入力データ

$ curl -sS "http://www.billboard.com/rss/charts/hot-100" | sed -n -e 's/<title>\([0-9]\+:.*\)<\/title>.*/\1/p' | head | cut -d: -f2 | tee billboardtop10.txt
 Bad And Boujee
 Shape Of You
 Closer
 Bad Things
 I Don&#039;t Wanna Live Forever (Fifty Shades Darker)
 Black Beatles
 Don&#039;t Wanna Know
 Starboy
 Fake Love
 Scars To Your Beautiful

sオプションを使用して、カンマ区切りで結合

$ paste -d, -s billboardtop10.txt
 Bad And Boujee, Shape Of You, Closer, Bad Things, I Don't Wanna Live Forever (Fifty Shades Darker), Black Beatles, Don't Wanna Know, Starboy, Fake Love, Scars To Your Beautiful