102.2 Lesson 1

The contents of the /boot partition may vary with system architecture or the boot loader in use, but on a x86-based system you will usually find the files below. Most of these are named with a -VERSION suffix, where -VERSION is the version of the corresponding Linux kernel. So, for example, a configuration file for the Linux kernel version 4.15.0-65-generic would be called config-4.15.0-65-generic.

GRUB 2 can be installed using the grub-install utility. If you have a non-booting system you will need to boot using a Live CD or rescue disc, find out which is the boot partition for your system, mount it and then run the utility.

The first disk on a system is usually the boot device and you may need to know whether there is a boot partition on the disk. This can be done with the fdisk utility. To list all the partitions on the first disk of your machine, use:

The boot partition is identified with the * under the boot column. In the example above, it is /dev/sda1.

Now, create a temporary directory under /mnt and mount the partition under it:

Then run grub-install, pointing it to the boot device (not the partition) and the directory where the boot partition is mounted. If your system does have a dedicated boot partition, the command is:

If you are installing to a system which does not have a boot partition, but just a /boot directory on the root filesystem, point grub-install to it. So, the command is:

The default configuration file for GRUB 2 is /boot/grub/grub.cfg. This file is automatically generated and manual editing is not recommended. To make changes to the GRUB configuration, you need to edit the file /etc/default/grub and then run the update-grub utility to generate a compliant file.

There are some options in the file /etc/default/grub that control the behaviour of GRUB 2, like the default kernel to boot, timeout, extra command line parameters, etc. The most important ones are:

When update-grub is run, GRUB 2 will scan for kernels and operating systems on the machine and generate the corresponding menu entries on the /boot/grub/grub.cfg file. New entries can be manually added to the script files within the /etc/grub.d directory.

These files should be executable, and are processed in numerical order by update-grub. Therefore 05_debian_theme is processed before 10_linux and so on. Custom menu entries are usually added to the 40_custom file.

The basic syntax for a menu entry is shown below:

The first line always starts with menuentry and ends with {. The text between quotes will be shown as the entry label on the GRUB 2 boot menu.

The set root parameter defines the disk and partition where the root file system for the operating system is located. Please note that on GRUB 2 disks are numbered from zero, so hd0 is the first disk (sda under Linux), hd1 the second, and so on. Partitions, however, are numbered starting from one. In the example above the root file system is located at the first disk (hd0), first partition (,1), or sda1.

Instead of directly specifying the device and partition you can also have GRUB 2 search for a file system with a specific label or UUID (Universally Unique Identifier). For that, use the parameter search --set=root followed by the --label parameter and file system label to search for, or --fs-uuid followed by the UUID of the file system.

You can find the UUID of a filesystem with the command below:

In the example above, the UUID for /dev/sda1 is ae71b214-0aec-48e8-80b2-090b6986b625. Should you wish to set it as the root device for GRUB 2, the command would be search --set=root --fs-uuid ae71b214-0aec-48e8-80b2-090b6986b625.

When using the search command it is common to add the --no-floppy parameter so GRUB will not waste time searching on floppy disks.

The linux line indicates where the kernel for the operating system is located (in this case, the vmlinuz file at the root of the file system). After that, you can pass command line parameters to the kernel.

In the example above we specified the root partition (root=/dev/sda1) and passed three kernel parameters: the root partition should be mounted read-only (ro), most of the log messages should be disabled (quiet) and a splash screen should be shown (splash).

The initrd line indicates where the initial RAM disk is located. In the example above the file is initrd.img, located at the root of the file system.

The last line of a menu entry should contain only the character }.

When booting a system with GRUB 2 you will see a menu of options. Use the arrow keys to select an option and Enter to confirm and boot the selected entry.

To edit an option, select it with the arrow keys and press E. This will show an editor window with the contents of the menuentry associated with that option, as defined in /boot/grub/grub.cfg.

After editing an option, type Ctrl+X or F10 to boot, or Esc to return to the menu.

To enter the GRUB 2 shell, press C on the menu screen (or Ctrl+C) on the editing window). You will see a command prompt like this: grub >

Type help to see a list of all available commands, or press Esc to exit the shell and return to the menu screen.

You can use the GRUB 2 shell to boot the system in case a misconfiguration in a menu entry causes it to fail.

The first thing you should do is find out where the boot partition is. You can do that with the command ls, which will show you a list of the partitions and disks GRUB 2 has found.

In the example above, things are easy. There is only one disk (hd0) with only one partition on it: (hd0,msdos1).

The disks and partitions listed will be different in your system. In our example the first partition of hd0 is called msdos1 because the disk was partitioned using the MBR partitioning scheme. If it were partitioned using GPT, the name would be gpt1.

