in reply to Re^2: Executing command from perl script with input/output
in thread Executing command from perl script with input/output

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

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Re^4: Executing command from perl script with input/output
by linuxfan (Beadle) on Aug 09, 2005 at 19:48 UTC
    Zentara,

    Thanks four your reply. After adding your changes to my script, I noticed that READ and ERROR do not have any data at all. This is weird because soon after I execute the binary (from a command line) it prompts with a list of choices to choose from. My suspicion is that the binary only sends out data if its STDOUT is connected to a terminal. I am not sure if open3 provides any connection to a terminal..

    I tried the write at the beginning of the program (as above) but neither of $answer or $error was populated.

    The pipe method you mention is what I've been using until now. I wanted to add more error checking by examining the choices the program gives the user before writing data back to its STDIN.

    Thanks for your time!

[reply]
      Hi, I have run into that "terminal" problem with a few scripts before. What I end up doing is the following "trick". Instead of opening $cmd with IPC::Open3, open "/bin/sh", then print $cmd to it. 
      my $pid=open3(\*WRITE, \*READ, \*ERROR, '/bin/bash');
print WRITE $cmd;

      [download]
      That usually works, because you are simualting a terminal.
      I'm not really a human, but I play one on earth. flash japh
[reply]
[d/l]
        Thanks Zentara,

        I tried the method you suggested above, but this still doesn't work for me :-( After writing the $cmd to WRITE, I created two variables $selread and $selerror (like earlier) - one each to read from READ and ERROR. A read on both of these did not yield anything at all.

        I want to use a debugger to see what is happening, but my program is split over several modules, and I am not sure how to use the debugger for something that spans over many modules.

        Regards,
        ravi

[reply]

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