How to run cron in every second | Interview Question for Linux Administrator

How to Run Cron every second, cron every second

Question 1: how  to run cron in every second



It's not possible to run cron in every second but we can run cron in every second  using this method, we use a sleep command before the echo date.






*    *    *    *    *       echo `date -I`  >> /tmp/cron.log



*    *    *    *    *       sleep 1 ; echo `date -I`  >> /tmp/cron.log



*    *    *    *    *       sleep 2 ; echo `date -I`  >> /tmp/cron.log



*    *    *    *    *       sleep 3 ; echo `date -I`  >> /tmp/cron.log





...........

...........



*    *    *    *    *       sleep 59 ; echo `date -I`  >> /tmp/cron.log



Question 2: Differences  between cron and crontab



Cron - Cron is command name that runs scheduled action using the crond  daemon.



Crontab - crontab is file name that  containing jobs scheduled details like



*    *    *    *    *      < CMD >



*  -Minute

* - Hour

* - Day

* - Month

*  -  Day of Week ( like Sunday 0, Monday 1, Tuesday 2 and so for

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Question 2 - Set cronjob to run command on every day 4:30 PM



30      16       *        *        *         /bin/bash    /opt/backup.sh



Question 3 - How to run cron after every reboot



@reboot           /bin/bash    /opt/start-vm.sh



Question 4 - deny particular user using from scheduling cron jobs?



Answer: Make entry of user in /etc/cron.deny file



Question 5 - Is required to restart cron service after any change 



Answer  - No