Overlays all videos in a directory on top of each other.

overlay.sh

#!/bin/bash

i=0
while read line
do
    array[ $i ]="$line"
    (( i++ ))
done < <(ls)

# Creates the base overlay.mkv file
ffmpeg -i "${array[0]}" -i "${array[1]}" \
        -filter_complex "[1:v]setpts=PTS-10/TB[a]; \
                         [0:v][a]overlay=enable=gte(t\,0):shortest=1[out]" \
        -map [out] -map 0:a \
        -c:v libx264 -crf 18 -pix_fmt yuv420p \
        -c:a copy \
        ~/overlay.mkv

# Overlays the 
for i in "${#array[@]}"; do \
        ffmpeg -i "${i}" -i ~/overlay.mkv \
        -filter_complex "[1:v]setpts=PTS-10/TB[a]; \
                         [0:v][a]overlay=enable=gte(t\,0):shortest=1[out]" \
        -map [out] -map 0:a \
        -c:v libx264 -crf 18 -pix_fmt yuv420p \
        -c:a copy \
        ~/overlay.mkv
done