ASSEMBLE_AND_EXTRACT.sh

#!/usr/bin/env bash
set -euo pipefail
# Reassemble .tar.gz stream from parts, verify checksums, then extract.
# Usage: ./ASSEMBLE_AND_EXTRACT.sh [DEST_DIR]
DEST="${1:-./extracted}"
mkdir -p "$DEST"

if [[ -f PARTS.SHA256SUMS ]]; then
  echo "Verifying part checksums..."
  sha256sum -c PARTS.SHA256SUMS
fi

if [[ -f TAR.SHA256 ]]; then
  echo "Verifying full tar.gz stream checksum..."
  expected="$(cut -d' ' -f1 TAR.SHA256)"
  actual="$(cat model.tar.gz.part.* | sha256sum | awk '{print $1}')"
  if [[ "$actual" != "$expected" ]]; then
    echo "ERROR: TAR stream checksum mismatch!"
    echo " expected: $expected"
    echo "   actual: $actual"
    exit 1
  fi
fi

echo "Checksum OK. Extracting..."
cat model.tar.gz.part.* | tar -xzf - -C "$DEST"
echo "✅ Done. Extracted to: $DEST"