切换ubuntu root账户

看了一篇文“ubuntu sudo不用输入密码的方法”，修改了/etc/sudoers文件，再试使用sudo时悲剧了，提示：

sudo: no valid sudoers sources found, quitting

 查了资料说要恢复到Recovery mode用root用户还原之前做的修改，但是重启按了shift或f12都没反应，最后用

$ su root

 输入root密码搞定，root密码同建立账户时第一次输入的密码。附/etc/sudoers源文件内容

#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#
Defaults	env_reset
Defaults	mail_badpass
Defaults	secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root	ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL
# Allow members of group sudo to execute any command
%sudo	ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:

#includedir /etc/sudoers.d