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+# Testers {#chap-testers}
+This chapter describes several testing builders which are available in the <literal>testers</literal> namespace.
+
+## `testVersion` {#tester-testVersion}
+
+Checks the command output contains the specified version
+
+Although simplistic, this test assures that the main program
+can run. While there's no substitute for a real test case,
+it does catch dynamic linking errors and such. It also provides
+some protection against accidentally building the wrong version,
+for example when using an 'old' hash in a fixed-output derivation.
+
+Examples:
+
+```nix
+passthru.tests.version = testVersion { package = hello; };
+
+passthru.tests.version = testVersion {
+  package = seaweedfs;
+  command = "weed version";
+};
+
+passthru.tests.version = testVersion {
+  package = key;
+  command = "KeY --help";
+  # Wrong '2.5' version in the code. Drop on next version.
+  version = "2.5";
+};
+```
+
+## `testEqualDerivation` {#tester-testEqualDerivation}
+
+Checks that two packages produce the exact same build instructions.
+
+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:
+
+```nix
+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:
+
+```nix
+tests.fetchgit = invalidateFetcherByDrvHash fetchgit {
+  name = "nix-source";
+  url = "https://github.com/NixOS/nix";
+  rev = "9d9dbe6ed05854e03811c361a3380e09183f4f4a";
+  sha256 = "sha256-7DszvbCNTjpzGRmpIVAWXk20P0/XTrWZ79KSOGLrUWY=";
+};
+```