How do I edit an existing buffer in a new tab in vim?

Suppose I have started vim like this:

vim foo bar

Now I decide that I want each of those files in its own tab. Is there a way to do that without exiting vim and adding the -p option to my command line?

Answer

When you start vim like that, you don't get a vim client, the text editor is using the terminal or cmd prompt - the two files are in two different buffers. Use :ls to list the buffers:

:ls
  1 %a   "foo"                 line 6
  2      "bar"             line 0

The %a is the active buffer. You can use :b2 to switch to buffer 2 or use :bn to cycle to the next or :bp for previous. I prefer (CTRL-W v) to split windows vertically, rather than (CTRL-W s), which splits horizontally.

If you have 2 files loaded & no tabs (yet), you can, :tabnew and in the new tab type :b2

If you want to always have buffers loaded into their own tabs, check out this article.