I learned to use the Terminal years ago when I first started using a personal computer - 1981. I taught myself, in part, and learned more from friends. I also learned Unix where you have to know the terminal. By the time I came to Mac OS, I had forgotten a lot of what I learned, so it's been slow catch-up but I had some background. I also spend time on Google looking for information about Linux and Macs, so I come across information such as the above command line. There are also several pretty good books on using the OS X Terminal. You will find them at Amazon Books. You also may find them through the Books app on your computer.
You can learn well from tutorials or books but unless you use the stuff everyday, it will be gone in no time. It happens to me because I so rarely use the Terminal.