参考：https://blog.csdn.net/signjing/article/details/69535226
正常的情况
$ echo "" | telnet 101.199.97.65 62715 Trying 101.199.97.65... Connected to 101.199.97.65. Escape character is '^]'. Connection closed by foreign host.
不正常的情况
$ echo "" | telnet 101.199.97.65 62715 Trying 101.199.97.65... telnet: connect to address 101.199.97.65: Connection refused
使用这种方式可以将标准输出和标准错误重定向到文件中，通过分析文件的内容来判断端口打开状态