Usually a Linux system boots with the latest installed kernel, which is set by default. It keeps maximum 3 recent versions of Linux Kernel boot entries.
Let’s say you’ve done the yum update, and a new kernel is installed as part of this update. This prevents the application from starting as this kernel is incompatible with the application.
To temporarily fix this problem, you should continue to boot the system into an older kernel.
In this article, we’ll show you how to set old kernel version as default on RHEL 8 and RHEL 9 systems using grubby utility.
* [**How to Upgrade from RHEL 7 to RHEL 8**][1]
### What is grubby
grubby is a command line tool used for updating and displaying information about the boot loaders configuration files across multiple architectures.
### Checking currently booted kernel
You can check booted/loaded kernel using **uname command** as shown below.
```
uname -r
4.18.0-477.13.1.el8_8.x86_64
```
### Listing the Default Kernel
To verify the default kernel version using grubby, run:
```
grubby --default-kernel
/boot/vmlinuz-4.18.0-477.13.1.el8_8.x86_64
```
To obtain the index number of the current default kernel, run:
```
grubby --default-index
0
```
### Checking installed kernels
To check list of installed kernels, run:
Let’s explain the below output. The latest kernel installed will have the **entry-index** of **`0`** , the next **older kernel** version will get **`1`** , second older kernel version will get **`2`** , and the **rescue kernel** will get an entry-index of **`3`**.
```
grubby --info=ALL | egrep -i 'index|title'
index=0
title="Red Hat Enterprise Linux (4.18.0-477.13.1.el8_8.x86_64) 8.8 (Ootpa)"
index=1
title="Red Hat Enterprise Linux (4.18.0-425.19.2.el8_7.x86_64) 8.7 (Ootpa)"
index=2
title="Red Hat Enterprise Linux (4.18.0-425.13.1.el8_7.x86_64) 8.7 (Ootpa)"
index=3
title="Red Hat Enterprise Linux (0-rescue-13iu76884ec5490puc67j8789s249b0c) 8.2 (Ootpa)"
```
### Changing default kernel Boot entry
This can be done in two ways either using **`'kernel filename'`** or **`'kernel entry-index'`**. We will be going to set ‘**index=1 (4.18.0-425.19.2.el8_7.x86_64)**‘ as default kernel due to application dependency.
