Made the template work in the translation command before splitting it.

This way you can use the template language to create the actual command
based on the data.

cli/filter_data_through_cmd.go

@@ -13,31 +13,27 @@ func filterDataThroughCmd(data []byte, filter string, subject, stream string) ([
 	if filter == "" {
 		return data, nil
 	}
-	parts, err := shlex.Split(filter)
-	if err != nil {
-		return nil, fmt.Errorf("the filter command line could not be parsed: %w", err)
-	}
-	cmd := parts[0]
-	args := parts[1:]
-
 	funcMap := template.FuncMap{
 		"Subject": func() string { return subject },
 		"Stream":  func() string { return stream },
 	}
 
-	for idx, arg := range args {
-		tmpl, err := template.New("translate").Funcs(funcMap).Parse(arg)
-		if err != nil {
-			return nil, err
-		}
-		var builder strings.Builder
-		err = tmpl.Execute(&builder, nil)
-		if err != nil {
-			return nil, err
-		}
+	tmpl, err := template.New("translate").Funcs(funcMap).Parse(filter)
+	if err != nil {
+		return nil, err
+	}
+	var builder strings.Builder
+	err = tmpl.Execute(&builder, nil)
+	if err != nil {
+		return nil, err
+	}
 
-		args[idx] = builder.String()
+	parts, err := shlex.Split(builder.String())
+	if err != nil {
+		return nil, fmt.Errorf("the filter command line could not be parsed: %w", err)
 	}
+	cmd := parts[0]
+	args := parts[1:]
 
 	runner := exec.Command(cmd, args...)
 	// pass the message as string to stdin