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 0xad0135af20fa82e106607257143d0060a7eb5cbf. Let's export our contracts and accounts as environment variables:

export ALICE=0xf39fd6e51aad88f6f4ce6ab8827279cfffb92266
export DAI=0x6b175474e89094c44da98b954eedeac495271d0f
export LUCKY_USER=0xad0135af20fa82e106607257143d0060a7eb5cbf

We can check Alice's balance using cast call:

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

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

$ cast call $DAI \
  "balanceOf(address)(uint256)" \
  $LUCKY_USER
71686045944718512103110072

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

# This calls Anvil and lets us impersonate our lucky user
$ cast rpc anvil_impersonateAccount $LUCKY_USER
$ cast send $DAI \
--from $LUCKY_USER \
  "transfer(address,uint256)(bool)" \
  $ALICE \
  1686045944718512103110072
blockHash               0xbf31c45f6935a0714bb4f709b5e3850ab0cc2f8bffe895fefb653d154e0aa062
blockNumber             15052891
...

Let's check that the transfer worked:

cast call $DAI \
  "balanceOf(address)(uint256)" \
  $ALICE
1686045944718512103110072

$ cast call $DAI \
  "balanceOf(address)(uint256)" \
  $LUCKY_USER
70000000000000000000000000

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