1. The MBR dominant partition is damaged
1. The dominant partition of the virtual machine can be overwritten
dd if=/dev/zero of=/dev/vda bs=446 count=1
The system will not be able to boot normally
2. Add an optical drive according to the virtual machine version (for example, the system optical drive of 7.0 is an image of version 7.0)
3. Select the repair option in the boot mode ##Note: CD-ROM boot
to view the partition where the boot program is located df
Install the program that dominates the partition grub2-install /dev/xda
2. The file boot is damaged
1. The boot file of the boot partition /boot/grub2/grub.cfg The
boot file specifies the location of the boor partition ;load kernel;start system initialization process
set root='hd0,msdos1'
linux16 /boot/vmlinuz-3.10.0-123.el7.x86_64 ro root=/dev/vda1
initrd16 /boot/initramfs-3.10.0-123.el7.x86_64.img
2. If you delete it accidentally, you can automatically create
grub2-mkconfig with the command > /boot/grub2/grub.cfg
[root@localhost grub2]# grub2-mkconfig > /boot/grub2/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.10.0-123.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-123.el7.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue -946cb0e817ea4adb916183df8c4fc817
Found initrd image: /boot/initramfs-0-rescue-946cb0e817ea4adb916183df8c4fc817.img
done
3. If the system does not have this file ( rm -fr /boot/grub2/grub.cfg manually delete the file) , it must be manually booted at startup
(Note: In this experiment, restart the virtual machine to boot directly from the hard disk, not from the CD-ROM)
grub > linux16 /boot/vmlinuz-3.10.0-123.el7.x86_64 ro root=/dev/vda1
grub > initrd16 /boot/initramfs-3.10.0-123.el7.x86_64.img
grub > boot
三、内核引导文件损坏 vmlinuz
1.内核引导文件vmlinuz
检测硬件设备；设备驱动初始化；将/只读挂载；载入初始进程 systemd
2.内核文件损坏，则无法正常开机，须重新安装内核软件
3.开机模式中选择修复选项 ##注意：光驱启动
进入系统后，打开shell chroot /mnt/sysimage
挂载光驱 mount /dev/sr0 /mnt
将光驱中内核软件拷贝至指定目录下
mkdir /kernel
cp /mnt/Packages/kernel-3.10.0-123.el7.x86_64.rpm /kernel
rpm2cpio kernel-3.10.0-123.el7.x86_64.rpm | cpio -id
cp ./boot/vmlinuz-3.10.0-123.el7.x86_64 /boot
查看/boot分区文件是否完整，ok后以虚拟硬盘驱动开机即可
四、img文件损坏
1.若不小心删除，可通过命令自动创建
2.img文件损坏后，无法正常开机，须重新建立img文件
进入系统后，打开shell chroot /mnt/sysimage
在/boot目录下自动建立文件 mkinitrd /boot/initramfs-`uname -r`.img `uname -r`
##uname -r 系统内核版本：3.10.0-123.el7.x86_64
五、systemd初始化
1.systemd初始化目录 /etc/systemd/system/multi-user.target.wants/
开启selinux和systemd-tools；加载内核参数；加载系统时钟，键盘，主机名称
读写挂载/文件系统；激活raid，lvm设备；激活系统配额；
启动/etc/systemd/system/multi-user.target.wants/中所有脚本
启用虚拟控台；启动图形；
2.若系统故障，开机自动关机
重启时按上下键，停止系统引导
开机时按e，在内核引导时设置图形化界面开启
正常开启后，修改systemd初始化默认参数,即可修复
systemctl set-default graphical.target
六、root用户密码修改
###root用户密码被更改或忘记
1.重启时按上下键，停止系统引导
2.按e，在内核引导时找到指定行linux16，从最后往前删除至ro
修改ro=rw rd.break
3.ctrl + x 启动修改设定后的系统引导
chroot /sysroot
passwd
5.建立文件 /.autorelabel
##当selinux关闭时，可不建立该文件；但当selinux开启时，必须建立该文件
建议：重启时默认执行此命令
6.输入2次exit退出、重新启动
