3 perlfaq8 - System Interaction ($Revision: 1.36 $, $Date: 1999/01/08 05:36:34 $)
7 This section of the Perl FAQ covers questions involving operating
8 system interaction. This involves interprocess communication (IPC),
9 control over the user-interface (keyboard, screen and pointing
10 devices), and most anything else not related to data manipulation.
12 Read the FAQs and documentation specific to the port of perl to your
13 operating system (eg, L<perlvms>, L<perlplan9>, ...). These should
14 contain more detailed information on the vagaries of your perl.
16 =head2 How do I find out which operating system I'm running under?
18 The $^O variable ($OSNAME if you use English) contains the operating
19 system that your perl binary was built for.
21 =head2 How come exec() doesn't return?
23 Because that's what it does: it replaces your currently running
24 program with a different one. If you want to keep going (as is
25 probably the case if you're asking this question) use system()
28 =head2 How do I do fancy stuff with the keyboard/screen/mouse?
30 How you access/control keyboards, screens, and pointing devices
31 ("mice") is system-dependent. Try the following modules:
37 Term::Cap Standard perl distribution
39 Term::ReadLine::Gnu CPAN
40 Term::ReadLine::Perl CPAN
45 Term::Cap Standard perl distribution
55 Some of these specific cases are shown below.
57 =head2 How do I print something out in color?
59 In general, you don't, because you don't know whether
60 the recipient has a color-aware display device. If you
61 know that they have an ANSI terminal that understands
62 color, you can use the Term::ANSIColor module from CPAN:
65 print color("red"), "Stop!\n", color("reset");
66 print color("green"), "Go!\n", color("reset");
70 use Term::ANSIColor qw(:constants);
71 print RED, "Stop!\n", RESET;
72 print GREEN, "Go!\n", RESET;
74 =head2 How do I read just one key without waiting for a return key?
76 Controlling input buffering is a remarkably system-dependent matter.
77 If most systems, you can just use the B<stty> command as shown in
78 L<perlfunc/getc>, but as you see, that's already getting you into
81 open(TTY, "+</dev/tty") or die "no tty: $!";
82 system "stty cbreak </dev/tty >/dev/tty 2>&1";
83 $key = getc(TTY); # perhaps this works
85 sysread(TTY, $key, 1); # probably this does
86 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
88 The Term::ReadKey module from CPAN offers an easy-to-use interface that
89 should be more efficient than shelling out to B<stty> for each key.
90 It even includes limited support for Windows.
97 However, that requires that you have a working C compiler and can use it
98 to build and install a CPAN module. Here's a solution using
99 the standard POSIX module, which is already on your systems (assuming
100 your system supports POSIX).
105 And here's the HotKey module, which hides the somewhat mystifying calls
106 to manipulate the POSIX termios structures.
112 @EXPORT = qw(cbreak cooked readkey);
115 use POSIX qw(:termios_h);
116 my ($term, $oterm, $echo, $noecho, $fd_stdin);
118 $fd_stdin = fileno(STDIN);
119 $term = POSIX::Termios->new();
120 $term->getattr($fd_stdin);
121 $oterm = $term->getlflag();
123 $echo = ECHO | ECHOK | ICANON;
124 $noecho = $oterm & ~$echo;
127 $term->setlflag($noecho); # ok, so i don't want echo either
128 $term->setcc(VTIME, 1);
129 $term->setattr($fd_stdin, TCSANOW);
133 $term->setlflag($oterm);
134 $term->setcc(VTIME, 0);
135 $term->setattr($fd_stdin, TCSANOW);
141 sysread(STDIN, $key, 1);
150 =head2 How do I check whether input is ready on the keyboard?
152 The easiest way to do this is to read a key in nonblocking mode with the
153 Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
160 if (defined ($char = ReadKey(-1)) ) {
161 # input was waiting and it was $char
163 # no input was waiting
166 ReadMode('normal'); # restore normal tty settings
168 =head2 How do I clear the screen?
