man pages section 1: User Commands

Exit Print View

Updated: July 2014

perlfaq8 (1)


perlfaq8 - System Interaction


Please see following description for synopsis


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
     (keyboard, screen and pointing devices), and most anything
     else not related to data manipulation.

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

  How do I find out which operating system I'm running 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?
     (contributed by brian d foy)

     The "exec" function's job is to turn your process into
     another command and never to return. If that's not what you
     want to do, don't use "exec". :)

     If you want to run an external command and still keep your
     Perl process going, look at a piped "open", "fork", or

  How do I do fancy stuff with the keyboard/screen/mouse?
     How you access/control keyboards, screens, and pointing
     devices ("mice") is system-dependent.  Try the following

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

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

                 Tk                      CPAN

perl v5.12.5         Last change: 2012-11-03                    1

Perl Programmers Reference Guide                      PERLFAQ8(1)

     Some of these specific cases are shown as examples in other
     answers in this section of the perlfaq.

  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

             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 system (assuming your system supports

             use HotKey;
             $key = readkey();

perl v5.12.5         Last change: 2012-11-03                    2

Perl Programmers Reference Guide                      PERLFAQ8(1)

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

             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;

perl v5.12.5         Last change: 2012-11-03                    3

Perl Programmers Reference Guide                      PERLFAQ8(1)


             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?
     (contributed by brian d foy)

     To clear the screen, you just have to print the special
     sequence that tells the terminal to clear the screen. Once
     you have that sequence, output it when you want to clear the

     You can use the "Term::ANSIScreen" module to get the special
     sequence. Import the "cls" function (or the ":screen" tag):

             use Term::ANSIScreen qw(cls);
             my $clear_screen = cls();

             print $clear_screen;

     The "Term::Cap" module can also get the special sequence if
     you want to deal with the low-level details of terminal
     control. The "Tputs" method returns the string for the given

             use Term::Cap;

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

             print $clear_screen;

     On Windows, you can use the "Win32::Console" module. After
     creating an object for the output filehandle you want to
     affect, call the "Cls" method:


             $OUT = Win32::Console->new(STD_OUTPUT_HANDLE);
             my $clear_string = $OUT->Cls;

             print $clear_screen;

     If you have a command-line program that does the job, you
     can call it in backticks to capture whatever it outputs so
     you can use it later:

perl v5.12.5         Last change: 2012-11-03                    4

Perl Programmers Reference Guide                      PERLFAQ8(1)

             $clear_string = `clear`;

             print $clear_string;

  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/';
             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";

  How do I ask the user for a password?
     (This question has nothing to do with the web.  See a
     different FAQ for that.)

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

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

             use Term::ReadKey;

             $password = ReadLine(0);

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

perl v5.12.5         Last change: 2012-11-03                    5

Perl Programmers Reference Guide                      PERLFAQ8(1)

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

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

     end of line
         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

                 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 $| variable
         to control autoflushing (see "$|" in perlvar and
         "select" in perlfunc, or perlfaq5, "How do I
         flush/unbuffer an output filehandle?  Why must I do

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

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

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

perl v5.12.5         Last change: 2012-11-03                    6

Perl Programmers Reference Guide                      PERLFAQ8(1)

         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 "<>", 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";

  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 do is check
     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.

perl v5.12.5         Last change: 2012-11-03                    7

Perl Programmers Reference Guide                      PERLFAQ8(1)

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

  How do I start a process in the background?
     (contributed by brian d foy)

     There's not a single way to run code in the background so
     you don't have to wait for it to finish before your program
     moves on to other tasks. Process management depends on your
     particular operating system, and many of the techniques are
     in perlipc.

     Several CPAN modules may be able to help, including
     "IPC::Open2" or "IPC::Open3", "IPC::Run", "Parallel::Jobs",
     "Parallel::ForkManager", "POE", "Proc::Background", and
     "Win32::Process". There are many other modules you might
     use, so check those namespaces for other options too.

     If you are on a Unix-like system, you might be able to get
     away with a system call where you put an "&" on the end of
     the command:

             system("cmd &")

     You can also try using "fork", as described in perlfunc
     (although this is the same thing that many of the modules
     will do for you).

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

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

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

                 $SIG{CHLD} = sub { wait };

perl v5.12.5         Last change: 2012-11-03                    8

Perl Programmers Reference Guide                      PERLFAQ8(1)

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

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

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

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

             # 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.

perl v5.12.5         Last change: 2012-11-03                    9

Perl Programmers Reference Guide                      PERLFAQ8(1)

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

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

     However, if all you want to do is change your time zone, you
     can probably get away with setting an environment 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 from CPAN, and starting from Perl 5.8
     "Time::HiRes" is part of the standard distribution).

