anacron command is used to execute commands periodically with a frequency specified in days. Its main advantage over cron is that it can be used on a machine which is not running continuously. In cron if a machine is not running on time of a scheduled job then it will skip it, but anacron is a bit different as it first checks for timestamp of the job then decides whether to run it or not and if its timestamp is >=n(n is defined number of days) then runs it after a specified time delay.
It mainly constitutes of two important Files:
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/etc/anacrontab : It contains specifications of job.
- See line SHELL=/bin/sh. It uses sh not bash you cannot run bash scripts.
- Table at the last represents different scheduling. The first column is the number of days(n) i.e. period and second is a time delay in minutes.
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/var/spool/anacron : This directory is used by Anacron for storing timestamp files. It represents timestamp for different category of jobs i.e. daily, weekly, monthly, etc.
Syntax:
anacron [-s] [-f] [-n] [-d] [-q] [-t anacrontab] [-S spooldir] [job] anacron [-S spooldir] -u [-t anacrontab] [job] ... anacron [-V|-h] anacron -T [-t anacrontab]
Options:
- f : Used to force execution of the jobs, ignoring the timestamps.
- u : Only update the timestamps of the jobs, to the current date, but don’t run anything.
- s : Serialize execution of jobs. Anacron will not start a new job before the previous one finished.
- n : Run jobs now.Ignore any delay.
- d : Don’t fork to the background. In this mode, Anacron will output informational messages to standard error, as well as to syslog. The output of jobs is mailed as usual.
- q : Suppress messages to standard error. Only applicable with -d.
- V (Use specified anacrontab) : Print version information and exit.
- h (Use specified anacrontab) : Print short usage message, and exit.
Note:
- You can add any script to etc/cron.daily or etc/cron.weekly or cron.monthly directory.but remember script should be sh not bash.
- Don’t forget to give executable permissions to your files sudo chmod +x filename.
- Scripts in directories automatically get executed depending upon which directory they are in.
Examples:
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Changing the timestamp of jobs. Observe the change in timestamp values.
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Performing force execution. Notice the time delay.
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Serialized Execution.
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Print version information.
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Print sort Usage message.
Note: Here we have used -d option with all commands to display on the screen what is actually happening.
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