170 If you only have to so infrequently, use C<system>:
174 If you have to do this a lot, save the clear string
175 so you can print it 100 times without calling a program
178 $clear_string = `clear`;
181 If you're planning on doing other screen manipulations, like cursor
182 positions, etc, you might wish to use Term::Cap module:
185 $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
186 $clear_string = $terminal->Tputs('cl');
188 =head2 How do I get the screen size?
190 If you have Term::ReadKey module installed from CPAN,
191 you can use it to fetch the width and height in characters
195 ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
197 This is more portable than the raw C<ioctl>, but not as
200 require 'sys/ioctl.ph';
201 die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
202 open(TTY, "+</dev/tty") or die "No tty: $!";
203 unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
204 die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
206 ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
207 print "(row,col) = ($row,$col)";
208 print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
211 =head2 How do I ask the user for a password?
213 (This question has nothing to do with the web. See a different
216 There's an example of this in L<perlfunc/crypt>). First, you put
217 the terminal into "no echo" mode, then just read the password
218 normally. You may do this with an old-style ioctl() function, POSIX
219 terminal control (see L<POSIX>, and Chapter 7 of the Camel), or a call
220 to the B<stty> program, with varying degrees of portability.
222 You can also do this for most systems using the Term::ReadKey module
223 from CPAN, which is easier to use and in theory more portable.
228 $password = ReadLine(0);
230 =head2 How do I read and write the serial port?
232 This depends on which operating system your program is running on. In
233 the case of Unix, the serial ports will be accessible through files in
234 /dev; on other systems, the devices names will doubtless differ.
235 Several problem areas common to all device interaction are the
242 Your system may use lockfiles to control multiple access. Make sure
243 you follow the correct protocol. Unpredictable behaviour can result
244 from multiple processes reading from one device.
248 If you expect to use both read and write operations on the device,
249 you'll have to open it for update (see L<perlfunc/"open"> for
250 details). You may wish to open it without running the risk of
251 blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
252 Fcntl module (part of the standard perl distribution). See
253 L<perlfunc/"sysopen"> for more on this approach.
257 Some devices will be expecting a "\r" at the end of each line rather
258 than a "\n". In some ports of perl, "\r" and "\n" are different from
259 their usual (Unix) ASCII values of "\012" and "\015". You may have to
260 give the numeric values you want directly, using octal ("\015"), hex
261 ("0x0D"), or as a control-character specification ("\cM").
263 print DEV "atv1\012"; # wrong, for some devices
264 print DEV "atv1\015"; # right, for some devices
266 Even though with normal text files, a "\n" will do the trick, there is
267 still no unified scheme for terminating a line that is portable
268 between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line
269 ends with "\015\012", and strip what you don't need from the output.
270 This applies especially to socket I/O and autoflushing, discussed
273 =item flushing output
275 If you expect characters to get to your device when you print() them,
276 you'll want to autoflush that filehandle. You can use select()
277 and the C<$|> variable to control autoflushing (see L<perlvar/$|>
278 and L<perlfunc/select>):
284 You'll also see code that does this without a temporary variable, as in
286 select((select(DEV), $| = 1)[0]);
288 Or if you don't mind pulling in a few thousand lines
289 of code just because you're afraid of a little $| variable:
294 As mentioned in the previous item, this still doesn't work when using
295 socket I/O between Unix and Macintosh. You'll need to hardcode your
296 line terminators, in that case.
298 =item non-blocking input
300 If you are doing a blocking read() or sysread(), you'll have to
301 arrange for an alarm handler to provide a timeout (see
302 L<perlfunc/alarm>). If you have a non-blocking open, you'll likely
303 have a non-blocking read, which means you may have to use a 4-arg
304 select() to determine whether I/O is ready on that device (see
305 L<perlfunc/"select">.
309 While trying to read from his caller-id box, the notorious Jamie Zawinski
310 <jwz@netscape.com>, after much gnashing of teeth and fighting with sysread,
311 sysopen, POSIX's tcgetattr business, and various other functions that
312 go bump in the night, finally came up with this:
316 my $stty = `/bin/stty -g`;
317 open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
318 # starting cu hoses /dev/tty's stty settings, even when it has
319 # been opened on a pipe...
