Você se esqueceu de citar o URL dado a ffmpeg
, de modo que o shell está consumindo alguns dos caracteres.
ffmpeg -i 'https://r1---sn-qxo7rn7e.googlevideo.com/videoplayback?signature=021CAFB9066554DD33675D89CC80D6E5FC616A7E.8A6222115FF91416C7F1B639B8F4A86671B40DD2&ipbits=0&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cexpire&fvip=1&source=youtube&id=o-AALFu428zi6lOqHvA5xGfADpvNCR_BXItpMzqWb73CSH&mm=31%2C26&expire=1520111723&dur=293.721&lmt=1508989837160273&key=yt6&ip=35.227.125.114&ms=au%2Conr&ei=C7yaWuSpMYj5qQWY_qH4DQ&mv=m&mt=1520090001&requiressl=yes&gir=yes&mn=sn-qxo7rn7e%2Csn-cvb7ln7l&clen=4618202&keepalive=yes&c=WEB&mime=audio%2Fwebm&pl=24&itag=251&ratebypass=yes' -f mp3 output.mp3
É claro que esse link provavelmente expirou e você precisará obter um novo em youtube-dl
.