cronjob
 

Who knows cronjob?

Can i make cronjob to run (for example) spider.php every 30 minutes?

I am going to make search engine's spider which will check links every 30 mins.

Everyone knows cronjob! ;) You have simply to do
the former will give a general description of the crontab, the latter will give a description of the crontab file format.

Regarding your requirements, you can edit your crontab (crontab -e) and add an entry like this:
where the first field (minutes) means "every 30 minutes". Substitute the /path/to/php with the real path on your machine and the trick is done.

10.X my friend ;)

Now please more practically

if i want to run http://www.xatchmeruk.com/cron_job.php every 30 minutes what i will write in command field.

I'm sorry, but I don't know much about php. It looks like you have to run a cron job remotely on a web server. The crontab entry on your local machine will be similar to the one above: the time and date fields
this will run every 30 minutes, each day of the year. Maybe some php expert can be more helpful.

You'll need the cmd line version of php, ie the one that doesn't rely on being called from/by Apache

If I would like to have my cron job consists of a number of commands, what should I do? It seems that I can only have a single command in my crontab for each job!