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68dc0745 1=head1 NAME
2
0bc0ad85 3perlfaq8 - System Interaction ($Revision: 1.6 $, $Date: 2002/01/28 04:17:27 $)
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
80portability snags.
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 = $terminal->Tputs('cl');
188
189=head2 How do I get the screen size?
190
191If you have Term::ReadKey module installed from CPAN,
192you can use it to fetch the width and height in characters
193and in pixels:
194
195 use Term::ReadKey;
196 ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
197
198This is more portable than the raw C<ioctl>, but not as
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()
278and the C<$|> variable to control autoflushing (see L<perlvar/$|>
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
349You could use
350
351 system("cmd &")
352
353or you could use fork as documented in L<perlfunc/"fork">, with
354further examples in L<perlipc>. Some things to be aware of, if you're
355on a Unix-like system:
356
357=over 4
358
c8db1d39 359=item STDIN, STDOUT, and STDERR are shared
68dc0745 360
361Both the main process and the backgrounded one (the "child" process)
362share the same STDIN, STDOUT and STDERR filehandles. If both try to
363access them at once, strange things can happen. You may want to close
364or reopen these for the child. You can get around this with
365C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this
366means that the child process cannot outlive the parent.
367
368=item Signals
369
370You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
371SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
372sent when you write to a filehandle whose child process has closed (an
373untrapped SIGPIPE can cause your program to silently die). This is
374not an issue with C<system("cmd&")>.
375
376=item Zombies
377
378You have to be prepared to "reap" the child process when it finishes
379
380 $SIG{CHLD} = sub { wait };
381
382See L<perlipc/"Signals"> for other examples of code to do this.
383Zombies are not an issue with C<system("prog &")>.
384
385=back
386
387=head2 How do I trap control characters/signals?
388
c8db1d39 389You don't actually "trap" a control character. Instead, that character
390generates a signal which is sent to your terminal's currently
391foregrounded process group, which you then trap in your process.
b73a15ae 392Signals are documented in L<perlipc/"Signals"> and the
393section on ``Signals'' in the Camel.
68dc0745 394
46fc3d4c 395Be warned that very few C libraries are re-entrant. Therefore, if you
68dc0745 396attempt to print() in a handler that got invoked during another stdio
397operation your internal structures will likely be in an
398inconsistent state, and your program will dump core. You can
399sometimes avoid this by using syswrite() instead of print().
400
401Unless you're exceedingly careful, the only safe things to do inside a
a6dd486b 402signal handler are (1) set a variable and (2) exit. In the first case,
68dc0745 403you should only set a variable in such a way that malloc() is not
404called (eg, by setting a variable that already has a value).
405
406For example:
407
408 $Interrupted = 0; # to ensure it has a value
409 $SIG{INT} = sub {
410 $Interrupted++;
411 syswrite(STDERR, "ouch\n", 5);
412 }
413
414However, because syscalls restart by default, you'll find that if
c47ff5f1 415you're in a "slow" call, such as <FH>, read(), connect(), or
68dc0745 416wait(), that the only way to terminate them is by "longjumping" out;
46fc3d4c 417that is, by raising an exception. See the time-out handler for a
b73a15ae 418blocking flock() in L<perlipc/"Signals"> or the section on ``Signals''
419in the Camel book.
68dc0745 420
421=head2 How do I modify the shadow password file on a Unix system?
422
a6dd486b 423If perl was installed correctly and your shadow library was written
c8db1d39 424properly, the getpw*() functions described in L<perlfunc> should in
425theory provide (read-only) access to entries in the shadow password
426file. To change the file, make a new shadow password file (the format
a6dd486b 427varies from system to system--see L<passwd(5)> for specifics) and use
d92eb7b0 428pwd_mkdb(8) to install it (see L<pwd_mkdb(8)> for more details).
68dc0745 429
430=head2 How do I set the time and date?
431
432Assuming you're running under sufficient permissions, you should be
433able to set the system-wide date and time by running the date(1)
434program. (There is no way to set the time and date on a per-process
435basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
436the VMS equivalent is C<set time>.
