Setting a cron

[rick]

Crontab

To display your cron file you run the following command:

crontab -l

The root user can see any users crontab file by adding “-u username”, for example:

crontab -u rick -l 

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Each line is a collection of six fields separated by spaces.

The fields are:

• The number of minutes after the hour (0 to 59)
• The hour in military time (24 hour) format (0 to 23)
• The day of the month (1 to 31)
• The month (1 to 12)
• The day of the week(0 or 7 is Sun, or use name)
• The command to run

*     *     *     *     *  Command string
-     -     -     -     -
|     |     |     |     |
|     |     |     |     +----- Day of week (0-7)
|     |     |     +------- Month (1 - 12)
|     |     +--------- Day of month (1 - 31)
|     +----------- Hour (0 - 23)
+------------- Min (0 - 59)

The ‘asterisks’ are where you enter day, time ect, however they can be left as ''s , meaning null*

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To edit crontab file run:

crontab -e

To enable your preferred editor follow this quick and simple guide: force-crontab-editor-default-editor

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To list all crons in the file:

crontab -l

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Example:

Reboot the machine every hour:

0   *   *   *   * /sbin/shutdown -r now

Reboot the machine at ten minutes past midnight every day:

10  0   *   *   * /sbin/shutdown -r now

Reboot the machine every monday at 2 AM

0   2   *   *   1  /sbin/shutdown -r now

Reboot the machine using range:

Reboot the machine within the range of hours matching 1, 2, 3 and 4AM

*   1-4   *   *   * /sbin/shutdown -r now