function run(callable $callback = null) {
        if (isset($this->res))
            return null;
        $this->res = proc_open($this->cmd, $this->descriptor_spec, $this->pipes, $this->cwd, $this->env);
        if ($this->res) {
            $this->stdout = new TcpConnection($this->pipes[1]);
            if (is_callable($this->onMessage))
                $this->stdout->onMessage = function ($conn, $data) {
                    ($this->onMessage)($this, $data);
                };
            if (is_callable($this->onClose))
                $this->stdout->onClose = function () {
                    ($this->onClose)($this);
                };
            stream_set_blocking($this->pipes[0], false);
            if (is_callable($callback))
                $callback($this);
            return true;
        }
        unset ($this->res);
        return false;
    }