getEnv package:shake

getEnv :: String -> Action (Maybe String)
shake Development.Shake
Return Just the value of the environment variable, or Nothing if the variable is not set. The environment variable is tracked as a dependency, and if it changes the rule will rerun in subsequent builds. This function is a tracked version of getEnv / lookupEnv from the base library. 
flags <- getEnv "CFLAGS"
cmd "gcc -c" [out] (maybe [] words flags)
getEnvError :: Partial => String -> Action String
shake Development.Shake
A partial variant of getEnv that returns the environment variable variable or fails.
getEnvWithDefault :: String -> String -> Action String
shake Development.Shake
getEnvWithDefault def var returns the value of the environment variable var, or the default value def if it is not set. Similar to getEnv. 
flags <- getEnvWithDefault "-Wall" "CFLAGS"
cmd "gcc -c" [out] flags