PERLFAQ8(1)      Perl Programmers Reference Guide     PERLFAQ8(1)

       perlfaq8 - System Interaction ($Revision: 1.36 $, $Date:
       1999/01/08 05:36:34 $)

       This section of the Perl FAQ covers questions involving
       operating system interaction.  This involves 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, the perlvms manpage,
       the perlplan9 manpage, ...).  These should contain more
       detailed information on the vagaries of your perl.

       HHooww ddoo II ffiinndd oouutt wwhhiicchh ooppeerraattiinngg ssyysstteemm II''mm rruunnnniinngg

       The $^O variable ($OSNAME if you use English) contains the
       operating system that your perl binary was built for.

       HHooww ccoommee 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.

       HHooww ddoo II ddoo ffaannccyy ssttuuffff wwiitthh tthhee kkeeyybbooaarrdd//ssccrreeeenn//mmoouussee??

       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.

       HHooww ddoo II pprriinntt ssoommeetthhiinngg oouutt iinn ccoolloorr??

       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!\n", color("reset");
           print color("green"), "Go!\n", color("reset");

       Or like this:

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

       HHooww ddoo II rreeaadd jjuusstt oonnee kkeeyy wwiitthhoouutt wwaaiittiinngg ffoorr aa rreettuurrnn

       Controlling input buffering is a remarkably system-
       dependent matter.  If most systems, you can just use the
       ssttttyy command as shown in the getc entry in the perlfunc
       manpage, 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 ssttttyy for each key.  It even includes limited support
       for Windows.

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

       However, that 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

           use HotKey;
           $key = readkey();

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

           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() }


       HHooww ddoo II cchheecckk wwhheetthheerr iinnppuutt iiss rreeaaddyy oonn tthhee kkeeyybbooaarrdd??

       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

       HHooww ddoo II cclleeaarr tthhee ssccrreeeenn??

       If you only have to 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');

       HHooww ddoo II ggeett tthhee ssccrreeeenn ssiizzee??

       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/';
           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: $!)\n", &TIOCGWINSZ;
           ($row, $col, $xpixel, $ypixel) = unpack('S4', $winsize);
           print "(row,col) = ($row,$col)";
           print "  (xpixel,ypixel) = ($xpixel,$ypixel)" if $xpixel || $ypixel;
           print "\n";

       HHooww ddoo II aasskk tthhee uusseerr ffoorr aa ppaasssswwoorrdd??

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

       There's an example of this in the crypt entry in the
       perlfunc manpage).  First, you put the terminal into "no
       echo" mode, then just read the password normally.  You may
       do this with an old-style ioctl() function, POSIX terminal
       control (see the POSIX manpage, and Chapter 7 of the
       Camel), or a call to the ssttttyy 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);

       HHooww ddoo II rreeaadd aanndd wwrriittee tthhee sseerriiaall ppoorrtt??

       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, the
       devices 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 behaviour 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 the
           section on open in the perlfunc manpage 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 the section
           on sysopen in the perlfunc manpage for more on this

       end of line
           Some devices will be expecting a "\r" at the end of
           each line rather than a "\n".  In some ports of perl,
           "\r" and "\n" are different from their usual (Unix)
           ASCII values of "\012" and "\015".  You may have to
           give the numeric values you want directly, using octal
           ("\015"), hex ("0x0D"), or as a control-character
           specification ("\cM").

               print DEV "atv1\012";       # wrong, for some devices
               print DEV "atv1\015";       # right, for some devices

           Even though with normal text files, a "\n" 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 "\015\012", 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 the section on $| in the
           perlvar manpage and the select entry in the perlfunc

               $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 hardcode your line terminators, in that

       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 the alarm entry in the perlfunc
           manpage).  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 the section on select in
           the perlfunc manpage.

       While trying to read from his caller-id box, the notorious
       Jamie Zawinski <>, 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 $stty = `/bin/stty -g`;
               open2( \*MODEM_IN, \*MODEM_OUT, "cu -l$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 `Connected'\n";

       HHooww ddoo II ddeeccooddee eennccrryypptteedd ppaasssswwoorrdd ffiilleess??

