3 perlfaq8 - System Interaction ($Revision: 1.4 $, $Date: 2001/11/09 08:06:04 $)
7 This section of the Perl FAQ covers questions involving operating
8 system interaction. Topics include 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 an indication of
19 the name of the operating system (not its release number) that your perl
22 =head2 How come exec() doesn't return?
24 Because that's what it does: it replaces your currently running
25 program with a different one. If you want to keep going (as is
26 probably the case if you're asking this question) use system()
29 =head2 How do I do fancy stuff with the keyboard/screen/mouse?
31 How you access/control keyboards, screens, and pointing devices
32 ("mice") is system-dependent. Try the following modules:
38 Term::Cap Standard perl distribution
40 Term::ReadLine::Gnu CPAN
41 Term::ReadLine::Perl CPAN
46 Term::Cap Standard perl distribution
56 Some of these specific cases are shown below.
58 =head2 How do I print something out in color?
60 In general, you don't, because you don't know whether
61 the recipient has a color-aware display device. If you
62 know that they have an ANSI terminal that understands
63 color, you can use the Term::ANSIColor module from CPAN:
66 print color("red"), "Stop!\n", color("reset");
67 print color("green"), "Go!\n", color("reset");
71 use Term::ANSIColor qw(:constants);
72 print RED, "Stop!\n", RESET;
73 print GREEN, "Go!\n", RESET;
75 =head2 How do I read just one key without waiting for a return key?
77 Controlling input buffering is a remarkably system-dependent matter.
78 On many systems, you can just use the B<stty> command as shown in
79 L<perlfunc/getc>, but as you see, that's already getting you into
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
86 sysread(TTY, $key, 1); # probably this does
87 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
89 The Term::ReadKey module from CPAN offers an easy-to-use interface that
90 should be more efficient than shelling out to B<stty> for each key.
91 It even includes limited support for Windows.
98 However, using the code requires that you have a working C compiler
99 and can use it to build and install a CPAN module. Here's a solution
100 using the standard POSIX module, which is already on your systems
101 (assuming your system supports POSIX).
106 And here's the HotKey module, which hides the somewhat mystifying calls
107 to manipulate the POSIX termios structures.
113 @EXPORT = qw(cbreak cooked readkey);
116 use POSIX qw(:termios_h);
117 my ($term, $oterm, $echo, $noecho, $fd_stdin);
119 $fd_stdin = fileno(STDIN);
120 $term = POSIX::Termios->new();
121 $term->getattr($fd_stdin);
122 $oterm = $term->getlflag();
124 $echo = ECHO | ECHOK | ICANON;
125 $noecho = $oterm & ~$echo;
128 $term->setlflag($noecho); # ok, so i don't want echo either
129 $term->setcc(VTIME, 1);
130 $term->setattr($fd_stdin, TCSANOW);
134 $term->setlflag($oterm);
135 $term->setcc(VTIME, 0);
136 $term->setattr($fd_stdin, TCSANOW);
142 sysread(STDIN, $key, 1);
151 =head2 How do I check whether input is ready on the keyboard?
153 The easiest way to do this is to read a key in nonblocking mode with the
154 Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
161 if (defined ($char = ReadKey(-1)) ) {
162 # input was waiting and it was $char
164 # no input was waiting
167 ReadMode('normal'); # restore normal tty settings
169 =head2 How do I clear the screen?
171 If you only have do so infrequently, use C<system>:
175 If you have to do this a lot, save the clear string
176 so you can print it 100 times without calling a program
179 $clear_string = `clear`;
182 If you're planning on doing other screen manipulations, like cursor
183 positions, etc, you might wish to use Term::Cap module:
186 $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
187 $clear_string = $terminal->Tputs('cl');
189 =head2 How do I get the screen size?
191 If you have Term::ReadKey module installed from CPAN,
192 you can use it to fetch the width and height in characters
196 ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
198 This is more portable than the raw C<ioctl>, but not as
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: $!)\n", &TIOCGWINSZ;
207 ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
208 print "(row,col) = ($row,$col)";
209 print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
212 =head2 How do I ask the user for a password?
214 (This question has nothing to do with the web. See a different
217 There's an example of this in L<perlfunc/crypt>). First, you put the
218 terminal into "no echo" mode, then just read the password normally.
219 You may do this with an old-style ioctl() function, POSIX terminal
220 control (see L<POSIX> or its documentation the Camel Book), or a call
221 to the B<stty> program, with varying degrees of portability.
