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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>
<para>Boot loading can be a complex area, so a few cautionary
words are in order. Be familiar with the current boot loader and any other
operating systems present on the hard drive(s) that need to be
bootable. Make sure that an emergency boot disk is ready to
<quote>rescue</quote> the computer if the computer becomes
unusable (un-bootable).</para>
<para>The procedure involves writing some special GRUB files to specific
locations on the hard drive. We highly recommend creating a GRUB boot
floppy diskette as a backup. Insert a blank floppy diskette and run the
following commands:</para>
<screen role="nodump"><userinput>cd /tmp
grub-mkrescue --image-type=floppy floppy.img
dd if=floppy.img of=/dev/fd0 bs=1440 count=1</userinput></screen>
<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>
<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>
</sect2>
<sect2>
<title>Setting Up the Configuration</title>
<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>:</para>
<screen role="nodump"><userinput>grub-install --grub-setup=/bin/true /dev/sda</userinput></screen>
<para>We use --grub-setup=/bin/true for now to prevent updating the
Master Boot Record (MBR). In this way, we can test our installation
before committing to a change that is hard to revert.</para>
<para>Generate <filename>/boot/grub/grub.cfg</filename>:</para>
<screen role="nodump"><userinput>grub-mkconfig -o /boot/grub/grub.cfg</userinput></screen>
<para>Here <command>grub-mkconfig</command> uses the files in <filename
class="directory">/etc/grub.d/</filename> to determine the contents
of this file. The configuration file will look something like:</para>
<screen><computeroutput>#
# DO NOT EDIT THIS FILE
#
# It is automatically generated by /usr/sbin/grub-mkconfig using templates
# from /etc/grub.d and settings from /etc/default/grub
#
### BEGIN /etc/grub.d/00_header ###
set default=0
set timeout=5
### END /etc/grub.d/00_header ###
### BEGIN /etc/grub.d/10_linux ###
menuentry "GNU/Linux, Linux 2.6.34-lfs" {
insmod ext2
set root=(hd0,2)
search --no-floppy --fs-uuid --set 915852a7-859e-45a6-9ff0-d3ebfdb5cea2
linux /boot/vmlinux-&linux-version;-lfs-&version; root=/dev/sda2 ro
}
menuentry "GNU/Linux, Linux 2.6.30.2-lfs65 (recovery mode)" {
insmod ext2
set root=(hd0,2)
search --no-floppy --fs-uuid --set 915852a7-859e-45a6-9ff0-d3ebfdb5cea2
linux /boot/vmlinux-&linux-version;-lfs-&version; root=/dev/sda2 ro single
}
menuentry "GNU/Linux, Linux 2.6.28-11-server" {
insmod ext2
set root=(hd0,2)
search --no-floppy --fs-uuid --set 6b4c0339-5501-4a85-8351-e398e5252be8
linux /boot/vmlinuz-2.6.28-11-server root=UUID=6b4c0339-5501-4a85-8351-e398e5252be8 ro
initrd /boot/initrd.img-2.6.28-11-server
}
menuentry "GNU/Linux, Linux 2.6.28-11-server (recovery mode)" {
insmod ext2
set root=(hd0,2)
search --no-floppy --fs-uuid --set 6b4c0339-5501-4a85-8351-e398e5252be8
linux /boot/vmlinuz-2.6.28-11-server root=UUID=6b4c0339-5501-4a85-8351-e398e5252be8 ro single
initrd /boot/initrd.img-2.6.28-11-server
}
### END /etc/grub.d/10_linux ###
### BEGIN /etc/grub.d/30_os-prober ###
### END /etc/grub.d/30_os-prober ###
### BEGIN /etc/grub.d/40_custom ###
# This file provides an easy way to add custom menu entries. Simply type the
# menu entries you want to add after this comment. Be careful not to change
# the 'exec tail' line above.
### END /etc/grub.d/40_custom ###
</computeroutput></screen>
<note>
<itemizedlist>
<listitem><para>Even though there is a warning not to edit the file, you
can do so as long as you do not re-run
<command>grub-mkconfig</command>.</para></listitem>
<listitem><para>The <emphasis>search</emphasis> lines are generally not
useful for LFS systems as that command only sets an internal GRUB
variable used to find the kernel image. The <emphasis>set root</emphasis>
command provides the same capability without the overhead of
searching.</para></listitem>
<listitem><para>The <emphasis>set root</emphasis> and
<emphasis>insmod ext2</emphasis> commands can be moved out of the
<emphasis>menuentry</emphasis> sections to apply to all sections of the file.
This leads to a simple section like:</para></listitem>
</itemizedlist>
<screen><computeroutput>menuentry "Linux &linux-version;-lfs-&version;" {
linux /boot/vmlinux-&linux-version;-lfs-&version; root=/dev/sda2 ro
}
</computeroutput></screen>
<itemizedlist>
<listitem><para>Passing a UUID to the kernel requires an initial ram disk
(initrd) not built by LFS.</para></listitem>
<listitem><para>If the <filename>/boot</filename> partition is installed
on a separate partition, the linux and initrd lines should not have the
string <emphasis>/boot</emphasis> prefixed to the file
names.</para></listitem>
<listitem><para>In this example the kernel files for a Ubuntu
installation are also found in <filename
class="directory">/boot</filename>.</para></listitem>
</itemizedlist>
</note>
</sect2>
<sect2>
<title>Testing the Configuration</title>
<para>The core image of GRUB is also a Multiboot kernel, so if you already
have GRUB Legacy loaded you can load GRUB-&grub-version; through your old
boot loader. To accomplish this, you will need to exit the
<command>chroot</command> environment and re-enter it to finish the
few remaining portions of the book.</para>
<screen role="nodump"><userinput>/sbin/reboot
...
grub> root (hd0,1)
grub> kernel /boot/grub/core.img
grub> boot</userinput></screen>
<para>Note that the GRUB commands above are assumed to be GRUB Legacy.
At this point the GRUB prompt will appear (very similar to GRUB Legacy) and
you can explore the interface or boot to one of the systems in the grub.cfg
file.</para>
</sect2>
<sect2>
<title>Updating the Master Boot Record</title>
<para>If you tested the GRUB configuration as specified above, re-enter
the <command>chroot</command> environment.</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>
<para>Update the MBR with:</para>
<screen role="nodump"><userinput>grub-setup '<DEVICE>'</userinput></screen>
<para>Change the DEVICE above to your boot disk, normally '(hd0)' or /dev/sda.
If using (hd0) be sure to escape the parentheses with backslashes or single
quotes to prevent the shell from interpreting them as a sub-shell.</para>
<para>This program uses the following defaults and are correct if you did not
deviate from the instructions above:</para>
<itemizedlist>
<listitem><para>boot image - boot.img </para></listitem>
<listitem><para>core image - core.img </para></listitem>
<listitem><para>directory - /boot/grub</para></listitem>
<listitem><para>device map - device.map</para></listitem>
<listitem><para>default root setting - guessed</para></listitem>
</itemizedlist>
<note><para>The root setting is the default value if a 'set root'
instruction is not found in grub.cfg. This is the partition that is
searched for the kernel and other supporting files. It is different from
the 'root=' parameter on the 'linux' line in the configuration line. The
latter is the partition the kernel mounts as '/'. In the example grub.cfg
above, both values point to /dev/sda2, but if there is a separate boot
partition, they will be different.</para></note>
</sect2>
</sect1>
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