Open your terminal and type as
```sh
lsof -i :8000
```
For example, with port 8000 (python3 -m http.server):
```sh
$ lsof -i :8000
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
python3 3269 user 3u IPv4 1783216 0t0 TCP *:8000 (LISTEN)
And port 22 (SSH):
```
```sh
$ sudo lsof -i :22
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
sshd 998 root 3u IPv4 1442116 0t0 TCP *:ssh (LISTEN)
sshd 998 root 4u IPv6 1442118 0t0 TCP *:ssh (LISTEN)
```sh
fuser 43796/tcp
```
https://askubuntu.com/questions/278448/how-to-know-what-program-is-listening-on-a-given-port
https://askubuntu.com/questions/227161/how-can-we-find-which-process-is-using-a-particular-port
```