Crontab Intro

This is the built-in automation tool for users of OpenBSD to set up task scheduling. Each user can have a separate crontab that holds several cronjobs to be executed periodically by a cron daemon. Synopsis:

$ crontab [-u (for user)] [-e (edit) | -l (list) | -r (remove)]

Synopsis of a command line in the contab (a cronjob):

Minute û Hour û Day of month û Month û Day of week /path/to/command /path/to/file

Minute: 0 û 59
Hour: 0 û 23
Day of month: 1 û 31
Month: 1 û 12
Day of week: 0 û 6, 0 = Sunday

Note: * = any value

User capacityÆs cronjobs:

# Refresh database tables everyday at 2:15am
15 2 * * * /usr/bin/perl /path/to/scripts/database_refresh.pl

# Backup table mytable and every Saturday 9:30am
30 9 * * 6 /usr/bin/perl /path/to/scripts/backup.pl

To edit a cronjob, do:
$crontab -l > temp.txt --> write current cronjob to file
$pico temp.txt (or use nano, of vi) --> make changes
$crontab temp.txt --> install back the job

Alternate way:
$export EDITOR=/path/to/pico (or nano, vi is default)
$crontab -e -->edit and save new jobs

Important: Cron has a different set of environmental variable settings from that of the user's. For example, the user's $PATH setting might have /sbin:/usr/sbin:/usr/local/sbin, but cron's $PATH might not. Sometimes extra environmental settings are required for a job to run inside cron. To make these setting, simply edit the cron, and add the neccessary settings on top of file (ex. add PATH=$PATH:/sbin:/usr/sbin)

Note: should a cronjob produce any error message, this message will be mailed to the owner of that cronjob. Simply type æmailÆ to open the email folder and view the cron daemon mail.