I forgot my sudo password and I need to change it with the terminal

Question

I have got a computer running Windows 8.1 and Ubuntu and I can't change it via grub.

Answer 1

Well if you really cannot go to the recovery mode, try doing the following:

a) boot using a live CD, mount the root FS in rw-mode

b) create a new password hash:

mkpasswd -m sha-512 -s

Enter the new password, you will get the hash string for it

c) manually put the hash string into the /etc/shadow file for whichever user you need it for, replacing the original string (to be inserted after the first colon :)

d) reboot and use the new password

e) rechange the password using passwd

Note that this is a pretty raw approach and you should make sure not to corrupt the shadow file. (Maybe a backup of the file would do good)