3 perlfaq8 - System Interaction ($Revision: 1.39 $, $Date: 1999/05/23 18:37:57 $)
7 This section of the Perl FAQ covers questions involving operating
8 system interaction. This involves interprocess communication (IPC),
9 control over the user-interface (keyboard, screen and pointing
10 devices), and most anything else not related to data manipulation.
12 Read the FAQs and documentation specific to the port of perl to your
13 operating system (eg, L<perlvms>, L<perlplan9>, ...). These should
14 contain more detailed information on the vagaries of your perl.
16 =head2 How do I find out which operating system I'm running under?
18 The $^O variable ($OSNAME if you use English) contains 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, $key, 1); # probably this does
87 system "stty -cbreak </dev/tty >/dev/tty 2>&1";
89 The Term::ReadKey module from CPAN offers an easy-to-use interface that
90 should be more efficient than shelling out to B<stty> for each key.
91 It even includes limited support for Windows.
98 However, that requires that you have a working C compiler and can use it
99 to build and install a CPAN module. Here's a solution using
100 the standard POSIX module, which is already on your systems (assuming
101 your system supports POSIX).
106 And here's the HotKey module, which hides the somewhat mystifying calls
107 to manipulate the POSIX termios structures.
113 @EXPORT = qw(cbreak cooked readkey);
116 use POSIX qw(:termios_h);
117 my ($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
218 the terminal into "no echo" mode, then just read the password
219 normally. You may do this with an old-style ioctl() function, POSIX
220 terminal control (see L<POSIX>, and Chapter 7 of the Camel), 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 $password = ReadLine(0);
231 =head2 How do I read and write the serial port?
233 This depends on which operating system your program is running on. In
234 the case of Unix, the serial ports will be accessible through files in
235 /dev; on other systems, the devices names will doubtless differ.
236 Several problem areas common to all device interaction are the
243 Your system may use lockfiles to control multiple access. Make sure
244 you follow the correct protocol. Unpredictable behaviour can result
245 from multiple processes reading from one device.
249 If you expect to use both read and write operations on the device,
250 you'll have to open it for update (see L<perlfunc/"open"> for
251 details). You may wish to open it without running the risk of
252 blocking by using sysopen() and C<O_RDWR|O_NDELAY|O_NOCTTY> from the
253 Fcntl module (part of the standard perl distribution). See
254 L<perlfunc/"sysopen"> for more on this approach.
258 Some devices will be expecting a "\r" at the end of each line rather
259 than a "\n". In some ports of perl, "\r" and "\n" are different from
260 their usual (Unix) ASCII values of "\012" and "\015". You may have to
261 give the numeric values you want directly, using octal ("\015"), hex
262 ("0x0D"), or as a control-character specification ("\cM").
264 print DEV "atv1\012"; # wrong, for some devices
265 print DEV "atv1\015"; # right, for some devices
267 Even though with normal text files, a "\n" will do the trick, there is
268 still no unified scheme for terminating a line that is portable
269 between Unix, DOS/Win, and Macintosh, except to terminate I<ALL> line
270 ends with "\015\012", and strip what you don't need from the output.
271 This applies especially to socket I/O and autoflushing, discussed
274 =item flushing output
276 If you expect characters to get to your device when you print() them,
277 you'll want to autoflush that filehandle. You can use select()
278 and the C<$|> variable to control autoflushing (see L<perlvar/$|>
279 and L<perlfunc/select>):
285 You'll also see code that does this without a temporary variable, as in
287 select((select(DEV), $| = 1)[0]);
289 Or if you don't mind pulling in a few thousand lines
290 of code just because you're afraid of a little $| variable:
295 As mentioned in the previous item, this still doesn't work when using
296 socket I/O between Unix and Macintosh. You'll need to hardcode your
297 line terminators, in that case.
299 =item non-blocking input
301 If you are doing a blocking read() or sysread(), you'll have to
302 arrange for an alarm handler to provide a timeout (see
303 L<perlfunc/alarm>). If you have a non-blocking open, you'll likely
304 have a non-blocking read, which means you may have to use a 4-arg
305 select() to determine whether I/O is ready on that device (see
306 L<perlfunc/"select">.
310 While trying to read from his caller-id box, the notorious Jamie Zawinski
311 <jwz@netscape.com>, after much gnashing of teeth and fighting with sysread,
312 sysopen, POSIX's tcgetattr business, and various other functions that
313 go bump in the night, finally came up with this:
317 my $stty = `/bin/stty -g`;
318 open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$modem_device -s2400 2>&1");
319 # starting cu hoses /dev/tty's stty settings, even when it has
320 # been opened on a pipe...
