Try putting the "print WRITE "0\n" before the select statements. I havn't tested your sub, but I have a feeling those "ifs" are not working, but I'm guessing. You also may need to insert a delay to let the program startup and return.

Try this: 

sub ExecCmd
{
   my $cmd = shift;
   my $pid = open3(\*WRITE,\*READ,\*ERROR,$cmd);
   unless (defined $pid) {
      LogError("Failed to execute $cmd");
      return undef;
   }
   LogInfo("PID is $pid");

select(undef,undef,undef, 1);  a little delay you can try

#print WRITE "0\n";   #try it here too

my $selread = new IO::Select();
my $selerror = new IO::Select();
$selread->add(\*READ);
$selerror->add(\*ERROR);
# may not be best use of IO::Select

my($error,$answer)=('','');

#see which filehandles have output 
if($selread->can_read(0)){print "ready->read\n"}
if($selerror->can_read(0)){print "ready->error\n"}

#get any output  
sysread(ERROR,$error,4096) if $selerror->can_read(0);
if($error){print "ERROR-> $error\n"}

sysread(READ,$answer,4096) if $selread->can_read(0);
if($answer){print "Response = $answer\n"}

($error,$answer)=('','');
 
print WRITE "0\n";

[download]

Also, if you don't care what the initial output of the program is, you can avoid IPC::Open3 altogether, and use a piped open. Like: 

my $pid = open( FH, "| $cmd") or warn "$!\n";
print FH "0\n";   #maybe need syswrite here

[download]
I'm not really a human, but I play one on earth. flash japh

In reply to Re^3: Executing command from perl script with input/output by zentara
in thread Executing command from perl script with input/output by linuxfan

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