4.5. Boot-time Kernel Options
The Loadlin and LILO sections of this chapter described some of the options
you can specify when you boot Linux. There are many more options that
can be specified. This section touches on the ways to pass options to the
kernel and then describes some of the kinds of parameters you might want to
use. The parameters in this section affect the kernel and therefore apply
regardless of which boot loader you use.
As always with Unix systems, there are a number of choices
for the boot process itself. If you are using Loadlin, you can
pass parameters to the kernel on the command line or in a file.
If LILO is your boot loader, you can add to or override the parameters
specified in /etc/lilo.conf during the boot process as follows:
If prompt is set in /etc/lilo.conf, LILO
always presents the boot prompt and waits for
input. At the prompt, you can choose the operating system to be booted. If
you choose Linux, you also can specify parameters.
If prompt isn't set, when the word "LILO" appears, press Control, Shift,
or Alt, and the boot prompt appears. You also can press the Scroll Lock key
before LILO is printed and not have to wait poised over the keyboard for the
At the boot prompt, specify the system you want to boot or press Tab to
get a list of the available choices. You then can enter the name of the
image to boot. For example:
LILO boot: <press Tab>
linux test dos
You also can add boot command options:
boot: linux single
If you don't provide any input, LILO waits the amount of time specified
in the delay parameter and then boots the default operating system with
the default parameters as set in /etc/lilo.conf.
Some of the boot parameters have been mentioned earlier. Many of the others
are hardware-specific and are too numerous to mention here. For a complete
list of parameters and a discussion of the booting process, see the
BootPrompt-HOWTO. Some of the parameters not shown earlier that
you might find useful are listed next; many more are covered in the HOWTO.
Most of the following parameters are used to
provide information or instructions for the kernel, rather than to LILO.
Prints all kernel messages to the console.
Specifies the hard drive geometry to the kernel. Useful
if Linux has trouble recognizing
the geometry of your drive, especially if it's an IDE drive with more than
Tells the kernel whether to load a RAM disk image for use during Linux
installation. Values of n are:
Don't try to load the image. This is the default.
Load the image from a floppy disk to the RAM disk.
Specifies the amount of system memory installed. Useful if your BIOS reports
memory only up to 64 MB and your system has more memory installed. Specify
as a number with M or k (case-insensitive) appended:
Because mem would have to be included on the command line for every
boot, it often is specified on a command line saved with lock
or with append to be added to the parameters passed to the kernel.
When set, disables the two-stage boot and preserves
the contents of /dev/initrd so the data is available
after the kernel has booted.
/dev/initrd can be read only once, and then its contents
are returned to the system.
Starts Linux at the runlevel specified by number. A runlevel is an
operating state that the system can be booted to, such as a multiuser
system or a system configuration running the X Window System. A runlevel
is generally one of the numbers from 1 to 6; the default usually is 3.
The runlevels and their corresponding states are defined in the file
/etc/inittab. See the manpage for /etc/inittab
for more information.
Mounts the root filesystem read-only. Used for doing system maintenance,
such as checking the filesystem integrity, when you don't want anything
written to the filesystem.
Mounts the root filesystem read-write. If neither ro nor rw is
specified, the default value (usually rw) stored in the kernel image
Starts Linux in single-user mode. This option is used for
system administration and recovery. It gives you a root prompt as
soon as the system boots, with minimal initialization. No other logins
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