Configuring cron jobs

Cron is a daemon that executes commands at specified intervals. These commands are called "cron jobs." Cron is available on Unix, Linux and Mac servers. Windows servers use a Scheduled Task to execute commands.

There is a video, How To: Setting up Drupal's Cron that talks about cron and shows various ways of configuring it.

The cron command

In the following example, the crontab command shown below will activate the cron tasks automatically on the hour:

0 * * * * wget -O - -q -t 1 http://www.example.com/cron.php

In the above sample, the 0 * * * * represents when the task should happen. The first figure represents minutes – in this case, on the "zero" minute, or top of the hour. (If the number were, say, 10, then the action would take place at 10 minutes past the hour.) The other figures represent, respectively, hour, day, month and day of the week. A * is a wildcard, meaning "every time."

The rest of the line basically tells the server to "ping" the url http://www.example.com/cron.php. NOTE: The -O is "dash capital O", not "dash zero". Yes, the capital O is important. Lowercase o won't work

Here is a diagram of the general crontab syntax, for illustration:

# +---------------- minute (0 - 59)
# | +------------- hour (0 - 23)
# | | +---------- day of month (1 - 31)
# | | | +------- month (1 - 12)
# | | | | +---- day of week (0 - 7) (Sunday=0 or 7)
# | | | | |
* * * * * command to be executed

Thus, the cron command example above means "ping http://www.example.com/cron.php at the zero minute on every hour of every day of every month of every day of the week."

How Drupal uses cron

Every Drupal install requires regular actions to handle maintenance tasks such as cleaning up log files and checking for updates. Cron.php is the file that Drupal uses to run the maintenance process.

For instance, if your site were www.example.com, loading the URL http://www.example.com/cron.php in your browser would run the maintenance.

This page is automatically set up when you install Drupal. Simply loading the URL will run the maintenance. Nothing more is required.

For a modest personal site, you might set up this cron job to run once a day. For a more active site you might want to run that job more often—perhaps every few hours or every hour. This regular visit will tell Drupal to perform its periodic tasks, and this will help keep your system running smoothly.

How to set up a cron job

Cron jobs are scheduled by setting up a "crontab." A crontab is a text file that contains the commands to be run. This file can be created and edited either through the command line interface, or, if you manage your website through a web-based control panel such as cpanel or Plesk, you will use the web interface. Check with your hosting company for detailed instructions if you are using a web-based control panel.

To edit a crontab through the command line, type:

crontab -e

If this fails, see the Troubleshooting Cron section below.

Add ONE of the following lines:

This would have a lynx, wget, or curl visit your cron page 45 minutes after every hour.

Three options are provided in case either wget, lynx or curl are not installed on the server. Any will do the job well.

Learn more about the crontab file syntax here to set up the cron job to run more or less often.

There are many ways to configure a cron job. If you have full access to crontab features, you should be able to simply paste in one of the above example commands – be sure to replace "example.com" with your own web domain or docroot.

If you're on shared hosting, you should be able to find cron job configuration somewhere in your hosting control panel. Some hosts even have cron "wizards" that walk you through the cron configuration, making it much easier if cron is new to you. On a Windows system you can accomplish the same thing with scheduled tasks to launch Internet Explorer pointed to the URL.

Some hosting companies do not permit local loopback, so using wget, curl or lynx will not work. If this is the case, and they run PHP as a CGI (check with your hosting company to see if this is the case), the following will run the cron locally:-

/usr/bin/php /home/sites/example.com/public_html/cron.php

Some hosting companies don’t allow access to cron

If your hosting company restricts access to cron you have many options.

• Ask the company to give you access, or to set up a cron job for you
• Ask someone else with access to a server to set up a cron job for you. Any Unix, Linux, or Mac server with access to the internet can have a cron job to regularly visit your site. There are also some companies that offer cron services.
• Use the Poor Man's Cron module.
• Find an online cron service. Many are free but with restrictions.

Cron doesn't guarantee your commands will run at the specified interval. But Drupal will try its best to come as close as possible. The more you visit cron.php, the more accurate cron will be.

Troubleshooting cron jobs

If you receive a permission denied error after starting crontab -e, you may need to use sudo:

sudo crontab -e

You may need to adjust the path to wget, lynx or curl in your crontab. For example, the cron example listed above contains the line:

45 * * * *  /usr/bin/lynx -source http://example.com/cron.php

However, Lynx may be in a different location on your server, or not installed at all. To find out where Lynx is installed, enter:

whereis lynx

or

which lynx

If it is not located at /usr/bin/lynx adjust the path as needed. The same applies for wget and curl. If none are installed ask a server administrator for help.

It may be necessary to change http://example.com/cron.php to the location of your Drupal installation. For example, if you have Drupal installed in a subdirectory, it might be http://www.example.com/drupal/cron.php).

Example scripts

Drupal ships with two example scripts in the scripts directory, cron-curl.sh and cron-lynx.sh. You can call these scripts from cron as well:

45 * * * * /home/www/drupal/scripts/cron-lynx.sh

Note that the scripts will need to be updated with the path to your directory and URL.

Running cron as an authenticated user

Security notes

It is possible to run cron.php directly via scripts/drupal.sh with Drupal 6. Drupal.sh allows a Drupal page to be executed from a shell script. To do so, add the following cron job to run as the Apache user.

/full/path/to/drupal.sh --root /full/path/to/site/root/ http://default/cron.php

Note that http://default/cron.php is NOT shown as an example, it should be used as is, without changes.

Taking this approach allows cron.php to be blocked to prevent remote access.

To block remote access to cron.php, in the server, .htaccess or vhost configuration file add this section:

If you take this approach and use drupal.sh to call cron.php, it is probably best not to use the root user to run the cron job. A non-privileged user account, or the Apache account user, for example http-service or www-data, is a better choice. To do so, call crontab -e when logged in as a non-privileged user, or for the Apache account on a Debian server, for example, you can add a user parameter:

sudo crontab -e -u www-data

The downside to this method is that any URLs generated by cron jobs using this method will not be properly formed, starting with "http://default/".

Tips and tricks

Cron Sandbox is a useful site for testing cron entries.

Multiple sites

If you run many sites, you can use this tip to make managing your cron jobs easier. To minimize the clutter, create a /etc/cron.5min directory and have crontab read this directory every five minutes.

*/5 * * * * root run-parts /etc/cron.5min

Then place multiple individual files into the /etc/cron.5min directory, one for each site. The files can be named "site1", "site2", etc. -- note that run-parts may fail to detect files which contain a dot (.) in their name. To make sure that all of your files are visible to cron, type this at a shell prompt:

$ sudo run-parts --test /etc/cron.5min
and make sure that all of your files are listed.

Each of the files in /etc/cron.5min should contain one line:

/usr/bin/lynx -source http://(full site URL)/cron.php > /dev/null 2>&1
or, alternatively, one of the curl or wget commands specified above.

If this doesn't work, try putting another line at the start of each file:

#!/bin/sh
and make sure that the files are executable by doing

$ sudo chmod u+x /etc/cron.5min/*

For more information about using cron in a multisite configuration, see the Multisite Cron section of this guide.

SSL

When using SSL, add one additional argument when calling wget: --no-check-certificate. Do not put “--no-check-certificate” between the -0 and the -.

45 * * * * /usr/bin/wget --no-check-certificate --quiet -O - https://example.com/cron.php

Configuring an editor for cron

You can specify which text editor (emacs, vi, nano, etc.) you want to use to edit the crontab. To tell the system which editor you want to use, type:

export EDITOR=nano