## `activeFork`

### Signature

```solidity
function activeFork() external view returns (uint256 forkId);
```

### Description

Returns the identifier for the currently active fork. Reverts if no fork is currently active.

### Returns

| Parameter | Type      | Description                           |
|-----------|-----------|---------------------------------------|
| `forkId`  | `uint256` | Identifier of the active fork         |

### Examples

```solidity [test/ActiveFork.t.sol]
function testActiveFork() public {
    uint256 mainnetFork = vm.createFork(MAINNET_RPC_URL);
    uint256 optimismFork = vm.createFork(OPTIMISM_RPC_URL);

    vm.selectFork(mainnetFork);
    assertEq(vm.activeFork(), mainnetFork);

    vm.selectFork(optimismFork);
    assertEq(vm.activeFork(), optimismFork);
}
```

### Related Cheatcodes

* [`createFork`](/reference/cheatcodes/create-fork) - Create a new fork
* [`selectFork`](/reference/cheatcodes/select-fork) - Select a fork
* [`createSelectFork`](/reference/cheatcodes/create-select-fork) - Create and select a fork