Linux startup process

Linux startup process is the multi-stage initialization process performed during booting a Linux installation. It is in many ways similar to the BSD and other Unix-style boot processes, from which it derives.

Booting a Linux installation involves multiple stages and software components, including firmware initialization, execution of a boot loader, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. For each of these stages and components there are different variations and approaches; for example, GRUB, LILO, SYSLINUX or Loadlin can be used as boot loaders, while the startup scripts can be either traditional init-style, or the system configuration can be performed through modern alternatives such as systemd or Upstart.

Early stages of the Linux startup process depend very much on the computer architecture. IBM PC compatible hardware is one architecture Linux is commonly used on; on these systems, the BIOS plays an important role, which might not have exact analogs on other systems. In the following example, IBM PC compatible hardware is assumed:

On shutdown, init is called to close down all user space functionality in a controlled manner. The init then terminates and the kernel executes its own shutdown.

The boot loader phase varies by computer architecture. Since the earlier phases are not specific to the operating system, the BIOS-based boot process for x86 and x86-64 architectures is considered to start when the master boot record (MBR) code is executed in real mode and the first-stage boot loader is loaded. In UEFI systems, a payload, such as the Linux kernel, can be executed directly. Thus no boot loader is necessary. Below is a summary of some popular boot loaders:

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