Textbook of Linux

GRUB (GRand Unified Bootloader) is the dominant bootloader on Linux systems. Developed under the GNU project, it loads the kernel and initial ramdisk from disk, passes them command-line parameters, and transfers control to the kernel. GRUB 2 (the current version, universally used despite the name "GRUB 2") supports a wide variety of filesystems and partition schemes, scripting, menus, chainloading into other operating systems, and both BIOS and UEFI firmware.

On a typical distribution, the core configuration is generated automatically:

sudo vim /etc/default/grub            # high-level options
sudo update-grub                       # Debian/Ubuntu
sudo grub2-mkconfig -o /boot/grub2/grub.cfg   # Fedora

This produces /boot/grub/grub.cfg (or similar), which lists available kernels and their parameters. Editing grub.cfg directly is discouraged; edit /etc/default/grub and the scripts in /etc/grub.d/ and then regenerate.

At boot you can press a key (often Shift or Esc) to reveal GRUB's menu, choose an older kernel, add a parameter like single for recovery mode, or open a command-line shell to type a manual boot sequence. On systems where something has gone wrong—a broken initramfs, a missing filesystem, a corrupted root partition—GRUB is often the last tool available before you reach for rescue media.