in reply to Executing command from perl script with input/output

Here is about the simplest example you can get using IPC::Open3. Of course a real world script may need alot more code. You can search here for other more complex examples, or search for them at groups.google.com 
#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;

#interface to "bc" calculator 
#my $pid = open3(\*WRITE, \*READ, \*ERROR,"bc"); 
my $pid = open3(\*WRITE, \*READ,0,"bc");
            #if \*ERROR is false, STDERR is sent to STDOUT  
while(1){
print "Enter expression for bc, i.e. 2 + 2\n";
chomp(my $query = <STDIN>);

#send query to bc 
print WRITE "$query\n";

#give bc time to output 
select(undef,undef,undef,.5);

#get the answer from bc 
chomp(my $answer = <READ>);
print "$query = $answer\n";

}

waitpid($pid, 1);
# It is important to waitpid on your child process,   
# otherwise zombies could be created.

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

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Re^2: Executing command from perl script with input/output
by linuxfan (Beadle) on Aug 08, 2005 at 18:21 UTC
    Thanks for your reply. I found some sample code on the web that uses IPC::Open3 and used it in my program. Is it requirs to write data to the WRITE descriptor before I can read from it? My binary writes data to STDOUT/STDERR first, and then reads STDIN. 
    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");
   my $select = new IO::Select();
   $select->add(\*READ);
   $select->add(\*ERROR);

   foreach my $handle ($select->can_read) {
      LogInfo("handle is $handle");
      my $buf = "";
      if($handle eq \*ERROR) {
         sysread(ERROR,$buf,BUFFER);
         close ERROR;
         if($buf) {
            LogInfo("ERROR -> $buf");
         }
      } else {
         sysread(READ,$buf,BUFFER);
         close READ;
         if($buf) {
            LogInfo("READ -> $buf");
         }
      }
   }
   print WRITE "0\n";

    [download]
    In the above, the program seems to be waiting for some kind of input after it prints the PID - never enters the foreach loop. How can I solve the above problem?

    Thanks!

[reply]
[d/l]
      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
[reply]
[d/l]
[select]
        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]

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