Linux Cron Jobs: From Basics to Advanced Automation

JOB AUTOMATION:
Automation with cron and at:
1. In any operating system, it is possible to create jobs that you want to reoccur. This process,
known as job scheduling, is usually done based on user-defined jobs. For Red Hat or any
other Linux, this process is handled by the cron service or a daemon called crond, which can
be used to schedule tasks (also called jobs). By default, Red Hat comes with a set of
predefined jobs that occur on the system (hourly, daily, weekly, monthly, and with arbitrary
periodicity).

Important Files related to cron and at :
1. /etc/crontab is the file which stores all scheduled jobs

2. /etc/cron.deny is the file used to restrict the users from using cron jobs.

3. /etc/cron.allow is used to allow only users whose names are mentioned in this file to use cron jobs.(this file does not exist by default)

4. /etc/at.deny same as cron.deny for restricting at jobs

5. /etc/at.allow same as cron.allow for allowing user to use at jobs.

LAB WORK:

CRON JOBS:
To check the assigned cron jobs of currently logged in user
• To check the cron jobs the command is
#crontab –l

To check the cron jobs of a particular user
• To check a user’s cron jobs, the syntax is
#crontab –l –u user name
#crontab –l –u ktuser
#crontab –lu ktuser

Setting a job to display the current date for every minute on present console
• To set the above job the steps are

• Check the console on which you are working by following command
#tty
/dev/pts/1

Note:- /dev/pts/1 is the console address

Schedule the task as shown below

#crontab –e and enter the field as shown below and save it as in VI editor
*/1 * * * * date /dev/pts/1

Note:- where * means every possible value.

Restart the cron services:
#service crond restart

Wait for a minute and check whether time is displaying or not. Every min time will be displayed as below.

Schedule a cron job to create a directory “ktdir” under “/root” on “Sunday 22 October at 1:30 AM”

• To schedule above job edit the crontab file as shown below and restart the service

#crontab –e
#mkdir /root/ktdir
#crontab -e

Note:- you can use 0 or 7 for Sunday.
Check whether it got created or not on scheduled day, if it created you can see the directory
otherwise a error mail will be generated to your mail.

Schedule a job to run the backup script “bkpscript.sh” on every “Saturday 12:30 PM”
• In order to schedule above job the steps are.
• Check the location of script and also check whether it is having execute permission or not.
If not then add the execute permissions to all user on it.

#ls
#pwd
#ls -l bkpscript.sh
#chmod a+x bkpscript.sh
#ls -l bkpscript.sh

Apply the job in crontab and restart the service
#crontab –l

#crontab –e
#!ser

Note:- !ser is the command to restart the last restarted service

Schedule a job so that a user “ktuser” should get a mail regarding meeting on 24th, 29th and
31st October at 2:25 PM.

• To set above task edit the crontab in following passion, and restart the service
#crontab –e –u user name
#crontab –e –u ktuser

#!ser

Schedule a job so that a user “ktuser” should get the mail from 15th to 20th and 25th to 30st
November as a reminder of some session at 2:25 PM
• This task is very much similar to the previous one but there is only a small change in format.

#crontab –e –u ktuser

There are still various method you can schedule the cron jobs, Do some R&D on it to find
out more.

Restrict users “ktuser” “amit” “vivek” from using cron jobs
• To restrict any user from using cron job facility, enter their names in /etc/cron.deny and save it

#vim /etc/cron.deny

Now login as one of those users and try to use crontab

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