3 perlfaq8 - System Interaction ($Revision: 1.25 $, $Date: 2005/06/04 04:12:40 $)
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/$E<verbar>>
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?
349 Several modules can start other processes that do not block
350 your Perl program. You can use IPC::Open3, Parallel::Jobs,
351 IPC::Run, and some of the POE modules. See CPAN for more
358 or you could use fork as documented in L<perlfunc/"fork">, with
359 further examples in L<perlipc>. Some things to be aware of, if you're
360 on a Unix-like system:
364 =item STDIN, STDOUT, and STDERR are shared
366 Both the main process and the backgrounded one (the "child" process)
367 share the same STDIN, STDOUT and STDERR filehandles. If both try to
368 access them at once, strange things can happen. You may want to close
369 or reopen these for the child. You can get around this with
370 C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this
371 means that the child process cannot outlive the parent.
375 You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
376 SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
377 sent when you write to a filehandle whose child process has closed (an
378 untrapped SIGPIPE can cause your program to silently die). This is
379 not an issue with C<system("cmd&")>.
383 You have to be prepared to "reap" the child process when it finishes.
385 $SIG{CHLD} = sub { wait };
387 $SIG{CHLD} = 'IGNORE';
389 You can also use a double fork. You immediately wait() for your
390 first child, and the init daemon will wait() for your grandchild once
393 unless ($pid = fork) {
395 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.
441 =head2 How do I modify the shadow password file on a Unix system?
443 If perl was installed correctly and your shadow library was written
444 properly, the getpw*() functions described in L<perlfunc> should in
445 theory provide (read-only) access to entries in the shadow password
446 file. To change the file, make a new shadow password file (the format
447 varies from system to system--see L<passwd> for specifics) and use
448 pwd_mkdb(8) to install it (see L<pwd_mkdb> for more details).
450 =head2 How do I set the time and date?
452 Assuming you're running under sufficient permissions, you should be
453 able to set the system-wide date and time by running the date(1)
454 program. (There is no way to set the time and date on a per-process
455 basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
456 the VMS equivalent is C<set time>.
458 However, if all you want to do is change your time zone, you can
459 probably get away with setting an environment variable:
461 $ENV{TZ} = "MST7MDT"; # unixish
462 $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
463 system "trn comp.lang.perl.misc";
465 =head2 How can I sleep() or alarm() for under a second?
467 If you want finer granularity than the 1 second that the sleep()
468 function provides, the easiest way is to use the select() function as
469 documented in L<perlfunc/"select">. Try the Time::HiRes and
470 the BSD::Itimer modules (available from CPAN, and starting from
471 Perl 5.8 Time::HiRes is part of the standard distribution).
473 =head2 How can I measure time under a second?
475 In general, you may not be able to. The Time::HiRes module (available
476 from CPAN, and starting from Perl 5.8 part of the standard distribution)
477 provides this functionality for some systems.
479 If your system supports both the syscall() function in Perl as well as
480 a system call like gettimeofday(2), then you may be able to do
483 require 'sys/syscall.ph';
487 $done = $start = pack($TIMEVAL_T, ());
489 syscall(&SYS_gettimeofday, $start, 0) != -1
490 or die "gettimeofday: $!";
492 ##########################
493 # DO YOUR OPERATION HERE #
494 ##########################
496 syscall( &SYS_gettimeofday, $done, 0) != -1
497 or die "gettimeofday: $!";
499 @start = unpack($TIMEVAL_T, $start);
500 @done = unpack($TIMEVAL_T, $done);
503 for ($done[1], $start[1]) { $_ /= 1_000_000 }
505 $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
507 ($start[0] + $start[1] );
509 =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
511 Release 5 of Perl added the END block, which can be used to simulate
512 atexit(). Each package's END block is called when the program or
513 thread ends (see L<perlmod> manpage for more details).
515 For example, you can use this to make sure your filter program
516 managed to finish its output without filling up the disk:
519 close(STDOUT) || die "stdout close failed: $!";
522 The END block isn't called when untrapped signals kill the program,
523 though, so if you use END blocks you should also use
525 use sigtrap qw(die normal-signals);
527 Perl's exception-handling mechanism is its eval() operator. You can
528 use eval() as setjmp and die() as longjmp. For details of this, see
529 the section on signals, especially the time-out handler for a blocking
530 flock() in L<perlipc/"Signals"> or the section on "Signals" in
533 If exception handling is all you're interested in, try the
534 exceptions.pl library (part of the standard perl distribution).
