How update-grub Rebuilds GRUB Configuration

The update-grub command is a vital utility in Debian-based Linux distributions used to regenerate the GRUB 2 bootloader configuration. Rather than executing a complex internal routine, it acts as a streamlined wrapper script that calls grub-mkconfig to inspect the system, discover installed kernels and alternative operating systems, and write a unified boot configuration file to /boot/grub/grub.cfg.

The Execution Process

When executed with root privileges, update-grub runs a single command under the hood:

grub-mkconfig -o /boot/grub/grub.cfg

This triggers a step-by-step process where the operating system compiles dynamic scripts and static settings into a single, static configuration that the GRUB bootloader can parse at machine startup.

1. Reading System Defaults

The generation process begins by loading settings from /etc/default/grub. This file contains the administrative preferences defined by the user or installation defaults, including:

• GRUB_DEFAULT: The default operating system or kernel to boot.
• GRUB_TIMEOUT: The duration the boot menu displays before booting the default entry.
• GRUB_CMDLINE_LINUX_DEFAULT: Kernel parameters passed to the Linux kernel at boot time (such as quiet splash).
• Visual preferences, including display resolutions, terminal output modes, and console themes.

2. Processing Modular Scripts in /etc/grub.d/

Once the general preferences are loaded, the system executes the scripts located in /etc/grub.d/ in numerical order. These executable shell scripts output GRUB script commands that construct the final /boot/grub/grub.cfg:

• 00_header: Establishes the GRUB environment, imports graphical modules, applies timeout settings, and loads basic variables.
• 05_debian_theme: Configures colors, fonts, and background images for the boot interface.
• 10_linux: Scans the /boot directory for installed Linux kernels (vmlinuz-*) and corresponding Initial RAM Disks (initrd.img-*). For each kernel found, it creates standard and recovery boot menu entries using the kernel parameters specified in /etc/default/grub.
• 20_memtest86+: Detects memory-testing utilities in /boot and adds them to the utility options if present.
• 30_os-prober: Utilizes the os-prober tool to scan other partitions and drives for foreign installations, such as Microsoft Windows or other Linux distributions, generating boot options for multi-boot configurations.
• 40_custom: Appends any user-written entries or configuration fragments that should remain persistent across regenerations.

3. Assembling and Writing the Configuration

As each script in /etc/grub.d/ runs, its standard output is captured and concatenated. The combined output forms the syntax of the GRUB scripting engine.

To prevent corruption during writes or interrupted executions, the output is typically constructed in a temporary stream before replacing /boot/grub/grub.cfg. Once written, GRUB reads this finalized file during the next system boot cycle without needing to re-scan the disks.