CRON

学习网址：

• https://www.marksanborn.net/linux/learning-cron-by-example/
• http://www.unixgeeks.org/security/newbie/unix/cron-1.html

If you are using a Linux system and want to schedule a task to run in the future you will probably need to know cron. Cron is much like Window’s Scheduled Tasks. The only difference is that cron is conifgured by a simple text file.

Now obviously cron is very dependent and sensitive to the time. If you want accurate results from cron you are going to want to setup your computer to sync its clock via NTP.

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$ sudo ntpdate pool.ntp.org

Editing Cron

There are more than one way to edit the cron config files; however many of them require you to restart the service.

Here is a method to add a task to cron without having to restart the deamon:

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$ crontab -e

Here is the basic structure for cron:

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m h dom mon dow command

where

• m - minutes
• h - hours (24 hours)
• dom - day of month
• mon - month
• dow - day of the week
• command - the command you want to run. This can contain spaces or point to a bash script.

Examples

*’s represent wildcards or any:

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10 * * * * echo “This command is run at 10 min past every hour”

22 7 * * * echo “This command is run daily at 7:22 am”

00 4 * * 0 echo “This command is run at 4 am every Sunday”

PostScript: In dow, 0 and 7 are both represent Sunday.

Using the - allows us to specify ranges of days (Execute at 5 pm only in weekdays):

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* 17 * * 1-5 /path/to/your/code

Using the , allows us to specify intervals without having to have multiple entries in cron: (This would execute the ask on the 1st, the 10th, the 20th and on the 30th of each month, at 17:59 PM.)

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59 17 1,10,20,30 * * /home/username/backupsite

Using the / allows us to divide the day into chunks: (Here, the tasks is executed every 4 hours (24⁄6 =4).)

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59 */6 * * * /home/username/backupsite

Start

Cron is a daemon, which means that it only needs to be started once, and will lay dormant until it is required.

The cron daemon, or crond, stays dormant until a time specified in one of the config files, or crontabs.