536 If you want the atexit() syntax (and an rmexit() as well), try the
537 AtExit module available from CPAN.
539 =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
541 Some Sys-V based systems, notably Solaris 2.X, redefined some of the
542 standard socket constants. Since these were constant across all
543 architectures, they were often hardwired into perl code. The proper
544 way to deal with this is to "use Socket" to get the correct values.
546 Note that even though SunOS and Solaris are binary compatible, these
547 values are different. Go figure.
549 =head2 How can I call my system's unique C functions from Perl?
551 In most cases, you write an external module to do it--see the answer
552 to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
553 However, if the function is a system call, and your system supports
554 syscall(), you can use the syscall function (documented in
557 Remember to check the modules that came with your distribution, and
558 CPAN as well---someone may already have written a module to do it. On
559 Windows, try Win32::API. On Macs, try Mac::Carbon. If no module
560 has an interface to the C function, you can inline a bit of C in your
561 Perl source with Inline::C.
563 =head2 Where do I get the include files to do ioctl() or syscall()?
565 Historically, these would be generated by the h2ph tool, part of the
566 standard perl distribution. This program converts cpp(1) directives
567 in C header files to files containing subroutine definitions, like
568 &SYS_getitimer, which you can use as arguments to your functions.
569 It doesn't work perfectly, but it usually gets most of the job done.
570 Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
571 but the hard ones like F<ioctl.h> nearly always need to hand-edited.
572 Here's how to install the *.ph files:
578 If your system supports dynamic loading, for reasons of portability and
579 sanity you probably ought to use h2xs (also part of the standard perl
580 distribution). This tool converts C header files to Perl extensions.
581 See L<perlxstut> for how to get started with h2xs.
583 If your system doesn't support dynamic loading, you still probably
584 ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
585 more information (in brief, just use B<make perl> instead of a plain
586 B<make> to rebuild perl with a new static extension).
588 =head2 Why do setuid perl scripts complain about kernel problems?
590 Some operating systems have bugs in the kernel that make setuid
591 scripts inherently insecure. Perl gives you a number of options
592 (described in L<perlsec>) to work around such systems.
594 =head2 How can I open a pipe both to and from a command?
596 The IPC::Open2 module (part of the standard perl distribution) is an
597 easy-to-use approach that internally uses pipe(), fork(), and exec() to do
598 the job. Make sure you read the deadlock warnings in its documentation,
599 though (see L<IPC::Open2>). See
600 L<perlipc/"Bidirectional Communication with Another Process"> and
601 L<perlipc/"Bidirectional Communication with Yourself">
603 You may also use the IPC::Open3 module (part of the standard perl
604 distribution), but be warned that it has a different order of
605 arguments from IPC::Open2 (see L<IPC::Open3>).
607 =head2 Why can't I get the output of a command with system()?
609 You're confusing the purpose of system() and backticks (``). system()
610 runs a command and returns exit status information (as a 16 bit value:
611 the low 7 bits are the signal the process died from, if any, and
612 the high 8 bits are the actual exit value). Backticks (``) run a
613 command and return what it sent to STDOUT.
615 $exit_status = system("mail-users");
616 $output_string = `ls`;
618 =head2 How can I capture STDERR from an external command?
620 There are three basic ways of running external commands:
622 system $cmd; # using system()
623 $output = `$cmd`; # using backticks (``)
624 open (PIPE, "cmd |"); # using open()
626 With system(), both STDOUT and STDERR will go the same place as the
627 script's STDOUT and STDERR, unless the system() command redirects them.
