Tuesday 4 April 2017

Crontab Concepts and details with examples

                               Crontab Concepts and details:
Hey BDA-Mates,
As we know that CRON is a Linux program and it is very helpful and useful for a programmer or DBA.

Crontab allows a task to be run automatically in background at mentioned date/time.
For example: 
We can schedule cron for backups, schedules update, synchronize files, Getting messages for completion tasks, getting alerts such as tablespace size details, blocking sessions, temp usage etc.

There are 5 entries in crontab.

For example:
1 2 3 4 5 /bin/ora-data.blogspot.com/scriptname.sh

Here:
1 : Minute (from 0 to 59)
2 : Hour (from 0 to 23)
3 : Date of the month ( from 1 to 31)
4 : Month ( from 1 to 12)
5 : Day of week (from 0 to 7, here 0 and 7 are Sunday)

If we want to check which crontabs are currently running on our server? We have to login to our server and run the below command:

$hostname;id
ora-data.blogspot;ora-data

$crontab -l
1 2 3 4 5 /bin/ora-data.blogspot.com/scriptname.sh

If you want to make changes or edit in crontab, follow below command:

$crontab -e
1 2 3 4 5 /bin/ora-data.blogspot.com/scriptname.sh

From above command, we will get all the crontab entries in which we have to find out the script name and edit or change the date and time as per your requirements, Save and then exit.

Note: Always be careful during edit the crontab on PROD.

Please find the below examples for references:

1. Suppose we want to execute any scripts every minute then we can use below commands:

* * * * * /bin/ora-data.blogspot.com/scriptname.sh

Here, * asterisk or start means every. So, this above command will run for every minute, every hour, and every date of month, every month and every day of week.

2. Example for execute on Friday at 7 AM, each date and every month.

0 7 * * 5 /bin/ora-data.blogspot.com/ora-data.sh

3. Suppose, we want to execute our scripts only on Friday and Saturday at 5 AM.

0 5 * * 5-6 /bin/ora-data.blogspot.com/ora-data.sh

4. Suppose, we want to execute at every 10 min.

0,10,20,30,40,50 * * * * /bin/ora-data.blogspot.com/ora-data.sh

5. Suppose, we want to check a particular running crontab.

$crontab -l |grep crontabname

Some more useful links:

Hope this will be useful and helpful. For any suggestion please write to us either in comment box or @Contact us: https://ora-data.blogspot.in/

Regards,
ora-data Team

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Thank you for your comments and suggestions. It's good to talk you.