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NAME    [Toc]    [Back]

       perlfaq8 - System Interaction ($Revision: 1.7 $, $Date:
       2004/08/09 18:10:15 $)

DESCRIPTION    [Toc]    [Back]

       This section of the Perl FAQ covers questions involving
       operating system interaction.  Topics include interprocess
       communication (IPC), control over the user-interface (keyboard,
 screen and pointing devices), and most anything
       else not related to data manipulation.

       Read the FAQs and documentation specific to the port of
       perl to your operating system (eg, perlvms, perlplan9,
       ...).  These should contain more detailed information on
       the vagaries of your perl.

       How do I find out which operating system I'm running

       The $^O variable ($OSNAME if you use English) contains an
       indication of the name of the operating system (not its
       release number) that your perl binary was built for.

       How come exec() doesn't return?

       Because that's what it does: it replaces your currently
       running program with a different one.  If you want to keep
       going (as is probably the case if you're asking this question)
 use system() instead.

       How do I do fancy stuff with the keyboard/screen/mouse?

       How you access/control keyboards, screens, and pointing
       devices ("mice") is system-dependent.  Try the following

               Term::Cap                   Standard perl  distribution
               Term::ReadKey               CPAN
               Term::ReadLine::Gnu         CPAN
               Term::ReadLine::Perl        CPAN
               Term::Screen                CPAN

               Term::Cap                    Standard perl distribution
               Curses                      CPAN
               Term::ANSIColor             CPAN

               Tk                          CPAN

       Some of these specific cases are shown below.
       How do I print something out in color?

       In general, you don't, because you don't know whether the
       recipient has a color-aware display device.  If you know
       that they have an ANSI terminal that understands color,
       you can use the Term::ANSIColor module from CPAN:

           use Term::ANSIColor;
           print color("red"), "Stop!0, color("reset");
           print color("green"), "Go!0, color("reset");

       Or like this:

           use Term::ANSIColor qw(:constants);
           print RED, "Stop!0, RESET;
           print GREEN, "Go!0, RESET;

       How do I read just one key without waiting for a return    [Toc]    [Back]

       Controlling input buffering is a remarkably system-dependent
 matter.  On many systems, you can just use the stty
       command as shown in "getc" in perlfunc, but as you see,
       that's already getting you into portability snags.

           open(TTY, "+</dev/tty") or die "no tty: $!";
           system "stty  cbreak </dev/tty >/dev/tty 2>&1";
           $key = getc(TTY);           # perhaps this works
           # OR ELSE
           sysread(TTY, $key, 1);      # probably this does
           system "stty -cbreak </dev/tty >/dev/tty 2>&1";

       The Term::ReadKey module from CPAN offers an easy-to-use
       interface that should be more efficient than shelling out
       to stty for each key.  It even includes limited support
       for Windows.

           use Term::ReadKey;
           $key = ReadKey(0);

       However, using the code requires that you have a working C
       compiler and can use it to build and install a CPAN module.
  Here's a solution using the standard POSIX module,
       which is already on your systems (assuming your system
       supports POSIX).

           use HotKey;
           $key = readkey();

       And here's the HotKey module, which hides the somewhat
       mystifying calls to manipulate the POSIX termios structures.

           # HotKey.pm
           package HotKey;

           @ISA = qw(Exporter);
           @EXPORT = qw(cbreak cooked readkey);

           use strict;
           use POSIX qw(:termios_h);
           my ($term, $oterm, $echo, $noecho, $fd_stdin);

           $fd_stdin = fileno(STDIN);
           $term     = POSIX::Termios->new();
           $oterm     = $term->getlflag();

           $echo     = ECHO | ECHOK | ICANON;
           $noecho   = $oterm & ~$echo;

           sub cbreak {
               $term->setlflag($noecho);   #  ok, so i don't want
echo either
               $term->setcc(VTIME, 1);
               $term->setattr($fd_stdin, TCSANOW);

           sub cooked {
               $term->setcc(VTIME, 0);
               $term->setattr($fd_stdin, TCSANOW);

           sub readkey {
               my $key = '';
               sysread(STDIN, $key, 1);
               return $key;

           END { cooked() }


       How do I check whether input is ready on the keyboard?

       The easiest way to do this is to read a key in nonblocking
       mode with the Term::ReadKey module from CPAN, passing it
       an argument of -1 to indicate not to block:

           use Term::ReadKey;

           if (defined ($char = ReadKey(-1)) ) {
               # input was waiting and it was $char
           } else {
               # no input was waiting

           ReadMode('normal');                   # restore normal
tty settings

       How do I clear the screen?

