freedombone-utils-cron

#!/bin/bash
#
# .---.                  .              .
# |                      |              |
# |--- .--. .-.  .-.  .-.|  .-. .--.--. |.-.  .-. .--.  .-.
# |    |   (.-' (.-' (   | (   )|  |  | |   )(   )|  | (.-'
# '    '     --'  --'  -' -  -' '  '   -' -'   -' '   -  --'
#
#                    Freedom in the Cloud
#
# Cron functions
#
# License
# =======
#
# Copyright (C) 2014-2016 Bob Mottram <bob@robotics.uk.to>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

function cron_add_mins {
	if ! grep -q "${2}" /etc/crontab; then
		echo "*/${1}            * *   *   *   root ${2}" >> /etc/crontab
		systemctl restart cron
	fi
}

function randomize_cron {
	# The predictable default timing of Debian cron jobs might
	# be exploitable knowledge. Avoid too much predictability
	# by randomizing the times when cron jobs run
	if grep -Fxq "randomize_cron" $COMPLETION_FILE; then
		return
	fi

	# randomize the day on which the weekly cron job runs
	randdow=$(($RANDOM%6+1))
	sed -i "s|\* \* 7|* * $randdow|g" /etc/crontab

	# randomize the time when the weekly cron job runs
	randmin=$(($RANDOM%60))
	randhr=$(($RANDOM%3+1))
	sed -i "s|47 6|$randmin $randhr|g" /etc/crontab

	# randomize the time when the daily cron job runs
	randmin=$(($RANDOM%60))
	randhr=$(($RANDOM%3+4))
	sed -i "s|25 6\t\* \* \*|$randmin $randhr\t* * *|g" /etc/crontab

	# randomize the time when the hourly cron job runs
	randmin=$(($RANDOM%60))
	sed -i "s|17 \*\t|$randmin *\t|g" /etc/crontab

	# randomize monthly cron job time and day
	randmin=$(($RANDOM%60))
	randhr=$(($RANDOM%22+1))
	randdom=$(($RANDOM%27+1))
	sed -i "s|52 6\t|$randmin $randhr\t|g" /etc/crontab
	sed -i "s|\t1 \* \*|\t$randdom * *|g" /etc/crontab

	systemctl restart cron

	echo 'randomize_cron' >> $COMPLETION_FILE
}

# NOTE: deliberately there is no "exit 0"