`rbenv shell -` allows you to switch to the previously activated ruby
version. Similar to `cd -` or `git checkout -`.
This tries to implement `rbenv shell -` as proposed in #854. However,
adding support seemed to break the "shell change version" test. I'm not
very good at Bash programming, can someone tell me what is wrong with
what I'm doing? I'd like to add a bit more functionality to this, but
I'm really just cargo cult programming Bash.
Thank you!
fix tests
`$SHELL` variable is a terrible way of detecting the current shell
because it's not even supposed to reflect the current shell; it's meant
for keeping the value of the default shell for programs to start.
If an explicit `<shell>` argument wasn't passed to `rbenv init`, it
tries to detect the shell by getting the name of its parent process. If
this fails, it falls back on the value of `$SHELL` as before.
Furthermore, `rbenv init` will set the RBENV_SHELL variable in the
current shell to the value of the detected shell so that `sh-shell` and
`sh-rehash` commands don't have to repeat the detection.
