Shell Scripting Newbie Help

I'm taking an online course, intro to unix, and have another assignment to complete before the semester ends. I understand most of it, but I have hit a brick wall on this one. Basically I have two make two scripts, with the second having an extra parted added which I cannot figure out.

Here is my first script:

I need to edit this so that it ONLY updates the file IF something was changed i.e. the two files are different. I have a decent understanding of how to use if statements and such in scripting and I've used commands such as "diff" straight in the shell, but I'm not sure how to tell the script if there is a difference or not. I don't necessarily need an answer given to me if that is too much to ask, but any pointers to guide me in the right direction would be great. Thanks in advance for any help!

I too am confused. You seem to have three parameters:
1 - a search string
2 - a replacement string
3 - a file to search/replace within

You mention two files, but you only reference one in the script, in effect. You mv the file - in essence doign a rename, thus you have just the one file. So, a bit hard to see if 'they' are different.

The only thing that would make sense would be if what you are after doing is the sed command (and maybe also the mv?) ONLY if the search and replacement strings are different.