I'm reading vim help about the line function and here is an example I found::let failed = append(line('$'), # THE END)In my vimrc I also found line('.'). The documentation says:line( {expr})           Number  line nr of cursor, last line or markI tried to invoke it as:echo :call line('$')but this printed the following error:E121: Undefined variable: :callE15: Invalid expression: :call line('$')What does '.' and '$' mean here and how to invoke it correctly?
Understanding the line() built-in function
vimscript
In :help line all possible values of {expr} are explained:.       the cursor position                                 $       the last line in the current buffer                 'x      position of mark x (if the mark is not set, 0 is returned)                                           w0      first line visible in current window                w$      last line visible in current window                 v       In Visual mode: the start of the Visual area (the           cursor is the end).  When not in Visual mode                returns the cursor position.  Differs from '< in            that it's updated right away.And to invoke that you just need::echo line('.')call is for running functions:function! func()    echo line('.')endfunctionIf you would have something like that in your .vimrc, then you could use call like so: :call func()