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After these two initial lines, there are three sections that each start with title. After the title, a string is specified that will be shown in the boot menu at boot time. For the two Linux configurations, there is then a kernel line that shows where the kernel image is loaded from. Everything else is passed directly to the kernel as boot parameters, including what the root device should be, and the terminal mode (vga=0x314). In the so-called failsafe configuration, we specify a lot of kernel parameters that turn off just about any kernel functionality that has the slightest chance of going wrong. Such a system will be slow and not have complete functionality, but if you have misconfigured something, the failsafe kernel may still get along and let you at least boot the system so that you can repair things.

If you type in those commands manually at the GRUB shell, you would have ended with the command boot. The kernel command loads the kernel into memory, but does not actually boot it; the boot command performs the actual booting process. However, if you use the GRUB menu system as we do here, the boot command is implicit and can be left out at the end of each section.

Loading Windows works differently. GRUB is not able to load operating systems other than Linux, the BSD family, and a few others directly. For those other systems, such as Windows, it invokes instead the bootloader that comes with those systems. This is called chain-loading, and the GRUB command to do this is not surprisingly called chainloader. The +1 means that GRUB can find the bootloader on the partition specified with the previous root command, one sector into the partition.

When you are satisfied with your GRUB setup, you still need to install it. This is best done with the command grub-install, which expects to be told the directory in which the stage files and kernel images can be found, and on which device to install the bootloader. This could look like this:

pooh:~ # grub-install --root-directory=/boot /dev/hda

This installs the bootloader on the first IDE hard drive (/dev/hda), which is typically the one that the computer's BIOS searches for finding the initial booting information.

This was just a short introduction into the world of GRUB. GRUB can do a lot more, such as booting kernel images from the network or over a serial line, providing fancy graphics, and so on. If you plan to do any serious work with GRUB (and since a bootloader is crucial for being able to use the system at all, any changes of configuration files beyond trivial things such as changing the strings of the menu entries, the default entry, or the timeout, can be considered serious in this context), we urge you to read the excellent GRUB documentation that comes as Info files, and that you can read nicely in KDE by typing info:grub in Konqueror's location bar.

Specifying boot-time options

When you first installed Linux, more than likely you booted either from a floppy or a CD-ROM, which very likely gave you a GRUB (or other bootloader) boot menu. Selecting an entry and typing e gets you a boot prompt. At this prompt you can enter several boot-time options, such as:

hd=cylinders,heads,sectors

to specify the hard drive geometry. Each time you boot Linux, it may be necessary to specify these parameters in order for your hardware to be detected correctly. If you are using GRUB to boot Linux from the hard drive, you can specify these parameters in the kernel line in the GRUB configuration file instead of entering them at the boot prompt each time. To the Linux entry, just add a line such as:

append = "hd=683,16,38"

This causes the system to behave as though hd=683,16,38 were entered at the GRUB boot prompt. If you wish to specify multiple boot options, you can do so with a single append line, as in:

append = "hd=683,16,38 hd=64,32,202"

In this case, we specify the geometry for the first and second hard drives, respectively. Once you are done with your