I have a raw img file. I resized the partition but now it doesn't fill the disk. How do I shrink the disk. (/dev/nbd0)

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"
GPT_BACKUP_SECTORS=33

# 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

# Attempt automatic detection of the backing file
IMG_FILE=""
if command -v qemu-nbd &>/dev/null; then
    IMG_FILE=$(qemu-nbd --list-connections 2>/dev/null | awk -v dev="$NBD_DEV" '$0 ~ dev {print $NF}' || true)
fi

# Fallback prompt if auto-detection doesn't return a valid file
if [[ -z "$IMG_FILE" || ! -f "$IMG_FILE" ]]; then
    read -rp "Enter full path to backing .img file: " RAW_INPUT
    # Extract only the file path if extra text or device names were pasted
    IMG_FILE=$(echo "$RAW_INPUT" | awk '{print $NF}')
fi

if [[ ! -f "$IMG_FILE" ]]; then
    echo "Error: Backing image file '$IMG_FILE' does not exist." >&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 sectors including GPT backup header padding
# Last partition end sector + 1 (0-indexed offset) + 33 sectors for secondary GPT table
REQUIRED_SECTORS=$((LAST_SECTOR + 1 + GPT_BACKUP_SECTORS))
REQUIRED_BYTES=$((REQUIRED_SECTORS * SECTOR_SIZE))

echo "=================================================="
echo " NBD Device          : $NBD_DEV"
echo " Backing File        : $IMG_FILE"
echo " Last Partition End  : Sector $LAST_SECTOR"
echo " GPT Backup Padding  : $GPT_BACKUP_SECTORS sectors"
echo " Total Target Sectors: $REQUIRED_SECTORS sectors"
echo " Target Size         : $REQUIRED_BYTES bytes ($((REQUIRED_BYTES / 1024 / 1024)) MiB)"
echo "=================================================="

# Interactive confirmation prompt
read -rp "Relocate secondary GPT header and truncate $IMG_FILE? (y/N): " CONFIRM
if [[ ! "$CONFIRM" =~ ^[Yy]$ ]]; then
    echo "Operation canceled by user."
    exit 0
fi

# Unmount active partitions
echo "[1/4] Unmounting active mounts on $NBD_DEV..."
umount ${NBD_DEV}p* 2>/dev/null || true

# Relocate secondary GPT header to fit newly calculated boundary
echo "[2/4] Relocating secondary GPT header..."
sgdisk -e "$NBD_DEV" 2>/dev/null || true

# Disconnect NBD device
echo "[3/4] Disconnecting $NBD_DEV..."
qemu-nbd --disconnect "$NBD_DEV"

# Truncate backing image file
echo "[4/4] Truncating $IMG_FILE to $REQUIRED_BYTES bytes..."
truncate -s "$REQUIRED_BYTES" "$IMG_FILE"

echo "--------------------------------------------------"
echo "Success: $IMG_FILE has been safely truncated with secondary GPT structures intact."