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Organizing Scheduled Tasks
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You can organize administrative cron tasks into two general groups: common administrative tasks that can be run at regular intervals, or specialized tasks that need to be run at a unique time. Unique tasks can be run as entries in the /etc/crontab file, as described in the next section. Common administrative tasks, though they can be run from the /etc/crontab file, are better organized into specialized cron directories. Within such directories, each task is placed in its own shell script that will invoke the task when run. For example, there may be several administrative tasks that all need to be run each week on the same day, say if maintenance for a system is scheduled on a Sunday morning. For these kinds of task, cron provides several specialized directories for automatic daily, weekly, monthly, and yearly tasks. Each contains a cron prefix and a suffix for the time interval. The /etc/cron.daily directory is used for tasks that need to be performed every day, whereas weekly task can be placed in the /etc/cron.weekly directory. The cron directories are listed in Table 1-2.
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cron files and directories /etc/crontab /etc/cron.d
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Description System crontab file, accessible only by the root user Directory containing multiple crontab files, accessible only by the root user Directory for tasks performed hourly Directory for tasks performed daily Directory for tasks performed weekly Directory for tasks performed monthly Directory for tasks performed yearly Directory for tasks performed hourly Users allowed to submit cron tasks
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/etc/cron.hourly /etc/cron.daily /etc/cron.weekly /etc/cron.monthly /etc/cron.yearly /etc/cron.hourly /etc/cron.allow
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/etc/cron.deny Users denied access to cron Table 1-2. Cron files and directories
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Running cron Directory Scripts
Each directory contains scripts that are all run at the same time. The scheduling for each group is determined by an entry in the /etc/crontab file. The actual execution of the scripts is performed by the /usr/bin/run-parts script, which runs all the scripts and programs in a given directory. Scheduling for all the tasks in a given directory is handled by an entry in the /etc/crontab file. Red Hat provides entries with designated times, which you may change for your own needs. The default Red Hat crontab file is shown here, with times for running scripts in the different cron directories. Here, you can see that most scripts are run at about 4 A.M., either daily (4:02), Sunday (4:22), or on the first day of each month (4:42). Hourly ones are run one minute after the hour. SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root HOME=/ # run-parts 01 * * * * root run-parts /etc/cron.hourly 02 4 * * * root run-parts /etc/cron.daily 22 4 * * 0 root run-parts /etc/cron.weekly 42 4 1 * * root run-parts /etc/cron.monthly
TIP Scripts within a cron directory are run alphabetically. If you need a certain script to run before any others, you may have to alter its name. One method is to prefix the name with a numeral. For example, in the /cron.weekly directory, the anacron script is named 0anacron so that it will run before any others.
Keep in mind, though, that these are simply directories that contain executable files. The actual scheduling is performed by the entries in the /etc/crontab file. For example, if the weekly field in the cron.weekly crontab entry is changed to * instead of 0, and the monthly field to 1 (22 4 1 * * instead of 22 4 * * 0), tasks in the cron.weekly file would end up running monthly instead of weekly.
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Cron Directory Names
The names used for these directories are merely conventions. They have no special meaning to the cron 1 daemon. You could, in fact, create your own directory, place scripts within it, and schedule run-parts to run those scripts at a given time. In the next example, scripts placed 1 in the /etc/cron.mydocs directory will run at 12 noon every Wednesday.
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