628 Backticks and open() read B<only> the STDOUT of your command.
630 You can also use the open3() function from IPC::Open3. Benjamin
631 Goldberg provides some sample code:
633 To capture a program's STDOUT, but discard its STDERR:
637 use Symbol qw(gensym);
638 open(NULL, ">", File::Spec->devnull);
639 my $pid = open3(gensym, \*PH, ">&NULL", "cmd");
643 To capture a program's STDERR, but discard its STDOUT:
647 use Symbol qw(gensym);
648 open(NULL, ">", File::Spec->devnull);
649 my $pid = open3(gensym, ">&NULL", \*PH, "cmd");
653 To capture a program's STDERR, and let its STDOUT go to our own STDERR:
656 use Symbol qw(gensym);
657 my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
661 To read both a command's STDOUT and its STDERR separately, you can
662 redirect them to temp files, let the command run, then read the temp
666 use Symbol qw(gensym);
668 local *CATCHOUT = IO::File->new_tempfile;
669 local *CATCHERR = IO::File->new_tempfile;
670 my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
672 seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
673 while( <CATCHOUT> ) {}
674 while( <CATCHERR> ) {}
676 But there's no real need for *both* to be tempfiles... the following
677 should work just as well, without deadlocking:
680 use Symbol qw(gensym);
682 local *CATCHERR = IO::File->new_tempfile;
683 my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
684 while( <CATCHOUT> ) {}
687 while( <CATCHERR> ) {}
689 And it'll be faster, too, since we can begin processing the program's
690 stdout immediately, rather than waiting for the program to finish.
692 With any of these, you can change file descriptors before the call:
694 open(STDOUT, ">logfile");
697 or you can use Bourne shell file-descriptor redirection:
699 $output = `$cmd 2>some_file`;
700 open (PIPE, "cmd 2>some_file |");
702 You can also use file-descriptor redirection to make STDERR a
705 $output = `$cmd 2>&1`;
706 open (PIPE, "cmd 2>&1 |");
708 Note that you I<cannot> simply open STDERR to be a dup of STDOUT
709 in your Perl program and avoid calling the shell to do the redirection.
712 open(STDERR, ">&STDOUT");
713 $alloutput = `cmd args`; # stderr still escapes
715 This fails because the open() makes STDERR go to where STDOUT was
716 going at the time of the open(). The backticks then make STDOUT go to
717 a string, but don't change STDERR (which still goes to the old
720 Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
721 backticks, not csh(1)! Details on why Perl's system() and backtick
722 and pipe opens all use the Bourne shell are in the
723 F<versus/csh.whynot> article in the "Far More Than You Ever Wanted To
724 Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz . To
725 capture a command's STDERR and STDOUT together:
727 $output = `cmd 2>&1`; # either with backticks
728 $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
729 while (<PH>) { } # plus a read
731 To capture a command's STDOUT but discard its STDERR:
733 $output = `cmd 2>/dev/null`; # either with backticks
734 $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
735 while (<PH>) { } # plus a read
737 To capture a command's STDERR but discard its STDOUT:
739 $output = `cmd 2>&1 1>/dev/null`; # either with backticks
740 $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
741 while (<PH>) { } # plus a read
743 To exchange a command's STDOUT and STDERR in order to capture the STDERR
744 but leave its STDOUT to come out our old STDERR:
746 $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
747 $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
748 while (<PH>) { } # plus a read
750 To read both a command's STDOUT and its STDERR separately, it's easiest
751 to redirect them separately to files, and then read from those files
752 when the program is done:
754 system("program args 1>program.stdout 2>program.stderr");
756 Ordering is important in all these examples. That's because the shell
757 processes file descriptor redirections in strictly left to right order.
759 system("prog args 1>tmpfile 2>&1");
760 system("prog args 2>&1 1>tmpfile");
762 The first command sends both standard out and standard error to the
763 temporary file. The second command sends only the old standard output
764 there, and the old standard error shows up on the old standard out.
766 =head2 Why doesn't open() return an error when a pipe open fails?
768 If the second argument to a piped open() contains shell
769 metacharacters, perl fork()s, then exec()s a shell to decode the
770 metacharacters and eventually run the desired program. If the program
771 couldn't be run, it's the shell that gets the message, not Perl. All
772 your Perl program can find out is whether the shell itself could be
773 successfully started. You can still capture the shell's STDERR and
774 check it for error messages. See L<"How can I capture STDERR from an
775 external command?"> elsewhere in this document, or use the
778 If there are no shell metacharacters in the argument of open(), Perl
779 runs the command directly, without using the shell, and can correctly
780 report whether the command started.
782 =head2 What's wrong with using backticks in a void context?
784 Strictly speaking, nothing. Stylistically speaking, it's not a good
785 way to write maintainable code. Perl has several operators for
786 running external commands. Backticks are one; they collect the output
787 from the command for use in your program. The C<system> function is
788 another; it doesn't do this.
