lsof

Network Port

What process is using the port 50000

lsof -i :50000

Mount Point

What process is using the mount-point

lsof | grep /path/to/mount-point

lsof +f -- /path/to/dir

Process

What files and directories are using by the process

lsof -p `pidof <app_name>`

User

List files opened by processes belonging to a specific user

lsof -u [user-name]

IP & Domain

List files based on their Internet address

# For IPv4
lsof -i 4

# For IPv6
lsof -i 6