`invalidateFetcherByDrvHash` has to invoke the fetcher function twice:
once to get a derivation hash, and again to produce the final fixed output derivation.

:::{.example #ex-invalidateFetcherByDrvHash-nix}

# Prevent nix from reusing the output of a fetcher

```nix
{
  tests.fetchgit = testers.invalidateFetcherByDrvHash fetchgit {
    name = "nix-source";
    url = "https://github.com/NixOS/nix";
    rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
    hash = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
  };
}
```

:::

## `runCommand` {#tester-runCommand}

`runCommand :: { name, script, stdenv ? stdenvNoCC, hash ? "...", ... } -> Derivation`

This is a wrapper around `pkgs.runCommandWith`, which
- produces a fixed-output derivation, enabling the command(s) to access the network ;
- salts the derivation's name based on its inputs, ensuring the command is re-run whenever the inputs change.

It accepts the following attributes:
- the derivation's `name` ;
- the `script` to be executed ;
- `stdenv`, the environment to use, defaulting to `stdenvNoCC` ;
- the derivation's output `hash`, defaulting to the empty file's.
  The derivation's `outputHashMode` is set by default to recursive, so the `script` can output a directory as well.

All other attributes are passed through to [`mkDerivation`](#sec-using-stdenv),
including `nativeBuildInputs` to specify dependencies available to the `script`.

:::{.example #ex-tester-runCommand-nix}

# Run a command with network access

```nix
testers.runCommand {
  name = "access-the-internet";
  script = ''
    curl -o /dev/null https://example.com
    touch $out
  '';
  nativeBuildInputs = with pkgs; [
    cacert
    curl
  ];
}
```

:::

## `runNixOSTest` {#tester-runNixOSTest}

A helper function that behaves exactly like the NixOS `runTest`, except it also assigns this Nixpkgs package set as the `pkgs` of the test and makes the `nixpkgs.*` options read-only.

If your test is part of the Nixpkgs repository, or if you need a more general entrypoint, see ["Calling a test" in the NixOS manual](https://nixos.org/manual/nixos/stable/index.html#sec-calling-nixos-tests).