ffmpeg -i input.mp4 outfile.srt
ffmpeg -i input.mp4 -map 0:s:0 sub.srt
----
===== Where multiple subtitle tracks exist =====
ffmpeg -i input.mp4 -map 0:s:0 eng.srt
ffmpeg -i input.mp4 -map 0:s:1 ita.srt
ffmpeg -i input.mp4 -map 0:s:2 fre.srt
ffmpeg -i input.mp4