By default, cron checks crontabs for cronjobs every minute. If you want to run a job every n seconds you need to use a simple workaround.

Method 1 : Using a start script

The easiest way to run a job every n seconds is to run a job every minute and, and sleep in a loop in n second intervals.

Every 5 seconds

i=0

while [ $i -lt 12 ]; do # 12 five-second intervals in 1 minute
  command/to/run & #run your command
  sleep 5
  i=$(( i + 1 ))
done

crontab -e

* * * * * start.sh

Every 15 seconds

i=0

while [ $i -lt 4 ]; do # 4 ten-second intervals in 1 minute
  command/to/run & #run your command
  sleep 15
  i=$(( i + 1 ))
done

crontab -e

* * * * * start.sh

Every 30 seconds

i=0

while [ $i -lt 2 ]; do # 2 ten-second intervals in 1 minute
  command/to/run & #run your command
  sleep 30
  i=$(( i + 1 ))
done

crontab -e

* * * * * start.sh

Method 2 : Specify multiple jobs with offsets

Let’s look at more examples. This one shows how to run a cron job every 10 seconds. The trick is to simply play around with the sleep command number of seconds:

* * * * * date>> /tmp/date.log
* * * * * sleep 10; date>> /tmp/date.log
* * * * * sleep 20; date>> /tmp/date.log
* * * * * sleep 30; date>> /tmp/date.log
* * * * * sleep 40; date>> /tmp/date.log
* * * * * sleep 50; date>> /tmp/date.log

Once again if we watch the /tmp/date.log file, it should be updated every 10 seconds based on the above crontab entries:

$ tail -f /tmp/date.log

Here is another example of executing the date command after every 15 seconds:

* * * * * date>> /tmp/date.log
* * * * * sleep 15; date>> /tmp/date.log
* * * * * sleep 30; date>> /tmp/date.log
* * * * * sleep 45; date>> /tmp/date.log

Finally, to run a cron job every 20 seconds, you can have something like this:

* * * * * date>> /tmp/date.log
* * * * * sleep 20; date>> /tmp/date.log
* * * * * sleep 40; date>> /tmp/date.log

Also, here are more articles for you to learn job scheduling using cron: