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.