Bash: Loop until command exit status equals 0

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

---

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

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!"