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---
Title : Bash Bits: Randomize a cronjob to run between 00:00 and 06:00 hours
Author : Remy van Elst
Date : 06-05-2019
URL : https://raymii.org/s/snippets/Bash_Bits_Randomize_cronjob_time.html
Format : Markdown/HTML
---

Bash Bits are small examples and tips for Bash Scripts. This bash bit shows you
how to randomize the time a cronjob runs in /etc/cron.d/

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### Randomize cronjob time

I've used this in the past in a [backup script][3] I wrote. During the
installation, a cronjob was placed and later the time was randomized between
00:00 AM and 06:00 AM. This way the load on the backup targets wouldn't be a
huge peak but more spread out.

First, place your cronjob in `/etc/cron.d/`, as a file. In this case,
`/etc/cron.d/my_example`. Use the regular `cron.d` format (include the username
between the time and executable):

#!/bin/bash
RANDM RANDH * * * root /usr/local/bin/my_binary

Note the two variables, `RANDM` and `RANDH`. These will be replaced to the
random hour and minute.

The following code will replace the variables with random digits, but in the
range you specify:

# use awk to get a number between 0 and 6 for the hour
RANDH="$(awk 'BEGIN{srand();print int(rand()*(0-6))+6 }')"
# and 0 to 59 for the minutes.
RANDM="$(awk 'BEGIN{srand();print int(rand()*(0-59))+59 }')"
# Replace it in the cronjob.
sed -i -e "s/RANDH/${RANDH}/g" -e "s/RANDM/${RANDM}/g" /etc/cron.d/my_example
# show the user
echo "Randomized cronjob time, will run on ${RANDH}:${RANDM}."

You could use the bash builtin `$RANDOM`, but then you cannot specify a range.
You could get an illigal time that way.

[1]: https://www.digitalocean.com/?refcode=7435ae6b8212
[2]: https://raymii.org/s/tags/bash-bits.html
[3]: https://github.com/RaymiiOrg/CloudVPS-Boss

---

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