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nix/doc/manual/src/language/builtin-constants.md
Valentin Gagarin 499ed26508 manual: remove "Writing Nix Expressions" chapter 
it is out of date, all over the place in level of detail, is really
about `nixpkgs`, and in general instructions should not be part of
a reference manual.

also:
- update redirects and internal links
- use "Nix language" consistently
2022-08-04 11:59:25 +02:00

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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]{#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".