Dynamically setting time of cron job based off of system variables

Question

I am wondering if there is a way to dynamically populate the time of a cron job off of an environment variable. For example:

export JOB_MINUTES=4

Then in the crontab

$JOB_MINUTES * * * * /tmp/my_script.sh

Resulting in a crontab of

4 * * * * /tmp/my_script.sh

Answer 1

Environment variables aren't expanded inside the crontab file. But you can use the environment variable when you're creating the crontab file, by piping an echo command to crontab.

{ crontab -l ; echo $JOB_MINUTES * * * * /tmp/my_script.sh; } | crontab -

The - filename argument means to get the crontab contents from standard input.