The crontab utility is used to schedule jobs to be run at specified times. Jobs are run by the cron daemon, which is started at boot time. The cron daemon stores a log of its activity in the /var/adm/cron/log file. If you want to execute a job only once, you use the at command. In case a job is to be run repeatedly, such as daily system backup, crontab files are used. Jobs in the crontab files are listed in a specified format. Each user has his or her own crontab file in the /var/spool/cron/crontabs directory. Access to the crontab files is restricted by using cron.allow and cron.deny files in the /var/adm/cron directory. Access to the at command is restricted using the at.allow and at.deny files in the same directory. To create, edit, list, and delete crontab files, you use the crontab command. To time a job for one-time execution, you use the at command. Job files created by the at command are stored in the /var/spool/cron/atjobs directory and contain all environment variable settings in addition to the command itself.
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