forking-mainnet-with-cast-anvil.md

Forking Mainnet with Cast and Anvil

Introduction

By combining Anvil and Cast, you can fork and test by interacting with contracts on a real network. The goal of this tutorial is to show you how to transfer Dai tokens from someone who holds Dai to an account created by Anvil.

Set Up

Let's start by forking mainnet.

anvil --fork-url https://mainnet.infura.io/v3/$INFURA_KEY

You will see 10 accounts are created with their public and private keys. We will work with 0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266 (Let's call this user Alice).

Transferring Dai

Go to Etherscan and search for holders of Dai tokens (here). Let's pick a random account. In this example we will be using 0xfc2eE3bD619B7cfb2dE2C797b96DeeCbD7F68e46. Let's export our contracts and accounts as environment variables:

export ALICE=0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266
export DAI=0x6b175474e89094c44da98b954eedeac495271d0f
export UNLUCKY_USER=0xfc2eE3bD619B7cfb2dE2C797b96DeeCbD7F68e46

We can check Alice's balance using cast call:

$ cast call $DAI \
  "balanceOf(address)(uint256)" \
  $ALICE
0

Similarly, we can also check our unlucky user's balance using cast call:

$ cast call $DAI \
  "balanceOf(address)(uint256)" \
  $UNLUCKY_USER
21840114973524208109322438

Let's transfer some tokens from the unlucky user to Alice using cast send:

# This calls Anvil and lets us impersonate our unlucky user
$ cast rpc anvil_impersonateAccount $UNLUCKY_USER
$ cast send $DAI \
--from $UNLUCKY_USER \
  "transfer(address,uint256)(bool)" \
  $ALICE \
  300000000000000000000000 \
  --unlocked
blockHash               0xbf31c45f6935a0714bb4f709b5e3850ab0cc2f8bffe895fefb653d154e0aa062
blockNumber             15052891
...

Let's check that the transfer worked:

cast call $DAI \
  "balanceOf(address)(uint256)" \
  $ALICE
300000000000000000000000

$ cast call $DAI \
  "balanceOf(address)(uint256)" \
  $UNLUCKY_USER
21540114973524208109322438