Find Which Process Is Using a Port (Linux)

The fastest reliable answer on a modern Linux is one ss command — here's the exact invocation, plus why netstat output misleads people.

What you'll see

'Address already in use' from a server you're starting
You need to know what owns :443 before restarting it

Root causes

Another process legitimately bound the port

Common double-booking: an old instance of the same app, or a second web server squatting on 80/443.

Docker's proxy process

dockerd/docker-proxy binds published ports — ps output shows 'docker-proxy', which confuses netstat readers.

Fix it

The one-liner (process name, PID, everything)
```
sudo ss -ltnp | grep -E ':443\b'
```
Map the PID to its full command and working dir
```
ps -fp <PID> && ls -l /proc/<PID>/cwd
```

If it's docker-proxy, find which container owns it

docker ps --format '{{.Names}} {{.Ports}}' | grep 443

Gracefully stop it (or repoint your new service)

sudo kill <PID>   # prefer systemctl stop <unit> when known

Field note

Use sudo with ss even as root — without it, process names for other users' sockets are hidden, which is exactly the case when you can't identify the holder. lsof -i :443 is the older equivalent if ss isn't available.

Common questions

What's the fastest way to see what's on a port?

ss -ltnp | grep :<port> (modern) or lsof -i :<port>. ss shows the process name/PID for sockets it owns; run with sudo to see processes owned by other users — the #1 reason people 'can't find' what's on the port.

Why does nothing show up on the port though the app says it's listening?

It's bound to a different interface (127.0.0.1 vs 0.0.0.0 — check ss for the bind address) or a different port (config mismatch). The bind address in ss output is the ground truth the app's config never shows you.

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Find Which Process Is Using a Port (Linux)

What you'll see

Root causes

Another process legitimately bound the port

Docker's proxy process

Fix it

Field note

Common questions

Related fixes

Ship it right the first time

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