What is GRUB ?

What is GRUB ?

GRUB (GRand Unified Bootloader) : A boot loader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to an operating system kernel software. The kernel, in turn, initializes the rest of the operating system. GRUB is a very powerful boot loader, which can load a wide variety of free operating systems, as well as proprietary operating systems with chain-loading (chain-load is the mechanism for loading unsupported operating systems by loading another boot loader. It is typically used for loading DOS or Windows). One of the important features in GRUB is flexibility; GRUB understands filesystems and kernel executable formats, so you can load an arbitrary operating system the way you like, without recording the physical position of your kernel on the disk.

Features:

1. Support multiple filesystem types like, ext2/3/4, DOS, FAT12/16/32, NTFS, ReiserFS and many more.
2. Suppoerts automatic & transparent decompression of files which were compressed by gzip.
3. Support reading data from any or all floppies or hard disk(s) recognized by the BIOS, independent of the setting of the root device.
4. Detect all installed RAM.
5. Support network booting, like loading OS images from a network by using the TFTP protocol etc.


Devices naming conventions:

"(fd0)" - Floppy Disk - 0 is the device number, it starts with "0 to 1,2,3 etc."

"(hd0,msdos2)" - Hard Disk  - This expression means the second partition of the first hard disk drive. In this case, GRUB uses one partition of the disk, instead of the whole disk.

"(hd0,msdos5)" - This specifies the first extended partition of the first hard disk drive. Note that the partition numbers for extended partitions are counted from ‘5’, regardless of the actual number of primary partitions on your hard disk.

"(hd0,msdos1)/vmlinuz" - This specifies the file named ‘vmlinuz’, found on the first partition of the first hard disk drive.

Grub Errors:

1 : Filename must be either an absolute filename or blocklist
This error is returned if a file name is requested which doesn't fit the syntax/rules.

2 : Bad file or directory type
This error is returned if a file requested is not a regular file, but something like a symbolic link, directory, or FIFO.

3 : Bad or corrupt data while decompressing file
This error is returned if the run-length decompression code gets an internal error. This is usually from a corrupt file.

4 : Bad or incompatible header in compressed file
This error is returned if the file header for a supposedly compressed file is bad.

5 : Partition table invalid or corrupt
This error is returned if the sanity checks on the integrity of the partition table fail. This is a bad sign.

6 : Mismatched or corrupt version of stage1/stage2
This error is returned if the install command is pointed to incompatible or corrupt versions of the stage1 or stage2. It can't detect corruption in general, but this is a sanity check on the version numbers, which should be correct.

7 : Loading below 1MB is not supported
This error is returned if the lowest address in a kernel is below the 1MB boundary. The Linux zImage format is a special case and can be handled since it has a fixed loading address and maximum size.

8 : Kernel must be loaded before booting
This error is returned if GRUB is told to execute the boot sequence without having a kernel to start.

9 : Unknown boot failure
This error is returned if the boot attempt did not succeed for reasons which are unknown.

10 : Unsupported Multiboot features requested
This error is returned when the Multiboot features word in the Multiboot header requires a feature that is not recognized. The point of this is that the kernel requires special handling which GRUB is likely unable to provide.

11 : Unrecognized device string
This error is returned if a device string was expected, and the string encountered didn't fit the syntax/rules listed in the section Filesystem syntax and semantics.

12 : Invalid device requested
This error is returned if a device string is recognizable but does not fall under the other device errors.

13 : Invalid or unsupported executable format
This error is returned if the kernel image being loaded is not recognized as Multiboot or one of the supported native formats (Linux zImage or bzImage, FreeBSD, or NetBSD).

14 : Filesystem compatibility error, cannot read whole file
Some of the filesystem reading code in GRUB has limits on the length of the files it can read. This error is returned when the user runs into such a limit.

15 : File not found
This error is returned if the specified file name cannot be found, but everything else (like the disk/partition info) is OK.

16 : Inconsistent filesystem structure
This error is returned by the filesystem code to denote an internal error caused by the sanity checks of the filesystem structure on disk not matching what it expects. This is usually caused by a corrupt filesystem or bugs in the code handling it in GRUB.

17 : Cannot mount selected partition
This error is returned if the partition requested exists, but the filesystem type cannot be recognized by GRUB.

18 : Selected cylinder exceeds maximum supported by BIOS
This error is returned when a read is attempted at a linear block address beyond the end of the BIOS translated area. This generally happens if your disk is larger than the BIOS can handle (512MB for (E)IDE disks on older machines or larger than 8GB in general).

19 : Linux kernel must be loaded before initrd
This error is returned if the initrd command is used before loading a Linux kernel. Similar to the above error, it only makes sense in that case anyway.

20 : Multiboot kernel must be loaded before modules
This error is returned if the module load command is used before loading a Multiboot kernel. It only makes sense in this case anyway, as GRUB has no idea how to communicate the presence of location of such modules to a non-Multiboot-aware kernel.

21 : Selected disk does not exist
This error is returned if the device part of a device- or full file name refers to a disk or BIOS device that is not present or not recognized by the BIOS in the system.

22 : No such partition
This error is returned if a partition is requested in the device part of a device- or full file name which isn't on the selected disk.

23 : Error while parsing number
This error is returned if GRUB was expecting to read a number and encountered bad data.

24 : Attempt to access block outside partition
This error is returned if a linear block address is outside of the disk partition. This generally happens because of a corrupt filesystem on the disk or a bug in the code handling it in GRUB (it's a great debugging tool).

25 : Disk read error
This error is returned if there is a disk read error when trying to probe or read data from a particular disk.

26 : Too many symbolic links
This error is returned if the link count is beyond the maximum (currently 5), possibly the symbolic links are looped.

27 : Unrecognized command
This error is returned if an unrecognized command is entered into the command-line or in a boot sequence section of a configuration file and that entry is selected.

28 : Selected item cannot fit into memory
This error is returned if a kernel, module, or raw file load command is either trying to load its data such that it won't fit into memory or it is simply too big.

29 : Disk write error
This error is returned if there is a disk write error when trying to write to a particular disk. This would generally only occur during an install of set active partition command.

30 : Invalid argument
This error is returned if an argument specified to a command is invalid.

31 : File is not sector aligned
This error may occur only when you access a ReiserFS partition by block-lists. In this case, you should mount the partition with the `-o notail' option.

32 : Must be authenticated
This error is returned if you try to run a locked entry. You should enter a correct password before running such an entry.