su -s /bin/bash nagios
$ /usr/bin/apt-get -o 'Debug::NoLocking=true' -s -qq upgrade
NOTE: This is only a simulation!
      apt-get needs root privileges for real execution.
      Keep also in mind that locking is deactivated,
      so don't depend on the relevance to the real current situation!
E: Opening /etc/apt/sources.list.d/icinga.list - ifstream::ifstream (13: Permission denied)
E: The list of sources could not be read.
E: Opening /etc/apt/sources.list.d/icinga.list - ifstream::ifstream (13: Permission denied)
E: The list of sources could not be read.