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After the server is turned on , Will be based on the motherboard BIOS The settings in are right for CPU、 Memory 、 The graphics card 、 Keyboard and other equipment for preliminary testing , After successful detection, the system control is transferred according to the preset startup sequence , Most of the time, it will be handed over to the local hard disk .
summary ： Detect the first device that can boot the system , Such as hard disk or optical drive

2、MBR guide

When booting the system from the local hard disk , First, according to the first sector of the hard disk MBR ( Master boot record ) Set up , Pass system control to the partition that contains the operating system boot file ; Or directly based on MBR The boot information in the record calls the boot menu ( Such as GRUB) .
summary ： Run on MBR Boot in sector GRUB Bootstrap

3、GRUB menu

about Linux For the operating system ,GRUB ( Unified boot loader ) Is the most widely used multi system bootloader program . Control of the system is passed on to GRUB in the future , The Startup menu will be displayed for the user to choose , And according to the selection ( Or use the default value ) load Linux Kernel files , Then transfer control of the system to the kernel .
CentOS 7 It's using GRUB2 Start the bootloader .
summary ：GRUB The bootstrapper reads GRUB The configuration file /boot/grub2/grub.cfg, To get the settings and path locations of the kernel and image file system

4、 Loading kernel （kernel）

Linux The kernel is a special pre compiled binary , Between various hardware resources and system programs , Responsible for resource allocation and scheduling . After the kernel takes control of the system , Will be in full control of the whole Linux The operation of the operating system , stay Centos In the system , The default kernel file is located in “/boot/vmlinuz-3.10.514.e17.x86_64”.
summary ： Load the kernel and image file system into memory

5、init Process initialization

In order to complete the further system boot process ,Linux The kernel first converts "/sbin/init" Program loaded into memory to run ( A running program is called a process ),init The process is responsible for the initialization of the whole system , Finally, wait for the user to log in .
summary ： Loading hardware drivers , The kernel puts init The process is loaded into memory to run

Linux Operating system boot process summary ：

1、 Self checking on startup ： Power on test hardware
2、MBR guide ： Know what system program to load （ It will refer to user-defined ---BIOS）
3、GRUB2 menu ： Submit menu （ Need to load 、 Program started ） Ready for the kernel
4、 Loading kernel ： Load the core program 、 Memory and image （ Put it in memory ）
5、init Process initialization ： Start the system program in the menu （ Put it in memory ）

Two 、 System initialization process

init process

from Linux The kernel loads and runs /sbin/init Program
init Process is the first process in the system
init Process PID（ Process tag ） The number will always be 1

Systemd

systemd yes linux One of the operating systems init Software
centos7 The new systemd Starting mode , Replace the traditional sysvinit
centos7 The first one running in init The process is /lib/systemd/system/
The first program loaded by the system is init
The tools of the system management program are systemd,systemd The command program of is ：systemctl.

Systemd Unit type

Unit type	Extension	explain
service	.service	Describe a system service
sockt	.sockt	Describes a socket for interprocess communication
device	.device	Describe a device file identified by the kernel
mount	.mount	Describe the mount point of a file system
automount	.automount	Describes the auto mount point of a file system
swap	.swap	Describe a memory swap device or swap file
path	.path	Describe a file or directory in a file system
timer	.timer	Describe a timer （ Used to implement similar cron Scheduling tasks for
snapshot	.snapshot	Used to save a systemd The state of
scope	.scope	Use systemd To create external processes programmatically
slice	.slice	Describe cgroup A group of management system processes that are organized hierarchically
target	.target	Describe a group systemd Unit of

Run level corresponding to Systemd The goal is

Operation level	Systemd Of target	explain
0	target	Shutdown status , Using this level will shut down the host
1	rescue.target	Single user mode , You can log in to the system without password verification , For system maintenance
2	multi-user.target	User defined / Domain specific runlevels . Default is equivalent to 3
3	multi-user.target	Full multi user mode of character interface , Most server hosts run at this level
4	multi-user.target	User defined / Domain specific runlevels . Default is equivalent to 3
5	graphical.target	The multi-user mode of graphical interface , Provides a graphical desktop operating environment
6	reboot.target	Restart , Using this level will restart the host