Linux Introduction-8 Linux system boot details

• System startup process
• Bios
• Mbr
• Grub
• KERNEL
• Init
• Single-User change root password
• GRUB Encryption

System startup process

• Bios
• Mbr:boot Code
• Execute Bootstrapper-GRUB
• Load kernel
• Execute init
• RunLevel

Bios

• Basic Input Output System, typically stored in the motherboard's BIOS chip
• Run the BIOS first when the computer starts, check the hardware and look for bootable devices
• Bootable devices are defined in BIOS settings, such as USB, CDROM, HD

Mbr

Different systems have different MBR

• Boot code is executed after the BIOS has found a bootable device
• The boot code is the first 446 bytes of the MBR

Grub

MBR generally acts as a jump, starting a more complex boot program, such as Grub

• Grub is now the mainstream boot program for Linux use
• Can be used to boot almost all of the operating systems now, PE is also grub boot
• Grub related files are saved in the /boot/grub directory
  Stage1 (MBR), xxx_stage1_5 (file system driver), Stage2
• The Grub configuration file is/boot/grub/grub.conf
• Configuration format:

default=0 # 第一个引导程序timeout=5 # 停留时间splashimage=(hd0,1)/boot/grub/splash.xpm.gz # 引导界面背景hiddenmenu # 默认隐藏菜单title CentOS # 操作系统名称    root (hd0,1) # 操作系统所在分区，第一块硬盘第二个分区    # 内核以及传递给内核的参数（ro root=...），用空格分开    kernel /boot/vmlinuz-2.6.32-431.el6.x86_64 ro root=...    # initial ram file system,内核模块打包文件    initrd /boot/initramfs-2.6.32-431.el6.x86_64.img

KERNEL

• The MBR boot code will be responsible for locating and loading the Linux kernel
• The Linux kernel is saved in/boot/vmlinuz-2.6.32-431.el6.x86_64
• Kernel module Packaging files are also typically loaded:/boot/initramfs-2.6.32-431.el6.x86_64.img
• Linux to keep the kernel of the simplification of some of the less common drivers, functions compiled into a module, when needed to dynamically load, and these modules are packaged to save as a Initramfs file
• Earlier versions of Linux use the Initrd file, Initramfs is a initrd alternative optimized version, more space-saving and more flexible than INITRD
• Command dmesg or /var/log/dmesg file to view the output information of the kernel at the start of this boot

Init

• Init ( /sbin/init ) is the first process running in a Linux system, the parent process of all processes.
• The call /etc/rc.d/rc.sysinit is responsible for initializing the system, mounting the file system, and starting the corresponding service based on the RunLevel
• Linux RunLevel (runlevel):
  • - 0Shutdown
  • - 1Single-User mode
  • - 2Multi-user mode with no network
  • - 3Multi-user mode
  • - 4Not used
  • - 5X11 Graphical mode
  • - 6Restart
• /etc/inittabThe default runlevel can be modified through the configuration file
• etc/init/The files under are other configuration information
• Each level corresponding to the startup service is saved in the /etc/rc.d/rc[0-6].d/
• runlevelcan display current and previous RunLevel
• initAvailable to change the current run level

Single-User change root password

• Pass parameters to the kernel 1 or single enter single-user mode
• No services are started in single-user mode
• Single-user mode log in directly with the root user and require a password
• You can use passwd to modify the root password

GRUB Encryption

• By adding the following parameters to the boot configuration in grub.conf
  password --md5 $1$aoWpTe$/MWUUYbBIF.2RUZK8nbWU0
• The encrypted password can be generated by the grub-md5-crypt

Alternatively, you can refer to Nanyi's blog:

1. How the computer is started
2. Linux Start-up process

Linux Introduction-8 Linux system boot details