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GRUB bootloader - Full tutorial

Updated: September 13, 2007; Nov 30, 2017

This tutorial focuses on GRUB legacy. If y'all're looking for a tutorial on Grub 2, the next generation of the pop bootloader, delight take a expect at this article.

One of the most frightening things about Linux is the horrible word bootloader. The primary reason for this is the fact that most new Linux users accept but always used Windows operating systems. In the Windows world, they have never bothered with bootloaders. For them, the issue of a system booting was a transparent one. At well-nigh, they would use Windows Recovery Console to fix issues for them. Thus, they accept been spared the need to learn about the unmarried most important piece of software on a computer - the fiddling program that makes it all work.

This article is supposed to provide you with basic understanding of the Grub bootloader. If you lot take read my other Linux manufactures, you are familiar with partitioning and Linux notation, as well as with the command line. The next logical step is to heighten this knowledge by taking i step further. Agreement how GRUB works and what boot entries in the boot menu mean volition help you understand how the operating systems piece of work, how to set up, recover or modify the GRUB menu to suit your needs, and how to setup different piece of work environments with several operating systems.

In the past, we have relied on the installation setups to make the hard work for u.s.a.. In fact, setting up Chow, while frightening, is a quite unproblematic and fully reversible process. Mastering the GRUB is a very important step in building up the confidence to apply Linux. So if you are prepare, read on.

But first of all, a foreword of wisdom. This commodity is a compilation of sources and examples that will help you learn about Grub. Of course, it's all out in that location somewhere, on the Net. However, new Linux users will probably find the notion of spending hours searching for relevant pieces of information (particularly if their PC won't boot) somewhat frustrating. The goal of this guide is to help provide uncomplicated and quick solutions to near mutual problems regarding multi-kick setups and installation of Linux operating systems.

Table of contents

1. Everything is a file
2. Introduction
   1. What about LILO?
3. How does GRUB piece of work?
4. GRUB notation
5. Grub configuration
   1. Installation of Grub
      1. GRUB on a floppy disk
      2. Installing GRUB natively
      3. Installing GRUB with grub-install
   2. Setting up GRUB manually (later on installation)
      1. Backup!
      2. Adding a new operating system to the Grub menu
6. Mutual problems
   1. GRUB got deleted; how to restore?
      1. The easy manner
      2. The hard way
   2. Windows is installed on a non-starting time hard disk (Swapping)
   3. There is more than one Windows operating system installed on one hd (Hide/Unhide)
   4. You lot resized a sectionalisation; GRUB is gone
7. Useful tools
8. Extras for special geeks
9. Links
   1. My other articles
   2. General information (by alphabetic order)

Everything is a file

To be able to successfully main the secrets of Chow, you lot must empathize 1 of the basic foundations of *Null-based operating systems. Everything is a file. Fifty-fifty hard disks and partitions are treated as files. There is no magic. If y'all remember this, y'all volition find the supposedly perilous task of tampering with partitions no dissimilar than playing with files using a file explorer (or the command line). At present that we take established this, we can move on to the more than technical parts of grubbing.

Introduction

GNU Grub is a bootloader (can as well be spelled boot loader) capable of loading a multifariousness of free and proprietary operating systems. Grub will work well with Linux, DOS, Windows, or BSD. GRUB stands for GRand Unified Bootloader.

Grub is dynamically configurable. This means that the user tin can make changes during the boot time, which include altering existing boot entries, adding new, custom entries, selecting different kernels, or modifying initrd. GRUB likewise supports Logical Block Accost fashion. This means that if your computer has a adequately modern BIOS that can access more than 8GB (showtime 1024 cylinders) of hard disk space, GRUB will automatically exist able to admission all of it.

GRUB can be run from or be installed to any device (floppy deejay, hard disk, CD-ROM, USB drive, network drive) and can load operating systems from just as many locations, including network drives. It tin can also decompress operating system images before booting them. Y'all tin can learn much, much more from the official GNU GRUB Manual 0.97.

What nearly LILO?

You may take heard about another Linux bootloader called LILO (stands for LInux LOader). While a sensible option for many Linux users, I believe that GRUB is a better choice, for several reasons:

• LILO supports only up to 16 different boot selections; GRUB supports an unlimited number of kick entries.
• LILO cannot boot from network; GRUB can.
• LILO must be written again every fourth dimension you change the configuration file; Grub does not.
• LILO does not have an interactive command interface.

