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[#]: collector: (lujun9972)
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[#]: translator: (MjSeven)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Scheduling tasks on Linux using the at command)
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[#]: via: (https://www.networkworld.com/article/3535808/scheduling-tasks-on-linux-using-the-at-command.html)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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Scheduling tasks on Linux using the at command
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======
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The at command makes it easy to schedule Linux tasks to be run at any time or date you choose. Check out what it can do for you.
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romkaz / Getty Images
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When you want commands or scripts to run at some particular time, you don’t have to sit with your fingers hovering over the keyboard waiting to press the enter key or even be at your desk at the right time. Instead, you can set your task to be run through the **at** command. In this post, we’ll look at how tasks are scheduled using **at**, how you can precisely select the time you want your process to run and how to view what’s been scheduled to run using **at**.
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### at vs cron
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For those who’ve been scheduling tasks on Linux systems using **cron**, the **at** command is something like **cron** in that you can schedule tasks to run at a selected time, but **cron** is used for jobs that are run periodically – even if that means only once a year. Most **cron** jobs are set up to be run daily, weekly or monthly, though you control how often and when.
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The **at** command, on the other hand, is used for tasks which are run only once. Want to reboot your system at midnight tonight? No problem, **at** can do that for you assuming you have the proper permissions. If you want the system rebooted every Saturday night at 2 a.m., use **cron** instead.
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### Using at
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The **at** command is easy to use ,and there are only a few things to remember. A simple use of **at** might look like this:
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```
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$ at 5:00PM
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at> date >> thisfile
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at> <EOT>
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```
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After typing “at” and the time the command should be run, **at** prompts you for the command to be run (in this case, the **date** command). Type **^D** to complete your request.
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Assuming we set up this **at** command earlier than 5 p.m., the date and time will be added to the end of a file named “thisfile” later the same day. Otherwise, the command will run at 5 p.m. the following day.
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You can enter more than one command when interacting with the **at** command. If you want more than one command to be run at the same time, simply specify more than one command line:
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[][1]
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```
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$ at 6:22
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warning: commands will be executed using /bin/sh
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at> echo first >> thisfile
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at> echo second >> thisfile
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at> <EOT>
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```
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In the commands above, we’re using a regular user account and adding some simple text to a file in that user’s home directory. If it’s after 6:22 a.m. when this command is run, the command will run the following day because 6:22 is taken to mean 6:22 a.m. If you want it to run at 6:22 p.m., either use 6:22 PM or 18:22. “6:22 PM” also works.
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You can use **at** to schedule commands to run on specific dates either by specifying the dates or specifying dates and times like “10:00AM April 15 2021” or “noon + 5 days” (run at noon five days from today). Here are some examples:
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```
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at 6PM tomorrow
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at noon April 15 2021
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at noon + 5 days
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at 9:15 + 1000 days
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```
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After you specify the command to run and press **^D**, you will notice that the **at** command has assigned a job number to each request. This number will show up in the **at** command's job queue.
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```
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$ at noon + 1000 days
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warning: commands will be executed using /bin/sh
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at> date >> thisfile
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at> <EOT>
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job 36 at Tue Dec 27 12:00:00 2022 <== job # is 36
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```
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### Checking the queue
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You can look at the queue of **at** jobs with the **atq** (at queue) command:
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```
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$ atq
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32 Thu Apr 2 03:06:00 2020 a shs
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35 Mon Apr 6 12:00:00 2020 a shs
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36 Tue Dec 27 12:00:00 2022 a shs
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34 Thu Apr 2 18:00:00 2020 a shs
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```
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If you need to cancel one of the jobs in the queue, use the **atrm** (at remove) command along with the job number.
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```
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$ atrm 32
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$ atq
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35 Mon Apr 6 12:00:00 2020 a shs
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36 Tue Dec 27 12:00:00 2022 a shs
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34 Thu Apr 2 18:00:00 2020 a shs
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```
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You can look into the details of a scheduled task using the **at -c** command. Additional details (the active search path, etc.) are also available, but the bottom lines of the output will show you what command has been scheduled to run.
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```
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$ at -c 36 | tail -6
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cd /home/shs || {
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echo 'Execution directory inaccessible' >&2
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exit 1
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}
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date >> thisfile
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```
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Notice that the command shown begins with testing whether the user’s directory can be entered with a **cd** command. The job will exit with an error if this is not the case. Otherwise the command specified when the **at** command was issued will be run. Read the command as "move into /home/shs OR exit with the error shown".
