This cron tutorial is divided into these subheadings for you to follow it easily:

• What is cron?
• What is cron tab?
• What cron will do for you?
• Have your web server cron enabled?
• What are the components of cron?
• Examples on how to set up cron
• Setting up the cron
  • METHOD ONE: Using Cron tab manager
  • METHOD TWO: Uploading ‘cron.txt’ file and checking using telnet
    • Telnet commands
    • Telnet steps (Four steps)
• Cron emails you everytime it runs. How to stop it?
• TIPS in setting up your cron jobs

 

What is cron?

Actually it is called ‘cron daemon’. Cron is an automatic task machine. You will use it on your Unix or Linux operating systems for doing some tasks at specific intervals with out your intervention every time. You set the clock and forget. The cron daemon runs the work for you.

What is cron tab?

‘Cron tab(CRON TABle)’ is a text file that contains a series of cron functions.

What cron job will do for you?

* If you want to send your email cources to your subscribers at 11.30 night, you will set the cron job on your server. And your cron manager sends one email every day at 11.30 until all the emails will be finished. If you want to send them on Sundays, you can schedule it with your cron.

* You can schedule it to delete your website members with expired accounts.

* You can schedule it to recieve an update on your subscribers from your mailing list manager.

* You can check your links on other websites in link exchange programms.

Have your web server cron enabled?

 

Have you seen any icon with a name ‘Cron Jobs’ in your control panel?

Have you seen a text link like ‘Cron Manager’ or ‘Cron jobs’?

If you see one, then you are cron enabled. Ask your host about your cron availability. Some host allows cron with out graphical interface. You have to access it through telnet. If you are hosted on Virtual hosting domains then your system administrator has to set up your cron tabs for you.

What are the components of cron?

 

1: Path to your php or cgi script
2: minutes (Value is 00 - 59. exact minute the cron executes)
3: hour (Value is 00 - 23. hour of the day the cron executes. 0 means midnight)
4: month (Value is 01 - 12. month of the year the cron executes)
5: day (Value is 01 - 31. day of the month the cron executes)
6: weekday (Value is 00 - 06. day of the week the cron executes. Sunday=0,Monday=1…)

Examples on how to set up cron:

If you have installed a php script in your public_html/www/html directory called members.php.

If you want to run this file each night at 11.30 PM every day:
  
30 23 * * * /home/username/www/members.php
30 - represents the minute of cron work 
23 - represents the hour of the day 
The * represent every day, month, and weekday.

If you want to set the cron job every sunday at midnight 11.30 PM: 
 
30 23 * * 0 /home/username/www/members.php
0 - represents the Sunday.  

If you want the cron job to run at 1:00 and 2:00 A.M:
  
* 1,2 * * * /home/username/www/members.php
This runs your cron at 1A.M and 2A.M every day, every month and every week.

If you want to run the above task only from Monday to Friday and at 11.30 P.M:

30 23 * * 1-5 /home/username/www/members.php
This runs your cron from Monday to Friday and everyday at 11.30 PM.

Setting up the cron:

• METHOD ONE: Using Cron tab manager

If you have a cron tab manager in your webpanel, it will be easy for you to set the cron tab function. You can click on the icon to open cron manager. There will be some textarea where you can enter the the script file path that you want the cron job to executes.

Your cron manager may be different from the above image, but basically it is easy to set up cron job with your cron manager. I highly recommend this unless you want to get to know about the Second Telnet Method.

• METHOD TWO: Uploading ‘cron.txt’ file and checking using telnet

* Cron Commands:

crontab filename  Install filename as your crontab file
crontab -e        Edit your crontab file, or create one if it doesn’t already exist. 
crontab -l        Display your crontab file.
crontab -r        Remove your crontab file.
crontab -v        Display the last time you edited your crontab file.      
                 
* Telnet Steps:

Basically FOUR steps:
1. Create cron.txt
2. Upload
3. Install the txt file as cron file with the command ‘crontab cron.txt’
4. Check your cron file

1. Open your notepad on your computer and write cron job following the guidelines explained above under ‘What are the components of cron?’.

For example - If you write your cron job like this:

30 23 * * * /home/username/www/members.php
After writing the above cron job PRESS return key so that a blank line will be there below the cron job line.

2. Always use absolute path for the command line. Turn off the ‘Word Wrap’ feature with the Notepad.

3. Save the file as ‘cron.txt’. Upload it to your root directory (’/') where you will see a list of folders like public_html,public_ftp,email etc,. Or ask your web host where to upload the cron files. Upload in ASCII mode.

 

When you ftp into your site, first you will land on your root directory.

4. Now telnet into your server.

5. You have to use telnet software like putty to login into your server.

 

At command prompt type -

crontab cron.txt

Press Enter.

You will be back at command prompt.

Then type: (l means lowercase L)

crontab -l

Press Enter.

You will see the list of cron jobs that you have entered in cron.txt.

That’s it!!

Cron emails you everytime it runs. How to stop it?

Add this tag at the end of your cron job:

>/dev/null 2>&1

Taking the above example your cron job looks like:

30 23 * * * /home/username/www/members.php>/dev/null 2>&1

TIPS in setting up your cron jobs:

* Ask your host about the procedure if you are not sure of a thing.

* Check your cron once in a while. For example you set your cron to send your ezine at 11.30 PM. Then you subscribe yourself to see it is working or not.

* Upload cron.txt in ASCII mode.

* If you are setting the script path in your schedule, the path SHOULD be from your hosts server root. NOT your domain path.

Ex:/home/user/www/scriptname.php
is correct.

http://www.yourdomain.com/scriptname.php
is incorrect.

* Script names are case sensitive on Unix. So be careful in entering the script name.

* If you are creating cron.txt file and uploading, you should not allow spaces within 1-5 components except between them. (For example 10,30 * * * * is correct not 10 ,30 * * * *)

Author: Radhika (c)
http://www.webmasters-central.com/

Tags: cron, cron command, cron daemon, cron job, cron job format, cron tab, linux cron job
Posted in: Tutorials