Exercise 2: Why the Two-Unit Pattern Makes Testing Genuinely Easier — Possible Solution
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Explanation:

Per this chapter's own opening point, the .timer/.service split is "a
deliberate design decision distinct from cron" specifically because
it separates "the what" (a real, independently-runnable systemd
service) from "the when" (the timer). The direct consequence: the
underlying task itself -- backup.service in the chapter's own
example -- is a genuine, standalone systemd unit that can be run
directly, on demand, at any moment, with a simple systemctl start
backup.service, completely independent of whatever schedule the
paired backup.timer defines.

A traditional crontab entry has no equivalent standalone unit to test
this way -- the scheduled command IS the crontab line itself, with no
separate, independently-invokable representation of "just the task,
please, right now." Testing a cron job typically means either waiting
for its actual scheduled time to arrive, or manually copy-pasting the
command out of the crontab and running it by hand in a way that may
not exactly replicate cron's own execution environment (its own PATH,
working directory, and environment variables often differ subtly
from an interactive shell).

With systemd's own approach, running systemctl start backup.service
executes the EXACT SAME unit, in the EXACT SAME way, that the timer
itself would eventually trigger -- there's no risk of a testing
discrepancy between "what I ran by hand to test it" and "what actually
runs on schedule," because they're genuinely the same operation,
just invoked two different ways (manually via systemctl start, or
automatically via the timer).

WHY THIS WORKS AS AN ANSWER
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This explains specifically why the systemd approach avoids the
testing-environment discrepancy risk inherent in manually replicating
a cron job's own execution context, directly grounding the explanation
in the chapter's own stated design rationale for splitting timer and
service into two units.