3 perlfaq8 - System Interaction ($Revision: 8539 $)
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 as examples in other answers
57 in this section of the perlfaq.
59 =head2 How do I print something out in color?
61 In general, you don't, because you don't know whether
62 the recipient has a color-aware display device. If you
63 know that they have an ANSI terminal that understands
64 color, you can use the Term::ANSIColor module from CPAN:
67 print color("red"), "Stop!\n", color("reset");
68 print color("green"), "Go!\n", color("reset");
72 use Term::ANSIColor qw(:constants);
73 print RED, "Stop!\n", RESET;
74 print GREEN, "Go!\n", RESET;
76 =head2 How do I read just one key without waiting for a return key?
78 Controlling input buffering is a remarkably system-dependent matter.
79 On many systems, you can just use the B<stty> command as shown in
80 L<perlfunc/getc>, but as you see, that's already getting you into
83 open(TTY, "+</dev/tty") or die "no tty: $!";
84 system "stty cbreak </dev/tty >/dev/tty 2>&1";
85 $key = getc(TTY); # perhaps this works
87 sysread(TTY, $key, 1); # probably this does
88 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
90 The Term::ReadKey module from CPAN offers an easy-to-use interface that
91 should be more efficient than shelling out to B<stty> for each key.
92 It even includes limited support for Windows.
99 However, using the code requires that you have a working C compiler
100 and can use it to build and install a CPAN module. Here's a solution
101 using the standard POSIX module, which is already on your systems
102 (assuming your system supports POSIX).
107 And here's the HotKey module, which hides the somewhat mystifying calls
108 to manipulate the POSIX termios structures.
114 @EXPORT = qw(cbreak cooked readkey);
117 use POSIX qw(:termios_h);
118 my ($term, $oterm, $echo, $noecho, $fd_stdin);
120 $fd_stdin = fileno(STDIN);
121 $term = POSIX::Termios->new();
122 $term->getattr($fd_stdin);
123 $oterm = $term->getlflag();
125 $echo = ECHO | ECHOK | ICANON;
126 $noecho = $oterm & ~$echo;
129 $term->setlflag($noecho); # ok, so i don't want echo either
130 $term->setcc(VTIME, 1);
131 $term->setattr($fd_stdin, TCSANOW);
135 $term->setlflag($oterm);
136 $term->setcc(VTIME, 0);
137 $term->setattr($fd_stdin, TCSANOW);
143 sysread(STDIN, $key, 1);
152 =head2 How do I check whether input is ready on the keyboard?
154 The easiest way to do this is to read a key in nonblocking mode with the
155 Term::ReadKey module from CPAN, passing it an argument of -1 to indicate
162 if (defined ($char = ReadKey(-1)) ) {
163 # input was waiting and it was $char
165 # no input was waiting
168 ReadMode('normal'); # restore normal tty settings
170 =head2 How do I clear the screen?
172 If you only have do so infrequently, use C<system>:
176 If you have to do this a lot, save the clear string
177 so you can print it 100 times without calling a program
180 $clear_string = `clear`;
183 If you're planning on doing other screen manipulations, like cursor
184 positions, etc, you might wish to use Term::Cap module:
187 $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
188 $clear_string = $terminal->Tputs('cl');
190 =head2 How do I get the screen size?
192 If you have Term::ReadKey module installed from CPAN,
193 you can use it to fetch the width and height in characters
197 ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();
199 This is more portable than the raw C<ioctl>, but not as
202 require 'sys/ioctl.ph';
203 die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
204 open(TTY, "+</dev/tty") or die "No tty: $!";
205 unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
206 die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
208 ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
209 print "(row,col) = ($row,$col)";
210 print " (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
213 =head2 How do I ask the user for a password?
215 (This question has nothing to do with the web. See a different
218 There's an example of this in L<perlfunc/crypt>). First, you put the
219 terminal into "no echo" mode, then just read the password normally.
220 You may do this with an old-style ioctl() function, POSIX terminal
221 control (see L<POSIX> or its documentation the Camel Book), or a call
222 to the B<stty> program, with varying degrees of portability.
224 You can also do this for most systems using the Term::ReadKey module
225 from CPAN, which is easier to use and in theory more portable.
230 $password = ReadLine(0);
232 =head2 How do I read and write the serial port?
234 This depends on which operating system your program is running on. In
235 the case of Unix, the serial ports will be accessible through files in
236 /dev; on other systems, device names will doubtless differ.
