#!/usr/bin/perl
# runN [-n N] CMD ARGS... - run up to N CMD in parallel command with ARGS
# http://perl.plover.com/classes/mybin/samples/slide147.html

use Getopt::Std;
my %opt = (n => 1);
getopts('n:v', \%opt) or usage();

sub spawn {
  my $pid = fork;
  die "fork: $!" unless defined $pid;
  return $pid if $pid;
  warn "@_\n" if $opt{v};
  exec @_;
  die "exec: $!";
}

my @cmd = split /\s+/, shift;

my %pid;
while (@ARGV) {
  if (keys(%pid) < $opt{n}) {
    $pid{spawn(@cmd, shift @ARGV)} = 1;
  } else {
    delete $pid{wait()};
  }
}

1 while wait() >= 0;