缩略图不是由ffmpeg生成的

Question

I have installed ffmpeg in my linux box and have this code to generate thumbnails while I upload videos to my webserver. I am trying to save thumbnails in the same folder where I am uploading my videos

php:

if (isset($_FILES['attachments'])) 
{
  $msg = "";
  $targetFile = "upload/" . basename($_FILES['attachments']['name'][0]);
  if (file_exists($targetFile))
  $msg = array("status" => 0, "msg" => "File already exists!");
  else if (move_uploaded_file($_FILES['attachments']['tmp_name'][0],$targetFile))
  $msg = array("status" => 1, "msg" => "File Has Been Uploaded", "path" => $targetFile);

  $ffmpeg = "upload/";
  $videoFile = $_FILES["attachments"]["tmp_name"];
  $size = "120x90";

    for($num = 1; $num <= 3; $num++)
    {
        $interval = $num * 3;
        shell_exec("$ffmpeg -i $videoFile -an -ss $interval -s $size $num.jpg");
        echo "Thumbnail Created! - $num.jpg<br />";
    }
}

But I am only getting my uploaded videos in that folder? I find the documentation to use ffmpeg is quite confusing and don't if I am doing it right or not.

Output :

and when I ran the "ffmpeg" in the command line: