# awk -F\' '$1=="menuentry " {print i++ " : " $2}' /etc/grub2.cfg
0 : CentOS Linux (3.10.0-957.5.1.el7.x86_64) 7 (Core)
1 : CentOS Linux (3.10.0-957.el7.x86_64) 7 (Core)
2 : CentOS Linux (0-rescue-2f0c165e79aa2eb8de65117302472e64) 7 (Core)

# grub2-editenv list
saved_entry=0

# grub2-set-default 1
# grub2-editenv list
saved_entry=1

サーバ起動しなくて、レスキューモードで変更する場合は、このを直接変更

# grep GRUB_DEFAULT /etc/default/grub
GRUB_DEFAULT=saved

# grubby --default-kernel
/boot/vmlinuz-4.16.12-1.choon.el7.x86_64

# grubby --default-index
0

# grubby --info=ALL
index=0
kernel=/boot/vmlinuz-4.16.12-1.choon.el7.x86_64
args="ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet biosdevname=0 net.ifnames=0 console=tty0 console=ttyS1,115200n8 spectre_v2=retpoline nopti LANG=en_US.UTF-8"
root=/dev/mapper/centos-root
initrd=/boot/initramfs-4.16.12-1.choon.el7.x86_64.img
title=CentOS Linux (4.16.12-1.choon.el7.x86_64) 7 (Core)
index=1
kernel=/boot/vmlinuz-3.10.0-1062.12.1.el7.x86_64
args="ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet biosdevname=0 net.ifnames=0 console=tty0 console=ttyS1,115200n8 spectre_v2=retpoline nopti"
root=/dev/mapper/centos-root
initrd=/boot/initramfs-3.10.0-1062.12.1.el7.x86_64.img
title=CentOS Linux (3.10.0-1062.12.1.el7.x86_64) 7 (Core)
index=2
kernel=/boot/vmlinuz-3.10.0-957.el7.x86_64
args="ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet biosdevname=0 net.ifnames=0 console=tty0 console=ttyS1,115200n8 spectre_v2=retpoline nopti"
root=/dev/mapper/centos-root
initrd=/boot/initramfs-3.10.0-957.el7.x86_64.img
title=CentOS Linux (3.10.0-957.el7.x86_64) 7 (Core)
index=3
kernel=/boot/vmlinuz-3.10.0-862.14.4.el7.centos.plus.x86_64
args="ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet biosdevname=0 net.ifnames=0 console=tty0 console=ttyS1,115200n8 spectre_v2=retpoline nopti"
root=/dev/mapper/centos-root
initrd=/boot/initramfs-3.10.0-862.14.4.el7.centos.plus.x86_64.img
title=CentOS Linux (3.10.0-862.14.4.el7.centos.plus.x86_64) 7 (Core)
index=4
kernel=/boot/vmlinuz-0-rescue-386cd61b700b4d7e96722ab240aea3fc
args="ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet biosdevname=0 net.ifnames=0 console=tty0 console=ttyS1,115200n8 spectre_v2=retpoline nopti"
root=/dev/mapper/centos-root
initrd=/boot/initramfs-0-rescue-386cd61b700b4d7e96722ab240aea3fc.img
title=CentOS Linux (0-rescue-386cd61b700b4d7e96722ab240aea3fc) 7 (Core)
index=5
non linux entry

インデックス1のカーネルに変更する場合

# grubby --set-default-index=1