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 ... |
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