790 Writing backticks in your program sends a clear message to the readers
791 of your code that you wanted to collect the output of the command.
792 Why send a clear message that isn't true?
798 You forgot to check C<$?> to see whether the program even ran
799 correctly. Even if you wrote
801 print `cat /etc/termcap`;
803 this code could and probably should be written as
805 system("cat /etc/termcap") == 0
806 or die "cat program failed!";
808 which will get the output quickly (as it is generated, instead of only
809 at the end) and also check the return value.
811 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 || 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/;
1007 open(TTY, "/dev/tty") or die $!;
1008 $tpgrp = tcgetpgrp(fileno(*TTY));
1010 if ($tpgrp == $pgrp) {
1011 print "foreground\n";
1013 print "background\n";
1016 =head2 How do I timeout a slow event?
1018 Use the alarm() function, probably in conjunction with a signal
1019 handler, as documented in L<perlipc/"Signals"> and the section on
1020 "Signals" in the Camel. You may instead use the more flexible
1021 Sys::AlarmCall module available from CPAN.
1023 The alarm() function is not implemented on all versions of Windows.
1024 Check the documentation for your specific version of Perl.
1026 =head2 How do I set CPU limits?
1028 Use the BSD::Resource module from CPAN.
1030 =head2 How do I avoid zombies on a Unix system?
1032 Use the reaper code from L<perlipc/"Signals"> to call wait() when a
1033 SIGCHLD is received, or else use the double-fork technique described
1034 in L<perlfaq8/"How do I start a process in the background?">.
1036 =head2 How do I use an SQL database?
1038 The DBI module provides an abstract interface to most database
1039 servers and types, including Oracle, DB2, Sybase, mysql, Postgresql,
1040 ODBC, and flat files. The DBI module accesses each database type
1041 through a database driver, or DBD. You can see a complete list of
1042 available drivers on CPAN: http://www.cpan.org/modules/by-module/DBD/ .
1043 You can read more about DBI on http://dbi.perl.org .
1045 Other modules provide more specific access: Win32::ODBC, Alzabo, iodbc,
1046 and others found on CPAN Search: http://search.cpan.org .
1048 =head2 How do I make a system() exit on control-C?
1050 You can't. You need to imitate the system() call (see L<perlipc> for
1051 sample code) and then have a signal handler for the INT signal that
1052 passes the signal on to the subprocess. Or you can check for it:
1055 if ($rc & 127) { die "signal death" }
1057 =head2 How do I open a file without blocking?
1059 If you're lucky enough to be using a system that supports
1060 non-blocking reads (most Unixish systems do), you need only to use the
1061 O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
1065 sysopen(FH, "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
1066 or die "can't open /foo/somefile: $!":
1068 =head2 How do I tell the difference between errors from the shell and perl?
1070 (answer contributed by brian d foy, C<< <bdfoy@cpan.org> >>
1072 When you run a Perl script, something else is running the script for you,
1073 and that something else may output error messages. The script might
1074 emit its own warnings and error messages. Most of the time you cannot
1077 You probably cannot fix the thing that runs perl, but you can change how
1078 perl outputs its warnings by defining a custom warning and die functions.
1080 Consider this script, which has an error you may not notice immediately.
1082 #!/usr/locl/bin/perl
1084 print "Hello World\n";
1086 I get an error when I run this from my shell (which happens to be
1087 bash). That may look like perl forgot it has a print() function,
1088 but my shebang line is not the path to perl, so the shell runs the
1089 script, and I get the error.
1092 ./test: line 3: print: command not found
1094 A quick and dirty fix involves a little bit of code, but this may be all
1095 you need to figure out the problem.
1100 $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
1101 $SIG{__DIE__} = sub{ print STDERR "Perl: ", @_; exit 1};
1108 The perl message comes out with "Perl" in front. The BEGIN block
1109 works at compile time so all of the compilation errors and warnings
1110 get the "Perl:" prefix too.
