You can find full documentation on GNU Bash on their site.
If you want to understand the basics in a few minutes check out: Learn X in Y Minutes: Bash.
One of the first things that confused me about Bash was its operators. One can use:
-ne= not equal
-lt= less than
-le= less than or equal
-gt= greater than
-ge= greater than or equal
When a bash command is entered it can contain parameters and one may want to do some conditional logic based on the number of parameters. One can get the number of parameters passed using