237 Several problem areas common to all device interaction are the
244 Your system may use lockfiles to control multiple access. Make sure
245 you follow the correct protocol. Unpredictable behavior can result
246 from multiple processes reading from one device.
250 If you expect to use both read and write operations on the device,
251 you'll have to open it for update (see L<perlfunc/"open"> for
252 details). You may wish to open it without running the risk of
253 blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
254 Fcntl module (part of the standard perl distribution). See
255 L<perlfunc/"sysopen"> for more on this approach.
259 Some devices will be expecting a "\r" at the end of each line rather
260 than a "\n". In some ports of perl, "\r" and "\n" are different from
261 their usual (Unix) ASCII values of "\012" and "\015". You may have to
262 give the numeric values you want directly, using octal ("\015"), hex
263 ("0x0D"), or as a control-character specification ("\cM").
265 print DEV "atv1\012"; # wrong, for some devices
266 print DEV "atv1\015"; # right, for some devices
268 Even though with normal text files a "\n" will do the trick, there is
269 still no unified scheme for terminating a line that is portable
270 between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line
271 ends with "\015\012", and strip what you don't need from the output.
272 This applies especially to socket I/O and autoflushing, discussed
275 =item flushing output
277 If you expect characters to get to your device when you print() them,
278 you'll want to autoflush that filehandle. You can use select()
279 and the C<$|> variable to control autoflushing (see L<perlvar/$E<verbar>>
280 and L<perlfunc/select>, or L<perlfaq5>, "How do I flush/unbuffer an
281 output filehandle? Why must I do this?"):
287 You'll also see code that does this without a temporary variable, as in
289 select((select(DEV), $| = 1)[0]);
291 Or if you don't mind pulling in a few thousand lines
292 of code just because you're afraid of a little $| variable:
297 As mentioned in the previous item, this still doesn't work when using
298 socket I/O between Unix and Macintosh. You'll need to hard code your
299 line terminators, in that case.
301 =item non-blocking input
303 If you are doing a blocking read() or sysread(), you'll have to
304 arrange for an alarm handler to provide a timeout (see
305 L<perlfunc/alarm>). If you have a non-blocking open, you'll likely
306 have a non-blocking read, which means you may have to use a 4-arg
307 select() to determine whether I/O is ready on that device (see
308 L<perlfunc/"select">.
312 While trying to read from his caller-id box, the notorious Jamie Zawinski
313 C<< <jwz@netscape.com> >>, after much gnashing of teeth and fighting with sysread,
314 sysopen, POSIX's tcgetattr business, and various other functions that
315 go bump in the night, finally came up with this:
319 my $stty = `/bin/stty -g`;
320 open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
321 # starting cu hoses /dev/tty's stty settings, even when it has
322 # been opened on a pipe...
323 system("/bin/stty $stty");
326 if ( !m/^Connected/ ) {
327 print STDERR "$0: cu printed `$_' instead of `Connected'\n";
331 =head2 How do I decode encrypted password files?
333 You spend lots and lots of money on dedicated hardware, but this is
334 bound to get you talked about.
336 Seriously, you can't if they are Unix password files--the Unix
337 password system employs one-way encryption. It's more like hashing than
338 encryption. The best you can check is whether something else hashes to
339 the same string. You can't turn a hash back into the original string.
341 can forcibly (and intelligently) try to guess passwords, but don't
342 (can't) guarantee quick success.
344 If you're worried about users selecting bad passwords, you should
345 proactively check when they try to change their password (by modifying
346 passwd(1), for example).
348 =head2 How do I start a process in the background?
350 Several modules can start other processes that do not block
351 your Perl program. You can use IPC::Open3, Parallel::Jobs,
352 IPC::Run, and some of the POE modules. See CPAN for more
359 or you could use fork as documented in L<perlfunc/"fork">, with
360 further examples in L<perlipc>. Some things to be aware of, if you're
361 on a Unix-like system:
365 =item STDIN, STDOUT, and STDERR are shared
367 Both the main process and the backgrounded one (the "child" process)
368 share the same STDIN, STDOUT and STDERR filehandles. If both try to
369 access them at once, strange things can happen. You may want to close
370 or reopen these for the child. You can get around this with
371 C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this
372 means that the child process cannot outlive the parent.
376 You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
377 SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
378 sent when you write to a filehandle whose child process has closed (an
379 untrapped SIGPIPE can cause your program to silently die). This is
380 not an issue with C<system("cmd&")>.
