diff --git a/generate-specific-bootdisk.sh b/generate-specific-bootdisk.sh
index d84cee5..6f620e8 100755
--- a/generate-specific-bootdisk.sh
+++ b/generate-specific-bootdisk.sh
@@ -51,8 +51,9 @@ Project: https://github.com/sickcodes/osx-serial-generator/
 License: GPLv3+
 "
 
-OPENCORE_IMAGE_MAKER_URL='https://raw.githubusercontent.com/sickcodes/osx-serial-generator/master/opencore-image-ng.sh'
-MASTER_PLIST_URL='https://raw.githubusercontent.com/sickcodes/osx-serial-generator/master/config-nopicker-custom.plist'
+OPENCORE_IMAGE_MAKER_URL="${OPENCORE_IMAGE_MAKER_URL:='https://raw.githubusercontent.com/sickcodes/osx-serial-generator/master/opencore-image-ng.sh'}"
+MASTER_PLIST_URL="${MASTER_PLIST_URL:='https://raw.githubusercontent.com/sickcodes/osx-serial-generator/master/config-nopicker-custom.plist'}"
+
 
 # gather arguments
 while (( "$#" )); do
diff --git a/generate-unique-machine-values.sh b/generate-unique-machine-values.sh
index a54f6f0..8b6c47c 100755
--- a/generate-unique-machine-values.sh
+++ b/generate-unique-machine-values.sh
@@ -56,8 +56,8 @@ Project: https://github.com/sickcodes/osx-serial-generator/
 License: GPLv3+
 "
 
-OPENCORE_IMAGE_MAKER_URL='https://raw.githubusercontent.com/sickcodes/osx-serial-generator/master/opencore-image-ng.sh'
-MASTER_PLIST_URL='https://raw.githubusercontent.com/sickcodes/osx-serial-generator/master/config-nopicker-custom.plist'
+OPENCORE_IMAGE_MAKER_URL="${OPENCORE_IMAGE_MAKER_URL:='https://raw.githubusercontent.com/sickcodes/osx-serial-generator/master/opencore-image-ng.sh'}"
+MASTER_PLIST_URL="${MASTER_PLIST_URL:='https://raw.githubusercontent.com/sickcodes/osx-serial-generator/master/config-nopicker-custom.plist'}"
 
 # gather arguments
 while (( "$#" )); do