320 system("/bin/stty $stty");
323 if ( !m/^Connected/ ) {
324 print STDERR "$0: cu printed `$_' instead of `Connected'\n";
328 =head2 How do I decode encrypted password files?
330 You spend lots and lots of money on dedicated hardware, but this is
331 bound to get you talked about.
333 Seriously, you can't if they are Unix password files - the Unix
334 password system employs one-way encryption. It's more like hashing than
335 encryption. The best you can check is whether something else hashes to
336 the same string. You can't turn a hash back into the original string.
338 can forcibly (and intelligently) try to guess passwords, but don't
339 (can't) guarantee quick success.
341 If you're worried about users selecting bad passwords, you should
342 proactively check when they try to change their password (by modifying
343 passwd(1), for example).
345 =head2 How do I start a process in the background?
351 or you could use fork as documented in L<perlfunc/"fork">, with
352 further examples in L<perlipc>. Some things to be aware of, if you're
353 on a Unix-like system:
357 =item STDIN, STDOUT, and STDERR are shared
359 Both the main process and the backgrounded one (the "child" process)
360 share the same STDIN, STDOUT and STDERR filehandles. If both try to
361 access them at once, strange things can happen. You may want to close
362 or reopen these for the child. You can get around this with
363 C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this
364 means that the child process cannot outlive the parent.
368 You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
369 SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
370 sent when you write to a filehandle whose child process has closed (an
371 untrapped SIGPIPE can cause your program to silently die). This is
372 not an issue with C<system("cmd&")>.
376 You have to be prepared to "reap" the child process when it finishes
378 $SIG{CHLD} = sub { wait };
380 See L<perlipc/"Signals"> for other examples of code to do this.
381 Zombies are not an issue with C<system("prog &")>.
385 =head2 How do I trap control characters/signals?
387 You don't actually "trap" a control character. Instead, that character
388 generates a signal which is sent to your terminal's currently
389 foregrounded process group, which you then trap in your process.
390 Signals are documented in L<perlipc/"Signals"> and chapter 6 of the Camel.
392 Be warned that very few C libraries are re-entrant. Therefore, if you
393 attempt to print() in a handler that got invoked during another stdio
394 operation your internal structures will likely be in an
395 inconsistent state, and your program will dump core. You can
396 sometimes avoid this by using syswrite() instead of print().
398 Unless you're exceedingly careful, the only safe things to do inside a
399 signal handler are: set a variable and exit. And in the first case,
400 you should only set a variable in such a way that malloc() is not
401 called (eg, by setting a variable that already has a value).
405 $Interrupted = 0; # to ensure it has a value
408 syswrite(STDERR, "ouch\n", 5);
411 However, because syscalls restart by default, you'll find that if
412 you're in a "slow" call, such as E<lt>FHE<gt>, read(), connect(), or
413 wait(), that the only way to terminate them is by "longjumping" out;
414 that is, by raising an exception. See the time-out handler for a
415 blocking flock() in L<perlipc/"Signals"> or chapter 6 of the Camel.
417 =head2 How do I modify the shadow password file on a Unix system?
419 If perl was installed correctly, and your shadow library was written
420 properly, the getpw*() functions described in L<perlfunc> should in
421 theory provide (read-only) access to entries in the shadow password
422 file. To change the file, make a new shadow password file (the format
423 varies from system to system - see L<passwd(5)> for specifics) and use
424 pwd_mkdb(8) to install it (see L<pwd_mkdb(5)> for more details).
426 =head2 How do I set the time and date?
428 Assuming you're running under sufficient permissions, you should be
429 able to set the system-wide date and time by running the date(1)
430 program. (There is no way to set the time and date on a per-process
431 basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
432 the VMS equivalent is C<set time>.
