Here are the constants built into the Nix expression evaluator:
builtins\ The set
builtinscontains all the built-in functions and values. You can use
builtinsto 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
currentSystemevaluates to the Nix platform identifier for the Nix installation on which the expression is being evaluated, such as