Well last issue first, you simply use [code][/code] tags around your code and it will appear like the other entries.
As for calling from another script in which you only do a cd, you can either add that to this script or just place the path within the script.
Yes all scripts demonstrated understand that you will be doing this everyday and on the 15th day the process should go back to day 1 file and overwrite.
Errors I can see is you have placed your variables in what look like back ticks. At best they could be double quotes but even they are not required.
The other main error is that your final line will always output as follows:
svnadmin dump /var/www/svn/main/ > /svndump/main1.svn_dump
ie it will always be number 1. This means the order is that the number 1 file will always be the most recent.
Let me know if I did not answer all your questions?