All in all, information technology seems that GRUB is the winner. Then let's see what this baby tin practice.

How does Chow work?

When a calculator boots, the BIOS transfers control to the starting time boot device, which can exist a hard disk drive, a floppy disk, a CD-ROM, or any other BIOS-recognized device. We'll concentrate on hard disks, for the sake of simplicity.

The first sector on a hard is called the Master Boot Tape (MBR). This sector is just 512 bytes long and contains a minor piece of code (446 bytes) called the primary boot loader and the partition table (64 bytes) describing the chief and extended partitions.

By default, MBR lawmaking looks for the segmentation marked as agile and in one case such a partition is found, it loads its kick sector into retentiveness and passes command to it.

GRUB replaces the default MBR with its ain code.

Furthermore, Chow works in stages.

Stage one is located in the MBR and mainly points to Stage 2, since the MBR is too small to contain all of the needed data.

Stage 2 points to its configuration file, which contains all of the complex user interface and options we are normally familiar with when talking about Chow. Stage 2 can be located anywhere on the disk. If Stage 2 cannot observe its configuration tabular array, GRUB volition cease the boot sequence and present the user with a command line for manual configuration.

Stage ane.5 likewise exists and might be used if the boot information is pocket-sized enough to fit in the area immediately after MBR.

The Phase architecture allows Chow to be large (~xx-30K) and therefore fairly complex and highly configurable, compared to most bootloaders, which are sparse and simple to fit within the limitations of the Partition Table.

GRUB note

This section is intended to help yous get familiar with Grub, without touching anything. The next section deals with bodily files and manual configuration. GRUB has its own notation, which is very similar even so somewhat different from the general notation a typical Linux user might exist used to. Now, here's an instance of a typical GRUB entry:

(hd0,1)

• The brackets are a must; all devices listed in Chow carte du jour must be enclosed in brackets.
• hd stands for hard disk; alternatively, fd stands for floppy disk, cd stands for CD-ROM etc.
• The first number (integer for geeks) refers to the physical difficult drive number; in this case, the first drive, as they are counted from zero up. For example, hd2 refers to the third physical hard drive.
• The 2nd number refers to the sectionalisation number of the selected hard drive; again, partitions are counted from zero up. In this case, i stands for the 2nd sectionalization.

From here, it is axiomatic that GRUB (menu) does not discriminate between IDE or SCSI drives or main or logical partitions. The chore of deciding which difficult bulldoze or partition may boot is left to BIOS and Stage one. As you run into, the notation is very simple.

Main partitions are marked from 0 to 3 (hard disk?,0), (hd?,1), (hard disk drive?,2), (hd?,three). Logical partitions in the extended segmentation are counted from 4 up, regardless of the bodily number of chief partitions on the hard disk drive, east.chiliad. (hd1,seven).

The entries alone are not enough to boot an operating system though. GRUB too needs to know what operating organisation images to load. These are assigned as parameters to each of the chosen devices, including special flags (switches). For example, Windows Rubber Mode is a special flag. Here is an instance of a Chow menu booting only Ubuntu.

Hither'due south an instance of a Chow menu from a reckoner with triple boot (SUSE 10.2, Ubuntu 6.10 and Windows XP). This is the actual menu.lst on i of my real machines.

Allow'southward try to understand what the entries mean. Simply uncommented lines count. Comments are marked with #. The lines that have a series of small-scale gray tens showing are in indication that text therein belongs to the line preceding it. In other words, there text simply dropped to another line to visually conform the text editor and screen resolution limitations.

default 0
timeout 8

The commencement line (default 0) ways that the first operating system listed will be booted. In this case, it's SUSE 10.2. The second line (timeout 8) tells how much time (seconds) the user has to make his pick before the default entry is loaded. Simple, isn't it?

gfxmenu (hd0,two)/boot/bulletin

The Chow menu can besides be graphic. The fancy stuff needed to nowadays the user with a colorful background and mayhap some extras is located on the starting time physical disk, third division (hd0,2). This is a chief partition, every bit we have seen earlier.

championship openSUSE 10.two
root (hd0,2)
kernel /kick/vmlinuz-...
initrd /boot/initrd.img-...

