[TP] Lab: A robust, scheduled backup script

What you will learn in this TP
Write a script with set -euo pipefail, trap and getopts
Check quality with ShellCheck
Schedule and restore

Introduction #

What you will learn in this section
Write a script with set -euo pipefail, trap and getopts
Check quality with ShellCheck
Schedule and restore

Context. Nobody backs up the server configuration. You write a reliable script that can be re-run safely and reports its failures.

Prerequisites: a disposable Ubuntu 24.04 virtual machine (Multipass, Vagrant or cloud) with a sudo account · Estimated time: 50 min

A verified, restorable daily backup of /etc.

Guided steps, with their solution.

1. Write the script
Why? Strict mode, automatic cleanup, a lock and options make the script safe to re-run and schedule.
Create /usr/local/bin/backup-etc.sh: a verified archive of /etc, options -n (dry run) and -r (retention in days), a flock lock, timestamped logging and deletion of old archives.
2. Check with ShellCheck
Why? ShellCheck catches missing quotes, undefined variables and classic pitfalls before they break production.
Install ShellCheck, analyse the script and fix any warnings. Check the syntax with bash -n.
3. Test normal and error cases
Why? A script is also tested on its failures: the exit code is what systemd or CI will see.
Run a dry run, a real backup, then trigger three errors (non-root, invalid retention, two simultaneous runs) showing the exit code.
4. Schedule with a timer
Why? The timer logs every run and marks the service failed if the script returns an error.
Create backup-etc.service and backup-etc.timer (daily at 2:30 with catch-up), trigger a manual run and read the journal.
5. Restore a file
Why? A backup is only worth something if you can restore it.
Modify /etc/motd, then restore the backed-up version from the latest archive to a temporary directory and compare before putting it back.

Difficulty level : ●●●○○ (3/5)