select an optional menu item, or interrupt the boot process. Interrupt the boot by pressing the Escape key. If a password is defined in the grub.conf file, press P, and enter the GRUB password. Then, press C for command mode, and a command line prompt appears. This is the boot prompt that allows arguments to be sent to the kernel using the kernel command interactively. The format of the kernel command is kernel file arguments where kernel is the command, file is the name of the file that contains the Linux kernel, and arguments are any optional arguments you wish to pass to the kernel. In the preceding kernel command example, ro root=/dev/hda3 are arguments that change the default boot behavior so that the root filesystem is mounted read-only. The possible arguments depend on the kernel, not on whether GRUB or LILO is used to control the boot process. Any of the kernel arguments described in this section can be sent to the kernel in this manner on a system that uses GRUB. The LILO boot prompt is different, but the function is the same. When the system is booted by LILO, the string boot: is displayed as the boot prompt. The operator can boot any operating system defined in the lilo.conf file by entering its name at the prompt (for example, linux, or dos). Arguments are passed to the selected operating system by placing them on the command line after the operating system name. An example of passing kernel parameters on a system booted by LILO is boot: linux panic=60 In this example, boot: is the prompt, linux is the kernel name, and panic=60 is the parameter passed to that kernel. The keyword linux is the label assigned to the Linux kernel in the LILO configuration. Use the label to tell LILO which kernel should receive the parameter. The panic argument changes the boot behavior after a system crash. It is possible for the Linux kernel to crash from an internal error, called a kernel panic. If the system crashes from a kernel panic, it does not automatically reboot it stops at the boot prompt waiting for instructions. Normally, this is a good idea. The exception is an unattended server. If you have a system that does not have an operator in attendance and that remote users rely on, it might be better to have it try an automatic reboot after it crashes. The example shown previously tells the system to wait 60 seconds and then reboot. Note This might surprise Windows administrators, but I have never had a Linux system crash. In fact, I had one specialized system (collecting network measurement data, and providing Web access to that data) that ran continuously for more than a year without a single problem. In a normal boot process, the kernel starts the /sbin/init program. Using the init argument, it is possible to tell the kernel to start another process instead of /sbin/init. For example, init=/bin/sh causes the system to run the shell program, which then can be used to repair the system if the / sbin/init program is corrupted. Booting directly to the shell looks very much like booting to single-user mode with the single argument, but there are differences. init=/bin/sh does not rely on the init program. single, on the other hand, is passed directly to init so that init can perform selected initialization procedures before placing the system into single-user mode. In both of these cases, the person who boots the computer is given password-free access to the shell unless password and restrict are defined in the lilo.conf file, as described in the previous section. 22
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