truffle-config.js

require('babel-register');
require('babel-polyfill');
require('dotenv').config();
const HDWalletProvider = require('truffle-hdwallet-provider-privkey');
const privateKeys =  process.env.PRIVATE_KEYS || ""

console.log(typeof privateKeys)

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)
     }, 

     kovan: {
      provider: function() {
        return new HDWalletProvider(
          privateKeys.split(','),//array of private keys
          `https://kovan.infura.io/v3/${process.env.INFURA_API_KEY}`//url to an Ethereum node
          )
      },
      gas: 5000000,
      gasPrice: 25000000000,
      network_id: 42
     }   
  },
contracts_directory: './src/contracts/',
contracts_build_directory: './src/abis/',
  // Set default mocha options here, use special reporters etc.
  mocha: {
    // timeout: 100000
  },

  // Configure your compilers
  compilers: {
    solc: {     
      optimizer: {
      enabled: true,
      runs: 200    
    }
  }
 }
}