anacron is a computer program that performs periodic command scheduling, which is traditionally done by cron, but without assuming that the system is running continuously. Thus, it can be used to control the execution of daily, weekly, and monthly jobs on systems that don't run 24 hours a day. anacron was originally conceived and implemented by Christian Schwarz in Perl, for the Unix operating system. It was later rewritten in C by Itai Tzur; maintainers have included Sean 'Shaleh' Perry and Pascal Hakim. It is a dependency of cronie which is maintained by Red Hat, and Red Hat updates to cronie have included updates to anacron as well.
Advantages
Unix systems commonly run "housekeeping chores" such as log rotation, unused files deletion, indexing local files for the search engine, sending disk usage reports, etc. A program called cron may be used to schedule these tasks. With cron, tasks are commonly scheduled to be executed when the system is expected to be idle. If the system is off atthe moment a task should be run, it will not be executed. Anacron is different and moves tasks to different moments so they are run when the system is on and utilising cron.
Disadvantages
Only the system administrator can configure anacron tasks. In contrast, cron allows non-admin users to configure scheduled tasks. If necessary, a non-admin user can use the at Unix command to request a scheduled task. Upon starting, this command can then automatically request to run itself at a future date, thus giving the appearance of a regularly scheduled task, with guaranteed execution. anacron can run tasks only once a day. In contrast, cron allows tasks to run as often as every minute. In practice, this is not usually an issue, since it is rare to have a task that must be guaranteed to run more often than once a day — especially on a system that is not necessarily running at all times. If the system is rebooted or started after midnight, the daily tasks are run after a user-specified delay in synchronized fashion, so only one anacron-task is running at a time. When that task is finished, the next one starts and so on. This behaviour can be overridden to allow concurrent tasks, but can consume system resources that are needed by the user to do work. In contrast, cron will only run daily tasks at the time set. fcron is another implementation of cron, which attempts to fulfill the roles of both Vixiecron and anacron.
