How can I edit files with a script? - Writing a "after install" script for my laptop

So I have been playing with BodHi for about a month now and really getting into it, but have yet to commit to it as a productive environment. I am still reinstalling. I have made a script to install all the software I have found I like and also set a lot of that software up... but one of the things I have to do myself is edit text files and stuff. Is there a way to edit text files with scripts? maybe searching and replacing a line, or adding to the bottom of the file or something? Or even making a new text file and adding lines to that?

Thanks!