Ytdyklly

We've lost the root password from our file server from supermicro server.

I tried to follow the common instructions to reboot with left shift and drop to root shell as it is described in this answer.

When I choose root in this menu:

Ubuntu still asks for root password or propose to press Ctrl+D to return to Recovery menu.

Are there any other ways to reset root password?

Since you have physical access to the machine you can do this via an Live USB/CD.

1. 
   

   Boot from your USB and chose 'Try Ubuntu' instead of 'Install Ubuntu'.

   
   
   

   

   
   


2. 
   

   Open a terminal (Ctrl+Alt+T) and first look what device handle your machine disk has. You can do that with lsblk which should yield an output like this (I used a live CD for this since I am reproducing the steps in a VM):

   
   
   

   $ lsblk
   
   NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
   
   sda      8:0    0 119,2G  0 disk 
   
   ├─sda1   8:2    0    16G  0 part [SWAP]
   
   └─sda2   8:3    0 103,2G  0 part /
   
   sr0     11:0    1  1024M  0 rom  /cdrom
   
   

   
   
   

   So in this case it would be /dev/sda2 but this could differ for your installation.

   
   


3. 
   

   Now mount the partition in question with:

   
   
   

   sudo mount /dev/sda2 /mnt
   
   

   
   


4. 
   

   Now coming to the part where you can finally¹chroot into it.

   
   
   

   sudo chroot /mnt
   
   

   
   
   

   You will see that your prompt has changed to something like root@ubuntu:/# and now the next steps are pretty straightforward.

   
   


5. 
   

   Change the password for your users with passwd:

   
   
   

   passwd root
   
   passwd <main-user>
   
   

   
   
   

   This should have done it already, but if that for whatever case setting passwords with the passwd command fails, you can go deep down the rabbit hole and change the /etc/shadow file, but Beware: this is quite dangerous and you do this at your own risk.

   
   



6. Exit the chrootby pressing Ctrl+D or type exit. Unmount the machine with sudo umount /mnt and then reboot by sudo reboot. You want to take the USB/CD out and make sure you're actually booting the machine in question.

¹ That chrooting method is sufficient to reset passwords, or even to add and remove users from groups, but it does not allow you to fully use the installed system through the chroot. Many other commands, such as apt, would fail if you ran them in a chroot set up that way.

If you ever need to perform more extensive repairs on an installed system that you are accessing from a live CD/DVD/USB--for example by installing, removing, or updating software--then you would want to set up some additional mounts before chrooting. You would do that by running these commands after running sudo mount /dev/sda2 /mnt but before running sudo chroot /mnt:

sudo mount -o bind /dev /mnt/dev

sudo mount -o bind /dev/pts /mnt/dev/pts

sudo mount -t sysfs /sys /mnt/sys

sudo mount -t proc /proc /mnt/proc

If you have multiple partitions for the different parts of the OS, like for example a separate /boot partition, then you would want to mount them to the right positions. For example, where sdX is the device name for that particular drive and n is the partition number:

sudo mount /dev/sdXn /mnt/boot

It is fine if you run those commands before chrooting in to reset passwords with the passwd command. It is not necessary, though.