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### Running jobs as root
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To run **at** jobs as root, simply use **sudo** with your **at** command like this:
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```
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$ sudo at 8PM
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[sudo] password for shs:
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warning: commands will be executed using /bin/sh
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at> reboot now
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at> <EOT>
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job 37 at Wed Apr 1 16:00:00 2020
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```
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Notice that the root task shows up in the queue with **root** as the one to execute it.
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```
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35 Mon Apr 6 12:00:00 2020 a shs
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36 Tue Dec 27 12:00:00 2022 a shs
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37 Wed Apr 1 20:00:00 2020 a root <==
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```
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### Running scripts
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You can also use the **at** command to run scripts. Here's an example:
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```
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$ at 4:30PM
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warning: commands will be executed using /bin/sh
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at> bin/tryme
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at> <EOT>
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```
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### Denying use of the at command
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The **/etc/at.deny** file provides a way to disallow users from being able to make use of the **at** command. By default, it will probably include a list of disallowed accounts like **ftp** and **nobody**. An **/etc/at.allow** file could be used to do the opposite but, generally, only the **at.deny** file is configured.
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**Wrap-Up**
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The **at** command is very versatile and easy to use when you want to schedule a one-time task – even if you want it to run this afternoon or years in the future.
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Join the Network World communities on [Facebook][2] and [LinkedIn][3] to comment on topics that are top of mind.
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3535808/scheduling-tasks-on-linux-using-the-at-command.html
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作者：[Sandra Henry-Stocker][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://www.networkworld.com/author/Sandra-Henry_Stocker/
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[b]: https://github.com/lujun9972
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[1]: https://www.networkworld.com/blog/itaas-and-the-corporate-storage-technology/?utm_source=IDG&utm_medium=promotions&utm_campaign=HPE22140&utm_content=sidebar (ITAAS and Corporate Storage Strategy)
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[2]: https://www.facebook.com/NetworkWorld/
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[3]: https://www.linkedin.com/company/network-world
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@ -0,0 +1,153 @@
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[#]: collector: (lujun9972)
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[#]: translator: (MjSeven)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (Scheduling tasks on Linux using the at command)
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[#]: via: (https://www.networkworld.com/article/3535808/scheduling-tasks-on-linux-using-the-at-command.html)
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[#]: author: (Sandra Henry-Stocker https://www.networkworld.com/author/Sandra-Henry_Stocker/)
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使用 at 命令在 Linu 上安排任务
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======
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at 命令可以很容易地安排 Linux 任务在你选择的任何时间或日期运行，让我们来看看它能为你做什么。
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![][0]
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romkaz / Getty Images
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当你希望命令或脚本在某个特定时间运行时，你不需要将手指放在键盘上盘旋等待按下回车键，或者是在特定时间坐在办公桌前。相反，你可以通过 **at** 命令来设置任务。在本文中，我们将研究如何使用 **at** 来安排任务，如何精确地选择任务希望运行的时间以及如何使用 **at** 来查看安排运行的任务。
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### at vs cron
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对于那些使用 **cron** 在 Linux 系统上安排任务的人来说，**at** 命令类似于 **cron**，因为你可以在选定的时间调度任务，但是 **cron** 用于定期运行的作业 - 甚至是每年仅一次。大多数 **cron** 作业的频率都设置为每天、每周或每月运行一次，不过你可以控制运行的频率和时间。
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另一方面，**at** 命令用于仅运行一次的任务。想在午夜重启系统？没问题，假设你有适当的权限，**at** 可以为你完成此操作。如果你希望系统在每个星期六凌晨 2 点重启，那么改用 **cron**。
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### 使用 at
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**at** 命令很容易使用，只需记住几件事。一个简单使用 **at** 的例子类似于这样：
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```
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$ at 5:00PM
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at> date >> thisfile
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at> <EOT>
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```
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在输入 "at" 和应该运行命令的时间，**at** 会提示你在设定时间会运行该命令（此例中是 **`date`** 命令）。输入 **^D**(Ctrl + d) 来完成请求。
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假设我们在下午 5 点之前设置这个 **at** 命令，那么日期和时间将在当天下午 5 点添加到名为 "thisfile" 文件的末尾。否则，该命令将在第二天下午 5 点运行。
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与 **at** 命令进行交互时，可以输入多个命令。如果你要同时运行多个命令，只需输入多个命令行即可：
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[][1]
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```
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$ at 6:22
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warning: commands will be executed using /bin/sh
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at> echo first >> thisfile
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at> echo second >> thisfile
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at> <EOT>
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```
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在上面的命令中，我们使用了一个普通的用户账户，将一些简单的文本添加到该用户主目录的文件中。如果在上午 6:22 之后运行这些命令，那么命令会在第二天运行，因为 6:22 表示上午 6:22。如果你想在下午 6:22 运行，使用 6:22 PM 或者 18:22，"6:22 PM" 也可以。