223 You can also do this for most systems using the Term::ReadKey module
224 from CPAN, which is easier to use and in theory more portable.
229 $password = ReadLine(0);
231 =head2 How do I read and write the serial port?
233 This depends on which operating system your program is running on. In
234 the case of Unix, the serial ports will be accessible through files in
235 /dev; on other systems, device names will doubtless differ.
236 Several problem areas common to all device interaction are the
243 Your system may use lockfiles to control multiple access. Make sure
244 you follow the correct protocol. Unpredictable behavior can result
245 from multiple processes reading from one device.
249 If you expect to use both read and write operations on the device,
250 you'll have to open it for update (see L<perlfunc/"open"> for
251 details). You may wish to open it without running the risk of
252 blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
253 Fcntl module (part of the standard perl distribution). See
254 L<perlfunc/"sysopen"> for more on this approach.
258 Some devices will be expecting a "\r" at the end of each line rather
259 than a "\n". In some ports of perl, "\r" and "\n" are different from
260 their usual (Unix) ASCII values of "\012" and "\015". You may have to
261 give the numeric values you want directly, using octal ("\015"), hex
262 ("0x0D"), or as a control-character specification ("\cM").
264 print DEV "atv1\012"; # wrong, for some devices
265 print DEV "atv1\015"; # right, for some devices
267 Even though with normal text files a "\n" will do the trick, there is
268 still no unified scheme for terminating a line that is portable
269 between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line
270 ends with "\015\012", and strip what you don't need from the output.
271 This applies especially to socket I/O and autoflushing, discussed
274 =item flushing output
276 If you expect characters to get to your device when you print() them,
277 you'll want to autoflush that filehandle. You can use select()
278 and the C<$|> variable to control autoflushing (see L<perlvar/$|>
279 and L<perlfunc/select>, or L<perlfaq5>, ``How do I flush/unbuffer an
280 output filehandle? Why must I do this?''):
286 You'll also see code that does this without a temporary variable, as in
288 select((select(DEV), $| = 1)[0]);
290 Or if you don't mind pulling in a few thousand lines
291 of code just because you're afraid of a little $| variable:
296 As mentioned in the previous item, this still doesn't work when using
297 socket I/O between Unix and Macintosh. You'll need to hard code your
298 line terminators, in that case.
300 =item non-blocking input
302 If you are doing a blocking read() or sysread(), you'll have to
303 arrange for an alarm handler to provide a timeout (see
304 L<perlfunc/alarm>). If you have a non-blocking open, you'll likely
305 have a non-blocking read, which means you may have to use a 4-arg
306 select() to determine whether I/O is ready on that device (see
307 L<perlfunc/"select">.
311 While 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,
313 sysopen, POSIX's tcgetattr business, and various other functions that
314 go bump in the night, finally came up with this:
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");
325 if ( !m/^Connected/ ) {
326 print STDERR "$0: cu printed `$_' instead of `Connected'\n";
330 =head2 How do I decode encrypted password files?
332 You spend lots and lots of money on dedicated hardware, but this is
333 bound to get you talked about.
335 Seriously, you can't if they are Unix password files--the Unix
336 password system employs one-way encryption. It's more like hashing than
337 encryption. The best you can check is whether something else hashes to
338 the same string. You can't turn a hash back into the original string.
340 can forcibly (and intelligently) try to guess passwords, but don't
341 (can't) guarantee quick success.
343 If you're worried about users selecting bad passwords, you should
344 proactively check when they try to change their password (by modifying
345 passwd(1), for example).
347 =head2 How do I start a process in the background?
353 or you could use fork as documented in L<perlfunc/"fork">, with
354 further examples in L<perlipc>. Some things to be aware of, if you're
355 on a Unix-like system:
359 =item STDIN, STDOUT, and STDERR are shared
361 Both the main process and the backgrounded one (the "child" process)
362 share the same STDIN, STDOUT and STDERR filehandles. If both try to
363 access them at once, strange things can happen. You may want to close
364 or reopen these for the child. You can get around this with
365 C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this
366 means that the child process cannot outlive the parent.
370 You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
371 SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
372 sent when you write to a filehandle whose child process has closed (an
373 untrapped SIGPIPE can cause your program to silently die). This is
374 not an issue with C<system("cmd&")>.
