Background – An Overview of the Crontab
Let’s face it, having the ability to schedule tasks to run in the background is just great! From backing up an SQL database, fetching / sending emails to running clean up tasks, analyzing performance, or even grabbing RSS feeds, cron jobs are fantastic!
Although the syntax of scheduling a new job may seem daunting at first glance, it’s actually relatively simple to understand once you break it down. A cron job will always have five columns each of which represent a chronological ‘operator’ followed by the full path and command to execute:
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* * * * * home/path/to/command/the_command.sh
* * * * * home/path/to/command/the_command.sh
Each of the chronological columns has a specific relevance to the schedule of the task. They are as follows:
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Minutes represents the minutes of a given hour, 0-59 respectively.
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Hours represents the hours of a given day, 0-23 respectively.
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Days represents the days of a given month, 1-31 respectively.
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Months represents the months of a given year, 1-12 respectively.
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Day of the Week represents the day of the week, Sunday through Saturday, numerically, as 0-6 respectively.
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Minutes [0-59]
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| Hours [0-23]
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| | Days [1-31]
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| | | Months [1-12]
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| | | | Days of the Week [Numeric, 0-6]
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| | | | |
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* * * * * home/path/to/command/the_command.sh
Minutes [0-59]
| Hours [0-23]
| | Days [1-31]
| | | Months [1-12]
| | | | Days of the Week [Numeric, 0-6]
| | | | |
* * * * * home/path/to/command/the_command.sh
So, for example, if one wanted to schedule a task for 12am on the first day of every month it would look something like this:
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0 0 1 * * home/path/to/command/the_command.sh
0 0 1 * * home/path/to/command/the_command.sh
If we wanted to schedule a task to run every Saturday at 8:30am we’d write it as follows:
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30 8 * * 6 home/path/to/command/the_command.sh
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