321 system("/bin/stty $stty");
324 if ( !m/^Connected/ ) {
325 print STDERR "$0: cu printed `$_' instead of `Connected'\n";
329 =head2 How do I decode encrypted password files?
331 You spend lots and lots of money on dedicated hardware, but this is
332 bound to get you talked about.
334 Seriously, you can't if they are Unix password files - the Unix
335 password system employs one-way encryption. It's more like hashing than
336 encryption. The best you can check is whether something else hashes to
337 the same string. You can't turn a hash back into the original string.
339 can forcibly (and intelligently) try to guess passwords, but don't
340 (can't) guarantee quick success.
342 If you're worried about users selecting bad passwords, you should
343 proactively check when they try to change their password (by modifying
344 passwd(1), for example).
346 =head2 How do I start a process in the background?
352 or you could use fork as documented in L<perlfunc/"fork">, with
353 further examples in L<perlipc>. Some things to be aware of, if you're
354 on a Unix-like system:
358 =item STDIN, STDOUT, and STDERR are shared
360 Both the main process and the backgrounded one (the "child" process)
361 share the same STDIN, STDOUT and STDERR filehandles. If both try to
362 access them at once, strange things can happen. You may want to close
363 or reopen these for the child. You can get around this with
364 C<open>ing a pipe (see L<perlfunc/"open">) but on some systems this
365 means that the child process cannot outlive the parent.
369 You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
370 SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
371 sent when you write to a filehandle whose child process has closed (an
372 untrapped SIGPIPE can cause your program to silently die). This is
373 not an issue with C<system("cmd&")>.
377 You have to be prepared to "reap" the child process when it finishes
379 $SIG{CHLD} = sub { wait };
381 See L<perlipc/"Signals"> for other examples of code to do this.
382 Zombies are not an issue with C<system("prog &")>.
386 =head2 How do I trap control characters/signals?
388 You don't actually "trap" a control character. Instead, that character
389 generates a signal which is sent to your terminal's currently
390 foregrounded process group, which you then trap in your process.
391 Signals are documented in L<perlipc/"Signals"> and chapter 6 of the Camel.
393 Be warned that very few C libraries are re-entrant. Therefore, if you
394 attempt to print() in a handler that got invoked during another stdio
395 operation your internal structures will likely be in an
396 inconsistent state, and your program will dump core. You can
397 sometimes avoid this by using syswrite() instead of print().
399 Unless you're exceedingly careful, the only safe things to do inside a
400 signal handler are: set a variable and exit. And in the first case,
401 you should only set a variable in such a way that malloc() is not
402 called (eg, by setting a variable that already has a value).
406 $Interrupted = 0; # to ensure it has a value
409 syswrite(STDERR, "ouch\n", 5);
412 However, because syscalls restart by default, you'll find that if
413 you're in a "slow" call, such as <FH>, read(), connect(), or
414 wait(), that the only way to terminate them is by "longjumping" out;
415 that is, by raising an exception. See the time-out handler for a
416 blocking flock() in L<perlipc/"Signals"> or chapter 6 of the Camel.
418 =head2 How do I modify the shadow password file on a Unix system?
420 If perl was installed correctly, and your shadow library was written
421 properly, the getpw*() functions described in L<perlfunc> should in
422 theory provide (read-only) access to entries in the shadow password
423 file. To change the file, make a new shadow password file (the format
424 varies from system to system - see L<passwd(5)> for specifics) and use
425 pwd_mkdb(8) to install it (see L<pwd_mkdb(8)> for more details).
427 =head2 How do I set the time and date?
429 Assuming you're running under sufficient permissions, you should be
430 able to set the system-wide date and time by running the date(1)
431 program. (There is no way to set the time and date on a per-process
432 basis.) This mechanism will work for Unix, MS-DOS, Windows, and NT;
433 the VMS equivalent is C<set time>.
