lsof is a very useful diagnostic command:
# show whose listening on sockets
lsof -i -P
# which processes are using a file
lsof /mnt
# which process is holding localhost:80
lsof -i TCP:80
# show handles associated with a process 30563
lsof -p 30563
# show handles associated with a process syslogd
lsof -c syslogd
# show files associated with user sekhonp
lsof -u sekhonp