I want to boot Linux distros from GRUB 2.0 command line. I've tried to do so for couple of distros and at "best" I receive initramfs prompt, no GUI which starts if distro is run stardard way. Resulting file system seems to have files contained in initrd file (less then 100Mb), but not in filesystem.squashfs (which is larger than 1Gb).

vmlinux, initrd and filesystem.squashfs files are in casper folder and linux command in menu entry in grub.cnf in distros contains boot=casper, I suspect folder name casper is not necessary for kernel option to work, casper is persistence something option related as far as I understood from wikipedia.

I've played several tries with setting root in GRUB and passing root parameter to vmlinuz to no success.

On last try: kernel boot starting, ending with many lines
then finally
and CLI prompt.

I've run grep -rnw 'initrd file loop mounted location' -e 'filesystem.squashfs' to find out how init process scripts find the file and got nothing.

How to make sure boot finds and merges filesystem.squashfs?

P.S. I want to make multi-boot USB using Linux, could not make work what I've read, so tackling the problem manually now and want to be able to configure freely.

ok I can help you with making a multidistro usb using many linux distros
What exact distros do you want on your usb and size of usb??

Thank you. I want to understand the process, to be able to go further to building distro myself. My current obstacle I explained in the question. As for practicality for now, I have 32gb so as a distro takes less than 2Gb as I see, I want to be able to download new one and add it to new 3-4Gb partition and to grub.cfg menu on USB. The ones I downloaded now - Ubuntu, Linux Lint, Fedora - some several versions each (e.g different desktops). To combine kernel etc. with desktop of my choice could be my next step.