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

# 0) Pick grub.cfg path (prefer grub2 layout)
if [ -r /boot/grub2/grub.cfg ]; then
  CFG="/boot/grub2/grub.cfg"
elif [ -r /boot/grub/grub.cfg ]; then
  CFG="/boot/grub/grub.cfg"
else
  echo "Could not find grub.cfg under /boot/grub2 or /boot/grub" >&2
  exit 1
fi

# 1) Read every top‑level menuentry title (handles leading whitespace & both quote styles)
mapfile -t ENTRIES < <(
  grep -E "^[[:space:]]*menuentry [\"'].*[\"']" "$CFG" \
    | sed -E "s/^[[:space:]]*menuentry [\"']([^\"']+)[\"'].*/\1/"
)

if [ ${#ENTRIES[@]} -eq 0 ]; then
  echo "No GRUB entries found in $CFG" >&2
  exit 1
fi

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

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

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

# 5) 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

# 6) Reboot into the selected title
TITLE="${ENTRIES[$CHOICE]}"
echo "Will reboot into \"$TITLE\" on next boot…"
"$GRUB_REBOOT_CMD" "$TITLE"
reboot