一.Linux 啟動流程

centos6啟動流程

1.載入BIOS的硬體資訊，獲取第一個啟動裝置

2.讀取第一個驅動裝置MBR的引導載入程式(grub)的啟動資訊

3.載入核心作業系統的核心資訊，核心開始解壓縮，並嘗試驅動所有硬體裝置

4.核心執行init程式，並獲取預設的執行資訊

5.init程式執行/etc/rc.d/rc.sysinit檔案，重新掛載根檔案系統

6.啟動核心的外掛模組

7.init執行執行的各個批處理檔案(scripts)

8.init執行/etc/rc.d/rc.local

9.執行/bin/login程式，等待使用者登入

10.登入之後開始以shell控制主機

centos7、8啟動流程

1.UEFI或BIOS初始化，執行POST開機自檢

2.選擇啟動裝置

3.引導裝載程式，centos7是grub2，載入裝載程式的配置檔案：

​ /etc/grub.d/

​ /etc/default/grub

​ /boot/grub2/grub.cfg

4.載入initramfs驅動模組

5.載入核心選項

6.核心初始化，centos7使用systemd代替init

7.執行initrd.target所有單元，包括掛載/etc/fstab

8.從initramfs根檔案系統切換到磁碟根目錄

9.systemd執行預設target配置，配置檔案/etc/systemd/system/default.target

10.systemd執行sysinit.target初始化系統及basic.target準備作業系統

11.systemd啟動multi-user.target下的本機與伺服器服務

12.systemd執行multi-user-target下的/etc/rc.d/rc.local

13.systemd執行multi-user.target下的getty.target及登入服務

14.systemd執行graphical需要的服務

二.修復grub第1階段故障

#破壞grub第1階段
[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位元組被清空

[root@centos6 ~]# reboot
 

一階段被破壞，系統認為硬碟不能啟動，自動從光碟啟動

#chroot /mnt/sysimage 切到根目錄
#grun-install /dev/sda 修復前446位元組
#sync  存檔
#exit  重啟

系統可以正常啟動

三.系統驅動檔案initramfs-2.6.32-754.el6.x86_64.img檔案刪除修復

[root@centos6 ~]# mv /boot/initramfs-2.6.32-754.el6.x86_64.img /data
[root@centos6 ~]# reboot

進入光碟救援模式

語言

鍵盤佈局

網路不配置

Continue

/ 根分割槽被掛載到/mnt/sysimage

chroot /mnt/sysimage 切換到根目錄

mkinitrd /boot/initramfs-uname -r uname -r 生成驅動檔案

sync 存檔

exit 重啟

系統可以正常啟動

四.centos6破解root密碼

按ESC，進入選單選擇，按a

在最後輸入1，表示臨時進入1模式，單使用者模式

不用密碼直接進來了，passwd 直接修改密碼

進入3模式

輸入新密碼 直接登入

五.centos6設定grub密碼

[root@centos6 ~]# grub-crypt 
Password: 
Retype password: 
$6$I2yXWff69mimF3TQ$WyJ63VaFZKWr.WPIVnDmcR84oGcsl8dXUjKrk3vOiu5MR.05uQNMDejjRjSFazVmgPLucmLZo7GwDmEPNPjQN0
#設定grub生成密碼

[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  	#在這裡設定grub密碼
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

輸入密碼

只有輸入密碼才可以進到這裡

忘記grub密碼，直接去vim /boot/grub/grub.conf檔案，把password --encrypt $6$I2yXWff69mimF3TQ$WyJ63VaFZKWr.WPIVnDmcR84oGcsl8dXUjKrk3vOiu5MR.05uQNMDejjRjSFazVmgPLucmLZo7GwDmEPNPjQN0 這行刪除就沒有grub密碼了

六.centos7、8設定錯啟動模式如何恢復

[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

現在系統會一直自動重啟

按e

在linux這行最後，加systemd.unit= multi-user.target，臨時切換模式，按ctrl+x

[root@centos8 ~]# systemctl get-default
reboot.target
[root@centos8 ~]# systemctl set-default multi-user.target

把預設啟動模式改回multi-user.target

七.破解centos7、8root密碼

按e

在linux這行最後，輸入rd.break，ctrl+x

mount -a remount,rw /sysroot 把根掛載成rw

chroot /sysroot 切根

passwd root 直接更改密碼

八.centos7、8設定grub密碼

[root@centos8 ~]# grub2-setpassword
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

必須要輸入使用者名稱和密碼

才可以進入grub

[root@centos8 ~]# rm -f /boot/grub2/user.cfg
#忘記密碼，刪了這個檔案就可以

九./boot分割槽被破壞修復

[root@centos8 ~]# rm -rf /boot/*
[root@centos8 ~]# reboot

系統不能啟動

光碟引導

選1

chroot /mnt/sysimage

grub2-install /dev/sda 安裝grub

mount /dev/sr0 /mnt 掛載光碟

rpm -ivh /mnt/BaseOS/Packages/kernel-core-4.18.0.-240.e18.x86_64.rpm –force，安裝核心

建立grub檔案

sync 存檔

exit 重啟

系統可以正常啟動