Let’s Encrypt client and ACME library written in Go.
Features

• ACME v2 [RFC 8555}(https://www.rfc-editor.org/rfc/rfc8555.html)
• Register with CA
• Obtain certificates, both from scratch or with an existing CSR
• Renew certificates
• Revoke certificates
• Robust implementation of all ACME challenges
  • HTTP (http-01)
  • DNS (dns-01)
  • TLS (tls-alpn-01)
• SAN certificate support
• Comes with multiple optional DNS providers
• Custom challenge solvers
• Certificate bundling
• OCSP helper function

github: LEGO

acme.sh --issue -d example.com --dns \
--yes-I-know-dns-manual-mode-enough-go-ahead-please

acme.sh --renew -d example.com \
  --yes-I-know-dns-manual-mode-enough-go-ahead-please

• An ACME protocol client written purely in Shell (Unix shell) language.
• Full ACME protocol implementation.
• Support ACME v1 and ACME v2
• Support ACME v2 wildcard certs
• Simple, powerful and very easy to use. You only need 3 minutes to learn it.
• Bash, dash and sh compatible.
• Purely written in Shell with no dependencies on python or the official Let's Encrypt client.
• Just one script to issue, renew and install your certificates automatically.
• DOES NOT require root/sudoer access.
• Docker friendly
• IPv6 support
• Cron job notifications for renewal or error etc.

It's probably the easiest & smartest shell script to automatically issue & renew the free certificates from Let's Encrypt.