This one for instance to grab my desktop quickly without starting another program when I want to make a quick demo for a colleague:

  #!/bin/bash

  size=${1:-"1920x1080"}
  offset=${2:-"0,0"}
  name=${3:-"video"}

  ffmpeg -video_size $size -framerate 25 -f x11grab -i :0.0+$offset -c:v libx264 -crf 0 -preset ultrafast "$name.mkv"
  ffmpeg -i "$name.mkv" -movflags faststart -pix_fmt yuv420p "$name.mp4"