How to overdesign a shell

Yesterday I stumpled upon this question, it asks about creating a OO shell with use of the command design pattern.

It made me curious, because I always hated my shells (a cascade of ifs and elses). I answered the question with a fully working example.

Here's a little excerpt that depicts the main idea

private final Writer writer = new BufferedWriter(new OutputStreamWriter(
            System.out));
    private boolean quit = false;

    private Map<String, Command> commands = new HashMap<>();
    {
        commands.put("create", new CreateChat(this));
        commands.put("join", new JoinChat(this));
        commands.put("exit", new ExitCommand(this));
    }

    public void run() throws IOException {
        try (Scanner s = new Scanner(System.in)) {
            writer.write("> ");
            writer.flush();
            while (!quit && s.hasNextLine()) {
                String input = s.nextLine().trim();

                // get or default is java8, alternatively you could check for null
                Command command = commands.getOrDefault(input, new UnknownCommand(this, input));
                command.execute();

                if (!quit)
                    writer.write("> ");
                writer.flush();
            }
        }
    }

Still, I'm not completely pleased. I'd prefer a declarative solution, where you can specify the commands and map them to a appropiate instance of the Command interface.

My problem herin is, that I'm not sure how to handle complex inputs, e.g.

collect sample <sequence<double>>

collect sample 1.0 1.5 1.33 1.45

or class teacher <name> pupils <name> <name> ...

I could hand the command instances a Reader object, but I feel it is not their responsibility to read their own input. They should have a CommandParameter object etc.

But I'm not sure how to design it. I once implemented a binary protocol parsing library which enabled the user to define the protocl via xml, that seems like a solution (but a complex one).

Another question is, how do I tell the user when - and where exactly - formatting errors occur, in case the CommandParameter can't be created?

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TL;DR

So to finish with a clear question: What are the design patterns one can utilize to create a clean shell (in a declarative way), while respecting all the common clean code rules (separation of concerns, single responsibility principle, etc.).