434 However, if all you want to do is change your timezone, you can
435 probably get away with setting an environment variable:
437 $ENV{TZ} = "MST7MDT"; # unixish
438 $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
439 system "trn comp.lang.perl.misc";
441 =head2 How can I sleep() or alarm() for under a second?
443 If you want finer granularity than the 1 second that the sleep()
444 function provides, the easiest way is to use the select() function as
445 documented in L<perlfunc/"select">. If your system has itimers and
446 syscall() support, you can check out the old example in
447 http://www.perl.com/CPAN/doc/misc/ancient/tutorial/eg/itimers.pl .
449 =head2 How can I measure time under a second?
451 In general, you may not be able to. The Time::HiRes module (available
452 from CPAN) provides this functionality for some systems.
454 If your system supports both the syscall() function in Perl as well as
455 a system call like gettimeofday(2), then you may be able to do
458 require 'sys/syscall.ph';
462 $done = $start = pack($TIMEVAL_T, ());
464 syscall( &SYS_gettimeofday, $start, 0) != -1
465 or die "gettimeofday: $!";
467 ##########################
468 # DO YOUR OPERATION HERE #
469 ##########################
471 syscall( &SYS_gettimeofday, $done, 0) != -1
472 or die "gettimeofday: $!";
474 @start = unpack($TIMEVAL_T, $start);
475 @done = unpack($TIMEVAL_T, $done);
478 for ($done[1], $start[1]) { $_ /= 1_000_000 }
480 $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
482 ($start[0] + $start[1] );
484 =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
486 Release 5 of Perl added the END block, which can be used to simulate
487 atexit(). Each package's END block is called when the program or
488 thread ends (see L<perlmod> manpage for more details).
490 For example, you can use this to make sure your filter program
491 managed to finish its output without filling up the disk:
494 close(STDOUT) || die "stdout close failed: $!";
497 The END block isn't called when untrapped signals kill the program, though, so if
498 you use END blocks you should also use
500 use sigtrap qw(die normal-signals);
502 Perl's exception-handling mechanism is its eval() operator. You can
503 use eval() as setjmp and die() as longjmp. For details of this, see
504 the section on signals, especially the time-out handler for a blocking
505 flock() in L<perlipc/"Signals"> and chapter 6 of the Camel.
507 If exception handling is all you're interested in, try the
508 exceptions.pl library (part of the standard perl distribution).
510 If you want the atexit() syntax (and an rmexit() as well), try the
511 AtExit module available from CPAN.
513 =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
515 Some Sys-V based systems, notably Solaris 2.X, redefined some of the
516 standard socket constants. Since these were constant across all
517 architectures, they were often hardwired into perl code. The proper
518 way to deal with this is to "use Socket" to get the correct values.
520 Note that even though SunOS and Solaris are binary compatible, these
521 values are different. Go figure.
523 =head2 How can I call my system's unique C functions from Perl?
525 In most cases, you write an external module to do it - see the answer
526 to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
527 However, if the function is a system call, and your system supports
528 syscall(), you can use the syscall function (documented in
531 Remember to check the modules that came with your distribution, and
532 CPAN as well - someone may already have written a module to do it.
534 =head2 Where do I get the include files to do ioctl() or syscall()?
536 Historically, these would be generated by the h2ph tool, part of the
537 standard perl distribution. This program converts cpp(1) directives
538 in C header files to files containing subroutine definitions, like
539 &SYS_getitimer, which you can use as arguments to your functions.
540 It doesn't work perfectly, but it usually gets most of the job done.
541 Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
542 but the hard ones like F<ioctl.h> nearly always need to hand-edited.
543 Here's how to install the *.ph files:
549 If your system supports dynamic loading, for reasons of portability and
550 sanity you probably ought to use h2xs (also part of the standard perl
551 distribution). This tool converts C header files to Perl extensions.
