一.Linux 启动流程
centos6启动流程
1.加载BIOS的硬件信息，获取第一个启动设备
2.读取第一个驱动设备MBR的引导加载程序(grub)的启动信息
3.加载核心操作系统的核心信息，核心开始解压缩，并尝试驱动所有硬件设备
4.核心执行init程序，并获取默认的运行信息
5.init程序执行/etc/rc.d/rc.sysinit文件，重新挂载根文件系统
6.启动核心的外挂模块
7.init执行运行的各个批处理文件(scripts)
8.init执行/etc/rc.d/rc.local
9.执行/bin/login程序，等待用户登录
10.登录之后开始以shell控制主机
centos7、8启动流程
1.UEFI或BIOS初始化，运行POST开机自检
2.选择启动设备
3.引导装载程序，centos7是grub2，加载装载程序的配置文件：
/etc/grub.d/
/etc/default/grub
/boot/grub2/grub.cfg
4.加载initramfs驱动模块
5.加载内核选项
6.内核初始化，centos7使用systemd代替init
7.执行initrd.target所有单元，包括挂载/etc/fstab
8.从initramfs根文件系统切换到磁盘根目录
9.systemd执行默认target配置，配置文件/etc/systemd/system/default.target
10.systemd执行sysinit.target初始化系统及basic.target准备操作系统
11.systemd启动multi-user.target下的本机与服务器服务
12.systemd执行multi-user-target下的/etc/rc.d/rc.local
13.systemd执行multi-user.target下的getty.target及登录服务
14.systemd执行graphical需要的服务
二.修复grub第1阶段故障
#破坏grub第1阶段
[root@centos6 ~]# dd if=/dev/zero of=/dev/sda bs=1 count=446
446+0 records in
446+0 records out
446 bytes (446 B) copied, 0.00150234 s, 297 kB/s
[root@centos6 ~]# hexdump -C -n 512 -v /dev/sda
00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000000a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000000b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000000c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000000d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000000e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000000f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000100 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000110 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000120 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000130 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000140 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000150 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000160 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000170 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000180 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
000001b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 80 20 |............... |
000001c0 21 00 83 aa 28 82 00 08 00 00 00 00 20 00 00 aa |!...(....... ...|
000001d0 29 82 83 fe ff ff 00 08 20 00 00 80 3c 0c 00 fe |)....... ...<...|
000001e0 ff ff 83 fe ff ff 00 88 5c 0c 00 00 40 06 00 fe |........\...@...|
000001f0 ff ff 05 fe ff ff 00 88 9c 12 00 78 63 06 55 aa |...........xc.U.|
00000200
#前446字节被清空
[root@centos6 ~]# reboot
一阶段被破坏，系统认为硬盘不能启动，自动从光盘启动
#chroot /mnt/sysimage 切到根目录
#grub-install /dev/sda 修复前446字节
#sync 存盘
#exit 重启
系统可以正常启动
三.系统驱动文件initramfs-2.6.32-754.el6.x86_64.img文件删除修复
[root@centos6 ~]# mv /boot/initramfs-2.6.32-754.el6.x86_64.img /data
[root@centos6 ~]# reboot
进入光盘救援模式
语言
键盘布局
网络不配置
Continue
/ 根分区被挂载到/mnt/sysimage
chroot /mnt/sysimage 切换到根目录
mkinitrd /boot/initramfs-`uname -r`.img `uname -r` 生成驱动文件
sync 存盘
exit 重启
系统可以正常启动
四.centos6破解root密码
按ESC，进入菜单选择，按a
在最后输入1，表示临时进入1模式，单用户模式
不用密码直接进来了，passwd 直接修改密码
进入3模式
输入新密码 直接登录
五.centos6设置grub密码
[root@centos6 ~]# grub-crypt
Password:
Retype password:
