$filename is handled as a shell variable.
What about
ffmpeg -i clip.mp4 fr1/clip_%d.jpg -hide_banner or
$mp4filename=clip ffmpeg -i $mp4filename.mp4 fr1/$mp4filename_%d.jpg -hide_banner ?
$filename is handled as a shell variable.
What about
ffmpeg -i clip.mp4 fr1/clip_%d.jpg -hide_banner or
$mp4filename=clip ffmpeg -i $mp4filename.mp4 fr1/$mp4filename_%d.jpg -hide_banner ?