Raspberry Pis are great, but sometimes their ability to keep running in the background can lead to forgotten root passwords. I've had more than one time where I was sure I knew the root password, only to learn that I had forgotten.
Luckily, Raspberry Pi has a "feature" that most Linux machines don't: very easily removable primary storage.
To reset your password:
- Power down and pull the SD card out from your Pi and put it into your computer.
- Open the file 'cmdline.txt' and add 'init=/bin/sh' to the end. This will cause the machine to boot to single user mode.
- Put the SD card back in the Pi and boot.
- When the prompt comes up, type 'su' to log in as root (no password needed).
- Type "passwd pi" and then follow the prompts to enter a new password.
- Shut the machine down, then pull the card again and put the cmdline.txt file back the way it was by removing the 'init=/bin/sh' bit.
The cmdline.txt should look something like this:
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait init=/bin/sh
It's worth noting that with this process being as easy as it is, to consider than a malicious person with physical access to your Raspberry Pi could do this as easily as you can.
ROOT ACCOUNT PROMPTING FOR PASSWORD:
If the root account is prompting for a password (not common) you can, back on your computer, open the /etc/shadow file and replace the root password in there with an asterisk. This will change the password to be blank.
ERROR WHEN CHANGING THE PASSWORD:
Note: Sometimes the password won't be able to be changed because the Pi will boot in a read-only mode. You'll get an error that you can't change the password. To fix this, remount the drive in read-write mode:
mount -o remount,rw /