# tested with: https://www.shutterstock.com/video/clip-3885476

TMP=.
SRC=countdown.mp4
DST=output.mp4
FREEZE_S=10

cd "$TMP"

# get last frame
NFRAMES=`ffprobe -show_streams "$SRC" 2> /dev/null | grep nb_frames | head -1 | cut -d \= -f 2`
let "LAST_FRAME = $NFRAMES - 1"
ffmpeg -i $SRC -vf select=\'eq\(n,$LAST_FRAME\) -vframes 1 LAST_FRAME.jpg


# make last-frame-frozen video
FRAMERATE=$(ffprobe -v 0 -of csv=p=0 -select_streams 0 -show_entries stream=r_frame_rate "$SRC")
ffmpeg  -loop 1  -framerate 1  -i LAST_FRAME.png  -r "$FRAMERATE"  -t "$FREEZE_S"  -pix_fmt yuv420p  LAST_FRAME.mp4

# Produces list.txt like this:
#     file foo.mp4
#     file 'bar with spaces.mp4'
printf "file %q\n"  "$SRC" LAST_FRAME.mp4  > list.txt

# concatenate
ffmpeg  -y  -f concat  -safe 0  -i list.txt  -c copy  "$DST"  # green-screen for LAST_FRAME.mp4 duration

# Alternative:
#    ffmpeg -i "concat:$SRC|LAST_FRAME.mp4"  -c:a copy  -c:v copy  "$DST"  # <-- no LAST_FRAME.mp4