AMIT SINGH

The crontab command is used to schedule jobs to be run in the future, usually on some regular schedule (such as every week).  The command is run with one of three command line arguments:

  crontab -l   View crontab file, if any
crontab -r   Remove crontab file, if any
crontab -e   Edit (or create) user's crontab file (starts the editor automatically)
crontab file   Replace existing crontab file (if any) with file

Field Descriptions:

minute  hour  dayOfMonth  month  dayOfWeek  command

where:
minute values range from 0 to 59,
hour values range from 0 to 23,
dayOfMonth values range from 1 to 31,
month values range from 1 to 12,
dayOfWeek values range from 0 to 6, with 0 meaning Sunday

Field Values:
  NUM A single value
  NUM-NUM A range of values
  NUM,NUM-NUM,... A comma separated list of values or ranges (remember no spaces after commas!)
  * wildcard, meaning match all possible values
(Note: Don't use a wildcard for the minute field, and rarely for the hour!) 

Examples:
# Example 1:  0,30 8-17 * * 1-5 cmd

Ans:- Run cmd on the half-hour from 8:00 AM to 5:30 PM, Monday thru Friday

Access Control:
This is controlled by a pair of files called cron.allow and cron.deny.  The location and even the exact use of these files vary from system to system.  For Linux these file exists in /etc.  On Linux:

•If cron.allow exists then only users listed in this file can use the crontab command.  Note if this file exists then any cron.deny file is ignored.
•If only cron.deny exists, then any user except those listed in this file can use the crontab command.
•If neither file exists then either the user root only can use the crontab command, or everyone can use it.  (This depends on the version of Unix or Linux you have.)