lsof – local open ports

A short one:

Command

lsof -i -P -n | grep LISTEN

This will output all the ports the system is listing on. You might have to prepend „sudo“.

So then the command will be:

sudo lsof -i -P -n | grep LISTEN

The list is long without the „grep“, it will filter the output and only show the lines which contain „LISTEN“.

Edit:
Oliver from Hubzilla (part of the Fediverse) pointed out that

netstat -anpl

might be helpful too. The command shows all the listing sockets of the Linux machine it is run on. (I added the „l“ parameter to just show listing sockets.