Command Line Courses?

I’m trying to get in to Linux system administration and was enrolled in the free introduction to Linux course by the Linux Foundation. It turned out to be too complicated for me in regard to the command line. I found it jumped from subject to subject too quickly. I’m wondering what you think is the best way to learn the command line/shell? Any books, free or paid courses or websites you would suggest? A specific emphasis on system administration would be a bonus!