CELE: Essential Linux concepts and architecture

This domain, Essential Linux concepts and architecture, tests whether you understand how a Linux system is assembled before you administer it: the kernel and user-space split, the boot chain, the filesystem hierarchy, and the service manager. Questions typically ask what happens at a given boot stage or which command reveals a kernel fact.

Core model

• The kernel is monolithic with loadable modules: inspect with lsmod, modinfo, load with modprobe. User programs reach hardware only through system calls.
• Boot order: firmware (UEFI/BIOS) → bootloader (GRUB2) → kernel plus initramfs → PID 1 (systemd) → targets. Targets replace SysV runlevels: multi-user.target is roughly runlevel 3, graphical.target roughly 5, rescue.target roughly 1.
• The FHS fixes meanings: /etc configuration, /var variable data, /usr programs and libraries, /proc and /sys kernel interfaces, /dev device nodes.
• dmesg or journalctl -k reads the kernel ring buffer; uname -r prints the running kernel; sysctl reads and writes kernel tunables.

Worked example

A server drops to emergency.target after an /etc/fstab edit. Reasoning: systemd reaches local-fs.target early, one mount unit failed, and the boot transaction could not complete. Remount / read-write with mount -o remount,rw /, fix the line, run systemctl daemon-reload, test with mount -a, then reboot. fstab is a boot dependency, not a runtime convenience.

Traps

• Confusing systemctl start with enable (now versus at boot).
• Treating /proc entries as ordinary files on disk.
• Editing fstab without a mount -a test or UUIDs.
• Assuming dmesg always covers the current boot after the ring buffer wraps.