Shell Programming and Scripting

Hi.. I am a beginner in the shell programming. And I have a homework about it. I will be very happy if someone can help me.
those are the questions:
1- Write a shell script which finds the sum of the numbers in the file "numbers.txt"

2-Write a shell script which changes the extensions of files in a directory.(using basename)

3- Write a shell script which checks whether a given file exist. If the file exist If the file exists add the extension ".bak" at the end of the file name.

Pls read the RULES first. No homework allowed here...

ok.. sorry.

But i need some help, where can i find any help for those scripts. I will write myself.

Try using google to look for some tutorials, there are quite a few out there

Use Google. A lot. You can also use some of the suggestions that are part of the problem statements that you have. For instance, the second question asks you to use basename. So use it. Check the man page if you don't know how.
The third question wants you to check for file existence in unix. So search on google for "check file existence in unix" or something similar. You will definitely get answers.
And for the first one, you can use the expr command. Check the man page for that too.