Job automation with cron
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).
The importance of the job scheduling is that the
critical tasks like taking backups, which the clients usually wants to be taken
in nights, can easily be performed without the intervention of the
administrator by scheduling a cron job. If the cron job is scheduled carefully
than the backup will be taken at any given time of the client and there will be
no need for the administrator to remain back at night s to take the backup.
Important
Files related to cron:-
/etc/crontab
:
is the file which stores all scheduled jobs
/etc/cron.deny
:
is the file used to restrict the users from using cron jobs
/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)
Crontab
format:
TO assign a job in the crontab file the format used
is the following.
|
***** Command to be executed
1>
* For
Minute (0-59)
2>
* For Hour
(0-23)
3>
* For Day
of Month (1 -31 )
4>
* For Month
(1-12) OR jan,feb,mar,….
5>
* For Day
of Week (0-6) (Sunday= 0 or 7) OR sun,mon.tue,wed,thu,fri,sat
|
Corntab
Commands:
|
Command
|
Explanation
|
|
Crontab –e
|
Edit
your crontab file, or create one If it doesn’t already exist.
|
|
Crontab –l
|
Display
your crontab file.
|
|
Crontab –r
|
Remove
your crontab file.
|
|
Crontab –u
|
If
combined with –e, edit a particular user’s Crontab file and if combined with
–l, display a particular user’s crontab file. If combined with –r , deletes a
particular user’s crontab file
|
How to check the assigned cron jobs of currently
logged in user.
For Check The cron Jobs the command is
#crontab
–l
|
[root@linuxelearn
~]# crontab -l
43
23 28 9 3 mkdir /root/rahull
[root@linuxelearn
~]#
|
How to check the cron jobs of a particular user.
To check a users cron jobs, the syntax is
#crontab
–l –u <username>
#crontab
–l –u user1
|
[root@linuxelearn
~]# crontab -l -u pooja
no
crontab for pooja
[root@linuxelearn
~]# crontab -lu pooja
no
crontab for pooja
[root@linuxelearn
~]#
|
How
to set automation job to display the current date for every minute on present
console.
TO set above job the steps are
Check the console on which you are currently working
by following command
#tty
|
[root@linuxelearn
~]# tty
/dev/pts/1
[root@linuxelearn
~]#
|
Note: /dev/pts/1
is the console address
Schedule the job as show below
#crontab
–e
and enter the field as shown below and save it as in VI editor.
|
[root@linuxelearn
~]# crontab –e
*/1
* * * * date > /dev/pts/1
~
~
~
~
[root@linuxelearn
~]# crontab -e
crontab:
installing new crontab
|
Note:
Here * means every possible value
Restart the cron services using following command.
#service
crond restart
|
[root@linuxelearn
~]# service crond restart
Stopping
crond:
[ OK ]
Starting
crond:
[ OK ]
[root@linuxelearn
~]#
|
Wait for a minute and check whether time is
displaying or not. Every min time will be displayed as below.
|
[root@linuxelearn
~]#
[root@linuxelearn
~]# Sun Jan 22 22:41:01 IST 2017
|
How
to remove a schedule job or task.
To remove a automation job use following command..
#
crontab -r
|
[root@linuxelearn
~]# crontab -r
[root@linuxelearn
~]#
[root@linuxelearn
~]# crontab -l
no
crontab for root
[root@linuxelearn
~]#
|
Using this method you can schedule more job in
Linux…
I hope You enjoyed reading this article………
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