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In the case of an installed Linux, such as Debian/Ubuntu or derivatives, it is possible to edit the Grub config file like this:

sudo vi /etc/default/grub

and comment out this line:

#GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

and replace it with this line:

GRUB_CMDLINE_LINUX_DEFAULT=""

and then run

sudo update-grub

which will print lots of output during the subsequent boot process instead of a splash screen, black screen, or circle, etc.;

I would like to know how can I get it to print this debug for a live USB?

I edited this file on the live USB:

 boot/grub/grub.cfg

and I was able to change the colors of the grub menu, but even if I remove the quiet splash words from the vmlinuz commandline, the boot process occurs with a blank screen, giving no output at all, no clue as to what is going on;

I also edited these files on the live Ubuntu 20.04 USB to remove quiet splash but it still tries to boot with a blank screen:

 syslinux.cfg
 isolinux/txt.cfg

The problem is that once I select to boot Ubuntu from the live USB grub menu, it accesses the USB for about 10 seconds (the USB has a LED indicating this), during which the screen is blank with no output, and then the boot process is canceled and it goes back to the Acer splash; now that is a separate question; in order to help figure that out, I would like to turn on the verbose output during the boot process, for a live USB, just like for an installed Linux.

I looked to see if there is any verbose option that may help but didn't find one, and the Grub for an installed system seems to be somewhat different than these config files on the live USB.

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  • edit the GRUB entry at boot time
    – jsotola
    Commented Dec 16, 2023 at 16:41
  • good idea, by pressing e to edit the grub menu entry at boot time, it is no longer necessary to guess which config file has to be edited; however even with the quiet splash removed, it doesn't print anything; hopefully there is an option or environment variable that can be set there to make it print something to give an idea why the boot from the live usb fails (the live usb boots fine on another laptop, the problem is on the acer aspire 515 and I need the verbose debug to try to figure it out)
    – jmarina
    Commented Dec 16, 2023 at 17:09
  • quiet and splash are two separate parameters
    – jsotola
    Commented Dec 16, 2023 at 17:24
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    remove quiet and try adding loglevel=4 and/or verbose
    – jsotola
    Commented Dec 16, 2023 at 17:29
  • with quiet and splash removed, in a laptop that has ubuntu installed and can boot from a live ubuntu usb, it does print the debug during boot; in the acer aspire 515 which has only uefi shell and does not boot from a live ubuntu usb (only getting as far as the grub menu) the debug does not print; I added loglevel=4 and verbose with quiet removed and still no debug - but now I think that is because it fails before the debug starts; on the laptop where it works, the debug starts after about 10sec; in the one where it does not work, after about 10sec the boot is canceled
    – jmarina
    Commented Dec 17, 2023 at 3:23

1 Answer 1

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pressing e to edit the grub menu entry to remove quiet and splash causes the debug to be printed during boot of a live linux usb, the file is boot/grub/grub.cfg in the case of an ubuntu live usb, similar to how it is printed for an installed linux after editing /etc/default/grub as described in the question, so this is the answer.

I wanted this debug to see why the aspire a515-48m refuses to boot linux, but in that case, it failed before printing the debug during boot starts; I figured out how to get it to boot linux

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