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The Boot Process
LILO: The Linux Loader
GRUB: The Grand Unified Bootloader
GRUB Commands
Loadlin: Booting from MS-DOS
Dual-Booting Linux and Windows NT/2000/XP
Boot-Time Kernel Options
initrd: Using a RAM Disk
This chapter describes some techniques for booting your Linux system. Depending on your hardware and whether you want to run any other operating systems, you can configure the system to boot Linux automatically or to provide a choice between several operating systems. Choosing between operating systems is generally referred to as dual booting, but you can actually boot more than two.
Once your Linux system is installed, rebooting the system is generally pretty straightforward. But with the wide variety of hardware and software in use, there are many possibilities for configuring your boot process. The most common choices are:
Boot Linux from a floppy or bootable CD, leaving any other operating system to boot from the hard drive.
Use the Linux Loader, LILO.[2] This is the traditional method of booting and lets you boot both Linux and other operating systems.
[2]LILO is a boot program for i386-architecture machines. On the Alpha, the equivalent boot program is called MILO (Mini Loader), and on the SPARC, it is SILO.
Use the Grand Unified Bootloader (GRUB), the GNU graphical boot loader and command shell. Like LILO, GRUB lets you boot both Linux and other operating systems. For now, GRUB runs only on i386-based systems.
Run Loadlin, which is an MS-DOS program that boots Linux from within DOS.
Other boot managers that can load Linux are available, but we don't discuss them here. We also won't talk further about booting from a floppy or CD, except to say that whatever method you choose for booting, be sure to have a working boot disk available for emergency use. In particular, don't experiment with the files and options in this chapter unless you have a boot disk, because any error could leave you unable to boot from the hard disk. Note, though, that one of the advantages of using GRUB is that if there is a problem booting from the menu, it drops you down to the command-line interface so you can enter commands directly and try to recover. Also, see Section 4.3.1.1 for information on making a GRUB boot floppy.[3]
[3]Unfortunately, there is no standard set of instructions we can provide for making a bootable CD. Your best bet is to use a bootable installation CD for your distribution. Also, instructions and utilities are available online for making bootable CDs.
On an x86-based PC, the first sector of every hard disk is known as the boot sector and contains the partition table for that disk and possibly also code for booting an operating system. The boot sector of the first hard disk is known as the master boot record (MBR) because when you boot the system, the BIOS transfers control to a program that lives on that sector along with the partition table. That code is the boot loader, the code that initiates an operating system. When you add Linux to the system, you need to modify the boot loader, replace it, or boot from a floppy or CD to start Linux.
In Linux, each disk and each partition on the disk is treated as a device. So, for example, the entire first hard disk is known as /dev/hda and the entire second hard disk is /dev/hdb. The first partition of the first hard drive is /dev/hda1, and the second partition is /dev/hda2. The first partition of the second hard drive is /dev/hdb1, and so on. If your drives are SCSI instead of IDE, the naming works the same way except that the devices are /dev/sda, /dev/sda1, and so on. Thus, if you want to specify that the Linux partition is the second partition of the first hard drive (as in the examples in this chapter), you refer to it as /dev/hda2. Note, though, that GRUB has its own disk naming convention, described in Section 4.3.
Once you've made the decision to install LILO or GRUB, you still need to decide how it should be configured. If you want your system to dual-boot Linux and Windows 95/98/ME, you can install LILO or GRUB on the MBR and set it up to let you select the system to boot. Dual-booting Linux and Windows NT/2000/XP is not quite as straightforward because they use the Windows NT loader, which is installed on the MBR and expects to be the one in charge. The standard solution described in this chapter is to add Linux as an option in the NT loader and install LILO or GRUB in the Linux partition as a secondary boot loader. The result is that the NT loader transfers control to the secondary loader, which then boots Linux. See Section 4.6 later in this chapter for more information. You can also install one of the Linux boot loaders in the MBR and use it to boot Windows—see the "Linux+WindowsNT" and the "Multiboot with GRUB" mini-HOWTOs if you're interested in doing that.
When you install the boot loader (either LILO or GRUB) on the MBR, it replaces the MS-DOS boot loader or any other boot loader that may be there, such as the Windows NT loader. If you have problems with your installation or you simply want to restore the original boot loader, you can do one of the following.
If you're running LILO, you can boot Linux from a floppy or CD and restore the boot sector, which LILO automatically backs up:
% /sbin/lilo -u
If you have the capability, boot to DOS and run the fdisk command with a special option that rebuilds the MBR:
C:> fdisk /mbr
For Windows 2000 and Windows XP, which do not have an fdisk command, boot your computer from the Windows CD (or the Windows boot floppies if you can't boot from your CD drive). When you see "Welcome to Setup," press R (for repair) and, in Windows 2000, you then press C. Select your Windows installation from the numbered list that is displayed (there may be only one entry) and enter the administrator password at the prompt. Enter the command fixmbr at the command-line prompt and confirm it with y. After the MBR has been restored, type exit to reboot.
The common element in all three methods is that they replace the boot loader on the MBR with the original Microsoft boot loader. The boot loader on the MBR is the one that will be used to boot the system. This means that if you want to switch from LILO to GRUB, say, or from GRUB to LILO, you don't need to uninstall the old loader; simply install the new one.
The rest of this chapter describes the various techniques for booting Linux and the options that you can specify to configure both the boot loader and the Linux kernel. Whether you use LILO, GRUB, or Loadlin, you can pass options to the loader and specify options for the kernel.
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| 3. Linux Commands |  | 4.2. LILO: The Linux Loader |
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