grub-boot-select.sh

#!/usr/bin/env bash
set -euo pipefail

# Check if running as root, if not, re-run with sudo
if [ "$EUID" -ne 0 ]; then
  echo "This script needs to read grub.cfg and modify boot settings."
  echo "Re-running with sudo..."
  exec sudo "$0" "$@"
fi

sudo grub2-mkconfig -o /boot/grub2/grub.cfg

# Arrays to hold all boot entries and their types
declare -a ALL_ENTRIES
declare -a ENTRY_TYPES
declare -a ENTRY_DATA

# 0) Pick grub.cfg path (prefer grub2 layout) and read GRUB entries
CFG=""
if [ -r /boot/grub2/grub.cfg ]; then
  CFG="/boot/grub2/grub.cfg"
elif [ -r /boot/grub/grub.cfg ]; then
  CFG="/boot/grub/grub.cfg"
fi

if [ -n "$CFG" ]; then
  # 1) Read every top‑level menuentry title (handles leading whitespace & both quote styles)
  mapfile -t GRUB_ENTRIES < <(
    grep -E "^[[:space:]]*menuentry [\"'].*[\"']" "$CFG" \
      | sed -E "s/^[[:space:]]*menuentry [\"']([^\"']+)[\"'].*/\1/"
  )
  
  for entry in "${GRUB_ENTRIES[@]}"; do
    ALL_ENTRIES+=("$entry")
    ENTRY_TYPES+=("GRUB")
    ENTRY_DATA+=("$entry")
  done
fi

# 2) Check for EFI boot entries
if command -v efibootmgr >/dev/null 2>&1; then
  # Parse efibootmgr output to get boot entries
  # Format: Boot0000* Ubuntu
  mapfile -t EFI_RAW < <(efibootmgr | grep -E "^Boot[0-9A-F]{4}\*?" || true)
  
  for line in "${EFI_RAW[@]}"; do
    # Extract boot number (e.g., "0000") and name
    if [[ "$line" =~ ^Boot([0-9A-F]{4})(\*?)[[:space:]]+(.+)$ ]]; then
      BOOT_NUM="${BASH_REMATCH[1]}"
      BOOT_NAME="${BASH_REMATCH[3]}"
      ALL_ENTRIES+=("[EFI] $BOOT_NAME")
      ENTRY_TYPES+=("EFI")
      ENTRY_DATA+=("$BOOT_NUM")
    fi
  done
fi

if [ ${#ALL_ENTRIES[@]} -eq 0 ]; then
  echo "No GRUB or EFI boot entries found" >&2
  exit 1
fi

# 3) Print numbered menu
echo "Select which OS (or kernel) to boot *next* time only:"
for i in "${!ALL_ENTRIES[@]}"; do
  printf "  %2d) %s\n" "$i" "${ALL_ENTRIES[$i]}"
done
echo

# 4) Prompt
read -rp "Enter choice number: " CHOICE

# 5) Validate
if ! [[ "$CHOICE" =~ ^[0-9]+$ ]] || (( CHOICE < 0 || CHOICE >= ${#ALL_ENTRIES[@]} )); then
  echo "Invalid selection." >&2
  exit 1
fi

# 6) Handle boot based on entry type
ENTRY_TYPE="${ENTRY_TYPES[$CHOICE]}"
ENTRY_INFO="${ENTRY_DATA[$CHOICE]}"

if [ "$ENTRY_TYPE" = "GRUB" ]; then
  # Find the proper grub reboot command
  if command -v grub-reboot >/dev/null 2>&1; then
    GRUB_REBOOT_CMD="grub-reboot"
  elif command -v grub2-reboot >/dev/null 2>&1; then
    GRUB_REBOOT_CMD="grub2-reboot"
  else
    echo "Neither grub-reboot nor grub2-reboot found in PATH." >&2
    exit 1
  fi
  
  echo "Will reboot into GRUB entry \"$ENTRY_INFO\" on next boot…"
  "$GRUB_REBOOT_CMD" "$ENTRY_INFO"
  
elif [ "$ENTRY_TYPE" = "EFI" ]; then
  echo "Will reboot into EFI entry \"${ALL_ENTRIES[$CHOICE]}\" on next boot…"
  efibootmgr --bootnext "$ENTRY_INFO"
fi

reboot