435 However, if all you want to do is change your timezone, you can
436 probably get away with setting an environment variable:
438 $ENV{TZ} = "MST7MDT"; # unixish
439 $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
440 system "trn comp.lang.perl.misc";
442 =head2 How can I sleep() or alarm() for under a second?
444 If you want finer granularity than the 1 second that the sleep()
445 function provides, the easiest way is to use the select() function as
446 documented in L<perlfunc/"select">. Try the Time::HiRes and
447 the BSD::Itimer modules (available from CPAN).
449 =head2 How can I measure time under a second?
451 In general, you may not be able to. The Time::HiRes module (available
452 from CPAN) provides this functionality for some systems.
454 If your system supports both the syscall() function in Perl as well as
455 a system call like gettimeofday(2), then you may be able to do
458 require 'sys/syscall.ph';
462 $done = $start = pack($TIMEVAL_T, ());
464 syscall(&SYS_gettimeofday, $start, 0) != -1
465 or die "gettimeofday: $!";
467 ##########################
468 # DO YOUR OPERATION HERE #
469 ##########################
471 syscall( &SYS_gettimeofday, $done, 0) != -1
472 or die "gettimeofday: $!";
474 @start = unpack($TIMEVAL_T, $start);
475 @done = unpack($TIMEVAL_T, $done);
478 for ($done[1], $start[1]) { $_ /= 1_000_000 }
480 $delta_time = sprintf "%.4f", ($done[0] + $done[1] )
482 ($start[0] + $start[1] );
484 =head2 How can I do an atexit() or setjmp()/longjmp()? (Exception handling)
486 Release 5 of Perl added the END block, which can be used to simulate
487 atexit(). Each package's END block is called when the program or
488 thread ends (see L<perlmod> manpage for more details).
490 For example, you can use this to make sure your filter program
491 managed to finish its output without filling up the disk:
494 close(STDOUT) || die "stdout close failed: $!";
497 The END block isn't called when untrapped signals kill the program, though, so if
498 you use END blocks you should also use
500 use sigtrap qw(die normal-signals);
502 Perl's exception-handling mechanism is its eval() operator. You can
503 use eval() as setjmp and die() as longjmp. For details of this, see
504 the section on signals, especially the time-out handler for a blocking
505 flock() in L<perlipc/"Signals"> and chapter 6 of the Camel.
507 If exception handling is all you're interested in, try the
508 exceptions.pl library (part of the standard perl distribution).
510 If you want the atexit() syntax (and an rmexit() as well), try the
511 AtExit module available from CPAN.
513 =head2 Why doesn't my sockets program work under System V (Solaris)? What does the error message "Protocol not supported" mean?
515 Some Sys-V based systems, notably Solaris 2.X, redefined some of the
516 standard socket constants. Since these were constant across all
517 architectures, they were often hardwired into perl code. The proper
518 way to deal with this is to "use Socket" to get the correct values.
520 Note that even though SunOS and Solaris are binary compatible, these
521 values are different. Go figure.
523 =head2 How can I call my system's unique C functions from Perl?
525 In most cases, you write an external module to do it - see the answer
526 to "Where can I learn about linking C with Perl? [h2xs, xsubpp]".
527 However, if the function is a system call, and your system supports
528 syscall(), you can use the syscall function (documented in
531 Remember to check the modules that came with your distribution, and
532 CPAN as well - someone may already have written a module to do it.
534 =head2 Where do I get the include files to do ioctl() or syscall()?
536 Historically, these would be generated by the h2ph tool, part of the
537 standard perl distribution. This program converts cpp(1) directives
538 in C header files to files containing subroutine definitions, like
539 &SYS_getitimer, which you can use as arguments to your functions.
540 It doesn't work perfectly, but it usually gets most of the job done.
541 Simple files like F<errno.h>, F<syscall.h>, and F<socket.h> were fine,
542 but the hard ones like F<ioctl.h> nearly always need to hand-edited.
543 Here's how to install the *.ph files:
549 If your system supports dynamic loading, for reasons of portability and
550 sanity you probably ought to use h2xs (also part of the standard perl
551 distribution). This tool converts C header files to Perl extensions.
