Unix and Linux with common boot stages

  
                         UNIX and LINUX are identified as multitasking, multiuser and timesharing operating system. Unix was developed in 1987, which started from an University, further worked by AT&T, BSD and other organizations, each developed their own OSes keeping same platform of UNIX. Eventually different variant of UNIX was merged into and combined to form System V release 4 which became a common base for all future UNIX OSes. These releases were proprietary.
                             Richard Stallman started free UNIX-like OS and worked upon by his students, one of them was Linus Torvalds, who helped to release the first version of Linux kernel. 

Both system have same elements which are Kernel, File System, Process and shell. Kernel is an interface between computer's hardware and its processes. Kernel allocates itself a space in memory called as kernel space and it uses this to manage process, memory usage, device drivers. system calls, and security. Rest of the memory is used for user processes this space is known as User space.

Basic and common stages of boot process:
  1. The system BIOS checks the system and launches the first stage boot loader on the MBR of the primary hard disk.
  2. The first stage boot loader loads itself into memory and launches the second stage boot loader from the /boot/ partition.
  3. The second stage boot loader loads the kernel into memory, which in turn loads any necessary modules and mounts the root partition read-only.
  4. The kernel transfers control of the boot process to the /sbin/init program.
  5. The /sbin/init program loads all services and user-space tools, and mounts all partitions listed in /etc/fstab.
  6. The user is presented with a login screen for the freshly booted Linux system.

With changing requirements the /sbin/init daemon had been replaced by Systemd.  As per man systemd - systemd is a system and service manager for Linux operating systems. When run as first process on boot (as PID 1), it acts as init system that brings up and maintains userspace services.





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