getenv

(getenv name)

Returns the value of the named environment variable as a string, or nil if the variable is unset. Works in all three backends; the WASM backend reads the real host environment in Preview 1 and wasi:cli/environment@0.3.0 in --component mode, so pass --env/-S inherit-env to wasmtime to make variables visible.

The result is whatever the host has assigned to the variable, so it is non-deterministic; (getenv "DEFINITELY_UNSET") returns nil.