Function std::os::getenv
pub fn getenv(n: &str) -> Option<~str>
Fetches the environment variable n
from the current process, returning
None if the variable isn't set.
Any invalid UTF-8 bytes in the value are replaced by \uFFFD. See
str::from_utf8_lossy()
for details.
Failure
Fails if n
has any interior NULs.