Unset a variable, cache variable, or environment variable.
unset(<variable> [CACHE | PARENT_SCOPE])
Removes the specified variable causing it to become undefined. If
CACHE is present then the variable is removed from the cache instead of the current scope.
PARENT_SCOPE is present then the variable is removed from the scope above the current scope. See the same option in the set command for further details.
<variable> can be an environment variable such as:
in which case the variable will be removed from the current environment.