How to Fix EADDRINUSE: Address Already in Use

Q: How do I find what process is using a port on Linux?

Use `ss -ltnp | grep :PORT` (shows process name and PID), `lsof -i :PORT`, or `fuser PORT/tcp` (prints only the PID). All three work; ss is the modern default on most distros.

Q: How do I kill a process on a port without knowing the PID?

Run `fuser -k PORT/tcp` - it finds every process listening on that TCP port and sends SIGKILL in one step. Or get the PID from ss/lsof and run `kill `.

EADDRINUSE / [Errno 98] Address already in use means a process is already bound to the port you want. Finding and killing that process is a three-command loop. Here's exactly how.

Fullstack Engineerfullstacklinuxnetworking

What EADDRINUSE actually means

When your server starts, it asks the OS to bind a port. If anything else is already listening there - a stale previous run, a python -m http.server left open, a misrouted service - the OS refuses with OSError: [Errno 98] Address already in use (EADDRINUSE on Linux, EADDRINUSE / WSAEADDRINUSE on macOS/Windows).

The bug is not in your code. The port is occupied.

Step 1: find what's holding the port

Three commands, any of them works - use whichever is on the box:

ss -ltnp | grep :8000       # modern Linux - shows process name + PID
lsof -i :8000               # classic - shows PID, user, and the full command
fuser 8000/tcp              # prints just the PID(s)

ss -ltnp output looks like:

LISTEN  0  5  0.0.0.0:8000  0.0.0.0:*  users:(("python3",pid=1842,fd=3))

The pid= field is the culprit. In this case it's PID 1842.

Step 2: kill the process

Once you have the PID:

kill 1842          # graceful SIGTERM
# if it doesn't stop:
kill -9 1842       # SIGKILL - guaranteed

Or let fuser do both steps in one shot:

fuser -k 8000/tcp   # finds and SIGKILLs every process on tcp/8000

Verify the port is free before restarting:

ss -ltnp | grep :8000    # should return nothing

Step 3: start your server

python3 app.py

The bind succeeds and your server is up.

Why does this keep happening?

Stale background processes - a previous nohup python3 app.py & that didn't get cleaned up. Always track long-running processes or use a process manager.
TIME_WAIT sockets - after a hard restart the OS holds the port in TIME_WAIT for ~60 s. SO_REUSEADDR in your socket options (Flask/uvicorn enable this by default) bypasses it for most cases.
Port conflicts - two services configured to the same port. Fix the config rather than killing one each time.

Quick reference

Command	What it does
`ss -ltnp \\| grep :PORT`	Shows PID + process name bound to PORT
`lsof -i :PORT`	Shows all processes with open sockets on PORT
`fuser PORT/tcp`	Prints PIDs on PORT
`fuser -k PORT/tcp`	Kills all processes on PORT

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What you'll practice

Using ss, lsof, and fuser to find the process holding a port
Killing a port-occupying process by PID and with fuser -k
Verifying a port is free before restarting a server

FAQ

What causes 'OSError: [Errno 98] Address already in use'?

Another process is already bound to the port your server is trying to open. The OS won't allow two processes to listen on the same TCP port. You need to find and stop the occupying process, or choose a different port.

How do I find what process is using a port on Linux?

Use ss -ltnp | grep :PORT (shows process name and PID), lsof -i :PORT, or fuser PORT/tcp (prints only the PID). All three work; ss is the modern default on most distros.

How do I kill a process on a port without knowing the PID?

Run fuser -k PORT/tcp - it finds every process listening on that TCP port and sends SIGKILL in one step. Or get the PID from ss/lsof and run kill <pid>.

How do I fix 'port already in use'?

Find the process holding the port with ss -ltnp, lsof -i, or fuser, then kill its PID - your server can bind the port again. Or start the app on a different port.

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