youtube-dl

Step 1: Download curl package

First download the curl package in the System

In RHEL/CentOS

yum install curl
In Debian/Ubuntu

sudo apt-get install curl
Step 2: Download youtube-dl script

sudo curl yt-dl.org/downloads/2014.03.12/youtube-dl -o /usr/local/bin/youtube-dl
Step 3: Give permission to youtube-dl script

sudo chmod a+x /usr/local/bin/youtube-dl
Step 4: Download all videos from youtube channel/Youtube User/Playlist

All videos from youtube channel

/usr/local/bin/youtube-dl -citw http://youtube-channel-URL
All videos of Youtube User

/usr/local/bin/youtube-dl -citw ytuser:User-name
All videos from Playlist

/usr/local/bin/youtube-dl -citw http://playlist-url
Note:
-c = force resume of partially downloaded files.By default, youtube-dl will resume downloads if possible
-w = do not overwrite files
-t = use title in file name (default)
-i = continue on download errors, for example to skip unavailable videos in a playlist

youtube-dl has many options,for further information run the command /usr/local/bin/youtube-dl --help