How to Check Which Process Is Using a Port in Ubuntu 25

When a port is already in use, services fail to start and return errors like address already in use. Identifying the process holding the port is the fastest fix.

Step 1: Check All Listening Ports

List all active listening ports and their associated processes.

ss -tulnp

This shows the port number, protocol, and process name.

Step 2: Check a Specific Port

If you already know the port number, filter the output.

sudo ss -tulnp | grep :3000

Replace 3000 with the port you are investigating.

Step 3: Identify the Process Using the Port

Use lsof to directly map the port to a running process.

sudo lsof -i :3000

The output shows the process name and PID.

Step 4: Stop the Process Safely

If the process is not needed, stop it gracefully.

sudo kill PID

Replace PID with the actual process ID.

Step 5: Force Kill a Stuck Process

If the process does not stop, force termination.

sudo kill -9 PID

Use this only when normal termination fails.

Optional Step: Check Common Ports

Quick checks for common services.

sudo ss -tulnp | grep :80
sudo ss -tulnp | grep :443
sudo ss -tulnp | grep :22

Helpful when debugging web servers, SSH, or reverse proxies.