extra code in pp_concat, Take 2
[p5sagit/p5-mst-13.2.git] / pod / perlfaq8.pod
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68dc0745 1=head1 NAME
2
7678cced 3perlfaq8 - System Interaction ($Revision: 1.23 $, $Date: 2005/01/03 18:43:37 $)
68dc0745 4
5=head1 DESCRIPTION
6
7This section of the Perl FAQ covers questions involving operating
a6dd486b 8system interaction. Topics include interprocess communication (IPC),
68dc0745 9control over the user-interface (keyboard, screen and pointing
10devices), and most anything else not related to data manipulation.
11
12Read the FAQs and documentation specific to the port of perl to your
46fc3d4c 13operating system (eg, L<perlvms>, L<perlplan9>, ...). These should
68dc0745 14contain more detailed information on the vagaries of your perl.
15
16=head2 How do I find out which operating system I'm running under?
17
d92eb7b0 18The $^O variable ($OSNAME if you use English) contains an indication of
19the name of the operating system (not its release number) that your perl
20binary was built for.
68dc0745 21
22=head2 How come exec() doesn't return?
23
24Because that's what it does: it replaces your currently running
25program with a different one. If you want to keep going (as is
26probably the case if you're asking this question) use system()
27instead.
28
29=head2 How do I do fancy stuff with the keyboard/screen/mouse?
30
31How you access/control keyboards, screens, and pointing devices
32("mice") is system-dependent. Try the following modules:
33
34=over 4
35
36=item Keyboard
37
38 Term::Cap Standard perl distribution
39 Term::ReadKey CPAN
40 Term::ReadLine::Gnu CPAN
41 Term::ReadLine::Perl CPAN
42 Term::Screen CPAN
43
44=item Screen
45
46 Term::Cap Standard perl distribution
47 Curses CPAN
48 Term::ANSIColor CPAN
49
50=item Mouse
51
52 Tk CPAN
53
54=back
55
c8db1d39 56Some of these specific cases are shown below.
57
58=head2 How do I print something out in color?
59
60In general, you don't, because you don't know whether
61the recipient has a color-aware display device. If you
62know that they have an ANSI terminal that understands
63color, you can use the Term::ANSIColor module from CPAN:
64
65 use Term::ANSIColor;
66 print color("red"), "Stop!\n", color("reset");
67 print color("green"), "Go!\n", color("reset");
68
69Or like this:
70
71 use Term::ANSIColor qw(:constants);
72 print RED, "Stop!\n", RESET;
73 print GREEN, "Go!\n", RESET;
74
75=head2 How do I read just one key without waiting for a return key?
76
77Controlling input buffering is a remarkably system-dependent matter.
d92eb7b0 78On many systems, you can just use the B<stty> command as shown in
c8db1d39 79L<perlfunc/getc>, but as you see, that's already getting you into
197aec24 80portability snags.
c8db1d39 81
82 open(TTY, "+</dev/tty") or die "no tty: $!";
83 system "stty cbreak </dev/tty >/dev/tty 2>&1";
84 $key = getc(TTY); # perhaps this works
85 # OR ELSE
86 sysread(TTY, $key, 1); # probably this does
87 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
88
89The Term::ReadKey module from CPAN offers an easy-to-use interface that
90should be more efficient than shelling out to B<stty> for each key.
91It even includes limited support for Windows.
92
93 use Term::ReadKey;
94 ReadMode('cbreak');
95 $key = ReadKey(0);
96 ReadMode('normal');
97
a6dd486b 98However, using the code requires that you have a working C compiler
99and can use it to build and install a CPAN module. Here's a solution
100using the standard POSIX module, which is already on your systems
101(assuming your system supports POSIX).
c8db1d39 102
103 use HotKey;
104 $key = readkey();
105
106And here's the HotKey module, which hides the somewhat mystifying calls
107to manipulate the POSIX termios structures.
108
109 # HotKey.pm
110 package HotKey;
111
112 @ISA = qw(Exporter);
113 @EXPORT = qw(cbreak cooked readkey);
114
115 use strict;
116 use POSIX qw(:termios_h);
117 my ($term, $oterm, $echo, $noecho, $fd_stdin);
118
119 $fd_stdin = fileno(STDIN);
120 $term = POSIX::Termios->new();
121 $term->getattr($fd_stdin);
122 $oterm = $term->getlflag();
123
124 $echo = ECHO | ECHOK | ICANON;
125 $noecho = $oterm & ~$echo;
126
127 sub cbreak {
128 $term->setlflag($noecho); # ok, so i don't want echo either
129 $term->setcc(VTIME, 1);
130 $term->setattr($fd_stdin, TCSANOW);
131 }
132
133 sub cooked {
134 $term->setlflag($oterm);
135 $term->setcc(VTIME, 0);
136 $term->setattr($fd_stdin, TCSANOW);
137 }
138
139 sub readkey {
140 my $key = '';
141 cbreak();
142 sysread(STDIN, $key, 1);
143 cooked();
144 return $key;
145 }
146
147 END { cooked() }
148
149 1;
150
151=head2 How do I check whether input is ready on the keyboard?
152
153The easiest way to do this is to read a key in nonblocking mode with the
154Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
155not to block:
156
157 use Term::ReadKey;
158
159 ReadMode('cbreak');
160
161 if (defined ($char = ReadKey(-1)) ) {
162 # input was waiting and it was $char
163 } else {
164 # no input was waiting
165 }
166
167 ReadMode('normal'); # restore normal tty settings
168
169=head2 How do I clear the screen?