552 See L<perlxstut> for how to get started with h2xs.
554 If your system doesn't support dynamic loading, you still probably
555 ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
556 more information (in brief, just use B<make perl> instead of a plain
557 B<make> to rebuild perl with a new static extension).
559 =head2 Why do setuid perl scripts complain about kernel problems?
561 Some operating systems have bugs in the kernel that make setuid
562 scripts inherently insecure. Perl gives you a number of options
563 (described in L<perlsec>) to work around such systems.
565 =head2 How can I open a pipe both to and from a command?
567 The IPC::Open2 module (part of the standard perl distribution) is an
568 easy-to-use approach that internally uses pipe(), fork(), and exec() to do
569 the job. Make sure you read the deadlock warnings in its documentation,
570 though (see L<IPC::Open2>). See L<perlipc/"Bidirectional Communication
571 with Another Process"> and L<perlipc/"Bidirectional Communication with
574 You may also use the IPC::Open3 module (part of the standard perl
575 distribution), but be warned that it has a different order of
576 arguments from IPC::Open2 (see L<IPC::Open3>).
578 =head2 Why can't I get the output of a command with system()?
580 You're confusing the purpose of system() and backticks (``). system()
581 runs a command and returns exit status information (as a 16 bit value:
582 the low 7 bits are the signal the process died from, if any, and
583 the high 8 bits are the actual exit value). Backticks (``) run a
584 command and return what it sent to STDOUT.
586 $exit_status = system("mail-users");
587 $output_string = `ls`;
589 =head2 How can I capture STDERR from an external command?
591 There are three basic ways of running external commands:
593 system $cmd; # using system()
594 $output = `$cmd`; # using backticks (``)
595 open (PIPE, "cmd |"); # using open()
597 With system(), both STDOUT and STDERR will go the same place as the
598 script's versions of these, unless the command redirects them.
599 Backticks and open() read B<only> the STDOUT of your command.
601 With any of these, you can change file descriptors before the call:
603 open(STDOUT, ">logfile");
606 or you can use Bourne shell file-descriptor redirection:
608 $output = `$cmd 2>some_file`;
609 open (PIPE, "cmd 2>some_file |");
611 You can also use file-descriptor redirection to make STDERR a
614 $output = `$cmd 2>&1`;
615 open (PIPE, "cmd 2>&1 |");
617 Note that you I<cannot> simply open STDERR to be a dup of STDOUT
618 in your Perl program and avoid calling the shell to do the redirection.
621 open(STDERR, ">&STDOUT");
622 $alloutput = `cmd args`; # stderr still escapes
624 This fails because the open() makes STDERR go to where STDOUT was
625 going at the time of the open(). The backticks then make STDOUT go to
626 a string, but don't change STDERR (which still goes to the old
629 Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
630 backticks, not csh(1)! Details on why Perl's system() and backtick
631 and pipe opens all use the Bourne shell are in
632 http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot .
633 To capture a command's STDERR and STDOUT together:
635 $output = `cmd 2>&1`; # either with backticks
636 $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
637 while (<PH>) { } # plus a read
639 To capture a command's STDOUT but discard its STDERR:
641 $output = `cmd 2>/dev/null`; # either with backticks
642 $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
643 while (<PH>) { } # plus a read
645 To capture a command's STDERR but discard its STDOUT:
647 $output = `cmd 2>&1 1>/dev/null`; # either with backticks
648 $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
649 while (<PH>) { } # plus a read
651 To exchange a command's STDOUT and STDERR in order to capture the STDERR
652 but leave its STDOUT to come out our old STDERR:
654 $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
655 $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
656 while (<PH>) { } # plus a read
658 To read both a command's STDOUT and its STDERR separately, it's easiest
659 and safest to redirect them separately to files, and then read from those
660 files when the program is done:
662 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
664 Ordering is important in all these examples. That's because the shell
665 processes file descriptor redirections in strictly left to right order.
