GNU / Linux

Bash

• The Bash Guide
• Shell Scripting Tutorial
• The Bash Hackers Wiki
• Bash scripting cheatsheet
• Shell scripting quick reference
• explainshell.com
• An A-Z Index of the Linux command line: bash + utilities.
• The Beginner’s Guide to Shell Scripting: The Basics
• Bourne-Again SHell and Linux CLI
• Linux commands
• Shell commands quick reference
• Useful Linux Command Tips & Tricks
• 30 Handy Bash Shell Aliases For Linux / Unix

String search / manipulation

• Regular Expressions
• Books about Regular Expressions
• Sed - An Introduction and Tutorial
• Sed, a stream editor
• Awk
• GNU Awk User’s Guide
• An introduction to grep & egrep - how to search for strings inside of files

Filesystem

• The Linux filesystem explained
• Inodes - an Introduction

Hardware

• 5 commands to check memory usage on Linux
• Display battery status and thermal temperature from command line
• Battery Features — TLP 1.3.1 documentation

Books

• Linux-insides

Apache

• Configure Multiple Sites with Apache
• Reverse Proxy Guide
• Change Apache Default Port To A Custom Port
• VirtualHost Examples
• Apache SSL/TLS Encryption
• SSL/TLS Strong Encryption: How-To
• How to Enable Apache Mod_Rewrite (Ubuntu Server)

Nginx

• How to Configure Nginx and Apache Together (Ubuntu Server)

Databases

• How To Install MariaDB on Debian
• How to see/get a list of MySQL/MariaDB users