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?
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.