cron and at

# vi /etc/crontab
Example of time specification
Create cronjob:
# crontab -e
For other user:
# su – username crontab -e
There are other configuration files in /etc/:
cron.daily, cron.monthly….those contain the scripts for daily bases…
The files are as script. There is no specific time. Can drop script there and will be executed.
The directory /etc/cron.d is like example cron file.
Can put a file into cron.d and it will be executed or use the command cron-e in order to create the file.

at run just one time.
# systemctl status atd -l
# at 14:30
at> logger hello at 2.30 from at
check the status:
# atq
# atrm –help
# atrm 1 (job ID)
# atq – check the queue of jobs.
All jobs are into: /var/spool/at/ directory.