170
d92eb7b0 171If you only have do so infrequently, use C<system>:
c8db1d39 172
173 system("clear");
174
175If you have to do this a lot, save the clear string
176so you can print it 100 times without calling a program
177100 times:
178
179 $clear_string = `clear`;
180 print $clear_string;
181
182If you're planning on doing other screen manipulations, like cursor
183positions, etc, you might wish to use Term::Cap module:
184
185 use Term::Cap;
186 $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
187 $clear_string = $terminal->Tputs('cl');
188
189=head2 How do I get the screen size?
190
197aec24 191If you have Term::ReadKey module installed from CPAN,
c8db1d39 192you can use it to fetch the width and height in characters
193and in pixels:
194
195 use Term::ReadKey;
196 ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
197
197aec24 198This is more portable than the raw C<ioctl>, but not as
c8db1d39 199illustrative:
200
201 require 'sys/ioctl.ph';
202 die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
203 open(TTY, "+</dev/tty") or die "No tty: $!";
204 unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
205 die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
206 }
207 ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
208 print "(row,col) = ($row,$col)";
209 print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
210 print "\n";
211
68dc0745 212=head2 How do I ask the user for a password?
213
214(This question has nothing to do with the web. See a different
215FAQ for that.)
216
a6dd486b 217There's an example of this in L<perlfunc/crypt>). First, you put the
218terminal into "no echo" mode, then just read the password normally.
219You may do this with an old-style ioctl() function, POSIX terminal
b73a15ae 220control (see L<POSIX> or its documentation the Camel Book), or a call
68dc0745 221to the B<stty> program, with varying degrees of portability.
222
223You can also do this for most systems using the Term::ReadKey module
224from CPAN, which is easier to use and in theory more portable.
225
c8db1d39 226 use Term::ReadKey;
227
228 ReadMode('noecho');
229 $password = ReadLine(0);
230
68dc0745 231=head2 How do I read and write the serial port?
232
233This depends on which operating system your program is running on. In
234the case of Unix, the serial ports will be accessible through files in
a6dd486b 235/dev; on other systems, device names will doubtless differ.
68dc0745 236Several problem areas common to all device interaction are the
a6dd486b 237following:
68dc0745 238
239=over 4
240
241=item lockfiles
242
243Your system may use lockfiles to control multiple access. Make sure
a6dd486b 244you follow the correct protocol. Unpredictable behavior can result
68dc0745 245from multiple processes reading from one device.
246
247=item open mode
248
249If you expect to use both read and write operations on the device,
250you'll have to open it for update (see L<perlfunc/"open"> for
251details). You may wish to open it without running the risk of
252blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
253Fcntl module (part of the standard perl distribution). See
254L<perlfunc/"sysopen"> for more on this approach.
255
256=item end of line
257
258Some devices will be expecting a "\r" at the end of each line rather
259than a "\n". In some ports of perl, "\r" and "\n" are different from
260their usual (Unix) ASCII values of "\012" and "\015". You may have to
261give the numeric values you want directly, using octal ("\015"), hex
262("0x0D"), or as a control-character specification ("\cM").
263
264 print DEV "atv1\012"; # wrong, for some devices
265 print DEV "atv1\015"; # right, for some devices
266
a6dd486b 267Even though with normal text files a "\n" will do the trick, there is
68dc0745 268still no unified scheme for terminating a line that is portable
46fc3d4c 269between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line
68dc0745 270ends with "\015\012", and strip what you don't need from the output.
271This applies especially to socket I/O and autoflushing, discussed
272next.
273
274=item flushing output
275
276If you expect characters to get to your device when you print() them,
c8db1d39 277you'll want to autoflush that filehandle. You can use select()
197aec24 278and the C<$|> variable to control autoflushing (see L<perlvar/$E<verbar>>
a6dd486b 279and L<perlfunc/select>, or L<perlfaq5>, ``How do I flush/unbuffer an
280output filehandle? Why must I do this?''):
68dc0745 281
282 $oldh = select(DEV);
283 $| = 1;
284 select($oldh);
285
286You'll also see code that does this without a temporary variable, as in
287
288 select((select(DEV), $| = 1)[0]);
289
c8db1d39 290Or if you don't mind pulling in a few thousand lines
291of code just because you're afraid of a little $| variable:
292
293 use IO::Handle;
294 DEV->autoflush(1);
295
68dc0745 296As mentioned in the previous item, this still doesn't work when using
8305e449 297socket I/O between Unix and Macintosh. You'll need to hard code your
68dc0745 298line terminators, in that case.
299
300=item non-blocking input
301
302If you are doing a blocking read() or sysread(), you'll have to
303arrange for an alarm handler to provide a timeout (see
304L<perlfunc/alarm>). If you have a non-blocking open, you'll likely
305have a non-blocking read, which means you may have to use a 4-arg
306select() to determine whether I/O is ready on that device (see
307L<perlfunc/"select">.
308
309=back
310
c8db1d39 311While trying to read from his caller-id box, the notorious Jamie Zawinski
312<jwz@netscape.com>, after much gnashing of teeth and fighting with sysread,
313sysopen, POSIX's tcgetattr business, and various other functions that
314go bump in the night, finally came up with this:
315
316 sub open_modem {
317 use IPC::Open2;
318 my $stty = `/bin/stty -g`;
319 open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
320 # starting cu hoses /dev/tty's stty settings, even when it has
321 # been opened on a pipe...
322 system("/bin/stty $stty");
323 $_ = <MODEM_IN>;
5b3eff12 324 chomp;
c8db1d39 325 if ( !m/^Connected/ ) {
326 print STDERR "$0: cu printed `$_' instead of `Connected'\n";
327 }
328 }
329
68dc0745 330=head2 How do I decode encrypted password files?
