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.

在Bash中，为TCP/UDP连接使用伪设备文件非常简单。以下是剧本:

#!/usr/bin/env bash
SERVER=example.com
PORT=80
</dev/tcp/$SERVER/$PORT
if [ "$?" -ne 0 ]; then
  echo "Connection to $SERVER on port $PORT failed"
  exit 1
else
  echo "Connection to $SERVER on port $PORT succeeded"
  exit 0
fi

测试:

$ ./test.sh 
Connection to example.com on port 80 succeeded

---

下面是一行代码(Bash语法):

</dev/tcp/localhost/11211 && echo Port open. || echo Port closed.

---

请注意，有些服务器可以通过防火墙保护免受SYN flood攻击，因此您可能会遇到TCP连接超时(~75秒)。要解决超时问题，请尝试:

timeout 1 bash -c "</dev/tcp/stackoverflow.com/81" && echo Port open. || echo Port closed.

参见:如何减少TCP connect()系统调用超时?

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。

在某些情况下，像curl, telnet, nc和nmap这样的工具是不可用的，你仍然有机会使用wget

if [[ $(wget -q -t 1 --spider --dns-timeout 3 --connect-timeout 10  host:port; echo $?) -eq 0 ]]; then echo "OK"; else echo "FAIL"; fi

我需要一个更灵活的解决方案来处理多个git存储库，所以我根据1和2编写了下面的sh代码。你可以用你的服务器地址代替gitlab.com，用你的端口代替22。

SERVER=gitlab.com
PORT=22
nc -z -v -w5 $SERVER $PORT
result1=$?

#Do whatever you want

if [  "$result1" != 0 ]; then
  echo  'port 22 is closed'
else
  echo 'port 22 is open'
fi

TOC:

使用bash和timeout 命令 例子 使用数控 命令 RHEL 6 (nc-1.84) 安装 例子 RHEL 7 (nmap-ncat-6.40) 安装 例子 讲话

使用bash和timeout:

请注意，超时应该在RHEL 6+中出现，或者在GNU coreutils 8.22中也可以找到。在MacOS上，使用brew install coreutils安装它，并将其用作gtimeout。

命令:

$ timeout $TIMEOUT_SECONDS bash -c "</dev/tcp/${HOST}/${PORT}"; echo $?

如果参数化主机和端口，请确保像上面那样将它们指定为${host}和${port}。不要仅仅将它们指定为$HOST和$PORT，即不带大括号;在这种情况下行不通。

例子:

成功:

$ timeout 2 bash -c "</dev/tcp/canyouseeme.org/80"; echo $?
0

失败:

$ timeout 2 bash -c "</dev/tcp/canyouseeme.org/81"; echo $?
124

如果必须保留bash的退出状态，

$ timeout --preserve-status 2 bash -c "</dev/tcp/canyouseeme.org/81"; echo $?
143

使用数控:

注意，在RHEL 7上安装了向后不兼容的nc版本。

命令:

注意，下面的命令是唯一的，因为它对于RHEL 6和7都是相同的。只是安装和输出不同而已。

$ nc -w $TIMEOUT_SECONDS -v $HOST $PORT </dev/null; echo $?

RHEL 6 （nc-1.84）：

安装:

$ sudo yum install nc

例子:

Success:

$ nc -w 2 -v canyouseeme.org 80 </dev/null; echo $?
Connection to canyouseeme.org 80 port [tcp/http] succeeded!
0

Failure:

$ nc -w 2 -v canyouseeme.org 81 </dev/null; echo $?
nc: connect to canyouseeme.org port 81 (tcp) timed out: Operation now in progress
1

如果主机名映射到多个ip，上述失败的命令将循环遍历多个或所有ip。例如:

$ nc -w 2 -v microsoft.com 81 </dev/null; echo $?
nc: connect to microsoft.com port 81 (tcp) timed out: Operation now in progress
nc: connect to microsoft.com port 81 (tcp) timed out: Operation now in progress
nc: connect to microsoft.com port 81 (tcp) timed out: Operation now in progress
nc: connect to microsoft.com port 81 (tcp) timed out: Operation now in progress
nc: connect to microsoft.com port 81 (tcp) timed out: Operation now in progress
1

RHEL 7 (nmap-ncat-6.40):

安装:

$ sudo yum install nmap-ncat

例子:

Success:

$ nc -w 2 -v canyouseeme.org 80 </dev/null; echo $?
Ncat: Version 6.40 ( http://nmap.org/ncat )
Ncat: Connected to 52.202.215.126:80.
Ncat: 0 bytes sent, 0 bytes received in 0.22 seconds.
0

Failure:

$ nc -w 2 -v canyouseeme.org 81 </dev/null; echo $?
Ncat: Version 6.40 ( http://nmap.org/ncat )
Ncat: Connection timed out.
1

如果主机名映射到多个ip，上述失败的命令将循环遍历多个或所有ip。例如:

$ nc -w 2 -v microsoft.com 81 </dev/null; echo $?
Ncat: Version 6.40 ( http://nmap.org/ncat )
Ncat: Connection to 104.43.195.251 failed: Connection timed out.
Ncat: Trying next address...
Ncat: Connection to 23.100.122.175 failed: Connection timed out.
Ncat: Trying next address...
Ncat: Connection to 23.96.52.53 failed: Connection timed out.
Ncat: Trying next address...
Ncat: Connection to 191.239.213.197 failed: Connection timed out.
Ncat: Trying next address...
Ncat: Connection timed out.
1

备注:

-v(——verbose)参数和回显$?命令当然只是为了说明。