3.3 GRUB 2 Configuration & Troubleshooting (102.2)

3.3 GRUB 2 Configuration & Troubleshooting (102.2)

Quick Summary: GRUB 2 (Grand Unified Bootloader version 2) is the default bootloader for modern Linux distributions. Unlike its legacy predecessor, GRUB 2 relies on an auto-generated configuration file located at /boot/grub/grub.cfg (or /boot/grub2/grub.cfg). Administrators control boot parameters, timeouts, and default kernels by editing /etc/default/grub and scripts in /etc/grub.d/, compiling them with grub-mkconfig. Mastering interactive editing (e), the GRUB command shell (c), and rescue recovery commands is essential for system maintenance and LPIC-1 certification.

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1. GRUB 2 Architecture and Component Hierarchy

GRUB 2 separates configuration source files from the active runtime configuration file. Direct manual edits to /boot/grub/grub.cfg will be completely overwritten during the next kernel update or configuration generation.

  ┌─────────────────────────┐      ┌─────────────────────────┐
  │    /etc/default/grub    │      │      /etc/grub.d/       │
  │  Key-Value Global Vars  │      │  00_header, 10_linux... │
  │  (TIMEOUT, CMDLINE...)  │      │  Executable Gen Scripts │
  └────────────┬────────────┘      └────────────┬────────────┘
               │                                │
               └───────────────┬────────────────┘
                               ▼
                  [ grub-mkconfig -o ... ]
                  [ (or update-grub)     ]
                               │
                               ▼
                  ┌─────────────────────────┐
                  │   /boot/grub/grub.cfg   │
                  │   (DO NOT EDIT DIRECT!) │
                  └─────────────────────────┘

Primary Configuration File Paths

• Debian / Ubuntu / openSUSE: /boot/grub/grub.cfg
• RHEL / CentOS / Fedora / Rocky Linux (BIOS): /boot/grub2/grub.cfg
• RHEL / CentOS / Fedora (UEFI): /boot/efi/EFI/redhat/grub.cfg or /boot/efi/EFI/centos/grub.cfg (or /boot/grub2/grub.cfg symlinked in modern releases)

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2. Configuration Templates: /etc/default/grub

The /etc/default/grub file contains global variables that define bootloader behavior, visual themes, and kernel command-line parameters.

# Sample /etc/default/grub configuration
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release 2>/dev/null || true)"
GRUB_DEFAULT=saved
GRUB_SAVEDEFAULT="true"
GRUB_DISABLE_SUBMENU="true"
GRUB_TERMINAL_OUTPUT="console"
GRUB_CMDLINE_LINUX="crashkernel=auto resume=/dev/sda2 quiet splash"
GRUB_DISABLE_RECOVERY="true"

Key Directives Reference

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3. Modular Script Generators: /etc/grub.d/

The /etc/grub.d/ directory contains shell scripts executed in alphanumerical order by grub-mkconfig to construct grub.cfg.

• 00_header: Sets environmental variables, terminal displays, graphic modes, and timeout settings from /etc/default/grub.
• 10_linux: Scans /boot for installed Linux kernels (vmlinuz-*) and creates corresponding boot menu entries.
• 20_memtest86: Detects standalone memory testing binaries (e.g., Memtest86+) and adds diagnostic menu entries.
• 30_os-prober: Executes os-prober to scan all attached storage drives for other operating systems (Windows, other Linux distributions) and generates dual-boot menu entries.
• 40_custom: A template script where administrators can append static, custom boot stanzas.
• 41_custom: Automatically sources /boot/grub/custom.cfg at boot time if it exists.

⚠️ LPIC-1 Trap: Any script in /etc/grub.d/ MUST have executable permissions (chmod +x /etc/grub.d/*). If a script lacks the +x bit, grub-mkconfig silently ignores it without generating an error, and its entries will not appear in grub.cfg.

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4. Generating Configuration and Installing the Bootloader

Compiling the GRUB 2 Configuration

# Standard generic command (RHEL, Fedora, CentOS, openSUSE)
sudo grub-mkconfig -o /boot/grub/grub.cfg
# OR on RHEL/CentOS systems
sudo grub2-mkconfig -o /boot/grub2/grub.cfg

# Debian / Ubuntu wrapper command
sudo update-grub

Installing the Bootloader to Disk

grub-install writes the core bootloader code and stage images to the Master Boot Record (MBR) or EFI System Partition (ESP).

# BIOS / MBR Installation (target the WHOLE DISK, not a partition)
sudo grub-install /dev/sda
# OR
sudo grub2-install /dev/sda

# UEFI Installation
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=GRUB

💡 LPIC-1 Exam Fill-in-the-Blank Alert: When asked for the exact command to install GRUB 2 to the master boot record of the primary SATA drive, the command is grub-install /dev/sda (never specify partition numbers like /dev/sda1 for BIOS MBR installs).

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5. Interactive Boot Menu Operations & Troubleshooting

When the system boots, the GRUB 2 visual menu presents several key interactive controls:

• e (Edit): Opens the inline editor for the highlighted boot entry. Allows administrators to modify kernel boot arguments dynamically for a single boot without modifying grub.cfg.
• c (Command-Line): Drops into the full interactive GRUB 2 command shell (grub>).
• Ctrl+X or F10: Boots the system with the edited in-memory configuration.
• Esc: Aborts editing and returns to the main menu.

Critical Kernel Override Arguments for Troubleshooting

To recover a system with a forgotten root password or broken systemd service, press e at the GRUB menu, navigate to the line beginning with linux (or linux16 / linuxefi), and append one of the following arguments:

# Bypass init and spawn a direct root shell
linux /vmlinuz-5.14.0 root=/dev/sda2 ro init=/bin/bash

# Boot into single-user / rescue target (systemd)
linux /vmlinuz-5.14.0 root=/dev/sda2 ro systemd.unit=rescue.target

# Emergency breakpoint in initramfs (RHEL/CentOS)
linux /vmlinuz-5.14.0 root=/dev/sda2 ro rd.break

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6. GRUB Rescue Mode Shell Recovery

If GRUB 2 cannot find its configuration file or secondary stage modules in /boot/grub, it drops into the minimal grub rescue> shell.

# 1. List all detected storage devices and partitions
grub rescue> ls
(hd0) (hd0,msdos1) (hd0,msdos2)

# 2. Browse filesystems to find the boot directory and grub modules
grub rescue> ls (hd0,msdos1)/
grub rescue> ls (hd0,msdos1)/boot/grub

# 3. Set the root and prefix environment variables
grub rescue> set root=(hd0,msdos1)
grub rescue> set prefix=(hd0,msdos1)/boot/grub

# 4. Load the normal execution module and launch the standard menu
grub rescue> insmod normal
grub rescue> normal

Once the system boots successfully into Linux, the administrator must repair the bootloader permanently by running grub-install /dev/sda followed by grub-mkconfig -o /boot/grub/grub.cfg.