       If you only have do so infrequently, use "system":


       If you have to do this a lot, save the clear string so you
       can print it 100 times without calling a program 100

           $clear_string = `clear`;
           print $clear_string;

       If you're planning on doing other screen manipulations,
       like cursor positions, etc, you might wish to use
       Term::Cap module:

           use Term::Cap;
           $terminal = Term::Cap->Tgetent( {OSPEED => 9600} );
           $clear_string = $terminal->Tputs('cl');

       How do I get the screen size?

       If you have Term::ReadKey module installed from CPAN, you
       can use it to fetch the width and height in characters and
       in pixels:

           use Term::ReadKey;
           ($wchar, $hchar, $wpixels,  $hpixels)  =  GetTerminalSize();

       This is more portable than the raw "ioctl", but not as

           require 'sys/ioctl.ph';
           die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
           open(TTY, "+</dev/tty")                     or die "No
tty: $!";
           unless (ioctl(TTY, &TIOCGWINSZ, $winsize='')) {
               die sprintf "$0:  ioctl  TIOCGWINSZ  (%08x:  $!)0,
           ($row,  $col,  $xpixel,  $ypixel) = unpack('S4', $winsize);
           print "(row,col) = ($row,$col)";
           print  "   (xpixel,ypixel)  =  ($xpixel,$ypixel)"   if
$xpixel || $ypixel;
           print "0;
       How do I ask the user for a password?

       (This question has nothing to do with the web.  See a different
 FAQ for that.)

       There's an example of this in "crypt" in perlfunc).
       First, you put the terminal into "no echo" mode, then just
       read the password normally.  You may do this with an oldstyle
 ioctl() function, POSIX terminal control (see POSIX
       or its documentation the Camel Book), or a call to the
       stty program, with varying degrees of portability.

       You can also do this for most systems using the
       Term::ReadKey module from CPAN, which is easier to use and
       in theory more portable.

           use Term::ReadKey;

           $password = ReadLine(0);

       How do I read and write the serial port?

       This depends on which operating system your program is
       running on.  In the case of Unix, the serial ports will be
       accessible through files in /dev; on other systems, device
       names will doubtless differ.  Several problem areas common
       to all device interaction are the following:

           Your system may use lockfiles to control multiple
           access.  Make sure you follow the correct protocol.
           Unpredictable behavior can result from multiple processes
 reading from one device.

       open mode
           If you expect to use both read and write operations on
           the device, you'll have to open it for update (see
           "open" in perlfunc for details).  You may wish to open
           it without running the risk of blocking by using
           sysopen() and "O_RDWR|O_NDELAY|O_NOCTTY" from the
           Fcntl module (part of the standard perl distribution).
           See "sysopen" in perlfunc for more on this approach.

       end of line                           " at the end of
           Some devices will be expecting a "
           e"candi"0 arehdifferent from theirmusualt(Unix)erl,
           ASCII values of " 12" and " 15".  You may have to
           give the numeric values you want directly, using octal
           (" 15"), hex ("0x0D"), or as a control-character
           specification ("
               print  DEV  "atv1 12";       # wrong, for some devices
               print DEV "atv1 15";       # right, for  some  devices

           Even though with normal text files a "0 will do the
           trick, there is still no unified scheme for terminating
 a line that is portable between Unix, DOS/Win, and
           Macintosh, except to terminate ALL line ends with
           " 15 12", and strip what you don't need from the
           output.  This applies especially to socket I/O and
           autoflushing, discussed next.

       flushing output
           If you expect characters to get to your device when
           you print() them, you'll want to autoflush that filehandle.
  You can use select() and the $| variable to
           control autoflushing (see "$|" in perlvar and "select"
           in perlfunc, or perlfaq5, ``How do I flush/unbuffer an
           output filehandle?  Why must I do this?''):

               $oldh = select(DEV);
               $| = 1;

           You'll also see code that does this without a temporary
 variable, as in

               select((select(DEV), $| = 1)[0]);

           Or if you don't mind pulling in a few thousand lines
           of code just because you're afraid of a little $|

               use IO::Handle;

           As mentioned in the previous item, this still doesn't
           work when using socket I/O between Unix and Macintosh.
           You'll need to hard code your line terminators, in
           that case.

       non-blocking input
           If you are doing a blocking read() or sysread(),
           you'll have to arrange for an alarm handler to provide
           a timeout (see "alarm" in perlfunc).  If you have a
           non-blocking open, you'll likely have a non-blocking
           read, which means you may have to use a 4-arg select()
           to determine whether I/O is ready on that device (see
           "select" in perlfunc.

       While trying to read from his caller-id box, the notorious
       Jamie Zawinski <jwz@netscape.com>, after much gnashing of
       teeth and fighting with sysread, sysopen, POSIX's tcgetattr
 business, and various other functions that go bump
       in the night, finally came up with this:
           sub open_modem {
               use IPC::Open2;
               my $stty = `/bin/stty -g`;
               open2(  ODEM_IN,  ODEM_OUT,  "cu   -l$modem_device
-s2400 2>&1");
               # starting cu hoses /dev/tty's stty settings, even
when it has
               # been opened on a pipe...
               system("/bin/stty $stty");
               $_ = <MODEM_IN>;
               if ( !m/^Connected/ ) {
                   print STDERR "$0: cu printed `$_'  instead  of

       How do I decode encrypted password files?

