How to run cron when the system is offline

Cron will skip the execution if the server is not online at the time that they are scheduled. This is default behavior and its not possible to run cron when the system is offline.

But Linux provides another utility called anacron which execute commands periodically with a frequency specified in days. If the machine if off when a scheduled job is due, it will execute a scheduled job when the machine is powered on the next time.

Anacron jobs can be configured in the file "/etc/anacrontab".

#vi /etc/anacrontab

The job syntax should be as follows.

period   delay   job-identifier   command

Where,

period – this is the frequency of job execution specified in days or as @daily, @weekly, or @monthly for once per day, week, or month. You can as well use numbers: 1 – daily, 7 – weekly, 30 – monthly and N – number of days.
delay – it’s the number of minutes to wait before executing a job.
job-id – it’s the distinctive name for the job written in log files.

The following is an example for anacron job configured to run daily.

@daily    10    backup   /bin/bash /root/backup.sh

That's all…