This chapter is structured in the
Linux operating system boot process to
eliminate startup faults,
service control and switch run levels to
optimize the startup process
Overview of the boot process of the Linux operating system
The boot process of the Linux operating system
1. Power-on self-check. After the
server is turned on, the CPU, memory, graphics card, keyboard and other devices will be preliminarily tested according to the settings in the motherboard BIOS. After the test is successful, the system control will be transferred according to the preset startup sequence. , Most of the time, it will be handed over to the local hard disk.
Summary: The first device that can boot 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 file according to the MBR (Master Boot Record) setting in the first sector of the hard disk; or directly according to the MBR record The boot information in the call boot menu (such as GRUB)
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.
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, which is between various hardware resources and system programs, and is responsible for resource allocation and scheduling. After the kernel took control of the system, will have complete control over the entire process of running the Linux operating system
Centos 7 system, the default kernel file is located in "/boot/vmlinuz-3.10.0-514.e17.x86_64"
Summary: kernel and image File system loaded into memory
5. Initialization of the init process
In order to complete the further system boot process, the Linux kernel first loads the "/sbin/init" program in the system into the memory to run (the running program is called a process), and the init process is responsible for completing the initialization of the entire system , And finally wait 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 The
init process
is loaded and run by the Linux kernel. /sbin/init program The
init process is the first process in the system and is the parent process of all processes. The
init process PID (process tag) number is always 1
Systemd
Systemd is an init software of the Linux operating system.
CentOS7 adopts a new Systemd startup method, replacing the traditional SysVinit
. The first init process running in CentOS7 is /lib/systemd/systemd
Traditional SysVinit relies on 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 can be started, thereby improving System startup speed
Systemd unit type
Systemd target corresponding to the run level
Repair MBR sector failure
cause of the malfunction
destroy viruses, Trojans, etc. caused by
incorrect operation of partition, disk read and write misuse
Trouble phenomenon: The
boot program
cannot be found, the operating system cannot be loaded after the startup is interrupted , and the screen is black after booting
Solution: Make a
backup file
in advance, boot
from the installation CD, enter the emergency mode , and restore from the backup file
MBR is located at the first physical sector of the first hard disk (/dev/sda), a total of 512 bytes
. 1. Backup 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, restore MBR sector data from the backup file,
first load the CD image, restart the operating system,
when the installation wizard interface appears, select the "Troubleshooting" option
and then select the "Rescue a CentOS Linux system" option, Enter the emergency mode,
select "1", select Continue and press Enter to continue,
press Enter again, and you will enter the Bash shell environment with "sh-4.2#" prompt
sh-4.2# mkdir /backupdir
sh-4.2# mount /dev/sdb1 /backupdir
#Mount the partition with backup files sh-4.2# dd if=/backupdir/mbr.bak of=/dev/sda #Restore the backup data
sh-4.2# exit #Execute the exit command to exit the temporary shell environment, the system will Will automatically restart
Repair GRUB boot failure
failure cause
The GRUB boot program in the MBR is damaged. The
grup.com file is missing, and the boot configuration is incorrect.
Symptom The
system boot stalls and the prompt "grup>" is displayed
Solution
Try to manually enter the boot command to
enter the emergency mode, rewrite or restore grup.conf from the backup
to rebuild the grup program in the MBR sector
The package in the /boot/grub/ directory is used for the background image and style of the boot menu
/boot/grub2/grub.cfg #GRUB configuration file
Method 1: Manually enter the boot command (clumsy and cumbersome, not recommended)
grub＞ insmod xfs
#Load the specified module into the kernel grub＞ linux16.......UTF-8
#Information such as the name and location of the kernel grub＞ initrd16 /initramfs-3.10.0-693.e17.x86_64.img #Mirror system file
grub＞ boot #boot 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 In the 447-510 bytes in the MBR sector,
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 MBR, but does not destroy the partition table
dd if=/dev/zero of=/dev/sda bs=446 count=1
#The boot interface enters the emergency mode and restores the GRUB boot program 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, rebuild the GRUB menu configuration file
rm -rf /boot/grub2/grub.cfg #Enter
emergency mode, load the CD image, switch to the system root environment
sh-4.2# chroot /mnt/sysimage
#Reinstall the GRUB boot program to the MBR sector of the first hard disk (/dev/sda)
bash-4.2# grub2-install /dev/sda
#Rebuild the configuration file of the GRUB
menubash-4.2# grub2-mkconfig -o /boot/grub2/grub.cfg
#Exit the chroot environment and restart
bash-4.2# exit
sh-4.2# reboot
Forgotten root user password
Failure reasons
Forgot root user password
Symptom
: Management operations that require root privileges cannot be performed.
If there is no other available account, the system cannot be logged in.
Solution ideas,
enter the emergency mode, reset the password
#Enter the emergency mode, load the system image, switch to the system root environment
sh-4.2# chroot /mnt/sysimage #Reset the
root user password
bash-4.1# passwd root
System service control
systemctl control type service name
Control type
start: start
stop: stop
restart: restart
reload: reload
status: view service status
······
The run level of the Linux system
view run level
runlevel command
systemctl command
Temporarily switch the run level
init command
systemctl command
Set the permanent run level
ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target
or
systemctl set-default multi-user.target
systemctl get-default #View the
current default level
Optimize the startup process. The
ntsysv tool
provides an interactive and visual window that
can be run in a character terminal to
facilitate centralized management of multiple services
to control whether the service starts automatically.
The systemctl tool
does not provide interactive, visual window
management for a single service, which is more efficient
Start and control
system services ntsysv system service management tool
ntsysv
ntsysv --level level list
Press the space to mark the service option,
press the ↑, ↓ key to switch the service option,
press the tab key to switch the cursor
View the startup status of system services
systemctl is-enabled service name
Set the self-starting state of the system service
systemctl enable service name
#enable automatic startup systemctl disable service name #turn off automatic startup
Permanently modify the host name
hostnamectl set-hostname newname
View the status of the host name
hostnamectl status
Set the system language to Chinese
localectl set-locale LANG=zh_CN.utf8
View the current system language
localectl [status]
View the system startup time
systemd-analyze