I develop web services using python flask. One of the common error that I see while deploying the application is “Gunicorn Connection in Use: (‘0.0.0.0’, 8000)”.

This means that the port 8000 is busy with some other running process. But when I check the status of the port with the following command, I get empty response. That means there are no active application using the port. Some stale process is making the port busy.

netstat -tulpn | grep 8000

I even tried with the ps command to see any active process, but that also did not help.

ps -aux

If the ps command list the process, we can kill the process directly using the kill command

kill -9 {PID}

In my case I do not have the PID. So the only option to kill these kind of zombie application by using the below command.

sudo fuser -k {PORT}/tcp

In my case, the port number is 8000, so the command will be.

sudo fuser -k 8000/tcp

This trick helped me several times, hope this helps someone else also.