postinst is in nagios3-core 3.5.1.dfsg-2.1ubuntu1.
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 | #!/bin/sh -e
#if we stop nagios3 in nagios3.prerm we should also start it here..
#(#481334)
if [ -x "/etc/init.d/nagios3" ]; then
	if ! /etc/init.d/nagios3 status >/dev/null; then
		if [ -x "`which invoke-rc.d 2>/dev/null`" ]; then
			invoke-rc.d nagios3 start || true
		else
			/etc/init.d/nagios3 start || true
		fi
	fi
fi
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