you can use the loops:

src_dir=~/test-dir
dest_dir=~/compressed-videos
for video in "$src_dir"/*.mp4; do
dest_video="$dest_dir/$(basename "$video")_compressed.mp4"
ffmpeg -i "$video" "$dest_video"
done


Sorry for being so late. I was little busy. To delete original files execute:
find ~/test-dir -name '*.mp4' ! -name '*_compressed.mp4' -exec rm {} \;
For being on safe side, you can use rm -i instead of rm to get confirmation before deleting the file.

I have tried it with my Lubuntu setup (on HCL Laptop) and it worked. May be there are some conflicts with other touchpad drivers installed by Lubuntu. So make sure those drivers are disabled/have less precedence.