MirBSD manpage: perlfaq8(1)

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


     perlfaq8 - System Interaction


     This section of the Perl FAQ covers questions involving
     operating system interaction.  Topics include interprocess
     communication (IPC), control over the user-interface (key-
     board, 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 under?

     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 run-
     ning program with a different one.  If you want to keep
     going (as is probably the case if you're asking this ques-
     tion) use system() instead.

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

     How you access/control keyboards, screens, and pointing dev-
     ices ("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 as examples in other
     answers in this section of the perlfaq.

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     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!\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;

     How do I read just one key without waiting for a return key?

     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 Win-

         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

         use HotKey;
         $key = readkey();

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

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         # 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;


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         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 times:

         $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

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

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     How do I ask the user for a password?

     (This question has nothing to do with the web.  See a dif-
     ferent 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 old-style ioctl()
     function, POSIX terminal control (see POSIX or its documen-
     tation 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 run-
     ning 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 syso-
         pen() 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
         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

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             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 out-
         put. This applies especially to socket I/O and auto-
         flushing, 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 perl-
         func, 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 $| variable:

             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

     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

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     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";

     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 pass-
     words, 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 pass-
     word (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

     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,

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         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 pro-
         cess 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 "Sig-
     nals" in perlipc and the section on "Signals" in the Camel.

     You can set the values of the %SIG hash to be the functions
     you want to handle the signal.  After perl catches the sig-
     nal, it looks in %SIG for a key with the same name as the
     signal, then calls the subroutine value for that key.

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             # as an anonymous subroutine

             $SIG{INT} = sub { syswrite(STDERR, "ouch\n", 5 ) };

             # or a reference to a function

             $SIG{INT} = \&ouch;

             # or the name of the function as a string

             $SIG{INT} = "ouch";

     Perl versions before 5.8 had in its C source code signal
     handlers which would catch the signal and possibly run a
     Perl function that you had set in %SIG.  This violated the
     rules of signal handling at that level causing perl to dump
     core. Since version 5.8.0, perl looks at %SIG *after* the
     signal has been caught, rather than while it is being
     caught. Previous versions of this answer were incorrect.

     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 perl-
     func 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 run-
     ning 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 variable:

         $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

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     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

     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

     For example, you can use this to make sure your filter pro-
     gram managed to finish its output without filling up the

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         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 per-
     lipc 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 distribu-

     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 compati-
     ble, 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 distribu-
     tion, 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 func-
     tion, you can inline a bit of C in your Perl source with

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

     Historically, these would be generated by the h2ph tool,
     part of the standard perl distribution.  This program con-
     verts cpp(1) directives in C header files to files contain-
     ing subroutine definitions, like &SYS_getitimer, which you
     can use as arguments to your functions. It doesn't work per-
     fectly, 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 por-
     tability 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 sys-

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

     The IPC::Open2 module (part of the standard perl distribu-
     tion) 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 IPC::Open3).

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     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 pro-
     cess 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 system() com-
     mand redirects them. Backticks and open() read only the
     STDOUT of your command.

     You can also use the open3() function from IPC::Open3.  Ben-
     jamin 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 $pid = open3(gensym, \*PH, ">&NULL", "cmd");
         while( <PH> ) { }
         waitpid($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 $pid = open3(gensym, ">&NULL", \*PH, "cmd");
         while( <PH> ) { }
         waitpid($pid, 0);

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

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         use IPC::Open3;
         use Symbol qw(gensym);
         my $pid = open3(gensym, ">&STDERR", \*PH, "cmd");
         while( <PH> ) { }
         waitpid($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_tmpfile;
         local *CATCHERR = IO::File->new_tmpfile;
         my $pid = open3(gensym, ">&CATCHOUT", ">&CATCHERR", "cmd");
         waitpid($pid, 0);
         seek $_, 0, 0 for \*CATCHOUT, \*CATCHERR;
         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_tmpfile;
         my $pid = open3(gensym, \*CATCHOUT, ">&CATCHERR", "cmd");
         while( <CATCHOUT> ) {}
         waitpid($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:

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         $output = `$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 syn-
     tax in backticks, not csh(1)!  Details on why Perl's sys-
     tem() 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 cap-
     ture 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:

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         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

     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 started.

