To be very clear, I don’t use this, but I confirmed it does work and generates the same codes as the linked article. Requires Bash (3+) and doesn’t work as written in Zsh.

https://gist.github.com/jsjohnst/95f34bc1f6ab46fd5c038138f0e...

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   read -r -p 'Secret Key: ' secret_key; dgst=$(printf '%016X' $(($(date -u '+%s') / ${TOTP_PERIOD_SECONDS:=30})) | xxd -r -p | openssl dgst -sha1 -mac hmac -macopt "hexkey:$(printf $secret_key | tr '[:lower:]' '[:upper:]' | base32 -d | xxd -p)"); offset=$(( 2 * 16#${dgst: -1} )); token=$(( ( 16#${dgst:offset:8} & 0x7fffffff ) % 10**${TOTP_DIGITS:=6} )); printf "%0${TOTP_DIGITS}d\n" "$token"