O_VID_FORMAT="mp4"
VID_URL="https://www.youtube.com/watch?v=jKPlGznbfZ0"
youtube-dl -f bestvideo+bestaudio --merge-output-format $O_VID_FORMAT $VID_URL
# For getting MP4 version
youtube-dl -f 'bestvideo[ext=mp4]+bestaudio[ext=m4a]/best[ext=mp4]/best' $VID_URL
ffmpeg -i FILE_NAME.mkv -codec copy FILE_NAME.mp4