Textbook of Linux

← Software

GRUB

The GNU GRand Unified Bootloader.

GRUB (the GNU GRand Unified Bootloader) loads the operating system kernel from disk into memory after firmware (BIOS or UEFI) hands off control. It supports a configurable boot menu, multiple kernels, multiple operating systems, a wide range of filesystems, and chain-loading of other bootloaders for unsupported OSes. GRUB is the default bootloader on most mainstream Linux distributions.

Erich Boleyn wrote the first version of GRUB in the mid-1990s. The current implementation is GRUB 2 (often just called GRUB), which is a complete rewrite started in 1999. Configuration is mostly auto-generated from a /etc/default/grub file plus a collection of /etc/grub.d scripts, with a final update-grub command that emits the actual grub.cfg.

GRUB has accumulated a reputation for occasional fragility during dual-boot setups, kernel-update misfires, and EFI/BIOS quirks. Modern alternatives — systemd-boot for UEFI-only systems, rEFInd for multi-OS Macs and PCs — have gained ground, but GRUB remains the universal default and is the bootloader that most distribution installers configure automatically.

Already installed on most Linux systems. To reinstall:
  Debian/Ubuntu: sudo apt install grub2
  Fedora/RHEL:   sudo dnf install grub2