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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.
GRUB allows a number of useful commands in its command line
interface. Some of the commands accept options after their name;
these options should be separated from the command and other
options on that line by space characters.
The following is a list of useful commands:
-
boot — Boots the operating system
or chain loader that was last loaded.
-
chainloader </path/to/file> — Loads the
specified file as a chain loader. If the file is located on the
first sector of the specified partition, use the blocklist
notation, +1, instead of the file
name.
The following is an example chainloader
command:
-
displaymem — Displays the current
use of memory, based on information from the BIOS. This is useful
to determine how much RAM a system has prior to booting it.
-
initrd </path/to/initrd> — Enables
users to specify an initial RAM disk to use when booting. An
initrd is necessary when the kernel needs
certain modules in order to boot properly, such as when the root
partition is formatted with the ext3 file system.
The following is an example initrd
command:
initrd /initrd-2.6.8-1.523.img
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-
install <stage-1> <install-disk> <stage-2> p
config-file — Installs
GRUB to the system MBR.
-
<stage-1> — Signifies a
device, partition, and file where the first boot loader image can
be found, such as (hd0,0)/grub/stage1.
-
<install-disk> — Specifies the
disk where the stage 1 boot loader should be installed, such as
(hd0).
-
<stage-2> — Passes the stage 2
boot loader location to the stage 1 boot loader, such as (hd0,0)/grub/stage2.
-
p <config-file> — This option
tells the install command to look for the
menu configuration file specified by <config-file>,
such as (hd0,0)/grub/grub.conf.
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The install command overwrites any
information already located on the MBR.
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-
kernel </path/to/kernel> <option-1> <option-N> ... — Specifies the
kernel file to load when booting the operating system. Replace
</path/to/kernel> with an
absolute path from the partition specified by the root command.
Replace <option-1> with
options for the Linux kernel, such as root=/dev/hda5 to specify the device on which the
root partition for the system is located. Multiple options can be
passed to the kernel in a space separated list.
The following is an example kernel
command:
kernel /vmlinuz-2.4.21 root=/dev/hda5
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The option in the previous example specifies that the root file
system for Linux is located on the hda5
partition.
-
root (<device-type><device-number>,<partition>) — Configures the
root partition for GRUB, such as (hd0,0),
and mounts the partition.
The following is an example root
command:
-
rootnoverify (<device-type><device-number>,<partition>) — Configures the
root partition for GRUB, just like the root command, but does not mount the partition.
Other commands are also available; type help
--all for a full list of commands. For a description of all
GRUB commands, refer to the documentation available online at
https://www.gnu.org/software/grub/manual/.
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