How do you use grub2 commands and options to boot into different modes and kernels?

3 How to boot into recovery mode or single-user mode?

Recovery mode and single-user mode are special modes that allow you to perform troubleshooting or maintenance tasks on your system. Recovery mode is a menu option that loads a minimal GNU/Linux environment with some tools and options, such as repairing file systems, updating packages, or resetting passwords. Single-user mode is a command line option that loads the system with only the root user and no graphical interface, network, or services. To boot into recovery mode or single-user mode, you need to access the grub2 menu or command line and select or type the appropriate option. For example, on Ubuntu, you can select Advanced options for Ubuntu and then choose a kernel with (recovery mode) at the end. Alternatively, you can edit the kernel parameters by pressing E on the menu or typing e on the command line, and then add the word single or recovery to the end of the line that starts with linux.

4 How to boot into an older kernel version?

Sometimes, you may want to boot into an older kernel version if the current one causes problems or does not support some hardware or software. To boot into an older kernel version, you need to access the grub2 menu or command line and select or type the option that corresponds to the kernel version you want. For example, on Ubuntu, you can select Advanced options for Ubuntu and then choose a kernel with a lower number. Alternatively, you can edit the kernel parameters by pressing E on the menu or typing e on the command line, and then change the number after vmlinuz or initrd to match the kernel version you want.

5 How to change the default boot option or timeout?

If you want to change the default boot option or timeout, which are the option and time that grub2 uses if you do not make a selection, you need to edit the grub2 configuration file. The file is usually located at /etc/default/grub and you need root privileges to edit it. You can use any text editor, such as nano or vi, to open the file and make changes. The most important variables are GRUB_DEFAULT and GRUB_TIMEOUT. GRUB_DEFAULT sets the default boot option by its number, name, or id. GRUB_TIMEOUT sets the number of seconds that grub2 waits before booting the default option. You can also use GRUB_SAVEDEFAULT to make grub2 remember the last option you chose. After you save the file, you need to run the command sudo update-grub to apply the changes.

6 How to use grub2 commands and options to troubleshoot boot issues?

Sometimes, you may experience boot issues, such as a black screen, a kernel panic, or a missing file. In these cases, you can use grub2 commands and options to troubleshoot and fix the problem. The set command allows you to check or change the location of your boot files or the resolution of your screen. The ls command lists the devices and partitions that grub2 can access, so you can identify your disk and partition names and numbers. The search command helps you to find your root partition or your boot files. The linux command sets the kernel to boot, so you can specify the kernel file and parameters, such as single or nomodeset. The initrd command sets the initial ramdisk to boot, so you can specify the initrd file that matches your kernel. Lastly, the boot command boots the system with the kernel and initrd that you set after setting the linux and initrd commands.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?