This is the commencement operating system entry in the carte du jour.

• title is as elementary as it sounds. It'due south merely a cord that'due south meant to help the user read the menu in human terminology.
• root (hd0,2) tells GRUB where its configuration files are located. In this case, they tin can exist establish under (hd0,two)/kick/grub.
• kernel /kicking/vmlinuz-... boots the actual kernel image. There can be many such images bachelor. The fact there is no device specified before the /boot/vmlinuz indicates the epitome is located on the same partition as the Chow itself. This is oftentimes the default case for your primary selection of operating arrangement.
• initrd /boot/initrd.img-... is the temporary file arrangement that makes system preparations - adapts generic kernel image to specific hardware - before the existent root is loaded.

The extra switches used subsequently the kernel indicate where the actual root is located, what graphic style is used and where the bandy partition resides. This article volition not go into detail regarding the more advanced GRUB configurations.

title Ubuntu, kernel ... (/dev/sda9)
kernel (hd0,eight)/boot/vmlinuz-...
initrd (hd0,8)/boot/initrd.img-...

• Once more, title indicates a name, in this case fancily adorned with technical details.
• kernel (hd0,eight)/boot/vmlinuz-... points to the 9th sectionalization on the first hard disk (hd0,viii). Accidentally, the root flag (/dev/sda9) indicates that the root partition is the aforementioned every bit the one containing the kernel image. Usually, this is the example, and for the sake of simplicity, you will want this option during your installations.
• initrd (hd0,8)/boot/initrd.img-... nix new hither.

Note regarding kernel images and root partitions: On older computers with BIOS that practice non support access to more than the start 1024 cylinders, y'all might setup a kicking partition that contains the kernel image, while the root itself is located elsewhere. But people with computers younger than the botched Y2K crunch need non worry.

Another thing you lot might notice is that the Ubuntu entry is fairly detailed. This characteristic is called Multi-boot Compliance; openSUSE recognizes Ubuntu and tin accurately call its images (including special switches) and mount the partitions. Withal, well-nigh operating systems are only partially multi-boot compliant. A little after on, I volition show you how the Ubuntu entry could have been treated differently, with the aforementioned results.

title Windows
rootnoverify (hd0,0)
chainloader (hd0,0)+i

• rootnoverify (hd0,0) ways that openSUSE cannot sympathize Windows operating arrangement, i.e. no multi-kicking compliance. Therefore, the operating system is called without whatsoever fore-knowledge of the kernel. GRUB assumes that the relevant boot images will be constitute on the target partition and mounted past the other operating system bootloader. Every bit y'all can see, Windows was installed on the beginning partition of the starting time difficult deejay. This is the well-nigh convenient option.
• chainloader (hd0,0)+one feature is used for operating systems that cannot be booted straight. Not surprisingly, Windows operating systems cannot be booted directly. They are booted by the method of chainloading. As the name implies, Grub passes the control of the boot sequence to another bootloader, located on the device to which the menu entry points. This can be a Windows operating organization, but also any other, including Linux.

Back to Ubuntu. You remember that I accept told nosotros could have booted Ubuntu in an alternative style? Indeed, we could have simply chainloaded it, just like Windows.

title Ubuntu
root (hd0,8)
chainloader (hd0,8)+1

This would take worked equally well. The last choice in the menu allows yous to boot SUSE in the failsafe mode (sort of Safe Way, no graphics) and does non contribute to our knowledge, therefore we will skip information technology at the moment. Let'southward see some more than examples. The example below is really written in the Ubuntu menu.lst (commented, of form).

championship Windows 95/98/NT/2000
root (hd0,0)
makeactive
chainloader +1

championship Linux
root (hd0,1)
kernel /vmlinuz root=/dev/hda3 ro

Past now, you should be able to "read" Grub language with ease. We brainstorm with the championship. root specifies the sectionalisation where we expect to discover the Windows kernel and mounts it (rootnoverify would leave this chore to the Windows bootloader). makeactive command sets the active partition on the root disk (above) to Chow root device. This ways that the next command, chainloader, is executed without the target partition specified (equally the target partition is the aforementioned, at present).

