百度得到的答案都是通过以前的yum安装crontab

yum -y install vixie-cron
...
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但是在centos8(阿里云)上始终报错,无法安装.然后谷歌得到下面一篇非常好的博客教程：

How To Use Cron to Automate Tasks on CentOS 8

Introduction

Cron is a time-based job scheduling daemon found in Unix-like operating systems, including Linux distributions. Cron runs in the background and tasks scheduled with cron, referred to as “cron jobs,” are executed automatically, making cron useful for automating maintenance-related tasks.

This guide provides an overview of how to schedule tasks using cron’s special syntax. It also goes over a few shortcuts one can use to make job schedules easier to write and understand.（这篇引介提供了一个如何使用cron的特殊语法来执行任务的概览。它也同时涉及一些使工作时间表更容易编写和理解的捷径）

Prerequisites

To complete this guide, you’ll need access to a computer running CentOS 8. This could be your local machine, a virtual machine, or a virtual private server.（需要Centos8）

Regardless of what kind of computer you use to follow this guide, it should have a non-root user with administrative privileges configured. To set this up, follow our Initial Server Setup guide for CentOS 8.(拥有管理权限的账户)

Installing Cron

Almost every Linux distribution has some form of cron installed by default. However, if you’re using a CentOS machine on which cron isn’t installed, you can install it using dnf.（使用dnf安装）

Before installing cron on a CentOS machine, update the computer’s local package index:（先更新dnf）

sudo dnf update

Then install the cron daemon with the following command:

sudo dnf install crontabs

This command will prompt you to confirm that you want to install the crontabs package and its dependencies. Do so by pressing y then ENTER.

This will install cron on your system, but you’ll need to start the daemon manually. You’ll also need to make sure it’s set to run whenever the server boots. You can perform both of these actions with the systemctl command.

To start the cron daemon, run the following command:(启动服务)

sudo systemctl start crond.service

To set cron to run whenever the server starts up, type:（开机自启）

sudo systemctl enable crond.service

Following that, cron will be installed on your system and ready for you to start scheduling jobs.

Understanding How Cron Works（理解Cron的工作原理）

Cron jobs are recorded and managed in a special file known as a crontab. Each user profile on the system can have their own crontab where they can schedule jobs, which is stored under /var/spool/cron/.（Cron被一个叫crontab的特殊文件记录和管理着。系统中每一个用户配置文件都可以拥有自己的crontab，可以在其中调度作业，它被存放在 /var/spool/cron/中）

To schedule a job, you just need to open up your crontab for editing and add a task written in the form of a cron expression. The syntax for cron expressions can be broken down into two elements: the schedule and the command to run.

The command can be virtually any command you would normally run on the command line. The schedule component of the syntax is broken down into 5 different fields, which are written in the following order:

Field	Allowed Values
minute	0-59
hour	0-23
Day of the month	1-31
month	1-12 or JAN-DEC
Day of the week	0-6 or SUN-SAT

Together, tasks scheduled in a crontab are structured like this:

 minute hour day_of_month month day_of_week command_to_run

Here’s a functional example of a cron expression. This expression runs the command curl http://www.google.com every Tuesday at 5:30 PM:

30 17 * * 2 curl http://www.google.com

There are also a few special characters you can include in the schedule component of a cron expression to make scheduling easier:

• *: In cron expressions, an asterisk is a wildcard variable that represents “all.” Thus, a task scheduled with * * * * * … will run every minute of every hour of every day of every month.
• ,: Commas break up scheduling values to form a list. If you want to have a task run at the beginning and middle of every hour, rather than writing out two separate tasks (e.g., 0 * * * * … and 30 * * * * …), you could achieve the same functionality with one (0,30 * * * * …).
• -: A hyphen represents a range of values in the schedule field. Instead of having 30 separate scheduled tasks for a command you want to run for the first 30 minutes of every hour (as in 0 * * * * …, 1 * * * * …, 2 * * * * …, and so on), you could just schedule it as 0-29 * * * * …
• /: You can use a forward slash with an asterisk to express a step value. For example, instead of writing out eight separate separate cron tasks to run a command every three hours (as in, 0 0 * * * …, 0 3 * * * …, 0 6 * * * …, and so on), you could schedule it to run like this: 0 */3 * * * …
  Note: You cannot express step values arbitrarily; you can only use integers that divide evenly into the range allowed by the field in question. For instance, in the “hours” field you could only follow a forward slash with 1, 2, 3, 4, 6, 8, or 12.

Here are some more examples of how to use cron’s scheduling component:

• */1 * * * * or * * * * * - Run the command every minute.
• 12 * * * * - Run the command 12 minutes after every hour.
• 0,15,30,45 * * * * - Run the command every 15 minutes.
• */15 * * * * - Run the command every 15 minutes.
• 0 4 * * * - Run the command every day at 4:00 AM.
• 0 4 * * 2-4 - Run the command every Tuesday, Wednesday, and Thursday at 4:00 AM.
• 20,40 */8 * 7-12 * - Run the command on the 20th and 40th minute of every 8th hour every day of the last 6 months of the year.

If you find any of this confusing or if you’d like help writing schedules for your own cron tasks, Cronitor provides a handy cron schedule expression editor named “Crontab Guru” which you can use to check whether your cron schedules are valid.

Managing Crontabs

权限管理部分就直接上图了