384 You have to be prepared to "reap" the child process when it finishes.
386 $SIG{CHLD} = sub { wait };
388 $SIG{CHLD} = 'IGNORE';
390 You can also use a double fork. You immediately wait() for your
391 first child, and the init daemon will wait() for your grandchild once
394 unless ($pid = fork) {
396 exec "what you really wanna do";
403 See L<perlipc/"Signals"> for other examples of code to do this.
404 Zombies are not an issue with C<system("prog &")>.
408 =head2 How do I trap control characters/signals?
410 You don't actually "trap" a control character. Instead, that character
411 generates a signal which is sent to your terminal's currently
412 foregrounded process group, which you then trap in your process.
413 Signals are documented in L<perlipc/"Signals"> and the
414 section on "Signals" in the Camel.
416 You can set the values of the %SIG hash to be the functions you want
417 to handle the signal. After perl catches the signal, it looks in %SIG
418 for a key with the same name as the signal, then calls the subroutine
421 # as an anonymous subroutine
423 $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };
425 # or a reference to a function
429 # or the name of the function as a string
433 Perl versions before 5.8 had in its C source code signal handlers which
434 would catch the signal and possibly run a Perl function that you had set
435 in %SIG. This violated the rules of signal handling at that level
436 causing perl to dump core. Since version 5.8.0, perl looks at %SIG
437 *after* the signal has been caught, rather than while it is being caught.
438 Previous versions of this answer were incorrect.
440 =head2 How do I modify the shadow password file on a Unix system?
442 If perl was installed correctly and your shadow library was written
443 properly, the getpw*() functions described in L<perlfunc> should in
444 theory provide (read-only) access to entries in the shadow password
445 file. To change the file, make a new shadow password file (the format
446 varies from system to system--see L<passwd> for specifics) and use
447 pwd_mkdb(8) to install it (see L<pwd_mkdb> for more details).
449 =head2 How do I set the time and date?
451 Assuming you're running under sufficient permissions, you should be
452 able to set the system-wide date and time by running the date(1)
453 program. (There is no way to set the time and date on a per-process
454 basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
455 the VMS equivalent is C<set time>.
457 However, if all you want to do is change your time zone, you can
458 probably get away with setting an environment variable:
460 $ENV{TZ} = "MST7MDT"; # unixish
461 $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
462 system "trn comp.lang.perl.misc";
464 =head2 How can I sleep() or alarm() for under a second?
466 If you want finer granularity than the 1 second that the sleep()
467 function provides, the easiest way is to use the select() function as
468 documented in L<perlfunc/"select">. Try the Time::HiRes and
469 the BSD::Itimer modules (available from CPAN, and starting from
470 Perl 5.8 Time::HiRes is part of the standard distribution).
472 =head2 How can I measure time under a second?
474 In general, you may not be able to. The Time::HiRes module (available
475 from CPAN, and starting from Perl 5.8 part of the standard distribution)
476 provides this functionality for some systems.
478 If your system supports both the syscall() function in Perl as well as
479 a system call like gettimeofday(2), then you may be able to do
482 require 'sys/syscall.ph';
486 $done = $start = pack($TIMEVAL_T, ());
488 syscall(&SYS_gettimeofday, $start, 0) != -1
489 or die "gettimeofday: $!";
491 ##########################
492 # DO YOUR OPERATION HERE #
493 ##########################
495 syscall( &SYS_gettimeofday, $done, 0) != -1
496 or die "gettimeofday: $!";
498 @start = unpack($TIMEVAL_T, $start);
499 @done = unpack($TIMEVAL_T, $done);
502 for ($done[1], $start[1]) { $_ /= 1_000_000 }
504 $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
506 ($start[0] + $start[1] );
508 =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
510 Release 5 of Perl added the END block, which can be used to simulate
511 atexit(). Each package's END block is called when the program or
512 thread ends (see L<perlmod> manpage for more details).
514 For example, you can use this to make sure your filter program
515 managed to finish its output without filling up the disk:
518 close(STDOUT) || die "stdout close failed: $!";
521 The END block isn't called when untrapped signals kill the program,
522 though, so if you use END blocks you should also use
524 use sigtrap qw(die normal-signals);
526 Perl's exception-handling mechanism is its eval() operator. You can
527 use eval() as setjmp and die() as longjmp. For details of this, see
528 the section on signals, especially the time-out handler for a blocking
529 flock() in L<perlipc/"Signals"> or the section on "Signals" in
532 If exception handling is all you're interested in, try the
533 exceptions.pl library (part of the standard perl distribution).
