read Unix and Linux System Administration Handbook

Chapter 1 - Where to Start
Chapter 2 - Booting and System Management Daemons
Chapter 3 - Access Control and Rootly Powers
Chapter 4 - Process Control
Chapter 5 - The Filesystem
Chapter 6 - Software Installation and Management
Chapter 7 - Scripting and the Shell
Chapter 8 - User Management
Chapter 9 - Cloud Computing
Chapter 10 - Logging
Chapter 11 - Drivers and the Kernel
Chapter 12 - Printing
Chapter 13 - TCP/IP Networking
Chapter 14 - Physical Networking
Chapter 15 - IP Routing
Chapter 16 - DNS: The Domain Name System
Chapter 17 - Single Sign-On
Chapter 18 - Electronic Mail
Chapter 19 - Web Hosting
Chapter 20 - Storage
Chapter 21 - The Network File System
Chapter 22 - SMB
Chapter 23 - Configuration Management
Chapter 24 - Virtualization
Chapter 25 - Containers
Chapter 26 - Continuous Integration and Delivery
Chapter 27 - Security
Chapter 28 - Monitoring
Chapter 29 - Performance Analysis
Chapter 30 - Data Center Basics
Chapter 31 - Methodology, Policy, and Politics