       You spend lots and lots of money on dedicated hardware,
       but this is bound to get you talked about.

       Seriously, you can't if they are Unix password files--the
       Unix password system employs one-way encryption.  It's
       more like hashing than encryption.  The best you can check
       is whether something else hashes to the same string.  You
       can't turn a hash back into the original string.  Programs
       like Crack can forcibly (and intelligently) try to guess
       passwords, but don't (can't) guarantee quick success.

       If you're worried about users selecting bad passwords, you
       should proactively check when they try to change their
       password (by modifying passwd(1), for example).

       How do I start a process in the background?

       Several modules can start other processes that do not
       block your Perl program.  You can use IPC::Open3, Parallel::Jobs,
 IPC::Run, and some of the POE modules.  See
       CPAN for more details.

       You could also use

           system("cmd &")

       or you could use fork as documented in "fork" in perlfunc,
       with further examples in perlipc.  Some things to be aware
       of, if you're on a Unix-like system:

       STDIN, STDOUT, and STDERR are shared
           Both the main process and the backgrounded one (the
           "child" process) share the same STDIN, STDOUT and
           STDERR filehandles.  If both try to access them at
           once, strange things can happen.  You may want to
           close or reopen these for the child.  You can get
           around this with "open"ing a pipe (see "open" in perlfunc)
 but on some systems this means that the child
           process cannot outlive the parent.

           You'll have to catch the SIGCHLD signal, and possibly
           SIGPIPE too.  SIGCHLD is sent when the backgrounded
           process finishes.  SIGPIPE is sent when you write to a
           filehandle whose child process has closed (an
           untrapped SIGPIPE can cause your program to silently
           die).  This is not an issue with "system("cmd&")".

           You have to be prepared to "reap" the child process
           when it finishes.

               $SIG{CHLD} = sub { wait };

               $SIG{CHLD} = 'IGNORE';

           You can also use a double fork. You immediately wait()
           for your first child, and the init daemon will wait()
           for your grandchild once it exits.

                   unless ($pid = fork) {
                           unless (fork) {
                       exec "what you really wanna do";
                       die "exec failed!";
                   exit 0;

           See "Signals" in perlipc for other examples of code to
           do this.  Zombies are not an issue with "system("prog

       How do I trap control characters/signals?

       You don't actually "trap" a control character.  Instead,
       that character generates a signal which is sent to your
       terminal's currently foregrounded process group, which you
       then trap in your process.  Signals are documented in
       "Signals" in perlipc and the section on ``Signals'' in the

       Be warned that very few C libraries are re-entrant.
       Therefore, if you attempt to print() in a handler that got
       invoked during another stdio operation your internal
       structures will likely be in an inconsistent state, and
       your program will dump core.  You can sometimes avoid this
       by using syswrite() instead of print().

       Unless you're exceedingly careful, the only safe things to
       do inside a signal handler are (1) set a variable and (2)
       exit.  In the first case, you should only set a variable
       in such a way that malloc() is not called (eg, by setting
       a variable that already has a value).

       For example:

           $Interrupted = 0;   # to ensure it has a value
           $SIG{INT} = sub {
               syswrite(STDERR, "ouch0, 5);

       However, because syscalls restart by default, you'll find
       that if you're in a "slow" call, such as <FH>, read(),
       connect(), or wait(), that the only way to terminate them
       is by "longjumping" out; that is, by raising an exception.
       See the time-out handler for a blocking flock() in "Signals"
 in perlipc or the section on ``Signals'' in the
       Camel book.

       How do I modify the shadow password file on a Unix system?

       If perl was installed correctly and your shadow library
       was written properly, the getpw*() functions described in
       perlfunc should in theory provide (read-only) access to
       entries in the shadow password file.  To change the file,
       make a new shadow password file (the format varies from
       system to system--see passwd for specifics) and use
       pwd_mkdb(8) to install it (see pwd_mkdb for more details).

       How do I set the time and date?

       Assuming you're running under sufficient permissions, you
       should be able to set the system-wide date and time by
       running the date(1) program.  (There is no way to set the
       time and date on a per-process basis.)  This mechanism
       will work for Unix, MS-DOS, Windows, and NT; the VMS
       equivalent is "set time".

       However, if all you want to do is change your time zone,
       you can probably get away with setting an environment

           $ENV{TZ} = "MST7MDT";                  # unixish
           $ENV{'SYS$TIMEZONE_DIFFERENTIAL'}="-5" # vms
           system "trn comp.lang.perl.misc";

       How can I sleep() or alarm() for under a second?

