Bash脚本故障：端口检测无效输出且循环无法终止

Hey there! Let's troubleshoot why your port-checking Bash script isn't working as intended. You said it should stop at the first open port (which should be 22, since all except 8080 are open) but instead it's spitting out invalid output and looping endlessly. Let's break down the common issues and fix this.

First, let's assume you might have a script that looks something like this (a common mistake):

ports=(22 80 443 8080)
for port in "${ports[@]}"
do
    nc -zv localhost $port
    if [ $? -eq 0 ]; then
        echo "succeeded $port"
    fi
done

What's going wrong here? #

1. Unsuppressed output: The nc -zv command prints its own connection messages (like Connection to localhost 22 port [tcp/ssh] succeeded!) directly to the terminal—this is your "invalid output".
2. No loop termination: Without a break statement inside the if block, the script will keep checking every port in your list even after finding the first open one.
3. Potential hanging: If you're using a tool like telnet instead of nc (or an nc version without timeout), the command might hang indefinitely waiting for a response, causing the loop to get stuck.

Here's the fixed script #

#!/bin/bash
# Define your list of ports to check
ports=(22 80 443 8080)

for port in "${ports[@]}"
do
    # Use nc with a 1-second timeout, suppress all output
    nc -zv -w1 localhost "$port" > /dev/null 2>&1
    # Check if the command succeeded (port is open)
    if [ $? -eq 0 ]; then
        echo "succeeded $port"
        # Immediately exit the loop once we find the first open port
        break
    fi
done

Key fixes explained: #

• -w1: Adds a 1-second timeout to nc—this prevents the command from hanging if a port doesn't respond quickly.
• > /dev/null 2>&1: Redirects both standard output and error to /dev/null, so you only get your custom succeeded $port message.
• break: Stops the loop as soon as we find the first open port, so the script doesn't waste time checking the rest.

If you were using telnet instead of nc, switch to nc (it's more reliable for scripted port checks). If you must use telnet, you can add a timeout with something like timeout 1 telnet localhost $port > /dev/null 2>&1.

This script will output exactly succeeded 22 and stop, just like you expected.

内容的提问来源于stack exchange，提问作者Eka