331
332You spend lots and lots of money on dedicated hardware, but this is
333bound to get you talked about.
334
a6dd486b 335Seriously, you can't if they are Unix password files--the Unix
c8db1d39 336password system employs one-way encryption. It's more like hashing than
337encryption. The best you can check is whether something else hashes to
338the same string. You can't turn a hash back into the original string.
339Programs like Crack
340can forcibly (and intelligently) try to guess passwords, but don't
341(can't) guarantee quick success.
68dc0745 342
343If you're worried about users selecting bad passwords, you should
344proactively check when they try to change their password (by modifying
345passwd(1), for example).
346
347=head2 How do I start a process in the background?
348
49d635f9 349Several modules can start other processes that do not block
350your Perl program. You can use IPC::Open3, Parallel::Jobs,
351IPC::Run, and some of the POE modules. See CPAN for more
352details.
353
354You could also use
68dc0745 355
356 system("cmd &")
357
358or you could use fork as documented in L<perlfunc/"fork">, with
359further examples in L<perlipc>. Some things to be aware of, if you're
360on a Unix-like system:
361
362=over 4
363
c8db1d39 364=item STDIN, STDOUT, and STDERR are shared
68dc0745 365
366Both the main process and the backgrounded one (the "child" process)
367share the same STDIN, STDOUT and STDERR filehandles. If both try to
368access them at once, strange things can happen. You may want to close
369or reopen these for the child. You can get around this with
370C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this
371means that the child process cannot outlive the parent.
372
373=item Signals
374
375You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
376SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
377sent when you write to a filehandle whose child process has closed (an
378untrapped SIGPIPE can cause your program to silently die). This is
379not an issue with C<system("cmd&")>.
380
381=item Zombies
382
49d635f9 383You have to be prepared to "reap" the child process when it finishes.
68dc0745 384
385 $SIG{CHLD} = sub { wait };
197aec24 386
49d635f9 387 $SIG{CHLD} = 'IGNORE';
197aec24 388
389You can also use a double fork. You immediately wait() for your
390first child, and the init daemon will wait() for your grandchild once
49d635f9 391it exits.
392
393 unless ($pid = fork) {
394 unless (fork) {
395 exec "what you really wanna do";
396 die "exec failed!";
397 }
398 exit 0;
399 }
400 waitpid($pid,0);
401
68dc0745 402
403See L<perlipc/"Signals"> for other examples of code to do this.
404Zombies are not an issue with C<system("prog &")>.
405
406=back
407
408=head2 How do I trap control characters/signals?
409
c8db1d39 410You don't actually "trap" a control character. Instead, that character
411generates a signal which is sent to your terminal's currently
412foregrounded process group, which you then trap in your process.
b73a15ae 413Signals are documented in L<perlipc/"Signals"> and the
414section on ``Signals'' in the Camel.
68dc0745 415
c98c5709 416You can set the values of the %SIG hash to be the functions you want
417to handle the signal. After perl catches the signal, it looks in %SIG
418for a key with the same name as the signal, then calls the subroutine
419value for that key.
420
421 # as an anonymous subroutine
422
423 $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
424
425 # or a reference to a function
426
427 $SIG{INT} = \&ouch;
428
429 # or the name of the function as a string
430
431 $SIG{INT} = "ouch";
432
433Perl versions before 5.8 had in its C source code signal handlers which
434would catch the signal and possibly run a Perl function that you had set
435in %SIG. This violated the rules of signal handling at that level
436causing perl to dump core. Since version 5.8.0, perl looks at %SIG
437*after* the signal has been caught, rather than while it is being caught.
438Previous versions of this answer were incorrect.
68dc0745 439
68dc0745 440
441=head2 How do I modify the shadow password file on a Unix system?
442
a6dd486b 443If perl was installed correctly and your shadow library was written
c8db1d39 444properly, the getpw*() functions described in L<perlfunc> should in
445theory provide (read-only) access to entries in the shadow password
446file. To change the file, make a new shadow password file (the format
197aec24 447varies from system to system--see L<passwd> for specifics) and use
448pwd_mkdb(8) to install it (see L<pwd_mkdb> for more details).
68dc0745 449
450=head2 How do I set the time and date?
451
452Assuming you're running under sufficient permissions, you should be
453able to set the system-wide date and time by running the date(1)
454program. (There is no way to set the time and date on a per-process
455basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
456the VMS equivalent is C<set time>.
457
8305e449 458However, if all you want to do is change your time zone, you can
68dc0745 459probably get away with setting an environment variable:
460
461 $ENV{TZ} = "MST7MDT"; # unixish
462 $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
c8db1d39 463 system "trn comp.lang.perl.misc";
68dc0745 464
465=head2 How can I sleep() or alarm() for under a second?
466
467If you want finer granularity than the 1 second that the sleep()
468function provides, the easiest way is to use the select() function as
0325b4c4 469documented in L<perlfunc/"select">. Try the Time::HiRes and
83df6a1d 470the BSD::Itimer modules (available from CPAN, and starting from
471Perl 5.8 Time::HiRes is part of the standard distribution).
68dc0745 472
473=head2 How can I measure time under a second?
474
65acb1b1 475In general, you may not be able to. The Time::HiRes module (available
83df6a1d 476from CPAN, and starting from Perl 5.8 part of the standard distribution)
477provides this functionality for some systems.
