#!/bin/bash

input_fn='input.mov'

image_fn='output.png'

frame_count=`ffprobe -v error -count_frames -select_streams v:0 -show_entries stream=nb_read_frames -of default=nokey=1:noprint_wrappers=1 $input_fn`

ffmpeg -i $input_fn -vf "select='eq(n,$frame_count-1)'" -vframes 1 "$image_fn" 2> /dev/null