Essential System Administration, 3rd Edition

Essential Administrative Techniques

In this section, we consider several system facilities with which system administrators need to be intimately familiar.

Periodic Program Execution: The cron Facility

cron is a Unix facility that allows you to schedule programs for periodic execution. For example, you can use cron to call a particular remote site every hour to exchange email, to clean up editor backup files every night, to back up and then truncate system log files once a month, or to perform any number of other tasks. Using cron, administrative functions are performed without any explicit action by the system administrator (or any other user).^[6]

For administrative purposes, cron is useful for running commands and scripts according to a preset schedule. cron can send the resulting output to a log file, as a mail or terminal message, or to a different host for centralized logging. The cron command starts the crond daemon, which has no options. It is normally started automatically by one of the system initialization scripts.

Table 3-3 lists the components of the cron facility on the various Unix systems we are considering. We will cover each of them in the course of thissection.

Table 3-3. Variations on the cron facility

Component	Location and information
crontab files	Usual: /var/spool/cron/crontabs FreeBSD: /var/cron/tabs, /etc/crontab Linux: /var/spool/cron (Red Hat), /var/spool/cron/tabs (SuSE), /etc/crontab (both)
crontab format	Usual: System V (no username field) BSD: /etc/crontab ...