packer: no colored output if machine-readable [GH-684]

packer.go

@@ -133,6 +133,13 @@ func wrappedMain() int {
 		envConfig.Ui = &packer.MachineReadableUi{
 			Writer: os.Stdout,
 		}
+
+		// Set this so that we don't get colored output in our machine-
+		// readable UI.
+		if err := os.Setenv("PACKER_NO_COLOR", "1"); err != nil {
+			fmt.Fprintf(os.Stderr, "Packer failed to initialize UI: %s\n", err)
+			return 1
+		}
 	}
 
 	env, err := packer.NewEnvironment(envConfig)

packer/ui.go

@@ -110,6 +110,11 @@ func (u *ColoredUi) colorize(message string, color UiColor, bold bool) string {
 }
 
 func (u *ColoredUi) supportsColors() bool {
+	// Never use colors if we have this environmental variable
+	if os.Getenv("PACKER_NO_COLOR") != "" {
+		return false
+	}
+
 	// For now, on non-Windows machine, just assume it does
 	if runtime.GOOS != "windows" {
 		return true