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; using network configuration and security firewall; managing users and groups; managing files and directories; managing NTFS, CIFS, EXT4, LBA, LVM file systems; installing CentOS systems; using SELinux (Security-Enhanced Linux) system; DNF/YUM software package manager; managing MySQL server; developing Python and PHP scripts; using GCC C/C++ compilers; managing vsftpd - Very Secure FTP daemon; managing Postfix and Dovecot servers for emails; managing directory service with OpenLDAP; running Conda - Environment and Package Manager. Updated in 2021 (Version v5.34) with FTP and Mail server tutorials. For latest updates and free sample chapters, visit http://www.herongyang.com/Linux.