diff --git a/image_builder/manual.sh b/image_builder/manual.sh
index 9200995f..185ce3e3 100755
--- a/image_builder/manual.sh
+++ b/image_builder/manual.sh
@@ -1,26 +1,20 @@
 #! /bin/bash
 
-DEBIAN_FRONTEND='noninteractive'
-LANG='C.UTF-8'
-LC_ALL='C.UTF-8'
+[ "${DEBIAN_FRONTEND:-}" ] || DEBIAN_FRONTEND='noninteractive'
+[ "${LANG:-}" ] || LANG='C.UTF-8'
+[ "${LC_ALL:-}" ] || LC_ALL='C.UTF-8'
 
-is_set() {
-  if ! [[ -z $$1 ]];
-  then $1="NOT_SET"
-  fi
-}
+[ "${TARGET_REPO_URL:-}" ] || TARGET_REPO_URL='https://github.com/urpylka/clever.git'
+[ "${TARGET_REPO_REF:-}" ] || TARGET_REPO_REF='qemu_test_2'
+[ "${TARGET_REPO_PATH:-}" ] || TARGET_REPO_PATH='image_builder'
 
-TARGET_REPO_URL='https://github.com/urpylka/clever.git'
-TARGET_REPO_REF='qemu_test_2'
-TARGET_REPO_PATH='image_builder'
+[ "${IMAGE_NAME:-}" ] || IMAGE_NAME='clever_docker.img'
+[ "${CLEVER_VERSION:-}" ] || CLEVER_VERSION='NO_VERSION'
 
-IMAGE_NAME='clever_docker.img'
-CLEVER_VERSION='NO_VERSION'
-
-SOURCE_IMAGE='https://downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2017-12-01/2017-11-29-raspbian-stretch-lite.zip'
-NUMBER_THREADS=4
-DISCOVER_ROS_PACKAGES=False
-SHRINK=True
+[ "${SOURCE_IMAGE:-}" ] || SOURCE_IMAGE='https://downloads.raspberrypi.org/raspbian_lite/images/raspbian_lite-2017-12-01/2017-11-29-raspbian-stretch-lite.zip'
+[ "${NUMBER_THREADS:-}" ] || NUMBER_THREADS=4
+[ "${DISCOVER_ROS_PACKAGES:-}" ] || DISCOVER_ROS_PACKAGES=False
+[ "${SHRINK:-}" ] || SHRINK=True
 
 set -e