But which command returns the details of a transaction (confirmation status, trunk transaction, branch transaction, message etc)?
getTrytes is the endpoint for getting transaction details from the ledger. It returns raw trytes that can be converted to a transaction object.
You can check out implementation of asTransactionObject for trytes to plain ...
Which operating system did you install it on?
Did you configure anything so far? (for example changed any ports or added any neighbors so far?)
By the way this reminds me of something similar I have opened an issue a while back https://github.com/iotaledger/iri/issues/1678
Maybe you've added a neighbor with an invalid port? Please check /etc/default/iri