Fresh Ubuntu 18.04 LTS Installation will only boot into grub mode

Here is what worked for me. (Needs internet connection):

1. Workaround to boot into Ubuntu 18.04 LTS

   (@jas- is right. Tab key autocomplete helps!)

   From the grub> prompt:

   configfile /efi/grub/grub.cfg
   

   alternatively

   configfile (hd0,gpt1)/efi/grub/grub.cfg
   

2. Once logged into the running system, from a terminal (Ctrl+Alt+T), reinstall Grub for UEFI boot:

   sudo su -
   apt install --reinstall grub-efi-amd64 linux-generic linux-headers-generic
   

   At the prompt for which configuration file to use, I selected the first option, use the package maintainer's files.

   Then

   update-initramfs -c -k all
   

3. Done!

Command Sources

• Workaround for Grub booting found in Ubuntu Documentation > Grub2/Troubleshooting > Specific Troubleshooting > grub> > Quick Fix
  https://help.ubuntu.com/community/Grub2/Troubleshooting#grub.3E-1

• Reinstall steps modified from System76 support article. Did not need to chroot to fix, since I could boot into the actual system. GRUB - Repair GRUB Bootloader
  https://support.system76.com/articles/grub/#for-sata-drives

• May be related to:

  • Doing an "Entire disk" installation over an existing LVM installation will fail because the installer selects the wrong boot device (1724417) Use custom partitioning instead and manually select the right boot device in the combo box.
    https://wiki.ubuntu.com/BionicBeaver/ReleaseNotes#Desktop

Grub understands most Linux filesystems; you will want to use commands like the following to identify the filename for your kernel and your initrd and your root device:

• ls (hd0,gpt3)/
• ls (hd0,gpt2)/
• ls (hd0,gpt1)/

Use that approach to find your kernel, and you should be able to boot with a set of commands like this, I believe:

• set root=(hd0,gpt3)
• linux /boot/vmlinuz-4.888.35-14-generic root=/dev/sda1
• initrd /boot/initrd.img-4.888.35-14-generic
• boot