Built-in Constants

These constants are built into the Nix language evaluator:

builtins (attribute set)

Contains all the built-in functions and values, in order to avoid polluting the global scope.

Since built-in functions were added over time, testing for attributes in builtins can be used for graceful fallback on older Nix installations:
```
if builtins ? getEnv then builtins.getEnv "PATH" else ""
```
builtins.currentSystem (string)

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".

Not available in pure evaluation mode.
builtins.currentTime (integer)

Return the Unix time at first evaluation. Repeated references to that name will re-use the initially obtained value.

Example:
```
$ nix repl
Welcome to Nix 2.15.1 Type :? for help.

nix-repl> builtins.currentTime
1683705525

nix-repl> builtins.currentTime
1683705525
```
The store path of a derivation depending on currentTime will differ for each evaluation.

Not available in pure evaluation mode.

Nix Reference Manual