Linux auto start system by cron

$ ls -l /etc/crontab
-rw-r--r-- 1 root root 701 Jan 11 11:04 /etc/crontab

$ cat /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# For details see man 4 crontabs
# Example of job definition:
# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed
 30   *  *  *  * ec2-user /home/user/cicd.sh

# Directories of different times.

/etc/cron.d/     #all kind files

/etc/cron.hourly/   

/etc/cron.daily/       

/etc/cron.weekly/

/etc/cron.monthly/

#user list of allow or deny.

/etc/cron.allow
/etc/cron.deny

#check at service

$ systemctl status atd

● atd.service - Job spooling tools

   Loaded: loaded (/usr/lib/systemd/system/atd.service; enabled; vendor preset: enabled)

   Active: active (running) since Mon 2022-01-17 09:01:43 JST; 55min ago

 Main PID: 4523 (atd)

    Tasks: 1

   Memory: 484.0K

   CGroup: /system.slice/atd.service

           mq4523 /usr/sbin/atd -f

Warning: Journal has been rotated since unit was started. Log output is incomplete or unavailable.

#run atd if not started.

$ systemctl start atd

#auto started with os.

$ systemctl enable atd

#Example: run backup.sh at 2016/10/17 19:15 

$ at "19:15 17.10.16"  -f backup.sh                           

$ at 10pm -f backup.sh      #run backup.sh at 22:00           
$ at now + 4days backup.sh  #run at the same time after 4days.
$ at 9pm + 3weeks           #run at 21:00 after 3weeks