Editing GNU GRUB (section)

=== Identifying partitions (UUID workaround) ===
A computer can have multiple hard disks connected to it. These could be identified via their SATA port. Each time the computer [[Power-on self-test|POSTs]], the hard disk connected to a specific motherboard port ''could'' be assigned the same identifier, for example {{code|hd0, hd1, …}}. But what if such consistency cannot be guaranteed? What if the constellation of connected hard disks changed from one start up to another? What if a hard disk will be connected to another computer?

By entering {{code|ls}} into either the ''GRUB rescue console'' (available after loading <code>core.img</code> ) or the ''GRUB console'' (available after loading <code>normal.mod</code>) a list of all available hard disks and partitions can be obtained. For example, {{code|ls (hd0,5)/}}) will show numbers that can be assigned to actual hard disks and partitions.

As it cannot be guaranteed that the "<code>hd0"</code>style numbering of hard disks via device numbers is consistent, GNU GRUB can use a [[Universally Unique Identifier]] (UUID) to identify partitions (actually file system instances).

The file systems ext2, ext3, ext4 and xfs use a UUID to uniquely identify an instance. The UUID is created when a partition is formatted. The UUID is part of the file system and written to the [[Superblock (file system)|superblock]]. All operations other than formatting should leave the UUID unaltered. It is possible to change the UUID or duplicate it by using [[dd (Unix)|dd]] to clone an entire partition.

The file {{code|grub.cfg}} is used to configure GRUB. It contains commands to be executed during each start-up. Without an existing and valid {{code|grub.cfg}}, GRUB will present a prompt.

An absolute minimal {{code|grub.cfg}} might contain only the following two commands (cf. [[initial ramdisk]]):
<pre>
linux (hd0,1)/kernel/vmlinuz-3.20.1-4 ro  # use the file name "vmlinuz-…" located in the directory /kernel on the first partition of the first hard disk as linux kernel image
initrd (hd0,1)/boot/initrd.img-3.20.1-4   # use the file named "initrd.img–…" located in the directory /boot on the first partition of the first hard disk as initial ramdisk
</pre>

A fancier {{code|grub.cfg}} will describe a menu to be presented, use multiple colors, and may specify a background picture.