AWS Lambda Cannot Execute External Commands

exports.handler=function(event, context){
  var exec = require('child_process').exec;
  exec('./youtube-dl --version', function(error, stdout, stderr){
    if(stdout){
      console.log('stdout:'+stdout);
    }
    if(stderr){
      console.log('stderr:'+stderr);
    }
    if(error!==null){
      console.log('Exec error:'+error);
    }
    context.succeed(stdout);
  });
};