       If you want finer granularity than the 1 second that the
       sleep() function provides, the easiest way is to use the
       select() function as documented in "select" in perlfunc.
       Try the Time::HiRes and the BSD::Itimer modules (available
       from CPAN, and starting from Perl 5.8 Time::HiRes is part
       of the standard distribution).
       How can I measure time under a second?

       In general, you may not be able to.  The Time::HiRes module
 (available from CPAN, and starting from Perl 5.8 part
       of the standard distribution) provides this functionality
       for some systems.

       If your system supports both the syscall() function in
       Perl as well as a system call like gettimeofday(2), then
       you may be able to do something like this:

           require 'sys/syscall.ph';

           $TIMEVAL_T = "LL";

           $done = $start = pack($TIMEVAL_T, ());

           syscall(&SYS_gettimeofday, $start, 0) != -1
                      or die "gettimeofday: $!";

              # DO YOUR OPERATION HERE #

           syscall( &SYS_gettimeofday, $done, 0) != -1
                  or die "gettimeofday: $!";

           @start = unpack($TIMEVAL_T, $start);
           @done  = unpack($TIMEVAL_T, $done);

           # fix microseconds
           for ($done[1], $start[1]) { $_ /= 1_000_000 }

           $delta_time = sprintf "%.4f", ($done[0]  + $done[1]  )
                                        ($start[0] + $start[1] );

       How can I do an atexit() or setjmp()/longjmp()? (Exception

       Release 5 of Perl added the END block, which can be used
       to simulate atexit().  Each package's END block is called
       when the program or thread ends (see perlmod manpage for
       more details).

       For example, you can use this to make sure your filter
       program managed to finish its output without filling up
       the disk:

           END {
               close(STDOUT) || die "stdout close failed: $!";

       The END block isn't called when untrapped signals kill the
       program, though, so if you use END blocks you should also

               use sigtrap qw(die normal-signals);

       Perl's exception-handling mechanism is its eval() operator.
  You can use eval() as setjmp and die() as longjmp.
       For details of this, see the section on signals, especially
 the time-out handler for a blocking flock() in
       "Signals" in perlipc or the section on ``Signals'' in the
       Camel Book.

       If exception handling is all you're interested in, try the
       exceptions.pl library (part of the standard perl distribution).

       If you want the atexit() syntax (and an rmexit() as well),
       try the AtExit module available from CPAN.

       Why doesn't my sockets program work under System V
       (Solaris)?  What does the error message "Protocol not sup-
       ported" mean?

       Some Sys-V based systems, notably Solaris 2.X, redefined
       some of the standard socket constants.  Since these were
       constant across all architectures, they were often hardwired
 into perl code.  The proper way to deal with this is
       to "use Socket" to get the correct values.

       Note that even though SunOS and Solaris are binary compatible,
 these values are different.  Go figure.

       How can I call my system's unique C functions from Perl?

       In most cases, you write an external module to do it--see
       the answer to "Where can I learn about linking C with
       Perl? [h2xs, xsubpp]".  However, if the function is a system
 call, and your system supports syscall(), you can use
       the syscall function (documented in perlfunc).

       Remember to check the modules that came with your distribution,
 and CPAN as well---someone may already have written
 a module to do it. On Windows, try Win32::API.  On
       Macs, try Mac::Carbon.  If no module has an interface to
       the C function, you can inline a bit of C in your Perl
       source with Inline::C.

       Where do I get the include files to do ioctl() or

       Historically, these would be generated by the h2ph tool,
       part of the standard perl distribution.  This program converts
 cpp(1) directives in C header files to files containing
 subroutine definitions, like &SYS_getitimer, which
       you can use as arguments to your functions.  It doesn't
       work perfectly, but it usually gets most of the job  done.
       Simple files like errno.h, syscall.h, and socket.h were
       fine, but the hard ones like ioctl.h nearly always need to
       hand-edited.  Here's how to install the *.ph files:

           1.  become super-user
           2.  cd /usr/include
           3.  h2ph *.h */*.h

       If your system supports dynamic loading, for reasons of
       portability and sanity you probably ought to use h2xs
       (also part of the standard perl distribution).  This tool
       converts C header files to Perl extensions.  See perlxstut
       for how to get started with h2xs.

       If your system doesn't support dynamic loading, you still
       probably ought to use h2xs.  See perlxstut and ExtUtils::MakeMaker
 for more information (in brief, just use
       make perl instead of a plain make to rebuild perl with a
       new static extension).

       Why do setuid perl scripts complain about kernel problems?

       Some operating systems have bugs in the kernel that make
       setuid scripts inherently insecure.  Perl gives you a number
 of options (described in perlsec) to work around such

       How can I open a pipe both to and from a command?

