Ways to Automatically Executing System Tasks

You can set up many system tasks to execute automatically. Some of these tasks should occur at regular intervals. Other tasks need to run only once, perhaps during off hours such as evenings or weekends.

This section contains overview information about two commands, crontab and at, which enable you to schedule routine tasks to execute automatically. The crontab command schedules repetitive commands. The at command schedules tasks that execute once.

The following table summarizes crontab and at commands, as well as the files that enable you to control access to these commands.

Table 4-1 Command Summary: Scheduling System Tasks

For Scheduling Repetitive Jobs: crontab

You can schedule routine system administration tasks to execute daily, weekly, or monthly by using the crontab command.

Daily crontab system administration tasks might include the following:

• Removing files more than a few days old from temporary directories

• Executing accounting summary commands

• Taking snapshots of the system by using the df and ps commands

• Performing daily security monitoring

• Running system backups

Weekly crontab system administration tasks might include the following:

• Rebuilding the catman database for use by the man -k command

• Running the fsck -n command to list any disk problems

Monthly crontab system administration tasks might include the following:

• Listing files not used during a specific month

• Producing monthly accounting reports

Additionally, users can schedule crontab commands to execute other routine system tasks, such as sending reminders and removing backup files.

For step-by-step instructions on scheduling crontab jobs, see How to Create or Edit a crontab File.

For Scheduling a Single Job: at

The at command allows you to schedule a job for execution at a later time. The job can consist of a single command or a script.

Similar to crontab, the at command allows you to schedule the automatic execution of routine tasks. However, unlike crontab files, at files execute their tasks once. Then, they are removed from their directory. Therefore, the at command is most useful for running simple commands or scripts that direct output into separate files for later examination.

Submitting an at job involves typing a command and following the at command syntax to specify options to schedule the time your job will be executed. For more information about submitting at jobs, see Description of the at Command.

The at command stores the command or script you ran, along with a copy of your current environment variable, in the /var/spool/cron/atjobs directory. Your at job file name is given a long number that specifies its location in the at queue, followed by the .a extension, such as 793962000.a.

The cron daemon checks for at jobs at startup and listens for new jobs that are submitted. After the cron daemon executes an at job, the at job's file is removed from the atjobs directory. For more information, see the at(1) man page.

For step-by-step instructions on scheduling at jobs, see How to Create an at Job.