Upgrade only packages from the grml-stable Debian repository:
echo 'deb http://deb.grml.org/ grml-stable main' > /etc/apt/grml-stable.list
-apt-get -o Dir::Etc::SourceList=/etc/apt/grml-stable.list update
+apt-get -o Dir::Etc::sourcelist=/etc/apt/grml-stable.list -o Dir::Etc::sourceparts=/doesnotexist update
apt-get upgrade
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