packer: don't output colors if Windows and not Cygwin

CHANGELOG.md

@@ -9,6 +9,7 @@ FEATURES:
 
 IMPROVEMENTS:
 
+* core: Colors won't be outputted on Windows unless in Cygwin.
 * builder/amazon/all: Added `iam_instance_profile` to launch the source
   image with a given IAM profile. [GH-226]
 

packer/ui.go

@@ -10,6 +10,7 @@ import (
 	"os/signal"
 	"strings"
 	"sync"
+	"runtime"
 	"unicode"
 )
 
@@ -80,6 +81,10 @@ func (u *ColoredUi) Error(message string) {
 }
 
 func (u *ColoredUi) colorize(message string, color UiColor, bold bool) string {
+	if !u.supportsColors() {
+		return message
+	}
+
 	attr := 0
 	if bold {
 		attr = 1
@@ -88,6 +93,20 @@ func (u *ColoredUi) colorize(message string, color UiColor, bold bool) string {
 	return fmt.Sprintf("\033[%d;%d;40m%s\033[0m", attr, color, message)
 }
 
+func (u *ColoredUi) supportsColors() bool {
+	// For now, on non-Windows machine, just assume it does
+	if runtime.GOOS != "windows" {
+		return true
+	}
+
+	// On Windows, if we appear to be in Cygwin, then it does
+	cygwin := os.Getenv("CYGWIN") != "" ||
+		os.Getenv("OSTYPE") == "cygwin" ||
+		os.Getenv("TERM") == "cygwin"
+
+	return cygwin
+}
+
 func (u *PrefixedUi) Ask(query string) (string, error) {
 	return u.Ui.Ask(u.prefixLines(u.SayPrefix, query))
 }