Random delay for cron.daily, cron.weekly, cron.monthly

cron-logoWouldn’t it be nice if cron’s daily, weekly and monthly jobs could be run with a slight offset? At least that’s what I thought when 20+ servers were hitting my backup infrastructure at once. The scripts in /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly are triggered directly from crontab at fixed times. Here’s what /etc/crontab looks like in Ubuntu Server 16.04:

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user	command
17 *	* * *	root    cd / && run-parts --report /etc/cron.hourly
25 6	* * *	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6	* * 7	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6	1 * *	root	test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#

I’ve found several tips which suggested to use a RANDOM_DELAY variable in crontab. Unfortunately, this variable doesn’t seem to be implemented in Debian/Ubuntu’s version of crontab at this time. I even checked the source code, there’s no RANDOM_DELAY variable to be found.

Here’s the solution I came up with. I’m using a combination of sleep and numrandom with a time range between 0 and 30 minutes.

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user	command
17 *	* * *	root    cd / && run-parts --report /etc/cron.hourly
25 6	* * *	root	sleep `numrandom /0..30/`m ; test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6	* * 7	root	sleep `numrandom /0..30/`m ; test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6	1 * *	root	sleep `numrandom /0..30/`m ; test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
#

In order to use the numrandom command, you have to apt-get -y install num-utils it first.

I didn’t delay the cron.hourly execution but the same sleep/numrandom combo could be used for it as well, just maybe replace the m (minutes) with s (seconds).