437
8305e449 438However, if all you want to do is change your time zone, you can
68dc0745 439probably get away with setting an environment variable:
440
441 $ENV{TZ} = "MST7MDT"; # unixish
442 $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
c8db1d39 443 system "trn comp.lang.perl.misc";
68dc0745 444
445=head2 How can I sleep() or alarm() for under a second?
446
447If you want finer granularity than the 1 second that the sleep()
448function provides, the easiest way is to use the select() function as
0325b4c4 449documented in L<perlfunc/"select">. Try the Time::HiRes and
83df6a1d 450the BSD::Itimer modules (available from CPAN, and starting from
451Perl 5.8 Time::HiRes is part of the standard distribution).
68dc0745 452
453=head2 How can I measure time under a second?
454
65acb1b1 455In general, you may not be able to. The Time::HiRes module (available
83df6a1d 456from CPAN, and starting from Perl 5.8 part of the standard distribution)
457provides this functionality for some systems.
68dc0745 458
65acb1b1 459If your system supports both the syscall() function in Perl as well as
460a system call like gettimeofday(2), then you may be able to do
461something like this:
68dc0745 462
463 require 'sys/syscall.ph';
464
465 $TIMEVAL_T = "LL";
466
467 $done = $start = pack($TIMEVAL_T, ());
468
d92eb7b0 469 syscall(&SYS_gettimeofday, $start, 0) != -1
68dc0745 470 or die "gettimeofday: $!";
471
472 ##########################
473 # DO YOUR OPERATION HERE #
474 ##########################
475
476 syscall( &SYS_gettimeofday, $done, 0) != -1
477 or die "gettimeofday: $!";
478
479 @start = unpack($TIMEVAL_T, $start);
480 @done = unpack($TIMEVAL_T, $done);
481
482 # fix microseconds
483 for ($done[1], $start[1]) { $_ /= 1_000_000 }
484
485 $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
486 -
487 ($start[0] + $start[1] );
488
489=head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
490
491Release 5 of Perl added the END block, which can be used to simulate
492atexit(). Each package's END block is called when the program or
c8db1d39 493thread ends (see L<perlmod> manpage for more details).
494
495For example, you can use this to make sure your filter program
496managed to finish its output without filling up the disk:
497
498 END {
499 close(STDOUT) || die "stdout close failed: $!";
500 }
501
a6dd486b 502The END block isn't called when untrapped signals kill the program,
503though, so if you use END blocks you should also use
68dc0745 504
505 use sigtrap qw(die normal-signals);
506
507Perl's exception-handling mechanism is its eval() operator. You can
508use eval() as setjmp and die() as longjmp. For details of this, see
46fc3d4c 509the section on signals, especially the time-out handler for a blocking
b73a15ae 510flock() in L<perlipc/"Signals"> or the section on ``Signals'' in
511the Camel Book.
68dc0745 512
513If exception handling is all you're interested in, try the
514exceptions.pl library (part of the standard perl distribution).
515
516If you want the atexit() syntax (and an rmexit() as well), try the
517AtExit module available from CPAN.
518
a6dd486b 519=head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
68dc0745 520
521Some Sys-V based systems, notably Solaris 2.X, redefined some of the
522standard socket constants. Since these were constant across all
523architectures, they were often hardwired into perl code. The proper
524way to deal with this is to "use Socket" to get the correct values.
525
526Note that even though SunOS and Solaris are binary compatible, these
527values are different. Go figure.
528
529=head2 How can I call my system's unique C functions from Perl?
530
a6dd486b 531In most cases, you write an external module to do it--see the answer
68dc0745 532to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
533However, if the function is a system call, and your system supports
534syscall(), you can use the syscall function (documented in
535L<perlfunc>).
536
537Remember to check the modules that came with your distribution, and
a6dd486b 538CPAN as well--someone may already have written a module to do it.
68dc0745 539
540=head2 Where do I get the include files to do ioctl() or syscall()?