     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

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

     Consider this line:

         `cat /etc/termcap`;

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

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         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 value.

     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 argu-
     ments. 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 vex-
     ing 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.

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     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 try-
     ing 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/netstuff/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:

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         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: $!";
         if (fork()) {               # XXX: undef means failure
             print while <STDIN>;    # everything from stdin to socket
         } else {
             print while <$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 avail-
     able 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 details.

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     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 (disasso-
     ciated 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.

     +   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.

     +   Change directory to /

     +   Reopen STDIN, STDOUT, and STDERR so they're not con-
         nected to the old tty.

     +   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:

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         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";

     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 CPAN.

     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 double-fork
     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 data-
     base 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 per-
     lipc 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:

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         $rc = system($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 tell the difference between errors from the shell
     and perl?

     (answer contributed by brian d foy, "<bdfoy@cpan.org>"

     When you run a Perl script, something else is running the
     script for you, and that something else may output error
     messages.  The script might emit its own warnings and error
     messages.  Most of the time you cannot tell who said what.

     You probably cannot fix the thing that runs perl, but you
     can change how perl outputs its warnings by defining a cus-
     tom warning and die functions.

     Consider this script, which has an error you may not notice


             print "Hello World\n";

     I get an error when I run this from my shell (which happens
     to be bash).  That may look like perl forgot it has a
     print() function, but my shebang line is not the path to
     perl, so the shell runs the script, and I get the error.

             $ ./test
             ./test: line 3: print: command not found

     A quick and dirty fix involves a little bit of code, but
     this may be all you need to figure out the problem.

             #!/usr/bin/perl -w

             BEGIN {
             $SIG{__WARN__} = sub{ print STDERR "Perl: ", @_; };
             $SIG{__DIE__}  = sub{ print STDERR "Perl: ", @_; exit 1};

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             $a = 1 + undef;
             $x / 0;

     The perl message comes out with "Perl" in front.  The BEGIN
     block works at compile time so all of the compilation errors
     and warnings get the "Perl:" prefix too.

             Perl: Useless use of division (/) in void context at ./test line 9.
             Perl: Name "main::a" used only once: possible typo at ./test line 8.
             Perl: Name "main::x" used only once: possible typo at ./test line 9.
             Perl: Use of uninitialized value in addition (+) at ./test line 8.
             Perl: Use of uninitialized value in division (/) at ./test line 9.
             Perl: Illegal division by zero at ./test line 9.
             Perl: Illegal division by zero at -e line 3.

     If I don't see that "Perl:", it's not from perl.

     You could also just know all the perl errors, and although
     there are some people who may know all of them, you probably
     don't.  However, they all should be in the perldiag manpage.
     If you don't find the error in there, it probably isn't a
     perl error.

     Looking up every message is not the easiest way, so let perl
     to do it for you.  Use the diagnostics pragma with turns
     perl's normal messages into longer discussions on the topic.

             use diagnostics;

     If you don't get a paragraph or two of expanded discussion,
     it might not be perl's message.

     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.


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             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 exten-
     sions. 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 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.

     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

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

         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 sub-
     directories. See Perl's lib for more information.

     How do I add the directory my program lives in to the
     module/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 path:

         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 network-
     ing constants.  Sometimes it is built using h2ph when Perl
     is installed, but other times it is not.  Modern programs
     "use Socket;" instead.
     Copyright (c) 1997-2006 Tom Christiansen, Nathan Torkington,
     and other authors as noted. All rights reserved.

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

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     Irrespective of its distribution, all code examples in this
     file are hereby placed into the public domain.  You are per-
     mitted 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

perl v5.8.8                2006-06-30                          26

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