使用netcat，你可以像这样检查端口是否打开:

nc my.example.com 80 < /dev/null

如果TCP端口被打开，nc的返回值为success，如果不能建立TCP连接，则返回值为failure(通常为返回码1)。

当你尝试这样做时，某些版本的nc会挂起，因为它们即使在从/dev/null.接收到文件结束后也不会关闭它们的套接字的发送部分在我自己的Ubuntu笔记本电脑(18.04)上，我安装的netcat-openbsd版本的netcat提供了一个变通方案:必须使用-N选项才能立即得到结果:

nc -N my.example.com 80 < /dev/null

使用bash检查端口

例子

$ ./test_port_bash.sh 192.168.7.7 22

端口“22”被打开

Code

HOST=$1
PORT=$2
exec 3> /dev/tcp/${HOST}/${PORT}
if [ $? -eq 0 ];then echo "the port $2 is open";else echo "the port $2 is closed";fi

我需要在cron中运行的短脚本，没有输出。我用nmap解决了这个问题

open=`nmap -p $PORT $SERVER | grep "$PORT" | grep open`
if [ -z "$open" ]; then
  echo "Connection to $SERVER on port $PORT failed"
  exit 1
else
  echo "Connection to $SERVER on port $PORT succeeded"
  exit 0
fi

要运行它，你应该安装nmap，因为它不是默认安装包。

我的机器不支持nc或/dev/tcp/$hostname/$port，但支持超时，所以我回到telnet，如下所示:

if echo "quit" | timeout 2 telnet $SERVER $PORT 2>&1 | grep -q 'Connected to'; then
    echo "Connection to $SERVER on port $PORT succeeded"
    exit 0
else
    echo "Connection to $SERVER on port $PORT failed"
    exit 1
fi

如果您正在使用ksh或bash，它们都支持使用/dev/tcp/ ip / port构造将IO重定向到/从套接字。在这个Korn shell示例中，我将从套接字重定向no-op的(:)标准输入:

W$ python -m SimpleHTTPServer &
[1]     16833
Serving HTTP on 0.0.0.0 port 8000 ...
W$ : </dev/tcp/127.0.0.1/8000

如果套接字没有打开，shell将打印一个错误:

W$ : </dev/tcp/127.0.0.1/8001
ksh: /dev/tcp/127.0.0.1/8001: cannot open [Connection refused]

因此，你可以在if条件中使用这个测试:

SERVER=127.0.0.1 PORT=8000
if (: < /dev/tcp/$SERVER/$PORT) 2>/dev/null
then
    print succeeded
else
    print failed
fi

no-op是在一个子shell中，所以如果stin重定向失败，我可以扔掉std-err。

我经常使用/dev/tcp来检查HTTP上资源的可用性:

W$ print arghhh > grr.html
W$ python -m SimpleHTTPServer &
[1]     16863
Serving HTTP on 0.0.0.0 port 8000 ...
W$ (print -u9 'GET /grr.html HTTP/1.0\n';cat <&9) 9<>/dev/tcp/127.0.0.1/8000
HTTP/1.0 200 OK
Server: SimpleHTTP/0.6 Python/2.6.1
Date: Thu, 14 Feb 2013 12:56:29 GMT
Content-type: text/html
Content-Length: 7
Last-Modified: Thu, 14 Feb 2013 12:55:44 GMT

arghhh
W$ 

这一行代码打开文件描述符9，用于读取和写入套接字，将HTTP GET打印到套接字，并使用cat从套接字读取。

对nc使用-z和-w TIMEOUT选项很容易，但并不是所有的系统都安装了nc。如果你有一个最新版本的bash，这将工作:

# Connection successful:
$ timeout 1 bash -c 'cat < /dev/null > /dev/tcp/google.com/80'
$ echo $?
0

# Connection failure prior to the timeout
$ timeout 1 bash -c 'cat < /dev/null > /dev/tcp/sfsfdfdff.com/80'
bash: sfsfdfdff.com: Name or service not known
bash: /dev/tcp/sfsfdfdff.com/80: Invalid argument
$ echo $?
1

# Connection not established by the timeout
$ timeout 1 bash -c 'cat < /dev/null > /dev/tcp/google.com/81'
$ echo $?
124

What's happening here is that timeout will run the subcommand and kill it if it doesn't exit within the specified timeout (1 second in the above example). In this case bash is the subcommand and uses its special /dev/tcp handling to try and open a connection to the server and port specified. If bash can open the connection within the timeout, cat will just close it immediately (since it's reading from /dev/null) and exit with a status code of 0 which will propagate through bash and then timeout. If bash gets a connection failure prior to the specified timeout, then bash will exit with an exit code of 1 which timeout will also return. And if bash isn't able to establish a connection and the specified timeout expires, then timeout will kill bash and exit with a status of 124.