How is a job run periodically in unix system?

If you want a job to run only once in the future use the 'at' command.

For more than a one-time execution, use the 'cron' facility with the 'crontab' command to create the schedule. Cron executions are specified one per line, with the following fields:

field allowed values
----- --------------
minute 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)

The use of an asterisk '*' indicates all possible values. Ranges may be used as well.

An example would be:

15,30 10-12 * 12 * myjob.ksh

Which would run every 15 and 30 minutes past the hour, between 10 a.m. and noon, any day of the month in December on any day of the week, with the job called 'myjob.ksh'