# Use perl-regex.
ffmpeg -i "input.mkv" -c copy -f null /dev/null 2>&1 | tail -n 1 | grep -Po "(?<=time=)([0-9]{1,2}:[0-9]{2}:[0-9]{2})"
 
or
 
# Use extended-regex.
ffmpeg -i "input.mkv" -c copy -f null /dev/null 2>&1 | tail -n 1 | grep -oE "[0-9]{1}:[0-9]{2}:[0-9]{2}"
 
or
 
ffmpeg -i "input.mkv" -c copy -f null /dev/null 2>&1 | grep -oE "[0-9]{1}:[0-9]{2}:[0-9]{2}" | tail -n 1
 
or
 
ffmpeg -i "input.mkv" 2>&1 | grep -oE "[0-9]{1}:[0-9]{2}:[0-9]{2}" | head -n 1

ffprobe -v error -select_streams v:0 -show_entries stream=duration -of default=noprint_wrappers=1:nokey=1 "input.mkv"

ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 "input.mkv"
 
ffprobe -v error -select_streams v:0 -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 "input.mkv"

This method reads each video packet, individually, and totals up each packet size.

• Sums up the duration_time of the -show_entries command.

video_pkt_durations=$(ffprobe -v error -select_streams v:0 -skip_frame nokey -show_entries packet=duration_time -of default=nokey=1:noprint_wrappers=1 "input.mkv")
 
sum_video_pkt_duration=0
precision=6
count_video_pkt_duration=0
 
#echo "Calculating video pkt_duration..."
for video_pkt_duration in $video_pkt_durations; do
  sum_video_pkt_duration=$(echo "${sum_video_pkt_duration} + ${video_pkt_duration}" | bc)
 
  (( count_video_pkt_duration++ ))
done;
 
video_duration=$( echo "scale=${precision}; ${sum_video_pkt_duration} " | bc)
 
echo "DURATION=${video_duration}"
echo "VIDEO FRAMES=${count_video_pkt_duration}"

duration=$( echo "scale=6; (${video_size} * 8) / ${bitrate}" | bc)

duration = NUMBER_OF_FRAMES / FPS