A boot loader lets you choose when and how to boot the bootable operating systems installed on your computer’s hard disks. Most Linux systems give you the opportunity to use GRUB or LILO boot loaders. The following sections describe both GRUB and LILO boot loaders. Booting Your Computer with GRUB With multiple operating systems installed and several partitions set up, how does your computer know which operating system to start? To select and manage which partition is booted and how it is booted, you need a boot loader. The boot loader that is installed by default with Fedora and other Linux systems is the GRand Unified Boot loader (GRUB).
GRUB is a GNU bootloader (www.gnu.org/software/grub) that replaced LILO as the default boot loader in many Linux systems, including Fedora and Ubuntu. GRUB offers the following features:
* Support for multiple executable formats.
* Support for multi-boot operating systems (such as Fedora, FreeBSD, NetBSD, OpenBSD, and other Linux systems).
* Support for non–multi-boot operating systems (such as Windows 95, Windows 98, Windows NT, Windows ME, Windows XP, and OS/2) via a chain-loading function. Chain-loading is the act of loading another boot loader (presumably one that is specific to the proprietary operating system) from GRUB to start the selected operating system.
* Support for multiple file system types.
* Support for automatic decompression of boot images.
* Support for downloading boot images from a network.
For more information on how GRUB works, type man grub or info grub. The info command contains more details about GRUB.
Booting with GRUB
When you install Linux, you are typically given the option to configure the information needed to boot your computer (with one or more operating systems) into the default boot loader. With GRUB configured, when you boot your computer, the first thing you see after the BIOS loads is the GRUB boot screen (it says GRUB at the top and lists bootable partitions below it). Do one of the following:
* Default—If you do nothing, the default operating system will boot automatically after a few seconds. (The timeout is set by the timeout value, in seconds, in the grub.conf file.)Select an operating system—Use the up and down arrow keys to select any of the titles, representing operating systems you can boot, that are shown on the screen. Then press Enter to boot that operating system.
* Edit the boot process—If you want to change any of the options used during the boot process, use the arrow keys to highlight the operating system you want and type e to select it. Follow the next procedure to change your boot options temporarily.
If you want to change your boot options so that they take effect every time you boot your computer, see the section on permanently changing boot options. Changing those options involves editing the /boot/grub/grub.conf file.
Temporarily Changing Boot Options
From the GRUB boot screen, you can select to change or add boot options for the current boot session. First, select the operating system you want (using the arrow keys) and type e (as described earlier). You will see a graphical screen that looks like the one shown in Figure 7-2.
There are three lines in the example of the GRUB editing screen that identify the boot process for the operating system you chose. The first line (beginning with root) shows that the entry for the GRUB boot loader is on the seventh partition of the first hard disk (hd0,6). GRUB represents the hard disk as hd, regardless of whether it is a SCSI, IDE, or other type of disk. You just count the drive number and partition number, starting from zero (0).
The second line of the example (beginning with kernel) identifies the boot image (/boot/vmlinuz-
2.6.20-1.3104.fc7) and several options. The options identify the partition as initially being loaded ro (read-only) and the location of the root file system on a partition with the label root=LABEL=/123. The third line (starting with initrd) identifies the location of the initial RAM disk, which contains the minimum files and directories needed during the boot process.
If you are going to change any of the lines related to the boot process, you would probably change only the second line to add or remove boot options. Here is how you do that:
1. Position the cursor on the kernel line and type e.
2. Either add or remove options after the name of the boot image. You can use a minimal set of bash shell command-line editing features to edit the line. You can even use command completion (type part of a filename and press Tab to complete it).
3. Press Enter to return to the editing screen.
4. Type b to boot the computer with the new options. The next time you boot your computer,
the new options will not be saved. To add options so they are saved permanently,
see the next section.
* Support for multiple executable formats.
* Support for multi-boot operating systems (such as Fedora, FreeBSD, NetBSD, OpenBSD, and other Linux systems).
* Support for non–multi-boot operating systems (such as Windows 95, Windows 98, Windows NT, Windows ME, Windows XP, and OS/2) via a chain-loading function. Chain-loading is the act of loading another boot loader (presumably one that is specific to the proprietary operating system) from GRUB to start the selected operating system.
* Support for multiple file system types.
* Support for automatic decompression of boot images.
* Support for downloading boot images from a network.
For more information on how GRUB works, type man grub or info grub. The info command contains more details about GRUB.
Booting with GRUB
When you install Linux, you are typically given the option to configure the information needed to boot your computer (with one or more operating systems) into the default boot loader. With GRUB configured, when you boot your computer, the first thing you see after the BIOS loads is the GRUB boot screen (it says GRUB at the top and lists bootable partitions below it). Do one of the following:
* Default—If you do nothing, the default operating system will boot automatically after a few seconds. (The timeout is set by the timeout value, in seconds, in the grub.conf file.)Select an operating system—Use the up and down arrow keys to select any of the titles, representing operating systems you can boot, that are shown on the screen. Then press Enter to boot that operating system.
* Edit the boot process—If you want to change any of the options used during the boot process, use the arrow keys to highlight the operating system you want and type e to select it. Follow the next procedure to change your boot options temporarily.
If you want to change your boot options so that they take effect every time you boot your computer, see the section on permanently changing boot options. Changing those options involves editing the /boot/grub/grub.conf file.
Temporarily Changing Boot Options
From the GRUB boot screen, you can select to change or add boot options for the current boot session. First, select the operating system you want (using the arrow keys) and type e (as described earlier). You will see a graphical screen that looks like the one shown in Figure 7-2.
There are three lines in the example of the GRUB editing screen that identify the boot process for the operating system you chose. The first line (beginning with root) shows that the entry for the GRUB boot loader is on the seventh partition of the first hard disk (hd0,6). GRUB represents the hard disk as hd, regardless of whether it is a SCSI, IDE, or other type of disk. You just count the drive number and partition number, starting from zero (0).
The second line of the example (beginning with kernel) identifies the boot image (/boot/vmlinuz-
2.6.20-1.3104.fc7) and several options. The options identify the partition as initially being loaded ro (read-only) and the location of the root file system on a partition with the label root=LABEL=/123. The third line (starting with initrd) identifies the location of the initial RAM disk, which contains the minimum files and directories needed during the boot process.
If you are going to change any of the lines related to the boot process, you would probably change only the second line to add or remove boot options. Here is how you do that:
1. Position the cursor on the kernel line and type e.
2. Either add or remove options after the name of the boot image. You can use a minimal set of bash shell command-line editing features to edit the line. You can even use command completion (type part of a filename and press Tab to complete it).
3. Press Enter to return to the editing screen.
4. Type b to boot the computer with the new options. The next time you boot your computer,
the new options will not be saved. To add options so they are saved permanently,
see the next section.
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