541
542Historically, these would be generated by the h2ph tool, part of the
543standard perl distribution. This program converts cpp(1) directives
544in C header files to files containing subroutine definitions, like
545&SYS_getitimer, which you can use as arguments to your functions.
546It doesn't work perfectly, but it usually gets most of the job done.
547Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
548but the hard ones like F<ioctl.h> nearly always need to hand-edited.
549Here's how to install the *.ph files:
550
46fc3d4c 551 1. become super-user
68dc0745 552 2. cd /usr/include
553 3. h2ph *.h */*.h
554
555If your system supports dynamic loading, for reasons of portability and
556sanity you probably ought to use h2xs (also part of the standard perl
557distribution). This tool converts C header files to Perl extensions.
558See L<perlxstut> for how to get started with h2xs.
559
560If your system doesn't support dynamic loading, you still probably
561ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
562more information (in brief, just use B<make perl> instead of a plain
563B<make> to rebuild perl with a new static extension).
564
565=head2 Why do setuid perl scripts complain about kernel problems?
566
567Some operating systems have bugs in the kernel that make setuid
568scripts inherently insecure. Perl gives you a number of options
569(described in L<perlsec>) to work around such systems.
570
571=head2 How can I open a pipe both to and from a command?
572
573The IPC::Open2 module (part of the standard perl distribution) is an
c8db1d39 574easy-to-use approach that internally uses pipe(), fork(), and exec() to do
575the job. Make sure you read the deadlock warnings in its documentation,
13a2d996 576though (see L<IPC::Open2>). See
577L<perlipc/"Bidirectional Communication with Another Process"> and
578L<perlipc/"Bidirectional Communication with Yourself">
c8db1d39 579
580You may also use the IPC::Open3 module (part of the standard perl
581distribution), but be warned that it has a different order of
582arguments from IPC::Open2 (see L<IPC::Open3>).
68dc0745 583
3fe9a6f1 584=head2 Why can't I get the output of a command with system()?
585
46fc3d4c 586You're confusing the purpose of system() and backticks (``). system()
587runs a command and returns exit status information (as a 16 bit value:
c8db1d39 588the low 7 bits are the signal the process died from, if any, and
46fc3d4c 589the high 8 bits are the actual exit value). Backticks (``) run a
3fe9a6f1 590command and return what it sent to STDOUT.
591
46fc3d4c 592 $exit_status = system("mail-users");
593 $output_string = `ls`;
3fe9a6f1 594
68dc0745 595=head2 How can I capture STDERR from an external command?
596
597There are three basic ways of running external commands:
598
599 system $cmd; # using system()
600 $output = `$cmd`; # using backticks (``)
601 open (PIPE, "cmd |"); # using open()
602
603With system(), both STDOUT and STDERR will go the same place as the
a6dd486b 604script's STDOUT and STDERR, unless the system() command redirects them.
68dc0745 605Backticks and open() read B<only> the STDOUT of your command.
606
607With any of these, you can change file descriptors before the call:
608
609 open(STDOUT, ">logfile");
610 system("ls");
611
612or you can use Bourne shell file-descriptor redirection:
613
614 $output = `$cmd 2>some_file`;
615 open (PIPE, "cmd 2>some_file |");
616
617You can also use file-descriptor redirection to make STDERR a
618duplicate of STDOUT:
619
620 $output = `$cmd 2>&1`;
621 open (PIPE, "cmd 2>&1 |");
622
623Note that you I<cannot> simply open STDERR to be a dup of STDOUT
624in your Perl program and avoid calling the shell to do the redirection.
625This doesn't work:
626
627 open(STDERR, ">&STDOUT");
628 $alloutput = `cmd args`; # stderr still escapes
629
630This fails because the open() makes STDERR go to where STDOUT was
631going at the time of the open(). The backticks then make STDOUT go to
632a string, but don't change STDERR (which still goes to the old
633STDOUT).