The second entry is fifty-fifty simpler. We name a Linux, we telephone call its partition and we boot the kernel. In this case, we see the very interesting example where the kernel epitome and root (/) partition of the operating system are Non located on the aforementioned partition. This would be a very likely case for older computers - or ones with a specific boot partition. Finally, the crazy stuff: A GRUB carte du jour booting 100+ systems of DOS, Windows, Linux, BSD, and Solaris.

Yous see, it's very simple! Once you get the hang of information technology, information technology actually becomes fun. For more data, you must read the GNU Grub Manual 0.97. In particular, yous need to familiar with The list of control line and menu entry commands.

GRUB configuration

Most of the time, yous will not want or need to touch GRUB. When you install operating systems, especially in the right sequence (inconsiderate OS showtime - like Windows, flexible Bone later - Linux and family), the Chow will be installed automatically and relevant entries appended.

Nonetheless, y'all should know when and what to do if things go wrong.

Installation of Grub

Chow can be installed to a variety of devices. About people will be interested in setting upwards Chow on their hard deejay. Still, it does not injure to know learn about other options. Of class, you tin can skip frontwards. Before we install annihilation, nosotros need to know where our files are. If installed, Grub carte is located on the root partitioning under:

/boot/grub/menu.lst

E'er, always back this file up before making whatsoever changes! The Chow files can exist found in the image of your operating system (usually a CD), under:

/usr/lib/grub/i386-pc

You should also be aware of the basic Linux commands regarding the hard disks and partitions. To this end, y'all might desire to try my commodity Highly useful Linux commands & configurations. Nevertheless, for those who do not fancy reading yet another article, here's a brief version:

You lot can display your PC environment data either through final or a text editor. Files through text editor are invoked by specifying the text editor and a target file (with su or sudo for arrangement files).

sudo six /etc/fstab
OR
sudo gedit /etc/fstab

Alternatively, you can simply print the contents of the files in the last by using the cat (concatenate) control.

(sudo) cat /etc/fstab

Some useful commands that one might need when dealing with boot, hard drives and partitions: Brandish the segmentation table:

fdisk -l

Display the mounted partitions:

cat /etc/fstab

At present that nosotros know what we demand and where to notice information technology, it's fourth dimension to install Grub.

GRUB on a floppy deejay

To create Chow on a floppy deejay, we must copy the Stage 1 and Stage 2 files from the paradigm directory to the first and second sector of the floppy disk. Nosotros'll employ the dd command, which copies information sector past sector. Hither's the prepare of commands that you need to execute (taken from the Manual):

cd /usr/lib/grub/i386-pc
dd if=stage1 of=/dev/fd0 bs=512 count=1
dd if=stage2 of=/dev/fd0 bs=512 seek=one

Only a curt explanation: if stands for input file, of stands for output file, /dev/fd0 refers to the beginning mounted floppy device, bs specifies the block size (in bytes), count instructs how many blocks should be copied, and seek tells how many blocks should be skipped before writing. Not surprisingly, since we demand to write the files to the first two sectors of the floppy disk, we write stage1 to the first sector, and copy stage2 to the second (skip kickoff, then write). At present that we accept taken this deep fancy for the control line, I'll get out you lot with the nuances of dissimilar commands for homework. More on dd on Wikipedia.

Regarding the GRUB on floppy, that's it. Since we have copies the stages from OS epitome, all of the required data is there. Of course, you can always brand manual adjustments. Simply that's the next chapter.

Installing Grub natively

Native install ways placing Grub Phase ane in the first sector of the difficult disk (MBR or Partition Table). This means you will be able to boot without a secondary device, like a floppy deejay (which have go a rarity nowadays). Still, this as well means that if you lot install an inconsiderate OS subsequently on (similar Windows) or try to repair the MBR for some reason (by running fdisk /MBR from DOS prompt), you will erase the Grub and return all systems listed in the menu.lst unbootable.