       The IPC::Open2 module (part of the standard perl distribution)
 is an easy-to-use approach that internally uses
       pipe(), fork(), and exec() to do the job.  Make sure you
       read the deadlock warnings in its documentation, though
       (see IPC::Open2).  See "Bidirectional Communication with
       Another Process" in perlipc and "Bidirectional Communication
 with Yourself" in perlipc

       You may also use the IPC::Open3 module (part of the standard
 perl distribution), but be warned that it has a different
 order of arguments from IPC::Open2 (see

       Why can't I get the output of a command with system()?

       You're confusing the purpose of system() and backticks
       (``).  system() runs a command and returns exit status
       information (as a 16 bit value: the low 7 bits are the
       signal the process died from, if any, and the high 8 bits
       are the actual exit value).  Backticks (``) run a command
       and return what it sent to STDOUT.
           $exit_status   = system("mail-users");
           $output_string = `ls`;

       How can I capture STDERR from an external command?

       There are three basic ways of running external commands:

           system $cmd;                # using system()
           $output = `$cmd`;           # using backticks (``)
           open (PIPE, "cmd |");       # using open()

       With system(), both STDOUT and STDERR will go the same
       place as the script's STDOUT and STDERR, unless the sys-
       tem() command redirects them.  Backticks and open() read
       only the STDOUT of your command.

       You can also use the open3() function from IPC::Open3.
       Benjamin Goldberg provides some sample code:

       To capture a program's STDOUT, but discard its STDERR:

           use IPC::Open3;
           use File::Spec;
           use Symbol qw(gensym);
           open(NULL, ">", File::Spec->devnull);
           my $pid = open3(gensym, H, ">&NULL", "cmd");
           while( <PH> ) { }
           waitpid($pid, 0);

       To capture a program's STDERR, but discard its STDOUT:

           use IPC::Open3;
           use File::Spec;
           use Symbol qw(gensym);
           open(NULL, ">", File::Spec->devnull);
           my $pid = open3(gensym, ">&NULL", H, "cmd");
           while( <PH> ) { }
           waitpid($pid, 0);

       To capture a program's STDERR, and let its STDOUT go to
       our own STDERR:

           use IPC::Open3;
           use Symbol qw(gensym);
           my $pid = open3(gensym, ">&STDERR", H, "cmd");
           while( <PH> ) { }
           waitpid($pid, 0);

       To read both a command's STDOUT and its STDERR separately,
       you can redirect them to temp files, let the command run,
       then read the temp files:
           use IPC::Open3;
           use Symbol qw(gensym);
           use IO::File;
           local *CATCHOUT = IO::File->new_tempfile;
           local *CATCHERR = IO::File->new_tempfile;
           my  $pid  =  open3(gensym, ">&CATCHOUT", ">&CATCHERR",
           waitpid($pid, 0);
           seek $_, 0, 0 for ATCHOUT, ATCHERR;
           while( <CATCHOUT> ) {}
           while( <CATCHERR> ) {}

       But there's no real need for *both* to be tempfiles... the
       following should work just as well, without deadlocking:

           use IPC::Open3;
           use Symbol qw(gensym);
           use IO::File;
           local *CATCHERR = IO::File->new_tempfile;
           my $pid = open3(gensym, ATCHOUT, ">&CATCHERR", "cmd");
           while( <CATCHOUT> ) {}
           waitpid($pid, 0);
           seek CATCHERR, 0, 0;
           while( <CATCHERR> ) {}

       And it'll be faster, too, since we can begin processing
       the program's stdout immediately, rather than waiting for
       the program to finish.

       With any of these, you can change file descriptors before
       the call:

           open(STDOUT, ">logfile");

       or you can use Bourne shell file-descriptor redirection:

           $output = `$cmd 2>some_file`;
           open (PIPE, "cmd 2>some_file |");

       You can also use file-descriptor redirection to make
       STDERR a duplicate of STDOUT:

           $output = `$cmd 2>&1`;
           open (PIPE, "cmd 2>&1 |");

       Note that you cannot simply open STDERR to be a dup of
       STDOUT in your Perl program and avoid calling the shell to
       do the redirection.  This doesn't work:

           open(STDERR, ">&STDOUT");
           $alloutput = `cmd args`;  # stderr still escapes

       This fails because the open() makes STDERR go to where
       STDOUT was going at the time of the open().  The backticks
       then make STDOUT go to a string, but don't change STDERR
       (which still goes to the old STDOUT).

