i was installed apex in r12 environmentYou mean EBS R12, right? How/where did you deploy APEX Listener to?
but i am getting error 503 service unavailable in apex listener by standalone installation in linux forStandalone Mode means you've started APEX Listener next to your EBS deployment. Did you make sure port 8080 hasn't been used by any of your EBS components before?
also get this error 404 service unavailable in apex listener by standalone installation in linux forAs long APEX Listener hasn't been configured, this URL won't work. But I'd expect a different error message, which makes me think you have a port conflict there. Do you know which service is actually issuing the 404?