postrm is in nagios3-common 3.5.1.dfsg-2.1ubuntu1.3.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 | #!/bin/sh -e
# some shorthands for sanity
en="/etc/nagios3"
enc="/etc/nagios3/conf.d"
usn="/usr/share/nagios3"
case "$1" in
  remove)
  	;;
  purge)
        for file in $en/resource.cfg /var/log/nagios3 /var/run/nagios3 \
	            /var/lib/nagios3 /var/lib/nagios3/spool \
		    /var/lib/nagios3/spool/checkresults \
		    /var/cache/nagios3 /var/lib/nagios3/rw; do
	    dpkg-statoverride --force --remove $file 2>/dev/null || true
	done
        rm -rf /var/run/nagios3 /var/log/nagios3 /var/lib/nagios3 \
	       /var/cache/nagios3
	rm -f /etc/nagios3/conf.d/host-gateway_nagios3.cfg
	#rm -f /etc/nagios3/conf.d/extcommands_nagios3.cfg
	if which ucf >/dev/null 2>&1; then
		ucf --purge /etc/nagios3/conf.d/host-gateway_nagios3.cfg
		#ucf --purge /etc/nagios3/conf.d/extcommands_nagios3.cfg
	fi
    ;;
esac
# Automatically added by dh_installinit
if [ "$1" = "purge" ] ; then
	update-rc.d nagios3 remove >/dev/null
fi
# In case this system is running systemd, we make systemd reload the unit files
# to pick up changes.
if [ -d /run/systemd/system ] ; then
	systemctl --system daemon-reload >/dev/null || true
fi
# End automatically added section
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