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bash does not support floating point operations, so at least for a short video `$interval` will always be `0`. That way you always get the same frame, which by chance probably was black for your video.

You notice that if you place an `echo` in front of your `ffmpeg` call to check the actual command. This resulted in this output for me:

```
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame0.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame1.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame2.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame3.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame4.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame5.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame6.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame7.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame8.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame9.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame10.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame11.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame12.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame13.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame14.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame15.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame16.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame17.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame18.jpg
ffmpeg -ss 0 -i Example.mp4 -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 Example_frame19.jpg
```

As you can see, `-ss` is always `0`.

You can use for example [`bc`](https://linux.die.net/man/1/bc) to do the calculations (it's possible that you need to install it):

```shell
interval=$(echo "$duration / 20" | bc)
```

Afterwards the rest of your script mostly worked for me.

![Output image of the script, a 5x4 grid of video images](https://linux.codidact.com/uploads/zqf5my353fnvcnm8mwgsxyo9ujgb)

More optimizations:

```shell
for i in {0..19}; do
    ffmpeg -ss $((interval * i)) -i "$1" -vf scale=200:-1 -q:v 2 -frames:v 1 -loglevel 0 "${1%.*}_frame$i.jpg"
done
```

- By using `0..19` instead of `1..20` you can clean up the calculations for the wanted second.
- `-frames:v 1` removes an error message
- `-loglevel 0` removes unnecessary clutter

```shell
rm "${1%.*}"_frame*.jpg
```

fixes the non working removal of the temporary images.