378 You have to be prepared to "reap" the child process when it finishes
380 $SIG{CHLD} = sub { wait };
382 See L<perlipc/"Signals"> for other examples of code to do this.
383 Zombies are not an issue with C<system("prog &")>.
387 =head2 How do I trap control characters/signals?
389 You don't actually "trap" a control character. Instead, that character
390 generates a signal which is sent to your terminal's currently
391 foregrounded process group, which you then trap in your process.
392 Signals are documented in L<perlipc/"Signals"> and the
393 section on ``Signals'' in the Camel.
395 Be warned that very few C libraries are re-entrant. Therefore, if you
396 attempt to print() in a handler that got invoked during another stdio
397 operation your internal structures will likely be in an
398 inconsistent state, and your program will dump core. You can
399 sometimes avoid this by using syswrite() instead of print().
401 Unless you're exceedingly careful, the only safe things to do inside a
402 signal handler are (1) set a variable and (2) exit. In the first case,
403 you should only set a variable in such a way that malloc() is not
404 called (eg, by setting a variable that already has a value).
408 $Interrupted = 0; # to ensure it has a value
411 syswrite(STDERR, "ouch\n", 5);
414 However, because syscalls restart by default, you'll find that if
415 you're in a "slow" call, such as <FH>, read(), connect(), or
416 wait(), that the only way to terminate them is by "longjumping" out;
417 that is, by raising an exception. See the time-out handler for a
418 blocking flock() in L<perlipc/"Signals"> or the section on ``Signals''
421 =head2 How do I modify the shadow password file on a Unix system?
423 If perl was installed correctly and your shadow library was written
424 properly, the getpw*() functions described in L<perlfunc> should in
425 theory provide (read-only) access to entries in the shadow password
426 file. To change the file, make a new shadow password file (the format
427 varies from system to system--see L<passwd(5)> for specifics) and use
428 pwd_mkdb(8) to install it (see L<pwd_mkdb(8)> for more details).
430 =head2 How do I set the time and date?
432 Assuming you're running under sufficient permissions, you should be
433 able to set the system-wide date and time by running the date(1)
434 program. (There is no way to set the time and date on a per-process
435 basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
436 the VMS equivalent is C<set time>.
438 However, if all you want to do is change your time zone, you can
439 probably get away with setting an environment variable:
441 $ENV{TZ} = "MST7MDT"; # unixish
442 $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
443 system "trn comp.lang.perl.misc";
445 =head2 How can I sleep() or alarm() for under a second?
447 If you want finer granularity than the 1 second that the sleep()
448 function provides, the easiest way is to use the select() function as
449 documented in L<perlfunc/"select">. Try the Time::HiRes and
450 the BSD::Itimer modules (available from CPAN, and starting from
451 Perl 5.8 Time::HiRes is part of the standard distribution).
453 =head2 How can I measure time under a second?
455 In general, you may not be able to. The Time::HiRes module (available
456 from CPAN, and starting from Perl 5.8 part of the standard distribution)
457 provides this functionality for some systems.
459 If your system supports both the syscall() function in Perl as well as
460 a system call like gettimeofday(2), then you may be able to do
463 require 'sys/syscall.ph';
467 $done = $start = pack($TIMEVAL_T, ());
469 syscall(&SYS_gettimeofday, $start, 0) != -1
470 or die "gettimeofday: $!";
472 ##########################
473 # DO YOUR OPERATION HERE #
474 ##########################
476 syscall( &SYS_gettimeofday, $done, 0) != -1
477 or die "gettimeofday: $!";
479 @start = unpack($TIMEVAL_T, $start);
480 @done = unpack($TIMEVAL_T, $done);
483 for ($done[1], $start[1]) { $_ /= 1_000_000 }
485 $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
487 ($start[0] + $start[1] );
489 =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
491 Release 5 of Perl added the END block, which can be used to simulate
492 atexit(). Each package's END block is called when the program or
493 thread ends (see L<perlmod> manpage for more details).
495 For example, you can use this to make sure your filter program
496 managed to finish its output without filling up the disk:
499 close(STDOUT) || die "stdout close failed: $!";
502 The END block isn't called when untrapped signals kill the program,
503 though, so if you use END blocks you should also use
505 use sigtrap qw(die normal-signals);
507 Perl's exception-handling mechanism is its eval() operator. You can
508 use eval() as setjmp and die() as longjmp. For details of this, see
509 the section on signals, especially the time-out handler for a blocking
510 flock() in L<perlipc/"Signals"> or the section on ``Signals'' in
513 If exception handling is all you're interested in, try the
514 exceptions.pl library (part of the standard perl distribution).