634
635Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
636backticks, not csh(1)! Details on why Perl's system() and backtick
06a5f41f 637and pipe opens all use the Bourne shell are in the
638F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To
639Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz . To
640capture a command's STDERR and STDOUT together:
68dc0745 641
c8db1d39 642 $output = `cmd 2>&1`; # either with backticks
643 $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
644 while (<PH>) { } # plus a read
645
646To capture a command's STDOUT but discard its STDERR:
647
648 $output = `cmd 2>/dev/null`; # either with backticks
649 $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
650 while (<PH>) { } # plus a read
651
652To capture a command's STDERR but discard its STDOUT:
653
654 $output = `cmd 2>&1 1>/dev/null`; # either with backticks
655 $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
656 while (<PH>) { } # plus a read
657
658To exchange a command's STDOUT and STDERR in order to capture the STDERR
659but leave its STDOUT to come out our old STDERR:
660
661 $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
662 $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
663 while (<PH>) { } # plus a read
664
665To read both a command's STDOUT and its STDERR separately, it's easiest
666and safest to redirect them separately to files, and then read from those
667files when the program is done:
668
669 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
670
671Ordering is important in all these examples. That's because the shell
672processes file descriptor redirections in strictly left to right order.
673
674 system("prog args 1>tmpfile 2>&1");
675 system("prog args 2>&1 1>tmpfile");
676
677The first command sends both standard out and standard error to the
678temporary file. The second command sends only the old standard output
679there, and the old standard error shows up on the old standard out.
68dc0745 680
681=head2 Why doesn't open() return an error when a pipe open fails?
682
65acb1b1 683Because the pipe open takes place in two steps: first Perl calls
684fork() to start a new process, then this new process calls exec() to
685run the program you really wanted to open. The first step reports
686success or failure to your process, so open() can only tell you
687whether the fork() succeeded or not.
688
689To find out if the exec() step succeeded, you have to catch SIGCHLD
690and wait() to get the exit status. You should also catch SIGPIPE if
691you're writing to the child--you may not have found out the exec()
3fe9a6f1 692failed by the time you write. This is documented in L<perlipc>.
68dc0745 693
65acb1b1 694In some cases, even this won't work. If the second argument to a
695piped open() contains shell metacharacters, perl fork()s, then exec()s
696a shell to decode the metacharacters and eventually run the desired
697program. Now when you call wait(), you only learn whether or not the
a6dd486b 698I<shell> could be successfully started...it's best to avoid shell
65acb1b1 699metacharacters.
700
68dc0745 701On systems that follow the spawn() paradigm, open() I<might> do what
65acb1b1 702you expect--unless perl uses a shell to start your command. In this
68dc0745 703case the fork()/exec() description still applies.
704
705=head2 What's wrong with using backticks in a void context?
706
707Strictly speaking, nothing. Stylistically speaking, it's not a good
708way to write maintainable code because backticks have a (potentially
d92eb7b0 709humongous) return value, and you're ignoring it. It's may also not be very
68dc0745 710efficient, because you have to read in all the lines of output, allocate
711memory for them, and then throw it away. Too often people are lulled
712to writing:
713
714 `cp file file.bak`;
715
716And now they think "Hey, I'll just always use backticks to run programs."
717Bad idea: backticks are for capturing a program's output; the system()
718function is for running programs.
719
720Consider this line:
721
722 `cat /etc/termcap`;
723
724You haven't assigned the output anywhere, so it just wastes memory
a6dd486b 725(for a little while). You forgot to check C<$?> to see whether
726the program even ran correctly, too. Even if you wrote
68dc0745 727
728 print `cat /etc/termcap`;
729
a6dd486b 730this code could and probably should be written as
68dc0745 731
732 system("cat /etc/termcap") == 0
733 or die "cat program failed!";
734
a6dd486b 735which will get the output quickly (as it is generated, instead of only
c8db1d39 736at the end) and also check the return value.
68dc0745 737
738system() also provides direct control over whether shell wildcard
739processing may take place, whereas backticks do not.
740
741=head2 How can I call backticks without shell processing?
