Technically . is an expression language with a very simple syntax. It is case sensitive as are most UNIX based packages, so A and a are different symbols and would refer to different variables.
Elementary commands consist of either expressions or assignments. If an expression is given as a command, it is evaluated, printed, and the value is lost. An assignment also evaluates an expression and passes the value to a variable but the result is not automatically printed.
Commands are separated either by a semi-colon, ;, or by a newline. If a command is not complete at the end of a line, . will give a different prompt, for example
+
on second and subsequent lines and continue to read input until the command is syntactically complete. This prompt may be changed by the user. In these notes we will generally omit the continuation prompt and indicate continuation by simple indenting.