How to find which process is using a partitucalr Port.

Issue - While installing OEM agent configuration has been failed with
below error.

INFO: oracle.sysman.top.agent:Validating OMS_HOST and EM_UPLOAD_PORT
INFO: oracle.sysman.top.agent:EM Protocol Switch determined: https
INFO: oracle.sysman.top.agent:Performing free port detection..
INFO: oracle.sysman.top.agent:Trying for host : rhel11gr2rac1.manzoor.com/192.168.0.20 and port : 3872
INFO: oracle.sysman.top.agent:isPortFree:Port=3872 is busy. Exception ioe=Address already in use
SEVERE: oracle.sysman.top.agent:SEVERE: Agent configuration has failed while performing free port detection with the message:The Agent Port that is set will be:3872
INFO: oracle.sysman.top.agent:Recommendation:  If there are no free ports available, then try any of the following actions:
1. Stop any processes that are using port 3872 or any port between 1830-1849, so that there is a port available for use by the agent.

-- The 3872 port been used by another process because of which the installation failed.

[root@rhel11gr2rac1 ~]# netstat -tulpn | grep :3872

tcp        0      0 :::3872                     :::*                        LISTEN      3330/java

-- from the above we could see that an java process is using that port. Killed the process 3330 and then started it.

http://www.cyberciti.biz/faq/what-process-has-open-linux-port/