lsof – list open files for more information, please see the man page 😉
List process running on a port
root# lsof -i :port_number
Lists IPv4, use 6 for IPv6
root# lsof -i 4
Lists open files for TCP port ranges 1-1024
root# lsof -i :1-1024
Lists by pid
root# lsof -p PID
List files opened by a specific user
root# lsof -u username
Kill all activity for a particular user
root# killall -9 `lsof -t -u username`
List all network connections
root# lsof -i
List the opened files in a directory
root# lsof +D path_of_the_directory
List the opened files based on process names
root# lsof -c process_name