516 If you want the atexit() syntax (and an rmexit() as well), try the
517 AtExit module available from CPAN.
519 =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
521 Some Sys-V based systems, notably Solaris 2.X, redefined some of the
522 standard socket constants. Since these were constant across all
523 architectures, they were often hardwired into perl code. The proper
524 way to deal with this is to "use Socket" to get the correct values.
526 Note that even though SunOS and Solaris are binary compatible, these
527 values are different. Go figure.
529 =head2 How can I call my system's unique C functions from Perl?
531 In most cases, you write an external module to do it--see the answer
532 to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
533 However, if the function is a system call, and your system supports
534 syscall(), you can use the syscall function (documented in
537 Remember to check the modules that came with your distribution, and
538 CPAN as well--someone may already have written a module to do it.
540 =head2 Where do I get the include files to do ioctl() or syscall()?
542 Historically, these would be generated by the h2ph tool, part of the
543 standard perl distribution. This program converts cpp(1) directives
544 in C header files to files containing subroutine definitions, like
545 &SYS_getitimer, which you can use as arguments to your functions.
546 It doesn't work perfectly, but it usually gets most of the job done.
547 Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
548 but the hard ones like F<ioctl.h> nearly always need to hand-edited.
549 Here's how to install the *.ph files:
555 If your system supports dynamic loading, for reasons of portability and
556 sanity you probably ought to use h2xs (also part of the standard perl
557 distribution). This tool converts C header files to Perl extensions.
558 See L<perlxstut> for how to get started with h2xs.
560 If your system doesn't support dynamic loading, you still probably
561 ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
562 more information (in brief, just use B<make perl> instead of a plain
563 B<make> to rebuild perl with a new static extension).
565 =head2 Why do setuid perl scripts complain about kernel problems?
567 Some operating systems have bugs in the kernel that make setuid
568 scripts inherently insecure. Perl gives you a number of options
569 (described in L<perlsec>) to work around such systems.
571 =head2 How can I open a pipe both to and from a command?
573 The IPC::Open2 module (part of the standard perl distribution) is an
574 easy-to-use approach that internally uses pipe(), fork(), and exec() to do
575 the job. Make sure you read the deadlock warnings in its documentation,
576 though (see L<IPC::Open2>). See
577 L<perlipc/"Bidirectional Communication with Another Process"> and
578 L<perlipc/"Bidirectional Communication with Yourself">
580 You may also use the IPC::Open3 module (part of the standard perl
581 distribution), but be warned that it has a different order of
582 arguments from IPC::Open2 (see L<IPC::Open3>).
584 =head2 Why can't I get the output of a command with system()?
586 You're confusing the purpose of system() and backticks (``). system()
587 runs a command and returns exit status information (as a 16 bit value:
588 the low 7 bits are the signal the process died from, if any, and
589 the high 8 bits are the actual exit value). Backticks (``) run a
590 command and return what it sent to STDOUT.
592 $exit_status = system("mail-users");
593 $output_string = `ls`;
595 =head2 How can I capture STDERR from an external command?
597 There are three basic ways of running external commands:
599 system $cmd; # using system()
600 $output = `$cmd`; # using backticks (``)
601 open (PIPE, "cmd |"); # using open()
603 With system(), both STDOUT and STDERR will go the same place as the
604 script's STDOUT and STDERR, unless the system() command redirects them.
605 Backticks and open() read B<only> the STDOUT of your command.
607 With any of these, you can change file descriptors before the call:
609 open(STDOUT, ">logfile");
612 or you can use Bourne shell file-descriptor redirection:
614 $output = `$cmd 2>some_file`;
615 open (PIPE, "cmd 2>some_file |");
617 You can also use file-descriptor redirection to make STDERR a
620 $output = `$cmd 2>&1`;
621 open (PIPE, "cmd 2>&1 |");
623 Note that you I<cannot> simply open STDERR to be a dup of STDOUT
624 in your Perl program and avoid calling the shell to do the redirection.
