Job Scheduling or Scheduling jobs Manually

Job Scheduling-

As a system administrator some tasks are respective like backup.

monitoring, log files.
  To automate them below the commands or keys

-at 
-batch
-crontab

Crontab: to run some tasks automatically 
to set a crontab for an user

#crontab -e -u <user name>
* * * * * /bin/echo "hello"

A crontab file contains instructions to the cron deamon of the general form 

"run this command at this time on this date"
Cron examines entries once every minute

Time mentioned here

 field        allowed values
 -----               
  --------------
 minutes               0-59
 hour                  0-23
 day of month     1-31
 month               1-12 (or names, see below)
 day of week      0-7 (0 or 7 is Sun, or use names)

     A field may be an asterisk (*), which always stands for 'first-last’.

       Ranges  of  numbers  are allowed.  Ranges are two numbers separated with a
       hyphen.  The specified range is  inclusive.   For  example,  8-11  for  an
       ‘‘hours’’ entry specifies execution at hours 8, 9, 10 and 11.

       Lists  are  allowed.   A list is a set of numbers (or ranges) separated by
       commas.  Examples: ‘‘1,2,5,9’’, ‘‘0-4,8-12’’.


Ex- Cron job

       # use /bin/sh to run commands, no matter what /etc/passwd says
       SHELL=/bin/sh
       # mail any output to ‘paul’, no matter whose crontab this is
       MAILTO=paul
       #
       # run five minutes after midnight, every day
       5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
       # run at 2:15pm on the first of every month -- output mailed to hanif
       15 14 1 * *     $HOME/bin/monthly
       # run at 10 pm on weekdays, annoy Joe
       0 22 * * 1-5   mail -s "It’s 10pm" joe%Joe,%%Where are your kids?%
       23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
       5 4 * * sun     echo "run at 5 after 4 every Monday"


To set a crontab for an user
#crontab -e -u <user name>
ex:crontab -e -u hanif

To remove a crontab for an user
#crontab -r -u <user name>
ex:crontab -r -u hanif