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.
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 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:
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
With lots of daemons, (e.g. httpd anUsing 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:d 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 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
Labels: automate tasks with cron, Centos 6, RHEL