diff --git a/pkg/shell/shell.go b/pkg/shell/shell.go
index 719c0e68..d24b880a 100644
--- a/pkg/shell/shell.go
+++ b/pkg/shell/shell.go
@@ -44,6 +44,7 @@ func QuoteSplit(s string) []string {
 	return a
 }
 
+// ReplaceEnvVars - support format ${CAMERA_PASSWORD} and ${RTSP_USER:admin}
 func ReplaceEnvVars(text string) string {
 	re := regexp.MustCompile(`\${([^}{]+)}`)
 	return re.ReplaceAllStringFunc(text, func(match string) string {