LICM: The Linux Operating System study guide

Domain 4 - The Linux Operating System

This domain tests how the system fits together: kernel and user space, hardware devices, the boot sequence, processes, and where files live in the filesystem hierarchy.

Core rules

The kernel manages CPU, memory and devices, and exposes hardware as device files under /dev: /dev/sda is the first disk, /dev/sda1 its first partition, /dev/null discards writes.
Boot order: firmware (BIOS/UEFI) → bootloader (GRUB) → kernel plus initramfs → init, which on modern systems is systemd running as PID 1. systemctl status, start, stop, enable and disable manage units.
Every process has a PID, owner, priority and state; ps aux and top show them. kill -TERM (15) asks politely, kill -9 (SIGKILL) cannot be caught. /proc holds live kernel and process data as pseudo-files.
FHS essentials: /etc configuration, /var variable data such as logs and spools, /home user data, /tmp scratch space, /usr programs and libraries, /bin and /sbin essential binaries, /boot kernel and GRUB files, /opt add-on software, /srv service data.

Worked example

A web service fails at boot. systemctl status nginx reports the failed unit; journalctl -u nginx shows the error, typically a bad path or directive in /etc/nginx/. Reasoning: configuration lives in /etc, logs in /var/log or the journal, and systemd reports unit state - check the unit, then the config, rather than reinstalling first.

Traps

Treating /proc or /sys as real files stored on disk.
Confusing the kernel with the distribution that ships it.
Reaching for kill -9 first; it prevents the process from cleaning up.
Editing files under /usr by hand instead of using the package manager.