       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).

       HHooww ddoo II ssttaarrtt aa pprroocceessss iinn tthhee bbaacckkggrroouunndd??

       You could use

           system("cmd &")

       or you could use fork as documented in the section on fork
       in the perlfunc manpage, with further examples in the
       perlipc manpage.  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 opening a pipe (see the section on
           open in the perlfunc manpage) but on some systems this
           means that the child process cannot outlive the

           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 };

           See the section on Signals in the perlipc manpage for
           other examples of code to do this.  Zombies are not an
           issue with system("prog &").

       HHooww ddoo II ttrraapp ccoonnttrrooll cchhaarraacctteerrss//ssiiggnnaallss??

       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 the
       section on Signals in the perlipc manpage and chapter 6 of
       the Camel.

       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: set a variable and exit.
       And 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, "ouch\n", 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 the
       section on Signals in the perlipc manpage or chapter 6 of
       the Camel.

       HHooww ddoo II mmooddiiffyy tthhee sshhaaddooww ppaasssswwoorrdd ffiillee oonn aa UUnniixx ssyysstteemm??

       If perl was installed correctly, and your shadow library
       was written properly, the getpw*() functions described in
       the perlfunc manpage 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 the passwd(5) manpage
       for specifics) and use pwd_mkdb(8) to install it (see the
       pwd_mkdb(5) manpage for more details).

       HHooww ddoo II sseett tthhee ttiimmee aanndd ddaattee??

       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 timezone,
       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";

       HHooww ccaann II 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 the section on select
       in the perlfunc manpage.  If your system has itimers and
       syscall() support, you can check out the old example in

       HHooww ccaann II mmeeaassuurree ttiimmee uunnddeerr aa sseeccoonndd??

       In general, you may not be able to.  The Time::HiRes
       module (available from CPAN) 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/';

           $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] );

       HHooww ccaann II ddoo aann 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 the perlmod manpage
       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
       the section on Signals in the perlipc manpage and chapter
       6 of the Camel.

       If exception handling is all you're interested in, try the library (part of the standard perl

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

       WWhhyy ddooeessnn''tt mmyy ssoocckkeettss pprrooggrraamm wwoorrkk uunnddeerr SSyysstteemm VV
       ((SSoollaarriiss))?? WWhhaatt ddooeess tthhee eerrrroorr mmeessssaaggee """"PPrroottooccooll nnoott
       ssuuppppoorrtteedd"""" mmeeaann??

       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.

       HHooww ccaann II ccaallll mmyy ssyysstteemm''ss uunniiqquuee CC ffuunnccttiioonnss ffrroomm PPeerrll??

       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 the 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.

       WWhheerree ddoo II ggeett tthhee iinncclluuddee ffiilleess ttoo ddoo 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 the
       perlxstut manpage for how to get started with h2xs.

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

       WWhhyy ddoo sseettuuiidd ppeerrll ssccrriippttss ccoommppllaaiinn aabboouutt kkeerrnneell pprroobblleemmss??

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

       HHooww ccaann II ooppeenn aa ppiippee bbootthh ttoo aanndd ffrroomm aa ccoommmmaanndd??

       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 the IPC::Open2 manpage).  See the section on
       Bidirectional Communication with Another Process in the
       perlipc manpage and the section on Bidirectional
       Communication with Yourself in the perlipc manpage

       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 the
       IPC::Open3 manpage).

       WWhhyy ccaann''tt II ggeett tthhee oouuttppuutt ooff aa ccoommmmaanndd wwiitthh 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`;

       HHooww ccaann II ccaappttuurree SSTTDDEERRRR ffrroomm aann eexxtteerrnnaall ccoommmmaanndd??

       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 versions of these, unless the
       command redirects them.  Backticks and open() read oonnllyy
       the STDOUT of your command.

       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 .
       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 STDERR:

           $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 and safest to redirect them separately to
       files, and then read from those files when the program is

           system("program args 1>/tmp/program.stdout 2>/tmp/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.

       WWhhyy ddooeessnn''tt open() return an error when a pipe open fails?

