diff --git a/setup.sh b/setup.sh
index 8f94135d..84a5d4fd 100755
--- a/setup.sh
+++ b/setup.sh
@@ -127,7 +127,7 @@ _docker_image_exists() {
   fi
 }
 
-if [ -t 1 ] ; then
+if tty -s ; then
   USE_TTY="-ti"
 fi