If we have more than one OS in our HD or SSD (either dual or multi boot) we may find ourselves in the scenario where, depending on a number of reasons, the default boot OS is not the one we want or changes when a major upgrade takes place. However we can tell grub2 to boot always the same OS (to avoid changing it ourselves during grub2’s timeout screen). To do this, we have to check on /boot/grub/grub.cfg & do some tweaking on /etc/default/grub.

Checking how many boot OS entries we have in /boot/grub/grub.cfg

We will run the following command (to grep the relevant information — field #2 in lines starting with menuentry):

grep "^menuentry" /boot/grub/grub.cfg |cut -d "'" -f2

Here’s the output I obtained:

Zorin
Memory test (memtest86+)
Memory test (memtest86+, serial console 115200)
Windows Recovery Environment (loader) (en /dev/sda1)
Windows 7 (loader) (en /dev/sda3)
Linux Mint 18.1 Serena (18.1) (en /dev/sda5)

As you can see, I have 3 OSs on my HD: Zorin (default), Windows 7 and Linux Mint 18.1 Serena. Now let’s suppose I want to make Linux Mint the new default …

Making a specific OS the default at boot in /etc/default/grub

The important parameter here is GRUB_DEFAULT. Let’s grep /etc/default/grub for this parameter and its value:

grep GRUB_DEFAUL /etc/default/grub

We get the following output:

GRUB_DEFAULT=0

Than means Zorin — being the main OS — is also the boot default. Now to set Linux Mint as the new boot default, we have to use our favourite text editor and change the value of GRUB_DEFAULT as follows:

GRUB_DEFAULT="Linux Mint 18.1 Serena (18.1) (en /dev/sda5)"

It’s a good practice to copy and paste the value from the output of grep “^menuentry” /boot/grub/grub.cfg |cut -d “’” -f2 to avoid spelling mistakes.

The only thing left to do now is updating grub2 with:

sudo update-grub

or …

sudo update-grub2

… since update-grub2 is linked to update-grub.

Halof!!!

Sources & References:

Grub2 Quick Start: https://wiki.gentoo.org/wiki/GRUB2_Quick_Start

GNU Grub Manual: https://www.gnu.org/software/grub/manual/grub/