Exercise 1: A .socket Unit for Port 9000 — Possible Solution
====================================================================

myapp.socket:

[Unit]
Description=Socket for myapp

[Socket]
ListenStream=9000

[Install]
WantedBy=sockets.target

Command to enable and start it:

sudo systemctl enable --now myapp.socket

Explanation:

This follows the chapter's own myapp.socket structure exactly, with
only the port number changed to 9000 per the exercise's own
requirement (ListenStream=9000 in place of the chapter's own
ListenStream=8080). The [Unit] and [Install] sections are unchanged
from the chapter's own example, since nothing in the exercise's own
requirements calls for a different description or a different target
-- WantedBy=sockets.target is still the correct target for a socket
unit, matching the chapter's own description of it as parallel to
timers.target.

systemctl enable --now myapp.socket is used rather than targeting
myapp.service directly, matching the chapter's own explicit
instruction to "enable and start the SOCKET, not the service
directly" -- the service itself is never enabled or started
directly; it only comes up on-demand once a real connection actually
arrives at the socket.

WHY THIS WORKS AS AN ANSWER
------------------------------
This builds a correctly structured .socket unit using the chapter's
own established fields with only the port number changed per the
exercise, and uses the correct systemctl target (the socket, not the
service) matching the chapter's own explicit instruction.