68dc0745 478
65acb1b1 479If your system supports both the syscall() function in Perl as well as
480a system call like gettimeofday(2), then you may be able to do
481something like this:
68dc0745 482
483 require 'sys/syscall.ph';
484
485 $TIMEVAL_T = "LL";
486
487 $done = $start = pack($TIMEVAL_T, ());
488
d92eb7b0 489 syscall(&SYS_gettimeofday, $start, 0) != -1
68dc0745 490 or die "gettimeofday: $!";
491
492 ##########################
493 # DO YOUR OPERATION HERE #
494 ##########################
495
496 syscall( &SYS_gettimeofday, $done, 0) != -1
497 or die "gettimeofday: $!";
498
499 @start = unpack($TIMEVAL_T, $start);
500 @done = unpack($TIMEVAL_T, $done);
501
502 # fix microseconds
503 for ($done[1], $start[1]) { $_ /= 1_000_000 }
504
505 $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
506 -
507 ($start[0] + $start[1] );
508
509=head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
510
511Release 5 of Perl added the END block, which can be used to simulate
512atexit(). Each package's END block is called when the program or
197aec24 513thread ends (see L<perlmod> manpage for more details).
c8db1d39 514
515For example, you can use this to make sure your filter program
516managed to finish its output without filling up the disk:
517
518 END {
519 close(STDOUT) || die "stdout close failed: $!";
197aec24 520 }
c8db1d39 521
a6dd486b 522The END block isn't called when untrapped signals kill the program,
523though, so if you use END blocks you should also use
68dc0745 524
525 use sigtrap qw(die normal-signals);
526
527Perl's exception-handling mechanism is its eval() operator. You can
528use eval() as setjmp and die() as longjmp. For details of this, see
46fc3d4c 529the section on signals, especially the time-out handler for a blocking
b73a15ae 530flock() in L<perlipc/"Signals"> or the section on ``Signals'' in
531the Camel Book.
68dc0745 532
533If exception handling is all you're interested in, try the
534exceptions.pl library (part of the standard perl distribution).
535
536If you want the atexit() syntax (and an rmexit() as well), try the
537AtExit module available from CPAN.
538
a6dd486b 539=head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
68dc0745 540
541Some Sys-V based systems, notably Solaris 2.X, redefined some of the
542standard socket constants. Since these were constant across all
543architectures, they were often hardwired into perl code. The proper
544way to deal with this is to "use Socket" to get the correct values.
545
546Note that even though SunOS and Solaris are binary compatible, these
547values are different. Go figure.
548
549=head2 How can I call my system's unique C functions from Perl?
550
a6dd486b 551In most cases, you write an external module to do it--see the answer
68dc0745 552to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
553However, if the function is a system call, and your system supports
554syscall(), you can use the syscall function (documented in
555L<perlfunc>).
556
557Remember to check the modules that came with your distribution, and
197aec24 558CPAN as well---someone may already have written a module to do it. On
559Windows, try Win32::API. On Macs, try Mac::Carbon. If no module
560has an interface to the C function, you can inline a bit of C in your
561Perl source with Inline::C.
68dc0745 562
563=head2 Where do I get the include files to do ioctl() or syscall()?
564
565Historically, these would be generated by the h2ph tool, part of the
566standard perl distribution. This program converts cpp(1) directives
567in C header files to files containing subroutine definitions, like
568&SYS_getitimer, which you can use as arguments to your functions.
569It doesn't work perfectly, but it usually gets most of the job done.
570Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
571but the hard ones like F<ioctl.h> nearly always need to hand-edited.
572Here's how to install the *.ph files:
573
46fc3d4c 574 1. become super-user
68dc0745 575 2. cd /usr/include
576 3. h2ph *.h */*.h
577
578If your system supports dynamic loading, for reasons of portability and
579sanity you probably ought to use h2xs (also part of the standard perl
580distribution). This tool converts C header files to Perl extensions.
581See L<perlxstut> for how to get started with h2xs.
582
583If your system doesn't support dynamic loading, you still probably
584ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
585more information (in brief, just use B<make perl> instead of a plain
586B<make> to rebuild perl with a new static extension).
587
588=head2 Why do setuid perl scripts complain about kernel problems?
589
590Some operating systems have bugs in the kernel that make setuid
591scripts inherently insecure. Perl gives you a number of options
592(described in L<perlsec>) to work around such systems.
593
594=head2 How can I open a pipe both to and from a command?
595
596The IPC::Open2 module (part of the standard perl distribution) is an
c8db1d39 597easy-to-use approach that internally uses pipe(), fork(), and exec() to do
598the job. Make sure you read the deadlock warnings in its documentation,
197aec24 599though (see L<IPC::Open2>). See
600L<perlipc/"Bidirectional Communication with Another Process"> and
13a2d996 601L<perlipc/"Bidirectional Communication with Yourself">
c8db1d39 602
603You may also use the IPC::Open3 module (part of the standard perl
604distribution), but be warned that it has a different order of
605arguments from IPC::Open2 (see L<IPC::Open3>).
68dc0745 606
3fe9a6f1 607=head2 Why can't I get the output of a command with system()?
608
46fc3d4c 609You're confusing the purpose of system() and backticks (``). system()
610runs a command and returns exit status information (as a 16 bit value:
c8db1d39 611the low 7 bits are the signal the process died from, if any, and
46fc3d4c 612the high 8 bits are the actual exit value). Backticks (``) run a
3fe9a6f1 613command and return what it sent to STDOUT.
614
46fc3d4c 615 $exit_status = system("mail-users");
616 $output_string = `ls`;
3fe9a6f1 617
68dc0745 618=head2 How can I capture STDERR from an external command?
619
620There are three basic ways of running external commands:
621
622 system $cmd; # using system()
623 $output = `$cmd`; # using backticks (``)
624 open (PIPE, "cmd |"); # using open()
625
626With system(), both STDOUT and STDERR will go the same place as the
a6dd486b 627script's STDOUT and STDERR, unless the system() command redirects them.
