Linux basic boot process and service control

The boot process of the Linux operating system

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 cards, keyboards and other devices are faulty. If there is no fault, the system will be transferred according to the preset startup sequence. This is the normal situation. Machine hard drive.
• Summary: The first device capable of booting the system is detected, such as a hard disk or an optical drive

2. MBR boot

• When starting the system from the local hard disk, first transfer the system control right to the partition containing the operating system boot files 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 (such as 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 the 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

System initialization process

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

Systemd

• Systemd is a kind of 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

• Traditional sysVinit relies on the serial execution of shell scripts to start services, resulting in low efficiency and slow system startup
• systemd can start more service processes in parallel, and has the ability to start services on demand, so that fewer processes are started, thereby increasing the system startup speed.

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 that reside in Cgroup through hierarchical organization
Targe	.target	Describe a set of systemd units

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 equal 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, when this level is used, the host will be shut down and restarted normally

Troubleshoot startup faults

Repair MBR sector failure

cause of issue:

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

Trouble phenomenon:

• The boot program is not found, the startup is interrupted
• Unable to load operating system, black screen after boot

Solutions:

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

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

Load the CD image first, restart the operating system,
when the installation wizard interface appears, select the "Troubleshooting" option,
and then select the "Rescue a CentOS Linux system" option to enter the emergency mode.
Select "1", select Continue and press Enter to continue
pressing again After the Enter key, you will enter the Bash Shell environment with "sh-4.2#" prompt

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

GRUB boot failure

cause of issue

• 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)

Failure phenomenon

• System booting stalls and the "grub>" prompt is displayed

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

Fix GRUB boot failure

/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

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

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

Method 2: Enter the emergency mode and restore the 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 Medium, so the first 446 bytes are backed up here, and the previous partition table is retained.

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

• Simulates 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
  

Method 3: Boot the interface into emergency mode and rebuild the GRUB menu configuration file

• Delete the grub.cfg file to simulate failure

  rm -rf /boot/grub2/grub.cfg
  

• Enter the emergency mode, load the system image, and 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
  

Forgot root user password

cause of issue

• Failure to forget the password of the root user
  
• Cannot perform management operations that require root privileges
• If there is no other available account, you will not be able to log in to the system.
  Solution
• Enter emergency mode, reset password

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

System service control

System service control

systemctl 控制类型 服务名称

Control type

start：启动
stop：停止
restart：重新启动
reload：重新加载
status：查看服务状态

Run level of Linux system

View run level

• runlevel command
• systemctl tool

Temporarily switch run level

• init command
• systemctl tool

Commands corresponding to the run level

init 0	systemctl isolate poweroff.target	systemctl poweroff	shutdown -h now
init 1	systemctl isolate rescue.target				
init 3	systemctl isolate multi-user.target				
init 5	systemctl isolate graphical.target				
init 6	systemctl isolate reboot.target	    systemctl reboot	shutdown -r now

Set permanent run level

ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target      #设置永久运行级别
或
systemctl set-default multi-user.target    #设置永久运行级别

systemctl get-default  #查看系统默认的运行级别

Optimize the startup process

ntsysv tool

• Provide an interactive, visual window
• Can be run in a character terminal
• Convenient for centralized management of multiple services
• Used to control whether the service starts automatically after booting

systemct tool

• No interactive, visual window is provided
• Manage a single service more efficiently

Start and control of system services

ntsysv system service management tool

ntsysv
ntsysv --level 级别列表

• Press the space bar to mark the service option
• Press ↑, ↓ to switch service options
• Press the tab key to switch the cursor

Start and control of system services

• View the startup status of system services

  systemctl is-enabled 服务名称
  

• The service name sets the startup status of the system service

  systemctl enable 服务名称       #开启开机自启动
  systemctl disable 服务名称       #关闭开机自启动