How can I check which process is using an ip/port?

Question

I have a program that occasionally fails to start because an IP/port that it wants to connect to is already in use. How can I programatically check which process is using the port? I can manually run lsof after I notice the failure, but whatever process is using the port must be using it for a very short time because by the time I run lsof, it says nothing is using the port. Thus, I'd like to programmatically find which process is using the port, immediately after the failure to connect, within the program that failed. How can I do this? It must be possible, because lsof does it.

Answer 1

You can check the process using netstat. It will give the pid of the process.

To check the port 8080 , you can do

nabil@LAPTOP~$ netstat -natup  | grep 8080
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
tcp        0      0 0.0.0.0:8080            0.0.0.0:*               LISTEN      63610/test-py

Here an example

nabil@LAPTOP:~$ netstat -natupe
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       User       Inode      PID/Program name
tcp        0      0 127.0.0.1:5010          0.0.0.0:*               LISTEN      1000       261855     66050/confd.smp
tcp        0      0 127.0.0.53:53           0.0.0.0:*               
tcp6       0      0 127.0.0.1:50736         127.0.0.1:4569          ESTABLISHED 1000       697733     63666/java
tcp6       0      0 127.0.0.1:41145         127.0.0.1:49159         TIME_WAIT   0          0          -
tcp6       0      0 127.0.0.1:41145         127.0.0.1:44619         TIME_WAIT   0          0          -
udp        0      0 127.0.0.53:53           0.0.0.0:*                           101        23138      -