#!/usr/bin/env perl

use strict;
use warnings FATAL => 'all';
use Time::HiRes qw(sleep time);

if ( !@ARGV || @ARGV < 2 ) {
   print STDERR "Usage: wait-to-exec WAIT EXEC [RUNTIME [INTERVAL]]\n",
                "When the WAIT script exits 0, execute the EXEC script.\n";
   exit 1;
}

my ($wait, $exec, $runtime, $interval) = @ARGV;
$runtime  ||= 1.0;
$interval ||= 0.1;

#warn "wait: $wait\n";
#warn "exec: $exec\n";

my $t_start = time;
while ( time - $t_start < $runtime ) {
   if ( exit_status($wait) ) {
      sleep $interval;
   }
   else {
      # exec does not return; this script becomes the exec process.
      exec($exec);
   }
}

warn "Never observed wait condition";
exit 1;

sub exit_status {
   my ($wait) = @_;
   system($wait);
   # Like grep, exit status 0 means "match found", so stop waiting.
   my $exit_status = $? >> 8;
   return $exit_status;
}