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Diffstat (limited to 'doc/manual/src/expressions/builtin-constants.md')
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diff --git a/doc/manual/src/expressions/builtin-constants.md b/doc/manual/src/expressions/builtin-constants.md new file mode 100644 index 000000000..1404289e5 --- /dev/null +++ b/doc/manual/src/expressions/builtin-constants.md @@ -0,0 +1,20 @@ +# 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., + + ```nix + 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"`. |