How to setup a Cron Job in Linux and Plesk

Wouldn’t it be great if something could be triggered even when you’re not around? Say once an hour, once a day, once a week or whenever you like in predetermined intervals?

Then you want to do this with what’s known as a Cron Job, or Scheduled Task.

Unfortunately, this is a bit beyond what WordPress can do, and it means getting down to the nitty gritty of the internal workings  of your server (after all, that’s where WordPress lives). Bear with me here, I’ll try my best to explain and show solutions.

I’m using a Cron Job with Manu Flury’s excellent Photo Q Plugin. It posts one of my pictures over at http://www.versluis.com every so many hours. But for this to work properly, both the WordPress Plugin and my (Linux) server need to be setup correctly.

Some WordPress Plugins (like Rob Felty’s Postie or Charles Johnson’s Feed WordPress) have similar functionality built in, but they rely on a visitor coming to your site at predetermined intervals. That’s not something you can control really. In most cases it works reliable enough for these plugins to work, however many others just don’t have that functionality, or require more accurate control. That’s where your Cron Job comes in.

So what on earth is a Cron Job?

In a nutshell, it’s a task that’s triggered at predetermined intervals. But it’s a bit like sitting in front of a Linux Prompt on your SSH connection, and all you have at your displosal is a keyboard with a black screen and white text to type in. What’s worse, your server doesn’t speak “WordPress”, or PHP for that matter. So all you can do really is to give him Linux commands.

In all likelyhood, you probably want to call a PHP file so that WordPress does something for you (such as check if it’s time for a new post, or maybe a database backup). And you can’t just tell Linux to go to that file, becasue it wouldn’t know what to do with it. I’ve tried this without success many times over (they nearly put me in a mental institution, seriously… I can assure you I’m much better now though).

So you need to find a command that calls your PHP file as if it were a browser. Lucky for us, the command “wget” will do the trick.

Wget is really designed for downloading a file to your server, but it’ll work fine for triggering a PHP file, just as a browser would do. The command for calling the Photo Q file for example looks like this:

/usr/bin/wget -O - -q -t 1 http://www.yourdomain.com/wimpq-cronpost.php

Confused? Don’t be! Let me explain:

• “/usr/bin/wget” tells the server where the wget command is (it’s a path to a file if you hadn’t guessed)
• “-O – -q -t -1” are some random parameters, let’s not concern ourselves with those right now (if you really want to find out, type in “wget –help” at your SSH prompt)
• “http://www.yourdomain.com/wimpq-cronpost.php” is the actual file you want to call, just like what you’d type into your browser

Now that we know how to call upon a PHP file from our command prompt, we need to tell our server to do this without us being there, and at what times he needs to do this. He’ll be more than happy to oblige, after all, that’s what he was designed to do.

How to setup a Cron Job in Plesk

I’ll focus on how to do this in Plesk 9 here, which refers to it as Scheduled Tasks (earlier versions of Plesk call it Crontab).

Here’s how you get there:

• from the main menu on the left, select HOME
• select DOMAINS
• select the DOMAIN you want to run this task on
• under Additional Tools, select SCHEDULED TASKS
• choose the SYSTEM USER you would like this task to be run as
• select SCHEDULE NEW TASK

OK, this was complicated enough to figure out – now comes the part nobody ever really talks about. It’s hard to explain, so please bear with me if I’m not making a whole lot of sense at first. In essence, you’re telling your server WHEN to do something, followed by WHAT to do.

• Each field requires an entry. Don’t leave them blank.
• The first tick box is to “switch on” the task. That’s what you want, unless you want to suspend the task. Tick it for now.
• Now tell your server WHEN your task shall be run.
• a “*” (i.e. star or asterisk) means “every”. So a star in minute would run the task “every minute”, likewise for hours and days of the month
• You can also create the command “*/5” if you want your task run every 5 hours, every 5 days, every 5 minutes – you get the drift
• Alternatively, put in the specific date (i.e. “Monday” and “17” for Monday at 5pm)

After all that, you’re left with one last line, which is the actual command or task you’d like to be run.

Like I explained above, for a PHP file to be called, use the wget command like so:

/usr/bin/wget -O - -q -t 1 http://www.yourdomain.com/wimpq-cronpost.php

Obviously change the path to your file, including “http://”

Click OK on the bottom and your task should be setup. No need to reboot anything.

If at any time you feel you want to amend the task, you can do so by just modifying the parameters, or uncheck the “run task” tickbox to suspend the task completely.

If you want to know even MORE about Cron Jobs, check this out:

http://www.hingham-ma.com/sendstudionx/admin/resources/tutorials/cron_intro.html

Good Luck, and have fun 😉