552 See L<perlxstut> for how to get started with h2xs.
554 If your system doesn't support dynamic loading, you still probably
555 ought to use h2xs. See L<perlxstut> and L<ExtUtils::MakeMaker> for
556 more information (in brief, just use B<make perl> instead of a plain
557 B<make> to rebuild perl with a new static extension).
559 =head2 Why do setuid perl scripts complain about kernel problems?
561 Some operating systems have bugs in the kernel that make setuid
562 scripts inherently insecure. Perl gives you a number of options
563 (described in L<perlsec>) to work around such systems.
565 =head2 How can I open a pipe both to and from a command?
567 The IPC::Open2 module (part of the standard perl distribution) is an
568 easy-to-use approach that internally uses pipe(), fork(), and exec() to do
569 the job. Make sure you read the deadlock warnings in its documentation,
570 though (see L<IPC::Open2>). See L<perlipc/"Bidirectional Communication
571 with Another Process"> and L<perlipc/"Bidirectional Communication with
574 You may also use the IPC::Open3 module (part of the standard perl
575 distribution), but be warned that it has a different order of
576 arguments from IPC::Open2 (see L<IPC::Open3>).
578 =head2 Why can't I get the output of a command with system()?
580 You're confusing the purpose of system() and backticks (``). system()
581 runs a command and returns exit status information (as a 16 bit value:
582 the low 7 bits are the signal the process died from, if any, and
583 the high 8 bits are the actual exit value). Backticks (``) run a
584 command and return what it sent to STDOUT.
586 $exit_status = system("mail-users");
587 $output_string = `ls`;
589 =head2 How can I capture STDERR from an external command?
591 There are three basic ways of running external commands:
593 system $cmd; # using system()
594 $output = `$cmd`; # using backticks (``)
595 open (PIPE, "cmd |"); # using open()
597 With system(), both STDOUT and STDERR will go the same place as the
598 script's versions of these, unless the command redirects them.
599 Backticks and open() read B<only> the STDOUT of your command.
601 With any of these, you can change file descriptors before the call:
603 open(STDOUT, ">logfile");
606 or you can use Bourne shell file-descriptor redirection:
608 $output = `$cmd 2>some_file`;
609 open (PIPE, "cmd 2>some_file |");
611 You can also use file-descriptor redirection to make STDERR a
614 $output = `$cmd 2>&1`;
615 open (PIPE, "cmd 2>&1 |");
617 Note that you I<cannot> simply open STDERR to be a dup of STDOUT
618 in your Perl program and avoid calling the shell to do the redirection.
621 open(STDERR, ">&STDOUT");
622 $alloutput = `cmd args`; # stderr still escapes
624 This fails because the open() makes STDERR go to where STDOUT was
625 going at the time of the open(). The backticks then make STDOUT go to
626 a string, but don't change STDERR (which still goes to the old
629 Note that you I<must> use Bourne shell (sh(1)) redirection syntax in
630 backticks, not csh(1)! Details on why Perl's system() and backtick
631 and pipe opens all use the Bourne shell are in
632 http://www.perl.com/CPAN/doc/FMTEYEWTK/versus/csh.whynot .
633 To capture a command's STDERR and STDOUT together:
635 $output = `cmd 2>&1`; # either with backticks
636 $pid = open(PH, "cmd 2>&1 |"); # or with an open pipe
637 while (<PH>) { } # plus a read
639 To capture a command's STDOUT but discard its STDERR:
641 $output = `cmd 2>/dev/null`; # either with backticks
642 $pid = open(PH, "cmd 2>/dev/null |"); # or with an open pipe
643 while (<PH>) { } # plus a read
645 To capture a command's STDERR but discard its STDOUT:
647 $output = `cmd 2>&1 1>/dev/null`; # either with backticks
648 $pid = open(PH, "cmd 2>&1 1>/dev/null |"); # or with an open pipe
649 while (<PH>) { } # plus a read
651 To exchange a command's STDOUT and STDERR in order to capture the STDERR
652 but leave its STDOUT to come out our old STDERR:
654 $output = `cmd 3>&1 1>&2 2>&3 3>&-`; # either with backticks
655 $pid = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with an open pipe
656 while (<PH>) { } # plus a read
658 To read both a command's STDOUT and its STDERR separately, it's easiest
659 and safest to redirect them separately to files, and then read from those
660 files when the program is done:
662 system("program args 1>/tmp/program.stdout 2>/tmp/program.stderr");
664 Ordering is important in all these examples. That's because the shell
665 processes file descriptor redirections in strictly left to right order.
