This is under Debian wheezy with the Debmon packages. <pre><code># /usr/lib/nagios/plugins/check_apt APT OK: 0 packages available for upgrade (0 critical updates). |available_upgrades=0;;;0 critical_updates=0;;;0 # su -c /usr/lib/nagios/plugins/check_apt -s /bin/sh - nagios APT WARNING: 6 packages available for upgrade (0 critical updates). |available_upgrades=6;;;0 critical_updates=0;;;0 </code></pre> Related to the problem seems to be this: <pre><code># # apt-cache policy icinga2-bin icinga2-bin: Installed: 2.1.1-1~debmon70+1 Candidate: 2.1.1-1~debmon70+1 Version table: *** 2.1.1-1~debmon70+1 0 400 http://debmon.org/debmon/ debmon-wheezy/main amd64 Packages 100 /var/lib/dpkg/status 2.1.1-1~bpo70+1 0 100 http://ftp.de.debian.org/debian/ wheezy-backports/main amd64 Packages N: Ignoring file '20auto-upgrades.ucf-dist' in directory '/etc/apt/apt.conf.d/' as it has an invalid filename extension </code></pre> Where: <pre><code># cat /etc/apt/preferences.d/debmon-pin-400 # prefer Debian Packages over Debmon packages # do not automatically update Package: * Pin: release o=debmon.org Pin-Priority: 400 </code></pre> I'm thinking that it's related to this, because there are 6 packages coming from the debmon Repository that have really similar versions in the backports repository. However neither "apt-cache policy", nor "dpkg --compare-versions" seem to behave differently when run as the nagios user or root. So at this point I have no idea why check_apt behaves differently under the nagios user. — <a href="https://github.com/monitoring-plugins/monitoring-plugins/issues/1294">Reply to this email on GitHub</a>.<img alt="" height="1" src="https://github.com/notifications/beacon/AFQl2SPHBMaVfI2KjCVG1c22Lzmohuq8ks5nGTT8gaJpZM4CySWy.gif" width="1" /> <script type="application/ld+json">{"@context":"http://schema.org","@type":"EmailMessage","description":"View this Issue on GitHub","action":{"@type":"ViewAction","url":"https://github.com/monitoring-plugins/monitoring-plugins/issues/1294","name":"View Issue"}}</script>