       Note that you must use Bourne shell (sh(1)) redirection
       syntax in backticks, not csh(1)!  Details on why Perl's
       system() and backtick and pipe opens all use the Bourne
       shell are in the versus/csh.whynot article in the "Far
       More Than You Ever Wanted To Know" collection in
       http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz .  To capture
 a command's STDERR and STDOUT together:

           $output = `cmd 2>&1`;                        #  either
with backticks
           $pid  = open(PH, "cmd 2>&1 |");              # or with
an open pipe
           while (<PH>) { }                            #     plus
a read

       To capture a command's STDOUT but discard its STDERR:

           $output  =  `cmd 2>/dev/null`;                # either
with backticks
           $pid = open(PH, "cmd 2>/dev/null |");       # or  with
an open pipe
           while  (<PH>) { }                            #    plus
a read

       To capture a command's STDERR but discard its STDOUT:

           $output = `cmd 2>&1 1>/dev/null`;            #  either
with backticks
           $pid  = open(PH, "cmd 2>&1 1>/dev/null |");  # or with
an open pipe
           while (<PH>) { }                            #     plus
a read

       To exchange a command's STDOUT and STDERR in order to capture
 the STDERR but leave its STDOUT to come out our old

           $output = `cmd 3>&1 1>&2 2>&3 3>&-`;         #  either
with backticks
           $pid  = open(PH, "cmd 3>&1 1>&2 2>&3 3>&-|");# or with
an open pipe
           while (<PH>) { }                            #     plus
a read

       To read both a command's STDOUT and its STDERR separately,
       it's easiest to redirect them separately to files, and
       then read from those files when the program is done:

           system("program    args    1>program.stdout     2>program.stderr");

       Ordering is important in all these examples.  That's
       because the shell processes file descriptor redirections
       in strictly left to right order.

           system("prog args 1>tmpfile 2>&1");
           system("prog args 2>&1 1>tmpfile");

       The first command sends both standard out and standard
       error to the temporary file.  The second command sends
       only the old standard output there, and the old standard
       error shows up on the old standard out.
       Why doesn't open() return an error when a pipe open fails?

       If the second argument to a piped open() contains shell
       metacharacters, perl fork()s, then exec()s a shell to
       decode the metacharacters and eventually run the desired
       program.  If the program couldn't be run, it's the shell
       that gets the message, not Perl. All your Perl program can
       find out is whether the shell itself could be successfully
       started.  You can still capture the shell's STDERR and
       check it for error messages.  See "How can I capture
       STDERR from an external command?" elsewhere in this document,
 or use the IPC::Open3 module.

       If there are no shell metacharacters in the argument of
       open(), Perl runs the command directly, without using the
       shell, and can correctly report whether the command

       What's wrong with using backticks in a void context?

       Strictly speaking, nothing.  Stylistically speaking, it's
       not a good way to write maintainable code.  Perl has several
 operators for running external commands.  Backticks
       are one; they collect the output from the command for use
       in your program.  The "system" function is another; it
       doesn't do this.

       Writing backticks in your program sends a clear message to
       the readers of your code that you wanted to collect the
       output of the command.  Why send a clear message that
       isn't true?

       Consider this line:

           `cat /etc/termcap`;

       You forgot to check $? to see whether the program even ran
       correctly.  Even if you wrote

           print `cat /etc/termcap`;

       this code could and probably should be written as

           system("cat /etc/termcap") == 0
               or die "cat program failed!";

       which will get the output quickly (as it is generated,
       instead of only at the end) and also check the return

       system() also provides direct control over whether shell
       wildcard processing may take place, whereas backticks do
       How can I call backticks without shell processing?

       This is a bit tricky.  You can't simply write the command
       like this:

           @ok = `grep @opts '$search_string' @filenames`;

       As of Perl 5.8.0, you can use open() with multiple arguments.
  Just like the list forms of system() and exec(),
       no shell escapes happen.

          open( GREP, "-|", 'grep', @opts, $search_string, @filenames );
          chomp(@ok = <GREP>);
          close GREP;

       You can also:

           my @ok = ();
           if (open(GREP, "-|")) {
               while (<GREP>) {
                   push(@ok, $_);
               close GREP;
           } else {
               exec 'grep', @opts, $search_string, @filenames;

       Just as with system(), no shell escapes happen when you
       exec() a list.  Further examples of this can be found in
       "Safe Pipe Opens" in perlipc.

       Note that if you're use Microsoft, no solution to this
       vexing issue is even possible.  Even if Perl were to emulate
 fork(), you'd still be stuck, because Microsoft does
       not have a argc/argv-style API.

       Why can't my script read from STDIN after I gave it EOF
       (^D on Unix, ^Z on MS-DOS)?

       Some stdio's set error and eof flags that need clearing.
       The POSIX module defines clearerr() that you can use.
       That is the technically correct way to do it.  Here are
       some less reliable workarounds:

       1   Try keeping around the seekpointer and go there, like

               $where = tell(LOG);
               seek(LOG, $where, 0);

       2   If that doesn't work, try seeking to a different part
           of the file and then back.
       3   If that doesn't work, try seeking to a different part
           of the file, reading something, and then seeking back.

