How to boot system normally from grub prompt?

Question

How can I boot Linux from the grub> prompt, as if I were booting from cold?

I have full disk encryption. When I fat-finger the encryption password, it bumps me to grub rescue (bonus points if you can tell me how to have it give 2 attempts to enter the password ... :) ) From here, I can unlock the disk (cryptomount hdX,gpt1).

After doing this (demonstrating keyboard is working, I'm typing the correct key, not asleep, etc) - I just want to boot the system, with the Linux argument line as normal. The system has no issues, obviously, so this will work (I had just fat-fingered the passphrase).

As it may be necessary in order to disambiguate what my system does, here is the first entry from my grub.cfg

menuentry 'Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-320f<GUID...>' {
        recordfail
        load_video
        gfxmode $linux_gfx_mode
        insmod gzio
        if [ x$grub_platform = xxen ]; then insmod xzio; insmod lzopio; fi
        insmod part_gpt
        insmod cryptodisk
        insmod luks
        insmod gcry_rijndael
        insmod gcry_rijndael
        insmod gcry_sha256
        insmod ext2
        cryptomount -u b6b7<32_char_part_id>
        set root='cryptouuid/b6b7<32_char_part_id>'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint='cryptouuid/b6b7<32_char_part_id>'  76f4<GUID...>
        else
          search --no-floppy --fs-uuid --set=root 76f4<GUID...>
        fi
        linux   /vmlinuz-5.15.0-84-generic root=/dev/mapper/ubuntu--vg-root ro rootflags=subvol=@  quiet splash $vt_handoff
        initrd  /initrd.img-5.15.0-84-generic
}

Answer 1

The command normal should return to the menu. You should probably do this before manually unlocking the volume, as it's likely not smart enough to "resume" – it'll only you reinvoke the whole menu item (all or nothing).

There's no direct "just boot" command as the OS kernel hasn't been loaded yet at this point (it would've been the subsequent command in the menu item), so the only alternative is to manually load the kernel and initramfs using the same commands as the menu item has.

I'd probably avoid encrypting the /boot filesystem in general – let the initramfs prompt for the / password instead. If you can ensure that GRUB itself hasn't been tampered with, you can use the same method to ensure the kernel and initramfs haven't been tampered with, without necessarily having to encrypt them.

Answer 2

The grub command prompt and grub configuration language are both basically the same.

The simple way you can find out how to do this will be to read the menuenry {} from the file /boot/grub/grub.cfg. There's one command hiding named boot which is implied by the menuentry.

On many / most systems you can use:

linux (hdX,gptY)/boot/vmlinuz_kernel_file root=root_fs_device
initrd (hdX,gptY)/boot/initramfs_file
boot

The other trick you can use us to call configfile to execute your grub.cfg file again as it's own script. This should display the menu you usually use.

configfile (hdX,gptY)/boot/grub.cfg

---

Note there are commands that can help you find the placeholders above:

search let's you find a partition by label or UUID

ls let's you list partitions or files on a partition. To list files on a partition Y on device X you would use ls (hdX,gptY)/ or ls (hdX,gptY)/bar/ to list directory "bar".

cat lets you read a file, so may be useful for reading your grub.cfg if you don't have a note of what it says.