How to customize at and cron scheduled tasks in Linux

There are two types of scheduled tasks in Linux system, one is the at scheduled task which will only be executed once, and the other is the cron scheduled task which can be executed periodically.

at One-time scheduled tasks

describe

Execute a specific command (single time) at a specified time, also called a delayed action task.

usage

at time [Enter]

parameter

at -l #List single scheduled tasks at -r task id ##Delete the task with this id at -d task id ##Delete the task with this id at -m #Make the command without output generate email at -M #Make the command with output generate email at -c task id ##View the action of the task with this id at -f file ##Use the commands recorded in the file as the execution commands of the delayed task

Examples

First check the system time

[root@test ~]# date
Thu May 17 18:40:11 CST 2018

Specify a single scheduled task (delayed action)

[root@test ~]# at 18:45 ##Plan executed at 18:45 on the same day (18:45 2018-05-17)
at> date ##Scheduled task content at> shutdown -h now ##Scheduled task content at> <EOT> ##Command input ends, press Ctrl+D to exit job 10 at Thu May 17 18:45:00 2018 ##The system prompts that there is a scheduled task numbered 10

Cron periodic scheduled tasks

To use cron to schedule tasks, you need to ensure that the crond service is enabled and starts automatically at boot.

# systemctl start crond
# systemctl enable crond

describe

Maintain a periodic cron job file for each user.

usage

crontab [ -u user ] [ -l|-r|-e ]

Options

corn plan task format

If you need to specify a time period, you can use a hyphen (-) to indicate a continuous period of time, a comma (,) to indicate several discontinuous periods of time, an asterisk (*) to indicate all periods of time, and a division sign (/) to indicate intervals of time.

Example

[root@test ~]# crontab -e
: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

Cron scheduled task permissions

To prevent users from arbitrarily defining their own scheduled tasks, administrators can implement ACL access control. The control files for the at scheduled tasks are /etc/at.allow and /etc/at.deny. By default, at.llow does not exist. The control files for cron scheduled tasks are /etc/cron.allow and /etc/cron.deny. By default, cron.allow does not exist.

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