Version: Current

Hello World Pt. 2 - Local Development

The Flow Command Line Interface (CLI) is a set of tools that developers can use to interact with the Flow blockchain by managing accounts, sending transactions, deploying smart contracts, running the emulator, and more. This quickstart will get you familiar with its main concepts and functionality.

Installation

The first thing you'll need to do is install the Flow CLI. If you have homebrew installed you can run:

brew install flow-cli

For other ways of installing, please refer to the installation guide.

Configuration

Let's first create a directory, then set up a project.

mkdir cli-quickstart

And then let's navigate to our new directory:

cd cli-quickstart

Let's first create a flow.json file for our project. We'll use this in a later step, but this is a good spot to bring it up. The flow.json file is a configuration file used by the Flow CLI when interacting with the Flow blockchain. It helps manage various project-specific settings, such as network configurations, account details, contract names and source files, and deployment targets. To create one, run:

flow init

If you look at flow.json now, you'll see its listed access nodes for networks and accounts. The default emulator-account create will come in handy when we want to run the emulator.

Running Scripts

On Flow, scripts are used to read data from the Flow blockchain. There is no state modification. In our case, we are going to read a greeting from a contract deployed to testnet called HelloWorld. (You can view the contract here)

Let's create a script file:

touch script.cdc

Then in the script file, let's put the following code:

import HelloWorld from 0x9dca641e9a4b691b
pub fun main(): String {
  return HelloWorld.greeting
}

In the above example, 0x9dca641e9a4b691b is the address where the HelloWorld contract has been deployed to on testnet. (Note: if you'll like to learn more about writing scripts, please read here).

To run the script, we'll run this from the CLI:

flow scripts execute script.cdc --network testnet

You should see the result of the greeting. Result: "Hello, world!"

Running the Contract Locally on Emulator

The Flow Emulator is a local, lightweight, and standalone version of the Flow blockchain. It's designed to provide developers with a local environment for testing and development purposes without having to interact with the mainnet or testnet. This makes it easier and faster to develop, test, and debug smart contracts and apps.

In order to use it, let's update our project configuration.

Let's create a local version of the HelloWorld contract. We'll deploy it to the emulator, Run:

touch HelloWorld.cdc

Copy the contract to HelloWorld.cdc. Then let's add the contract into our project by updating flow.json.

pub contract HelloWorld {
  pub var greeting: String
  pub fun changeGreeting(newGreeting: String) {
    self.greeting = newGreeting
  }
  init() {
    self.greeting = "Hello, World!"
  }
}

Next we'll add a contracts section to our flow.json configuration that will describe our project setup. We'll state the contract file location lives with source and then define aliases for the addresses of the deployed contracts.

// flow.json
{
...
"contracts": {
  "HelloWorld": {
    "source": "HelloWorld.cdc",
    "aliases": {
      "testnet": "0x9dca641e9a4b691b"
    }
  }
}
...
}

We're also going to change the imports of our script so that there are no hardcoded network specific addresses. The CLI can figure out how to interact with the network (i.e. emulator, testnet or mainnet) based on our configuration.

// script.cdc
import "HelloWorld"
pub fun main(): String {
  return HelloWorld.greeting
}

Next, we'll add a deployments section to flow.json and define what account we'd like what contract deployed to and on what network. In this case, let's deploy the HelloWorld contract to the emulator network and on emulator-account provided.

// flow.json
{
...
"deployments": {
  "emulator": {
    "emulator-account": ["HelloWorld"]
}
...
}

Next let's run the emulator in a new terminal with:

flow emulator start

And then deploy the contract with:

flow project deploy

Now if we run the following script we should see the result of the script against our emulator deployed contract.

flow scripts execute script.cdc --network emulator

Creating an Account and Running a Transaction

To change state on the Flow Blockchain, you need to run a transaction. Let's create a simple transaction file. We can use to modify the greeting on the HelloWorld contract.

First, create a file called transaction.cdc from the root of your project:

touch transaction.cdc

Then copy the following code:

import "HelloWorld"
transaction(greeting: String) {
  prepare(acct: AuthAccount) {
    log(acct.address)
  }
  execute {
    HelloWorld.changeGreeting(newGreeting: greeting)
  }
}

This will log the account signing the transaction, call the changeGreeting method of the HelloWorld contract, and pass in the new greeting. (If you want to learn more about writing transactions, please read here).

In order to run a transaction, the signing account needs to pay for it. We could run a transaction on emulator using the default emulator-account account. Let's learn one more command for creating accounts.

The easiest way to create an account using CLI is by running (remember, your emulator should still be running at this point in another terminal):

flow accounts create

Once that runs, select Emulator as the network and give your account the name emulator-tester. You'll now see this account in your flow.json.

To run a transaction with this new account, you can run the following:

flow transactions send ./transaction.cdc "Hello, me" --signer emulator-tester --network emulator

You've just modified the state of the Flow Blockchain!

Next Steps

Dive deeper by checking out the scaffolds generated by the Flow CLI. They can serve as a great starting point for any project you're trying to create. See how to create a scaffold here.

Hello World Pt. 2 - Local Development

Installation​

Configuration​

Running Scripts​

Running the Contract Locally on Emulator​

Creating an Account and Running a Transaction​

Next Steps​