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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: (perfiffer)
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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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108
translated/tech/20210726 How to use cron on Linux.md
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108
translated/tech/20210726 How to use cron on Linux.md
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@ -0,0 +1,108 @@
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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: (perfiffer)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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在 Linux 上怎么使用 cron 定时器
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======
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cron 定时器是一个可以按照计划自动运行命令的工具。
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![Cron 表达式][1]
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cron定时器是一个可以按照计划自动运行命令的工具。这个定时器作业被叫做 cronjob，它被创建在 crontab 文件中。这是用户自动操作电脑的最简单也是最古老的方法。
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### 创建一个 cronjob
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要创建一个 cronjob，你可以使用 crontab 命令，并添加 `-e` 选项：
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```
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`$ crontab -e`
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```
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这会使用默认的文本编辑器打开 crontab。要显示设置文本编辑器，请使用 `EDITOR` [环境变量][1]：
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```
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`$ EDITOR=nano crontab -e`
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```
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### Cron 语法
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要调度一个 cronjob，你需要提供给计算机你想要执行的命令，然后提供一个 cron 表达式。cron 表达式在命令调度时运行：
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* minute (0 到 59)
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* hour (0 到 23, 0 代表午夜执行)
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* day of month (1 到 31)
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* month (1 到 12)
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* day of week (0 到 6, 星期天是 0)
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星号 （`*`） 代表的是“每一个”。例如，下面的表达式在每月每日每小时的 0 分钟运行备份脚本：
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```
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`/opt/backup.sh 0 * * * *`
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```
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下面的表达式在周日的凌晨 3:30 运行备份脚本：
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```
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`/opt/backup.sh 30 3 * * 0`
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```
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### 简写语法
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现代的 cron 实现接收简化的宏，而不是 cron 表达式：
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* `@hourly` 在每天的每小时的 0 分运行
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* `@daily` 在每天的 0 时 0 分运行
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* `@weekly` 在周日的 0 时 0 分运行
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* `@monthly` 在每月的第一天的 0 时 0 分运行
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例如，下面的 crontab 命令在每天的 0 时运行备份脚本：
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```
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`/opt/backup.sh @daily`
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```
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### 如何停止一个 cronjob
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一旦你开始了一个 cronjob，它就会永远按照计划运行。想要在启动后停止 cronjob，你必须编辑 crontab，删除触发该作业的命令行，然后保存文件。
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```
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`$ EDITOR=nano crontab -e`
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```
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要停止一个正在运行的作业，可以[使用标准的Linux进程命令][3]来停止一个正在运行的进程。
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### 它是自动的
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一旦你编写完 crontab，保存了文件并且退出了编辑器。你的 cronjob 就已经被调度了，剩下的工作都由 cron 完成。
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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：[perfiffer](https://github.com/perfiffer)
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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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