Question

I know you can use

set foldcolumn=1

to enable fold column

but is there a way to automatic turn it on only when there are folds exist in the file?

Answer 1

Most probably you can create a function to check if the file has any folds, like:

function HasFoldedLine() 
    let lnum=1 
    while lnum <= line("$") 
        if (foldclosed(lnum) > -1) 
            return 1 
        endif 
        let lnum+=1 
    endwhile 
    return 0 
 endfu 

Now you can use it with some autocommand, e.g.:

au CursorHold * if HasFoldedLine() == 1 | set fdc=1 | else |set fdc=0 | endif 

HTH