Learn how to configure scheduled tasks in Linux: using cron and anacron

Introduction

In this article, we will explain cron and anacron and show you how to set up anacron in Linux. We will also compare these two tools.

cron settings in Linux

cron - is a daemon process used to run scheduled tasks such as system backups, updates, etc. It is suitable for scheduled tasks that run on machines that run 24X7, such as servers.

Commands/scripts are written in cron task scripts, which are scheduled in crontab files. The system default cromtab file is /etc/crontab, but each user can also create their own cromtab file to run user-defined commands at specific times.

To create a personal crontab file, just enter:

$ crontab -e

How to set up anacron in Linux

anacron is used to run commands at a frequency in days. It works slightly differently from cron in that it assumes the machine won't be on all the time.

Cron is also suitable for running daily, weekly and monthly scheduled tasks on machines that do not run 24X7, such as laptops and desktop computers (LCTT translation: not suitable for executing tasks by hours and minutes).

Suppose you have a scheduled task (such as a backup script) that you want to run every day using cron in the middle of the night, maybe while you are asleep and your desktop/laptop is shut down by then. Your backup script will not be run.

However, if you use anacron, you can ensure that the backup script will be executed the next time you turn on your desktop/laptop.

How anacron works on Linux

anacron tasks are listed in /etc/anacrontab, and tasks can be scheduled using the following format (comments in the anacron file must start with #).

period   delay   job-identifier   command

From the format above:

• period - This is the frequency of the task, specified in days, or @daily, @weekly, @monthly Stands for daily, weekly, monthly. You can also use numbers: 1 - every day, 7 - every week, 30 - every month, or N - days.
• delay - This is the number of minutes to wait before executing a task.
• job-id - This is the unique name of the job written in the log file.
• command - This is the command or shell script to be executed.

To browse the sample files, enter:

$ ls -l /var/spool/anacron/
total 12
-rw------- 1 root root 9 Jun  1 10:25 cron.daily
-rw------- 1 root root 9 May 27 11:01 cron.monthly
-rw------- 1 root root 9 May 30 10:28 cron.weekly

This is what actually happened:

• anacron 会检查任务是否已经在 period 字段指定的时间被被执行了。如果没有，则在等待 delay 字段中指定的分钟数后，执行 command字段中指定的命令。
• 一旦任务被执行了，它会使用 job-id （时间戳文件名）字段中指定的名称将日期记录在 /var/spool/anacron 目录中的时间戳文件中。

现在让我们看一个例子。这个会每天运行 /home/aaronkilik/bin/backup.sh 脚本：

 @daily    10    example.daily   /bin/bash /home/aaronkilik/bin/backup.sh

当机器在 backup.sh 期望被运行时是关机的，anacron 会在机器开机十分钟之后运行它，而不用再等待 7 天。

这里有两个你应该理解的 anacrontab 文件的重要变量：

• START_HOURS_RANGE - 这个设置任务开始运行的时间范围（也就是任务只在这几个小时内运行）。
• RANDOM_DELAY - 这定义添加到用户定义的任务延迟的最大随机延迟（默认为 45）。

这是你的 anacrontab 文件可能看上去的样子。

Anacron – /etc/anacrontab ：

# /etc/anacrontab: configuration file for anacron
# See anacron(8) and anacrontab(5) for details.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
HOME=/root
LOGNAME=root
# These replace cron's entries
1       5       cron.daily      run-parts --report /etc/cron.daily
7       10      cron.weekly     run-parts --report /etc/cron.weekly
@monthly        15      cron.monthly    run-parts --report /etc/cron.monthly
@daily    10    example.daily   /bin/bash /home/aaronkilik/bin/backup.sh                                                                      

下面是 cron 以及 anacron 的比较，帮助你理解何时用他们其中一个。

cron	anacron
它是守护进程	它不是守护进程
适合服务器	适合桌面/笔记本电脑
可以让你以分钟级运行计划任务	只能让你以天为基础来运行计划任务
关机时不会执行计划任务	如果计划任务到期，机器是关机的，那么它会在机器下次开机后执行计划任务
普通用户和 root 用户都可以使用	只有 root 用户可以使用（使用特定的配置启动普通任务）

The main difference between cron and anacron is that cron can run effectively on machines that are continuously running, while anacron is targeted at machines that will shut down within a day or a week.

If you know of other methods, please share them with us in the comment box.

Original address: https://www.tecmint.com/cron-vs-anacron-schedule-jobs-using-anacron-on-linux/‎

This article’s address: https://www.linuxprobe.com/cron-anacron-work.html ‎Editor: Zhang Xiong, Reviewer: Pang Zengbao

Original address of this article: https://www.linuxprobe.com/cron-anacron-work.htmlEditor: Problem Terminator, Auditor: None

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