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


       perlfaq8 - System Interaction


       version 5.20210411


       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 "system".

   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 modules:

               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
               Wx                      CPAN
               Gtk2                    CPAN
               Qt4                     kdebindings4 package

       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 Windows.

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

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

           use HotKey;
           $key = readkey();

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

           package HotKey;

           use strict;
           use warnings;

           use parent 'Exporter';
           our @EXPORT = qw(cbreak cooked readkey);

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

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

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

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

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

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

           END { cooked() }


   How do I check whether input is ready on the keyboard?
       The easiest way to do this is to read a key in nonblocking mode with
       the Term::ReadKey module from CPAN, passing it an argument of -1 to
       indicate not to block:

           use Term::ReadKey;


           if (defined (my $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 screen.

       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 capability:

           use Term::Cap;

           my $terminal = Term::Cap->Tgetent( { OSPEED => 9600 } );
           my $clear_screen = $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"


           my $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:

           my $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;
           my ($wchar, $hchar, $wpixels, $hpixels) = GetTerminalSize();

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

           require './sys/';
           die "no TIOCGWINSZ " unless defined &TIOCGWINSZ;
           open(my $tty_fh, "+</dev/tty")                     or die "No tty: $!";
           unless (ioctl($tty_fh, &TIOCGWINSZ, $winsize='')) {
               die sprintf "$0: ioctl TIOCGWINSZ (%08x: $!)\n", &TIOCGWINSZ;
           my ($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

       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;

           my $password = ReadLine(0);

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

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

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

       end of line
           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 "\015" and
           "\012". You may have to give the numeric values you want directly,
           using octal ("\015"), hex ("0x0D"), or as a control-character
           specification ("\cM").

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

           Even though with normal text files a "\n" will do the trick, there
           is still no unified scheme for terminating a line that is portable
           between Unix, DOS/Win, and Macintosh, except to terminate ALL line
           ends with "\015\012", and strip what you don't need from the
           output.  This applies especially to socket I/O and autoflushing,
           discussed next.

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

               my $old_handle = select($dev_fh);
               $| = 1;

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

               select((select($deb_handle), $| = 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 case.

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

       While trying to read from his caller-id box, the notorious Jamie
       Zawinski "<>", 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

           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.

       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 covered 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

           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

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

       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.

   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(1) for specifics) and use
       pwd_mkdb(8) to install it (see pwd_mkdb(8) 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 handling)
       You can use the "END" block to simulate "atexit()". Each package's
       "END" block is called when the program or thread ends. See the perlmod
       manpage for more details about "END" blocks.

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

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

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

           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 Try::Tiny.

       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

       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()?
       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 the 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 systems.

   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 IPC::Open3).

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

           my $exit_status   = system("mail-users");
           my $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()
           my $output = `$cmd`;        # using backticks (``)
           open (my $pipe_fh, "$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;
           my $in = '';
           open(NULL, ">", File::Spec->devnull);
           my $pid = open3($in, \*PH, ">&NULL", "cmd");
           while( <PH> ) { }
           waitpid($pid, 0);

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

           use IPC::Open3;
           use File::Spec;
           my $in = '';
           open(NULL, ">", File::Spec->devnull);
           my $pid = open3($in, ">&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;
           my $in = '';
           my $pid = open3($in, ">&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

           use IPC::Open3;
           use IO::File;
           my $in = '';
           local *CATCHOUT = IO::File->new_tmpfile;
           local *CATCHERR = IO::File->new_tmpfile;
           my $pid = open3($in, ">&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;
           my $in = '';
           use IO::File;
           local *CATCHERR = IO::File->new_tmpfile;
           my $pid = open3($in, \*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

       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 STDERR:

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

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

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

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

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

       The first command sends both standard out and standard error to the
       temporary file. The second command sends only the old standard output
       there, and the old standard error shows up on the old standard out.

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

       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 this.

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

       Consider this line:

           `cat /etc/termcap`;

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

           print `cat /etc/termcap`;

       this code could and probably should be written as

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

       which will 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 not.

   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

           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

       Note that if you're using Windows, no solution to this vexing issue is
       even possible. Even though Perl emulates "fork()", you'll 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
       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 this:

               my $where = tell($log_fh);
               seek($log_fh, $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

   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
           my $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.

   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

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

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

       o   Open /dev/tty and use the TIOCNOTTY ioctl on it. See tty(1) for
           details. Or better yet, you can just use the "POSIX::setsid()"
           function, so you don't have to worry about process groups.

       o   Change directory to /

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

       o   Background yourself like this:

               fork && exit;

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

   How do I find out if I'm running interactively or not?
       (contributed by brian d foy)

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

       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

           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 output:

           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 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?
       (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

   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" }

   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

           use Fcntl;
           sysopen(my $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)

       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

       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;

       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 that 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 stop):

           $ cpan .

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

       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-time:

           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!

       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 modules.

       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

       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 option:

           perl Build.PL --install_base /mydir/perl

       You can configure "" to automatically use this option too:

           % 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

       There is one caveat with INSTALL_BASE, though, since it acts
       differently from 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 whether you really want that
       and, if you do, use the older PREFIX and LIB settings. See the
       ExtUtils::Makemaker documentation for more details.

   How do I add the directory my program lives in to the module/library search
       (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 the

           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

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

       See the local::lib documentation for more details.

   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";

   Where are modules installed?
       Modules are installed on a case-by-case basis (as provided by the
       methods described in the previous section), and in the operating
       system. All of these paths are stored in @INC, which you can display
       with the one-liner

           perl -e 'print join("\n",@INC,"")'

       The same information is displayed at the end of the output from the

           perl -V

       To find out where a module's source code is located, use

           perldoc -l Encode

       to display the path to the module. In some cases (for example, the
       "AutoLoader" module), this command will show the path to a separate
       "pod" file; the module itself should be in the same directory, with a
       'pm' file extension.

   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 should use "use Socket;"


       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 v5.34.0                      2021-05-04                     PERLFAQ8(1pm)

perl 5.34.0 - Generated Fri Feb 25 19:51:05 CST 2022
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