To install GRUB in MBR, you will demand to kick from external media (floppy, live Linux CD). Then, one time you lot reach the Grub prompt, execute the following commands:

Detect the GRUB root device:

observe /kicking/grub/stage1

Chow will search for all available Stages one and present them. If yous take more than one operating system image present (due east.thousand. SUSE, Kubuntu, Mandriva), you will have more than than i stage1 available. Example - Let's say the calculator has the post-obit operating systems installed on different partitions:

• SUSE on (hd0,one)
• Kubuntu on (hd0,ii)
• Mandriva on (hd0,4)

All these will exist returned as potential roots for Grub device (as each Os has its own files). If you wish to use SUSE Grub, then you will setup the Grub root device to (hd0,1):

root (hd0,1)

If you want Mandriva's Chow, then:

root (hd0,4)

If yous know in advance what yous want to do, yous tin skip the find command. Once you take decided on the root, you need to write the data to the MBR:

setup (hd0)

Finally, quit the Grub prompt:

quit

As a sequence, the commands that you demand are:

detect /boot/grub/stage1 (optional)
root (hdX,Y)
setup (hd0)
quit

You can besides setup Chow on another drive or partition, but then you will have to chainload Chow to another bootloader for this to work. That's it. Like shooting fish in a barrel peasy orangish squeasy!

Installing GRUB with chow-install

This method is considered less safe (co-ordinate to the Manual), every bit information technology guesses the mapping. However, for total newbs in need of dire assistance, this might exist the preferred method. Yous only need to invoke a single command - namely, where to install the bootloader. Furthermore, this command tin can be written in several ways, all equivalent:

grub-install /dev/hda

grub-install /dev/hd0

grub-install '(hd0)'

chow-install hd0

Later y'all take installed the Grub, your operating systems should boot. Once booted, y'all can once once again start playing with GRUB, manually irresolute settings - adding and removing entries, chainloading other bootloaders, or even hiding and unhiding partitions.

Setting upward Chow manually (afterward installation)

You can reconfigure or reinstall Grub at any moment.

Fill-in!

First, before y'all make any changes to the Grub configuration file, it is almost warmly recommended that you fill-in the existing menu. You might even consider copying to another car or printing the bill of fare, in case things go bunkerous.

cp /kick/grub/menu.lst /boot/chow/bill of fare.lst_backup

To access GRUB, execute the following command in Linux terminal:

sudo chow

After a few moments, the GRUB command line should show up. You can place it past the chow> prompt.

Alternatively, you can reach the Grub command line during bootup. When the GRUB bill of fare loads, press C on the keyboard.

Adding a new operating system to the GRUB menu

Let's say yous have installed yet another operating system on your machine, Sabayon. During the installation, yous skip the Chow setup. This means that the original GRUB remains untouched - and information technology does non incorporate an entry nigh Sabayon. For all applied purposes, Sabayon is non bootable.

Annotation: Chow menu entries are called stanzas (probably a twist of instance?). And so, we need to add Sabayon to the list. If you know where Sabayon is installed, y'all just need to add its entry to the menu.lst.

title Sabayon
root (hdX,Y)
chainloader +1

That'south it. Y'all can also do this while booting the computer, without editing the menu. When the original GRUB comes up, press C to attain the command line. And then:

root (hdX,Y)
chainloader +1

Optionally, yous will apply the find command to get around. Basically, this is the whole of GRUB magic. As you can see, information technology'due south very very simple. Merely for people who have never heard of Chow and see long lists of strange commands, the prospect tin be daunting. Now that we take covered the basics of grubbing, it'southward time for extras and some more common problems.

Common problems

GRUB got deleted; how to restore?

This will often happen if y'all install Windows later on Linux. Windows assumes information technology's the only operating system in the world and does non endeavor to live with existing data present in the MBR; it overwrites information technology. For people with dual-boot and very little cognition of Linux, this is a disaster. Luckily, it's very easily recoverable i.

The easy way

Use Super Chow Disk. I have written most this tool in detail in my article A (cool) listing of Linux tools, under Rescue.

The difficult manner

• Boot from floppy deejay or CD (whatsoever Linux live CD should practice).
• Get to the grub command line.

find /boot/grub/stage1 (optional)
root (hdX,Y)
setup (hd0)
quit

The aforementioned every bit before!

Windows is installed on a non-first hard disk drive (Swapping)

Chow cannot directly boot Microsoft operating systems. And sometimes, even chainloading may not work, particularly if Windows is not installed on the first hd. One time again, you should retrieve that you should ever install Windows first, on the first difficult disk, on the showtime partition (the dominion of three showtime). Nevertheless, even if you lot have Windows installed on a dissever disk, you can solve the problem past swapping. You need to perform a virtual swap between hard disks. Similar this:

map (hd0) (hd1)
map (hd1) (hd0)

Later on you add these ii lines, you should be able to boot into Windows (or DOS, for that thing).

