3 files changed, 21 insertions, 7 deletions
diff --git a/main/src/mill/MillMain.scala b/main/src/mill/MillMain.scala
index 421b9f20..1598d5f3 100644
--- a/main/src/mill/MillMain.scala
+++ b/main/src/mill/MillMain.scala
@@ -64,8 +64,18 @@ object MillMain {
       }
     )
 
+    var debugLog = false
+    val debugLogSignature = Arg[Config, Unit](
+      name = "debug", shortName = Some('d'),
+      doc = "Show debug output on STDOUT",
+      (c, v) => {
+        debugLog = true
+        c
+      }
+    )
+
     val millArgSignature =
-      Cli.genericSignature.filter(a => !removed(a.name)) ++ Seq(interactiveSignature, disableTickerSignature)
+      Cli.genericSignature.filter(a => !removed(a.name)) ++ Seq(interactiveSignature, disableTickerSignature, debugLogSignature)
 
     Cli.groupArgs(
       args.toList,
@@ -105,7 +115,8 @@ object MillMain {
                   |  interp.colors(),
                   |  repl.pprinter(),
                   |  build.millSelf.get,
-                  |  build.millDiscover
+                  |  build.millDiscover,
+                  |  $debugLog
                   |)
                   |repl.pprinter() = replApplyHandler.pprinter
                   |import replApplyHandler.generatedEval._
@@ -120,7 +131,8 @@ object MillMain {
             stdout, stderr, stdin,
             stateCache,
             env,
-            setIdle
+            setIdle,
+            debugLog
           )
 
           if (mill.main.client.Util.isJava9OrAbove) {
diff --git a/main/src/mill/main/MainRunner.scala b/main/src/mill/main/MainRunner.scala
index 34151ab4..4f31a724 100644
--- a/main/src/mill/main/MainRunner.scala
+++ b/main/src/mill/main/MainRunner.scala
@@ -24,7 +24,8 @@ class MainRunner(val config: ammonite.main.Cli.Config,
                  stdIn: InputStream,
                  stateCache0: Option[Evaluator.State] = None,
                  env : Map[String, String],
-                 setIdle: Boolean => Unit)
+                 setIdle: Boolean => Unit,
+                 debugLog: Boolean)
   extends ammonite.MainRunner(
     config, outprintStream, errPrintStream,
     stdIn, outprintStream, errPrintStream
@@ -81,7 +82,7 @@ class MainRunner(val config: ammonite.main.Cli.Config,
             errPrintStream,
             errPrintStream,
             stdIn,
-            debugEnabled = false // TODO: read from cmdline args
+            debugEnabled = debugLog
           ),
           env
         )
diff --git a/main/src/mill/main/ReplApplyHandler.scala b/main/src/mill/main/ReplApplyHandler.scala
index 2cf0c6f1..af69c761 100644
--- a/main/src/mill/main/ReplApplyHandler.scala
+++ b/main/src/mill/main/ReplApplyHandler.scala
@@ -16,7 +16,8 @@ object ReplApplyHandler{
                colors: ammonite.util.Colors,
                pprinter0: pprint.PPrinter,
                rootModule: mill.define.BaseModule,
-               discover: Discover[_]) = {
+               discover: Discover[_],
+               debugLog: Boolean) = {
     new ReplApplyHandler(
       pprinter0,
       new Evaluator(
@@ -32,7 +33,7 @@ object ReplApplyHandler{
           System.err,
           System.err,
           System.in,
-          debugEnabled = false
+          debugEnabled = debugLog
         )
       )
     )