source: chapter10/grub.xml

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<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>

  <note>
    <para>
      If your system has UEFI support and you wish to boot LFS with UEFI,
      you should skip the instructions in this page but still learn the
      syntax of <filename>grub.cfg</filename> and the method to specify
      a partition in the file from this page, and configure GRUB with UEFI
      support using the instructions provided in
      <ulink url="&blfs-book;postlfs/grub-setup.html">the BLFS page</ulink>.
    </para>
  </note>

  <sect2>
    <title>Introduction</title>

    <warning><para>Configuring GRUB incorrectly can render your system
    inoperable without an alternate boot device such as a CD-ROM or bootable
    USB drive. 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="&blfs-book;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 numbers start from zero, but the partition numbers
    start from one for normal partitions (from 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>

  </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 200 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>Leaving <filename class="directory">/boot</filename> on
    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>

    <note>
      <para>If the system has been booted using UEFI,
      <command>grub-install</command> will try to install files for the
      <emphasis>x86_64-efi</emphasis> target, but those files
      have not been installed in <xref linkend="chapter-building-system"/>.
      If this is the case, add <option>--target i386-pc</option> to the
      command above.</para>
    </note>

  </sect2>

  <sect2 id="grub-cfg">
    <title>Creating the GRUB Configuration File</title>

    <para>Generate <filename>/boot/grub/grub.cfg</filename>:</para>

    <screen><userinput>cat &gt; /boot/grub/grub.cfg &lt;&lt; "EOF"
<literal># Begin /boot/grub/grub.cfg
set default=0
set timeout=5

insmod part_gpt
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>

    <para>
      The <command>insmod</command> commands load the
      <application>GRUB</application> modules named
      <filename>part_gpt</filename> and <filename>ext2</filename>.
      Despite the naming, <filename>ext2</filename> actually supports
      <systemitem class='filesystem'>ext2</systemitem>,
      <systemitem class='filesystem'>ext3</systemitem>, and
      <systemitem class='filesystem'>ext4</systemitem> filesystems.
      The <command>grub-install</command> command has embedded some modules
      into the main <application>GRUB</application> image (installed into
      the MBR or the GRUB BIOS partition) to access the other modules
      (in <filename class='directory'>/boot/grub/i386-pc</filename>) without
      a chicken-or-egg issue, so with a typical configuration these two
      modules are already embedded and those two <command>insmod</command>
      commands will do nothing.  But they do no harm anyway, and they may
      be needed with some rare configurations.
    </para>

    <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>

    <note>
      <para>The GRUB designator for a partition may change if you added or
      removed some disks (including removable disks like USB thumb devices).
      The change may cause boot failure because
      <filename>grub.cfg</filename> refers to some <quote>old</quote>
      designators.  If you wish to avoid such a problem, you may use
      the UUID of a partition and the UUID of a filesystem instead of a GRUB designator to
      specify a device.
      Run <command>lsblk -o UUID,PARTUUID,PATH,MOUNTPOINT</command> to show
      the UUIDs of your filesystems (in the <literal>UUID</literal> column) and
      partitions (in the <literal>PARTUUID</literal> column).  Then replace
      <literal>set root=(hdx,y)</literal> with
      <literal>search --set=root --fs-uuid <replaceable>&lt;UUID of the filesystem
      where the kernel is installed&gt;</replaceable></literal>, and replace
      <literal>root=/dev/sda2</literal> with
      <literal>root=PARTUUID=<replaceable>&lt;UUID of the partition where LFS 
      is built&gt;</replaceable></literal>.</para>
      <para>Note that the UUID of a partition is completely different from the
      UUID of the filesystem in this partition.  Some online resources may
      instruct you to use
      <literal>root=UUID=<replaceable>&lt;filesystem UUID&gt;</replaceable></literal>
      instead of
      <literal>root=PARTUUID=<replaceable>&lt;partition UUID&gt;</replaceable></literal>,
      but doing so will require an initramfs, which is beyond the scope of
      LFS.</para>
      <para>The name of the device node for a partition in
      <filename class='directory'>/dev</filename> may also change (this is less
      likely than a GRUB designator change).  You can also replace
      paths to device nodes like <literal>/dev/sda1</literal> with
      <literal>PARTUUID=<replaceable>&lt;partition UUID&gt;</replaceable></literal>,
      in <filename>/etc/fstab</filename>, to avoid a potential boot failure
      in case the device node name has changed.</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>

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