Here is a nice tool to download youtube videos. It’s called youtube-dl. You can install it as follows.

sudo curl https://yt-dl.org/downloads/2014.05.05/youtube-dl -o /usr/local/bin/youtube-dl
sudo chmod a+x /usr/local/bin/youtube-dl

Or if you have apt-get, simply do sudo apt-get install youtube-dl.

Here is the documentation on how to use it. Below is a simple command to get you started. =)

# using the video title as filename
youtube-dl -o "%(title)s.%(ext)s" "https://www.youtube.com/watch?v=R9ohnM69JQ0" --restrict-filenames

The –restrict-filenames flag will take away the special characters in output filename.