docs: function exec (cmd, callback) {
    debug('executing cmd', cmd)
    child_process.exec(cmd, function (err, stdout, stderr) {
      debug('command completed')
      if (err) {
        if (callback) {
          callback(err)
        }
      } else {
        if (callback) {
          callback(null, stdout, stderr)
        }
      }
    })
  }