       Because the pipe open takes place in two steps: first Perl
       calls fork() to start a new process, then this new process
       calls exec() to run the program you really wanted to open.
       The first step reports success or failure to your process,
       so open() can only tell you whether the fork() succeeded
       or not.

       To find out if the exec() step succeeded, you have to
       catch SIGCHLD and wait() to get the exit status.  You
       should also catch SIGPIPE if you're writing to the
       child--you may not have found out the exec() failed by the
       time you write.  This is documented in the perlipc

       In some cases, even this won't work.  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.
       Now when you call wait(), you only learn whether or not
       the shell could be successfully started.  Best to avoid
       shell metacharacters.

       On systems that follow the spawn() paradigm, open() might
       do what you expect--unless perl uses a shell to start your
       command. In this case the fork()/exec() description still

       WWhhaatt''ss wwrroonngg wwiitthh uussiinngg bbaacckkttiicckkss iinn aa vvooiidd ccoonntteexxtt??

       Strictly speaking, nothing.  Stylistically speaking, it's
       not a good way to write maintainable code because
       backticks have a (potentially humungous) return value, and
       you're ignoring it.  It's may also not be very efficient,
       because you have to read in all the lines of output,
       allocate memory for them, and then throw it away.  Too
       often people are lulled to writing:

           `cp file file.bak`;

       And now they think "Hey, I'll just always use backticks to
       run programs."  Bad idea: backticks are for capturing a
       program's output; the system() function is for running

       Consider this line:

           `cat /etc/termcap`;

       You haven't assigned the output anywhere, so it just
       wastes memory (for a little while).  Plus you forgot to
       check $? to see whether the program even ran correctly.
       Even if you wrote

           print `cat /etc/termcap`;

       In most cases, this could and probably should be written

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

       Which will get the output quickly (as its 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

       HHooww ccaann II ccaallll bbaacckkttiicckkss wwiitthhoouutt sshheellll pprroocceessssiinngg??

       This is a bit tricky.  Instead of writing

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

       You have to do this:

           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.

       There are more examples of this the section on Safe Pipe
       Opens in the perlipc manpage.

       WWhhyy ccaann''tt mmyy ssccrriipptt rreeaadd ffrroomm SSTTDDIINN aafftteerr II ggaavvee iitt EEOOFF
       ((^^DD oonn UUnniixx,, ^^ZZ oonn MMSS--DDOOSS))??

       Because 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.

       HHooww ccaann II ccoonnvveerrtt mmyy sshheellll ssccrriipptt ttoo ppeerrll??

       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.

       CCaann II uussee ppeerrll ttoo rruunn aa tteellnneett oorr ffttpp sseessssiioonn??

       Try the Net::FTP, TCP::Client, and Net::Telnet modules
       (available from CPAN).
       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('')
                   || die "can't connect to port 80 on $!";
           if (fork()) {               # XXX: undef means failure
               print while <STDIN>;    # everything from stdin to socket
           } else {
               print while <$handle>;  # everything from socket to stdout
           close $handle;

       HHooww ccaann II wwrriittee eexxppeecctt iinn PPeerrll??

       Once upon a time, there was a library called
       (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.

       IIss tthheerree aa wwaayy ttoo hhiiddee ppeerrll''ss ccoommmmaanndd lliinnee ffrroomm pprrooggrraammss
       ssuucchh aass """"ppss""""??

       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 the perlvar manpage.
       This won't work on all operating systems, though.  Daemon
       programs like sendmail place their state there, as in:

           $0 = "orcus [accepting connections]";

       II {{cchhaannggeedd ddiirreeccttoorryy,, mmooddiiffiieedd mmyy eennvviirroonnmmeenntt}} iinn aa ppeerrll
       ssccrriipptt..  HHooww ccoommee tthhee cchhaannggee ddiissaappppeeaarreedd wwhheenn II eexxiitteedd tthhee
       ssccrriipptt??  HHooww ddoo II ggeett mmyy cchhaannggeess ttoo bbee vviissiibbllee??

           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 its own 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

       HHooww ddoo II cclloossee aa pprroocceessss''ss ffiilleehhaannddllee wwiitthhoouutt wwaaiittiinngg ffoorr
       iitt ttoo ccoommpplleettee??

