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boot loader 主要功能

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核心模块的加载与移除

boot loader 的两个 stage

grup2配置文件  /boot/grub2/grub.cfg

grub2识别磁盘

grub2 配置文件  /etc/default/grub 和 /etc/grub.d

[root@localhost grub.d]# cat /boot/grub2/grub.cfg
#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by grub2-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#

### BEGIN /etc/grub.d/00_header ###
set pager=

if [ -s $prefix/grubenv ]; then
  load_env
fi
if [ "${next_entry}" ] ; then
   set default="${next_entry}"
   set next_entry=
   save_env next_entry
   set boot_once=true
else
   set default="${saved_entry}"
fi

if [ x"${feature_menuentry_id}" = xy ]; then
  menuentry_id_option="--id"
else
  menuentry_id_option=""
fi

export menuentry_id_option

if [ "${prev_saved_entry}" ]; then
  set saved_entry="${prev_saved_entry}"
  save_env saved_entry
  set prev_saved_entry=
  save_env prev_saved_entry
  set boot_once=true
fi

function savedefault {
  if [ -z "${boot_once}" ]; then
    saved_entry="${chosen}"
    save_env saved_entry
  fi
}

function load_video {
  if [ x$feature_all_video_module = xy ]; then
    insmod all_video
  else
    insmod efi_gop
    insmod efi_uga
    insmod ieee1275_fb
    insmod vbe
    insmod vga
    insmod video_bochs
    insmod video_cirrus
  fi
}

terminal_output console
if [ x$feature_timeout_style = xy ] ; then
  set timeout_style=menu
  set timeout=
# Fallback normal timeout code in case the timeout_style feature is
# unavailable.
else
  set timeout=
fi
### END /etc/grub.d/00_header ###

### BEGIN /etc/grub.d/00_tuned ###
set tuned_params=""
set tuned_initrd=""
### END /etc/grub.d/00_tuned ###

### BEGIN /etc/grub.d/01_users ###
if [ -f ${prefix}/user.cfg ]; then
  source ${prefix}/user.cfg
  if [ -n "${GRUB2_PASSWORD}" ]; then
    set superusers="root"
    export superusers
    password_pbkdf2 root ${GRUB2_PASSWORD}
  fi
fi
  # END /etc/grub.d/01_users ###
▽
### BEGIN /etc/grub.d/10_linux ###
menuentry 'CentOS Linux (3.10.0-957.el7.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-957.el7.x86_64-advanced-ccdda529-f59c-4990-9165-b6d1cd522034' {
        load_video
        set gfxpayload=keep
        insmod gzio
        insmod part_msdos
        insmod xfs
        set root='hd0,msdos1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1'  ba80a371-e434-431e--df4b2827efad
        else
          search --no-floppy --fs-uuid --set=root ba80a371-e434-431e--df4b2827efad
        fi
        linux16 /vmlinuz-3.10.-.el7.x86_64 root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet LANG=zh_CN.UTF-
        initrd16 /initramfs-3.10.-.el7.x86_64.img
}
menuentry 'CentOS Linux (0-rescue-f7819fbfbe0b4074b5dfebfab540afcb) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-f7819fbfbe0b4074b5dfebfab540afcb-advanced-ccdda529-f59c-4990-9165-b6d1cd522034' {
        load_video
        insmod gzio
        insmod part_msdos
        insmod xfs
        set root='hd0,msdos1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1'  ba80a371-e434-431e--df4b2827efad
        else
          search --no-floppy --fs-uuid --set=root ba80a371-e434-431e--df4b2827efad
        fi
        linux16 /vmlinuz--rescue-f7819fbfbe0b4074b5dfebfab540afcb root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet
        initrd16 /initramfs--rescue-f7819fbfbe0b4074b5dfebfab540afcb.img
}

### END /etc/grub.d/10_linux ###

### BEGIN /etc/grub.d/20_linux_xen ###
### END /etc/grub.d/20_linux_xen ###

### BEGIN /etc/grub.d/20_ppc_terminfo ###
### END /etc/grub.d/20_ppc_terminfo ###

### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###

### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###

### BEGIN /etc/grub.d/41_custom ###
if [ -f  ${config_directory}/custom.cfg ]; then
  source ${config_directory}/custom.cfg
elif [ -z "${config_directory}" -a -f  $prefix/custom.cfg ]; then
  source $prefix/custom.cfg;
fi
### END /etc/grub.d/41_custom ###
[root@localhost grub.d]# vim /etc/grub.d/40_custom
#!/bin/sh
exec tail -n + $
# This file provides an easy way to add custom menu entries.  Simply type the
# menu entries you want to add after this comment.  Be careful not to change
# the 'exec tail' line above.
menuentry 'My graphical Centos' --class centos --class gnu-linux --class gnu --class os --unrestricted --id 'mygraphical' $menuentry_id_option 'gnulinux-3.10.0-957.el7.x86_64-advanced-ccdda529-f59c-4990-9165-b6d1cd522034' {
        load_video
        set gfxpayload=keep
        insmod gzio
        insmod part_msdos
        insmod xfs
        set root='hd0,msdos1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,msdos1 --hint-efi=hd0,msdos1 --hint-baremetal=ahci0,msdos1 --hint='hd0,msdos1'  ba80a371-e434-431e--df4b2827efad
        else
          search --no-floppy --fs-uuid --set=root ba80a371-e434-431e--df4b2827efad
        fi
        linux16 /vmlinuz-3.10.-.el7.x86_64 root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet LANG=zh_CN.UTF- systemd.unit=graphical.target
        initrd16 /initramfs-3.10.-.el7.x86_64.img
}
~
"40_custom" 20L, 1185C 已写入
[root@localhost grub.d]# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.10.-.el7.x86_64
Found initrd image: /boot/initramfs-3.10.-.el7.x86_64.img
Found linux image: /boot/vmlinuz--rescue-f7819fbfbe0b4074b5dfebfab540afcb
Found initrd image: /boot/initramfs--rescue-f7819fbfbe0b4074b5dfebfab540afcb.img
done
[root@localhost grub.d]# reboot

选择之后就会直接进入我们的图形界面

设置默认开机画面为字符

[root@localhost /]# sudo systemctl set-default multi-user.target
Removed symlink /etc/systemd/system/default.target.
Created symlink from /etc/systemd/system/default.target to /usr/lib/systemd/system/multi-user.target.

以前我们设置的默认开机方式是图形界面，经过上面设置之后，默认设置就变成了命令行模式。

但是当我们选择第三项，我们自己的设置的时候，他还是会进入图形界面。有点意思。

透过chainloader 的方式移交lader 控制权

上面主要介绍的做双系统的相关知识（我觉得），还是需要实践，看是看不会的。。。。下面有时间会找台机器做测试。来做双系统试一下