Resetting forgotten user passwords under Linux

Using a Live system

  1. Boot the Live system and open a prompt.
  2. If not already mounted, mount the file system in question. 
    sudo mkdir /mnt/sda3
sudo mount /dev/sda3 /mnt/sda3
  3. Chroot into the system. 
    sudo chroot /mnt/sda3
  4. Change the password. 
    passwd unluckyuser

Using Recovery mode

  1. Hold Shift when booting to bring up GRUB menu
  2. Select a kernel with recovery mode
  3. In the recovery menu, select root (Drop to root shell prompt)
  4. Confirm with Enter
  5. Mount / as writable 
    mount -o remount,rw /
  6. Change the password 
    passwd unluckyuser