Write a bash script to calculate the last sector of /dev/nbd0 and automatically truncate the backing raw img file

sudo fdisk -l /dev/nbd0

# Unmount any active mounts
sudo umount /dev/nbd0p* 2>/dev/null

# Disconnect NBD device
sudo qemu-nbd --disconnect /dev/nbd0

# Truncate image to exact sector boundary (replace <BYTES> with calculated value)
truncate -s <BYTES> /path/to/your_image.img

sudo qemu-nbd --connect=/dev/nbd0 /path/to/your_image.img
sudo fdisk -l /dev/nbd0

sudo qemu-nbd --disconnect /dev/nbd0

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

NBD_DEV="/dev/nbd0"

# Check root privileges
if [[ $EUID -ne 0 ]]; then
    echo "Error: This script must be run with sudo or as root." >&2
    exit 1
fi

# Ensure NBD device is connected
if ! lsblk "$NBD_DEV" &>/dev/null; then
    echo "Error: $NBD_DEV is not active or connected." >&2
    exit 1
fi

# Find the backing image file
IMG_FILE=$(qemu-nbd --list-connections 2>/dev/null | awk -v dev="$NBD_DEV" '$0 ~ dev {print $NF}' || true)

# Fallback check via sysfs if qemu-nbd output isn't parsed
if [[ -z "$IMG_FILE" && -b "$NBD_DEV" ]]; then
    # Retrieve backing file path using losetup/sysfs inspection if applicable
    if [[ -f "/sys/block/${NBD_DEV#/dev/}/size" ]]; then
        read -rp "Enter the full path to the raw .img file: " IMG_FILE
    fi
fi

if [[ ! -f "$IMG_FILE" ]]; then
    echo "Error: Could not locate backing image file '$IMG_FILE'." >&2
    exit 1
fi

# Find the highest end sector across all partitions
LAST_SECTOR=$(fdisk -l "$NBD_DEV" 2>/dev/null | awk -v dev="$NBD_DEV" '$1 ~ dev {print $3}' | sort -n | tail -n 1)

if [[ -z "$LAST_SECTOR" || ! "$LAST_SECTOR" =~ ^[0-9]+$ ]]; then
    echo "Error: Could not determine last sector from partition table on $NBD_DEV." >&2
    exit 1
fi

# Get sector size (usually 512 bytes)
SECTOR_SIZE=$(blockdev --getss "$NBD_DEV")

# Calculate total required bytes (Last Sector + 1) * Sector Size
REQUIRED_SECTORS=$((LAST_SECTOR + 1))
REQUIRED_BYTES=$((REQUIRED_SECTORS * SECTOR_SIZE))

echo "--------------------------------------------------"
echo "Backing File   : $IMG_FILE"
echo "Last End Sector: $LAST_SECTOR"
echo "Sector Size    : $SECTOR_SIZE bytes"
echo "Target Size    : $REQUIRED_BYTES bytes ($((REQUIRED_BYTES / 1024 / 1024)) MiB)"
echo "--------------------------------------------------"

# Unmount active partitions on nbd0
echo "[1/3] Unmounting active mounts on $NBD_DEV..."
umount ${NBD_DEV}p* 2>/dev/null || true

# Disconnect NBD device
echo "[2/3] Disconnecting $NBD_DEV..."
qemu-nbd --disconnect "$NBD_DEV"

# Truncate image file
echo "[3/3] Truncating $IMG_FILE to $REQUIRED_BYTES bytes..."
truncate -s "$REQUIRED_BYTES" "$IMG_FILE"

echo "Done. $IMG_FILE has been successfully resized."