Linux Commands-crontab: Task Scheduler

Cron is a timed execution tool under Linux that can run a job without human intervention. The daemon cron reads the crontab file and executes the task at a specified time, depending on the configuration.
The Contab command is used to add, delete, and display cron task tables.

(Ubuntu Environment) can start and stop the Cron service via service command:
Service Cron Status # View Cron service state
Service Cron Start # start cron service
Service cron Stop # Stop cron service
Service Cron Restart # restart Cron Service
Service Cron Reload # reload Cron service configuration

crontab Command Command format

crontab [-u user] File
crontab [-u user] [-l |-r |-e] [-I.] [-S]

Command parameters

-U
Specify user

-L
Displays the contents of the current Crontab file.

-R
Deletes the contents of the current Crontab file.

-E
Edits the current crontab file content and creates a new file if the file does not exist.

-I.
Be prompted when you delete the contents of the crontab file.

crontab file

format:minute hour day_of_month month Day_of_week command

Field meaning

Field	Meaning
Minute	Minutes, the value range is (0-59)
Hour	Hours, the value range is (0-23)
Day_of_month	Date, value range is (0-31)
Month	month, the value range is (1-12), you can also use jan,feb,mar,apr ... Said
Day_of_week	Week, the value range is (0-6), of which Sunday is 0 or 7, can also be expressed in Sun,mon,tue,wed,thu,fri,sat
Command	The command to execute

Special Symbolic meaning

Special symbols	Description
*	Pass all possible values
,	Specify a list of values, such as 1,3,5,7,9
-	Specifies an integer range, such as 1-5
/	Specify interval frequency

Cron Expression Example

Example	Description
*/15 * * * *	Perform tasks every 15 minutes
0 */2 * * *	Perform the task every 2 hours on the hour
0 3 * * * *	Perform tasks three o'clock in the morning every day.
0 0 5,15,25 * *	Every month, number 5th, 15th, Morning 25th.
15 12 * 1	Every Monday, 12:15 noon, the mission is performed.
0,30 12-20 * * *	Every day from 12:00 to 20:00, at the hour or half of the time to perform tasks

Instance

a) adding cron tasks

echo "*/3 * * * * date >> ~/huey/linux/cmdline/cron.out" > Mycron crontab Mycron

b) Delete Cron task

Crontab-r

c) Display cron task

crontab-l*/3 * * * * Date >> ~/huey/linux/cmdline/cron.out

Linux Commands-crontab: Task Scheduler