# versionCheckHook {#versioncheckhook}

This hook adds a `versionCheckPhase` to the [`preInstallCheckHooks`](#ssec-installCheck-phase) that runs the main program of the derivation with a `--help` or `--version` argument, and checks that the `${version}` string is found in that output. If this check fails then the whole build will fail. _(A softer option is [`testers.testVersion`](#tester-testVersion).)_

You use it like this:

```nix
{
  lib,
  stdenv,
  versionCheckHook,
# ...
}:

stdenv.mkDerivation (finalAttrs: {
  # ...

  nativeInstallCheckInputs = [
    versionCheckHook
  ];
  doInstallCheck = true;

  # ...
})
```

Note that for [`buildPythonPackage`](#buildpythonpackage-function) and [`buildPythonApplication`](#buildpythonapplication-function), `doInstallCheck` is enabled by default.

It does so in a clean environment (using `env --ignore-environment`), and it checks for the `${version}` string in both the `stdout` and the `stderr` of the command. It will report to you in the build log the output it received and it will fail the build if it failed to find `${version}`.

The variables that this phase control are:

- `dontVersionCheck`: Disable adding this hook to the [`preInstallCheckHooks`](#ssec-installCheck-phase). Useful if you do want to load the bash functions of the hook, but run them differently.
- `versionCheckProgram`: The full path to the program that should print the `${version}` string. Defaults roughly to `${placeholder "out"}/bin/${pname}`. Using `$out` in the value of this variable won't work, as environment variables from this variable are not expanded by the hook. Hence using `placeholder` is unavoidable.
- `versionCheckProgramArg`: The argument that needs to be passed to `versionCheckProgram`. If undefined the hook tries first `--help` and then `--version`. Examples: `version`, `-V`, `-v`.
- `preVersionCheck`: A hook to run before the check is done.
- `postVersionCheck`: A hook to run after the check is done.

This check assumes the executable is _hermetic_. If environment variables such as `PATH` or `HOME` are required for the program to function, then [`testers.testVersion`](#tester-testVersion) is currently the better alternative.