Home machine, single user account, and I can't remember the login password. No other admin user exists. Is there a recovery mode way to reset it without reinstalling?
Recovery mode does it, no reinstall needed.
1. Reboot and bring up the GRUB menu. Hold Shift while it starts (older BIOS machines) or tap Esc (UEFI).
2. Advanced options for Ubuntu > the entry ending in (recovery mode).
3. Pick root (Drop to root shell prompt).
4. The disk is mounted read-only at this point, so run: mount -o remount,rw /
5. Then: passwd yourusername (your login name, all lowercase) and type the new password twice.
6. Type reboot.
Two catches. If you set up full disk encryption at install, you still need that disk passphrase to get anywhere, recovery mode won't skip it. And after logging in you'll probably get a prompt about the login keyring, because it's still locked with the old password. Easiest is to let it create a new keyring, you just lose saved wifi/browser passwords stored in it.
