1. The boot process of Linux operating system
2. System initialization process
3. Eliminate startup faults
Four, system service control
- 1. System service control
- 2. Control type
Five, the operating level of the Linux system
Six, optimize the startup process
- 1. ntsysv tool
- 2. Systemct tool
7. Start and control of system services
- 1. ntsysv system service management tool
- 2. Start and control system services

1. The boot process of Linux operating system

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1. POST

When the server is turned on, it will perform a power-on self-test to detect whether hardware devices such as CPU, memory, graphics card, keyboard and other devices are faulty. If there is no fault, the system will be transferred according to the preset startup sequence, which is the normal situation. Machine hard drive.
Summary: The first device capable of booting the system is detected, such as a hard disk or optical drive

2. MBR boot

When starting the system from the local hard disk, first transfer system control to the partition containing the operating system boot file according to the MBR (Master Boot Record) setting in the first sector of the hard disk; or directly according to the boot in the MBR record The information calls the boot menu (eg GRUB).
The MBR is 512 bytes in size and stores pre-boot information, partition table and other information.
Summary: Run the boot GRUB boot program placed in the MBR sector

3. GRUB menu

For the Linux operating system, GRUB (Unified Boot Loader) is the most widely used multi-system bootloader program. After the system control is passed to GRUB, the boot menu will be displayed for the user to choose, and the Linux kernel file will be loaded according to the selected option (or the default value), and then the system control will be transferred to the kernel.
CentOS 7 uses GRUB2 boot loader.
GRUB is the first 446 bytes of the MBR program.
Summary: The GRUB boot program reads the GRUB configuration file /boot/grub2/grub.cfg to obtain the kernel and mirror file system settings and path locations

4. Load the Linux kernel

The Linux kernel is a pre-compiled special binary file, between various hardware resources and system programs, responsible for resource allocation and scheduling. After the kernel takes over the control of the system, it will fully control the running process of the entire Linux operating system.
In CentOS 7 system, the default kernel file is located at "/boot/vmlinuz-3.10.0-514.el7.x86_64".
Summary: Load the kernel and mirror file system into memory

5. init process initialization

After the kernel is loaded, the first program to run is "/sbin/init" (the running program is called a process). The init process is responsible for completing the initialization of the entire system and finally waiting for the user to log in.
Summary: Load the hardware driver, the kernel loads the init process into the memory to run

2. System initialization process

1. The init process

Load and run the /sbin/init program by the Linux kernel
The init process is the first process in the system and the parent process of all processes
The PID (Process ID) number of the init process is always 1

2 、 Systemd

Systemd is an init software of Linux operating system
CentOS7 uses a new Systemd startup method to replace the traditional SysVinit
The first init process running in CentOS7 is /lib/systemd/systemd

Comparison of traditional sysvinit and systemd

name	Start method	effectiveness	Start speed
Traditional sysvinit	Rely on the serial execution of the Shell script to start the service (there is a dependency between each file, start according to the dependency, in order)	All services are started, resulting in low efficiency	System startup is slow
system	Start more service processes in parallel (ignore the dependencies first, start them at the same time, and establish the dependencies after the corresponding dependent software is started)	Provides the ability to start services on demand, so that fewer processes are started, and efficiency is high	Fast system startup

3. Systemd unit type

Unit type	extension name	Description
Service	.service	Describe a system service
Socket	.socket	Describe a socket (IP port) for inter-process communication
Device	.device	Describe a device file recognized by the kernel
Mount	.mount	Describe the mount point of a file system
Automount	.automount	Describe the automatic 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 cron-like scheduling tasks)
Snapshot	.snapshot	Used to save the state of a systemd
Scope	.scope	Use systemd's bus interface to programmatically create external processes
Slice	.slice	Describe a group of management system processes organized by hierarchy in Cgroup
Targe	.target	Describe a set of systemd units

The three main unit types are Service, Socket, and Targe, and the others are also required to see what they mean.
These unit types are appended to the resource name in the form of a suffix, such as network service network.service.

4. The Systemd target corresponding to the run level

init run level	systemd的target	Description
0	target	Shutdown state, the host will be shut down when using this level
1	rescue.target	Single user mode, you can log in to the system without password verification, mostly used for system maintenance
2	multi-user.target	User-defined/domain-specific run level, the default is equivalent to 3, compared to 3, no network
3	multi-user.target	Full user mode with character interface, most server hosts run at this level
4	multi-user.target	User-defined/domain-specific run level, not used by default in centos6, reserved, it is equivalent to 3 in centos7
5	graphical.target	The multi-user mode of the graphical interface provides a graphical desktop operating environment
6	reboot.target	Restart, the host will be shut down and restarted normally when using this level

The most used are 0, 3, 5, 6, 3 for servers, 5 for personal use, and 1 for system maintenance. The higher levels 2 and 4 use very little.

