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Cron is very useful tool  used in Linux Operating Systems. It is used for scheduling the jobs, For ex. sysadmin jobs. Most of Linux users use Crontab for Schedule There task. there is another utility is available for Scheduling Future task at one time the utility called AT.

Schedule Cron Job using Crontab

To run the job continuously & repetitively in future at specific time period. Daemon Crond must be running for scheduling the future task.Scheduled jobs are stored in separate files for each user in /var/spool/cron/ directory. Users can set their scheduled jobs with crontab command as well as the at command.

Time for the execution of cron job is given in a specific order.

* * * * *
|_  First Star from left side is represent Minute (0-59)
* * * * *
  |_ Second star from left side is represent Hour (0-23)
* * * * *
    |_ Middle star is represent the Day of Month i,e (1-31)
* * * * *
      |_ Second star from right side is represent a Month (1-12) Or (Jan, Feb,-----, Dec) 
* * * * *
        |_ First star from right side is represent the Day of week (SUN=0, MON=1, ---, SAT=6) Should be written as (0,7)

Here, Some examples is given for better understanding of time format.

Every user can set their cronjobs.

* * * * * 
using this user can perform their job at every second.

8 4 * * * 
Using this cron perform the job at 04:08

8,15 17,23 * * * 
Using this cron perform the job at 17:8 , 23:8 , 17:15, 23:15
 
12 20 14 * * 
Cronjob set on Every 14th at 20:12 

*/20 10 8  * 
Cronjob set on every 8th at 10:20 10:40 

30 */5 8 12 * 
Cronjob set on Dec 8 at 00:30, 5:30, 10:30, 15:30, 20:30

30 10,12 8 12 0,3 
Cronjob set on Dec 8 at 10:30 and 12:30 if the day is sunday or wednesday

 

Step-1 : To Set Cron Job for scheduling future task

# crontab -e
30 17 * * * echo "I LOVE TECHBROWN"

:wq

It prints I LOVE TECHBROWN daily at 17:30

Step-2 : To check Cron Job for scheduling future task

# crontab -l

Step-3 : To schedule cron Job for a particular user

# crontab -eu techbrown
*/30 11,15 * * * /home/student/pro.sh

:wq

# crontab -lu techbrown

Step-4 : Check the cron file is created or not

# ls /var/spool/cron

Step-5 : It is possible to restrict a user from using cron service by writing an entry in /etc/cron.deny

# vim /etc/cron.deny
techbrown
:wq

Schedule Cron Job using AT

Step-1 : To run task only once in future at specified time.

Here, scheduling cron jobs to print I LOVE TECHBROWN at 10:30 on terminal 2

# at 10:30

at> echo "I LOVE TECHBROWN" > /dev/tty2

at> Ctrl+d

Using Ctrl+d the shell would exit with the output shown below

at > <EOT>
job 2 at 10:30 (also have day and year here)

Step-2 : The atq command is used to list the pending jobs which created by user.

# atq

Step-3 : Cat the job listed in command line to standard output.

# at -c 2

Step-4 : The atrm command is used to remove the jobs. Here, this command will remove job number 2.

# atrm 2

That’s all for now.

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