Bash：循环直到命令退出状态等于 0

Question

I have a netcat installed on my local machine and a service running on port 25565. Using the command:

nc 127.0.0.1 25565 < /dev/null; echo $?

Netcat checks if the port is open and returns a 0 if it open, and a 1 if it closed.

I am trying to write a bash script to loop endlessly and execute the above command every second until the output from the command equals 0 (the port opens).

My current script just keeps endlessly looping "...", even after the port opens (the 1 becomes a 0).

until [ "nc 127.0.0.1 25565 < /dev/null; echo $?" = "0" ]; do
         echo "..."
         sleep 1
     done
echo "The command output changed!"

What am I doing wrong here?

Answer 1

Keep it Simple

until nc -z 127.0.0.1 25565
do
    echo ...
    sleep 1
done

Just let the shell deal with the exit status implicitly

The shell can deal with the exit status (recorded in $? ) in two ways, explicit, and implicit.

Explicit: status=$? , which allows for further processing.

Implicit:

For every statement, in your mind , add the word "succeeds" to the command, and then add if , until or while constructs around them, until the phrase makes sense.

until nc succeeds ; do ...; done ; do ...; done

---

The -z option will stop nc from reading stdin, so there's no need for the < /dev/null redirect.

Answer 2

You could try something like

while true; do
    nc 127.0.0.1 25565 < /dev/null
    if [ $? -eq 0 ]; then
        break
    fi
    sleep 1
done
echo "The command output changed!"