diff --git a/scripts/upgrade b/scripts/upgrade
index a099077..c9ea744 100755
--- a/scripts/upgrade
+++ b/scripts/upgrade
@@ -34,13 +34,13 @@ ynh_script_progression "Upgrading system configurations related to $app..."
 
 ynh_config_add_nginx
 
-ynh_add_systemd_config --service= "$app-backend" --template="$backend"
+ynh_add_systemd_config --service= "${app}-backend" --template="$backend"
 
-ynh_add_systemd_config --service= "$app-frontend" --template="$frontend"
+ynh_add_systemd_config --service= "${app}-frontend" --template="$frontend"
 
-yunohost service add "$app-backend" --description="a beautiful and performant vault to save all your files in the cloud" --log="/var/log/$app/$app.log"
+yunohost service add "${app}-backend" --description="a beautiful and performant vault to save all your files in the cloud" --log="/var/log/$app/$app.log"
 
-yunohost service add "$app-frontend" --description="a beautiful and performant vault to save all your files in the cloud" --log="/var/log/$app/$app.log"
+yunohost service add "${app}-frontend" --description="a beautiful and performant vault to save all your files in the cloud" --log="/var/log/$app/$app.log"
 
 ynh_script_progression "Installing $app..."
 
@@ -56,9 +56,9 @@ popd
 #=================================================
 ynh_script_progression "Starting $app's systemd service..."
 
-ynh_systemctl --service="$app-backend" --action="start"
+ynh_systemctl --service="${app}-backend" --action="start"
 
-ynh_systemctl --service="$app-frontend" --action="start"
+ynh_systemctl --service="${app}-frontend" --action="start"
 
 #=================================================
 # END OF SCRIPT