Question

Write a bash script that change the grub default boot to the highest kernel (not necessary the first entry) version. The OS is CentOS 6.x. Manual steps will not be accepted.

Answer 1

Hey, It seems like you want to change the Grand unified bootloader(GRUB) using scripting of BASH.

For simplicity purposes, I had divided the explanation into parts. So, let's start:

PART ONE: UNDERSTANDING THE REQUIREMENT AND INPUTS REQUIRED:

This part will discuss the requirements to automatically change the default boot of GRUB to the user-specific choice.

Since manual steps are not entertained, I will bring my approach solely on the basis of the scripting.

The requirement is as simple as that to change the default GRUB boot loader to a choice of the user.

PART TWO: APPROACH:

The approach will be as follows:

1. Find the path where this default boot loading path information is kept. (/etc/default/grub)

GRUB_DEFAULT=0

2. Edit the path using the root-level access and change the default grub to the choice of yours. (vim /etc/default/grub)

3. Search inside the file for GRUB_DEFAULT and change its value.

GRUB_DEFAULT=5

where 5 is the index of GRUB_MENU in which mode you would like to boot by default. The menu item starts with 0. Hence, to boot with the 6th item, you have to set the GRUB_DEFAULT = 5 and so on.

If you don't know the sequence of boot loaders, please open the /boot/grub/grub.cfg file and find the order.

In this config file, the name and other details of bootloaders will be present in an ordered manner.

In place of the index, you can also pass the name like,

GRUB_DEFAULT='Windows8'

4. Update GRUB with root-level privileges.

- sudo update grub

PART THREE: FULL IMPLEMENTATION:

Considering you want to change the default GRUB to user-defined. The present value as a GRUB_DEFAULT will be 0 and you need to give a value as an input or can make constant in the script so that the bootloader will be selected.

sed -i 's/GRUB DEFAULT=0/GRUB DEFAULT=1/' /etc/default/grub

sed -i 's/GRUB_DEFAULT=0/GRUB_DEFAULT=1/' /etc/default/grub

Save this file as a bash script and it will do the needful for changing the bootloader without human intervention of manual things.

In place of 1, you can give the index or name of the bootloader which you wish to change.

Make sure to run this script as a root-user only.

*****************SCRIPT**************

#!/bin/bash
sed -i 's/GRUB_DEFAULT=0/GRUB_DEFAULT=1/' /etc/default/grub

*****************SCRIPT***************

I hope it will help you in completing the requirement.

Please don't forget to thumbs up if it helped you.

Keep learning and keep HomeworkLibing :)

Thanks and All the best

******************************************COMPLETE************************************