The Linux System Stack
A running Linux system is layered. Understanding the layers is the foundation every certification builds on.
| Layer | Role | Examples |
|---|---|---|
| Hardware | Physical CPU, RAM, disks, NICs | - |
| Firmware | Initialises hardware, finds a bootloader | BIOS, UEFI |
| Bootloader | Loads the kernel | GRUB2, systemd-boot |
| Kernel | Manages memory, processes, drivers, syscalls | vmlinuz |
| init system | First userspace process (PID 1), starts services | systemd, SysVinit |
| Userspace | Shells, daemons, applications | bash, nginx |
Exam tip: The kernel is not the operating system by itself - the OS is the kernel plus the userland (GNU tools, libraries, init). This is why it is called GNU/Linux.