Ubuntu – How to download a youtube playlist with numbered prefix via youtube-dl

You are seeing the default behavior of youtube-dl where since April 2015 and version 2015.04.26 unless told otherwise youtube-dl will download the best quality video stream and the best quality audio stream.

Two cases arise then:

1. If you have either ffmpeg or avconv installed the two best quality streams will be downloaded and then muxed together. If the codecs are incompatible together in either flv or mp4 they will be joined in mkv. This is happening in your case.
2. If you don't have either ffmpeg or avconv installed the best quality will be downloaded that is available as a single file.

So, you are seeing normal behavior...

References:

• youtube-dl FAQ: Format Selection

Playlist

youtube-dl -f FORMAT -ciw --output '%(title)s.%(ext)s' --playlist-start NUMBER-START --playlist-end NUMBER-END <url-of-playlist>  

...where <url-of-playlist> is replaced by the URL of the playlist, replace FORMAT with any available video format, for example 18, NUMBER-START is the number of the video in the playlist to start downloading first, and NUMBER-END is the number of the video in the playlist to download last.

Channel

If a channel has more than one playlist, click on the first playlist and download all the videos in the selected playlist using the above command. Then repeat for each playlist in the channel.

Explanation

-f, --format FORMAT
    video format code. The -F option (capital F) displays all available video  
    formats for a video link. Example: youtube-dl -F <url-of-video>

-c, --continue                   
    force resume of partially downloaded files

-i, --ignore-errors              
    continue on download errors, for example to skip unavailable videos  
    in a channel   

-w, --no-overwrites
    do not overwrite files 

Convert all the video titles to lowercase

youtube-dl -f FORMAT -ciw --output '%(title)s.%(ext)s' --playlist-start NUMBER-START --playlist-end NUMBER-END <url-of-playlist>     
find -type f -exec rename 'y/A-Z/a-z/' {} +

Explanation

--output '%(title)s.%(ext)s'  
    output file name(s) as the name of the video, followed by a dot character and the video's extension  

find -type f 
    Find all files.

y/source/destination/  
    Transliterate the characters in the pattern space which appear in source   
    to the corresponding character in destination.