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117 lines
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[#]: subject: (How to use cron on Linux)
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[#]: via: (https://opensource.com/article/21/7/cron-linux)
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[#]: author: (Seth Kenlon https://opensource.com/users/seth)
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[#]: collector: (lujun9972)
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[#]: translator: ( )
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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How to use cron on Linux
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======
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The cron system is a method to automatically run commands on a schedule.
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![Cron expression][1]
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The cron system is a method to automatically run commands on a schedule. A scheduled job is called a _cronjob_, and it’s created in a file called a _crontab_. It’s the easiest and oldest way for a computer user to automate their computer.
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### Writing a cronjob
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To create a cronjob, you edit your `crontab` using the `-e` option:
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```
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`$ crontab -e`
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```
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This opens your crontab your default text editor. To set the text editor explicitly, use the `EDITOR` [environment variable][2]:
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```
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`$ EDITOR=nano crontab -e`
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```
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### Cron syntax
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To schedule a cronjob, you provide the command you want your computer to execute, followed by a cron expression. The cron expression schedules when the command gets run:
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* minute (0 to 59)
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* hour (0 to 23, with 0 being midnight)
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* day of month (1 to 31)
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* month (1 to 12)
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* day of week (0 to 6, with Sunday being 0)
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An asterisk (`*`) in a field translates to "every." For example, this expression runs a backup script at the 0th minute of _every_ hour on _every_ day of _every_ month:
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```
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`/opt/backup.sh 0 * * * *`
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```
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This expression runs a backup script at 3:30 AM on Sunday:
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```
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`/opt/backup.sh 30 3 * * 0`
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```
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### Simplified syntax
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Modern cron implementations accept simplified macros instead of a cron expression:
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* `@hourly` runs at the 0th minute of every hour of every day
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* `@daily` runs at the 0th minute of the 0th hour of every day
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* `@weekly` runs at the 0th minute of the 0th hour on Sunday
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* `@monthly` runs at the 0th minute of the 0th hour on the first day of the month
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For example, this crontab line runs a backup script every day at midnight:
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```
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`/opt/backup.sh @daily`
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```
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### How to stop a cronjob
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Once you've started a cronjob, it's designed to run on schedule forever. To stop a cronjob once you've started it, you must edit your crontab, remove the line that triggers the job, and then save the file.
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```
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`$ EDITOR=nano crontab -e`
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```
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To stop a job that's actively running, [use standard Linux process commands][3] to stop a running process.
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### It’s automated
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Once you’ve written your crontab, save the file and exit your editor. Your cronjob has been scheduled, so cron does the rest.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/7/cron-linux
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作者：[Seth Kenlon][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/seth
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/cron-splash.png?itok=AoBigzts (Cron expression)
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[2]: https://opensource.com/article/19/8/what-are-environment-variables
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[3]: https://opensource.com/article/18/5/how-kill-process-stop-program-linux
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