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".