AdminSysGNULinux

Source : http://dennisk.freeshell.org/cis238dl_grub.html

GRUB2 is the bootloader responsible for finding the Linux kernel and booting the operating system. GRUB2 has a powerful command line that can be used to rescue a non-booting system. In this assignment you will use GRUB2 commands to boot a CentOS 7 installation.

When you have successfully completed this assignment you will be able to :

• Install CentOS 7.
• Identify the GRUB2 configuration files.
• Access the GRUB2 prompt.
• Use GRUB commands to find and boot the Linux kernel.

3 Terms You Should Know

/etc/default/grub \ Editable GRUB2 configuration file.

/etc/grub.d/ \ Directory for customization files.

/boot/grub/grub.cfg \ Main GRUB2 configuration file. Do not edit directly.

Initial Ram File System \ Provides an initial filesystem until the root partition can be accessed.

GRUB2 \ The GRand Unified Bootloader

For this assignment you need to install CentOS 7 Minimal as a virtual machine.

• After installation reboot the virtual machine.
• As soon as the GRUB menu appears press c to access the GRUB2 prompt.
• Press Tab to list available commands.
• GRUB2 supports tab completion for commands and file names.

CentOS uses Logical Volume Management (LVM). Before GRUB2 can access files on this filesystems a kernel module needs to be loaded.

grub> set pager=1
grub> insmod lvm

Use the ls command to list the devices on the virtual machine. Make a note of the LVM logical volume names as they may be different on your virtual machine.

grub> ls
(hd0) (hd0,msdos2) (hd0,msdos1) (lvm/centos-root) (lvm/centos-swap)

(hd0,msdos1) and (hd0,msdos2) refer to /dev/sda1 and /dev/sda2. The other entries refer to LVM Logical Volumes. GRUB2 uses the drive names provided by the BIOS.

Use ls to list the files on the /boot partition.

grub> ls (hd0,1)/

Root is mounted as a LVM logical volume. Use this command to list the files.

grub> ls (lvm/centos-root)/

Now that you know where to find the Linux kernel and the root partition you can run commands to boot the system. Tab completion makes it easy to find the correct kernel and matching initial ram filesystem. Use tab completion to find a matching kernel and initramfs.

grub> set root=(hd0,1)
grub> linux /vmlinuz-<version>-<arch> root=/dev/mapper/centos-root
grub> initrd /initramfs-<version>-<arch>.img
grub> boot

Submit a screenshot of the GRUB2 commands you used to boot the system.

Resources

• man grub2-mkconfig
• initramfs
• Working with the GRUB2 Boot Loader

Creative Commons License
This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License

Date: 2015-04-27T12:11-0700

Author: Dennis Kibbe