chore(gitlab): improve notes about backups

knowledge base/cron.md

@@ -1,29 +1,56 @@
-# Title
-
-## Table of contents <!-- omit in toc -->
+# Cron
 
 1. [TL;DR](#tldr)
-1. [Sources](#sources)
+1. [Further readings](#further-readings)
+   1. [Sources](#sources)
 
 ## TL;DR
 
-Files in `/etc/cron.hourly` and similar need to:
+Files in `/etc/cron.hourly` and similar must:
 
-- be executable,
-- match the Debian cron script namespace (`^[a-zA-Z0-9_-]+$`, so script **with an extension** won't work).
+- Be **scripts**.<br/>
+  Normal crontab files will error out.
+- Be **executable**.<br/>
+  If not executable, execution will silently fail.
+- Match the Debian cron script namespace (`^[a-zA-Z0-9_-]+$`).<br/>
+  Files **with** an extension **won't** work.
 
 ```sh
+systemctl --enable --now 'crond.service'
+journalctl -xefu 'crond.service'
+
+# Validate crontab files.
+crontab -T '/etc/cron.d/prometheus-backup'
+
+# List files in given directories.
+# Kinda… useless?
+run-parts --list '/etc/cron.daily'
+
 # Print the names of the scripts which would be invoked.
-sudo run-parts --report --test '/etc/cron.hourly'
+# '--report' is *not* available everywhere.
+run-parts --report --test '/etc/cron.hourly'
+
+# Manually run crontab files in given directories.
+run-parts '/etc/cron.weekly'
 ```
 
-## Sources
+## Further readings
+
+- [Crontab]
+
+### Sources
 
 - [Function of /etc/cron.hourly]
+- [Use anacron for a better crontab]
 
 <!--
-  References
+  Reference
+  ═╬═Time══
   -->
 
+<!-- Knowledge base -->
+[crontab]: crontab.md
+
 <!-- Others -->
-[Function of /etc/cron.hourly]: https://askubuntu.com/questions/7676/function-of-etc-cron-hourly#607974
+[function of /etc/cron.hourly]: https://askubuntu.com/questions/7676/function-of-etc-cron-hourly#607974
+[use anacron for a better crontab]: https://opensource.com/article/21/2/linux-automation