742
743This is a bit tricky. Instead of writing
744
745 @ok = `grep @opts '$search_string' @filenames`;
746
747You have to do this:
748
749 my @ok = ();
750 if (open(GREP, "-|")) {
751 while (<GREP>) {
752 chomp;
753 push(@ok, $_);
754 }
755 close GREP;
756 } else {
757 exec 'grep', @opts, $search_string, @filenames;
758 }
759
760Just as with system(), no shell escapes happen when you exec() a list.
d92eb7b0 761Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
68dc0745 762
d92eb7b0 763Note that if you're stuck on Microsoft, no solution to this vexing issue
764is even possible. Even if Perl were to emulate fork(), you'd still
765be hosed, because Microsoft gives no argc/argv-style API. Their API
766always reparses from a single string, which is fundamentally wrong,
767but you're not likely to get the Gods of Redmond to acknowledge this
768and fix it for you.
c8db1d39 769
54310121 770=head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
68dc0745 771
a6dd486b 772Some stdio's set error and eof flags that need clearing. The
68dc0745 773POSIX module defines clearerr() that you can use. That is the
774technically correct way to do it. Here are some less reliable
775workarounds:
776
777=over 4
778
779=item 1
780
781Try keeping around the seekpointer and go there, like this:
782
783 $where = tell(LOG);
784 seek(LOG, $where, 0);
785
786=item 2
787
788If that doesn't work, try seeking to a different part of the file and
789then back.
790
791=item 3
792
793If that doesn't work, try seeking to a different part of
794the file, reading something, and then seeking back.
795
796=item 4
797
798If that doesn't work, give up on your stdio package and use sysread.
799
800=back
801
802=head2 How can I convert my shell script to perl?
803
804Learn Perl and rewrite it. Seriously, there's no simple converter.
805Things that are awkward to do in the shell are easy to do in Perl, and
806this very awkwardness is what would make a shell->perl converter
807nigh-on impossible to write. By rewriting it, you'll think about what
808you're really trying to do, and hopefully will escape the shell's
46fc3d4c 809pipeline datastream paradigm, which while convenient for some matters,
68dc0745 810causes many inefficiencies.
811
812=head2 Can I use perl to run a telnet or ftp session?
813
46fc3d4c 814Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
a93751fa 815CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar
46fc3d4c 816will also help for emulating the telnet protocol, but Net::Telnet is
817quite probably easier to use..
818
819If all you want to do is pretend to be telnet but don't need
820the initial telnet handshaking, then the standard dual-process
821approach will suffice:
822
823 use IO::Socket; # new in 5.004
824 $handle = IO::Socket::INET->new('www.perl.com:80')
825 || die "can't connect to port 80 on www.perl.com: $!";
826 $handle->autoflush(1);
827 if (fork()) { # XXX: undef means failure
828 select($handle);
829 print while <STDIN>; # everything from stdin to socket
830 } else {
831 print while <$handle>; # everything from socket to stdout
832 }
833 close $handle;
834 exit;
68dc0745 835
836=head2 How can I write expect in Perl?
837
838Once upon a time, there was a library called chat2.pl (part of the
c8db1d39 839standard perl distribution), which never really got finished. If you
840find it somewhere, I<don't use it>. These days, your best bet is to
841look at the Expect module available from CPAN, which also requires two
842other modules from CPAN, IO::Pty and IO::Stty.
68dc0745 843
844=head2 Is there a way to hide perl's command line from programs such as "ps"?
845
846First of all note that if you're doing this for security reasons (to
847avoid people seeing passwords, for example) then you should rewrite
848your program so that critical information is never given as an
849argument. Hiding the arguments won't make your program completely
850secure.
851
852To actually alter the visible command line, you can assign to the
853variable $0 as documented in L<perlvar>. This won't work on all
854operating systems, though. Daemon programs like sendmail place their
855state there, as in:
856
857 $0 = "orcus [accepting connections]";
858
859=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?
