#!/bin/bash -x

# Set grub to boot into a different distro, and reboot unless -r
# $0 [DISTRO_NAME]
# with no argument, print available distros

# DISTRO_NAME is based on the partition names in /boot. eg boot_debianjessie

set -eE -o pipefail
trap 'echo "$0:$LINENO:error: \"$BASH_COMMAND\" returned $?"' ERR

[[ $EUID == 0 ]] || exec sudo "$BASH_SOURCE" "$@"

reboot=true
while [[ $1 == -* ]]; do
    case $1 in
        -r) reboot=false; shift ;;
        --) break ;;
    esac
done


distro=$1

###### end command line parsing #####

if [[ ! $distro ]]; then
    echo "available distros:"
    btrfs subvolume list /boot | sed -rn 's/^.*boot_(.*)/\1/p'
    exit 0
fi

if ! btrfs subvolume list /boot | grep "_$distro$" &>/dev/null; then
    echo "$0: error: _$distro$ not found in btrfs subvolume list /boot:"
    btrfs subvolume list /boot
    exit 1
fi

e() { echo "$@"; "$@"; }

boot_dev=$(mount | sed -rn "s#^(\S+) on /boot .*#\1#p")

mount_point=$(mktemp -d)

e mount -o subvol=boot_$distro $boot_dev $mount_point

boot_disk=${boot_dev%%[0-9]*}

# arch doesn't have $mount_point/grub/device.map, accoring to the grub manual,
# it just generates one if the file doesn't exist.
# https://www.gnu.org/software/grub/manual/html_node/Device-map.html
e grub-bios-setup -d $mount_point/grub/i386-pc -s -m $mount_point/grub/device.map $boot_disk

e umount $mount_point
e rmdir $mount_point

if $reboot; then
    reboot now
fi