-echo "---> Updating operating system"
-apt-get update -qq
-DEBIAN_FRONTEND=noninteractive apt-get upgrade -y --force-yes -qq \
- -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold"
+# Ensure that necessary variables are set to enable noninteractive mode in
+# commands.
+export DEBIAN_FRONTEND=noninteractive