This book is a collection of notes and sample codes written by the author while he was learning Linux systems. Topics include using Cockpit Web portal for admin tasks; managing users and groups; managing files and directories; managing NTFS, CIFS, EXT4, LBA, LVM file systems; using network tools and security firewall; installing CentOS systems; using SELinux (Security-Enhanced Linux) system; DNF/YUM software package manager; SSH Server configuration and client tools; managing vsftpd - Very Secure FTP daemon; managing directory service with OpenLDAP; Updated in 2024 (Version v5.44) with email topics moved to 'Email Tutorials' book. For latest updates and free sample chapters, visit https://www.herongyang.com/Linux.