627 open(STDERR, ">&STDOUT");
628 $alloutput = `cmd args`; # stderr still escapes
630 This fails because the open() makes STDERR go to where STDOUT was
631 going at the time of the open(). The backticks then make STDOUT go to
632 a string, but don't change STDERR (which still goes to the old
635 Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
636 backticks, not csh(1)! Details on why Perl's system() and backtick
637 and pipe opens all use the Bourne shell are in
638 http://www.cpan.org/doc/FMTEYEWTK/versus/csh.whynot .
639 To capture a command's STDERR and STDOUT together:
641 $output = `cmd 2>&1`; # either with backticks
642 $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
643 while (<PH>) { } # plus a read
645 To capture a command's STDOUT but discard its STDERR:
647 $output = `cmd 2>/dev/null`; # either with backticks
648 $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
649 while (<PH>) { } # plus a read
651 To capture a command's STDERR but discard its STDOUT:
653 $output = `cmd 2>&1 1>/dev/null`; # either with backticks
654 $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
655 while (<PH>) { } # plus a read
657 To exchange a command's STDOUT and STDERR in order to capture the STDERR
658 but leave its STDOUT to come out our old STDERR:
660 $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
661 $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
662 while (<PH>) { } # plus a read
664 To read both a command's STDOUT and its STDERR separately, it's easiest
665 and safest to redirect them separately to files, and then read from those
666 files when the program is done:
668 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
670 Ordering is important in all these examples. That's because the shell
671 processes file descriptor redirections in strictly left to right order.
673 system("prog args 1>tmpfile 2>&1");
674 system("prog args 2>&1 1>tmpfile");
676 The first command sends both standard out and standard error to the
677 temporary file. The second command sends only the old standard output
678 there, and the old standard error shows up on the old standard out.
680 =head2 Why doesn't open() return an error when a pipe open fails?
682 Because the pipe open takes place in two steps: first Perl calls
683 fork() to start a new process, then this new process calls exec() to
684 run the program you really wanted to open. The first step reports
685 success or failure to your process, so open() can only tell you
686 whether the fork() succeeded or not.
688 To find out if the exec() step succeeded, you have to catch SIGCHLD
689 and wait() to get the exit status. You should also catch SIGPIPE if
690 you're writing to the child--you may not have found out the exec()
691 failed by the time you write. This is documented in L<perlipc>.
693 In some cases, even this won't work. If the second argument to a
694 piped open() contains shell metacharacters, perl fork()s, then exec()s
695 a shell to decode the metacharacters and eventually run the desired
696 program. Now when you call wait(), you only learn whether or not the
697 I<shell> could be successfully started...it's best to avoid shell
700 On systems that follow the spawn() paradigm, open() I<might> do what
701 you expect--unless perl uses a shell to start your command. In this
702 case the fork()/exec() description still applies.
704 =head2 What's wrong with using backticks in a void context?
706 Strictly speaking, nothing. Stylistically speaking, it's not a good
707 way to write maintainable code because backticks have a (potentially
708 humongous) return value, and you're ignoring it. It's may also not be very
709 efficient, because you have to read in all the lines of output, allocate
710 memory for them, and then throw it away. Too often people are lulled
715 And now they think "Hey, I'll just always use backticks to run programs."
716 Bad idea: backticks are for capturing a program's output; the system()
717 function is for running programs.
723 You haven't assigned the output anywhere, so it just wastes memory
724 (for a little while). You forgot to check C<$?> to see whether
725 the program even ran correctly, too. Even if you wrote
727 print `cat /etc/termcap`;
729 this code could and probably should be written as
731 system("cat /etc/termcap") == 0
732 or die "cat program failed!";
734 which will get the output quickly (as it is generated, instead of only
735 at the end) and also check the return value.
737 system() also provides direct control over whether shell wildcard
738 processing may take place, whereas backticks do not.
740 =head2 How can I call backticks without shell processing?
742 This is a bit tricky. Instead of writing
744 @ok = `grep @opts '$search_string' @filenames`;
749 if (open(GREP, "-|")) {
756 exec 'grep', @opts, $search_string, @filenames;
759 Just as with system(), no shell escapes happen when you exec() a list.
760 Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
762 Note that if you're stuck on Microsoft, no solution to this vexing issue
763 is even possible. Even if Perl were to emulate fork(), you'd still
764 be hosed, because Microsoft gives no argc/argv-style API. Their API
765 always reparses from a single string, which is fundamentally wrong,
766 but you're not likely to get the Gods of Redmond to acknowledge this
769 =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
771 Some stdio's set error and eof flags that need clearing. The
772 POSIX module defines clearerr() that you can use. That is the
773 technically correct way to do it. Here are some less reliable
780 Try keeping around the seekpointer and go there, like this:
783 seek(LOG, $where, 0);
787 If that doesn't work, try seeking to a different part of the file and
792 If that doesn't work, try seeking to a different part of
793 the file, reading something, and then seeking back.
