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Find and kill whatever is using a port

Updated 2026-07-05

Problem: Starting a service fails with "address already in use," and you need to find out what's already listening on that port before you can free it up.

Solution:

lsof -i :8080
COMMAND   PID USER FD   TYPE  DEVICE SIZE/OFF NODE NAME
python3 24862 node 3u  IPv4 1252652      0t0  TCP *:8080 (LISTEN)

How it works:

  • -i :8080 filters lsof's (list open files) output to sockets on port 8080, on any address.
  • The PID column is what you need next: kill 24862 stops that specific process. Try a plain kill first (sends SIGTERM, letting the process shut down cleanly) before reaching for kill -9 (SIGKILL, immediate and unconditional).

Variations:

# ss: no separate package needed, shows the same information
ss -ltnp 'sport = :8080'

# fuser: identify and kill in one step
fuser -k 8080/tcp
8080/tcp:            24862

ss ships with the base system on Debian and doesn't require installing anything, making it the first thing to reach for on a box you don't control. fuser -k skips the two-step "find the PID, then kill it" process entirely, sending SIGTERM straight to whatever's using the port; useful for a quick cleanup, but skip it when you specifically need to inspect the process (its command line, working directory, or owner) before deciding whether killing it is actually the right call.

All three commands default to TCP. A port bound over UDP (common for DNS resolvers or some monitoring agents) needs -i udp:8080 for lsof, sport = :8080 with -u instead of -t for ss, or 8080/udp for fuser. If kill doesn't work and the process lingers, it's usually stuck in an uninterruptible wait (disk I/O, most often), and kill -9 (SIGKILL) is the next step, bypassing the process's own shutdown handling entirely.