667 system("prog args 1>tmpfile 2>&1");
668 system("prog args 2>&1 1>tmpfile");
670 The first command sends both standard out and standard error to the
671 temporary file. The second command sends only the old standard output
672 there, and the old standard error shows up on the old standard out.
674 =head2 Why doesn't open() return an error when a pipe open fails?
676 Because the pipe open takes place in two steps: first Perl calls
677 fork() to start a new process, then this new process calls exec() to
678 run the program you really wanted to open. The first step reports
679 success or failure to your process, so open() can only tell you
680 whether the fork() succeeded or not.
682 To find out if the exec() step succeeded, you have to catch SIGCHLD
683 and wait() to get the exit status. You should also catch SIGPIPE if
684 you're writing to the child--you may not have found out the exec()
685 failed by the time you write. This is documented in L<perlipc>.
687 In some cases, even this won't work. If the second argument to a
688 piped open() contains shell metacharacters, perl fork()s, then exec()s
689 a shell to decode the metacharacters and eventually run the desired
690 program. Now when you call wait(), you only learn whether or not the
691 I<shell> could be successfully started. Best to avoid shell
694 On systems that follow the spawn() paradigm, open() I<might> do what
695 you expect--unless perl uses a shell to start your command. In this
696 case the fork()/exec() description still applies.
698 =head2 What's wrong with using backticks in a void context?
700 Strictly speaking, nothing. Stylistically speaking, it's not a good
701 way to write maintainable code because backticks have a (potentially
702 humongous) return value, and you're ignoring it. It's may also not be very
703 efficient, because you have to read in all the lines of output, allocate
704 memory for them, and then throw it away. Too often people are lulled
709 And now they think "Hey, I'll just always use backticks to run programs."
710 Bad idea: backticks are for capturing a program's output; the system()
711 function is for running programs.
717 You haven't assigned the output anywhere, so it just wastes memory
718 (for a little while). Plus you forgot to check C<$?> to see whether
719 the program even ran correctly. Even if you wrote
721 print `cat /etc/termcap`;
723 In most cases, this could and probably should be written as
725 system("cat /etc/termcap") == 0
726 or die "cat program failed!";
728 Which will get the output quickly (as it is generated, instead of only
729 at the end) and also check the return value.
731 system() also provides direct control over whether shell wildcard
732 processing may take place, whereas backticks do not.
734 =head2 How can I call backticks without shell processing?
736 This is a bit tricky. Instead of writing
738 @ok = `grep @opts '$search_string' @filenames`;
743 if (open(GREP, "-|")) {
750 exec 'grep', @opts, $search_string, @filenames;
753 Just as with system(), no shell escapes happen when you exec() a list.
754 Further examples of this can be found in L<perlipc/"Safe Pipe Opens">.
756 Note that if you're stuck on Microsoft, no solution to this vexing issue
757 is even possible. Even if Perl were to emulate fork(), you'd still
758 be hosed, because Microsoft gives no argc/argv-style API. Their API
759 always reparses from a single string, which is fundamentally wrong,
760 but you're not likely to get the Gods of Redmond to acknowledge this
763 =head2 Why can't my script read from STDIN after I gave it EOF (^D on Unix, ^Z on MS-DOS)?
765 Because some stdio's set error and eof flags that need clearing. The
766 POSIX module defines clearerr() that you can use. That is the
767 technically correct way to do it. Here are some less reliable
774 Try keeping around the seekpointer and go there, like this:
777 seek(LOG, $where, 0);
781 If that doesn't work, try seeking to a different part of the file and
786 If that doesn't work, try seeking to a different part of
787 the file, reading something, and then seeking back.
791 If that doesn't work, give up on your stdio package and use sysread.
795 =head2 How can I convert my shell script to perl?
797 Learn Perl and rewrite it. Seriously, there's no simple converter.
798 Things that are awkward to do in the shell are easy to do in Perl, and
799 this very awkwardness is what would make a shell->perl converter
800 nigh-on impossible to write. By rewriting it, you'll think about what
801 you're really trying to do, and hopefully will escape the shell's
802 pipeline datastream paradigm, which while convenient for some matters,
803 causes many inefficiencies.
