Reset Root Password

RedHat/CentOS

For CentOS/RHEL 7)

For GRUB Bootloader)
系統重開機 -> 進入開機選單

1. Select the kernel
2. Press the e key to edit the entry
3. Select second line (the line starting with the word kernel)
4. Press the e key to edit kernel entry so that you can append single user mode
5. Append the letter S (or word Single) to the end of the (kernel) line
6. Press ENTER key
7. Now press the b key to boot the Linux kernel into single user mode
8. At prompt type passwd command to reset password:

# mount -t proc proc /proc
# mount -o remount,rw /
# passwd
# sync
# reboot

For LILO Bootloader)

Boot: linux single 

# passwd
# sync
# reboot 

Ubuntu/Debian