667 system("prog args 1>tmpfile 2>&1");
668 system("prog args 2>&1 1>tmpfile");
670 The first command sends both standard out and standard error to the
671 temporary file. The second command sends only the old standard output
672 there, and the old standard error shows up on the old standard out.
674 =head2 Why doesn't open() return an error when a pipe open fails?
676 Because the pipe open takes place in two steps: first Perl calls
677 fork() to start a new process, then this new process calls exec() to
678 run the program you really wanted to open. The first step reports
679 success or failure to your process, so open() can only tell you
680 whether the fork() succeeded or not.
682 To find out if the exec() step succeeded, you have to catch SIGCHLD
683 and wait() to get the exit status. You should also catch SIGPIPE if
684 you're writing to the child--you may not have found out the exec()
685 failed by the time you write. This is documented in L<perlipc>.
687 In some cases, even this won't work. If the second argument to a
688 piped open() contains shell metacharacters, perl fork()s, then exec()s
689 a shell to decode the metacharacters and eventually run the desired
690 program. Now when you call wait(), you only learn whether or not the
691 I<shell> could be successfully started. Best to avoid shell
694 On systems that follow the spawn() paradigm, open() I<might> do what
695 you expect--unless perl uses a shell to start your command. In this
696 case the fork()/exec() description still applies.
698 =head2 What's wrong with using backticks in a void context?
700 Strictly speaking, nothing. Stylistically speaking, it's not a good
701 way to write maintainable code because backticks have a (potentially
702 humungous) return value, and you're ignoring it. It's may also not be very
703 efficient, because you have to read in all the lines of output, allocate
704 memory for them, and then throw it away. Too often people are lulled
709 And now they think "Hey, I'll just always use backticks to run programs."
710 Bad idea: backticks are for capturing a program's output; the system()
711 function is for running programs.
717 You haven't assigned the output anywhere, so it just wastes memory
718 (for a little while). Plus you forgot to check C<$?> to see whether
719 the program even ran correctly. Even if you wrote
721 print `cat /etc/termcap`;
723 In most cases, this could and probably should be written as
725 system("cat /etc/termcap") == 0
726 or die "cat program failed!";
728 Which will get the output quickly (as its generated, instead of only
729 at the end) and also check the return value.
731 system() also provides direct control over whether shell wildcard
732 processing may take place, whereas backticks do not.
734 =head2 How can I call backticks without shell processing?
736 This is a bit tricky. Instead of writing
738 @ok = `grep @opts '$search_string' @filenames`;
743 if (open(GREP, "-|")) {
750 exec 'grep', @opts, $search_string, @filenames;
753 Just as with system(), no shell escapes happen when you exec() a list.
755 There are more examples of this L<perlipc/"Safe Pipe Opens">.
757 =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
759 Because some stdio's set error and eof flags that need clearing. The
760 POSIX module defines clearerr() that you can use. That is the
761 technically correct way to do it. Here are some less reliable
768 Try keeping around the seekpointer and go there, like this:
771 seek(LOG, $where, 0);
775 If that doesn't work, try seeking to a different part of the file and
780 If that doesn't work, try seeking to a different part of
781 the file, reading something, and then seeking back.
785 If that doesn't work, give up on your stdio package and use sysread.
789 =head2 How can I convert my shell script to perl?
791 Learn Perl and rewrite it. Seriously, there's no simple converter.
792 Things that are awkward to do in the shell are easy to do in Perl, and
793 this very awkwardness is what would make a shell->perl converter
794 nigh-on impossible to write. By rewriting it, you'll think about what
795 you're really trying to do, and hopefully will escape the shell's
796 pipeline datastream paradigm, which while convenient for some matters,
797 causes many inefficiencies.
799 =head2 Can I use perl to run a telnet or ftp session?
801 Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
802 CPAN). http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar
803 will also help for emulating the telnet protocol, but Net::Telnet is
804 quite probably easier to use..
