Well I don't know about unencrypted filesystems but encrypted systems certainly do not give a prompt.
I have a small Howto on how to change a password on an existing system which --- can also be used to edit grub to reflect the wanted changes on a non-booting system but that would be a longer story.
resetting password using "chroot" on encrypted (lvm) filesystem.
Boot up live session ... in console mode will do for our purposes, which is a lot faster.
Become root with
"sudo su", elive will not prompt for a password and decrypt the filesystem (in my case /dev/sda3 as it's a uefi boot)
"cryptsetup luksOpen /dev/sda3 lost-machine"
and enter the decryption password when prompted.
Mount the unencrypted filesystem on "lost-macine" (you'll want the / filesystem where passwords are stored):
"mount /dev/mapper/elive_vg-toot_lv /mnt"
TIP: make use of hitting Tab (autocomplete) after /dev/mapper to show the system names available.
Check if it is indeed the wanted root filesystem with
"mount" to see if it is indeed mounted "rw" which is needed.
chroot into /mnt :
"chroot /mnt "
If you don't know the user name on the machine do:
"less /etc/group" where you'll probably be wanting the adm: user ... or to show other users do
"less /etc/group |grep 100"
To change the root password simply type
"passwd" and enter a password for root and wait for the
"successfully changed" message.
If you need to change a forgotten user password, like in my case for "triantares" you give the same command with the wanted username added:
"passwd triantares" and after entering the new passwords and wait for the success message
And reboot the machine with Ctrl,Altand Del combo.
You'll find the passwords changed as wanted.
Added this as a separate Howto.