How to enable crontab job

Setting Up Cron Job Using crontab:
Step 1: Open a Terminal Window (Command Line) in Linux.
Step 2: The following is a list of cron directories:
• /etc/cron.hourly
• /etc/cron.daily
• /etc/cron.weekly
• /etc/cron.monthly
Copy your shell script ‘script.sh` or ‘script’ into one of the directories above.
If you need to run the script hourly, place your script file in the “cron.hourly”
folder. For daily, place it inside the “cron.daily” and so forth.
Step 3: Give the shell script the correct permission. For example, if script is called
“script.sh”, set permission as follows:
cd /etc/cron.daily/
chmod 755 script.sh
Step 4: Add new cron job to crontab:
crontab –e
This opens vi editor for you. Create the cron command using the following syntax:
1. The number of minutes after the hour (0 to 59)
2. The hour in military time (24 hour) format (0 to 23)
3. The day of the month (1 to 31)
4. The month (1 to 12)
5. The day of the week(0 or 7 is Sun, or use name)
6. The command to run
More graphically they would look like this:
* * * * * Command to be executed
- - - - -
| | | | |
| | | | +----- Day of week (0-7)
| | | +------- Month (1 - 12)
| | +--------- Day of month (1 - 31)
| +----------- Hour (0 - 23)
+------------- Min (0 - 59)
An example command would be “0 0 * * * /etc/cron.daily/script.sh”. This
would mean that the shell script will exactly execute at midnight every
night.
To save the changes to the crontab that you just made, hit ESC key, and
then type :w followed by :q to exit.
To list existing cron jobs:
 crontab –l
To remove an existing cron job:
• Enter: crontab –e
• Delete the line that contains your cron job
• Hit ESC > :w > :q