68dc0745 628Backticks and open() read B<only> the STDOUT of your command.
629
49d635f9 630You can also use the open3() function from IPC::Open3. Benjamin
631Goldberg provides some sample code:
632
633To capture a program's STDOUT, but discard its STDERR:
634
635 use IPC::Open3;
636 use File::Spec;
637 use Symbol qw(gensym);
638 open(NULL, ">", File::Spec->devnull);
639 my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
640 while( <PH> ) { }
641 waitpid($pid, 0);
642
643To capture a program's STDERR, but discard its STDOUT:
644
645 use IPC::Open3;
646 use File::Spec;
647 use Symbol qw(gensym);
648 open(NULL, ">", File::Spec->devnull);
649 my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
650 while( <PH> ) { }
651 waitpid($pid, 0);
652
653To capture a program's STDERR, and let its STDOUT go to our own STDERR:
654
655 use IPC::Open3;
656 use Symbol qw(gensym);
657 my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
658 while( <PH> ) { }
659 waitpid($pid, 0);
660
661To read both a command's STDOUT and its STDERR separately, you can
662redirect them to temp files, let the command run, then read the temp
663files:
664
665 use IPC::Open3;
666 use Symbol qw(gensym);
667 use IO::File;
668 local *CATCHOUT = IO::File->new_tempfile;
669 local *CATCHERR = IO::File->new_tempfile;
670 my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
671 waitpid($pid, 0);
672 seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
673 while( <CATCHOUT> ) {}
674 while( <CATCHERR> ) {}
675
676But there's no real need for *both* to be tempfiles... the following
677should work just as well, without deadlocking:
678
679 use IPC::Open3;
680 use Symbol qw(gensym);
681 use IO::File;
682 local *CATCHERR = IO::File->new_tempfile;
683 my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
684 while( <CATCHOUT> ) {}
685 waitpid($pid, 0);
686 seek CATCHERR, 0, 0;
687 while( <CATCHERR> ) {}
688
689And it'll be faster, too, since we can begin processing the program's
690stdout immediately, rather than waiting for the program to finish.
691
68dc0745 692With any of these, you can change file descriptors before the call:
693
694 open(STDOUT, ">logfile");
695 system("ls");
696
697or you can use Bourne shell file-descriptor redirection:
698
699 $output = `$cmd 2>some_file`;
700 open (PIPE, "cmd 2>some_file |");
701
702You can also use file-descriptor redirection to make STDERR a
703duplicate of STDOUT:
704
705 $output = `$cmd 2>&1`;
706 open (PIPE, "cmd 2>&1 |");
707
708Note that you I<cannot> simply open STDERR to be a dup of STDOUT
709in your Perl program and avoid calling the shell to do the redirection.
710This doesn't work:
711
712 open(STDERR, ">&STDOUT");
713 $alloutput = `cmd args`; # stderr still escapes
714
715This fails because the open() makes STDERR go to where STDOUT was
716going at the time of the open(). The backticks then make STDOUT go to
717a string, but don't change STDERR (which still goes to the old
718STDOUT).
719
720Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
721backticks, not csh(1)! Details on why Perl's system() and backtick
06a5f41f 722and pipe opens all use the Bourne shell are in the
723F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To
49d635f9 724Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To
06a5f41f 725capture a command's STDERR and STDOUT together:
68dc0745 726
c8db1d39 727 $output = `cmd 2>&1`; # either with backticks
728 $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
729 while (<PH>) { } # plus a read
730
731To capture a command's STDOUT but discard its STDERR:
732
733 $output = `cmd 2>/dev/null`; # either with backticks
734 $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
735 while (<PH>) { } # plus a read
736
737To capture a command's STDERR but discard its STDOUT:
738
739 $output = `cmd 2>&1 1>/dev/null`; # either with backticks
740 $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
741 while (<PH>) { } # plus a read
742
743To exchange a command's STDOUT and STDERR in order to capture the STDERR
744but leave its STDOUT to come out our old STDERR:
745
746 $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
747 $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
748 while (<PH>) { } # plus a read
749
750To read both a command's STDOUT and its STDERR separately, it's easiest
2359510d 751to redirect them separately to files, and then read from those files
752when the program is done:
c8db1d39 753
2359510d 754 system("program args 1>program.stdout 2>program.stderr");
c8db1d39 755
756Ordering is important in all these examples. That's because the shell
757processes file descriptor redirections in strictly left to right order.
758
759 system("prog args 1>tmpfile 2>&1");
760 system("prog args 2>&1 1>tmpfile");
761
762The first command sends both standard out and standard error to the
763temporary file. The second command sends only the old standard output
764there, and the old standard error shows up on the old standard out.
68dc0745 765
766=head2 Why doesn't open() return an error when a pipe open fails?
767
49d635f9 768If the second argument to a piped open() contains shell
dfdf0730 769metacharacters, perl fork()s, then exec()s a shell to decode the
770metacharacters and eventually run the desired program. If the program
771couldn't be run, it's the shell that gets the message, not Perl. All
772your Perl program can find out is whether the shell itself could be
773successfully started. You can still capture the shell's STDERR and
774check it for error messages. See L<"How can I capture STDERR from an
775external command?"> elsewhere in this document, or use the
49d635f9 776IPC::Open3 module.
dfdf0730 777
49d635f9 778If there are no shell metacharacters in the argument of open(), Perl
dfdf0730 779runs the command directly, without using the shell, and can correctly
780report whether the command started.
68dc0745 781
782=head2 What's wrong with using backticks in a void context?
