#!/bin/bash

#

# For grub2

# ---------

# Script for selecting which kernel to boot on:

# but only once (single boot),

# useful if kernel will not boot (next power cycle, boot default kernel)

#

# Grub2 is a little more tricky than grub1

# ----------------------------------------

# [1] https://fedoraproject.org/wiki/GRUB_2?rd=Grub2#Setting_default_entry

#

# Due to grub2-mkconfig (and os-prober) we cannot predict the order of

# the entries in /boot/grub2/grub.cfg, so we set the default by

# name/title instead.

#

# Authors: Jesper Dangaard Brouer <brouer@redhat.com>

# sudo trick

function root_check_run_with_sudo() {

# Trick so, program can be run as normal user, will just use "sudo"

# call as root_check_run_as_sudo "$@"

if [ "$EUID" -ne 0 ]; then

if [ -x $0 ]; then # Directly executable use sudo

echo "(Not root, running with sudo)"

sudo "$0" "$@"

exit $?

fi

echo "cannot perform sudo run of $0"

exit 1

fi

}

root_check_run_with_sudo "$@"

# Source the default grub config

source /etc/default/grub

# Verify "saved" mode is enabled

if [[ "$GRUB_DEFAULT" != "saved" ]]; then

echo "***ERROR***: Please enable grub2 \"saved\" mode, by adding:"

echo " GRUB_DEFAULT=saved to /etc/default/grub"

echo "and regenerate grub2 config via cmd:"

echo " grub2-mkconfig -o /boot/grub2/grub.cfg"

exit 1

fi

# Verify submenu have been disabled

if [[ "$GRUB_DISABLE_SUBMENU" != "true" ]]; then

echo "***ERROR***: Please disable grub2 \"submenu\" mode, by adding:"

echo " GRUB_DISABLE_SUBMENU=true to /etc/default/grub"

echo "and regenerate grub2 config via cmd:"

echo " grub2-mkconfig -o /boot/grub2/grub.cfg"

echo "(This script does not support grub2 submenus)"

exit 2

fi

# Unfortunately output from grubenv or $(grub2-editenv list) cannot

# directly be sourced as shell variables due to missing quoting.

# It would have been as easy as:

# export $(grub2-editenv list)

# Instead extract each variable seperately

function get_grubenv()

{

local key=$1

value=$(grub2-editenv list | grep $key | cut -d "=" -f2)

echo "$value"

}

# Get current saved grub env variables

# export $(grub2-editenv list) #cannot be used due to missing quoting

# export $(grub2-editenv list | awk -F= '{print $1"=\""$2"\""}')

# export $(grub2-editenv list | sed -n 's/\(.*\)=\(.*\)/\1=\x27\2\x27/p')

saved_entry=$(get_grubenv saved_entry)

next_entry=$(get_grubenv next_entry)

echo "Select kernel to boot once (on next reboot):"

# Extract the avail kernels from grub.cfg

IFS=$'\n' KERNELS=( $(grep "^\menuentry" /boot/grub2/grub.cfg | cut -d "'" -f2) )

function select_kernel()

{

local sel

cnt=${#KERNELS[@]}

if [[ -n $1 ]]; then

sel=$1

else

for ((i = 0; i < cnt; i++)); do

flag=' '

[[ "${KERNELS[i]}" == "$saved_entry" ]] && flag='*'

[[ "${KERNELS[i]}" == "$next_entry" ]] && flag='+'

# todo: saved entry could be a number

# (( i == $saved_entry )) && flag='*'

echo "$flag$i: ${KERNELS[i]}" >&2

done

read -p 'Select a kernel: ' sel

fi

if (( sel == 0 || sel >= cnt )) && [[ $sel != 0 ]]; then

echo "Invalid input" >&2

exit 1

fi

echo "$sel"

}

# Make sure a default fallback kernel have been selected

if [[ -z "$saved_entry" ]]; then

echo "***WARNING***: You don't have a fallback saved entry"

read -p "Do you want to select a fallback now (y/[n])?"

if [ "$REPLY" == "y" ]; then

fallback=$(select_kernel)

if [[ $? -gt 0 ]]; then

echo "***ERROR*** selecting kernel, bailout"

exit 2;

fi

echo " - Selected fallback kernel: ${KERNELS[fallback]}"

grub2-set-default "${KERNELS[fallback]}"

saved_entry=$(get_grubenv saved_entry)

fi

echo "Now: Select kernel to boot once (on next reboot):"

else

echo " - Current fallback saved_entry: $saved_entry"

fi

if [[ -n "$next_entry" ]]; then

echo " - Currently already choose boot next: $next_entry"

fi

# Allow cmdline selecting kernel by index

if [[ -n $1 ]]; then

cmdline_select=$1

fi

# Choose kernel

index=$(select_kernel $cmdline_select)

if [[ $? -gt 0 ]]; then

echo "***ERROR*** selecting kernel, bailout"

exit 1;

fi

echo " - Selected boot once kernel: $index: \"${KERNELS[index]}\""

grub2-reboot "${KERNELS[index]}"

read -p "Do you want to reboot now (y/[n])?"

if [[ "$REPLY" == "y" ]]; then

reboot

fi