Cron Vs Anacron

Both Cron and Anacron are daemons used for job scheduling .Cron jobs can run as often as every minute. However, the utility assumes that the system is running continuously and if the system is not up at the time when a job is scheduled, the job is not executed. On the other hand, Anacron remembers the scheduled jobs if the system is not running at the time when the job is scheduled. The job is then exectuted as soon as the system is up. However, Anacron can only run a job once a day.The cron and anacron jobs are both picked by the crond  service. Just like how cron has /etc/crontab, anacron has /etc/anacrontab. 

/etc/anacrontab file has the anacron jobs mentioned in the following format.

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 period in days       delay in minutes       job-identifier        command

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period in days — frequency of job execution in days

The property value can be defined as an integer or a macro (@daily, @weekly, @monthly), where @daily denotes the same value as integer 1, @weekly the same as 7, and @monthly specifies that the job is run once a month regardless of the length of the month. 

delay in minutes — number of minutes anacron waits before executing the job 

The property value is defined as an integer. If the value is set to 0, no delay applies. 

job-identifier — unique name referring to a particular job used in the log files 

command — command to be executed 

The command can be either a command such as ls /proc >> /tmp/proc or a command which executes a custom script.