Print key frame numbers and intervals with ffprobe

print_key_frames

ffprobe in.mkv -v error -select_streams v:0 -show_frames -skip_frame nokey -print_format csv | gawk -F, "/^frame/{print $22,$22-n;n=$22}"
ffprobe in.mkv -v error -select_streams v:0 -show_frames -skip_frame nokey -show_entries frame=pts_time -print_format csv | gawk -F, "/^frame/{printf \"%.0f %.0f\n\",$2*23.976,($2-n)*23.976;n=$2}"
ffprobe in.mkv -v error -select_streams v:0 -show_entries packet=pts_time,flags -print_format csv | gawk -F, "$3~\"^K\"{printf \"%.0f %.0f\n\",$2*23.976,($2-n)*23.976;n=$2}"

Quoting is Windows-like.

1. Some video streams have $22 (coded_picture_number) always 0: use line 2 or 3 instead of line 1.
2. 23.976 fps
3. this seems faster than line 2