reference: https://superuser.com/questions/556029/how-do-i-convert-a-video-to-gif-using-ffmpeg-with-reasonable-quality

vid2gif.sh

#!/bin/sh
TMP_DIR="$HOME/${0##*/}.$(openssl rand -base64 12 | tr +/ -_)"
mkdir "$TMP_DIR"
do_exit() {
	rm -r "$TMP_DIR"
	exit
}
trap 'do_exit' 1 2 3 15
[ ! -f "$1" ] && do_exit

SRC="${1##*/}"
SRC_NAME="${SRC%.*}"
ffmpeg -i "$1" -vf \
	fps=10,scale=iw:-1:flags=lanczos,palettegen "${TMP_DIR}/palette.png"
ffmpeg -i "$1" -i "${TMP_DIR}/palette.png" -filter_complex \
	fps=10,scale=iw:-1:flags=lanczos[x]\;[x][1:v]paletteuse "${SRC_NAME}.gif"

do_exit