postinst is in at 3.1.20-3.1ubuntu2.
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 | #!/bin/sh
set -e
if [ "$1" = "configure" ]; then
    SEQFILE=/var/spool/cron/atjobs/.SEQ
    if [ ! -f $SEQFILE ]; then
	echo 0 > $SEQFILE
	chmod 600 $SEQFILE
	chown daemon:daemon $SEQFILE
    fi
fi
# Automatically added by dh_systemd_enable/11.1.4ubuntu1
if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ] ; then
	# This will only remove masks created by d-s-h on package removal.
	deb-systemd-helper unmask 'atd.service' >/dev/null || true
	# was-enabled defaults to true, so new installations run enable.
	if deb-systemd-helper --quiet was-enabled 'atd.service'; then
		# Enables the unit on first installation, creates new
		# symlinks on upgrades if the unit file has changed.
		deb-systemd-helper enable 'atd.service' >/dev/null || true
	else
		# Update the statefile to add new symlinks (if any), which need to be
		# cleaned up on purge. Also remove old symlinks.
		deb-systemd-helper update-state 'atd.service' >/dev/null || true
	fi
fi
# End automatically added section
# Automatically added by dh_installinit/11.1.4ubuntu1
if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ] ; then
	if [ -x "/etc/init.d/atd" ]; then
		update-rc.d atd defaults 89 11 >/dev/null
		invoke-rc.d atd start || exit 1
	fi
fi
# End automatically added section
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