The Learn Ubuntu Linux App is a great guide for the Ubuntu Operating System.
It includes not only Ubuntu Linux Operating System commands, but also provides a complete Ubuntu operating system guide for both Desktop and Server.

Main Features
✔ Dark Mode support
✔ Circular slider to track learning progress
✔ Percentage-based topic completion tracking
✔ Mobile-friendly reading experience

Unix,Linux Basics & Tutorials
✔ Basics 1 - One line Commands
✔ Basics 2 - Unix
✔ Basics 3 - Linux

Ubuntu Desktop Guide & Tutorials
✔ Desktop Commands and Guide

Ubuntu Server Guide DB,Web Server,Network and More
✔ Server Guide DB
✔ Web Server & More

Editors, Utils and More (Unix Administrative and Networking Commands)
✔ Ubuntu Editors
✔ Various OS Commands
✔ Ubuntu Utils
✔ Advanced Commands