Port Already in Use

This operation is to free a selected port already in use. A simple alternative is to start your app on a different port but in this case we will see how to free up the port your app wants to use.

Quick reference

sudo lsof -i :8000        # find process + PID on port 8000
sudo ss -ltnp | grep 8000 # alternative
sudo fuser 8000/tcp       # one-shot: just print the PID
sudo fuser -k 8000/tcp    # one-shot: find and kill in one step

kill <PID>                # stop the process
kill -9 <PID>             # force-kill if it ignores kill

Why sudo is needed

Running ss or lsof without sudo shows the port is occupied but hides the PID and process name. You need elevated privileges to see which process owns the socket.

sudo ss -ltnp | grep 8000
sudo lsof -i :8000

The output will show the process name (e.g. python, uvicorn, docker-proxy) and the PID.

Kill by PID

kill <PID>
kill -9 <PID>   # if the process ignores the first kill

One-shot with fuser

fuser skips the find then kill two step run:

sudo fuser 8000/tcp        # print PID only
sudo fuser -k 8000/tcp    # kill it directly