lsof -i :portnumber
lsof -i tcp:portnumber
lsof -i udp:portnumber
fuser portnumber/tcp
netstat -tulpn | grep :portnumber
ps -ef | grep PID
ps -p PID (gives name of the command that started this process)
pwdx PID
alternatively, you can browse through /proc/PID/cwd
dpkg -S /bin/ls