Why does Ubuntu show boot messages when starting up?

short answer: yes! :) you can customize Ubuntu or Fedora whatever way you like. I don't know what version you are running, but under any normal installation i believe plymouth should be installed.

Plymouth is the application which provides the graphical "splash" screen when booting and shutting down an Ubuntu system.

There is an entire Wiki on this topic on the ubuntu website here: https://wiki.ubuntu.com/Plymouth I think the reason yours is not working is because either the kernel is not set up to use plymouth correctly, or your grub bootloader was set up to boot the os in text-mode.

Another issue i've encountered before myself, is that my Ubuntu wasn't shut down properly, which causes it to boot in a system check, and that also shows the terminal output.

Those fancy OK/FAIL messages that you see are actually boot messages that are output to TTY1 during the boot process.

To answer your question, that technically is not a shell, but a separate program, namely the systemd init system that puts out those messages.

You can hide those messages but i need to look it up. Since i never hide it, i like it actually.

Edit :

To hide those messages edit the /etc/default/grub

sudo nano /etc/default/grub

Change these lines :

GRUB_CMDLINE_LINUX_DEFAULT=quiet   #Don't show Ubuntu bootup text
GRUB_CMDLINE_LINUX="console=tty12" #Don't show kernel text

Update grub :

sudo update-grub

Instead of hiding use a splash screen to cover it up.

Using plymouth, a graphical startup animator, you can provide a pretty bootscreen that is well suited to desktop machines.

sudo apt-get install plymouth-theme-ubuntu-logo

You will find a how-to on this link : Boot your Linux silently