How do I differentiate between vim and vimdiff from within vimrc?

Question

I have a function in vim which will open all buffers as tabs. However after I add this function, vimdiff too opens the files to be compared as different tabs and hence doesn't show the differences. Is there a way where I can find out if its vim or vimdiff which is invoking my vimrc file?

function! OpenTabs()
    let bf=len(filter(range(1, bufnr('$')), 'buflisted(v:val)'))
    if bf > 1
      exe "tab 5sball"
    endif
endfunction

Answer 1

:help diff covers this:

In your .vimrc file you could do something special when Vim was started in diff mode. You could use a construct like this:

   if &diff
      setup for diff mode
   else
      setup for non-diff mode
   endif

Answer 2

You can use the &diff option like in your vimrc:

if &diff
    echo "in diff mode"
else
    echo "not in diff mode"
endif

See :h 'diff'