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

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

NNAAMMEE #

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

SSYYNNOOPPSSIISS #

     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;

DDEESSCCRRIIPPTTIIOONN #

 The ooppeenn22(()) 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 dduupp(2) instead of
 a ppiippee(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.

 ooppeenn22(()) 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.

 ooppeenn22(()) 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 information.

 This whole affair is quite dangerous, as you may block forever.  It
 assumes it's going to talk to something like bbcc(1), both writing to it
 and reading from it.  This is presumably safe because you "know" that
 commands like bbcc(1) will read a line at a time and output a line at a
 time.  Programs like ssoorrtt(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.

WWAARRNNIINNGG #

 The order of arguments differs from that of ooppeenn33(()).

SSEEEE AALLSSOO #

 See IPC::Open3 for an alternative that handles STDERR as well.  This
 function is really just a wrapper around ooppeenn33(()).

perl v5.36.3 2023-02-15 IPC::Open2(3p)