797 If that doesn't work, give up on your stdio package and use sysread.
801 =head2 How can I convert my shell script to perl?
803 Learn Perl and rewrite it. Seriously, there's no simple converter.
804 Things that are awkward to do in the shell are easy to do in Perl, and
805 this very awkwardness is what would make a shell->perl converter
806 nigh-on impossible to write. By rewriting it, you'll think about what
807 you're really trying to do, and hopefully will escape the shell's
808 pipeline datastream paradigm, which while convenient for some matters,
809 causes many inefficiencies.
811 =head2 Can I use perl to run a telnet or ftp session?
813 Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
814 CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar
815 will also help for emulating the telnet protocol, but Net::Telnet is
816 quite probably easier to use..
818 If all you want to do is pretend to be telnet but don't need
819 the initial telnet handshaking, then the standard dual-process
820 approach will suffice:
822 use IO::Socket; # new in 5.004
823 $handle = IO::Socket::INET->new('www.perl.com:80')
824 || die "can't connect to port 80 on www.perl.com: $!";
825 $handle->autoflush(1);
826 if (fork()) { # XXX: undef means failure
828 print while <STDIN>; # everything from stdin to socket
830 print while <$handle>; # everything from socket to stdout
835 =head2 How can I write expect in Perl?
837 Once upon a time, there was a library called chat2.pl (part of the
838 standard perl distribution), which never really got finished. If you
839 find it somewhere, I<don't use it>. These days, your best bet is to
840 look at the Expect module available from CPAN, which also requires two
841 other modules from CPAN, IO::Pty and IO::Stty.
843 =head2 Is there a way to hide perl's command line from programs such as "ps"?
845 First of all note that if you're doing this for security reasons (to
846 avoid people seeing passwords, for example) then you should rewrite
847 your program so that critical information is never given as an
848 argument. Hiding the arguments won't make your program completely
851 To actually alter the visible command line, you can assign to the
852 variable $0 as documented in L<perlvar>. This won't work on all
853 operating systems, though. Daemon programs like sendmail place their
856 $0 = "orcus [accepting connections]";
858 =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?
864 In the strictest sense, it can't be done--the script executes as a
865 different process from the shell it was started from. Changes to a
866 process are not reflected in its parent--only in any children
867 created after the change. There is shell magic that may allow you to
868 fake it by eval()ing the script's output in your shell; check out the
869 comp.unix.questions FAQ for details.
873 =head2 How do I close a process's filehandle without waiting for it to complete?
875 Assuming your system supports such things, just send an appropriate signal
876 to the process (see L<perlfunc/"kill">). It's common to first send a TERM
877 signal, wait a little bit, and then send a KILL signal to finish it off.
879 =head2 How do I fork a daemon process?
881 If by daemon process you mean one that's detached (disassociated from
882 its tty), then the following process is reported to work on most
883 Unixish systems. Non-Unix users should check their Your_OS::Process
884 module for other solutions.
890 Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
891 for details. Or better yet, you can just use the POSIX::setsid()
892 function, so you don't have to worry about process groups.
896 Change directory to /
900 Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
905 Background yourself like this:
911 The Proc::Daemon module, available from CPAN, provides a function to
912 perform these actions for you.
914 =head2 How do I find out if I'm running interactively or not?
916 Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
919 if (-t STDIN && -t STDOUT) {
923 On POSIX systems, you can test whether your own process group matches
924 the current process group of your controlling terminal as follows:
926 use POSIX qw/getpgrp tcgetpgrp/;
927 open(TTY, "/dev/tty") or die $!;
928 $tpgrp = tcgetpgrp(fileno(*TTY));
930 if ($tpgrp == $pgrp) {
931 print "foreground\n";
933 print "background\n";
936 =head2 How do I timeout a slow event?
938 Use the alarm() function, probably in conjunction with a signal
939 handler, as documented in L<perlipc/"Signals"> and the section on
940 ``Signals'' in the Camel. You may instead use the more flexible
941 Sys::AlarmCall module available from CPAN.
943 =head2 How do I set CPU limits?
