If I boot into a linux kernel using a grub command like "linux /vmlinuz; inirtd /initrd; boot", how do I shutdown linux itself (but do not power off or reboot the computer hardware) and return back to the grub directly?
What I meant is something equivalent to the "exit" command in most shells: If you are within one shell and you enter another shell, then if you want to quit the second shell and return back to the first shell, just simply type exit. All env variables and commands you previous typed in the first shell are completely intact.
Suppose I am now in the grub shell, and then I boot into a tiny linux kernel, say, a busybox shell, then what can I do to exit the second shell (i.e. busybox shell) and return to the first shell (i.e. grub shell)? Of course I can enter the grub shell again by simply rebooting the hardware, but this is not what I want.