1112 Perl: Useless use of division (/) in void context at ./test line 9.
1113 Perl: Name "main::a" used only once: possible typo at ./test line 8.
1114 Perl: Name "main::x" used only once: possible typo at ./test line 9.
1115 Perl: Use of uninitialized value in addition (+) at ./test line 8.
1116 Perl: Use of uninitialized value in division (/) at ./test line 9.
1117 Perl: Illegal division by zero at ./test line 9.
1118 Perl: Illegal division by zero at -e line 3.
1120 If I don't see that "Perl:", it's not from perl.
1122 You could also just know all the perl errors, and although there are
1123 some people who may know all of them, you probably don't. However, they
1124 all should be in the perldiag manpage. If you don't find the error in
1125 there, it probably isn't a perl error.
1127 Looking up every message is not the easiest way, so let perl to do it
1128 for you. Use the diagnostics pragma with turns perl's normal messages
1129 into longer discussions on the topic.
1133 If you don't get a paragraph or two of expanded discussion, it
1134 might not be perl's message.
1136 =head2 How do I install a module from CPAN?
1138 The easiest way is to have a module also named CPAN do it for you.
1139 This module comes with perl version 5.004 and later.
1141 $ perl -MCPAN -e shell
1143 cpan shell -- CPAN exploration and modules installation (v1.59_54)
1144 ReadLine support enabled
1146 cpan> install Some::Module
1148 To manually install the CPAN module, or any well-behaved CPAN module
1149 for that matter, follow these steps:
1155 Unpack the source into a temporary area.
1175 If your version of perl is compiled without dynamic loading, then you
1176 just need to replace step 3 (B<make>) with B<make perl> and you will
1177 get a new F<perl> binary with your extension linked in.
1179 See L<ExtUtils::MakeMaker> for more details on building extensions.
1180 See also the next question, "What's the difference between require
1183 =head2 What's the difference between require and use?
1185 Perl offers several different ways to include code from one file into
1186 another. Here are the deltas between the various inclusion constructs:
1188 1) do $file is like eval `cat $file`, except the former
1189 1.1: searches @INC and updates %INC.
1190 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
1192 2) require $file is like do $file, except the former
1193 2.1: checks for redundant loading, skipping already loaded files.
1194 2.2: raises an exception on failure to find, compile, or execute $file.
1196 3) require Module is like require "Module.pm", except the former
1197 3.1: translates each "::" into your system's directory separator.
1198 3.2: primes the parser to disambiguate class Module as an indirect object.
1200 4) use Module is like require Module, except the former
1201 4.1: loads the module at compile time, not run-time.
1202 4.2: imports symbols and semantics from that package to the current one.
1204 In general, you usually want C<use> and a proper Perl module.
1206 =head2 How do I keep my own module/library directory?
1208 When you build modules, use the PREFIX and LIB options when generating
1211 perl Makefile.PL PREFIX=/mydir/perl LIB=/mydir/perl/lib
1213 then either set the PERL5LIB environment variable before you run
1214 scripts that use the modules/libraries (see L<perlrun>) or say
1216 use lib '/mydir/perl/lib';
1218 This is almost the same as
1221 unshift(@INC, '/mydir/perl/lib');
1224 except that the lib module checks for machine-dependent subdirectories.
1225 See Perl's L<lib> for more information.
1227 =head2 How do I add the directory my program lives in to the module/library search path?
1230 use lib "$FindBin::Bin";
1231 use your_own_modules;
1233 =head2 How do I add a directory to my include path (@INC) at runtime?
1235 Here are the suggested ways of modifying your include path:
1237 the PERLLIB environment variable
1238 the PERL5LIB environment variable
1239 the perl -Idir command line flag
1240 the use lib pragma, as in
1241 use lib "$ENV{HOME}/myown_perllib";
1243 The latter is particularly useful because it knows about machine
1244 dependent architectures. The lib.pm pragmatic module was first
1245 included with the 5.002 release of Perl.
1247 =head2 What is socket.ph and where do I get it?
1249 It's a perl4-style file defining values for system networking
1250 constants. Sometimes it is built using h2ph when Perl is installed,
1251 but other times it is not. Modern programs C<use Socket;> instead.
1253 =head1 AUTHOR AND COPYRIGHT
1255 Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
1256 other authors as noted. All rights reserved.
1258 This documentation is free; you can redistribute it and/or modify it
1259 under the same terms as Perl itself.
1261 Irrespective of its distribution, all code examples in this file
1262 are hereby placed into the public domain. You are permitted and
1263 encouraged to use this code in your own programs for fun
1264 or for profit as you see fit. A simple comment in the code giving
1265 credit would be courteous but is not required.