Exercise 3: A Laptop Missing Its Nightly Backup, Then Turned Back On — Possible Solution
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What happens: the moment the laptop is turned back on and boots up,
systemd checks backup.timer's own scheduled trigger against when the
system was actually running, recognizes that the scheduled 3am trigger
was missed entirely (the laptop was off at that exact moment), and --
because Persistent=true is set -- runs backup.service once,
immediately, as part of this boot, rather than silently skipping that
day's backup and simply waiting for the next regularly scheduled
trigger.

Explanation:

This is precisely the "real, standout advantage over cron" the
chapter names directly: Persistent=true "tells systemd to check, at
boot, whether this timer's scheduled trigger was missed while the
system was off... and if so, run it once, immediately, upon boot,
rather than silently skipping that day's backup entirely" -- using
almost exactly this chapter's own laptop-shutdown example. The backup
still happens, just later than originally scheduled (at boot time that
afternoon, rather than 3am), rather than being lost for an entire
missed cycle.

-- Contrast with what a traditional cron job would have done in the
same situation --
--
-- Per the chapter's own comparison, "a missed cron job (system off at
-- the scheduled time) is just... missed, silently, until the next
-- scheduled occurrence." If this had been a cron-based nightly
-- backup instead of a systemd timer, the laptop being off at the
-- scheduled time would have meant that night's backup simply never
-- happened at all, with no automatic catch-up mechanism -- the next
-- backup wouldn't occur until the FOLLOWING night's scheduled time,
-- leaving a genuine gap in backup coverage that Persistent=true
-- specifically closes.

WHY THIS WORKS AS AN ANSWER
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This directly applies the chapter's own laptop-shutdown example and
explicit Persistent=true behavior to the exercise's own scenario,
predicts the concrete outcome (a catch-up run at boot), and explicitly
contrasts it against what would have happened under traditional cron
using the chapter's own stated comparison.