$6$I2yXWff69mimF3TQ$WyJ63VaFZKWr.WPIVnDmcR84oGcsl8dXUjKrk3vOiu5MR.05uQNMDejjRjSFazVmgPLucmLZo7GwDmEPNPjQN0
#生成grub密码
[root@centos6 ~]# vim /boot/grub/grub.conf
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/sda2
# initrd /initrd-[generic-]version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
password --encrypt $6$I2yXWff69mimF3TQ$WyJ63VaFZKWr.WPIVnDmcR84oGcsl8dXUjKrk3vOiu5MR.05uQNMDejjRjSFazVmgPLucmLZo7GwDmEPNPjQN0 #在这里设置grub密码
title CentOS 6 (2.6.32-754.el6.x86_64)
root (hd0,0)
kernel /vmlinuz-2.6.32-754.el6.x86_64 ro root=UUID=55ed4e69-a262-485e-a2c8-9df9ac044aec rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF
-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 crashkernel=auto KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM rhgb quiet
initrd /initramfs-2.6.32-754.el6.x86_64.img
[root@centos6 ~]# reboot
按p
输入密码
只有输入密码才可以进到这里
忘记grub密码，直接去vim /boot/grub/grub.conf文件，把password --encrypt $6$I2yXWff69mimF3TQ$WyJ63VaFZKWr.WPIVnDmcR84oGcsl8dXUjKrk3vOiu5MR.05uQNMDejjRjSFazVmgPLucmLZo7GwDmEPNPjQN0 这行删除就没有grub密码了
六.centos7、8设置错启动模式如何恢复
[root@centos8 ~]# systemctl get-default
multi-user.target
[root@centos8 ~]# systemctl set-default reboot.target
Removed /etc/systemd/system/default.target.
Created symlink /etc/systemd/system/default.target → /usr/lib/systemd/system/reboot.target.
[root@centos8 ~]# systemctl get-default
reboot.target
[root@centos8 ~]# reboot
现在系统会一直自动重启
按e
在linux这行最后，加systemd.unit= multi-user.target，临时切换模式，按ctrl+x
[root@centos8 ~]# systemctl get-default
reboot.target
[root@centos8 ~]# systemctl set-default multi-user.target
把默认启动模式改回multi-user.target
七.破解centos7、8root密码
按e
在linux这行最后，输入rd.break，ctrl+x
mount -a remount,rw /sysroot 把根挂载成rw
chroot /sysroot 切根
passwd root 直接更改密码
touch ./autorelabel 如果开启了SELinux请执行这步，没有就不用执行
exit 重启
八.centos7、8设置grub密码
[root@centos8 ~]# grub2-setpassword #设置grub密码
Enter password:
Confirm password:
[root@centos8 ~]# ll /boot/grub2
total 32
-rw-r--r--. 1 root root 64 Dec 21 17:02 device.map
drwxr-xr-x. 2 root root 25 Dec 21 17:02 fonts
-rw-r--r--. 1 root root 6605 Dec 23 14:39 grub.cfg
-rw-r--r--. 1 root root 1024 Dec 23 14:39 grubenv
drwxr-xr-x. 2 root root 8192 Dec 21 17:02 i386-pc
-rw------- 1 root root 298 Dec 23 14:54 user.cfg
[root@centos8 ~]#
[root@centos8 ~]# cat /boot/grub2/user.cfg
GRUB2_PASSWORD=grub.pbkdf2.sha512.10000.00F10E32D3BBA86820ED6C1944C80CBEE10D48CAACD0DD70872DFBE22080507B70AABC39C4F3B0D2323E518FCE0F37F8C05884F5FE66F7663129353617772001.70F58B75EF579D9A358C003EA5466F790E6FD061BD28520EC5A44D99F36500E16BC50D8050A559A43A45DD4D060A56CD12B034BF47679AC8C2D730009DB81DF3
[root@centos8 ~]# reboot
按e
必须要输入用户名和密码
才可以进入grub
[root@centos8 ~]# rm -f /boot/grub2/user.cfg
忘记密码，删了这个文件就可以
九./boot分区被破坏修复
[root@centos8 ~]# rm -rf /boot/*
[root@centos8 ~]# reboot
系统不能启动
光盘引导
选1
chroot /mnt/sysimage
grub2-install /dev/sda 安装grub
mount /dev/sr0 /mnt 挂载光盘
rpm -ivh /mnt/BaseOS/Packages/kernel-core-4.18.0.-240.e18.x86_64.rpm –force，安装内核
grub2-mkconfig -o /boot/grub2/grub.cfg 创建grub文件
sync 存盘
exit 重启
系统可以正常启动