To boot Linux, we need a kernel and initial RAM disk (initrd). Let us check the contents of (hd0,msdos1):

You can add the -l parameter to ls to get a long listing, similar to what you would get on a Linux terminal. Use Tab to autocomplete disk, partition and file names.

Note that we have a kernel (vmlinuz) and initrd (initrd.img) images right on the root directory. If not, we could check the contents of /boot with list (hd0,msdos1)/boot/.

Now, set the boot partition:

Load the Linux kernel with the linux command, followed by the path to the kernel and the root= option to tell the kernel where the root filesystem for the operating system is located.

Load the initial RAM disk with initrd, followed by the full path to the initrd.img file:

Now, boot the system with boot.

In case of a boot failure GRUB 2 may load a rescue shell, a simplified version of the shell we mentioned before. You will recognize it by the command prompt, which displays as grub rescue>.

The process to boot a system from this shell is almost the same as shown before. However, you will need to load a few GRUB 2 modules to make things work.

After finding out which partition is the boot partition (with ls, as shown before), use the set prefix= command, followed by the full path to the directory containing the GRUB 2 files. Usually /boot/grub. In our example:

Now, load the modules normal and linux with the insmod command:

Next, set the boot partition with set root= as directed before, load the linux kernel (with linux), the initial RAM disk (initrd) and try to boot with boot.

To install GRUB Legacy on a disk from a running system we will use the grub-install utility. The basic command is grub-install DEVICE where DEVICE is the disk where you wish to install GRUB Legacy. An example would be /dev/sda.

Note that you need to specify the device where GRUB Legacy will be installed, like /dev/sda/, not the partition as in /dev/sda1.

By default GRUB will copy the needed files to the /boot directory on the specified device. If you wish to copy them to another directory, use the --boot-directory= parameter, followed by the full path to where the files should be copied to.

If you cannot boot the system for some reason and need to reinstall GRUB Legacy, you can do so from the GRUB shell on a GRUB Legacy boot disk.

From the GRUB shell (type c at the boot menu to get to the grub> prompt), the first step is to set the boot device, which contains the /boot directory. For example, if this directory is in the first partition of the first disk, the command would be:

If you do not know which device contains the /boot directory, you can ask GRUB to search for it with the find command, like below:

Then, set the boot partition as instructed above and use the setup command to install GRUB Legacy to the MBR and copy the needed files to the disk:

When finished reboot the system and it should boot normally.

GRUB Legacy menu entries and settings are stored in the file /boot/grub/menu.lst. This is a simple text file with a list of commands and parameters, which can be edited directly with your favourite text editor.

Lines starting with # are considered comments, and blank lines are ignored.

A menu entry has at least three commands. The first one, title, sets the title of the operating system on the menu screen. The second one, root, tells GRUB Legacy which device or partition to boot from.

The third entry, kernel, specifies the full path to the kernel image that should be loaded when the corresponding entry is selected. Note that this path is relative to the device specified on the root parameter.

A simple example follows:

Unlike GRUB 2, in GRUB Legacy both disks and partitions are numbered from zero. So, the command root (hd0,0) will set the boot partition as the first partition (0) of the first disk (hd0).

You can omit the root statement if you specify the boot device before the path on the kernel command. The syntax is the same, so:

is equivalent to:

Both will load the file vmlinuz from the root directory (/) of the first partition of the first disk (hd0,0).

The parameter root=/dev/hda1 after the kernel command tells the Linux kernel which partition should be used as the root filesystem. This is a Linux kernel parameter, not a GRUB Legacy command.

You may need to specify the location of the initial RAM Disk image for the operating system with the initrd parameter. The full path to the file can be specified like in the kernel parameter, and you can also specify a device or partition before the path, e.g.:

GRUB Legacy has a modular design, where modules (usually stored as .mod files in /boot/grub/i386-pc) can be loaded to add extra features, like support for unusual hardware, filesystems or new compression algorithms.

Modules are loaded using the module command, followed by the full path to the corresponding .mod file. Keep in mind that, like kernels and initrd images, this path is relative to the device specified in the root command.

The example below will load the module 915resolution, needed to correctly set the framebuffer resolution on systems with Intel 800 or 900 series video chipsets.

GRUB Legacy can be used to load unsupported operating systems, like Windows, using a process called chainloading. GRUB Legacy is loaded first, and when the corresponding option is selected the bootloader for the desired system is loaded.

A typical entry for chainloading Windows would look like the one below:

Let us walk through each parameter. As before, root (hd0,1) specifies the device and partition where the boot loader for the operating system we wish to load is located. In this example, the second partition of the first disk.