       4   If that doesn't work, give up on your stdio package
           and use sysread.

       How can I convert my shell script to perl?

       Learn Perl and rewrite it.  Seriously, there's no simple
       converter.  Things that are awkward to do in the shell are
       easy to do in Perl, and this very awkwardness is what
       would make a shell->perl converter nigh-on impossible to
       write.  By rewriting it, you'll think about what you're
       really trying to do, and hopefully will escape the shell's
       pipeline datastream paradigm, which while convenient for
       some matters, causes many inefficiencies.

       Can I use perl to run a telnet or ftp session?

       Try the Net::FTP, TCP::Client, and Net::Telnet modules
       (available from CPAN).  http://www.cpan.org/scripts/net-
       stuff/telnet.emul.shar will also help for emulating the
       telnet protocol, but Net::Telnet is quite probably easier
       to use..

       If all you want to do is pretend to be telnet but don't
       need the initial telnet handshaking, then the standard
       dual-process approach will suffice:

           use IO::Socket;             # new in 5.004
           $handle = IO::Socket::INET->new('www.perl.com:80')
                   ||  die  "can't  connect   to   port   80   on
www.perl.com: $!";
           if (fork()) {               # XXX: undef means failure
               print while <STDIN>;    # everything from stdin to
           } else {
               print  while  <$handle>;  # everything from socket
to stdout
           close $handle;

       How can I write expect in Perl?

       Once upon a time, there was a library called chat2.pl
       (part of the standard perl distribution), which never
       really got finished.  If you find it somewhere, don't use
       it.  These days, your best bet is to look at the Expect
       module available from CPAN, which also requires two other
       modules from CPAN, IO::Pty and IO::Stty.
       Is there a way to hide perl's command line from programs
       such as "ps"?

       First of all note that if you're doing this for security
       reasons (to avoid people seeing passwords, for example)
       then you should rewrite your program so that critical
       information is never given as an argument.  Hiding the
       arguments won't make your program completely secure.

       To actually alter the visible command line, you can assign
       to the variable $0 as documented in perlvar.  This won't
       work on all operating systems, though.  Daemon programs
       like sendmail place their state there, as in:

           $0 = "orcus [accepting connections]";

       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?

           In the strictest sense, it can't be done--the script
           executes as a different process from the shell it was
           started from.  Changes to a process are not reflected
           in its parent--only in any children created after the
           change.  There is shell magic that may allow you to
           fake it by eval()ing the script's output in your
           shell; check out the comp.unix.questions FAQ for

       How do I close a process's filehandle without waiting for
       it to complete?

       Assuming your system supports such things, just send an
       appropriate signal to the process (see "kill" in perlfunc).
  It's common to first send a TERM signal, wait a
       little bit, and then send a KILL signal to finish it  off.

       How do I fork a daemon process?

       If by daemon process you mean one that's detached (disassociated
 from its tty), then the following process is
       reported to work on most Unixish systems.  Non-Unix users
       should check their Your_OS::Process module for other solutions.

       o   Open /dev/tty and use the TIOCNOTTY ioctl on it.  See
           tty for details.  Or better yet, you can just use the
           POSIX::setsid() function, so you don't have to worry
           about process groups.

       o   Change directory to /
       o   Reopen STDIN, STDOUT, and STDERR so they're not connected
 to the old tty.

       o   Background yourself like this:

               fork && exit;

       The Proc::Daemon module, available from CPAN, provides a
       function to perform these actions for you.

       How do I find out if I'm running interactively or not?

       Good question.  Sometimes "-t STDIN" and "-t STDOUT" can
       give clues, sometimes not.

           if (-t STDIN && -t STDOUT) {
               print "Now what? ";

       On POSIX systems, you can test whether your own process
       group matches the current process group of your controlling
 terminal as follows:

           use POSIX qw/getpgrp tcgetpgrp/;
           open(TTY, "/dev/tty") or die $!;
           $tpgrp = tcgetpgrp(fileno(*TTY));
           $pgrp = getpgrp();
           if ($tpgrp == $pgrp) {
               print "foreground0;
           } else {
               print "background0;

       How do I timeout a slow event?

       Use the alarm() function, probably in conjunction with a
       signal handler, as documented in "Signals" in perlipc and
       the section on ``Signals'' in the Camel.  You may instead
       use the more flexible Sys::AlarmCall module available from

       The alarm() function is not implemented on all versions of
       Windows.  Check the documentation for your specific version
 of Perl.

       How do I set CPU limits?

       Use the BSD::Resource module from CPAN.

       How do I avoid zombies on a Unix system?

       Use the reaper code from "Signals" in perlipc to call
       wait() when a SIGCHLD is received, or else use the doublefork
 technique described in "How do I start a process in
       the background?" in perlfaq8.