535 If you want the atexit() syntax (and an rmexit() as well), try the
536 AtExit module available from CPAN.
538 =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
540 Some Sys-V based systems, notably Solaris 2.X, redefined some of the
541 standard socket constants. Since these were constant across all
542 architectures, they were often hardwired into perl code. The proper
543 way to deal with this is to "use Socket" to get the correct values.
545 Note that even though SunOS and Solaris are binary compatible, these
546 values are different. Go figure.
548 =head2 How can I call my system's unique C functions from Perl?
550 In most cases, you write an external module to do it--see the answer
551 to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
552 However, if the function is a system call, and your system supports
553 syscall(), you can use the syscall function (documented in
556 Remember to check the modules that came with your distribution, and
557 CPAN as well--someone may already have written a module to do it. On
558 Windows, try Win32::API. On Macs, try Mac::Carbon. If no module
559 has an interface to the C function, you can inline a bit of C in your
560 Perl source with Inline::C.
562 =head2 Where do I get the include files to do ioctl() or syscall()?
564 Historically, these would be generated by the h2ph tool, part of the
565 standard perl distribution. This program converts cpp(1) directives
566 in C header files to files containing subroutine definitions, like
567 &SYS_getitimer, which you can use as arguments to your functions.
568 It doesn't work perfectly, but it usually gets most of the job done.
569 Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
570 but the hard ones like F<ioctl.h> nearly always need to hand-edited.
571 Here's how to install the *.ph files:
577 If your system supports dynamic loading, for reasons of portability and
578 sanity you probably ought to use h2xs (also part of the standard perl
579 distribution). This tool converts C header files to Perl extensions.
580 See L<perlxstut> for how to get started with h2xs.
582 If your system doesn't support dynamic loading, you still probably
583 ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
584 more information (in brief, just use B<make perl> instead of a plain
585 B<make> to rebuild perl with a new static extension).
587 =head2 Why do setuid perl scripts complain about kernel problems?
589 Some operating systems have bugs in the kernel that make setuid
590 scripts inherently insecure. Perl gives you a number of options
591 (described in L<perlsec>) to work around such systems.
593 =head2 How can I open a pipe both to and from a command?
595 The IPC::Open2 module (part of the standard perl distribution) is an
596 easy-to-use approach that internally uses pipe(), fork(), and exec() to do
597 the job. Make sure you read the deadlock warnings in its documentation,
598 though (see L<IPC::Open2>). See
599 L<perlipc/"Bidirectional Communication with Another Process"> and
600 L<perlipc/"Bidirectional Communication with Yourself">
602 You may also use the IPC::Open3 module (part of the standard perl
603 distribution), but be warned that it has a different order of
604 arguments from IPC::Open2 (see L<IPC::Open3>).
606 =head2 Why can't I get the output of a command with system()?
608 You're confusing the purpose of system() and backticks (``). system()
609 runs a command and returns exit status information (as a 16 bit value:
610 the low 7 bits are the signal the process died from, if any, and
611 the high 8 bits are the actual exit value). Backticks (``) run a
612 command and return what it sent to STDOUT.
614 $exit_status = system("mail-users");
615 $output_string = `ls`;
617 =head2 How can I capture STDERR from an external command?
619 There are three basic ways of running external commands:
621 system $cmd; # using system()
622 $output = `$cmd`; # using backticks (``)
623 open (PIPE, "cmd |"); # using open()
625 With system(), both STDOUT and STDERR will go the same place as the
626 script's STDOUT and STDERR, unless the system() command redirects them.
