Only prompt for interface when the terminal is a TTY

... otherwise we shouldn't bother trying to prompt the user, because the
environment running the command can't be expected to allow the user to respond.

arguments/parser.go

@@ -64,6 +64,12 @@ func (argParser *argumentParser) ParseArguments(args ...string) ParsedArguments
 }
 
 func (parser *argumentParser) PromptUserForInterfaceName(filepath string) string {
+	if !(parser.ui.TerminalIsTTY()) {
+		parser.ui.WriteLine("Cowardly refusing to prompt user for an interface name in a non-tty environment")
+		parser.failHandler("Perhaps you meant to invoke counterfeiter with more than one argument?")
+		return ""
+	}
+
 	parser.ui.WriteLine("Which interface to counterfeit?")
 
 	interfacesInPackage := parser.interfaceLocator.GetInterfacesFromFilePath(filepath)

arguments/parser_test.go

@@ -75,6 +75,17 @@ var _ = Describe("parsing arguments", func() {
 
 			interfaceLocator.GetInterfacesFromFilePathReturns([]string{"Foo", "Bar"})
 			ui.ReadLineFromStdinReturns("1")
+			ui.TerminalIsTTYReturns(true)
+		})
+
+		Context("but the connecting terminal is not a TTY", func() {
+			BeforeEach(func() {
+				ui.TerminalIsTTYReturns(false)
+			})
+
+			It("should invoke the fail handler", func() {
+				Expect(failWasCalled).To(BeTrue())
+			})
 		})
 
 		It("prompts the user for which interface they want", func() {

integration/integration_test.go

@@ -117,7 +117,10 @@ func startCounterfeiterWithStdinPipe(workingDir string, stdin io.Reader, args ..
 	cmd := exec.Command(pathToCounterfeiter, args...)
 	cmd.Stdin = stdin
 	cmd.Dir = workingDir
-	cmd.Env = []string{"GOPATH=" + absPathWithSymlinks}
+	cmd.Env = []string{
+		"GOPATH=" + absPathWithSymlinks,
+		"COUNTERFEITER_INTERACTIVE=1",
+	}
 
 	session, err := gexec.Start(cmd, GinkgoWriter, GinkgoWriter)
 	Expect(err).ToNot(HaveOccurred())

scripts/test.sh

@@ -21,6 +21,7 @@ $counterfeiter /tmp/symlinked_fixtures Something >/dev/null
 
 go build ./fixtures/...
 
-go test -race -v ./...
+go test -race -v . ./arguments ./integration
 
 rm /tmp/symlinked_fixtures
+rm -rf fixtures/fixtures

terminal/ui.go

@@ -22,7 +22,9 @@ func NewUI() UI {
 type ui struct{}
 
 func (ui *ui) TerminalIsTTY() bool {
-	return terminal.IsTerminal(int(os.Stdin.Fd()))
+	isTTY := terminal.IsTerminal(int(os.Stdin.Fd()))
+	hasOverride := os.Getenv("COUNTERFEITER_INTERACTIVE") == "1"
+	return isTTY || hasOverride
 }
 
 func (ui *ui) ReadLineFromStdin() string {