On Linux, jobs that run regularly are `cron``` One-time job reservations can be scheduled using the ```at command
`!
System management costs can be reduced by automating the tasks that are performed on a regular basis.
cron
cron consists of `crond```, which is a daemon that manages schedules (a process that runs on Unix-like multitasking operating systems), and
crontab command
, which edits scheduling. The crond daemon checks the crontab file every minute and will run any jobs that you want to run! The crontab file is located under the / var / spool / cron directory and cannot be edited directly with an editor. So use the
crontab command
```.
cron job settings
$ crontab -e
You can edit it when crontab opens in your default editor. Edit and save, then finish.
Check the list of reserved jobs
$ crontab -l
Delete all reserved cron jobs
$ crontab -r
Format crontab file
Minutes, hours, days, months, days of the week commands
For fields,
--Min: Integer from 0-59 --Hour: Integer from 0-23 --Day: Integer from 1-31 --Month: An integer from 1-12, or a character string such as Sun, Mon, etc. --Command: Command to execute
Example crontab file (run program daily at 22:30)
Dairy Backup
30 22 * * * /usr/local/bin/backup
There is no need to restart and the job will run automatically!
The at command `` `runs a one-time schedule, whereas cron runs the job repeatedly on a regular basis.
at command```The at daemon must be running to perform scheduling by!
Format when reserving a job with the at command
at hh:mm MMDDYY
For example, if you book to run at 22:30 on January 28, 2020 ...
$ at 22:30 012820
The at command inputs commands interactively, so
at>
When prompted, specify the command.
$ at 22:30 012820
at> /usr/local/sbin/backup
at> ^D
When specifying a command in a file, write the command in the file (jobs file this time) and write it.
$ at -f jobs 22:30
Check the list of reserved jobs
$ at -l
Or
$ atq
Delete reserved job (specify job ID)
$ at -d 3
Or
$ atrm 3
--Write a user who is allowed to use cron in /etc/cron.allow
--In /etc/cron.deny
, describe the user who refuses to use cron.
If there is ** /etc/cron.allow **, only the users described there can use cron, but if not, it refers to ** /etc/cron.deny ** and is not described there. All users can use cron.
--In /etc/at.allow
, describe the users who are allowed to use at.
--In / etc / at / deny
, describe the user who refuses to use at.
If there is ** /etc/at.allow **, only the users described there can use at, but if not, it refers to ** / etc / at / deny ** and is not described there. All users can use at.
Without either, only the root user can use at.
Linux textbook LPIC Level 1 Version 5.0 compatible
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