Red Hat System Administration II

Objectives

After completing this section, students should be able to fix boot loader issues.

The boot loader used by default on Red Hat Enterprise Linux 7 is grub2, the second major version of the GRand Unified Bootloader.

grub2 can be used to boot on both BIOS and UEFI systems, and supports booting almost any operating system that runs on modern hardware.

The main configuration file for grub2 is /boot/grub2/grub.cfg, but administrators are not supposed to edit this file directly. Instead, a tool called grub2-mkconfig is used to generate that configuration using a set of different configuration files, and the list of installed kernels.

grub2-mkconfig will look at /etc/default/grub for options such as the default menu timeout and kernel command line to use, then use a set of scripts in /etc/grub.d/ to generate a configuration file.

To make permanent changes to the boot loader configuration, an administrator needs to edit the configuration files listed previously, then run the following command:

[root@serverX ~]# grub2-mkconfig > /boot/grub2/grub.cfg

In those cases where major changes have been made, an administrator might prefer to run that command without the redirection so that the results can be inspected first.

Important directives

To troubleshoot a broken grub2 configuration, an administrator will need to understand the syntax of /boot/grub2/grub.cfg first. Actual bootable entries are encoded inside menuentry blocks. In these blocks, linux16 and initrd16 lines point to the kernel to be loaded from disk (along with the kernel command line) and the initramfs to be loaded. During interactive editing at boot, Tab completion is available to find these files.

The set root lines inside those blocks do not point to the root file system for the Red Hat Enterprise Linux 7 system, but instead point to the file system from which grub2 should load the kernel and initramfs files. The syntax is harddrive,partition, where hd0 is the first hard drive in the system, hd1 is the second, etc. The partitions are indicated as msdos1 for the first MBR partition, or gpt1 for the first GPT partition on that drive.

Reinstalling the boot loader

In those cases where the boot loader itself has become corrupted, it can be reinstalled using the grub2-install command. On BIOS systems, the disk where grub2 should be installed in the MBR should be provided as an argument. On UEFI systems, no arguments are necessary when the EFI system partition is mounted on /boot/efi.