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Setting Up grub2 on CentOS 7

This article is largely based on Things to Know About Grub2 and is intended for those who open /etc/grub2.cfg(or /boot/grub2/grub.cfg)in an attempt to edit the boot menu.

See also the upstream document on grub2 and the grub manual.

1. The Boot Menu is Automatically Created

Do not attempt to manually edit the boot menu because it is automatically created from files within the /boot/ directory tree. However you can adjust the /etc/default/grub file that defines the general settings and the /etc/grub.d/40_custom file to add specific custom entries.

The content of the /etc/default/grub file looks like this:

GRUB_CMDLINE_LINUX="crashkernel=auto rhgb quiet"

Kernel options that are common to each entry are defined on the GRUB_CMDLINE_LINUX line. For example, if you like to see the full detailed boot messages, delete rhgb quiet. If you like to see the standard boot messages, just delete rhgb. Changes to the setting is made effective by executing the following command:

[root@host ~]# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.10.0-229.14.1.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-229.14.1.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-229.4.2.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-229.4.2.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-229.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-229.el7.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue-605f01abef434fb98dd1309e774b72ba
Found initrd image: /boot/initramfs-0-rescue-605f01abef434fb98dd1309e774b72ba.img

On UEFI-based systems, the command will be grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg

2. How to Define the Default Entries

To list all the menu entries that will be displayed at system boot, issue the following command:

[root@host ~]# awk -F\' '$1=="menuentry " {print i++ " : " $2}' /etc/grub2.cfg
0 : CentOS Linux 7 (Core), with Linux 3.10.0-229.14.1.el7.x86_64
1 : CentOS Linux 7 (Core), with Linux 3.10.0-229.4.2.el7.x86_64
2 : CentOS Linux 7 (Core), with Linux 3.10.0-229.el7.x86_64
3 : CentOS Linux 7 (Core), with Linux 0-rescue-605f01abef434fb98dd1309e774b72ba

Or alternatively:

[root@host ~]# grep "^menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2

The file name /etc/grub2.cfg is a symbolic link to the grub.cfg file, whose location is architecture dependent. It is better to use absolute paths when accessing to a file, as in the example above, especially when repairing a system. The default entry is defined by the GRUB_DEFAULT line in the /etc/default/grub file. However, if the GRUB_DEFAULT line is set as saved, the parameter is stored in the /boot/grub2/grubenv file. It may be viewed by:

[root@host ~]# grub2-editenv list
saved_entry=CentOS Linux (3.10.0-229.14.1.el7.x86_64) 7 (Core)

Another useful entry in the /etc/default/grub file is:


Together with GRUB_DEFAULT=saved, it makes sure that the currently used boot menu entry is automatically marked for the next boot - useful e.g. on a dual-boot system where updating Windows needs a number of reboots.

The /boot/grub2/grubenv file cannot be manually edited. Use the following command instead:

[root@host ~]# grub2-set-default 2
[root@host ~]# grub2-editenv list

Note that the first entry in the output of the awk command, above, is index number 0.

Now reboot the system.

3. Rescue Mode and Emergency Mode

The "Linux 0-rescue-..." menu entry is to boot the system in rescue mode. This is equivalent to the single-user mode.

In addition, with CentOS 7, an emergency mode is also available. In this latter mode, a shell is provided immediately after systemd has started. No other processes will exist and the root filesystem will be mounted as read-only. No other filesystems will be mounted.

To boot into emergency mode, enter the setup edit display by typing "e" in the grub2 menu. Then add "systemd.unit=emergency.target" to the end of the kernel option.

[If you have a friendly relationship with systemd :-) you can see what happens in emergency mode by viewing the /usr/lib/systemd/system/emergency.service file.]

4. Placement of Stage 1.5 (core.img)

The entity of grub/grub2 is in the /boot file system. In a classic (non-UEFI) BIOS environment, a bootstrap image (stage1 in grub, boot.img in grub2) is loaded, followed by loading of stage 1.5 (core.img in grub2) which boots the entity of the /boot filesystem. The stage 1.5 is placed either immediately after the MBR or somewhere in a partition where the /boot filesystem resides in. In CentOS 7, the default is immediately after the MBR. See this figure.

2023-09-11 07:22