At the beginning of a migration file, we tell truffle which contracts we'd like to interact with via the artifacts.require() method. This method is similar to Node's require, but in our case, it specifically returns a contract abstraction that we can use within the rest of our deployment script.

All migrations must export a function via the module.exports syntax. The function exported by each migration should accept a deployer object as its first parameter. This object assists in deployment both by providing a clear API to deploy smart contracts as well as performing some of the deployment's more mundane duties, such as saving deployed artifacts in the artifacts files for later use, linking libraries, and so on. The deployer object is your main interface for the staging of deployment tasks.

Here are the methods of the deployer object. All the methods are synchronous:

• deployer.deploy(contractAbstraction, args..., options): Deploys a specific contract specified by the contract abstraction object, with optional constructor arguments. This is useful for singleton contracts, so that only one instance of this contract exists for your DApp. This will set the address of the contract after deployment (that is, the address property in the artifacts file will equal the newly deployed address), and it will override any previous address stored. You can optionally pass an array of contracts, or an array of arrays, to speed up the deployment of multiple contracts. Additionally, the last argument is an optional object that can contain a single key, overwrite. If overwrite is set to false, the deployer won't deploy this contract if one has already been deployed. This method returns a promise.
• deployer.link(library, destinations): Links an already deployed library to a contract or multiple contracts. The destinations argument can be a single contract abstraction or an array of multiple contract abstractions. If any contract within the destination doesn't rely on the library being linked, the deployer will ignore that contract. This method returns a promise.
• deployer.then(function(){}): This is used to run an arbitrary deployment step. Use it to call specific contract functions during your migration to add, edit, and reorganize contract data. Inside the callback function, you would use the contract abstraction APIs to deploy and link contracts.

It is possible to run the deployment steps conditionally based on the network being deployed to. To conditionally stage the deployment steps, write your migrations so that they accept a second parameter called network. One example use case can be that many of the popular libraries are already deployed to the main network; therefore, when using these networks, we will not deploy the libraries again and just link them instead. Here is a code example:

module.exports = function(deployer, network) { 
  if (network != "live") { 
   // Perform a different step otherwise. 
  } else { 
    // Do something specific to the network named "live". 
  } 
}

In the project, you will find two migration files, that is, 1_initial_migration.js and 2_deploy_contracts.js. The first file shouldn't be edited unless you know what you are doing. You are free to do anything with the other file. Here is the code for the 2_deploy_contracts.js file:

var ConvertLib = artifacts.require("./ConvertLib.sol"); 
var MetaCoin = artifacts.require("./MetaCoin.sol"); 

module.exports = function(deployer) { 
  deployer.deploy(ConvertLib); 
  deployer.link(ConvertLib, MetaCoin); 
  deployer.deploy(MetaCoin); 
};

Here, we are creating abstractions for the CovertLib library and the MetaCoin contract at first. Regardless of which network is being used, we are deploying the ConvertLib library and then linking the library to the MetaCoin network and finally deploying the MetaCoin network.

To run the migrations, that is, to deploy the contracts, run this command:

truffle migrate --network development

Here, we are telling truffle to run migrations on the development network. If we don't provide the --network option, then it will use the network with the name development by default.

After you run the preceding command, you will notice that truffle will automatically update the ConvertLib library and MetaCoin contract addresses in the artifacts files and also update the links.

Here are some other important options you can provide to the migrate sub-command:

• --reset: Runs all migrations from the beginning instead of running from the last completed migration.
• -f number: Runs contracts from a specific migration.