Built-in Constants
Here are the constants built into the Nix expression evaluator:
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builtins
The setbuiltinscontains all the built-in functions and values. You can usebuiltinsto 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.
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builtins.currentSystem
The built-in valuecurrentSystemevaluates to the Nix platform identifier for the Nix installation on which the expression is being evaluated, such as"i686-linux"or"x86_64-darwin".