Crontab: Scheduling tasks

In this post we will see how to schedule task in unix systems with the use of crontab.

Frequently we need to run processes in the background in out web servers like generate thumbnails, execute mantenance MySQl queries, etc. Unix systems have a great tool named cron. This tool allows us to run processes automatically in regular intervals. Also, we can create backups, synchronize files and a much more. To do all these stuff we have crontab.

The crontab command is used in Unix systems to schedule other programs to be executed periodicaly. To see what crontabs are currently running in our system, we can run:

user@unix:~$ crontab -l

To edit the crontabs we can run:

user@unix:~$ crontab -e

This command will open our editor by default to allow us to edit the crontab. To write our crontabs we can use this format:

user@unix:~$ * * * * * php /var/www/hasheado/script.php

Explaining the format

As we can see in the above command there are 5 asterisks. The asterisk represent different date and time parts:

  1. minutes (from 0 to 59)
  2. hour (from 0 to 23)
  3. day of month (from 1 to 31)
  4. month(from 1 to 12)
  5. day of the week (from 0 to 6) (0 = Sunday)

Executing a command every minute

If we use an asterisk in all parts:

user@unix:~$ * * * * * php /var/www/hasheado/script.php

This will execute the script /var/www/hasheado/script.php:

  1. every minute
  2. every hour
  3. every day of the month
  4. every month
  5. every day of the week

Well that is, I hope it’s helpful, crontab is so powerful.

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