860
861=over 4
862
863=item Unix
864
a6dd486b 865In the strictest sense, it can't be done--the script executes as a
68dc0745 866different process from the shell it was started from. Changes to a
a6dd486b 867process are not reflected in its parent--only in any children
68dc0745 868created after the change. There is shell magic that may allow you to
869fake it by eval()ing the script's output in your shell; check out the
92c2ed05 870comp.unix.questions FAQ for details.
68dc0745 871
68dc0745 872=back
873
874=head2 How do I close a process's filehandle without waiting for it to complete?
875
876Assuming your system supports such things, just send an appropriate signal
a6dd486b 877to the process (see L<perlfunc/"kill">). It's common to first send a TERM
68dc0745 878signal, wait a little bit, and then send a KILL signal to finish it off.
879
880=head2 How do I fork a daemon process?
881
882If by daemon process you mean one that's detached (disassociated from
883its tty), then the following process is reported to work on most
884Unixish systems. Non-Unix users should check their Your_OS::Process
885module for other solutions.
886
887=over 4
888
889=item *
890
b5a41e52 891Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
c8db1d39 892for details. Or better yet, you can just use the POSIX::setsid()
893function, so you don't have to worry about process groups.
68dc0745 894
895=item *
896
897Change directory to /
898
899=item *
900
901Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
902tty.
903
904=item *
905
906Background yourself like this:
907
908 fork && exit;
909
910=back
911
1a91aff4 912The Proc::Daemon module, available from CPAN, provides a function to
913perform these actions for you.
914
68dc0745 915=head2 How do I find out if I'm running interactively or not?
916
917Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
918sometimes not.
919
920 if (-t STDIN && -t STDOUT) {
921 print "Now what? ";
922 }
923
924On POSIX systems, you can test whether your own process group matches
925the current process group of your controlling terminal as follows:
926
927 use POSIX qw/getpgrp tcgetpgrp/;
928 open(TTY, "/dev/tty") or die $!;
65acb1b1 929 $tpgrp = tcgetpgrp(fileno(*TTY));
68dc0745 930 $pgrp = getpgrp();
931 if ($tpgrp == $pgrp) {
932 print "foreground\n";
933 } else {
934 print "background\n";
935 }
936
937=head2 How do I timeout a slow event?
938
939Use the alarm() function, probably in conjunction with a signal
b73a15ae 940handler, as documented in L<perlipc/"Signals"> and the section on
941``Signals'' in the Camel. You may instead use the more flexible
942Sys::AlarmCall module available from CPAN.
68dc0745 943
944=head2 How do I set CPU limits?
945
946Use the BSD::Resource module from CPAN.
947
948=head2 How do I avoid zombies on a Unix system?
949
950Use the reaper code from L<perlipc/"Signals"> to call wait() when a
951SIGCHLD is received, or else use the double-fork technique described
952in L<perlfunc/fork>.
953
954=head2 How do I use an SQL database?
955
956There are a number of excellent interfaces to SQL databases. See the
a93751fa 957DBD::* modules available from http://www.cpan.org/modules/DBD .
213329dd 958A lot of information on this can be found at http://dbi.perl.org/
68dc0745 959
960=head2 How do I make a system() exit on control-C?
961
962You can't. You need to imitate the system() call (see L<perlipc> for
963sample code) and then have a signal handler for the INT signal that
c8db1d39 964passes the signal on to the subprocess. Or you can check for it:
965
966 $rc = system($cmd);
967 if ($rc & 127) { die "signal death" }
68dc0745 968
969=head2 How do I open a file without blocking?
970
971If you're lucky enough to be using a system that supports
972non-blocking reads (most Unixish systems do), you need only to use the
973O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
974sysopen():
975
976 use Fcntl;
977 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
978 or die "can't open /tmp/somefile: $!":
979
d92eb7b0 980=head2 How do I install a module from CPAN?
981
982The easiest way is to have a module also named CPAN do it for you.
983This module comes with perl version 5.004 and later. To manually install
984the CPAN module, or any well-behaved CPAN module for that matter, follow
985these steps:
68dc0745 986
987=over 4
988
989=item 1
990
991Unpack the source into a temporary area.
