Ubuntu 开机引导文件 /etc/default/grub

 # If you change this file, run 'update-grub' afterwards to update
 # /boot/grub/grub.cfg.  

 GRUB_DEFAULT=saved
 GRUB_SAVEDEFAULT=true
 #GRUB_HIDDEN_TIMEOUT=
 GRUB_HIDDEN_TIMEOUT_QUIET=true
 GRUB_TIMEOUT=
 GRUB_DISTRIBUTOR=`lsb_release -i -s > /dev/null || echo Debian`
 GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
 GRUB_CMDLINE_LINUX=""  

 # Uncomment to enable BadRAM filtering, modify to suit your needs
 # This works with Linux (no patch required) and with any kernel that obtains
 # the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
 #GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"  

 # Uncomment to disable graphical terminal (grub-pc only)
 #GRUB_TERMINAL=console  

 # The resolution used on graphical terminal
 # note that you can use only modes which your graphic card supports via VBE
 # you can see them in real GRUB with the command `vbeinfo'
 #GRUB_GFXMODE=640x480  

 # Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
 #GRUB_DISABLE_LINUX_UUID=true  

 # Uncomment to disable generation of recovery mode menu entries
 #GRUB_DISABLE_LINUX_RECOVERY="true"  

 # Uncomment to get a beep at grub start
 #GRUB_INIT_TUNE="480 440 1"  

对于grub2，ubuntu给了一个官方的配置文件/etc/default/grub。大部分情况下grub2的设置都可以在这个文件中搞定，而且这个文件结构也比较简单，修改起来也容易。完全没有必要直接改/boot/grub/grub.cfg或者/etc/grub.d/下的文件。

修改/etc/default/grub只需简单一个命令：sudo gedit /etc/default/grub
下面是系统默认的内容，以及最常用的修改菜单显示时间和默认操作系统的方法：
# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.

GRUB_DEFAULT=0 #将0改为saved，可让grub记住上次启动时选择的系统
GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT="5" #显示启动选择菜单的时间
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entrys
#GRUB_DISABLE_LINUX_RECOVERY="true"

修改完成后使用命令：
代码:sudo update-grub
会自动更新/boot/grub/grub.cfg，这样做最大的好处是当系统更新内核时，你的设置不会被覆盖掉。

另外对于从9.10 beta及以前一路升级上来的朋友，如果确定想使用grub2，手动运行：

代码:sudo apt-get install grub2
升级并确保grub2正确配置。（我全新安装的9.10 beta，一路升级上来后，系统中的grub工具居然还是老版，运行这个命令后才正常）

其实Ubuntu官方wiki上对这些都有说明：
https://wiki.ubuntu.com/Grub2

PS：   官方文件只说/boot/grub/grub.cfg不要手工修改，这个文件是运行update-grub自动生成的，/etc/default/grub和/etc/grub.d/ (folder)都很重要。
要修改配置文件的只要打开/boot/grub/grub.cfg文件，找到想修改的地方，然后根据注释找到相应的/etc/default/grub或/etc/grub.d/ (folder)进行修改，很方便滴。

File Structure
/boot/grub/grub.cfg This is in keeping with the intent that the file should not be edited manually. 
/etc/default/grub
/etc/grub.d/ (folder)

转自：http://blog.csdn.net/yuanchaonll/article/details/8275601