GRUB

GRUB, short for Grand Unified Bootloader, is a boot loader package from the GNU Project that bootstraps one or more operating systems on a computer. It is commonly used as the primary boot loader on GNU/Linux systems and is considered the successor to LILO due to its greater flexibility and modular design. GRUB is available in two major flavors: GRUB Legacy (0.x) and GRUB 2 (the current implementation).

GRUB operates at boot time by loading code from firmware into memory and then loading the kernel and initramfs for the selected operating system. In BIOS-based systems GRUB 2 uses a small core image plus modular components; in UEFI systems it runs as a signed EFI application (grubx64.efi). The configuration is typically in /boot/grub/grub.cfg and is usually generated by tools such as grub-mkconfig or update-grub; menu entries define which kernel to boot, pass parameters, and specify root and initrd options. GRUB also supports scripting for dynamic menus and advanced setups and can chainload other bootloaders.

Features include support for numerous filesystems (ext2/3/4, btrfs, xfs, ntfs, vfat), multi-boot configurations, device mapping, and access to logical volumes via LVM or RAID. It can boot Linux kernels, BSDs, Windows, and other systems, and can boot encrypted or LUKS-protected volumes. GRUB offers password protection for menu entries and optional Secure Boot support via signed shim on modern systems. It is maintained by the GNU Project and is the default bootloader on many Linux distributions.