nix/doc/manual/src/language/builtin-constants.md

# 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`]{#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"`.
-												Move primop docs inline

This makes them available to 'nix repl'.

											
										
										
											2020-08-24 08:31:10 -04:00
+								# Built-in Constants
 								Here are the constants built into the Nix expression evaluator:
-												Replace the trailing markdown spaces by a backslash

They are equivalent according to
<https://spec.commonmark.org/0.29/#hard-line-breaks>,
and the trailing spaces tend to be a pain (because the make git
complain, editors tend to want to remove them − the `.editorconfig`
actually specifies that − etc..).

											
										
										
											2021-04-23 08:30:42 -04:00
+								  - `builtins`\
-												Move primop docs inline

This makes them available to 'nix repl'.

											
										
										
											2020-08-24 08:31:10 -04:00
+								    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.
-												doc: Add anchors to long lists

Added using the following sed scripts:

- For command-ref/opt-common.md:

    s~- `(--?)([^`]+)`~- [`\1\2`]{#opt-\2}~g

- For expressions/builtin-constants.md:

    s~- `(builtins\.?)([^`]+)`~- [`\1\2`]{#builtins-\2}~g

- For expressions/advanced-attributes.md

    s~^  - `([^`]+)`~  - [`\1`]{#adv-attr-\1}~g

  and manually adjusted outputHashAlgo & outputHashMode.

- For glossary.md

    s~^  - (`([^`]+)`|(.+)) ?\\~  - [\1]{#gloss-\2\3}\\~g;
    s~(gloss-\w+) ~\1-~g

  and manually adjusted anchors for Nix expression, user environment, NAR, ∅ and ε.

- For command-ref/env-common.md

    s~^  - `([^`]+)`~  - [`\1`]{#env-\1}~g'

											
										
										
											2022-05-25 06:36:46 -04:00
+								  - [`builtins.currentSystem`]{#builtins-currentSystem}\
-												Move primop docs inline

This makes them available to 'nix repl'.

											
										
										
											2020-08-24 08:31:10 -04:00
+								    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"`.