#!/bin/bash
#from
video_folder="~/video/"
#to
screenshot_folder="~/screenshot/"

for i in $video_folder/*.mkv
do
i=$(echo $i | sed 's/.mkv//' | sed 's%^.*/%%')
    then
    ffmpeg -i ${video_folder}$i.mkv -f image2 -vf "select='eq(pict_type,PICT_TYPE_I)'" -vsync vfr ${films_folder}_{$i}_thumb%04d.png
fi
done