This is a simple script to grab frames from a video file at 3 second intervals using FFMPEG

framegrabber.sh

#!/bin/bash
# this is a simple script to grab frames from a MP4 file (GOPR5934) using FFMPEG
# at a range of seconds 180 secs to  540 secs in 3 second intervals
# Modify the seconds and the file as you wish. I will improve it through time :) 
for i in {180..540};
do if (( $(($i % 3)) == 0 ));
   then
	   ffmpeg -accurate_seek -ss $i -i GOPR5934.MP4  -frames:v 1 GOPR5934_$i.jpg
   fi;
done