Recently, I get a timed check in requirement. On Windows we have lots of applications which provide reminder feature or timed tasks. On Linux, we can use Crontab and at to achieve the same thing.

Requirements

Installation:

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pacman -S cronie at

Start and enable the daemon, this will make the service survive from the system reboot:

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systemctl start cronie at
systemctl enable cronie at

Usage

Both crontab and at provide a timed task function. Here I will simply introduce them and then give an example.

crontab

First edit a cron file like mycron to describe a cron job:

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# start a job at midnight, every day 
0 0 * * * /path/to/your/script

Then use crontab mycron to load the job, you can use crontab -l to list all the cron jobs. And to verify this job has been started, check the /var/spool/cron directory, the job configuration file should be copied here by it before the job started.

at

The command usage is:

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at yourscript time # this will start a job

and atq will list all the jobs, at -c job_no will show the job information, atrm job_no will cancel the job.

Example

I want to automatically start a job randomly between 0:00 and 6:00 every day. Then I edit a script named at.sh

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#!/bin/bash
script="/path/to/your/script.sh"      #insert the path to your script here
min=$(( 6 * 60 ))
rmin=$(( $RANDOM % $min ))
at -f "$script" now+${rmin}min

and then start a crontab with the configuration like:

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# start a job at midnight, every day 
0 0 * * * /path/to/at.sh