CODEWITHSUNDEEP
linuxbashloopsnetcat
Snaacky
Snaacky

Reputation: 642

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 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?

Upvotes: 52

Views: 61513

Answers (3)

Henk Langeveld
Henk Langeveld

Reputation: 8446

Keep it Simple

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

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

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

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

Upvotes: 123

Jesse Taube
Jesse Taube

Reputation: 492

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

Upvotes: 6

Lee Duhem
Lee Duhem

Reputation: 15121

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!"

Upvotes: 23

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