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<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
<!ENTITY % general-entities SYSTEM "../general.ent">
%general-entities;
]>
<sect1 id="ch-bootable-grub" role="wrap">
<?dbhtml filename="grub.html"?>
<sect1info condition="script">
<productname>grub</productname>
<productnumber>&grub-version;</productnumber>
<address>&grub-url;</address>
</sect1info>
<title>Using GRUB to Set Up the Boot Process</title>
<sect2>
<title>Introduction</title>
<warning><para>Configuring GRUB incorrectly can render your system
inoperable without an alternate boot device such as a CD-ROM. This
section is not required to boot your LFS system. You may just
want to modify your current boot loader, e.g. Grub-Legacy, GRUB2, or
LILO.</para></warning>
<para> Ensure that an emergency boot disk is ready to <quote>rescue</quote>
the computer if the computer becomes unusable (un-bootable). If you do not
already have a boot device, you can create one. In order for the procedure
below to work, you need to jump ahead to BLFS and install
<userinput>xorriso</userinput> from the <ulink
url="http://www.linuxfromscratch.org/blfs/view/svn/multimedia/libisoburn.html">
libisoburn</ulink> package.</para>
<screen role="nodump"><userinput>cd /tmp &&
grub-mkrescue --output=grub-img.iso &&
xorriso -as cdrecord -v dev=/dev/cdrw blank=as_needed grub-img.iso</userinput></screen>
</sect2>
<sect2>
<title>GRUB Naming Conventions</title>
<para>GRUB uses its own naming structure for drives and partitions in
the form of <emphasis>(hdn,m)</emphasis>, where <emphasis>n</emphasis>
is the hard drive number and <emphasis>m</emphasis> is the partition
number. The hard drive number starts from zero, but the partition number
starts from one for normal partitions and five for extended partitions.
Note that this is different from earlier versions where
both numbers started from zero. For example, partition <filename
class="partition">sda1</filename> is <emphasis>(hd0,1)</emphasis> to
GRUB and <filename class="partition">sdb3</filename> is
<emphasis>(hd1,3)</emphasis>. In contrast to Linux, GRUB does not
consider CD-ROM drives to be hard drives. For example, if using a CD
on <filename class="partition">hdb</filename> and a second hard drive
on <filename class="partition">hdc</filename>, that second hard drive
would still be <emphasis>(hd1)</emphasis>.</para>
<para>You can determine what GRUB thinks your disk devices are by running:</para>
<screen role="nodump"><userinput>grub-mkdevicemap --device-map=device.map
cat device.map</userinput></screen>
</sect2>
<sect2>
<title>Setting Up the Configuration</title>
<para>GRUB works by writing data to the first physical track of the
hard disk. This area is not part of any file system. The programs
there access GRUB modules in the boot partition. The default location
is /boot/grub/.</para>
<para>The location of the boot partition is a choice of the user that
affects the configuration. One recommendation is to have a separate small
(suggested size is 100 MB) partition just for boot information. That way
each build, whether LFS or some commercial distro, can access the same boot
files and access can be made from any booted system. If you choose to do
this, you will need to mount the separate partition, move all files in the
current <filename class="directory">/boot</filename> directory (e.g. the
linux kernel you just built in the previous section) to the new partition.
You will then need to unmount the partition and remount it as <filename
class="directory">/boot</filename>. If you do this, be sure to update
<filename>/etc/fstab</filename>.</para>
<para>Using the current lfs partition will also work, but configuration
for multiple systems is more difficult.</para>
<para>Using the above information, determine the appropriate
designator for the root partition (or boot partition, if a separate
one is used). For the following example, it is assumed that the root
(or separate boot) partition is <filename
class="partition">sda2</filename>.</para>
<para>Install the GRUB files into <filename
class="directory">/boot/grub</filename> and set up the boot track:</para>
<warning>
<para>The following command will overwrite the current boot loader. Do not
run the command if this is not desired, for example, if using a third party
boot manager to manage the Master Boot Record (MBR).</para>
</warning>
<screen role="nodump"><userinput>grub-install /dev/sda</userinput></screen>
<!-- This does not seem to be true any more
<note><para><application>grub-install</application> is a script and calls another
program, grub-probe, that may fail with a message "cannot stat `/dev/root'".
If so, create a temporary symbolic link from your root partition to /dev/root:</para>
<screen role="nodump"><userinput>ln -sv /dev/sda2 /dev/root</userinput></screen>
<para>The symbolic link will only be present until the system is rebooted.
The link is only needed for the installation procedure.
</para></note>
-->
</sect2>
<sect2>
<title>Creating the Configuration File</title>
<para>Generate <filename>/boot/grub/grub.cfg</filename>:</para>
<screen><userinput>cat > /boot/grub/grub.cfg << "EOF"
<literal># Begin /boot/grub/grub.cfg
set default=0
set timeout=5
insmod ext2
set root=(hd0,2)
menuentry "GNU/Linux, Linux &linux-version;-lfs-&version;" {
linux /boot/vmlinuz-&linux-version;-lfs-&version; root=/dev/sda2 ro
}</literal>
EOF</userinput></screen>
<note><para>From <application>GRUB</application>'s perspective, the
kernel files are relative to the partition used. If you
used a separate /boot partition, remove /boot from the above
<emphasis>linux</emphasis> line. You will also need to change the
<emphasis>set root</emphasis> line to point to the boot partition.
</para></note>
<para>GRUB is an extremely powerful program and it provides a tremendous
number of options for booting from a wide variety of devices, operating
systems, and partition types. There are also many options for customization
such as graphical splash screens, playing sounds, mouse input, etc. The
details of these options are beyond the scope of this introduction.</para>
<caution><para>There is a command, <application>grub-mkconfig</application>, that
can write a configuration file automatically. It uses a set of scripts in
/etc/grub.d/ and will destroy any customizations that you make. These scripts
are designed primarily for non-source distributions and are not recommended for
LFS. If you install a commercial Linux distribution, there is a good chance
that this program will be run. Be sure to back up your grub.cfg file.</para></caution>
</sect2>
</sect1>
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