See the
1219       ExtUtils::Makemaker documentation for more details.
1220
1221   How do I add the directory my program lives in to the module/library search
1222       path?
1223       (contributed by brian d foy)
1224
1225       If you know the directory already, you can add it to @INC as you would
1226       for any other directory. You might "use lib" if you know the directory
1227       at compile time:
1228
1229           use lib $directory;
1230
1231       The trick in this task is to find the directory. Before your script
1232       does anything else (such as a "chdir"), you can get the current working
1233       directory with the "Cwd" module, which comes with Perl:
1234
1235           BEGIN {
1236               use Cwd;
1237               our $directory = cwd;
1238           }
1239
1240           use lib $directory;
1241
1242       You can do a similar thing with the value of $0, which holds the script
1243       name. That might hold a relative path, but "rel2abs" can turn it into
1244       an absolute path. Once you have the
1245
1246           BEGIN {
1247               use File::Spec::Functions qw(rel2abs);
1248               use File::Basename qw(dirname);
1249
1250               my $path   = rel2abs( $0 );
1251               our $directory = dirname( $path );
1252           }
1253
1254           use lib $directory;
1255
1256       The FindBin module, which comes with Perl, might work. It finds the
1257       directory of the currently running script and puts it in $Bin, which
1258       you can then use to construct the right library path:
1259
1260           use FindBin qw($Bin);
1261
1262       You can also use local::lib to do much of the same thing. Install
1263       modules using local::lib's settings then use the module in your
1264       program:
1265
1266            use local::lib; # sets up a local lib at ~/perl5
1267
1268       See the local::lib documentation for more details.
1269
1270   How do I add a directory to my include path (@INC) at runtime?
1271       Here are the suggested ways of modifying your include path, including
1272       environment variables, run-time switches, and in-code statements:
1273
1274       the "PERLLIB" environment variable
1275               $ export PERLLIB=/path/to/my/dir
1276               $ perl program.pl
1277
1278       the "PERL5LIB" environment variable
1279               $ export PERL5LIB=/path/to/my/dir
1280               $ perl program.pl
1281
1282       the "perl -Idir" command line flag
1283               $ perl -I/path/to/my/dir program.pl
1284
1285       the "lib" pragma:
1286               use lib "$ENV{HOME}/myown_perllib";
1287
1288       the local::lib module:
1289               use local::lib;
1290
1291               use local::lib "~/myown_perllib";
1292
1293   Where are modules installed?
1294       Modules are installed on a case-by-case basis (as provided by the
1295       methods described in the previous section), and in the operating
1296       system. All of these paths are stored in @INC, which you can display
1297       with the one-liner
1298
1299           perl -e 'print join("\n",@INC,"")'
1300
1301       The same information is displayed at the end of the output from the
1302       command
1303
1304           perl -V
1305
1306       To find out where a module's source code is located, use
1307
1308           perldoc -l Encode
1309
1310       to display the path to the module. In some cases (for example, the
1311       "AutoLoader" module), this command will show the path to a separate
1312       "pod" file; the module itself should be in the same directory, with a
1313       'pm' file extension.
1314
1315   What is socket.ph and where do I get it?
1316       It's a Perl 4 style file defining values for system networking
1317       constants. Sometimes it is built using h2ph when Perl is installed, but
1318       other times it is not. Modern programs should use "use Socket;"
1319       instead.
1320
1322       Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other
1323       authors as noted. All rights reserved.
1324
1325       This documentation is free; you can redistribute it and/or modify it
1326       under the same terms as Perl itself.
1327
1328       Irrespective of its distribution, all code examples in this file are
1329       hereby placed into the public domain. You are permitted and encouraged
1330       to use this code in your own programs for fun or for profit as you see
1331       fit. A simple comment in the code giving credit would be courteous but
1332       is not required.
1333
1334
1335
1336perl v5.34.0                      2022-01-21                       perlfaq8(3)
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