grub: Features
1.4 GRUB features
=================
The primary requirement for GRUB is that it be compliant with the
"Multiboot Specification", which is described in ⇒Multiboot
Specification (multiboot)Top.
The other goals, listed in approximate order of importance, are:
* Basic functions must be straightforward for end-users.
* Rich functionality to support kernel experts and designers.
* Backward compatibility for booting FreeBSD, NetBSD, OpenBSD, and
Linux. Proprietary kernels (such as DOS, Windows NT, and OS/2) are
supported via a chain-loading function.
Except for specific compatibility modes (chain-loading and the Linux
"piggyback" format), all kernels will be started in much the same state
as in the Multiboot Specification. Only kernels loaded at 1 megabyte or
above are presently supported. Any attempt to load below that boundary
will simply result in immediate failure and an error message reporting
the problem.
In addition to the requirements above, GRUB has the following
features (note that the Multiboot Specification doesn't require all the
features that GRUB supports):
Recognize multiple executable formats
Support many of the "a.out" variants plus "ELF". Symbol tables are
also loaded.
Support non-Multiboot kernels
Support many of the various free 32-bit kernels that lack Multiboot
compliance (primarily FreeBSD, NetBSD(1) (⇒
Features-Footnote-1), OpenBSD, and Linux). Chain-loading of
other boot loaders is also supported.
Load multiples modules
Fully support the Multiboot feature of loading multiple modules.
Load a configuration file
Support a human-readable text configuration file with preset boot
commands. You can also load another configuration file dynamically
and embed a preset configuration file in a GRUB image file. The
list of commands (⇒Commands) are a superset of those
supported on the command-line. An example configuration file is
provided in ⇒Configuration.
Provide a menu interface
A menu interface listing preset boot commands, with a programmable
timeout, is available. There is no fixed limit on the number of
boot entries, and the current implementation has space for several
hundred.
Have a flexible command-line interface
A fairly flexible command-line interface, accessible from the menu,
is available to edit any preset commands, or write a new boot
command set from scratch. If no configuration file is present,
GRUB drops to the command-line.
The list of commands (⇒Commands) are a subset of those
supported for configuration files. Editing commands closely
resembles the Bash command-line (⇒Bash (features)Command Line
Editing.), with <TAB>-completion of commands, devices, partitions,
and files in a directory depending on context.
Support multiple filesystem types
Support multiple filesystem types transparently, plus a useful
explicit blocklist notation. The currently supported filesystem
types are "Amiga Fast FileSystem (AFFS)", "AtheOS fs", "BeFS",
"BtrFS" (including raid0, raid1, raid10, gzip and lzo), "cpio"
(little- and big-endian bin, odc and newc variants), "Linux
ext2/ext3/ext4", "DOS FAT12/FAT16/FAT32", "exFAT", "F2FS", "HFS",
"HFS+", "ISO9660" (including Joliet, Rock-ridge and multi-chunk
files), "JFS", "Minix fs" (versions 1, 2 and 3), "nilfs2", "NTFS"
(including compression), "ReiserFS", "ROMFS", "Amiga Smart
FileSystem (SFS)", "Squash4", "tar", "UDF", "BSD UFS/UFS2", "XFS",
and "ZFS" (including lzjb, gzip, zle, mirror, stripe, raidz1/2/3
and encryption in AES-CCM and AES-GCM). ⇒Filesystem, for
more information.
Support automatic decompression
Can decompress files which were compressed by 'gzip' or 'xz'(2)
(⇒Features-Footnote-2). This function is both automatic and
transparent to the user (i.e. all functions operate upon the
uncompressed contents of the specified files). This greatly
reduces a file size and loading time, a particularly great benefit
for floppies.(3) (⇒Features-Footnote-3)
It is conceivable that some kernel modules should be loaded in a
compressed state, so a different module-loading command can be
specified to avoid uncompressing the modules.
Access data on any installed device
Support reading data from any or all floppies or hard disk(s)
recognized by the BIOS, independent of the setting of the root
device.
Be independent of drive geometry translations
Unlike many other boot loaders, GRUB makes the particular drive
translation irrelevant. A drive installed and running with one
translation may be converted to another translation without any
adverse effects or changes in GRUB's configuration.
Detect all installed RAM
GRUB can generally find all the installed RAM on a PC-compatible
machine. It uses an advanced BIOS query technique for finding all
memory regions. As described on the Multiboot Specification (⇒
Multiboot Specification (multiboot)Top.), not all kernels make use
of this information, but GRUB provides it for those who do.
Support Logical Block Address mode
In traditional disk calls (called "CHS mode"), there is a geometry
translation problem, that is, the BIOS cannot access over 1024
cylinders, so the accessible space is limited to at least 508 MB
and to at most 8GB. GRUB can't universally solve this problem, as
there is no standard interface used in all machines. However,
several newer machines have the new interface, Logical Block
Address ("LBA") mode. GRUB automatically detects if LBA mode is
available and uses it if available. In LBA mode, GRUB can access
the entire disk.
Support network booting
GRUB is basically a disk-based boot loader but also has network
support. You can load OS images from a network by using the "TFTP"
protocol.
Support remote terminals
To support computers with no console, GRUB provides remote terminal
support, so that you can control GRUB from a remote host. Only
serial terminal support is implemented at the moment.
Info Catalog
grub: Changes from GRUB Legacy
grub: Introduction
grub: Role of a boot loader