992
993=item 2
994
995 perl Makefile.PL
996
997=item 3
998
999 make
1000
1001=item 4
1002
1003 make test
1004
1005=item 5
1006
1007 make install
1008
1009=back
1010
1011If your version of perl is compiled without dynamic loading, then you
1012just need to replace step 3 (B<make>) with B<make perl> and you will
1013get a new F<perl> binary with your extension linked in.
1014
c8db1d39 1015See L<ExtUtils::MakeMaker> for more details on building extensions.
a6dd486b 1016See also the next question, ``What's the difference between require
1017and use?''.
c8db1d39 1018
1019=head2 What's the difference between require and use?
1020
1021Perl offers several different ways to include code from one file into
1022another. Here are the deltas between the various inclusion constructs:
1023
a6dd486b 1024 1) do $file is like eval `cat $file`, except the former
5e3006a4 1025 1.1: searches @INC and updates %INC.
c8db1d39 1026 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
1027
a6dd486b 1028 2) require $file is like do $file, except the former
c8db1d39 1029 2.1: checks for redundant loading, skipping already loaded files.
1030 2.2: raises an exception on failure to find, compile, or execute $file.
1031
a6dd486b 1032 3) require Module is like require "Module.pm", except the former
c8db1d39 1033 3.1: translates each "::" into your system's directory separator.
1034 3.2: primes the parser to disambiguate class Module as an indirect object.
1035
a6dd486b 1036 4) use Module is like require Module, except the former
c8db1d39 1037 4.1: loads the module at compile time, not run-time.
1038 4.2: imports symbols and semantics from that package to the current one.
1039
1040In general, you usually want C<use> and a proper Perl module.
46fc3d4c 1041
1042=head2 How do I keep my own module/library directory?
1043
1044When you build modules, use the PREFIX option when generating
1045Makefiles:
1046
1047 perl Makefile.PL PREFIX=/u/mydir/perl
1048
1049then either set the PERL5LIB environment variable before you run
1050scripts that use the modules/libraries (see L<perlrun>) or say
1051
1052 use lib '/u/mydir/perl';
1053
a6dd486b 1054This is almost the same as
65acb1b1 1055
1056 BEGIN {
1057 unshift(@INC, '/u/mydir/perl');
1058 }
1059
1060except that the lib module checks for machine-dependent subdirectories.
46fc3d4c 1061See Perl's L<lib> for more information.
1062
1063=head2 How do I add the directory my program lives in to the module/library search path?
1064
1065 use FindBin;
7b8d334a 1066 use lib "$FindBin::Bin";
46fc3d4c 1067 use your_own_modules;
1068
1069=head2 How do I add a directory to my include path at runtime?
1070
1071Here are the suggested ways of modifying your include path:
1072
1073 the PERLLIB environment variable
1074 the PERL5LIB environment variable
c2611fb3 1075 the perl -Idir command line flag
46fc3d4c 1076 the use lib pragma, as in
1077 use lib "$ENV{HOME}/myown_perllib";
1078
1079The latter is particularly useful because it knows about machine
1080dependent architectures. The lib.pm pragmatic module was first
1081included with the 5.002 release of Perl.
68dc0745 1082
65acb1b1 1083=head2 What is socket.ph and where do I get it?
1084
1085It's a perl4-style file defining values for system networking
1086constants. Sometimes it is built using h2ph when Perl is installed,
1087but other times it is not. Modern programs C<use Socket;> instead.
1088
fc36a67e 1089=head1 AUTHOR AND COPYRIGHT
1090
0bc0ad85 1091Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.
5a964f20 1092All rights reserved.
1093
5a7beb56 1094This documentation is free; you can redistribute it and/or modify it
1095under the same terms as Perl itself.
5a964f20 1096
1097Irrespective of its distribution, all code examples in this file
1098are hereby placed into the public domain. You are permitted and
1099encouraged to use this code in your own programs for fun
1100or for profit as you see fit. A simple comment in the code giving
1101credit would be courteous but is not required.