linux · medium

Who Owns This Port / FD? — lsof & /proc/PID/fd

lsof maps descriptors to resources: find a port's owner (lsof -i / fuser), detect a leak by counting /proc/PID/fd against ulimit -n (ends in EMFILE), and solve 'df full but du empty' — a deleted-but-still-open file held by a process (lsof +L1).

The code

sudo lsof -i :8080      # which process owns a port
sudo fuser 8080/tcp     # just the PID (-k to kill)

ls /proc/1234/fd | wc -l   # fd count vs ulimit -n  (leak check)

# df full but du empty? a deleted file is still open:
sudo lsof +L1           # files with link count 0

What this lesson walks through

1. 01Which process owns a port?
2. 02Detect a descriptor leak
3. 03'Disk full' but du finds nothing

lsof -i :PORT or fuser PORT/tcp names the process and PID holding a port — fuser -k even kills it.