錯誤提示

Error: Mnemonic invalid or undefined

原因：.env檔案命名錯誤

將process.env檔案名直接改成.env后再執行 truffle console --network kovan命令即可

truffle 連接公鏈步驟

1. 安裝dotenv和truffle-hdwallet-provider

npm install dotenv --save-dev -g
npm install truffle-hdwallet-provider --save-dev -g

注意：windows 系統可能需要重啟這兩個依賴包才會在專案中生效

2. 在專案根目錄中創建一個.env檔案，在.env檔案中以鍵值對的形式寫入Mnemonic和INFURA_API_KEY

INFURA_API_KEY= your infura_api_key
MNEMONIC="your mnemonic"

3. 在truffle-config.js或者truffle.js檔案中進行配置：

// 匯入dotenv庫創用于讀取`.env`檔案中的設定
require('dotenv').config();
// 匯入truffle-hdwallet-provider庫重建錢包
const HDWalletProvider = require('truffle-hdwallet-provider');

module.exports = {
  networks: {
    development: {
     host: "127.0.0.1",     // Localhost (default: none)
     port: 7545,            // Standard Ethereum port (default: none)
     network_id: "*",       // Any network (default: none)
    },
    // Useful for deploying to a public network.
    // NB: It's important to wrap the provider as a function.
    kovan: {
      provider: () => new HDWalletProvider(
        process.env.MNEMONIC, 
        process.env.INFURA_API_KEY
      ),
      gas: 5000000,
      gasPrice: 25000000000,
      network_id: 42
    },
  },
  solc: {
    optimizer: {
      enabled: true,
      runs: 200
    }
  }
}

4. 在控制臺中輸入truffle console --network kovan，即可連接到公鏈，可以輸入web3.eth.getBlock('latest').then(console.log)進行驗證，若回傳以下內容則表示連接成功：

{ author: '0x03801efb0efe2a25ede5dd3a003ae880c0292e4d',
  difficulty: '340282366920938463463374607431768211454',
  extraData:
   '0xde830206028f5061726974792d457468657265756d86312e33362e30826c69',
  gasLimit: '0x7a1200',
  gasUsed: '0x17d23',
  hash:
   '0xc7390c4f492c8c1da60608135fc9e05930123b645b39f221cba33d8b3c577b2a',
  logsBloom:
   '0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000800000000000000000000000000080000000000000000000100000008000000000000000000080000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000400800000000000010000000000000080000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000009000000008000000',
  receiptsRoot:
   '0x3d05bb2ed4fcc90234eea6d840e7d0e3ce7f598a15e5314536b17bcd11c78b5b',
  sealFields:
   [ '0x84175e8801',
     '0xb84155a8cdb108dccec1d314124058fa6f22e7400ee200db0a94b7b165e4c3454c1818cc05f815cb7ce48f7a88b8401515740311a3566d9cf079428d506a6daca50101' ],
  sha3Uncles:
   '0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347',
  signature:
   '55a8cdb108dccec1d314124058fa6f22e7400ee200db0a94b7b165e4c3454c1818cc05f815cb7ce48f7a88b8401515740311a3566d9cf079428d506a6daca50101',
  size: 877,
  stateRoot:
   '0x03af5adce52a81ce5d332cddb9955e344214bff00859b78868116e1e839efdf7',
  step: '392071169',
  timestamp: 1568284676,
  totalDifficulty: '4524524338444961608702071789512829094373049115',
  transactions:
   [ '0xded7fed0842fd65ec808bc3652ec4175bc190acc11345c49c44b1fb5d954610f',
     '0x7e9112a46fa3c07aad813ea86355b15eebb44023c040d198ee7d15d379bbc2be' ],
  transactionsRoot:
   '0x0dd10d90686dda2684bd0ba70d1c9e1d9a5302c30ca75eb2c5b07a7b6e4498b9',
  uncles: [] }

附上：

1. .env檔案使用講解

2. 以太坊連接公鏈教程