Download & Slowdown a YouTube video using ffmpeg && youtube-dl

youtube_slowed.bash

# add this to yer .bash_profile ==>

function youtube_slower {
    if [ ! -z $2 ]; then
        tempo="$2"
    else
        tempo="0.8"
    fi
    base_filename="$(youtube-dl "$1" --get-filename)"
    extensionless="${base_filename%.*}"
    output_filename="$extensionless"".slowed.mp3";
    youtube-dl $1 -q -o - | ffmpeg -y -v quiet -i - -filter:a "atempo=$tempo" "$output_filename";
    echo "opening \"$output_filename\"";
    if [ -f "$output_filename" ]; then
            open "$output_filename";
    fi
}
# usage:
# youtube-slow "http://www.youtube.com/watch?v=drbnCPq5YHk"
alias youtube-slow=youtube_slower
 
function youtube_munch {
	while read p; do
		echo "munching $p...";
		youtube-slow "$p"
	done < <(grep '' $1)
}
alias youtube-munch=youtube_munch
# usage:
# youtube-munch path-to-text-file-full-of-youtube-urls.txt

requires ffmpeg and youtube-dl, installable via HomeBrew on the Mac:

brew install ffmpeg
brew install youtube-dl

homebrew may require the Command Line Tools, which can be installed thusly:

xcode-select --install