How can I show all kernels in the Grub2 menu, ie disable submenu ("Previous Linux Versions")?

In the latest Ubuntu (14.04), one can specify:

GRUB_DISABLE_SUBMENU=y

in /etc/default/grub.

Then apply with

sudo update-grub

Deprecated

The answer below applies to Ubuntu versions older than 14.04

gksu gedit /etc/grub.d/10_linux

At the end of the file:

  if [ "$list" ] && ! $in_submenu; then
    echo "submenu \"Previous Linux versions\" {"
    in_submenu=:
  fi
done

if $in_submenu; then
  echo "}"

add the "#" character in front of some lines to disable (i.e. comment out) their effect, make it look like this (4 lines changed):

  if [ "$list" ] && ! $in_submenu; then
    #echo "submenu \"Previous Linux versions\" {"
    in_submenu=:
  fi
done

#if $in_submenu; then
  #echo "}"
#fi

Then run sudo update-grub to apply the changes.

Well, if you don't mind adding a PPA then yes!

Grub Customizer:

Grub Customizer is a graphical interface to configure the grub2/burg settings.It allows you to edit the GRUB2 menu entries: reorder, rename or add/remove entries.

Installation:

sudo add-apt-repository ppa:danielrichter2007/grub-customizer
sudo apt-get update
sudo apt-get install grub-customizer

How to disable Grub2 submenu?

Once installed, Hit Alt+F2, type grub-customizer and hit Enter.

Select the submenu entries one by one and click on ↑(Up) button in toolbar to move them to main menu.

And no more submenu! Save your settings by clicking on save button in toolbar. That's it!

---

To remove Grub Customizer run following commands in terminal

sudo apt-get autoremove --purge grub-customizer
sudo add-apt-repository -r ppa:danielrichter2007/grub-customizer
sudo apt-get update