It is not possible to specify a cron job to run in every 30 seconds or so using the time field parameter of the crontab.

However, we can still do this using the following entries.

* * * * * /scripts/script.sh
* * * * * sleep 30; /scripts/script.sh

There are actually two different entries needed for this purpose.

The first crontab entry tells cron to run the script at every minute.

And the second entry makes cron run the next one after a 30-second pause.