Due to the outdated dependencies for tileshrink it is difficult to install this on e.g. macOS. This image make it easy to run it in Docker.
Map your current directory where you have your tiles files into the container like this
docker run -ti --rm -v $(pwd):/data oozberg/tileshrink
You can then run tileshrink command and access your files in the /data directory, e.g
tileshrink --shrink 4 /data/input.mbtiles /data/output.mbtiles