  How can I measure time under a second?
     (contributed by brian d foy)

     The "Time::HiRes" module (part of the standard distribution
     as of Perl 5.8) measures time with the "gettimeofday()"
     system call, which returns the time in microseconds since
     the epoch. If you can't install "Time::HiRes" for older
     Perls and you are on a Unixish system, you may be able to
     call gettimeofday(2) directly. See "syscall" in perlfunc.

  How can I do an atexit() or setjmp()/longjmp()? (Exception
     You can use the "END" block to simulate "atexit()". Each
     package's "END" block is called when the program or thread
     ends See perlmod manpage for more details about "END"

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

perl v5.12.5         Last change: 2012-11-03                   10

Perl Programmers Reference Guide                      PERLFAQ8(1)

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

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

             use sigtrap qw(die normal-signals);

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

     If exception handling is all you're interested in, use one
     of the many CPAN modules that handle exceptions, such as

     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 supported" mean?

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

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

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

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

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

perl v5.12.5         Last change: 2012-11-03                   11

Perl Programmers Reference Guide                      PERLFAQ8(1)

     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 be hand-edited.  Here's how to install the *.ph files:

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

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

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

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

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

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

  Why can't I get the output of a command with system()?
     You're confusing the purpose of "system()" and backticks
     (``).  "system()" runs a command and returns exit status
     information (as a 16 bit value: the low 7 bits are the
     signal the process died from, if any, and the high 8 bits
     are the actual exit value).  Backticks (``) run a command

perl v5.12.5         Last change: 2012-11-03                   12

Perl Programmers Reference Guide                      PERLFAQ8(1)

     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()" command redirects them.  Backticks and "open()"
     read only the STDOUT of your command.

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

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

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

             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:

perl v5.12.5         Last change: 2012-11-03                   13

Perl Programmers Reference Guide                      PERLFAQ8(1)

             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:

             $output = `$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

perl v5.12.5         Last change: 2012-11-03                   14

Perl Programmers Reference Guide                      PERLFAQ8(1)

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

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

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

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

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

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

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

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

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

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

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

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

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

     The first command sends both standard out and standard error
     to the temporary file.  The second command sends only the

perl v5.12.5         Last change: 2012-11-03                   15

Perl Programmers Reference Guide                      PERLFAQ8(1)

     old standard output there, and the old standard error shows
     up on the old standard out.

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

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

             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 echo the cat command's output as it is generated,
     instead of waiting until the program has completed to print
     it out. It also checks the return value.

     "system" also provides direct control over whether shell
     wildcard processing may take place, whereas backticks do

perl v5.12.5         Last change: 2012-11-03                   16

Perl Programmers Reference Guide                      PERLFAQ8(1)


  How can I call backticks without shell processing?
     This is a bit tricky.  You can't simply write the command
     like this:

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

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

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

     You can also:

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

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

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

  Why can't my script read from STDIN after I gave it EOF (^D on
     Unix, ^Z on MS-DOS)?
     This happens only if your perl is compiled to use stdio
     instead of perlio, which is the default. Some (maybe all?)
     stdios set error and eof flags that you may need to clear.
     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);

perl v5.12.5         Last change: 2012-11-03                   17

Perl Programmers Reference Guide                      PERLFAQ8(1)

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

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

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

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

  Can I use perl to run a telnet or ftp session?
     Try the "Net::FTP", "TCP::Client", and "Net::Telnet" modules
     (available from CPAN). 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('')
                 or 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;

  How can I write expect in Perl?
     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".

perl v5.12.5         Last change: 2012-11-03                   18

Perl Programmers Reference Guide                      PERLFAQ8(1)

  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.

  How do I close a process's filehandle without waiting for it to
     Assuming your system supports such things, just send an
     appropriate signal to the process (see "kill" in perlfunc).
     It's common to first send a TERM signal, wait a little bit,
     and then send a KILL signal to finish it off.

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

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

     o   Change directory to /

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

     o   Background yourself like this:

perl v5.12.5         Last change: 2012-11-03                   19

Perl Programmers Reference Guide                      PERLFAQ8(1)

                 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?
     (contributed by brian d foy)

     This is a difficult question to answer, and the best answer
     is only a guess.

