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Newbie: Intro to cron Date: 30-Dec-99 Author: cogNiTioN Cron This file is an introduction to cron, it covers the basics of what cron does, and how to use it. What is cron? Cron is the name of program that enables unix users to execute commands or scripts (groups of commands) automatically at a specified time/date. It is normally used for sys admin commands, like makewhatis, which builds a search database for the man -k command, or for running a backup script, but can be used for anything. A common use for it today is connecting to the internet and downloading your email. This file will look at Vixie Cron, a version of cron authored by Paul Vixie. How to start Cron Cron is a daemon, which means that it only needs to be started once, and will lay dormant until it is required. A Web server is a daemon, it stays dormant until it gets asked for a web page. The cron daemon, or crond, stays dormant until a time specified in one of the config files, or crontabs. On most Linux distributions crond is automatically installed and entered into the start up scripts. To find out if it's running do the following: cog@pingu $ ps aux | grep crond root 311 0.0 0.7 1284 112 ? S Dec24 0:00 crond cog 8606 4.0 2.6 1148 388 tty2 S 12:47 0:00 grep crond The top line shows that crond is running, the bottom line is the search we just run. If it's not running then either you killed it since the last time you rebooted, or it wasn't started. To start it, just add the line crond to one of your start up scripts. The process automatically goes into the back ground, so you don't have to force it with &. Cron will be started next time you reboot. To run it without rebooting, just type crond as root: root@pingu # crond With lots of daemons, (e.g. httpd and syslogd) they need to be restarted after the config files have been changed so that the program has a chance to reload them. Vixie Cron will automatically reload the files after they have been edited with the crontab command. Some cron versions reload the files every minute, and some require restarting, but Vixie Cron just loads the files if they have changed. Using cron There are a few different ways to use cron (surprise, surprise). In the /etc directory you will probably find some sub directories called 'cron.hourly', 'cron.daily', 'cron.weekly' and 'cron.monthly'. If you place a script into one of those directories it will be run either hourly, daily, weekly or monthly, depending on the name of the directory. If you want more flexibility than this, you can edit a crontab (the name for cron's config files). The main config file is normally /etc/crontab. On a default RedHat install, the crontab will look something like this: root@pingu # cat /etc/crontab SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root HOME=/ # run-parts 01 * * * * root run-parts /etc/cron.hourly 02 4 * * * root run-parts /etc/cron.daily 22 4 * * 0 root run-parts /etc/cron.weekly 42 4 1 * * root run-parts /etc/cron.monthly The first part is almost self explanatory; it sets the variables for cron.
Approximate Word count = 2118
Approximate Pages = 8.5
(250 words per page double spaced)
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