Hello World Part 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.
Objectives
After completing this guide, you'll be able to:
- Create a Flow project using the Flow Command Line Interface
- Add an already-deployed contract to your project with the Dependency Manager
- Deploy a smart contract locally to the Flow Emulator
- Write and execute scripts to interact with a deployed smart contract
Installation
The first thing you'll need to do is install the Flow CLI. If you have homebrew installed you can run:
_10brew install flow-cli
For other ways of installing, please refer to the installation guide.
Configuration
Lets first create a project directory and navigate to it:
_10mkdir cli-quickstart_10cd cli-quickstart
Next, we'll initialize a new Flow project with the CLI:
_10flow init --config-only
This will create a flow.json file in your project directory. This file is used to configure your project and describe the setup of your contracts, networks, and accounts.
It will also have a default emulator-account created for you. We'll use this account to interact with the emulator later on.
For additional details on how flow.json is configured, review the configuration docs.
Grabbing the HelloWorld Contract
For this demo, we are going to be interacting with a simple HelloWorld contract, written in Cadence, that is already deployed on Flow's testnet network on account 0xa1296b1e2e90ca5b. In order to grab this project dependency, we'll use Flow's Dependency Manager to install it into our project using a source string that defines the network, address, and contract name of the contract we want to import.
_10flow dependencies add testnet://0xa1296b1e2e90ca5b.HelloWorld
This will add the HelloWorld contract and any of its dependencies to an imports directory in your project. We recommend adding this directory to your .gitignore file, which is done by the script by default. It will also add any dependencies to your flow.json file.
During the install you'll be prompted to specify which account to deploy the contracts to. For this tutorial, you can select the default emulator-account. Leave the alias address for HelloWorld on mainnet blank.
Review the 📝 Dependency Manager Actions Summary for a list of actions completed by the script.
Open imports/a1296b1e2e90ca5b/HelloWorld.cdc in your editor. You will see the following:
_14access(all) contract HelloWorld {_14_14 access(all)_14 var greeting: String_14_14 access(all)_14 fun changeGreeting(newGreeting: String) {_14 self.greeting = newGreeting_14 }_14_14 init() {_14 self.greeting = "Hello, World!"_14 }_14}
This contract has a greeting variable that can be read and changed. It also has a changeGreeting function that allows you to change the greeting.
Deploying the Contract to Emulator
The emulator is a local version of the Flow blockchain that you can use to test your contracts and scripts. It's a great way to develop and test your contracts locally - before you try them on the testnet or mainnet.
Before we deploy, let's open a new terminal window and run the emulator. From the root of your project directory, where your emulator-account.pkey and flow.json files are located, run:
_10flow emulator start
If you see a message that configuration is missing, you are in the wrong directory. Do not run flow init!.
🙏 Configuration is missing, initialize it with: 'flow init' and then rerun this command.
To deploy the HelloWorld contract to the emulator, return to your first terminal and run the following command:
_10flow project deploy
You should see:
_10🎉 All contracts deployed successfully
The contract will now have been deployed to the default emulator-account. You can now interact with it using a script.
Running Scripts
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 the HelloWorld contract.
Let's create a script file. We can generate a boilerplate script file with the following command:
_10flow generate script ReadGreeting
This will create a file called ReadGreeting.cdc in the cadence/scripts directory. Let's update the script to read the greeting from the HelloWorld contract. Replace the existing coded with:
_10import "HelloWorld"_10_10access(all) fun main(): String {_10 return HelloWorld.greeting_10}
The import syntax will automatically resolve the address of the contract on the network you are running the script on. This is determined by your flow.json configuration.
If you'll like to learn more about writing scripts, please check out the docs for basic scripts.
To run the script, we'll run this from the CLI:
_10flow scripts execute cadence/scripts/ReadGreeting.cdc
You should see the result of the greeting. Result: "Hello, world!"
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 cadence/transactions/ChangeGreeting.cdc with the following command:
_10flow generate transaction ChangeGreeting
Open the new file - cadence/transactions/ChangeGreeting.cdc. Update the boilerplate transaction to look like this:
_12import "HelloWorld"_12_12transaction(greeting: String) {_12_12 prepare(acct: &Account) {_12 log(acct.address)_12 }_12_12 execute {_12 HelloWorld.changeGreeting(newGreeting: greeting)_12 }_12}
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 the docs for basic transactions.
In order to run a transaction, the signing account needs to pay for it. You could run the transaction on emulator using the default emulator-account account, but a better test is to run it with a new test account.
Let's learn the command for creating accounts.
The easiest way to create an account using CLI is with:
_10flow accounts create
Remember, your emulator should still be running at this point in another terminal.
Give your account the name emulator-tester, then select Emulator as the network. You'll now see this account in your flow.json.
To run a transaction with this new account, you can run the following:
_10flow transactions send cadence/transactions/ChangeGreeting.cdc "Hello, me" --signer emulator-tester --network emulator
You've just modified the state of the Flow Blockchain! At least on the emulator. You'll know it worked if you see the receipt. Yours will be similar to:
_10Transaction ID: 2ff6cbb8125103595fca0abaead94cd00510d29902ceae9f5dc480e927ab7334_10_10Block ID 36bbf6fc573129fa9a3c78a43e257d3b627a3af78fd9e64eeb133d981819cc69_10Block Height 3_10Status ✅ SEALED_10ID 2ff6cbb8125103595fca0abaead94cd00510d29902ceae9f5dc480e927ab7334_10Payer 179b6b1cb6755e31_10Authorizers [179b6b1cb6755e31]
You can also re-run the ReadGreeting script with:
_10flow scripts execute cadence/scripts/ReadGreeting.cdc
You'll now see:
_10Result: "Hello, me"
More
If you want to continue on generating your own contracts, you can also use the the generate subcommand to create a new contract file. See more in the generate documentation.
After that, it's easy to add your contract to your project configuration using the Flow CLI config commands.