postinst is in nagios3-common 3.5.1-1ubuntu1.
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 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 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 | #!/bin/bash
set -e
# some shorthands for sanity
en="/etc/nagios3"
enc="/etc/nagios3/conf.d"
usn="/usr/share/nagios3"
. /usr/share/debconf/confmodule
if [ -n "$NAG3DEBUG" ]; then
  echo "now debugging $0 $@"
  set -x
fi
setperm() {
    local user="$1"
    local group="$2"
    local mode="$3"
    local file="$4"
    shift 4
    # only do something when no setting exists
    if ! dpkg-statoverride --list "$file" >/dev/null 2>&1; then
      chown "$user":"$group" "$file"
      chmod "$mode" "$file"
    fi
}
case "$1" in
  configure)
    if ! getent passwd nagios > /dev/null ; then
      echo 'Adding system-user for nagios' 1>&2
      adduser --system --group --home /var/lib/nagios \
              --disabled-login --force-badname nagios > /dev/null
    fi
	# explicitly set permissions on some files that are dependent
	# on the uid/gid of the nagios user, which is dynamically created.
        setperm root nagios 0640 $en/resource.cfg
        setperm nagios adm 2751 /var/log/nagios3
        setperm nagios adm 2751 /var/log/nagios3/archives
        setperm nagios nagios 0750 /var/lib/nagios3
	setperm nagios nagios 0750 /var/lib/nagios3/spool
	setperm nagios nagios 0750 /var/lib/nagios3/spool/checkresults
        setperm nagios www-data 02750 /var/cache/nagios3
        setperm nagios www-data 0700 /var/lib/nagios3/rw
	db_stop
    ;;
  abort-upgrade|abort-remove|abort-deconfigure)
    ;;
  *)
    echo "postinst called with unknown argument \$1'" >&2
    exit 1
    ;;
esac
# Automatically added by dh_installinit
if [ -x "/etc/init.d/nagios3" ] || [ -e "/etc/init/nagios3.conf" ]; then
	if [ ! -e "/etc/init/nagios3.conf" ]; then
		update-rc.d nagios3 defaults 30 18 >/dev/null
	fi
	invoke-rc.d nagios3 start || exit $?
fi
# End automatically added section
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