Sets default kernel and modifies update policy

set_default_kernel.sh

#!/bin/bash --
# requires bash 4.0+

### FUNCTIONS

# usage: error_exit message
function error_exit() {
  echo -e "\e[31m$1\e[0m"
  exit 1
}

# usage: get_command_option command option
function get_command_option() {
  [ -z "$1" -o -z "$2" ] && error_exit 'get_command_option: missing arguments...'
  value="$( $1 | grep "^$2=" )"
  echo "${value#$2=}"
}

# usage: get_file_option file option
function get_file_option() {
  [ -f "$1" ] || error_exit "get_file_option: file \"$1\" not found"
  get_command_option "cat $1" "$2"
}

function get_grub_default() {
  get_file_option '/etc/default/grub' 'GRUB_DEFAULT'
}

function update_grub_default() {
  grub_default="$( get_grub_default )"
  if [ ! "$grub_default" == 'saved' ]; then
    sudo sed -i 's|^\(GRUB_DEFAULT=\).*$|\1saved|' /etc/default/grub \
      && echo -e "\nUpdated grub to use \e[32m$( get_grub_default )\e[0m kernel version." \
      || error_exit '\nupdate_grub_default: cannot update /etc/default/grub to use saved kernel version...'
    [ $( which update-grub 2>/dev/null ) ] && sudo update-grub \
      || [ $( which grub2-mkconfig 2>/dev/null ) ] && sudo grub2-mkconfig -o /boot/grub2/grub.cfg \
      || error_exit 'update_grub_default: cannot reconfigure grub...'
  else
    echo -e "\nGrub is set to use \e[33m$( get_grub_default )\e[0m kernel version."
  fi
}

function get_saved_kernel() {
  get_command_option 'grub2-editenv list' 'saved_entry'
}

function set_saved_kernel() {
  [ -z "$1" ] && error_exit 'set_saved_kernel: missing argument...'
  sudo grub2-set-default "$1"
}

function get_kernel_update_default() {
  get_file_option '/etc/sysconfig/kernel' 'UPDATEDEFAULT'
}

function toggle_kernel_update_default() {
  kernel_update_default=$( get_kernel_update_default )
  if [ "${kernel_update_default,,}" == 'yes' ]; then
    sudo sed -i 's|^\(UPDATEDEFAULT=\).*$|\1no|' /etc/sysconfig/kernel
  else
    sudo sed -i 's|^\(UPDATEDEFAULT=\).*$|\1yes|' /etc/sysconfig/kernel
  fi
}

function get_kernels_limit() {
  get_file_option '/etc/yum.conf' 'installonly_limit'
}

function update_kernel_removing() {
  if [ -f /etc/yum.conf ]; then
    kernels_limit="$( get_kernels_limit )"
    if [[ "$kernels_limit" =~ ^[0-9]+$ ]]; then
      kernel_update_default=$( get_kernel_update_default )
      ok=0
      if [ "${kernel_update_default,,}" == 'yes' -a $kernels_limit -eq 0 ]; then
        sudo sed -i 's|^\(installonly_limit=\).*$|\13|' /etc/yum.conf && ok=1 || ok=-1
      elif [ "${kernel_update_default,,}" == "no" -a $kernels_limit -ne 0 ]; then
        sudo sed -i 's|^\(installonly_limit=\).*$|\10|' /etc/yum.conf && ok=1 || ok=-1
      fi
      case $ok in
        1) echo -e "\nUpdated kernel removing policy.\nThe number of kernels to keep set to:" \
                   "\e[32m$( get_kernels_limit | sed 's|^0$|infinity|' )\e[0m" ;;
        -1) error_exit '\nCannot update kernel removing policy...' ;;
        0) echo -e '\nNo updated kernel removing policy needed.' ;;
      esac
    fi
  fi
}

### MAIN

# check/update grub default kernel policy to make saved kernel default
update_grub_default

# show current saved kernel
echo -e "\nCurrent default kernel: \e[33m$( get_saved_kernel )\e[0m"

# obtain the list of available kernels
kernels=()
count=0
echo -e '\nAvailable kernels:'
while read s; do
  let count=$count+1
  kernels[$count]="$s"
  echo "$count: ${kernels[$count]}"
done < <( sudo cat /boot/grub2/grub.cfg \
             | grep '^menuentry.*[Ll]inux' \
             | grep -v 'rescue' \
             | sed "s|^\s*menuentry\s*['\"]\([^'\"]\+\)['\"].*$|\1|" \
        )
[ $count -eq 0 ] && error_exit 'No kernels found in /boot/grub2/grub.cfg...'

# ask for the number of the new default kernel
read -p "Enter kernel number to set as default (1-$count): [skip] " n
if [ -z "$n" ]; then
  :
elif ! [[ "$n" =~ ^[0-9]*$ ]] || [ $n -lt 1 -o $n -gt $count ]; then
  error_exit 'Wrong number.'
else
  # set the new default kernel
  kernel="${kernels[$n]}"
  echo -e "\nDefault kernel will be set as $kernel"
  read -p 'Confirm (y/n) ? [n] ' yn
  if [[ "$yn" =~ ^[Yy]$ ]]; then
    set_saved_kernel "$kernel"
    echo -e "\nCurrent default kernel: \e[32m$( get_saved_kernel )\e[0m"
  fi
fi

# set default kernel update behavior
echo -en "\nWhen the kernel gets updated it can automatically set the new kernel version as default." \
         "Current status of automatic default kernel updates: \e[33m$( get_kernel_update_default )\e[0m" \
         "\nDo you want to switch this behaviour (y/n) ? [n] "
read yn
if [[ "$yn" =~ ^[Yy]$ ]]; then
  toggle_kernel_update_default
  echo -e "\nCurrent status of automatic default kernel updates: \e[32m$( get_kernel_update_default )\e[0m"
  # prevent kernel removing if needed: yum only
  update_kernel_removing
fi