Linux grub2 change default kernel

Linux grub2 change default kernel

GRUB 2 is a rewrite of GRUB

1. The configuration file is grub.cfg rather than menu.lst or grub.conf

2. grub.cfg is typically automatically generated by grub-mkconfig

3. Partition numbers in GRUB device names now start at 1, not 0

4. Many more file systems are supported, including but not limited to ext4, HFS+, and NTFS.

5. GRUB 2 can read files directly from LVM and RAID devices.

6. GRUB 2 has more reliable ways to find its own files and those of target kernels on multiple-disk systems, 

GRUB 2 configuration file

GRUB 2 reads its configuration from the /boot/grub2/grub.cfg file on traditional BIOS-based machines and from the /boot/efi/EFI/redhat/grub.cfgfile on UEFI machines. This file contains menu information.

The GRUB 2 configuration file, grub.cfg, is generated during installation, or by invoking the /usr/sbin/grub2-mkconfig utility

[/etc/default/grub]

 : grub2-mkconfig tool

[root@dev proc]# cat /etc/default/grub

GRUB_TIMEOUT=5

GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"

GRUB_DEFAULT=saved

GRUB_DISABLE_SUBMENU=true

GRUB_TERMINAL_OUTPUT="console"

GRUB_CMDLINE_LINUX="crashkernel=auto rhgb quiet"

GRUB_DISABLE_RECOVERY="true"

* Do not attempt to manually edit the boot menu because it is automatically created from files within the /boot/ 

Configuring GRUB 2

[root@cdr]# grubby --default-kernel
/boot/vmlinuz-3.10.0-514.6.2.el7.x86_64

[root@cdr]# grubby --default-index
0

[root@cdr ]# ls -al /boot/vmlinuz-*
-rwxr-xr-x. 1 root root 5156528 Feb 24  2017 /boot/vmlinuz-0-rescue-37f277e0207041d0bdf740809887b2bb
-rwxr-xr-x. 1 root root 5392752 Feb 23  2017 /boot/vmlinuz-3.10.0-514.6.2.el7.x86_64
-rwxr-xr-x. 1 root root 6192432 Feb 20  2017 /boot/vmlinuz-4.10.0-1.el7.elrepo.x86_64

Changing the Default Boot Entry

[root@cdr]# grubby --set-default /boot/vmlinuz-4.10.0-1.el7.elrepo.x86_64
[root@cdr]# grubby --default-index
1

[root@cdr]#  grubby --info=ALL
index=0
kernel=/boot/vmlinuz-3.10.0-514.6.2.el7.x86_64
args="ro crashkernel=auto rhgb quiet LANG=en_US.UTF-8"
root=UUID=3022c111-bd96-410c-bef2-ab1aa5e1789b
initrd=/boot/initramfs-3.10.0-514.6.2.el7.x86_64.img
title=CentOS Linux (3.10.0-514.6.2.el7.x86_64) 7 (Core)
index=1
kernel=/boot/vmlinuz-4.10.0-1.el7.elrepo.x86_64
args="ro crashkernel=auto rhgb quiet LANG=en_US.UTF-8"
root=UUID=3022c111-bd96-410c-bef2-ab1aa5e1789b
initrd=/boot/initramfs-4.10.0-1.el7.elrepo.x86_64.img
title=CentOS Linux (4.10.0-1.el7.elrepo.x86_64) 7 (Core)
index=2
kernel=/boot/vmlinuz-0-rescue-37f277e0207041d0bdf740809887b2bb
args="ro crashkernel=auto rhgb quiet"
root=UUID=3022c111-bd96-410c-bef2-ab1aa5e1789b
initrd=/boot/initramfs-0-rescue-37f277e0207041d0bdf740809887b2bb.img
title=CentOS Linux (0-rescue-37f277e0207041d0bdf740809887b2bb) 7 (Core)
index=3
non linux entry1

Refence : 

1. http://www.gnu.org/software/grub/manual/grub/grub.html

2. https://wiki.centos.org/HowTos/Grub2