805 =head2 Can I use perl to run a telnet or ftp session?
807 Try the Net::FTP, TCP::Client, and Net::Telnet modules (available from
808 CPAN). http://www.perl.com/CPAN/scripts/netstuff/telnet.emul.shar
809 will also help for emulating the telnet protocol, but Net::Telnet is
810 quite probably easier to use..
812 If all you want to do is pretend to be telnet but don't need
813 the initial telnet handshaking, then the standard dual-process
814 approach will suffice:
816 use IO::Socket; # new in 5.004
817 $handle = IO::Socket::INET->new('www.perl.com:80')
818 || die "can't connect to port 80 on www.perl.com: $!";
819 $handle->autoflush(1);
820 if (fork()) { # XXX: undef means failure
822 print while <STDIN>; # everything from stdin to socket
824 print while <$handle>; # everything from socket to stdout
829 =head2 How can I write expect in Perl?
831 Once upon a time, there was a library called chat2.pl (part of the
832 standard perl distribution), which never really got finished. If you
833 find it somewhere, I<don't use it>. These days, your best bet is to
834 look at the Expect module available from CPAN, which also requires two
835 other modules from CPAN, IO::Pty and IO::Stty.
837 =head2 Is there a way to hide perl's command line from programs such as "ps"?
839 First of all note that if you're doing this for security reasons (to
840 avoid people seeing passwords, for example) then you should rewrite
841 your program so that critical information is never given as an
842 argument. Hiding the arguments won't make your program completely
845 To actually alter the visible command line, you can assign to the
846 variable $0 as documented in L<perlvar>. This won't work on all
847 operating systems, though. Daemon programs like sendmail place their
850 $0 = "orcus [accepting connections]";
852 =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?
858 In the strictest sense, it can't be done -- the script executes as a
859 different process from the shell it was started from. Changes to a
860 process are not reflected in its parent, only in its own children
861 created after the change. There is shell magic that may allow you to
862 fake it by eval()ing the script's output in your shell; check out the
863 comp.unix.questions FAQ for details.
867 =head2 How do I close a process's filehandle without waiting for it to complete?
869 Assuming your system supports such things, just send an appropriate signal
870 to the process (see L<perlfunc/"kill">. It's common to first send a TERM
871 signal, wait a little bit, and then send a KILL signal to finish it off.
873 =head2 How do I fork a daemon process?
875 If by daemon process you mean one that's detached (disassociated from
876 its tty), then the following process is reported to work on most
877 Unixish systems. Non-Unix users should check their Your_OS::Process
878 module for other solutions.
884 Open /dev/tty and use the TIOCNOTTY ioctl on it. See L<tty(4)>
885 for details. Or better yet, you can just use the POSIX::setsid()
886 function, so you don't have to worry about process groups.
890 Change directory to /
894 Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
899 Background yourself like this:
905 The Proc::Daemon module, available from CPAN, provides a function to
906 perform these actions for you.
908 =head2 How do I find out if I'm running interactively or not?
910 Good question. Sometimes C<-t STDIN> and C<-t STDOUT> can give clues,
913 if (-t STDIN && -t STDOUT) {
917 On POSIX systems, you can test whether your own process group matches
918 the current process group of your controlling terminal as follows:
920 use POSIX qw/getpgrp tcgetpgrp/;
921 open(TTY, "/dev/tty") or die $!;
922 $tpgrp = tcgetpgrp(fileno(*TTY));
924 if ($tpgrp == $pgrp) {
925 print "foreground\n";
927 print "background\n";
930 =head2 How do I timeout a slow event?
932 Use the alarm() function, probably in conjunction with a signal
933 handler, as documented in L<perlipc/"Signals"> and chapter 6 of the
934 Camel. You may instead use the more flexible Sys::AlarmCall module
937 =head2 How do I set CPU limits?
939 Use the BSD::Resource module from CPAN.
941 =head2 How do I avoid zombies on a Unix system?
943 Use the reaper code from L<perlipc/"Signals"> to call wait() when a
944 SIGCHLD is received, or else use the double-fork technique described
947 =head2 How do I use an SQL database?
949 There are a number of excellent interfaces to SQL databases. See the
950 DBD::* modules available from http://www.perl.com/CPAN/modules/DBD .
951 A lot of information on this can be found at
952 http://www.symbolstone.org/technology/perl/DBI/
954 =head2 How do I make a system() exit on control-C?
