Chapter 2. Specifying the RHEL kernel to run

You can boot any installed kernel, standard or Real Time. You can select the required kernel manually in the GRUB menu during booting. You can also configure which kernel boot by default.

When the real-time kernel is installed, it is automatically set to be the default kernel and is used on the next boot.

2.1. Displaying the default kernel

You can display the kernel configured to boot by default.

Procedure

  • To view the default kernel: 

    ~]# grubby --default-kernel
/boot/vmlinuz-4.18.0-80.rt9.138.el8.x86_64

    The rt in the output of the command shows that the default kernel is a real time kernel.

2.2. Displaying the running kernel

You can display the currently running kernel

Procedure

  • To show which kernel the system is currently running. 

    ~]# uname -a
Linux rt-server.example.com 4.18.0-80.rt9.138.el8.x86_64 …
    Note

    When the system receives a minor update, for example, from 8.3 to 8.4, the default kernel might automatically change from the Real Time kernel back to the standard kernel.

2.3. Configuring the default kernel

You can configure the default boot kernel.

Procedure

  1. List the installed Real Time kernels. 

    ~]# ls /boot/vmlinuz*rt*
/boot/vmlinuz-4.18.0-80.rt9.138.el8.x86_64

  2. Set the default kernel to the listed Real Time kernel. 

    ~]# grubby --set-default real-time-kernel

    Replace real-time-kernel with the Real Time kernel version. For example: 

    ~]# grubby --set-default /boot/vmlinuz-4.18.0-80.rt9.138.el8.x86_64

Verification steps

  • Display the default kernel: 

    ~]# grubby --default-kernel
/boot/vmlinuz-4.18.0-80.rt9.138.el8.x86_64