Linux operating system boot process and system service control

Hardware settings check, transfer system control
After the server host is turned on, a preliminary test of the CPU, memory, graphics card, keyboard and other devices will be carried out based on the settings in the motherboard BIOS. The test is successful. and then transfers system control according to the preset startup sequence. Most of the time, it will be transferred to the local hard disk, because the operating system is usually installed on the hard disk

Summary: Detect the first device that can boot the system, such as a hard disk or optical drive

2.2 MBR boot

Run the startup GRUB boot program placed in the MBR sector.
When the system is booted from the local hard disk, system control is first transferred to the partition containing the operating system boot file according to the setting of the MBR (Master Boot Record) in the first sector of the hard disk. ; Or directly call the boot menu based on the boot information in the MBR record. For example: GRUB, etc.

Summary: Run the startup GRUB boot program placed in the MBR sector

2.3 GRUB menu

The GRUB boot program obtains the settings and path locations of the kernel and image file systems by reading the GRUB configuration file /boot/grub2/grub.cfg.
For Linux operating systems, GRUB (Unified Boot Loader) is the most widely used multi-system boot program. After system control is passed to FRUB, the boot menu will be displayed for the user to choose. , and loads the Linux kernel file according to the selected options (or adopts the default value), and then transfers system control to the kernel.

GRUB startup phase

primary boot loader:
1st stage.…The first 446 bytes of MBR boot the hardware to find the kernel

5 stage:.. The sectors after MBR allow the bootloader in stage1 to identify the file system on the partition where stage2 is located.

secondary boot loader: 2nd stage, partition file/boot/grub2/grub.cfg
(CentOS 7 uses the GRUB2 boot loader)

Summary: The GRUB boot program obtains the settings and path locations of the kernel and image file systems by reading the GRUB configuration file /boot/grub2/grub.cfg.

2.4 Loading the Linux kernel

Load the kernel and image file system into memory.
The Linux kernel is a pre-compiled special binary file, which is between the respective hardware resources and system programs. It replicates resource allocation and scheduling. After the kernel takes over the control of the system, it will fully control the entire system. The running process of the Linux operating system.

(In CentOS systems, the default kernel file is located at "/boot/vmlinuz-3.10.0-514.e17.x86_64")

Summary: Load the kernel and image files into memory

2.5 init process initialization

loads the hardware driver, and the kernel loads the init process into the memory to run.
In order to complete the further system boot process, the Linux kernel first loads the "/sbin/init" program in the system into the memory and runs it (the running program is called a process), and the init process is responsible for completing Initialization of the entire system, and finally waiting for the user to log in.

Summary: Load the hardware driver, and the kernel loads the init process into memory and runs it.

3. System initialization process

3.1 init process (serial startup)

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

3.2 Systemd can replace service monitoring (start in parallel)

Systemd is an init software for Linux operating system; (the first process of centos7 is systemd, the first process of centos6 and before is init)
Used in CentOS 7 The authorized Systemd (parallel startup of service processes) startup method replaces the traditional SysVinit (serial execution of shell scripts to start services);
The first init process running in CentOS 7 is /lib/systemd /systemd
Traditional sysVinit relies on serial execution of Shell scripts to start services, resulting in low efficiency and slow system startup speed
systemd can parallelize more service processes startup, and has the ability to provide on-demand startup services, allowing fewer processes to be started, thereby improving system startup speed

4.Systemd unit type

5. Systemd target corresponding to the run level

2. Repair MBR sector failure

The MBR is located in the first physical sector of the first hard disk (/dev/sda), with a total of 512 bytes. The first 446 bytes are the GRUB program, and the next 64 bytes are the partition table.

1. Cause of failure

Damage caused by viruses, Trojans, etc.
Incorrect partition operations, disk read and write errors

2. Fault phenomenon

The bootloader cannot be found and the startup is interrupted
The operating system cannot be loaded and the screen goes black after booting

3. Solution ideas

Prepare to add another hard disk as a backup;
Make a backup file in advance;
Boot into the rescue mode with the installation CD;
Restore from backup file.

4. Specific steps to repair MBR sectors

4.1 Add hard disk and partition

4.2 Back up MBR sector data to other disks (/dev/sdb1) (cannot back up on the same hard disk)

dd if=/dev/sda of=/mnt/mdr.bak bs=512 count=1

(Copy and overwrite, copy sda under dev to mnt and name it mbr.bak, 512 bytes at a time, copy once)

4.3 Simulate the destruction of MBR boot sector

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

(Write 512 bytes of space to sda once)

hexdump -C -n 512 /dev/sda

Check the first 512 bytes of the SDA hard disk and find that it has been damaged. At this time, you need to restart and enter the emergency mode. At this time, the system of the optical drive is read.

4.4 Boot the interface into emergency mode, restore the MBR sector data from the backup file and load the disc image first.

1. When the installation wizard interface appears, select the "Troubleshooting" option.

2. Select the "Rescue a CentOS Linux system" option to enter rescue mode

3. Select "1", select Continue and press Enter to continue.

4. Press the Enter key again and you will enter the Bash Shell environment with the "sh-4.2#" prompt.

4.5 Enter the shell interface, mount the partition with backup, restore the static backup data to the damaged hard disk, and restart

sh-4.2# mkdir bak
sh-4.2# mount /dev/sdb1 /bak
#Mount the partition with backup files < /span> sh-4.2# reboot Check the first 512 bytes of the sda hard disk and find that it has been repaired. sh -4.2# hexdump -C -n 512 /dev/sda #Restore backup data
sh-4.2# dd if=/bak/mdr.bak of=/dev/ sda