kernel

#!/bin/bash

# This scripts lists all kernels and indicates which kernel is selected to boot.
# You can also set the entry you want to boot next.

# Tested on Centos 7; quit on other distros
if [ -f '/etc/os-release' ] ; then
  source '/etc/os-release'
  if [ "$PRETTY_NAME" != "CentOS Linux 7 (Core)" ]  ; then
    echo "This script was tested on Centos 7. Use on other distros may be DANGEROUS!"
    exit 1
  fi
else
  echo "Could not determine distro."
  exit 1
fi

# Refresh kernel list
grub2-mkconfig -o /boot/grub2/grub.cfg 2> /dev/null

KERNEL_LIST=$(
  grep -P -o "menuentry '.*?'" /boot/grub2/grub.cfg \
  | sed -e "s/menuentry '\(.*\)'/\1/" \
  | nl -v 0 -w 2 -s ' '
)

# What to do, provide a list or set the kernel?
if [ -z "$1" ] ; then
  # Show which kernel of the list is set to boot.
  grub_default=$(grep GRUB_DEFAULT /etc/default/grub | sed -e 's/GRUB_DEFAULT=//')
  if [ "$grub_default" = "saved" ] ; then
    echo "Default kernel: saved ($(grep saved_entry /boot/grub2/grubenv))"
    grub_default=999  # Don't show highlighted item
  fi
  echo "$KERNEL_LIST" | sed -e 's/^/ /' -e "$((grub_default + 1))s/^ />/"
else
  # If a number is provided, select the kernel with that number for boot.
  case $1 in
    *[!0-9]* )
      echo "Please provide a number to set the boot kernel."
      exit
      ;;
  esac
  echo "Selecting kernel $1..."
  sed -i -e "s/GRUB_DEFAULT=.*/GRUB_DEFAULT=$1/" /etc/default/grub
  grub2-mkconfig -o /boot/grub2/grub.cfg 2> /dev/null
  grub_default=$(grep GRUB_DEFAULT /etc/default/grub | sed -e 's/GRUB_DEFAULT=//')
  grep -P -o "menuentry '.*?'" /boot/grub2/grub.cfg \
  | sed -e "s/menuentry '\(.*\)'/\1/" \
  | nl -v 0 -w 2 -s ' ' \
  | sed -e 's/^/ /' -e "$((grub_default + 1))s/^ />/"
fi