       How do I use an SQL database?

       The DBI module provides an abstract interface to most
       database servers and types, including Oracle, DB2, Sybase,
       mysql, Postgresql, ODBC, and flat files.  The DBI module
       accesses each database type through a database driver, or
       DBD.  You can see a complete list of available drivers on
       CPAN: http://www.cpan.org/modules/by-module/DBD/ .  You
       can read more about DBI on http://dbi.perl.org .

       Other modules provide more specific access: Win32::ODBC,
       Alzabo, iodbc, and others found on CPAN Search:
       http://search.cpan.org .

       How do I make a system() exit on control-C?

       You can't.  You need to imitate the system() call (see
       perlipc for sample code) and then have a signal handler
       for the INT signal that passes the signal on to the subprocess.
  Or you can check for it:

           $rc = system($cmd);
           if ($rc & 127) { die "signal death" }

       How do I open a file without blocking?

       If you're lucky enough to be using a system that supports
       non-blocking reads (most Unixish systems do), you need
       only to use the O_NDELAY or O_NONBLOCK flag from the Fcntl
       module in conjunction with sysopen():

           use Fcntl;
           sysopen(FH,  "/foo/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
               or die "can't open /foo/somefile: $!":

       How do I install a module from CPAN?

       The easiest way is to have a module also named CPAN do it
       for you.  This module comes with perl version 5.004 and

           $ perl -MCPAN -e shell

           cpan shell -- CPAN exploration and  modules  installation (v1.59_54)
           ReadLine support enabled

           cpan> install Some::Module

       To manually install the CPAN module, or any well-behaved
       CPAN module for that matter, follow these steps:

       1   Unpack the source into a temporary area.
               perl Makefile.PL


               make test

               make install

       If your version of perl is compiled without dynamic loading,
 then you just need to replace step 3 (make) with make
       perl and you will get a new perl binary with your extension
 linked in.

       See ExtUtils::MakeMaker for more details on building
       extensions.  See also the next question, ``What's the difference
 between require and use?''.

       What's the difference between require and use?

       Perl offers several different ways to include code from
       one file into another.  Here are the deltas between the
       various inclusion constructs:

           1)  do $file is like eval `cat $file`, except the former
               1.1: searches @INC and updates %INC.
               1.2: bequeaths an *unrelated* lexical scope on the
eval'ed code.

           2)  require $file is like do $file, except the former
               2.1:  checks  for  redundant loading, skipping already loaded files.
               2.2: raises an exception on failure to find,  compile, or execute $file.

           3)  require Module is like require "Module.pm", except
the former
               3.1: translates each "::" into your  system's  directory separator.
               3.2:  primes the parser to disambiguate class Module as an indirect object.

           4)  use Module is like require Module, except the former
               4.1:  loads  the  module at compile time, not runtime.
               4.2: imports symbols and semantics from that package to the current one.

       In general, you usually want "use" and a proper Perl module.

       How do I keep my own module/library directory?

       When you build modules, use the PREFIX and LIB options
       when generating Makefiles:

           perl     Makefile.PL    PREFIX=/mydir/perl    LIB=/mydir/perl/lib

       then either set the PERL5LIB environment variable before
       you run scripts that use the modules/libraries (see perlrun)
 or say

           use lib '/mydir/perl/lib';

       This is almost the same as

           BEGIN {
               unshift(@INC, '/mydir/perl/lib');

       except that the lib module checks for machine-dependent
       subdirectories.  See Perl's lib for more information.

       How do I add the directory my program lives in to the mod-    [Toc]    [Back]
       ule/library search path?

           use FindBin;
           use lib "$FindBin::Bin";
           use your_own_modules;

       How do I add a directory to my include path (@INC) at run-

       Here are the suggested ways of modifying your include

           the PERLLIB environment variable
           the PERL5LIB environment variable
           the perl -Idir command line flag
           the use lib pragma, as in
               use lib "$ENV{HOME}/myown_perllib";

       The latter is particularly useful because it knows about
       machine dependent architectures.  The lib.pm pragmatic
       module was first included with the 5.002 release of  Perl.

       What is socket.ph and where do I get it?

       It's a perl4-style file defining values for system networking
 constants.  Sometimes it is built using h2ph when
       Perl is installed, but other times it is not.  Modern programs
 "use Socket;" instead.


       Copyright (c) 1997-2003 Tom Christiansen and Nathan Torkington.
  All rights reserved.

       This documentation is free; you can redistribute it and/or
       modify it under the same terms as Perl itself.

       Irrespective of its distribution, all code examples in
       this file are hereby placed into the public domain.  You
       are permitted and encouraged to use this code in your own
       programs for fun or for profit as you see fit.  A simple
       comment in the code giving credit would be courteous but
       is not required.

perl v5.8.5                 2002-11-06                         24
[ Back ]
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