806 If all you want to do is pretend to be telnet but don't need
807 the initial telnet handshaking, then the standard dual-process
808 approach will suffice:
810 use IO::Socket; # new in 5.004
811 $handle = IO::Socket::INET->new('www.perl.com:80')
812 || die "can't connect to port 80 on www.perl.com: $!";
813 $handle->autoflush(1);
814 if (fork()) { # XXX: undef means failure
816 print while <STDIN>; # everything from stdin to socket
818 print while <$handle>; # everything from socket to stdout
823 =head2 How can I write expect in Perl?
825 Once upon a time, there was a library called chat2.pl (part of the
826 standard perl distribution), which never really got finished. If you
827 find it somewhere, I<don't use it>. These days, your best bet is to
828 look at the Expect module available from CPAN, which also requires two
829 other modules from CPAN, IO::Pty and IO::Stty.
831 =head2 Is there a way to hide perl's command line from programs such as "ps"?
833 First of all note that if you're doing this for security reasons (to
834 avoid people seeing passwords, for example) then you should rewrite
835 your program so that critical information is never given as an
836 argument. Hiding the arguments won't make your program completely
839 To actually alter the visible command line, you can assign to the
840 variable $0 as documented in L<perlvar>. This won't work on all
841 operating systems, though. Daemon programs like sendmail place their
844 $0 = "orcus [accepting connections]";
846 =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?
852 In the strictest sense, it can't be done -- the script executes as a
853 different process from the shell it was started from. Changes to a
854 process are not reflected in its parent, only in its own children
855 created after the change. There is shell magic that may allow you to
856 fake it by eval()ing the script's output in your shell; check out the
857 comp.unix.questions FAQ for details.
861 =head2 How do I close a process's filehandle without waiting for it to complete?
863 Assuming your system supports such things, just send an appropriate signal
864 to the process (see L<perlfunc/"kill">. It's common to first send a TERM
865 signal, wait a little bit, and then send a KILL signal to finish it off.
867 =head2 How do I fork a daemon process?
869 If by daemon process you mean one that's detached (disassociated from
870 its tty), then the following process is reported to work on most
871 Unixish systems. Non-Unix users should check their Your_OS::Process
872 module for other solutions.
878 Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
879 for details. Or better yet, you can just use the POSIX::setsid()
880 function, so you don't have to worry about process groups.
884 Change directory to /
888 Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
893 Background yourself like this:
899 =head2 How do I make my program run with sh and csh?
901 See the F<eg/nih> script (part of the perl source distribution).
903 =head2 How do I find out if I'm running interactively or not?
905 Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
908 if (-t STDIN && -t STDOUT) {
912 On POSIX systems, you can test whether your own process group matches
913 the current process group of your controlling terminal as follows:
915 use POSIX qw/getpgrp tcgetpgrp/;
916 open(TTY, "/dev/tty") or die $!;
917 $tpgrp = tcgetpgrp(fileno(*TTY));
919 if ($tpgrp == $pgrp) {
920 print "foreground\n";
922 print "background\n";
925 =head2 How do I timeout a slow event?
927 Use the alarm() function, probably in conjunction with a signal
928 handler, as documented L<perlipc/"Signals"> and chapter 6 of the
929 Camel. You may instead use the more flexible Sys::AlarmCall module
932 =head2 How do I set CPU limits?
934 Use the BSD::Resource module from CPAN.
936 =head2 How do I avoid zombies on a Unix system?
938 Use the reaper code from L<perlipc/"Signals"> to call wait() when a
939 SIGCHLD is received, or else use the double-fork technique described
942 =head2 How do I use an SQL database?
944 There are a number of excellent interfaces to SQL databases. See the
945 DBD::* modules available from
946 http://www.perl.com/CPAN/modules/dbperl/DBD .
947 A lot of information on this can be found at
948 http://www.hermetica.com/technologia/perl/DBI/index.html .
950 =head2 How do I make a system() exit on control-C?
