User UID test in Linux leads to system error

Today, under the Ubuntu14.04 a small test, that is, to modify the system file/etc/passwd user UID value, but caused the system bug, not normal use. Because modifying /etc/passwd requires root privileges, when you are ready to use the sudo command again to restore the file, error:

Sudo:unknown uid 1000:who is you?

After rebooting, it goes directly to the least privileged Guest account, while using the sudo command is an error:

sudo:unable to the root gid:operation not permitted still cannot be resolved.

Attempts to use CTRL+ALT+F1 to enter Tty1 mode are still unavailable. A whole day in Baidu to solve this problem. Through Baidu, found that there are other people have encountered the same problem, but most of the reinstallation of the system to solve. I think it's going to be a sad reminder. Brainwave, suddenly think can use PE into the system and then modify the file? And then did it, the result was successful, so happy!

User UID test in Linux leads to system error