Anacron vs Cron

One of cron's biggest weaknesses is that it assumes that your server or computer is always on. If your machine is off and you have a task scheduled during that time, the task will never run.

Anacron stands for anachronistic, and it is used compensate for this problem with cron. The smallest increment that anacron understands is days. This means that anacron should be used to complement cron, not to replace it.

Anacron's advantage is that it uses time-stamped files to find out when the last time its commands were executed. This means, if a task is scheduled to be run daily and the computer was turned off during that time, when anacron is run, it can see that the task was last run more than 24 hours ago and execute the task correctly.

The anacron utility has a scheduling table /etc/anacrontab


The following runs every day, five minutes after anacron is called:

1       5       cron.daily       nice run-parts --report /etc/cron.daily

anacron is set to run some of the same scripts that are run by cron. Distributions handle this clash differently, by creating a preference for either cron or anacron and making the other program not execute the rule.

For instance, on Ubuntu, the "/etc/crontab" tests if anacron is available on the system, and only executes the scripts in the cron.* directories with cron if anacron is not found.

Other distributions have cron update the anacron's time-stamps every time it runs the contents of these directories, so that anacron does not execute when it is called.

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