perl - How to pass a variable from a child process (fork by Parallel::ForkManager)?

Question

My query:

In the following code i had tried to bring the print $commandoutput[0] to be shifted or passed into the upcoming subroutine.i tried the shift to pass it.But i failed with it.Can you please help me the right way to follow?

Code:

my $max_forks = 4;

#createThreads();
my %commandData;
my @arr = (
   'bhappy',  'bload -m all -l -res CPUSTEAL',
   'bqueues', 'bjobs -u all -l -hfreq 101'
);

#print @arr;
my $fork = new Parallel::ForkManager($max_forks);
$fork->run_on_start(
   sub {
      my $pid = shift;
   }
);
$fork->run_on_finish(
   sub {
      my ( $pid, $exit, $ident, $signal, $core ) = @_;
      if ($core) {
         print "PID $pid core dumped.
";
      }
      else { }
   }
);
my @Commandoutput;
my $commandposition = 0;
for my $command (@arr) {
   $fork->start and next;
   my @var = split( " ", $command );
   $commandoutput[$commandposition] = `$command`;
   $commandposition++;
   $line = $commandoutput[0];

# print $line;
   $fork->finish;
}
$fork->wait_all_children;

#print Dumper(\%commandData);
print $commandoutput[0];

Here i had tried to store the print $commandoutput[0] in the variable inside the subroutine.I gated here how to pass the variables from outside to inside the subroutine.

sub gen_help_data
{
  my $lines=shift;
  print $lines;
}

Answer 1

I think you're misunderstanding what a fork does. When you successfully fork, you're creating a subprocess, independent from the process you started with, to continue doing work. Because it's a separate process, it has its own memory, variables, etc., even though some of these started out as copies from the parent process.

So you're setting $commandoutput[0] in each subprocess, but then, when that subprocess dies, so does the content of its copy of @commandoutput.

You can either run each command serially, or you can use threads (which comes with a host of other issues - your code would need some significant redesign to work even with threads), or you can use events (POE, AnyEvent, etc., and this will be another significant redesign). Or you could run each command with its output put into temporary files, then, once all the children are done, read each file and continue. This also comes with issues, but generally fewer issues than the others.