When running `rbenv init`, typically during rbenv setup, users expected
their shell environment to be modified permanently. Instead, what the
command would do is print the instructions to the user and expect them
to edit their shell initialization files accordingly. This proved to
be unintuitive.
Now, running `rbenv init <shells>...` will modify the shell initialization
files of the following shells:
- bash: `~/.bash_profile` or `~/.bashrc` if the latter exists but the former does not
- zsh: `~/.zprofile` or `~/.zshrc` if the latter exists and mentions "rbenv"
- fish: `~/.config/fish/config.fish`
If no shells were specified on the command line, rbenv will try to detect
the current shell.
It should be safe to run `rbenv init` multiple times, as the command will
avoid modifying any shell startup file that already mentions "rbenv init".
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