In that location is more than ane Windows operating system installed on i hard disk (Hide/Unhide)

Again, Windows can crusade problems if there's more than than ane instance present on a difficult disk, especially if installed on principal partitions. When y'all use the chainloader command, the command is transferred to the Windows boot loader, simply which one?

The problem is easily solved by hiding and unhiding partitions. If GRUB hides a partition, it volition not be visible by Windows and prevent a possible confusion. Vice versa, if you lot unhide a partitioning, Windows will detect information technology and be able to boot from it, without getting confused.

Hither's the example taken from the Manual that demonstrates this signal. Allow's say we have Windows installed on the first and the second division of your difficult deejay. We wish to boot the get-go instance of Windows. And then, nosotros need to execute the following set up of commands to brand information technology all work:

unhide (hd0,0)
hide (hd0,i)
rootnoverify (hd0,0)
chainloader +i
makeactive

You resized a sectionalization; GRUB is gone

This is an interesting case. It can happen later on you utilise a division or an imaging software to brand changes to the Partition Table. Usually, this volition happen when you brand such a change in Windows. Linux volition not be informed of the alter, because Windows is blind to anything else on the machine relieve Microsoft thingies. Consequently, Chow volition suffer.

The solution is that almost likely the filesystem is damaged and needs to exist repaired. Boot from a live CD and execute the following commands, assuming yous know where Linux partitions are. Check the filesystem:

fsck.ext2 /dev/hdXY

Replace 10 and Y with hd letter and partitioning number that you lot want to check (hda2, hdb3, etc.). Create the ext2/three filesystem parameters once again.

tune2fs -j /dev/hdXY

Now mount the root filesystem on /mnt/sysimage and run grub:

mount -t ext2 /dev/hdXY /mnt/sysimage
OR
mount -t ext2 /dev/hdXY /mnt/sysimage

cd /mnt/sysimage/sbin
grub

You're dorsum to the familiar grounds. For more information on Linux commands, including fsck.ext2, tune2fs and others, meet Manual pages on Linux Reviews.

Finally, some more tricky questions answered in Ubuntu wiki:

How to restore GRUB menu later on Windows installation

How to identify the name of the boot drive (hd0, hd1, hd2, etc)

How to add Windows entry into Chow menu

How to boot into Windows installed on a split up SATA drive

Useful tools

GParted

GParted is a Gnome Partitioning Editor. Although Linux-based, it fully supports the NTFS file organization. Information technology runs from a CD.

SystemRescueCD

This is a live CD specifically geared toward rescue and recovery. The tools packet includes some of the nigh important tools available for Linux user, similar GParted, QTParted, Partimage, Grub, Lilo, sfdisk, security tools, network tools, and more.

Super Chow Deejay

Super Grub Disk is intended to run from a floppy disk or CD and is used for organization rescue. Nigh chiefly, information technology tin be used to restore boot loaders, including GRUB, LILO and even Windows boot loader.

TestDisk

TestDisk is a utility particularly designed to recover lost partitions and make non-booting disks bootable over again. This highly useful tool tin be constitute on a number of alive CDs, including Knoppix and GParted. Information technology supports NTFS partitions and besides comes in Windows flavor.

Extras for special geeks

Multiboot with GRUB Mini-HOWTO

How To Multi-kicking Operating Systems

How to boot Ubuntu from the Windows Bootloader

Filesystems and Mounting

Links:

GNU GRUB

GNU Chow FAQ

GNU GRUB Transmission 0.97

GNU GRUB on Wikipedia

The list of command line and menu entry commands

A Chow menu booting 100+ systems of DOS, Windows, Linux, BSD, and Solaris

My other articles:

Highly useful Linux commands & configurations

A (cool) listing of Linux tools

Full general data:

cat

dd

initrd

LILO

NTLDR

Logical Block Address

Transmission pages (man) on Linux Reviews

That'due south it. Savor! P.S. If y'all find this article useful, please support Dedoimedo!

Cheers.

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Source: https://www.dedoimedo.com/computers/grub.html

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