956 You can't. You need to imitate the system() call (see L<perlipc> for
957 sample code) and then have a signal handler for the INT signal that
958 passes the signal on to the subprocess. Or you can check for it:
961 if ($rc & 127) { die "signal death" }
963 =head2 How do I open a file without blocking?
965 If you're lucky enough to be using a system that supports
966 non-blocking reads (most Unixish systems do), you need only to use the
967 O_NDELAY or O_NONBLOCK flag from the Fcntl module in conjunction with
971 sysopen(FH, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
972 or die "can't open /tmp/somefile: $!":
977 =head2 How do I install a module from CPAN?
979 The easiest way is to have a module also named CPAN do it for you.
980 This module comes with perl version 5.004 and later. To manually install
981 the CPAN module, or any well-behaved CPAN module for that matter, follow
988 Unpack the source into a temporary area.
1008 If your version of perl is compiled without dynamic loading, then you
1009 just need to replace step 3 (B<make>) with B<make perl> and you will
1010 get a new F<perl> binary with your extension linked in.
1012 See L<ExtUtils::MakeMaker> for more details on building extensions.
1013 See also the next question.
1015 =head2 What's the difference between require and use?
1017 Perl offers several different ways to include code from one file into
1018 another. Here are the deltas between the various inclusion constructs:
1020 1) do $file is like eval `cat $file`, except the former:
1021 1.1: searches @INC and updates %INC.
1022 1.2: bequeaths an *unrelated* lexical scope on the eval'ed code.
1024 2) require $file is like do $file, except the former:
1025 2.1: checks for redundant loading, skipping already loaded files.
1026 2.2: raises an exception on failure to find, compile, or execute $file.
1028 3) require Module is like require "Module.pm", except the former:
1029 3.1: translates each "::" into your system's directory separator.
1030 3.2: primes the parser to disambiguate class Module as an indirect object.
1032 4) use Module is like require Module, except the former:
1033 4.1: loads the module at compile time, not run-time.
1034 4.2: imports symbols and semantics from that package to the current one.
1036 In general, you usually want C<use> and a proper Perl module.
1038 =head2 How do I keep my own module/library directory?
1040 When you build modules, use the PREFIX option when generating
1043 perl Makefile.PL PREFIX=/u/mydir/perl
1045 then either set the PERL5LIB environment variable before you run
1046 scripts that use the modules/libraries (see L<perlrun>) or say
1048 use lib '/u/mydir/perl';
1050 This is almost the same as:
1053 unshift(@INC, '/u/mydir/perl');
1056 except that the lib module checks for machine-dependent subdirectories.
1057 See Perl's L<lib> for more information.
1059 =head2 How do I add the directory my program lives in to the module/library search path?
1062 use lib "$FindBin::Bin";
1063 use your_own_modules;
1065 =head2 How do I add a directory to my include path at runtime?
1067 Here are the suggested ways of modifying your include path:
1069 the PERLLIB environment variable
1070 the PERL5LIB environment variable
1071 the perl -Idir command line flag
1072 the use lib pragma, as in
1073 use lib "$ENV{HOME}/myown_perllib";
1075 The latter is particularly useful because it knows about machine
1076 dependent architectures. The lib.pm pragmatic module was first
1077 included with the 5.002 release of Perl.
1079 =head2 What is socket.ph and where do I get it?
1081 It's a perl4-style file defining values for system networking
1082 constants. Sometimes it is built using h2ph when Perl is installed,
1083 but other times it is not. Modern programs C<use Socket;> instead.
1085 =head1 AUTHOR AND COPYRIGHT
1087 Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
1088 All rights reserved.
1090 When included as part of the Standard Version of Perl, or as part of
1091 its complete documentation whether printed or otherwise, this work
1092 may be distributed only under the terms of Perl's Artistic License.
1093 Any distribution of this file or derivatives thereof I<outside>
1094 of that package require that special arrangements be made with
1097 Irrespective of its distribution, all code examples in this file
1098 are hereby placed into the public domain. You are permitted and
1099 encouraged to use this code in your own programs for fun
1100 or for profit as you see fit. A simple comment in the code giving
1101 credit would be courteous but is not required.