783
784Strictly speaking, nothing. Stylistically speaking, it's not a good
dfdf0730 785way to write maintainable code. Perl has several operators for
786running external commands. Backticks are one; they collect the output
787from the command for use in your program. The C<system> function is
197aec24 788another; it doesn't do this.
68dc0745 789
dfdf0730 790Writing backticks in your program sends a clear message to the readers
791of your code that you wanted to collect the output of the command.
792Why send a clear message that isn't true?
68dc0745 793
794Consider this line:
795
796 `cat /etc/termcap`;
797
dfdf0730 798You forgot to check C<$?> to see whether the program even ran
799correctly. Even if you wrote
68dc0745 800
801 print `cat /etc/termcap`;
802
a6dd486b 803this code could and probably should be written as
68dc0745 804
805 system("cat /etc/termcap") == 0
806 or die "cat program failed!";
807
a6dd486b 808which will get the output quickly (as it is generated, instead of only
c8db1d39 809at the end) and also check the return value.
68dc0745 810
811system() also provides direct control over whether shell wildcard
812processing may take place, whereas backticks do not.
813
814=head2 How can I call backticks without shell processing?
815
49d635f9 816This is a bit tricky. You can't simply write the command
817like this:
68dc0745 818
819 @ok = `grep @opts '$search_string' @filenames`;
820
49d635f9 821As of Perl 5.8.0, you can use open() with multiple arguments.
822Just like the list forms of system() and exec(), no shell
823escapes happen.
824
825 open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
826 chomp(@ok = <GREP>);
827 close GREP;
828
829You can also:
68dc0745 830
831 my @ok = ();
832 if (open(GREP, "-|")) {
833 while (<GREP>) {
834 chomp;
835 push(@ok, $_);
836 }
837 close GREP;
838 } else {
839 exec 'grep', @opts, $search_string, @filenames;
840 }
841
842Just as with system(), no shell escapes happen when you exec() a list.
d92eb7b0 843Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
68dc0745 844
49d635f9 845Note that if you're use Microsoft, no solution to this vexing issue
d92eb7b0 846is even possible. Even if Perl were to emulate fork(), you'd still
49d635f9 847be stuck, because Microsoft does not have a argc/argv-style API.
c8db1d39 848
54310121 849=head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
68dc0745 850
a6dd486b 851Some stdio's set error and eof flags that need clearing. The
68dc0745 852POSIX module defines clearerr() that you can use. That is the
853technically correct way to do it. Here are some less reliable
854workarounds:
855
856=over 4
857
858=item 1
859
860Try keeping around the seekpointer and go there, like this:
861
862 $where = tell(LOG);
863 seek(LOG, $where, 0);
864
865=item 2
866
867If that doesn't work, try seeking to a different part of the file and
868then back.
869
870=item 3
871
872If that doesn't work, try seeking to a different part of
873the file, reading something, and then seeking back.
874
875=item 4
876
877If that doesn't work, give up on your stdio package and use sysread.
878
879=back
880
881=head2 How can I convert my shell script to perl?
882
883Learn Perl and rewrite it. Seriously, there's no simple converter.
884Things that are awkward to do in the shell are easy to do in Perl, and
885this very awkwardness is what would make a shell->perl converter
886nigh-on impossible to write. By rewriting it, you'll think about what
887you're really trying to do, and hopefully will escape the shell's
46fc3d4c 888pipeline datastream paradigm, which while convenient for some matters,
68dc0745 889causes many inefficiencies.
890
891=head2 Can I use perl to run a telnet or ftp session?
892
46fc3d4c 893Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
a93751fa 894CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar
46fc3d4c 895will also help for emulating the telnet protocol, but Net::Telnet is
896quite probably easier to use..
897
898If all you want to do is pretend to be telnet but don't need
899the initial telnet handshaking, then the standard dual-process
900approach will suffice:
901
902 use IO::Socket; # new in 5.004
903 $handle = IO::Socket::INET->new('www.perl.com:80')
904 || die "can't connect to port 80 on www.perl.com: $!";
905 $handle->autoflush(1);
906 if (fork()) { # XXX: undef means failure
907 select($handle);
908 print while <STDIN>; # everything from stdin to socket
909 } else {
910 print while <$handle>; # everything from socket to stdout
911 }
912 close $handle;
913 exit;
68dc0745 914
915=head2 How can I write expect in Perl?
916
917Once upon a time, there was a library called chat2.pl (part of the
c8db1d39 918standard perl distribution), which never really got finished. If you
919find it somewhere, I<don't use it>. These days, your best bet is to
920look at the Expect module available from CPAN, which also requires two
921other modules from CPAN, IO::Pty and IO::Stty.
68dc0745 922
923=head2 Is there a way to hide perl's command line from programs such as "ps"?
924
925First of all note that if you're doing this for security reasons (to
926avoid people seeing passwords, for example) then you should rewrite
927your program so that critical information is never given as an
928argument. Hiding the arguments won't make your program completely
929secure.
930
931To actually alter the visible command line, you can assign to the
932variable $0 as documented in L<perlvar>. This won't work on all
933operating systems, though. Daemon programs like sendmail place their
934state there, as in:
935
936 $0 = "orcus [accepting connections]";
937
938=head2 I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible?
939
940=over 4
941
942=item Unix
943
a6dd486b 944In the strictest sense, it can't be done--the script executes as a
68dc0745 945different process from the shell it was started from. Changes to a
a6dd486b 946process are not reflected in its parent--only in any children
68dc0745 947created after the change. There is shell magic that may allow you to
948fake it by eval()ing the script's output in your shell; check out the
197aec24 949comp.unix.questions FAQ for details.
