Bash: Loop until command exit status equals 0

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

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

What am I doing wrong here?


  • Keep it Simple

    Bash and other sh shells provide an until ... do ... done loop:

    until nc -z 25565
        echo ...
        sleep 1

    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.


    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

    * Bash manual: until ... do ... loop

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