627 Backticks and open() read B<only> the STDOUT of your command.
629 You can also use the open3() function from IPC::Open3. Benjamin
630 Goldberg provides some sample code:
632 To capture a program's STDOUT, but discard its STDERR:
636 use Symbol qw(gensym);
637 open(NULL, ">", File::Spec->devnull);
638 my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
642 To capture a program's STDERR, but discard its STDOUT:
646 use Symbol qw(gensym);
647 open(NULL, ">", File::Spec->devnull);
648 my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
652 To capture a program's STDERR, and let its STDOUT go to our own STDERR:
655 use Symbol qw(gensym);
656 my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
660 To read both a command's STDOUT and its STDERR separately, you can
661 redirect them to temp files, let the command run, then read the temp
665 use Symbol qw(gensym);
667 local *CATCHOUT = IO::File->new_tmpfile;
668 local *CATCHERR = IO::File->new_tmpfile;
669 my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
671 seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
672 while( <CATCHOUT> ) {}
673 while( <CATCHERR> ) {}
675 But there's no real need for *both* to be tempfiles... the following
676 should work just as well, without deadlocking:
679 use Symbol qw(gensym);
681 local *CATCHERR = IO::File->new_tmpfile;
682 my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
683 while( <CATCHOUT> ) {}
686 while( <CATCHERR> ) {}
688 And it'll be faster, too, since we can begin processing the program's
689 stdout immediately, rather than waiting for the program to finish.
691 With any of these, you can change file descriptors before the call:
693 open(STDOUT, ">logfile");
696 or you can use Bourne shell file-descriptor redirection:
698 $output = `$cmd 2>some_file`;
699 open (PIPE, "cmd 2>some_file |");
701 You can also use file-descriptor redirection to make STDERR a
704 $output = `$cmd 2>&1`;
705 open (PIPE, "cmd 2>&1 |");
707 Note that you I<cannot> simply open STDERR to be a dup of STDOUT
708 in your Perl program and avoid calling the shell to do the redirection.
711 open(STDERR, ">&STDOUT");
712 $alloutput = `cmd args`; # stderr still escapes
714 This fails because the open() makes STDERR go to where STDOUT was
715 going at the time of the open(). The backticks then make STDOUT go to
716 a string, but don't change STDERR (which still goes to the old
719 Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
720 backticks, not csh(1)! Details on why Perl's system() and backtick
721 and pipe opens all use the Bourne shell are in the
722 F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To
723 Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To
724 capture a command's STDERR and STDOUT together:
726 $output = `cmd 2>&1`; # either with backticks
727 $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
728 while (<PH>) { } # plus a read
730 To capture a command's STDOUT but discard its STDERR:
732 $output = `cmd 2>/dev/null`; # either with backticks
733 $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
734 while (<PH>) { } # plus a read
736 To capture a command's STDERR but discard its STDOUT:
738 $output = `cmd 2>&1 1>/dev/null`; # either with backticks
739 $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
740 while (<PH>) { } # plus a read
742 To exchange a command's STDOUT and STDERR in order to capture the STDERR
743 but leave its STDOUT to come out our old STDERR:
745 $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
746 $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
747 while (<PH>) { } # plus a read
749 To read both a command's STDOUT and its STDERR separately, it's easiest
750 to redirect them separately to files, and then read from those files
751 when the program is done:
753 system("program args 1>program.stdout 2>program.stderr");
755 Ordering is important in all these examples. That's because the shell
756 processes file descriptor redirections in strictly left to right order.
758 system("prog args 1>tmpfile 2>&1");
759 system("prog args 2>&1 1>tmpfile");
761 The first command sends both standard out and standard error to the
762 temporary file. The second command sends only the old standard output
763 there, and the old standard error shows up on the old standard out.
765 =head2 Why doesn't open() return an error when a pipe open fails?
767 If the second argument to a piped open() contains shell
768 metacharacters, perl fork()s, then exec()s a shell to decode the
769 metacharacters and eventually run the desired program. If the program
770 couldn't be run, it's the shell that gets the message, not Perl. All
771 your Perl program can find out is whether the shell itself could be
772 successfully started. You can still capture the shell's STDERR and
773 check it for error messages. See L<"How can I capture STDERR from an
774 external command?"> elsewhere in this document, or use the
777 If there are no shell metacharacters in the argument of open(), Perl
778 runs the command directly, without using the shell, and can correctly
779 report whether the command started.
781 =head2 What's wrong with using backticks in a void context?
783 Strictly speaking, nothing. Stylistically speaking, it's not a good
784 way to write maintainable code. Perl has several operators for
785 running external commands. Backticks are one; they collect the output
786 from the command for use in your program. The C<system> function is
787 another; it doesn't do this.
789 Writing backticks in your program sends a clear message to the readers
790 of your code that you wanted to collect the output of the command.
