Your biggest hurdle will probably be coming up with the method of backing up: straight copy, tarball, cpio, or something like the Amanda program. The shell script itself will be easy. A good guide commonly referenced is the Advanced Bash Scripting Guide
. That link provides access to a number of guides, and offers the Bash guide in a number of different formats you can view online or download if you like. Here's a link to the HTML
version if you want to jump right in.
You won't find a whole lot of similarity between shell scripting and C++, but the basic concepts of programming will help you hit the ground running.