This can be used to make sure that a certain difference of configuration, such as the presence of an overlay does not cause a cache miss.

When the derivations are equal, the return value is an empty file.
Otherwise, the build log explains the difference via `nix-diff`.

:::{.example #ex-testEqualDerivation-hello}

# Check that two packages produce the same derivation

```nix
testers.testEqualDerivation "The hello package must stay the same when enabling checks." hello (
  hello.overrideAttrs (o: {
    doCheck = true;
  })
)
```

:::

## `invalidateFetcherByDrvHash` {#tester-invalidateFetcherByDrvHash}

Use the derivation hash to invalidate the output via name, for testing.

Type: `(a@{ name, ... } -> Derivation) -> a -> Derivation`

Normally, fixed output derivations can and should be cached by their output hash only, but for testing we want to re-fetch everytime the fetcher changes.

Changes to the fetcher become apparent in the drvPath, which is a hash of how to fetch, rather than a fixed store path.
By inserting this hash into the name, we can make sure to re-run the fetcher every time the fetcher changes.

This relies on the assumption that Nix isn't clever enough to reuse its database of local store contents to optimize fetching.

You might notice that the "salted" name derives from the normal invocation, not the final derivation.
`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 changes.

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),