sudoers文件更改错误，root权限无法使用的解决方案

/etc/sudoers: syntax error near line

sudo: parse error in /etc/sudoers near line 25

sudo: no valid sudoers sources found, quitting

终极解决方案：

1. shift进入recovery模式

2. 选中root账号

3. chmod 666 /dev/null

mount -o remount rw /

4. nano /etc/sudoers

恢复本文件内容并存盘。

# /etc/sudoers

#

# This file MUST be edited with the 'visudo' command as root.

#

# See the man page for details on how to write a sudoers file.

#

Defaults env_reset

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification

root ALL=(ALL) ALL

# Allow members of group sudo to execute any command after they have

# provided their password

# (Note that later entries override this, so you might need to move

# it further down)

%sudo ALL=(ALL) ALL

#

#includedir /etc/sudoers.d

# Members of the admin group may gain root privileges

%admin ALL=(ALL) ALL

参考：http://psychocats.net/ubuntu/fixsudo
http://my5622576.iteye.com/blog/662158