Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
PERLFAQ8(1)	       Perl Programmers	Reference Guide		   PERLFAQ8(1)

       perlfaq8	- System Interaction

       version 5.021011

       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
	   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";

       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.

   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 v5.28.3			  2020-05-14			   PERLFAQ8(1)


Want to link to this manual page? Use this URL:

home | help