Extract video closed captions and convert them to subrip format using ffmpeg.
# ffmpeg version n4.2.1
ffmpeg -f lavfi -i 'movie=video.file[out0+subcc]' -map s output.srt
Will extract the closed captions from the video file video.file into the output file output.srt in subrip format.
Closed captions are not the same as subtitle streams. Use this if the video stream info says Closed Captions:
Stream #0:0[0x100]: Video: mpeg2video (Main) ([2][0][0][0] / 0x0002),
yuv420p(tv, top first), 1920x1080 [SAR 1:1 DAR 16:9], Closed Captions,
29.97 fps, 29.97 tbr, 90k tbn, 59.94 tbc
You can then embed the converted closed captions as subtitles into another file:
ffmpeg -i input_video.mkv -i output.srt output_video.mkv