791 Why send a clear message that isn't true?
797 You forgot to check C<$?> to see whether the program even ran
798 correctly. Even if you wrote
800 print `cat /etc/termcap`;
802 this code could and probably should be written as
804 system("cat /etc/termcap") == 0
805 or die "cat program failed!";
807 which will echo the cat command's output as it is generated, instead
808 of waiting until the program has completed to print it out. It also
809 checks the return value.
811 C<system> also provides direct control over whether shell wildcard
812 processing may take place, whereas backticks do not.
814 =head2 How can I call backticks without shell processing?
816 This is a bit tricky. You can't simply write the command
819 @ok = `grep @opts '$search_string' @filenames`;
821 As of Perl 5.8.0, you can use open() with multiple arguments.
822 Just like the list forms of system() and exec(), no shell
825 open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
832 if (open(GREP, "-|")) {
839 exec 'grep', @opts, $search_string, @filenames;
842 Just as with system(), no shell escapes happen when you exec() a list.
843 Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
845 Note that if you're use Microsoft, no solution to this vexing issue
846 is even possible. Even if Perl were to emulate fork(), you'd still
847 be stuck, because Microsoft does not have a argc/argv-style API.
849 =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
851 Some stdio's set error and eof flags that need clearing. The
852 POSIX module defines clearerr() that you can use. That is the
853 technically correct way to do it. Here are some less reliable
860 Try keeping around the seekpointer and go there, like this:
863 seek(LOG, $where, 0);
867 If that doesn't work, try seeking to a different part of the file and
872 If that doesn't work, try seeking to a different part of
873 the file, reading something, and then seeking back.
877 If that doesn't work, give up on your stdio package and use sysread.
881 =head2 How can I convert my shell script to perl?
883 Learn Perl and rewrite it. Seriously, there's no simple converter.
884 Things that are awkward to do in the shell are easy to do in Perl, and
885 this very awkwardness is what would make a shell->perl converter
886 nigh-on impossible to write. By rewriting it, you'll think about what
887 you're really trying to do, and hopefully will escape the shell's
888 pipeline datastream paradigm, which while convenient for some matters,
889 causes many inefficiencies.
891 =head2 Can I use perl to run a telnet or ftp session?
893 Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
894 CPAN). http://www.cpan.org/scripts/netstuff/telnet.emul.shar
895 will also help for emulating the telnet protocol, but Net::Telnet is
896 quite probably easier to use..
898 If all you want to do is pretend to be telnet but don't need
899 the initial telnet handshaking, then the standard dual-process
900 approach will suffice:
902 use IO::Socket; # new in 5.004
903 $handle = IO::Socket::INET->new('www.perl.com:80')
904 or die "can't connect to port 80 on www.perl.com: $!";
905 $handle->autoflush(1);
906 if (fork()) { # XXX: undef means failure
908 print while <STDIN>; # everything from stdin to socket
910 print while <$handle>; # everything from socket to stdout
915 =head2 How can I write expect in Perl?
917 Once upon a time, there was a library called chat2.pl (part of the
918 standard perl distribution), which never really got finished. If you
919 find it somewhere, I<don't use it>. These days, your best bet is to
920 look at the Expect module available from CPAN, which also requires two
921 other modules from CPAN, IO::Pty and IO::Stty.
923 =head2 Is there a way to hide perl's command line from programs such as "ps"?
925 First of all note that if you're doing this for security reasons (to
926 avoid people seeing passwords, for example) then you should rewrite
927 your program so that critical information is never given as an
928 argument. Hiding the arguments won't make your program completely
931 To actually alter the visible command line, you can assign to the
932 variable $0 as documented in L<perlvar>. This won't work on all
933 operating systems, though. Daemon programs like sendmail place their
936 $0 = "orcus [accepting connections]";
938 =head2 I {changed directory, modified my environment} in a perl script. How come the change disappeared when I exited the script? How do I get my changes to be visible?
944 In the strictest sense, it can't be done--the script executes as a
945 different process from the shell it was started from. Changes to a
946 process are not reflected in its parent--only in any children
947 created after the change. There is shell magic that may allow you to
948 fake it by eval()ing the script's output in your shell; check out the
949 comp.unix.questions FAQ for details.
953 =head2 How do I close a process's filehandle without waiting for it to complete?
955 Assuming your system supports such things, just send an appropriate signal
956 to the process (see L<perlfunc/"kill">). It's common to first send a TERM
957 signal, wait a little bit, and then send a KILL signal to finish it off.
959 =head2 How do I fork a daemon process?
