1. Overview of the boot process
2. System initialization process
1. The init process
(1) The /sbin/init program is loaded and run by the Linux kernel;
(2) The init process is the first process in the system;
(3) The PID (process tag) number of the init process is always 1
2. Systemd
(1) Systemd is a kind of init software of the Linux operating system;
(2) CentOS7 adopts a brand-new Systemd startup method to replace the traditional SysVinit;
(3) The first init process running in CentOS7 is /lib/ system/system
Three, Systemd unit type
Fourth, the Systemd target corresponding to the run level
5. Repair MBR sector failure
1. Reasons for failure:
(1) Destruction caused by viruses, Trojan horses, etc.;
(2) Misoperations such as incorrect partition operation, disk read and write, etc.
2. Fault phenomenon:
(1) The boot program cannot be found, and the startup is interrupted;
(2) The operating system cannot be loaded, and the screen goes black after booting
3. Solutions:
(1) Make a backup file in advance;
(2) Boot into the emergency mode with the installation CD;
(3) Restore from the backup file
Operation steps to simulate MBR sector experiment
Add a hard disk to experiment.
Enter the system and check the disk
fdisk -l ##See if there is sdb
mkfs.ext4 /dev/sdb ##Format sdb
mount mount /dev/sdb /opt
1. Backup MBR sector data
dd if=/dev/sda of=/opt/mbr.bak bs=512 count=1
2. Simulate MBR sector failure
dd if=/dev/zero of=/dev/sda bs=512 count=1
Restart to see if there is a problem.
Boot from the CD, enter the emergency mode, and prompt the operation
. Recover the MBR sector from the backup file. The
steps are as follows:
1. Load the CD and select troubleshooting on the CD boot interface.
2. Enter the troubleshooting menu and select Rescue a centos system
3. To enter the boot, you can press Enter, or wait
Press "1", then press Enter to enter sh-4.2#
4. Mount sdb in emergency mode
mkdir /sdd
mount /dev/sdb /sdd
cd /sdd ##Query the backup Whether the file exists
5. dd if=/sdd/mbr.bak of=/dev/sda count=1 bs=512
6. Reboot and enter the system to test whether it is normal
Fix GRUB boot failure
1. Reasons for failure:
(1) The GRUB boot program in MBR is damaged;
(2) The grub.conf file is missing, and the boot configuration is incorrect
2. Trouble phenomenon:
(1) The system boot stalls and the "grub>" prompt is displayed;
3. Solution:
(1) Try to manually enter the boot command;
(2) Enter the emergency mode, rewrite or restore grub.conf from the backup;
(3) Rebuild the grub program into the MBR sector
Grub repair:
cd /boot/grub2 ##Enter grub
rm –rf grub.cfg ##Delete grub.cfg, simulate failure
init 6 ##Restart
There is a failure, as follows:
"grub>"
The repair is as follows:
1. Boot the emergency mode, load the system image
chroot /mnt/sysimage/
2. Re-establish and load the sda partition
grub2 --install /dev/sda
3. Rebuild the grub menu configuration file
grub2 --mkconofig --o /boot/grub2/grub.cfg
4, exit bash environment
exit
5. Restart
reboot
Forgot root password
sh-4.2# chroot /mnt/sysimage ##Load system image
bash-4.2# passwd root
new:
retry:
exit bash environment: exit
restart: reboot