Use bash from $SHELL when it makes sense

[roberth]

This fixes file completion in cases where a virtual environment with a non-interactive bash is loaded with direnv, nix-shell or similar.

It solves the error:

$ mycommand --file <TAB>bash: compgen: command not found
bash: compgen: command not found
bash: compgen: command not found

[HuwCampbell]

Hi thanks for contributing to optparse!

I'll be honest, I'm very conflicted on this, though happy to discuss further.

What you've shown in your terminal printout is awful! We just regurgitate stderr of the base process into the user's shell when really, they don't want that. What I'm thinking is changing the bashCompleter function to something like this:

bashCompleter :: String -> Completer
bashCompleter action = Completer $ \word -> do
  let cmd = unwords ["compgen", "-A", action, "--", requote word]
  (ec, _, result) <-
    readProcessWithExitCode "bash" ["-c", cmd] ""

  return $
    if ec == ExitSuccess then
      lines result
    else
      []

Here we get rid of a exception handler and won't spew error messages to their terminal when they hit tab. Your solution won't fix that in the general case, but could reduce it for some.

I don't know, is that acceptable?

[roberth]

A change like that will make it practically impossible to troubleshoot, unless someone is dead set on solving a minor inconvenience.