961 If by daemon process you mean one that's detached (disassociated from
962 its tty), then the following process is reported to work on most
963 Unixish systems. Non-Unix users should check their Your_OS::Process
964 module for other solutions.
970 Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty>
971 for details. Or better yet, you can just use the POSIX::setsid()
972 function, so you don't have to worry about process groups.
976 Change directory to /
980 Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
985 Background yourself like this:
991 The Proc::Daemon module, available from CPAN, provides a function to
992 perform these actions for you.
994 =head2 How do I find out if I'm running interactively or not?
996 Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
999 if (-t STDIN && -t STDOUT) {
1003 On POSIX systems, you can test whether your own process group matches
1004 the current process group of your controlling terminal as follows:
1006 use POSIX qw/getpgrp tcgetpgrp/;
1008 # Some POSIX systems, such as Linux, can be
1009 # without a /dev/tty at boot time.
1010 if (!open(TTY, "/dev/tty")) {
1013 $tpgrp = tcgetpgrp(fileno(*TTY));
1015 if ($tpgrp == $pgrp) {
1016 print "foreground\n";
1018 print "background\n";
1022 =head2 How do I timeout a slow event?
1024 Use the alarm() function, probably in conjunction with a signal
1025 handler, as documented in L<perlipc/"Signals"> and the section on
1026 "Signals" in the Camel. You may instead use the more flexible
1027 Sys::AlarmCall module available from CPAN.
1029 The alarm() function is not implemented on all versions of Windows.
1030 Check the documentation for your specific version of Perl.
1032 =head2 How do I set CPU limits?
1034 Use the BSD::Resource module from CPAN.
1036 =head2 How do I avoid zombies on a Unix system?
1038 Use the reaper code from L<perlipc/"Signals"> to call wait() when a
1039 SIGCHLD is received, or else use the double-fork technique described
1040 in L<perlfaq8/"How do I start a process in the background?">.
1042 =head2 How do I use an SQL database?
1044 The DBI module provides an abstract interface to most database
1045 servers and types, including Oracle, DB2, Sybase, mysql, Postgresql,
1046 ODBC, and flat files. The DBI module accesses each database type
1047 through a database driver, or DBD. You can see a complete list of
1048 available drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ .
1049 You can read more about DBI on http://dbi.perl.org .
1051 Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc,
1052 and others found on CPAN Search: http://search.cpan.org .
1054 =head2 How do I make a system() exit on control-C?
1056 You can't. You need to imitate the system() call (see L<perlipc> for
1057 sample code) and then have a signal handler for the INT signal that
1058 passes the signal on to the subprocess. Or you can check for it:
1061 if ($rc & 127) { die "signal death" }
1063 =head2 How do I open a file without blocking?
1065 If you're lucky enough to be using a system that supports
1066 non-blocking reads (most Unixish systems do), you need only to use the
1067 O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
1071 sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
1072 or die "can't open /foo/somefile: $!":
1074 =head2 How do I tell the difference between errors from the shell and perl?
1076 (answer contributed by brian d foy, C<< <bdfoy@cpan.org> >>
1078 When you run a Perl script, something else is running the script for you,
1079 and that something else may output error messages. The script might
1080 emit its own warnings and error messages. Most of the time you cannot
1083 You probably cannot fix the thing that runs perl, but you can change how
1084 perl outputs its warnings by defining a custom warning and die functions.
1086 Consider this script, which has an error you may not notice immediately.
1088 #!/usr/locl/bin/perl
1090 print "Hello World\n";
1092 I get an error when I run this from my shell (which happens to be
1093 bash). That may look like perl forgot it has a print() function,
1094 but my shebang line is not the path to perl, so the shell runs the
1095 script, and I get the error.
1098 ./test: line 3: print: command not found
1100 A quick and dirty fix involves a little bit of code, but this may be all
1101 you need to figure out the problem.
1106 $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
1107 $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1};
1114 The perl message comes out with "Perl" in front. The BEGIN block
1115 works at compile time so all of the compilation errors and warnings
1116 get the "Perl:" prefix too.
1118 Perl: Useless use of division (/) in void context at ./test line 9.
1119 Perl: Name "main::a" used only once: possible typo at ./test line 8.
1120 Perl: Name "main::x" used only once: possible typo at ./test line 9.
1121 Perl: Use of uninitialized value in addition (+) at ./test line 8.
1122 Perl: Use of uninitialized value in division (/) at ./test line 9.
1123 Perl: Illegal division by zero at ./test line 9.
1124 Perl: Illegal division by zero at -e line 3.
1126 If I don't see that "Perl:", it's not from perl.
1128 You could also just know all the perl errors, and although there are
1129 some people who may know all of them, you probably don't. However, they
1130 all should be in the perldiag manpage. If you don't find the error in
1131 there, it probably isn't a perl error.
