Firewalld

The rwx permission model, chmod symbolic vs. octal notation, chown/chgrp, setuid/setgid/sticky bits, and how umask sets your defaults.

Crontab syntax, special strings, run-parts directories, and modern systemd timer units — with a direct comparison of when to use each.

Disabling password auth, key-based login, sshd_config hardening, fail2ban jails, and bastion-host architecture for production access.

Real questions asked in Linux SysAdmin, DevOps, and RHCSA-track interviews — filesystem, permissions, systemd, networking, storage, and troubleshooting.

Core concepts: the shell, commands, filesystem hierarchy, and how distributions differ.

The FHS layout, mount points, inodes, and navigating the filesystem like an administrator.