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20 lines
764 B
Markdown
20 lines
764 B
Markdown
# Built-in Constants
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Here are the constants built into the Nix expression evaluator:
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- `builtins`\
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The set `builtins` contains all the built-in functions and values.
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You can use `builtins` to test for the availability of features in
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the Nix installation, e.g.,
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```nix
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if builtins ? getEnv then builtins.getEnv "PATH" else ""
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```
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This allows a Nix expression to fall back gracefully on older Nix
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installations that don’t have the desired built-in function.
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- [`builtins.currentSystem`]{#builtins-currentSystem}\
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The built-in value `currentSystem` evaluates to the Nix platform
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identifier for the Nix installation on which the expression is being
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evaluated, such as `"i686-linux"` or `"x86_64-darwin"`.
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