lsof lists open files — and on Unix, sockets and network connections count as files too, making it the go-to tool for finding which process holds a port or a file handle open.
Find What’s Using a TCP Port
-i filters by network address/port, :tcp or :udp scope it to a protocol. Combine with -P (skip port-name resolution, e.g. show 80 not http) and -n (skip host-name resolution) for fast, unambiguous output.
lsof -i :<port> # any protocol on that port
lsof -i tcp:<port> # TCP only
lsof -nP -i tcp:<port> # fast, no DNS/service-name lookupsFind All Open Files for a Process
-p restricts output to a given PID.
lsof -p <pid>Find Which Process Has a File Open
Pass the path directly — useful for “device busy” or “resource in use” errors before unmounting.
lsof <path>Kill Whatever Owns a Port
Combine -t (terse: PIDs only) with kill to free a stuck port in one line.
kill -9 $(lsof -t -i tcp:<port>)Cheatsheet
lsof -i :<port> # who's using this port (any protocol)
lsof -nP -i tcp:<port> # who's using this TCP port, fast
lsof -i tcp -s tcp:listen # all listening TCP sockets
lsof -p <pid> # all open files for a process
lsof <path> # who has this file open
lsof -u <user> # all open files owned by a user
lsof -t -i tcp:<port> # PID only, for scripting
kill -9 $(lsof -t -i tcp:<port>) # kill whatever owns a port