The grub.cfg file contains Bash-like code and a list of installed kernels in an array ordered by sequence of installation. The / boot/grub2/ grub.cfg file is first generated during Linux installation and regenerated when a new kernel is installed. It is generated by the grub2-mkconfig program using a set of primary configuration files and the grub default file as a source for user configuration specifications. The grub.cfg file is the GRUB configuration file. Unfortunately, this rescue kernel does not boot to rescue mode.
The GRUB menu also provides a "rescue" kernel, in for use when troubleshooting or when the regular kernels don't complete the boot process for some reason. Pressing the Enter key immediately continues the boot process with either the default kernel or an optionally selected one.
Pressing any key on the keyboard except the Enter key terminates the countdown timer which is displayed on the console. The GRUB menu also provides a timeout so that, if the user does not make any other selection, GRUB will continue to boot with the default kernel without user intervention. Using the up and down arrow keys allows you to select the desired kernel and pressing the Enter key continues the boot process using the selected kernel. The function of the GRUB menu is to allow the user to select one of the installed kernels to boot in the case where the default kernel is not the desired one. This has come in handy on a couple occasions when I had to revert to a kernel that was several versions down-level. I usually change the installonly_limit value to 9 to retain a total of nine kernels. The number of kernels to be kept before the package manager erases them is configurable in the /etc/dnf/dnf.conf or /etc/yum.conf files. By default, three kernels are kept–the newest and two previous–when yum or dnf are used to perform upgrades. Many times, booting to an older kernel can circumvent issues such as these. I have found this to be a very useful tool because there have been many instances that I have encountered problems with an application or system service that fails with a particular kernel version. GRUB supports multiple Linux kernels and allows the user to select between them at boot time using a menu. Once the kernel takes over, GRUB has done its job and it is no longer needed. Its function is to take over from BIOS at boot time, load itself, load the Linux kernel into memory, and then turn over execution to the kernel. GRUB stands for GRand Unified Bootloader. This article provides a quick introduction to configuring GRUB2, which I will mostly refer to as GRUB for simplicity. When researching my article from last month, An introduction to the Linux boot and startup process, I became interested in learning more about GRUB2. As long as you have not removed the older version of the kernel, you could configure GRUB to boot with that older kernel by default. More importantly, you do not want to disrupt the daily routine of other users whilst you are figuring things out. Furthermore, it might be that you cannot or do no want to solve this issue straight away. Occasionally, a new kernel might break a custom-installed package that does not belong to the standard repositories. By default, GRUB will choose the latest kernel to boot after its menu time-out. The upgrade also included a new version of the Linux kernel. Now, suppose you just upgraded your rolling release GNU/Linux distribution. It gives a user the choice to boot one of multiple operating systems or select a specific kernel configuration available on a disk partition of the computer system. GNU GRUB (short for GNU GRand Unified Bootloader) is the boot loader of most GNU/Linux distributions. Stroobandt Copyright 2013–2016, licensed under Creative Commons BY-NC-SA GRUB: Changing the Default Kernel Serge Y.
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