dehydrated
letsencrypt/acme client implemented as a shell-script
...It uses the OpenSSL utility for everything related to actually handling keys and certificates, so you need to have that installed. Other dependencies are cURL, sed, grep, mktemp (all found on almost any system, cURL being the only exception). Please keep in mind that this software and even the acme-protocol are relatively young and may still have some unresolved issues. Feel free to report any issues you find with this script or contribute by submitting a pull request. dehydrated is looking for a config file in a few different places, it will use the first one it can find.