Bash: Loop until command exit status equals 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?

You aren't actually RUNning the nc command, it's just a string. — Mark Setchell, Feb 24 '14 at 08:17
Just a suggestion, but have you tried using **back ticks** instead of double quotes around `nc 127.0.0.1 25565 < /dev/null; echo $?` ? — csiu, Feb 24 '14 at 08:18

score 104 · Accepted Answer · edited Aug 31 '23 at 12:46

Keep it Simple

Bash provides the until looping construct:

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.

Nice, you get to see the command output message while retrying. — Nik, Oct 27 '22 at 10:27

score 19 · Answer 2 · answered Feb 24 '14 at 08:14

19

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!"

answered Feb 24 '14 at 08:14

Lee Duhem

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3
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score 1 · Answer 3 · answered Mar 07 '23 at 17:02

I personally like @Lee Duhem solutions better, although I think I have a better way of executing it.

This will run until it exits success:

while true; do
nc 127.0.0.1 25565 < /dev/null && break
done

This will run until it exits failure:

while true; do
nc 127.0.0.1 25565 < /dev/null || break
done

And a one-line equivalent of the first:

while true; do nc 127.0.0.1 25565 < /dev/null && break; done

Bash: Loop until command exit status equals 0

3 Answers3

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