code: function(flvtool, cb) {
          if (flvtool.length) {
            return cb(null, flvtool);
          }
  
          if (process.env.FLVTOOL2_PATH) {
            fs.exists(process.env.FLVTOOL2_PATH, function(exists) {
              cb(null, exists ? process.env.FLVTOOL2_PATH : '');
            });
          } else {
            cb(null, '');
          }
        }
docs: Try FLVTOOL2_PATH