This is a text-only version of the following page on https://raymii.org:
---
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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[All Bash Bits can be found using this link][2]

### 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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