Exercise 1: A Complete backup-agent Unit File — Possible Solution
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[Unit]
Description=Backup Agent Service
After=network.target

[Service]
Type=simple
ExecStart=/opt/backup/agent
Restart=on-failure
RestartSec=10

[Install]
WantedBy=multi-user.target

Explanation:

This follows the chapter's own worked example structure exactly, with
values matching the exercise's own specific requirements. Type=simple
is the correct, default choice here since nothing in the exercise
description suggests this is a forking daemon, a one-shot command, or
an application supporting sd_notify -- it's described as a normal
long-running agent process, exactly what Type=simple is for.
ExecStart=/opt/backup/agent uses the exact absolute path specified in
the exercise, required per the chapter's own warning that no shell
PATH lookup happens for this field. Restart=on-failure and
RestartSec=10 together implement "restarts automatically on failure
after a 10-second delay" precisely as the exercise requests. After=
network.target implements "starts after the network is up," matching
the chapter's own [Unit]-section After= preview. WantedBy=
multi-user.target is included in [Install] so the service can actually
be enabled to start at boot, following the chapter's own complete
worked example rather than omitting it.

WHY THIS WORKS AS AN ANSWER
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This builds a complete, correctly structured unit file matching every
specific requirement in the exercise (restart-on-failure, 10-second
delay, after-network ordering), using the chapter's own field names
and syntax exactly.