public function beforeRun(Shell $shell)
    {
        list($up, $down) = \stream_socket_pair(STREAM_PF_UNIX, STREAM_SOCK_STREAM, STREAM_IPPROTO_IP);

        if (!$up) {
            throw new \RuntimeException('Unable to create socket pair');
        }

        $pid = \pcntl_fork();
        if ($pid < 0) {
            throw new \RuntimeException('Unable to start execution loop');
        } elseif ($pid > 0) {
            // This is the main thread. We'll just wait for a while.

            // We won't be needing this one.
            \fclose($up);

            // Wait for a return value from the loop process.
            $read   = [$down];
            $write  = null;
            $except = null;

            do {
                $n = @\stream_select($read, $write, $except, null);

                if ($n === 0) {
                    throw new \RuntimeException('Process timed out waiting for execution loop');
                }

                if ($n === false) {
                    $err = \error_get_last();
                    if (!isset($err['message']) || \stripos($err['message'], 'interrupted system call') === false) {
                        $msg = $err['message'] ?
                            \sprintf('Error waiting for execution loop: %s', $err['message']) :
                            'Error waiting for execution loop';
                        throw new \RuntimeException($msg);
                    }
                }
            } while ($n < 1);

            $content = \stream_get_contents($down);
            \fclose($down);

            if ($content) {
                $shell->setScopeVariables(@\unserialize($content));
            }

            throw new BreakException('Exiting main thread');
        }

        // This is the child process. It's going to do all the work.
        if (\function_exists('setproctitle')) {
            setproctitle('psysh (loop)');
        }

        // We won't be needing this one.
        \fclose($down);

        // Save this; we'll need to close it in `afterRun`
        $this->up = $up;
    }