kernel /boot/vmlinuz-2.4.7-10 ro root=/dev/hda3 initrd /boot/initrd-2.4.7-10.img title DOS rootnoverify (hd0,0) chainloader +1 The first title command defines the menu text Red Hat Linux (2.4.7 10). The next three lines define the operating system that is booted when that item is selected from the GRUB menu: root (hd0,2) Defines the physical location of the filesystem root for this operating system. The values defined for the root command are the disk device name and the partition number. Notice that GRUB device names are slightly different from normal Linux device names. GRUB calls the first hard disk hd0. Additionally, GRUB counts partitions differently than Linux does. GRUB counts from 0, whereas Linux counts from 1. Thus, the GRUB value hd0,2 on a Linux system that boots from an IDE drive is the same as the Linux value hda,3 partition number 3 on the first IDE drive. kernel /boot/vmlinuz-2.4.7-10 ro root=/dev/hda3 Identifies the file that contains the operating system that is to be started, and defines any arguments passed to that operating system at run time. In this case, GRUB will load the Linux kernel stored in vmlinuz-2.4.7-10, and it will pass the Linux kernel the arguments ro root=/dev/hda3, which tell the kernel where the filesystem root is located, and that it should be mounted as read-only. The ro option causes Linux to mount the root read-only during the initial phase of the boot. (Later, the rc.sysinit script changes it to read-write after successfully completing the filesystem checks.) initrd /boot/initrd-2.4.7-10.img Identifies a ramdisk file for Linux to use during the boot. Red Hat uses the ramdisk to provide Linux with critical modules that the kernel might need to access the disk drives. The last title command defines the DOS menu entry. Two commands define the operating system loaded when DOS is selected from the menu: rootnoverify (hd0,0) Like the root command, defines the physical location of the filesystem root for this operating system. But rootnoverify tells GRUB that the filesystem found at this location does not comply with the multiboot standards, and thus cannot be validated. chainloader +1 Emulates the function of the DOS MBR by simply loading the specified sector and passing boot responsibilities to the loader found there. The value +1 is a blocklist value, which defines the sector address of the loader relative to the partition defined by the rootnoverify command. +1 means the first sector of the partition. Taken together, the rootnoverify command and the chainloader command from our sample mean that GRUB will pass control to the loader found in the first sector of the first partition on the first IDE drive when DOS is selected from the GRUB menu. In this example, that partition contains the DOS boot loader that will be responsible for loading DOS. The grub.conf file on your system will be very similar to the one in this example. The location of files may be different, and a server system’s configuration usually won’t define multiple operating systems, but the commands will be essentially the same. GRUB is used with several different flavors of UNIX. It is not, however, the only boot loader used 16
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