Count open fds of a PID

Quick fd count to spot leaks vs ulimit -n; faster: ls /proc/<pid>/fd | wc -l

lsof -nP -p <pid> | wc -l

More lsof commands

List open files by process

lsof -p $(pgrep nginx) | head -30

Show all files (sockets, regular, pipes) opened by nginx
Who is using a port

lsof -i :8080

Find which process is listening on port 8080
List all network connections

lsof -i -n -P

Show all open network connections without DNS resolution
Find open files on a filesystem

lsof +D /var/log

Show all processes with open files under /var/log
Find deleted files still held open

lsof | grep '(deleted)'

Find files deleted but still held open by processes (disk bloat)