1133 Looking up every message is not the easiest way, so let perl to do it
1134 for you. Use the diagnostics pragma with turns perl's normal messages
1135 into longer discussions on the topic.
1139 If you don't get a paragraph or two of expanded discussion, it
1140 might not be perl's message.
1142 =head2 How do I install a module from CPAN?
1144 The easiest way is to have a module also named CPAN do it for you.
1145 This module comes with perl version 5.004 and later.
1147 $ perl -MCPAN -e shell
1149 cpan shell -- CPAN exploration and modules installation (v1.59_54)
1150 ReadLine support enabled
1152 cpan> install Some::Module
1154 To manually install the CPAN module, or any well-behaved CPAN module
1155 for that matter, follow these steps:
1161 Unpack the source into a temporary area.
1181 If your version of perl is compiled without dynamic loading, then you
1182 just need to replace step 3 (B<make>) with B<make perl> and you will
1183 get a new F<perl> binary with your extension linked in.
1185 See L<ExtUtils::MakeMaker> for more details on building extensions.
1186 See also the next question, "What's the difference between require
1189 =head2 What's the difference between require and use?
1191 Perl offers several different ways to include code from one file into
1192 another. Here are the deltas between the various inclusion constructs:
1194 1) do $file is like eval `cat $file`, except the former
1195 1.1: searches @INC and updates %INC.
1196 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
1198 2) require $file is like do $file, except the former
1199 2.1: checks for redundant loading, skipping already loaded files.
1200 2.2: raises an exception on failure to find, compile, or execute $file.
1202 3) require Module is like require "Module.pm", except the former
1203 3.1: translates each "::" into your system's directory separator.
1204 3.2: primes the parser to disambiguate class Module as an indirect object.
1206 4) use Module is like require Module, except the former
1207 4.1: loads the module at compile time, not run-time.
1208 4.2: imports symbols and semantics from that package to the current one.
1210 In general, you usually want C<use> and a proper Perl module.
1212 =head2 How do I keep my own module/library directory?
1214 When you build modules, use the PREFIX and LIB options when generating
1217 perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
1219 then either set the PERL5LIB environment variable before you run
1220 scripts that use the modules/libraries (see L<perlrun>) or say
1222 use lib '/mydir/perl/lib';
1224 This is almost the same as
1227 unshift(@INC, '/mydir/perl/lib');
1230 except that the lib module checks for machine-dependent subdirectories.
1231 See Perl's L<lib> for more information.
1233 =head2 How do I add the directory my program lives in to the module/library search path?
1236 use lib "$FindBin::Bin";
1237 use your_own_modules;
1239 =head2 How do I add a directory to my include path (@INC) at runtime?
1241 Here are the suggested ways of modifying your include path:
1243 the PERLLIB environment variable
1244 the PERL5LIB environment variable
1245 the perl -Idir command line flag
1246 the use lib pragma, as in
1247 use lib "$ENV{HOME}/myown_perllib";
1249 The latter is particularly useful because it knows about machine
1250 dependent architectures. The lib.pm pragmatic module was first
1251 included with the 5.002 release of Perl.
1253 =head2 What is socket.ph and where do I get it?
1255 It's a perl4-style file defining values for system networking
1256 constants. Sometimes it is built using h2ph when Perl is installed,
1257 but other times it is not. Modern programs C<use Socket;> instead.
1261 Revision: $Revision: 8539 $
1263 Date: $Date: 2007-01-11 00:07:14 +0100 (Thu, 11 Jan 2007) $
1265 See L<perlfaq> for source control details and availability.
1267 =head1 AUTHOR AND COPYRIGHT
1269 Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and
1270 other authors as noted. All rights reserved.
1272 This documentation is free; you can redistribute it and/or modify it
1273 under the same terms as Perl itself.
1275 Irrespective of its distribution, all code examples in this file
1276 are hereby placed into the public domain. You are permitted and
1277 encouraged to use this code in your own programs for fun
1278 or for profit as you see fit. A simple comment in the code giving
1279 credit would be courteous but is not required.