68dc0745 950
68dc0745 951=back
952
953=head2 How do I close a process's filehandle without waiting for it to complete?
954
955Assuming your system supports such things, just send an appropriate signal
a6dd486b 956to the process (see L<perlfunc/"kill">). It's common to first send a TERM
68dc0745 957signal, wait a little bit, and then send a KILL signal to finish it off.
958
959=head2 How do I fork a daemon process?
960
961If by daemon process you mean one that's detached (disassociated from
962its tty), then the following process is reported to work on most
963Unixish systems. Non-Unix users should check their Your_OS::Process
964module for other solutions.
965
966=over 4
967
968=item *
969
197aec24 970Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty>
c8db1d39 971for details. Or better yet, you can just use the POSIX::setsid()
972function, so you don't have to worry about process groups.
68dc0745 973
974=item *
975
976Change directory to /
977
978=item *
979
980Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
981tty.
982
983=item *
984
985Background yourself like this:
986
987 fork && exit;
988
989=back
990
1a91aff4 991The Proc::Daemon module, available from CPAN, provides a function to
992perform these actions for you.
993
68dc0745 994=head2 How do I find out if I'm running interactively or not?
995
996Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
997sometimes not.
998
999 if (-t STDIN && -t STDOUT) {
1000 print "Now what? ";
1001 }
1002
1003On POSIX systems, you can test whether your own process group matches
1004the current process group of your controlling terminal as follows:
1005
1006 use POSIX qw/getpgrp tcgetpgrp/;
1007 open(TTY, "/dev/tty") or die $!;
65acb1b1 1008 $tpgrp = tcgetpgrp(fileno(*TTY));
68dc0745 1009 $pgrp = getpgrp();
1010 if ($tpgrp == $pgrp) {
1011 print "foreground\n";
1012 } else {
1013 print "background\n";
1014 }
1015
1016=head2 How do I timeout a slow event?
1017
1018Use the alarm() function, probably in conjunction with a signal
b73a15ae 1019handler, as documented in L<perlipc/"Signals"> and the section on
1020``Signals'' in the Camel. You may instead use the more flexible
1021Sys::AlarmCall module available from CPAN.
68dc0745 1022
49d635f9 1023The alarm() function is not implemented on all versions of Windows.
1024Check the documentation for your specific version of Perl.
1025
68dc0745 1026=head2 How do I set CPU limits?
1027
1028Use the BSD::Resource module from CPAN.
1029
1030=head2 How do I avoid zombies on a Unix system?
1031
1032Use the reaper code from L<perlipc/"Signals"> to call wait() when a
1033SIGCHLD is received, or else use the double-fork technique described
49d635f9 1034in L<perlfaq8/"How do I start a process in the background?">.
68dc0745 1035
1036=head2 How do I use an SQL database?
1037
04d666b1 1038The DBI module provides an abstract interface to most database
1039servers and types, including Oracle, DB2, Sybase, mysql, Postgresql,
1040ODBC, and flat files. The DBI module accesses each database type
1041through a database driver, or DBD. You can see a complete list of
1042available drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ .
1043You can read more about DBI on http://dbi.perl.org .
1044
1045Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc,
1046and others found on CPAN Search: http://search.cpan.org .
68dc0745 1047
1048=head2 How do I make a system() exit on control-C?
1049
1050You can't. You need to imitate the system() call (see L<perlipc> for
1051sample code) and then have a signal handler for the INT signal that
c8db1d39 1052passes the signal on to the subprocess. Or you can check for it:
1053
1054 $rc = system($cmd);
197aec24 1055 if ($rc & 127) { die "signal death" }
68dc0745 1056
1057=head2 How do I open a file without blocking?
1058
1059If you're lucky enough to be using a system that supports
1060non-blocking reads (most Unixish systems do), you need only to use the
1061O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
1062sysopen():
1063
1064 use Fcntl;
2359510d 1065 sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
1066 or die "can't open /foo/somefile: $!":
68dc0745 1067
c98c5709 1068=head2 How do I tell the difference between errors from the shell and perl?
1069
1070(answer contributed by brian d foy, C<< <bdfoy@cpan.org> >>
1071
1072When you run a Perl script, something else is running the script for you,
1073and that something else may output error messages. The script might
1074emit its own warnings and error messages. Most of the time you cannot
1075tell who said what.
1076
1077You probably cannot fix the thing that runs perl, but you can change how
1078perl outputs its warnings by defining a custom warning and die functions.
1079
1080Consider this script, which has an error you may not notice immediately.
1081
1082 #!/usr/locl/bin/perl
1083
1084 print "Hello World\n";
1085
1086I get an error when I run this from my shell (which happens to be
1087bash). That may look like perl forgot it has a print() function,
1088but my shebang line is not the path to perl, so the shell runs the
1089script, and I get the error.
1090
1091 $ ./test
1092 ./test: line 3: print: command not found
1093
1094A quick and dirty fix involves a little bit of code, but this may be all
1095you need to figure out the problem.
1096
1097 #!/usr/bin/perl -w
1098
1099 BEGIN {
1100 $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
1101 $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1};
1102 }
1103
1104 $a = 1 + undef;
1105 $x / 0;
1106 __END__
1107
1108The perl message comes out with "Perl" in front. The BEGIN block
1109works at compile time so all of the compilation errors and warnings
1110get the "Perl:" prefix too.
1111
1112 Perl: Useless use of division (/) in void context at ./test line 9.
1113 Perl: Name "main::a" used only once: possible typo at ./test line 8.
1114 Perl: Name "main::x" used only once: possible typo at ./test line 9.
1115 Perl: Use of uninitialized value in addition (+) at ./test line 8.
1116 Perl: Use of uninitialized value in division (/) at ./test line 9.
1117 Perl: Illegal division by zero at ./test line 9.
1118 Perl: Illegal division by zero at -e line 3.
1119
1120If I don't see that "Perl:", it's not from perl.
