# Built-in Constants

Here are the constants built into the Nix expression evaluator:

- `builtins`
  The set `builtins` contains all the built-in functions and values. You can use `builtins` to test for the availability of features in the Nix installation, e.g.,

      if builtins ? getEnv then builtins.getEnv "PATH" else ""

  This allows a Nix expression to fall back gracefully on older Nix installations that don’t have the desired built-in function.

- `builtins.currentSystem`
  The built-in value `currentSystem` evaluates to the Nix platform identifier for the Nix installation on which the expression is being evaluated, such as `"i686-linux"` or `"x86_64-darwin"`.