What are cronjobs?

cron is a time-based scheduling service in Unix-like computer operating systems. 'cron' is short for 'chronograph', cron is driven by a crontab, a configuration file that specifies shell commands to run periodically on a given schedule. A job is executed when the time/date specification fields all match the current time and date, with the exception that either the "day of month" field or the "day of week" field must match the current day, even though the other of the two fields doesn't match the current day.

Ok... we already know what is a cron.

How we use cron?

On unix-based operative systems we have crontab. The crontab files are stored where the lists of jobs and other instructions to the cron daemon are kept. Users can have their own individual crontab files.

crontab syntax

There are several ways of specifying multiple date/time values in a field:

• The comma (',') operator specifies a list of values
• The dash ('-') operator specifies a range of values.
• The asterisk ('*') operator specifies all possible values for a field.

There is also an operator which some extended versions of cron support, the slash ('/') operator (called "step"), which can be used to skip a given number of values. For example, "*/3" in the hour time field is equivalent to "0,3,6,9,12,15,18,21". So "*" specifies 'every hour' but the "*/3" means only those hours divisible by 3.

These are the fields for a cronjob:

# +---------------- minute (0 - 59)
# |  +------------- hour (0 - 23)
# |  |  +---------- day of month (1 - 31)
# |  |  |  +------- month (1 - 12)
# |  |  |  |  +---- day of week (0 - 6) (Sunday=0 or 7)
# |  |  |  |  |
*  *  *  *  *  command to be executed

To edit the corn files we just type "crontab -e". If we need more help, just type "crontab --help".
Example

*/10 * * * * php synch.php > log.txt

This cron execute every 10 minutes a php script.

Some real world cases where we need to use cronjobs.

• When we need to create backups of our databases.
• When need to send newsletter.
• Sometimes we need to read log files to check fatal errors and send a notification to a administrator.
• When we need to synchronize to databases.
• If we have a robot we need to execute it automatically.

Share on Facebook