youtube-dl

download and install

sudo wget https://yt-dl.org/latest/youtube-dl -O /usr/local/bin/youtube-dl

sudo apt-get install ffmpeg

set ejecution

sudo chmod a+x /usr/local/bin/youtube-dl

update

sudo youtube-dl -U

help

youtube-dl -h

download type video

youtube-dl -F https://www.youtube.com/watch?v=j_JgXJ-apXs -ciw

download video best quality

youtube-dl -f bestvideo+bestaudio http://www.youtube.com/watch?v=P9pzm5b6FFY -ciw

171         webm      audio only  DASH webm audio , audio@ 48k (worst)
140         m4a       audio only  DASH audio , audio@128k (worst)
160         mp4       144p        DASH video , video only
133         mp4       240p        DASH video , video only
134         mp4       360p        DASH video , video only
135         mp4       480p        DASH video , video only
136         mp4       720p        DASH video , video only
17          3gp       176x144     
36          3gp       320x240     
5           flv       400x240     
43          webm      640x360     
18          mp4       640x360     
22          mp4       1280x720    (best)

youtube-dl -f 22 http://www.youtube.com/watch?v=P9pzm5b6FFY -ciw

If you want to download Youtube video in mp3 audio format, then it is also possible with the following command

youtube-dl https://www.youtube.com/watch?v=j_JgXJ-apXs -x --audio-format mp3 -ciw