952 You can't. You need to imitate the system() call (see L<perlipc> for
953 sample code) and then have a signal handler for the INT signal that
954 passes the signal on to the subprocess. Or you can check for it:
957 if ($rc & 127) { die "signal death" }
959 =head2 How do I open a file without blocking?
961 If you're lucky enough to be using a system that supports
962 non-blocking reads (most Unixish systems do), you need only to use the
963 O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
967 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
968 or die "can't open /tmp/somefile: $!":
970 =head2 How do I install a CPAN module?
972 The easiest way is to have the CPAN module do it for you. This module
973 comes with perl version 5.004 and later. To manually install the CPAN
974 module, or any well-behaved CPAN module for that matter, follow these
981 Unpack the source into a temporary area.
1001 If your version of perl is compiled without dynamic loading, then you
1002 just need to replace step 3 (B<make>) with B<make perl> and you will
1003 get a new F<perl> binary with your extension linked in.
1005 See L<ExtUtils::MakeMaker> for more details on building extensions.
1006 See also the next question.
1008 =head2 What's the difference between require and use?
1010 Perl offers several different ways to include code from one file into
1011 another. Here are the deltas between the various inclusion constructs:
1013 1) do $file is like eval `cat $file`, except the former:
1014 1.1: searches @INC and updates %INC.
1015 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
1017 2) require $file is like do $file, except the former:
1018 2.1: checks for redundant loading, skipping already loaded files.
1019 2.2: raises an exception on failure to find, compile, or execute $file.
1021 3) require Module is like require "Module.pm", except the former:
1022 3.1: translates each "::" into your system's directory separator.
1023 3.2: primes the parser to disambiguate class Module as an indirect object.
1025 4) use Module is like require Module, except the former:
1026 4.1: loads the module at compile time, not run-time.
1027 4.2: imports symbols and semantics from that package to the current one.
1029 In general, you usually want C<use> and a proper Perl module.
1031 =head2 How do I keep my own module/library directory?
1033 When you build modules, use the PREFIX option when generating
1036 perl Makefile.PL PREFIX=/u/mydir/perl
1038 then either set the PERL5LIB environment variable before you run
1039 scripts that use the modules/libraries (see L<perlrun>) or say
1041 use lib '/u/mydir/perl';
1043 This is almost the same as:
1046 unshift(@INC, '/u/mydir/perl');
1049 except that the lib module checks for machine-dependent subdirectories.
1050 See Perl's L<lib> for more information.
1052 =head2 How do I add the directory my program lives in to the module/library search path?
1055 use lib "$FindBin::Bin";
1056 use your_own_modules;
1058 =head2 How do I add a directory to my include path at runtime?
1060 Here are the suggested ways of modifying your include path:
1062 the PERLLIB environment variable
1063 the PERL5LIB environment variable
1064 the perl -Idir commpand line flag
1065 the use lib pragma, as in
1066 use lib "$ENV{HOME}/myown_perllib";
1068 The latter is particularly useful because it knows about machine
1069 dependent architectures. The lib.pm pragmatic module was first
1070 included with the 5.002 release of Perl.
1072 =head2 What is socket.ph and where do I get it?
1074 It's a perl4-style file defining values for system networking
1075 constants. Sometimes it is built using h2ph when Perl is installed,
1076 but other times it is not. Modern programs C<use Socket;> instead.
1078 =head1 AUTHOR AND COPYRIGHT
1080 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1081 All rights reserved.
1083 When included as part of the Standard Version of Perl, or as part of
1084 its complete documentation whether printed or otherwise, this work
1085 may be distributed only under the terms of Perl's Artistic License.
1086 Any distribution of this file or derivatives thereof I<outside>
1087 of that package require that special arrangements be made with
1090 Irrespective of its distribution, all code examples in this file
1091 are hereby placed into the public domain. You are permitted and
1092 encouraged to use this code in your own programs for fun
1093 or for profit as you see fit. A simple comment in the code giving
1094 credit would be courteous but is not required.