diff --git a/to_opus.sh b/to_opus.sh
index bc2f70f..78c0665 100644
--- a/to_opus.sh
+++ b/to_opus.sh
@@ -1,18 +1,28 @@
-#!/usr/bin/env bash
+#!/bin/bash
 
+# set shopt-options used by this shell-script
+# Note, 0 (true) from shopt -q is "false" in a math context.
 shopt -q globstar; globstar_set=$?
 ((globstar_set)) && shopt -s globstar
 shopt -q extglob; extglob_set=$?
 ((extglob_set)) && shopt -s extglob
-# Note, 0 (true) from shopt -q is "false" in a math context.
 
-FORMATS=$@
+# return default shopt-options
+function finish {
+	((globstar_set)) && shopt -u globstar
+	((extglob_set)) && shopt -u extglob
+}
+trap finish EXIT
 
-for file in **/*.@(${FORMATS// /|}); do
-	ffmpeg -threads 4 -i "$file" -c:a libopus -b:a 128k "${file%.*}.opus" && rm "$file"
+# setting default extension to all
+if [ $# -eq 0 ]; then
+	extension="*"
+else
+	extension=$@
+fi
+
+for file in **/*.@(${extension// /|}); do
+	ffmpeg -threads 4 -i $file -c:a libopus -b:a 128k "${file%.*}.opus" && rm "$file"
 done
 
-((globstar_set)) && shopt -u globstar
-((extglob_set)) && shopt -u extglob
-
 exit 0