     What do you really want to know? If you merely want to know
     if one of your filehandles is connected to a terminal, you
     can try the "-t" file test:

             if( -t STDOUT ) {
                     print "I'm connected to a terminal!\n";

     However, you might be out of luck if you expect that means
     there is a real person on the other side. With the "Expect"
     module, another program can pretend to be a person. The
     program might even come close to passing the Turing test.

     The "IO::Interactive" module does the best it can to give
     you an answer. Its "is_interactive" function returns an
     output filehandle; that filehandle points to standard output
     if the module thinks the session is interactive. Otherwise,
     the filehandle is a null handle that simply discards the

             use IO::Interactive;

             print { is_interactive } "I might go to standard output!\n";

     This still doesn't guarantee that a real person is answering
     your prompts or reading your output.

     If you want to know how to handle automated testing for your
     distribution, you can check the environment. The CPAN
     Testers, for instance, set the value of "AUTOMATED_TESTING":

             unless( $ENV{AUTOMATED_TESTING} ) {
                     print "Hello interactive tester!\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

perl v5.12.5         Last change: 2012-11-03                   20

Perl Programmers Reference Guide                      PERLFAQ8(1)

     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?
     (contributed by Xho)

     Use the "BSD::Resource" module from CPAN. As an example:

             use BSD::Resource;
             setrlimit(RLIMIT_CPU,10,20) or die $!;

     This sets the soft and hard limits to 10 and 20 seconds,
     respectively.  After 10 seconds of time spent running on the
     CPU (not "wall" time), the process will be sent a signal
     (XCPU on some systems) which, if not trapped, will cause the
     process to terminate.  If that signal is trapped, then after
     10 more seconds (20 seconds in total) the process will be
     killed with a non-trappable signal.

     See the "BSD::Resource" and your systems documentation for
     the gory details.

  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
     database servers and types, including Oracle, DB2, Sybase,
     mysql, Postgresql, ODBC, and flat files.  The DBI module
     accesses each database type through a database driver, or
     DBD.  You can see a complete list of available drivers on
     CPAN: .  You can
     read more about DBI on .

     Other modules provide more specific access: "Win32::ODBC",
     "Alzabo", "iodbc", and others found on CPAN Search: .

  How do I make a system() exit on control-C?
     You can't.  You need to imitate the "system()" call (see
     perlipc for sample code) and then have a signal handler for
     the INT signal that passes the signal on to the subprocess.
     Or you can check for it:

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

perl v5.12.5         Last change: 2012-11-03                   21

Perl Programmers Reference Guide                      PERLFAQ8(1)

  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
     (answer contributed by brian d foy)

     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
     custom 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};

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

perl v5.12.5         Last change: 2012-11-03                   22

Perl Programmers Reference Guide                      PERLFAQ8(1)

     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?
     (contributed by brian d foy)

     The easiest way is to have a module also named CPAN do it
     for you by using the "cpan" command the comes with Perl. You
     can give it a list of modules to install:

             $ cpan IO::Interactive Getopt::Whatever

     If you prefer "CPANPLUS", it's just as easy:

             $ cpanp i IO::Interactive Getopt::Whatever

     If you want to install a distribution from the current
     directory, you can tell "" to install "." (the full

             $ cpan .

     See the documentation for either of those commands to see
     what else you can do.

perl v5.12.5         Last change: 2012-11-03                   23

Perl Programmers Reference Guide                      PERLFAQ8(1)

     If you want to try to install a distribution by yourself,
     resolving all dependencies on your own, you follow one of
     two possible build paths.

     For distributions that use Makefile.PL:

             $ perl Makefile.PL
             $ make test install

     For distributions that use Build.PL:

             $ perl Build.PL
             $ ./Build test
             $ ./Build install

     Some distributions may need to link to libraries or other
     third-party code and their build and installation sequences
     may be more complicated.  Check any README or INSTALL files
     that you may find.

  What's the difference between require and use?
     (contributed by brian d foy)

     Perl runs "require" statement at run-time. Once Perl loads,
     compiles, and runs the file, it doesn't do anything else.
     The "use" statement is the same as a "require" run at
     compile-time, but Perl also calls the "import" method for
     the loaded package. These two are the same:

             use MODULE qw(import list);

             BEGIN {
                     require MODULE;
                     MODULE->import(import list);

     However, you can suppress the "import" by using an explicit,
     empty import list. Both of these still happen at compile-

             use MODULE ();

             BEGIN {
                     require MODULE;

     Since "use" will also call the "import" method, the actual
     value for "MODULE" must be a bareword. That is, "use" cannot
     load files by name, although "require" can:

             require "$ENV{HOME}/lib/"; # no @INC searching!

perl v5.12.5         Last change: 2012-11-03                   24

Perl Programmers Reference Guide                      PERLFAQ8(1)

     See the entry for "use" in perlfunc for more details.

