gistfile1.vim

function! s:session_write(spath)
  let ssop = &sessionoptions
  try
    set sessionoptions-=options
    execute 'mksession!' a:spath
  catch
    execute 'echoerr' string(v:exception)
  finally
    let &sessionoptions = ssop
  endtry

  if exists('g:startify_session_savevars') || exists('g:startify_session_savecmds')
    " put these commands into the session file
    let cmds = get(g:, 'startify_session_savecmds', [])
    " put these existing variables into the session file
    let vars = map(filter(get(g:, 'startify_session_savevars', []), 'exists(v:val)'),
          \ '"let ". v:val ." = ". strtrans(string(eval(v:val)))')

    let sessionlines = readfile(a:spath)
    call writefile(sessionlines[:-4] + vars + cmds + sessionlines[-3:], a:spath)
  endif
endfunction