explicitly expect a directory for logging

Verify the logging parameter is explicitly a directory.  Treat the path
as a directory for creating the logging files.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I84d6be79780df3f52deeb1ff1b7a50de6175c055
diff --git a/main.cpp b/main.cpp
index 3726f50..8c99154 100644
--- a/main.cpp
+++ b/main.cpp
@@ -72,7 +72,8 @@
                    "Optional parameter to specify configuration at run-time")
         ->check(CLI::ExistingFile);
     app.add_option("-l,--log", loggingPath,
-                   "Optional parameter to specify logging path");
+                   "Optional parameter to specify logging folder")
+        ->check(CLI::ExistingDirectory);
     app.add_flag("-t,--tuning", tuningEnabled, "Enable or disable tuning");
 
     loggingEnabled = (!loggingPath.empty());
diff --git a/pid/zone.hpp b/pid/zone.hpp
index fcadac0..53139ec 100644
--- a/pid/zone.hpp
+++ b/pid/zone.hpp
@@ -54,7 +54,7 @@
     {
         if (loggingEnabled)
         {
-            _log.open(loggingPath + std::to_string(zone));
+            _log.open(loggingPath + "/zone_" + std::to_string(zone) + ".log");
         }
     }