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IPC::Open2(3)	       Perl Programmers	Reference Guide		 IPC::Open2(3)

       IPC::Open2 - open a process for both reading and	writing	using open2()

	   use IPC::Open2;

	   my $pid = open2(my $chld_out, my $chld_in,
	     'some', 'cmd', 'and', 'args');
	   # or	passing	the command through the	shell
	   my $pid = open2(my $chld_out, my $chld_in, 'some cmd	and args');

	   # read from parent STDIN and	write to already open handle
	   open	my $outfile, '>', 'outfile.txt'	or die "open failed: $!";
	   my $pid = open2($outfile, '<&STDIN',	'some',	'cmd', 'and', 'args');

	   # read from already open handle and write to	parent STDOUT
	   open	my $infile, '<', 'infile.txt' or die "open failed: $!";
	   my $pid = open2('>&STDOUT', $infile,	'some',	'cmd', 'and', 'args');

	   # reap zombie and retrieve exit status
	   waitpid( $pid, 0 );
	   my $child_exit_status = $? >> 8;

       The open2() function runs the given command and connects	$chld_out for
       reading and $chld_in for	writing.  It's what you	think should work when
       you try

	   my $pid = open(my $fh, "|cmd	args|");

       The $chld_in filehandle will have autoflush turned on.

       If $chld_out is a string	(that is, a bareword filehandle	rather than a
       glob or a reference) and	it begins with ">&", then the child will send
       output directly to that file handle.  If	$chld_in is a string that
       begins with "<&", then $chld_in will be closed in the parent, and the
       child will read from it directly.  In both cases, there will be a
       dup(2) instead of a pipe(2) made.

       If either reader	or writer is the empty string or undefined, this will
       be replaced by an autogenerated filehandle.  If so, you must pass a
       valid lvalue in the parameter slot so it	can be overwritten in the
       caller, or an exception will be raised.

       open2() returns the process ID of the child process.  It	doesn't	return
       on failure: it just raises an exception matching	"/^open2:/".  However,
       "exec" failures in the child are	not detected.  You'll have to trap
       SIGPIPE yourself.

       open2() does not	wait for and reap the child process after it exits.
       Except for short	programs where it's acceptable to let the operating
       system take care	of this, you need to do	this yourself.	This is
       normally	as simple as calling "waitpid $pid, 0" when you're done	with
       the process.  Failing to	do this	can result in an accumulation of
       defunct or "zombie" processes.  See "waitpid" in	perlfunc for more

       This whole affair is quite dangerous, as	you may	block forever.	It
       assumes it's going to talk to something like bc(1), both	writing	to it
       and reading from	it.  This is presumably	safe because you "know"	that
       commands	like bc(1) will	read a line at a time and output a line	at a
       time.  Programs like sort(1) that read their entire input stream	first,
       however,	are quite apt to cause deadlock.

       The big problem with this approach is that if you don't have control
       over source code	being run in the child process,	you can't control what
       it does with pipe buffering.  Thus you can't just open a	pipe to	"cat
       -v" and continually read	and write a line from it.

       The IO::Pty and Expect modules from CPAN	can help with this, as they
       provide a real tty (well, a pseudo-tty, actually), which	gets you back
       to line buffering in the	invoked	command	again.

       The order of arguments differs from that	of open3().

       See IPC::Open3 for an alternative that handles STDERR as	well.  This
       function	is really just a wrapper around	open3().

perl v5.32.0			  2020-06-14			 IPC::Open2(3)


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