945 Use the BSD::Resource module from CPAN.
947 =head2 How do I avoid zombies on a Unix system?
949 Use the reaper code from L<perlipc/"Signals"> to call wait() when a
950 SIGCHLD is received, or else use the double-fork technique described
953 =head2 How do I use an SQL database?
955 There are a number of excellent interfaces to SQL databases. See the
956 DBD::* modules available from http://www.cpan.org/modules/DBD .
957 A lot of information on this can be found at http://dbi.perl.org/
959 =head2 How do I make a system() exit on control-C?
961 You can't. You need to imitate the system() call (see L<perlipc> for
962 sample code) and then have a signal handler for the INT signal that
963 passes the signal on to the subprocess. Or you can check for it:
966 if ($rc & 127) { die "signal death" }
968 =head2 How do I open a file without blocking?
970 If you're lucky enough to be using a system that supports
971 non-blocking reads (most Unixish systems do), you need only to use the
972 O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
976 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
977 or die "can't open /tmp/somefile: $!":
979 =head2 How do I install a module from CPAN?
981 The easiest way is to have a module also named CPAN do it for you.
982 This module comes with perl version 5.004 and later. To manually install
983 the CPAN module, or any well-behaved CPAN module for that matter, follow
990 Unpack the source into a temporary area.
1010 If your version of perl is compiled without dynamic loading, then you
1011 just need to replace step 3 (B<make>) with B<make perl> and you will
1012 get a new F<perl> binary with your extension linked in.
1014 See L<ExtUtils::MakeMaker> for more details on building extensions.
1015 See also the next question, ``What's the difference between require
1018 =head2 What's the difference between require and use?
1020 Perl offers several different ways to include code from one file into
1021 another. Here are the deltas between the various inclusion constructs:
1023 1) do $file is like eval `cat $file`, except the former
1024 1.1: searches @INC and updates %INC.
1025 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
1027 2) require $file is like do $file, except the former
1028 2.1: checks for redundant loading, skipping already loaded files.
1029 2.2: raises an exception on failure to find, compile, or execute $file.
1031 3) require Module is like require "Module.pm", except the former
1032 3.1: translates each "::" into your system's directory separator.
1033 3.2: primes the parser to disambiguate class Module as an indirect object.
1035 4) use Module is like require Module, except the former
1036 4.1: loads the module at compile time, not run-time.
1037 4.2: imports symbols and semantics from that package to the current one.
1039 In general, you usually want C<use> and a proper Perl module.
1041 =head2 How do I keep my own module/library directory?
1043 When you build modules, use the PREFIX option when generating
1046 perl Makefile.PL PREFIX=/u/mydir/perl
1048 then either set the PERL5LIB environment variable before you run
1049 scripts that use the modules/libraries (see L<perlrun>) or say
1051 use lib '/u/mydir/perl';
1053 This is almost the same as
1056 unshift(@INC, '/u/mydir/perl');
1059 except that the lib module checks for machine-dependent subdirectories.
1060 See Perl's L<lib> for more information.
1062 =head2 How do I add the directory my program lives in to the module/library search path?
1065 use lib "$FindBin::Bin";
1066 use your_own_modules;
1068 =head2 How do I add a directory to my include path at runtime?
1070 Here are the suggested ways of modifying your include path:
1072 the PERLLIB environment variable
1073 the PERL5LIB environment variable
1074 the perl -Idir command line flag
1075 the use lib pragma, as in
1076 use lib "$ENV{HOME}/myown_perllib";
1078 The latter is particularly useful because it knows about machine
1079 dependent architectures. The lib.pm pragmatic module was first
1080 included with the 5.002 release of Perl.
1082 =head2 What is socket.ph and where do I get it?
1084 It's a perl4-style file defining values for system networking
1085 constants. Sometimes it is built using h2ph when Perl is installed,
1086 but other times it is not. Modern programs C<use Socket;> instead.
1088 =head1 AUTHOR AND COPYRIGHT
1090 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1091 All rights reserved.
1093 This documentation is free; you can redistribute it and/or modify it
1094 under the same terms as Perl itself.
1096 Irrespective of its distribution, all code examples in this file
1097 are hereby placed into the public domain. You are permitted and
1098 encouraged to use this code in your own programs for fun
1099 or for profit as you see fit. A simple comment in the code giving
1100 credit would be courteous but is not required.