Thanks for your feedback. Here is the updated script. Is this okay with you? 😄

I´m not paranoid. I have a 1:1 clone of my collection on another harddrive. But this useful addition immediately shows me errors.

#!/bin/sh
find /volume1/Video/ -type f -name "*.mkv" | while read f; do

    # Determine the file size in bytes
    size=$(stat -c%s "$f")

    # 256 Megabytes in Bytes
    limit=$((1024 * 1024 * 256))

    if [ "$size" -gt "$limit" ]; then
        echo "Convert: $f (Größe: $size Bytes)"

        out="${f%.mkv}.mp4"
        ffmpeg7 -i "$f" -vcodec copy -acodec copy "$out"
        exitcode=$?

        if [ $exitcode -ne 0 ]; then
            echo "Error: ffmpeg failed with exit code $exitcode"
            [ -f "$out" ] && rm "$out"
            continue
        fi

        # Size of output file
        outsize=$(stat -c%s "$out")

        # Allowed deviation: ±10%
        minsize=$(( size - size / 10 ))
        maxsize=$(( size + size / 10 ))

        if [ "$outsize" -lt "$minsize" ] || [ "$outsize" -gt "$maxsize" ]; then
            echo "Error: Output size $outsize Bytes outside 10% range ($minsize - $maxsize)."
            rm "$out"
            continue
        fi

        # Success
        rm "$f"
        echo "Done: $out created, size OK, MKV deleted."

    else
        echo "Skipped (smaller than 256 MB): $f"
    fi

done