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Linux LPIC2 (Starting a system (The Linux boot process can be split into…

- - - - The Firmware Startup
    - - several choices of bootloaders
        
        ■■ Linux Loader (LILO)
        
        ■■ Grand Unified Bootloader (GRUB) Legacy
        
        ■■ GRUB2
      - The bootloader program helps bridge the gap between the system firmware and the full Linux operating system kernel.
- - - - The kernel is primarily responsible for four main functions:
        
        ■■ System memory management
        
        Memory management is the ability to control how programs and utilities run within the memory restrictions of the system.
        
        Not only does the kernel manage the physical memory available on the server, but it can also create and manage virtual memory, or memory that does not actually exist but is created on the hard drive and treated as real memory.
        
        The kernel keeps track of which memory pages are in use and automatically copies memory pages that have not been accessed for a period of time to the swap space area (this is called swapping out), even if there’s other memory available. When a program wants to access a memory page that has been swapped out, the kernel must make room for it in physical memory by swapping out a different memory page and swapping in the required page from the swap space. Obviously this process takes time, and it can slow down a running process. The process of swapping out memory pages for running applications continues for as long as the Linux
        system is running.
        
        You can see the current status of the virtual memory on your Linux system by viewing the special /proc/meminfo file.
        
        By default, each process running on the Linux system has its own private memory pages. One process cannot access memory pages being used by another process. The kernel maintains its own memory areas. For security purposes, no processes can access memory used by the kernel processes
        
        To facilitate data sharing, you can create shared memory pages. Multiple processes can read and write to and from a common shared memory area. The kernel maintains and administers the shared memory areas, and it allows individual processes access to the shared area
        
        By default, each process running on the Linux system has its own private memory pages. One process cannot access memory pages being used by another process. The kernel maintains its own memory areas. For security purposes, no processes can access memory used by the kernel processes.
        
        The special ipcs command allows you to view the current shared memory pages on the
        system.
        
        ipcs -m
        
        Each shared memory segment has an owner that created the segment. Each segment also has a standard Linux permissions setting that sets the availability of the segment for other users. The key value is used to allow other users to gain access to the shared memory segment.
        
        ■■ Software program management
        
        The Linux operating system calls a running program a process. A process can run in the foreground, displaying output on a display, or it can run in background behind the scenes. The kernel controls how the Linux system manages all of the processes running on the
        system.
        
        the Linux operating system uses an init system that utilizes either runlevel (for the SysV init method) or targets (for the systemd method) that determine which processes start. There are seven init run levels in the Linux operating system.
        
        At run level 1, only the basic system processes are started, along with one console terminal process. This is called single-user mode. Single-user mode is most often used for emergency file system maintenance when something is broken. Obviously, in this mode only one person (usually the administrator) can log into the system to manipulate data. For most Linux distributions, the standard init run level is 3. At this run level, most application software, such as network support software, is started. Another popular run level in Linux is run level 5. This is the run level where the system starts the graphical X Window software, and it allows you to log in using a graphical desktop window.
        
        The Linux system can control the overall system functionality by controlling the init run level. For example, by changing the run level from 3 to 5, the system can change from a console-based system to an advanced, graphical X Window System.
        
        ■■ Hardware management
        
        ■■ Filesystem management