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Home > Forums > MIS/IT > Operating Systems - UNIX based > Linux (server) > FAQs

Linux (server) FAQ

INSTALLATION

Crontab Intro by Faroon
faq54-4237
Posted: 29 Sep 03

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.

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