Understanding the line() built-in function

Question

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 mark

I tried to invoke it as

:echo :call line('$')

but this printed the following error:

E121: Undefined variable: :call
E15: Invalid expression: :call line('$')

What does '.' and '$' mean here and how to invoke it correctly?

Answer 1

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('.')
endfunction

If you would have something like that in your .vimrc, then you could use call like so: :call func()