diff --git a/files/yum_update.sh b/files/yum_update.sh
index 69722ed..1ea99a1 100755
--- a/files/yum_update.sh
+++ b/files/yum_update.sh
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 set -eux
 
@@ -11,7 +11,9 @@ fi
 
 packages_for_update=
 if [ -n "$1" ] && command -v repoquery >/dev/null 2>&1; then
-    packages_for_update=("$(repoquery --disablerepo='*' --enablerepo=$1 --qf %{NAME} -a)")
+    installed=$(rpm -qa --qf "%{NAME}\n" | sort)
+    available=$(repoquery --disablerepo='*' --enablerepo=$1 --qf %{NAME} -a | sort)
+    packages_for_update=$(comm -12 <(printf "%s\n" $installed) <(printf "%s\n" $available))
 fi
 
 if [ -z "$packages_for_update" ]; then