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

       HHooww ddoo II ffoorrkk aa ddaaeemmoonn pprroocceessss??

       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

       o   Open /dev/tty and use the TIOCNOTTY ioctl on it.  See
           the tty(4) manpage 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.

       HHooww ddoo II mmaakkee mmyy pprrooggrraamm rruunn wwiitthh sshh aanndd ccsshh??

       See the eg/nih script (part of the perl source

       HHooww ddoo II ffiinndd oouutt iiff II''mm rruunnnniinngg iinntteerraaccttiivveellyy oorr nnoott??

       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 "foreground\n";
           } else {
               print "background\n";

       HHooww ddoo II ttiimmeeoouutt aa ssllooww eevveenntt??

       Use the alarm() function, probably in conjunction with a
       signal handler, as documented the section on Signals in
       the perlipc manpage and chapter 6 of the Camel.  You may
       instead use the more flexible Sys::AlarmCall module
       available from CPAN.

       HHooww ddoo II sseett CCPPUU lliimmiittss??

       Use the BSD::Resource module from CPAN.

       HHooww ddoo II aavvooiidd zzoommbbiieess oonn aa UUnniixx ssyysstteemm??

       Use the reaper code from the section on Signals in the
       perlipc manpage to call wait() when a SIGCHLD is received,
       or else use the double-fork technique described in the
       fork entry in the perlfunc manpage.

       HHooww ddoo II uussee aann SSQQLL ddaattaabbaassee??

       There are a number of excellent interfaces to SQL
       databases.  See the DBD::* modules available from .  A lot of
       information on this can be found at .

       HHooww ddoo II mmaakkee aa system() exit on control-C?

       You can't.  You need to imitate the system() call (see the
       perlipc manpage 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" }

       HHooww ddoo II ooppeenn aa ffiillee wwiitthhoouutt bblloocckkiinngg??

       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, "/tmp/somefile", O_WRONLY|O_NDELAY|O_CREAT, 0644)
               or die "can't open /tmp/somefile: $!":

       HHooww ddoo II iinnssttaallll aa CCPPAANN mmoodduullee??

       The easiest way is to have the CPAN module do it for you.
       This module comes with perl version 5.004 and later.  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 (mmaakkee) with
       mmaakkee ppeerrll and you will get a new perl binary with your
       extension linked in.

       See the ExtUtils::MakeMaker manpage for more details on
       building extensions.  See also the next question.

       WWhhaatt''ss tthhee ddiiffffeerreennccee bbeettwweeeenn rreeqquuiirree aanndd uussee??

       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 "", 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 run-time.
               4.2: imports symbols and semantics from that package to the current one.

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

       HHooww ddoo II kkeeeepp mmyy oowwnn mmoodduullee//lliibbrraarryy ddiirreeccttoorryy??

       When you build modules, use the PREFIX option when
       generating Makefiles:

           perl Makefile.PL PREFIX=/u/mydir/perl

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

           use lib '/u/mydir/perl';

       This is almost the same as:

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

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

       HHooww ddoo II aadddd tthhee ddiirreeccttoorryy mmyy pprrooggrraamm lliivveess iinn ttoo tthhee
       mmoodduullee//lliibbrraarryy sseeaarrcchh ppaatthh??

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

       HHooww ddoo II aadddd aa ddiirreeccttoorryy ttoo mmyy iinncclluuddee ppaatthh aatt rruunnttiimmee??

       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 pragmatic
       module was first included with the 5.002 release of Perl.

       WWhhaatt iiss ssoocckkeett..pphh aanndd wwhheerree ddoo II ggeett iitt??

       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) 1997-1999 Tom Christiansen and Nathan
       Torkington.  All rights reserved.

       When included as part of the Standard Version of Perl, or
       as part of its complete documentation whether printed or
       otherwise, this work may be distributed only under the
       terms of Perl's Artistic Licence.  Any distribution of
       this file or derivatives thereof outside of that package
       require that special arrangements be made with copyright

       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.

27/Mar/1999            perl 5.005, patch 03                     1