Current linux systems use a simple scheme to ease the addition of cron jobs with a periodicity of an hour, a day, a week, or a month, while placing them (except the hourly job) under simultaneous control of cron and anacron. It relies on the command run-parts.

run-parts is a shell script that takes a directory argument. It simply runs each executable file in that directory, skipping those with extensions used by certain programs for installations and backups. (see the run-parts script)

The master anacrontab on linux includes three commands that use run-parts. (Remember, cron now runs jobs that are in anacrontab!) One command each is used to run daily, weekly and monthly jobs, giving run-parts a directory containing the appropriate set of tasks. (Thus, one command in anacrontab (with a period of 1 day) is used to run daily tasks, giving run-parts the /etc/cron.daily directory ...) In addition, a separate cron job (in the master cron directory /etc/cron.d) uses run-parts to run jobs that must be run hourly. (This last job cannot be in the anacrontab, as anacron only has a granularity of days.)

This automated use of a directory to hold these jobs to be run at regular intervals (hourly, daily, weekly or monthly) makes it easy to add such a job to the system. To add a job that must be executed daily, for example:

• create a shell script to run the job
• insure that the shell script does not generate standard output or standard error, and that it uses absolute paths for commands.
• place the executable shell script in the directory used by the crontab to run jobs daily.

Investigation of this helpful scheme is left to the reader. You should look at the run-parts command, at the master crontab and anacrontab, at the directories used with the run-parts commands and list (with attributes) the directory containing anacron time stamps.

Copyright 2011 Greg Boyd - All Rights Reserved.