What is 'Cron Jobs'?

I have seen this all over the place but what exactly is it and what does it do? Thanks in advance.

well you can compare cron jobs with task scheduling,

Its like you can have some task's scheduled to be done automatically, people use it to ceate backup of website or perform general cleanup sort of tasks.

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Do you know how to do backup with cron job ? If you know can you explain how to do it ? I think thats one of the best use we can get from cron jobs.

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You would need a script (either a shell script or PHP) that does the necessary copying of files. It is this script that would be put into the cron task. You might want to check out hotscripts.com, as they have backup management type scripts that would serve your purpose.

Cron jobs is to schedule a task for the server to do. PHP can schedule cron jobs to work, just places like hotscripts will fix you up.

thanks for the site infor brandon, till now i used only shell to create cron jobs, but this will of great help to me.

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The cron daemon is a long running process that executes commands at specific dates and times. To schedule one-time only tasks with cron, use at or batch. For commands that need to be executed repeatedly (e.g. hourly, daily or weekly), use crontab, which has the following options:
crontab filename Install filename as your crontab file.
crontab -e Edit your crontab file.
crontab -l Show your crontab file.
crontab -r Remove your crontab file.

The crontab command creates a crontab file containing commands and how often cron should execute them. Each entry in a crontab file consists of six fields, specified in the following order:

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.