Ubuntu Server: Commands to Check and Configure GRUB Kernel Boot

Managing your Ubuntu server efficiently often involves customizing boot options. GRUB (GRand Unified Bootloader) is crucial for selecting which operating system or kernel to boot. Understanding how to check and configure GRUB settings is essential for system administrators. This guide will walk you through the necessary commands to inspect and modify your GRUB configuration on Ubuntu Server, ensuring you can effectively manage your server’s boot process, including setting it to remember your last kernel choice.

Understanding GRUB Configuration Files

Before making changes, it’s important to know where GRUB configuration is stored. The primary configuration file is /etc/default/grub. This file contains settings that control how GRUB behaves. Changes here are not directly applied to the bootloader’s configuration. To apply modifications, you need to use the update-grub command, which generates the actual GRUB configuration file, typically located at /boot/grub/grub.cfg. It’s crucial to avoid directly editing grub.cfg as it’s auto-generated and changes will be overwritten.

[instruction]

Commands to Check Current GRUB Configuration

To begin, let’s explore commands to check your current GRUB configuration. This helps you understand the existing setup before making any modifications.

1. Displaying /etc/default/grub Contents

The simplest way to check your GRUB configuration is to view the contents of /etc/default/grub. Use the cat command:

cat /etc/default/grub

This command will output the content of the file, showing you variables like GRUB_DEFAULT, GRUB_TIMEOUT, and GRUB_CMDLINE_LINUX_DEFAULT. These settings dictate default boot behavior, timeout duration, and kernel command-line parameters.

2. Using grub-editenv to Check Environment Variables

GRUB stores some settings in an environment block file, often /boot/grub/grubenv. You can use the grub-editenv command to interact with this file. To list saved environment variables, use:

sudo grub-editenv list

This command is particularly useful for checking settings related to saved defaults, as we’ll see in the next section. You might see variables like saved_entry if you’ve previously configured GRUB to remember the last booted kernel.

Configuring GRUB to Remember Last Booted Kernel

A useful feature is to configure GRUB to remember the last kernel you manually selected at boot time. This is helpful if you often switch between kernels and want GRUB to default to your last choice instead of always booting the default kernel.

Steps to Enable “Saved Default”

1. Edit /etc/default/grub: Open the /etc/default/grub file with a text editor using sudo:

   sudo nano /etc/default/grub

2. Add or Modify Lines: Add or modify the following lines within the file:

   GRUB_SAVEDEFAULT=true
   GRUB_DEFAULT=saved

   GRUB_SAVEDEFAULT=true enables the “saved default” feature, telling GRUB to save the last selected boot entry. GRUB_DEFAULT=saved sets the default boot entry to be the one saved from the previous boot.

3. Update GRUB Configuration: After editing and saving the file, apply the changes by running:

   sudo update-grub

   This command regenerates the grub.cfg file, incorporating your changes from /etc/default/grub.

4. Reboot Your Server: Reboot your Ubuntu server for the changes to take effect:

   sudo reboot

Now, when you boot your server and manually select a kernel from the GRUB menu (e.g., under “Advanced options for Ubuntu”), GRUB will remember this selection. On subsequent reboots, it will automatically boot into the kernel you last chose.

Verifying the Saved Default

After rebooting, you can verify that GRUB is remembering your last selection.

1. Check grubenv with grub-editenv: Use the list command again:

   sudo grub-editenv list

   You should now see a saved_entry variable in the output, indicating the last booted entry. For example:

   saved_entry=gnulinux-advanced-xxxxxxxxxxxxxxxxxxxxx>gnulinux-5.15.0-xx-generic-advanced-xxxxxxxxxxxxxxxxxxxxx

2. Observe Boot Behavior: Reboot your server again. It should automatically boot into the kernel you selected in the previous boot.

Manually Setting a Saved Entry (Advanced)

In some cases, you might want to manually set the saved_entry. This is less common but can be useful for specific scenarios. You can use grub-editenv set for this. First, you need to identify the correct entry name from /boot/grub/grub.cfg.

1. Find the Entry Name: Open /boot/grub/grub.cfg and locate the menuentry for the kernel you want to set as the saved default. Look for the menuentry_id_option value. For submenu entries, note the submenu’s menuentry_id_option and the kernel’s menuentry_id_option, separated by >.

   For example, an entry might look like this in /boot/grub/grub.cfg:

   submenu 'Advanced options for Ubuntu' $menuentry_id_option 'gnulinux-advanced-xxxxxxxxxxxxxxxxxxxxx' {
     menuentry 'Ubuntu, with Linux 5.19.0-xx-generic' --class ubuntu ... $menuentry_id_option 'gnulinux-5.19.0-xx-generic-advanced-xxxxxxxxxxxxxxxxxxxxx' {
       ...
     }
   }

   The entry name would be: gnulinux-advanced-xxxxxxxxxxxxxxxxxxxxx>gnulinux-5.19.0-xx-generic-advanced-xxxxxxxxxxxxxxxxxxxxx.

2. Set the saved_entry: Use grub-editenv set with the entry name, enclosed in single quotes to handle the > character:

   sudo grub-editenv set 'saved_entry=gnulinux-advanced-xxxxxxxxxxxxxxxxxxxxx>gnulinux-5.19.0-xx-generic-advanced-xxxxxxxxxxxxxxxxxxxxx'

Important Note: Modifying GRUB configurations incorrectly can lead to boot issues. Always double-check your commands and configurations. If you encounter problems, you might need to boot into a recovery mode to fix GRUB settings.

Conclusion

Understanding and utilizing these Ubuntu Server Command To Check Grub And Kernel Configuration is vital for managing your server’s boot process effectively. By checking your current GRUB configuration and knowing how to configure options like “saved default,” you gain greater control over your Ubuntu server’s boot behavior, ensuring it boots into the desired kernel every time. This level of customization is invaluable for server administrators who need to manage different kernel versions or boot options regularly.