uiop:getenv

(uiop:getenv name) / (setf (uiop:getenv name) value)

Returns the value of the named environment variable as a string, or nil if the variable is unset. Common Lisp has no getenv, so this is homed in the uiop package -- the portable spelling implementation-independent libraries already use; there is no unqualified getenv. Works on all four backends; the WASM backend reads the real host environment in Preview 1 and wasi:cli/environment@0.3.0 in --component mode -- including a rontolisp:http-handler component under wasmtime serve, which imports the interface for it -- so pass --env/-S inherit-env to wasmtime to make variables visible.

(setf (uiop:getenv name) value) records an override that later reads consult before the host, and a nil value makes the variable read as unset. It does not change the process environment: no backend can (the JVM cannot at all, WASI's is read-only), so the override lives for this program run only -- see uiop/os.

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