Tente isto:
#!/bin/bash
echo 'Press "s" to pause loop or Ctrl+c to stop'
n=1
while true; do
read -r -t 0.5 -n 1 -s stop
[ -n "${stop}" ] && {
read -p 'Paused... press Enter button for continue' x
}
ffmpeg -re -i "input" output.mp4
n=$(( n+=1 ))
echo "Loop Counter: ${n}"
done