How to restore the /etc/passwd file?

I was doing some experiment about resource-accessing. By mistake, I executed this command, $ sudo mv /etc/passwd /etc/passwd.bak Then I could not execute any command with privilege(eg. sudo mv /etc/passwd.bak /etc/passwd). When I shut the system down, I could not boot it any more. Afterwards, I restored the passwd file with a LiveCD. Any other approaches could make it?

Maybe this: When the bootloader shows up, add

init=/bin/bash

to the kernel parameters. Instead of booting normally (with the init process spawning all the other services including login), you should end up in single-user mode with a root shell (no password is asked) and be able to restore the passwd file. On some distributions you may need to manually remount the root partition in read/write mode before moving the file:

mount -o remount,rw /dev/sda123

(I don't want to test your issue and see if this solution works :)