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你也可以通过使用 **at** 来安排命令在指定的日期或时间运行，例如 "10:00AM April 15 2021" 或 "noon + 5 days"（从今天起 5 天内的中午运行），以下是一些例子：
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```
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at 6PM tomorrow
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at noon April 15 2021
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at noon + 5 days
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at 9:15 + 1000 days
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```
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在指定要运行的命令并按下 **^D** 后，你会注意到 **at** 命令为每个请求分配了一个作业编号，这个数字将显示在 **at** 命令的作业队列中。
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```
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$ at noon + 1000 days
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warning: commands will be executed using /bin/sh
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at> date >> thisfile
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at> <EOT>
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job 36 at Tue Dec 27 12:00:00 2022 <== job # is 36
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```
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### 检查队列
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你可以使用 **atq**(at queue) 命令来查看 **at** 作业队列：
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```
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$ atq
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32 Thu Apr 2 03:06:00 2020 a shs
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35 Mon Apr 6 12:00:00 2020 a shs
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36 Tue Dec 27 12:00:00 2022 a shs
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34 Thu Apr 2 18:00:00 2020 a shs
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```
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如果你需要取消队列中的一个作业，使用 **atrm**(at remote) 命令和作业编号：
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```
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$ atrm 32
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$ atq
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35 Mon Apr 6 12:00:00 2020 a shs
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36 Tue Dec 27 12:00:00 2022 a shs
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34 Thu Apr 2 18:00:00 2020 a shs
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```
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你可以使用 **at -c** 命令来查看安排任务的详细信息，其它详细信息（活动的搜索路径等）也可以，但是输出的最后一行将显示计划运行的命令。
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```
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$ at -c 36 | tail -6
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cd /home/shs || {
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echo 'Execution directory inaccessible' >&2
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exit 1
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}
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date >> thisfile
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```
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注意，命令首先会测试是否可以通过 **cd** 命令进入用户目录。如果不可以，作业将退出并显示错误。如果可以，则运行在 **at** 中指定的命令。它将命令视为 “进入 /home/shs 或退出并显示错误”。
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### 以 root 身份运行作业
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要以 root 身份运行 **at** 作业，只需将 **sudo** 与你的 **at** 命令一起使用，如下所示：
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```
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$ sudo at 8PM
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[sudo] password for shs:
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warning: commands will be executed using /bin/sh
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at> reboot now
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at> <EOT>
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job 37 at Wed Apr 1 16:00:00 2020
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```
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注意，root 的任务以 **root** 作为执行者显示在队列中。
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```
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35 Mon Apr 6 12:00:00 2020 a shs
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36 Tue Dec 27 12:00:00 2022 a shs
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37 Wed Apr 1 20:00:00 2020 a root <==
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```
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### 运行脚本
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你还可以使用 **at** 命令来运行脚本，这里有一个例子：
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```
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$ at 4:30PM
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warning: commands will be executed using /bin/sh
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at> bin/tryme
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at> <EOT>
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```
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### 禁止使用 at 命令
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**/etc/at.deny** 文件提供了一种禁止用户使用 **at** 命令的方法。默认情况下，它可能会包含一个不允许的账户列表，例如 **ftp** 和 **nobody**。可以使用 **/etc/at.allow** 文件执行相反的操作，但是通常只配置 **at.deny** 文件。
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**总结**
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当你要安排一项一次性任务时，无论你是希望在今天下午或几年后运行，**at** 命令都是通用且易于使用的。
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--------------------------------------------------------------------------------
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via: https://www.networkworld.com/article/3535808/scheduling-tasks-on-linux-using-the-at-command.html
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|
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作者：[Sandra Henry-Stocker][a]
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选题：[lujun9972][b]
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译者：[MjSeven](https://github.com/MjSeven)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://www.networkworld.com/author/Sandra-Henry_Stocker/
|
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[b]: https://github.com/lujun9972
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[0]: https://images.idgesg.net/images/article/2019/08/gettyimages-140389307-100808283-large.jpg
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[1]: https://www.networkworld.com/blog/itaas-and-the-corporate-storage-technology/?utm_source=IDG&utm_medium=promotions&utm_campaign=HPE22140&utm_content=sidebar (ITAAS and Corporate Storage Strategy)
|
||||
[2]: https://www.facebook.com/NetworkWorld/
|
||||
[3]: https://www.linkedin.com/company/network-world
|
||||
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