A bash function to generate TOTP codes

totp.sh

#!/usr/bin/env bash
set -euo pipefail

# Dependencies: openssl, xxd, base32 (coreutils)

# generate_totp key [time_sec [digits]]
generate_totp() {
  key=$1
  key_hex=$(base32 -d <<<"$key" | xxd -plain)
  time_sec=${2:-$(date +%s)}
  digits=${3:-6}
  time_step=$((time_sec / 30))
  time_step_hex=$(printf "%016x" $time_step)
  hash_hex=$(xxd -revert -plain <<< "$time_step_hex" | openssl sha1 -mac HMAC -macopt "hexkey:$key_hex" -binary | xxd -plain)
  offset=$((16#${hash_hex:39}))
  offset_str="${hash_hex:$((offset * 2)):8}"
  byte0=$(((16#${offset_str:0:2} & 127) << 24))
  byte1=$((16#${offset_str:2:2} << 16))
  byte2=$((16#${offset_str:4:2} << 8))
  byte3=$((16#${offset_str:6:2}))
  bin_code=$((byte0 + byte1 + byte2 + byte3))
  printf "%0${digits}d" $((bin_code % (10 ** digits)))
}

generate_totp AC6ZWNRLFOG5JUQHC5YW6BXX2K675AZL | tee >(pbcopy)
printf "\n"