  How do I keep my own module/library directory?
     When you build modules, tell Perl where to install the

     If you want to install modules for your own use, the easiest
     way might be "local::lib", which you can download from CPAN.
     It sets various installation settings for you, and uses
     those same settings within your programs.

     If you want more flexibility, you need to configure your
     CPAN client for your particular situation.

     For "Makefile.PL"-based distributions, use the INSTALL_BASE
     option when generating Makefiles:

             perl Makefile.PL INSTALL_BASE=/mydir/perl

     You can set this in your "" configuration so modules
     automatically install in your private library directory when
     you use the shell:

             % cpan
             cpan> o conf makepl_arg INSTALL_BASE=/mydir/perl
             cpan> o conf commit

     For "Build.PL"-based distributions, use the --install_base

             perl Build.PL --install_base /mydir/perl

     You can configure "" to automatically use this option

             % cpan
             cpan> o conf mbuild_arg "--install_base /mydir/perl"
             cpan> o conf commit

     INSTALL_BASE tells these tools to put your modules into
     /mydir/perl/lib/perl5.  See "How do I add a directory to my
     include path (@INC) at runtime?" for details on how to run
     your newly installed modules.

     There is one caveat with INSTALL_BASE, though, since it acts
     differently than the PREFIX and LIB settings that older
     versions of "ExtUtils::MakeMaker" advocated. INSTALL_BASE
     does not support installing modules for multiple versions of
     Perl or different architectures under the same directory.
     You should consider if you really want that , and if you do,
     use the older PREFIX and LIB settings. See the
     "ExtUtils::Makemaker" documentation for more details.

perl v5.12.5         Last change: 2012-11-03                   25

Perl Programmers Reference Guide                      PERLFAQ8(1)

  How do I add the directory my program lives in to the
     module/library search path?
     (contributed by brian d foy)

     If you know the directory already, you can add it to @INC as
     you would for any other directory. You might <use lib> if
     you know the directory at compile time:

             use lib $directory;

     The trick in this task is to find the directory. Before your
     script does anything else (such as a "chdir"), you can get
     the current working directory with the "Cwd" module, which
     comes with Perl:

             BEGIN {
                     use Cwd;
                     our $directory = cwd;

             use lib $directory;

     You can do a similar thing with the value of $0, which holds
     the script name. That might hold a relative path, but
     "rel2abs" can turn it into an absolute path. Once you have

             BEGIN {
                     use File::Spec::Functions qw(rel2abs);
                     use File::Basename qw(dirname);

                     my $path   = rel2abs( $0 );
                     our $directory = dirname( $path );

             use lib $directory;

     The "FindBin" module, which comes with Perl, might work. It
     finds the directory of the currently running script and puts
     it in $Bin, which you can then use to construct the right
     library path:

             use FindBin qw($Bin);

     You can also use "local::lib" to do much of the same thing.
     Install modules using "local::lib"'s settings then use the
     module in your program:

              use local::lib; # sets up a local lib at ~/perl5

     See the "local::lib" documentation for more details.

perl v5.12.5         Last change: 2012-11-03                   26

Perl Programmers Reference Guide                      PERLFAQ8(1)

  How do I add a directory to my include path (@INC) at runtime?

     Here are the suggested ways of modifying your include path,
     including environment variables, run-time switches, and in-
     code statements:

     the "PERLLIB" environment variable
                 $ export PERLLIB=/path/to/my/dir
                 $ perl

     the "PERL5LIB" environment variable
                 $ export PERL5LIB=/path/to/my/dir
                 $ perl

     the "perl -Idir" command line flag
                 $ perl -I/path/to/my/dir

     the "lib" pragma:
                 use lib "$ENV{HOME}/myown_perllib";

     the "local::lib" module:
                 use local::lib;

                 use local::lib "~/myown_perllib";

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

  What is and where do I get it?
     It's a Perl 4 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) 1997-2010 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.

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

perl v5.12.5         Last change: 2012-11-03                   27

Perl Programmers Reference Guide                      PERLFAQ8(1)

     See attributes(5) for descriptions of the following

     |Availability   | runtime/perl-512 |
     |Stability      | Uncommitted      |
     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

perl v5.12.5         Last change: 2012-11-03                   28