I set up a vdi in VirtualBox, boot up using some linux live dvd.

Set up VirtualBox, enable EFI

Create GPT partition, 1st primary partition formatted as FAT32.

Copy EFI and kernels directories only.

My expectation would be

1. having a shell thats it.

But I got the following
Boot Failed. EFI DVD/CDROM
Welcome to GRUB!
_

what is your question?

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You can't just copy the EFI loader and the Kernel for a number of reasons.

First EFI needs to be registered using something like efibootmgr. Unless you specifically place the efi loader at the fall back location /bootx64.efi on the efi filesystem. So the EFI system probably has no idea what to load next as it is.

Second it takes more than the kernel to get a shell. The kernels is really process scheduling, filesystem handling, hardware abstraction, etc. Its not something use can really interact with as a user. You need a user space process to do that. On UNIX this is the shell.

To run the shell though it needs to live on a filesystem of itself. However you can't just park bash there. You will need a set of libraries and device nodes at a minimum. To that end it will have to be a separate filesystem from the EFI because it will need to be able to host devs and fat32 is not able to do this. You will also need to configure whatever bootloader you have chose to pass the kernel a command line argument for init.

Frankly all of this is rather complex; not beyond the ability to understand by any means; but its not like BIOS just loading the first 512 bytes of the disk which in turns reads MSDOS.SYS or similar of old. My advice is install Slackware using the installer! Boot of the installation media and follow the documentation. Once you have working booting system; than you can take time to learn and understand the boot process. It will all make more sense when you have something complete and correct to look at; rather than trying to assemble piecemeal

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