How to download a portion of a YouTube video

After a few days of searching, the best solution I've come across for this so far combines youtube-dl's ability to fetch the internal URLs of a YouTube video's individual streams with ffmpeg's ability to use those streams as input.

The quick (Bash) way

Run the following Bash magic to automatically populate the video_url and audio_url variables with the internal YouTube URLs for the target link's video and audio streams respectively:

# As scary as it looks, perfectly safe to run in a terminal 
{   
   read -r video_url  
   read -r audio_url
} < <(
   youtube-dl --get-url --youtube-skip-dash-manifest https://www.youtube.com/watch?v=MfnzBYV5fxs
)

Lastly, pass the timestamps through to download the portion of the video that you want:

# Download 2 minutes of the video between 5 mins in to 7 mins, using timestamps:
ffmpeg -ss 00:05:00.00 -to 00:07:00.00 -i "$video_url" -ss 00:05:00.00 -to 00:07:00.00 -i "$audio_url" output.mkv

Or if you prefer to crop a duration instead of the time between two timestamps:

# Download 2 minutes of the video between 5 mins in to 7 mins, using duration:  
ffmpeg -ss 00:05:00.00 -i "$video_url" -ss 00:05:00.00 -i "$audio_url" -t 2:00 output.mkv

The manual (non-Bash) way

If you're using any other shell apart from Bash, you'll have to do it the manual way:

Running the following will output YouTube's (very long) internal URLs for your target video's video and audio streams:

youtube-dl --get-url --youtube-skip-dash-manifest "https://www.youtube.com/watch?v=MfnzBYV5fxs"

Plug in these URLs into the following command, pasting them in place of <video_url> and <audio_url> respectively:

# Download 2 minutes of the video between 5 mins in to 7 mins, using timestamps:
ffmpeg -ss 00:05:00.00 -to 00:07:00.00 -i <video_url> -ss 00:05:00.00 -to 00:07:00.00 -i <audio_url> output.mkv

Or if you prefer to crop a duration instead of the time between two timestamps:

# Download 2 minutes of the video between 5 mins in to 7 mins, using duration:  
ffmpeg -ss 00:05:00.00 -i <video_url> -ss 00:05:00.00 -i <audio_url> -t 2:00 output.mkv