3. Eliminate startup faults

Demonstration of fault repair in the next blog: linux technical document: simulate and repair the fault

1. MBR sector failure analysis

1) Cause of failure

Damage caused by viruses, Trojan horses, etc.
Incorrect partition operation, disk read and write errors

2) Failure phenomenon

Cannot find the boot program, start interrupted
Unable to load operating system, black screen after boot

3) Solutions

Backup files should be made in advance
Boot into emergency mode with installation CD
Restore from backup file

2. Simulate MBR sector failure and repair

MBR is located at the first physical sector of the first hard disk (/dev/sda), a total of 512 bytes

1) Back up MBR sector data to other disks (/dev/sdb1)

mkdir /backup
mount /dev/sdb1 /backup
dd if=/dev/sda of=/backup/mbr.bak bs=512 count=1

2. Simulate destruction of MBR boot sector

dd if=/dev/zero of=/dev/sda bs=512 count=1

3. The boot interface enters emergency mode and restores MBR sector data from the backup file

先加载好光盘镜像，重启操作系统，
当出现安装向导界面时，选择“Troubleshooting”选项，
再选择“Rescue a CentOS Linux system”选项，进入急救模式
选择“1”选择 Continue并按 Enter 键继续
再次按 Enter 键后将进入带“sh-4.2#”提示符的 Bash Shell 环境
sh-4.2# mkdir /backupdir
sh-4.2# mount /dev/sdb1 /backupdir 				#挂载带有备份文件的分区
sh-4.2# dd if=/backupdir/mbr.bak of=/dev/sda	#恢复备份数据
sh-4.2# exit									#执行exit命令退出临时Shell 环境，系统将会自动重启

3. GRUB boot failure analysis

1) Cause of failure

The GRUB boot program in MBR is corrupted
The grub. cfg file is missing, and the boot configuration is incorrect (the location of the centos7 file is /boot/grub2/grub.cfg)

2) Failure phenomenon

System booting stalls, displaying "grub>" prompt

3) Solutions

Try to manually enter the boot command
Enter emergency mode, rewrite or restore grub.conf from backup
Rebuild the grub program into the MBR sector

4. Repair GRUB boot failure

1) Method 1: Manually enter the boot command (clumsy and cumbersome, not recommended)

/boot/grub/目录下的包是用于启动菜单的背景图片及样式
/boot/grub2/grub.cfg			#GRUB配置文件

grub> insmod xfs					#加载指定的模块到内核
grub> linux16 /vmlinuz-3.10.0-693.el7.x86_64 root=UUID=8fd74986-ae66-4ffd-b7d8-a19f2eca7b6f ro rhgb quiet LANG=zh_CN.UTF-8	 #内核的名字及位置等信息
grub> initrd16 /initramfs-3.10.0-693.el7.x86_64.img			#镜像系统文件
grub> boot							#引导boot

2) Method 2: Enter emergency mode and restore GRUB boot program

MBR is located at the first physical sector of the first hard disk (/dev/sda), a total of 512 bytes, the first 446 bytes are the master boot record, and the partition table is stored in the 477-510 bytes in the MBR sector So here we back up the first 446 bytes and keep the previous partition table.

mkdir /bak
mount /dev/sdb1 /bak
dd if=/dev/sda of=/bak/grub.bak bs=446 count=1

Simulate the destruction of the GRUB boot program in MRB, but does not destroy the partition table

dd if=/dev/zero of=/dev/sda bs=446 count=1

The boot interface enters emergency mode, and the GRUB boot program is restored from the backup file

sh-4.2# mkdir /backupdir
sh-4.2# mount /dev/sdb1 /backupdir 	
sh-4.2# dd if=/backupdir/grub.bak of=/dev/sda	
sh-4.2# exit

3) Method 3: The boot interface enters emergency mode, and the GRUB menu configuration file is rebuilt.
This method generally deals with the situation that the /boot/grub2/grub.cfg file is missing or the boot configuration is incorrect.

Delete the grub.cfg file to simulate failure

rm -rf /boot/grub2/grub.cfg

Enter emergency mode, load the system image, switch to the system root environment

sh-4.2# chroot /mnt/sysimage

Reinstall the GRUB boot program to the MRB sector of the first hard disk (/dev/sda)

bash-4.2# grub2-install /dev/sda

Rebuild the configuration file of the GRUB menu

bash-4.2# grub2-mkconfig -o /boot/grub2/grub.cfg

Exit the chroot environment and restart

bash-4.2# exit
sh-4.2# reboot

5. Password analysis of forgotten root user

1. The cause of the fault:
The password of the root user is forgotten.
2. The phenomenon of the fault
. The management operation that requires root privileges cannot be performed.
If there is no other available account, the system will not be able to log in.
3. Solutions to
enter the emergency mode and reset the password

6. Reset the password of the root user

sh-4.2# chroot /mnt/sysimage     #进入急救模式，加载系统镜像，切换到系统根环境
bash-4.1# passwd root      #重设 root 用户密码

Four, system service control

1. System service control

systemctl 控制类型 服务名称

2. Control type

start: start
stop: stop
restart: restart
reload: reload
status: view service status
...

Five, the operating level of the Linux system

1. View the run level

runlevel command
systemctl tool
--systemctl get-default (view system default run level)

2. Temporarily switch the run level

init command
systemctl tool

3. Commands corresponding to the run level

After the function is the command that can be run, and the function is the same

effect	init run level	systemd的target
Shutdown	init 0	systemctl isolate poweroff.target	systemctl poweroff	poweroff	shutdown -h time	halt
Single user mode	init 1	systemctl isolate rescue.target
Character interface	init 3	systemctl isolate multi-user.target
graphic interface	init 5	systemctl isolate graphical.target
Reboot	init 6	systemctl isolate reboot.target	systemctl reboot	reboot	shutdown -r time

Supplement: shutdown -h and -r must be followed by time, for example: now, shutdown or restart immediately; automatic shutdown or restart after 10 or 10 minutes; 12:00, shutdown or restart at 12:00. Command format: shutdown -r now
If the shutdown or restart is set through the shutdown command, you can use the shutdown -c command to cancel the restart.

4. Set permanent operation level

ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target      #设置永久运行级别
systemctl set-default multi-user.target    #设置永久运行级别
两者作用一样，选其一即可
systemctl get-default  #查看系统默认的运行级别