#!/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