1121
1122You could also just know all the perl errors, and although there are
1123some people who may know all of them, you probably don't. However, they
1124all should be in the perldiag manpage. If you don't find the error in
1125there, it probably isn't a perl error.
1126
1127Looking up every message is not the easiest way, so let perl to do it
1128for you. Use the diagnostics pragma with turns perl's normal messages
1129into longer discussions on the topic.
1130
1131 use diagnostics;
1132
1133If you don't get a paragraph or two of expanded discussion, it
1134might not be perl's message.
1135
d92eb7b0 1136=head2 How do I install a module from CPAN?
1137
1138The easiest way is to have a module also named CPAN do it for you.
197aec24 1139This module comes with perl version 5.004 and later.
76817d6d 1140
1141 $ perl -MCPAN -e shell
1142
1143 cpan shell -- CPAN exploration and modules installation (v1.59_54)
1144 ReadLine support enabled
1145
197aec24 1146 cpan> install Some::Module
76817d6d 1147
197aec24 1148To manually install the CPAN module, or any well-behaved CPAN module
76817d6d 1149for that matter, follow these steps:
68dc0745 1150
1151=over 4
1152
1153=item 1
1154
1155Unpack the source into a temporary area.
1156
1157=item 2
1158
1159 perl Makefile.PL
1160
1161=item 3
1162
1163 make
1164
1165=item 4
1166
1167 make test
1168
1169=item 5
1170
1171 make install
1172
1173=back
1174
1175If your version of perl is compiled without dynamic loading, then you
1176just need to replace step 3 (B<make>) with B<make perl> and you will
1177get a new F<perl> binary with your extension linked in.
1178
c8db1d39 1179See L<ExtUtils::MakeMaker> for more details on building extensions.
a6dd486b 1180See also the next question, ``What's the difference between require
1181and use?''.
c8db1d39 1182
1183=head2 What's the difference between require and use?
1184
1185Perl offers several different ways to include code from one file into
1186another. Here are the deltas between the various inclusion constructs:
1187
a6dd486b 1188 1) do $file is like eval `cat $file`, except the former
5e3006a4 1189 1.1: searches @INC and updates %INC.
c8db1d39 1190 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
1191
a6dd486b 1192 2) require $file is like do $file, except the former
c8db1d39 1193 2.1: checks for redundant loading, skipping already loaded files.
1194 2.2: raises an exception on failure to find, compile, or execute $file.
1195
a6dd486b 1196 3) require Module is like require "Module.pm", except the former
c8db1d39 1197 3.1: translates each "::" into your system's directory separator.
1198 3.2: primes the parser to disambiguate class Module as an indirect object.
1199
a6dd486b 1200 4) use Module is like require Module, except the former
c8db1d39 1201 4.1: loads the module at compile time, not run-time.
1202 4.2: imports symbols and semantics from that package to the current one.
1203
1204In general, you usually want C<use> and a proper Perl module.
46fc3d4c 1205
1206=head2 How do I keep my own module/library directory?
1207
e8c8d959 1208When you build modules, use the PREFIX and LIB options when generating
46fc3d4c 1209Makefiles:
1210
e8c8d959 1211 perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
46fc3d4c 1212
1213then either set the PERL5LIB environment variable before you run
1214scripts that use the modules/libraries (see L<perlrun>) or say
1215
e8c8d959 1216 use lib '/mydir/perl/lib';
46fc3d4c 1217
a6dd486b 1218This is almost the same as
65acb1b1 1219
1220 BEGIN {
e8c8d959 1221 unshift(@INC, '/mydir/perl/lib');
65acb1b1 1222 }
1223
1224except that the lib module checks for machine-dependent subdirectories.
46fc3d4c 1225See Perl's L<lib> for more information.
1226
1227=head2 How do I add the directory my program lives in to the module/library search path?
1228
1229 use FindBin;
7b8d334a 1230 use lib "$FindBin::Bin";
46fc3d4c 1231 use your_own_modules;
1232
f0d19b68 1233=head2 How do I add a directory to my include path (@INC) at runtime?
46fc3d4c 1234
1235Here are the suggested ways of modifying your include path:
1236
1237 the PERLLIB environment variable
1238 the PERL5LIB environment variable
c2611fb3 1239 the perl -Idir command line flag
46fc3d4c 1240 the use lib pragma, as in
1241 use lib "$ENV{HOME}/myown_perllib";
1242
1243The latter is particularly useful because it knows about machine
1244dependent architectures. The lib.pm pragmatic module was first
1245included with the 5.002 release of Perl.
68dc0745 1246
65acb1b1 1247=head2 What is socket.ph and where do I get it?
1248
1249It's a perl4-style file defining values for system networking
1250constants. Sometimes it is built using h2ph when Perl is installed,
1251but other times it is not. Modern programs C<use Socket;> instead.
1252
fc36a67e 1253=head1 AUTHOR AND COPYRIGHT
1254
7678cced 1255Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
1256other authors as noted. All rights reserved.
5a964f20 1257
5a7beb56 1258This documentation is free; you can redistribute it and/or modify it
1259under the same terms as Perl itself.
5a964f20 1260
1261Irrespective of its distribution, all code examples in this file
1262are hereby placed into the public domain. You are permitted and
1263encouraged to use this code in your own programs for fun
1264or for profit as you see fit. A simple comment in the code giving
1265credit would be courteous but is not required.