Wouldn’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).