I'm not real familiar with grub, other than I was going to use it to
start my OS.
For Linux, there is a command grub-mkconfig that sets the command line
in the grub.cfg file in /boot/grub/ directory.
As for DOS, i.e., GRUB4DOS, I have no idea.
So, I'd mostly have to refer you to the manual. The first link lists
and the second one explains.
There are some interesting commands there that may interest you, like
"boot" "chainloader" "initrd" "insmod" "linux" "loopback" "module"
e.g., "initrd" stands for init ramdisk, "insmod" for insert module.
What is hidden in the ground, when found, is hidden there again?