The current template generation has several issues:

1. Bug: The $ in template variables like $v.PkgName are not escaped. The shell will substitute them with empty strings, breaking the template and causing trivy to fail.
2. Incorrect Output: The template includes a static header row, which will be repeated in the output for every image.
3. Inefficiency: The template is regenerated on every script run.

I suggest replacing this block to create a header-less template only if it doesn't exist. Using <<'EOL' is a safer way to define the template content without needing to escape $ characters. You should then add a command like echo '"Image","Package","Version Installed","Vulnerability ID","Severity","Title"' before the main loop to print the CSV header once.

The main processing loop can be improved for robustness and to produce valid CSV output:

1. Incorrect Output: The awk command produces a non-standard CSV field Trivy:"<image>". The image name should be a proper, quoted CSV field.
2. Mixed Output: Diagnostic messages are sent to stdout, mixing them with the CSV data. They should be sent to stderr.
3. Robustness: The script doesn't filter out images with <none> tags and uses a fragile method for generating SBOM filenames.

Here is a suggested replacement for the loop that addresses these points. This assumes you also add a line to print the CSV header (e.g., echo '"Image","Package"...') before this loop, as suggested in another comment.

# Loop through each container image and process its SBOM
docker image ls --format "{{.Repository}}:{{.Tag}}" | grep -v "<none>" | sort -u | while read -r image; do
    # Generate SBOM filename from a hash of the image name for robustness
    sbom_file="$SBOM_DIR/$(echo -n "$image" | sha256sum | cut -d' ' -f1).sbom"

    # Generate SBOM if missing
    if [[ ! -f "$sbom_file" ]]; then
        echo "Generating SBOM for $image..." >&2
        if ! trivy image --quiet --format spdx-json --output "$sbom_file" "$image"; then
            echo "Failed to generate SBOM for $image. Skipping." >&2
            continue
        fi
    fi

    # Scan SBOM and prepend image info to each output line
    trivy sbom \
      --scanners vuln \
      --severity CRITICAL,HIGH \
      --ignore-unfixed \
      --quiet \
      --format template \
      --template "@$SBOM_DIR/trivy-custom.tmpl" \
      "$sbom_file" | \
    awk -v img="$image" 'NF > 0 {print "\"" img "\"," $0}'
done

The SBOM directory path is hardcoded. This reduces the script's flexibility. It's better to allow this to be configured via an environment variable, with the current path as a default.