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

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

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.

在投票最多的答案的基础上，这里有一个等待两个端口打开的函数，还有一个超时。请注意必须打开的两个端口，8890和1111，以及max_attempts(每秒1次)。

function wait_for_server_to_boot()
{
    echo "Waiting for server to boot up..."
    attempts=0
    max_attempts=30
    while ( nc 127.0.0.1 8890 < /dev/null || nc 127.0.0.1 1111 < /dev/null )  && [[ $attempts < $max_attempts ]] ; do
        attempts=$((attempts+1))
        sleep 1;
        echo "waiting... (${attempts}/${max_attempts})"
    done
}

如果你想使用nc，但没有支持-z的版本，尝试使用——send-only:

nc --send-only <IP> <PORT> </dev/null

和timeout:

nc -w 1 --send-only <IP> <PORT> </dev/null

如果是IP，则不需要DNS查询:

nc -n -w 1 --send-only <IP> <PORT> </dev/null

它根据是否可以连接返回-z代码。

这在幕后使用telnet，在mac/linux上似乎工作得很好。它不使用netcat，因为linux/mac版本之间的差异，这适用于默认的mac安装。

例子:

$ is_port_open.sh 80 google.com
OPEN

$ is_port_open.sh 8080 google.com
CLOSED

is_port_open.sh

PORT=$1
HOST=$2
TIMEOUT_IN_SEC=${3:-1}
VALUE_IF_OPEN=${4:-"OPEN"}
VALUE_IF_CLOSED=${5:-"CLOSED"}

function eztern()
{
  if [ "$1" == "$2" ]
  then
    echo $3
  else
    echo $4
  fi
}

# cross platform timeout util to support mac mostly
# https://gist.github.com/jaytaylor/6527607
function eztimeout() { perl -e 'alarm shift; exec @ARGV' "$@"; }

function testPort()
{
  OPTS=""

  # find out if port is open using telnet
  # by saving telnet output to temporary file
  # and looking for "Escape character" response
  # from telnet
  FILENAME="/tmp/__port_check_$(uuidgen)"
  RESULT=$(eztimeout $TIMEOUT_IN_SEC telnet $HOST $PORT &> $FILENAME; cat $FILENAME | tail -n1)
  rm -f $FILENAME;
  SUCCESS=$(eztern "$RESULT" "Escape character is '^]'." "$VALUE_IF_OPEN" "$VALUE_IF_CLOSED")

  echo "$SUCCESS"
}

testPort