JOB Scheduling in RHEL (crontab and at)

It is used for Task Automation and Task Scheduling.
Two command are used
1. at
2. crontab

syntax:
#at time
at>command I
at>command II
ctrl+D
Example:
#at 10:30
at>eject
at>eject –t
ctrl+D
Option used
#at now
#at now+5 minutes
#at now+5 hours
#at now+5 days
#at tomorrow
#at 10:30 july 16 2006

we can also restrict the user not to use the ‘at’ command. Restriction will be provided by
root login with administrator.
#vi /etc/at.deny
inside this file we have to just mentioned the name of the user in order to restrict him
not to use ‘at’ command.
Just save and exit form the file
By default all users are allowed to use ‘at’ command. ‘at.deny’ is default in system. We
can also create a file ‘at.allow’ file. Once we create this file, now by default all users will
be restricted to use ‘at’ command and only that user which have entry on ‘at.allow’ will
allow to use the ‘at’ command.
Another method is to make an entry inside the ‘crontab’ file. Entry will be done by the
root. Difference between these two method is crontab is used to perform the same task
many times whereas job through ‘at’ command will be removed after the task has been
performed.

Syntax: ( By root login)
#crontab –e
six field are listed
1 2 3 4 5 6
minute hour day of Month day of command
month Week
Example:
35 10 31 05 3 eject
36 10 31 05 3 eject –t
Minute : 0-59
Hours : 0-23
Day of Month : 1-31
Month : 1-12
Day of Week : 0-7
0 and 7 are Sunday
#service crond restart
#Whereis eject
this command display the path of the command. It is better to use the full path of the
command inside crontab instead of just the name of the command.
We can also make the entry as
35 10 * * * eject
36 10 * * * eject –t
In this case. These two jobs will be performed at 10:35 in every month.
Some command which are executed with ‘at’ or ‘crontab’ send their output to the user
mail box. These command are executed on background and their output is transferred to
the particular user’s mail box. In order to see the output of the command we use.
#mutt
This command will open the mailbox from where we check the output
Some of the switched used with the ‘crontab’
#crontab –l
List out all the job scheduled in crontab.
#crontab –r
Can remove the job from the crontab.
#atq : list the job number scheduled in at
#atrm jobno : to remove any job

Regards,
MMM