Cron - The Linux Task Scheduler

shiva · #1 03-20-2007, 03:35 AM

This article intends to provide a detailed overview on cron and its implementation.

Everyone would have seen the Windows Task Scheduler (Scheduling things at specified intervals). Cron is exactly the same in Linux. Basically Cron derives it's names from Chronology (i.e. doing things at the right time).

The Cron tool plays a very vital role in web applications such as allowing the administrator to send emails at a specific period of time, processing recurring credit card billing etc.

Let's think that you are runing a photo prints site and you are really in need of a daily report of the orders made in the site to be mailed to you at the end of the day automatically.Let's deal with writing a cron script for that.

Before that let's discuss about the various cron commands and the syntax.

crontab -e --> Edit the cron tab file

crontab -l --> List the entries in the crontab

crontab -r --> Remove the crontab

man 1 crontab --> Cron Tab Help

man 5 crontab --> Cron Tab Help

And the main syntax for a crontab goes like this:

Code:

minutes hours day_of_month month day_of_week command

Quote:

And here goes the detail about the fields and the allowed values in each filed of the above syntax.
field --> allowed values
minute --> 0-59
hour --> 0-23
day of month --> 1-31
month --> 1-12 (or names, see below)
day of week --> 0-7 (0 or 7 is Sun, or use names)
command --> specifies the command to be executed

Now let's look at different examples and explanation:

A script that runs exactly at 10:00 PM every night

Code:

0 22 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1

Here you've specified 22 at the hour field and 0 at the minutes field and the rest are specified as *(wildcard -meaning it will accept any value). SO here since you've specified * in day of week,month & day of month meaning this script will run on 10:00 PM on all days of the week in a month.

A script that runs exactly at 10:00 PM only on the weekdays

Code:

0 22 * * 1-5       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1

A script that runs exactly at 10:30 PM on the first of every month

Code:

30 22 1 * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1

A script that runs exactly at 10:30 PM on the first of every month

Code:

30 22 1 * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1

A script that runs for every one hour

Code:

0 * * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1

Even Ranges can be included, say if you need to run a script on every odd hour of the day

Code:

0 0-23/2 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1

this is same as that of "0-23/2" = 1,3,5,7,9,11....

So coming to our script where we need to mail the details of the orders made in the site at the end of everyday, here's how the cron script will look like

Code:

0 22 * * * w3m http://[your server name]/mailorderinfo.php >> /Library/Apache2/htdocs/[site_name]/admin/dailynotify.txt

Here we've included a log file to be updated with the status of every cron execution, so that if there's any problem in the php script, it will get automatically get updated in the log file for future reference.

Your queries and suggestions are always welcome!!!

Thanks
Shiva...

Sabari · #2 09-10-2007, 06:54 AM

is there any command to execute Cron tab from PHP?

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