Question

So I have the following code, the problem is that it exits before all the child processes (sort/gzip) are done. How can I instruct Perl to wait for all the descendent processes ?

#!/usr/bin/perl


use strict;
use warnings;

sub systemBash {
  my $cmd = shift;
  my @args = ( "bash", "-c", $cmd );
  print "command ".$cmd."\n";
  system(@args);
  if($? != 0){
    die "Command ".Dumper(@args)." failed";
  }
}
print "start";
systemBash("yes |head -n 1000000|awk '{print rand()}' > >(sort |gzip -9 -c > /dev/null)");
print "done";

Answer 1

I wrote the following hideous code to solve this problem:

sub findAllPIDs{
 my ($firstPID) = @_;
 my %hashPID;
 $hashPID{$firstPID}=1;
 my $found;
 do{
   $found=0;
   foreach my $key (keys %hashPID){
     my $cmd="ps -o pid --ppid ".$key;
     my $out=`$cmd`;
     foreach my $line (split("\n",$out)){
       $line =~ s/^\s+//;
       $line =~ s/\s+$//;
       if($line ne "PID"){
         if(!(exists $hashPID{$line})){
           $hashPID{$line}=1;
           $found=1;
         }
       }
     }
   }

 }while($found);
 return (keys %hashPID);
}

sub poll{
 my $proc=shift;
 while(1){
   my $cmd="ps  -o s --pid $proc";
   my $out=`$cmd`;
   my $found=0;
   my @arr=split("\n",$out);
   shift(@arr);
   foreach my $line (@arr){
     $line =~ s/^\s+//;
     $line =~ s/\s+$//;
     if($line ne "Z"  ){
       $found=1;
     }
   }
   if(!$found){
     return $proc;
   }
   sleep(2);
 }
}

sub systemBash {
 my $cmd = shift;
 my @args = ( "bash", "-c", $cmd );

 print "running ".$cmd."\n";
 if(!$fakeRun){
   my $pid=fork();
   if($pid == 0){
     exec(@args);
   }else{
     sleep(1);
     my @array=findAllPIDs($pid);
     foreach my $pid (@array){
       my $code =poll($pid);
     }
   }
 }
}