Add option to not install existing versions

[kmwhite]

The -f option allows for forcing installation of versions that already
exist in the environment. If that option isn't used, then an interactive
prompt is created which causes problems for non-interactive execution of
the command. One could use -f for non-interactive executions, but it
causes a significant slowdown in runtime if you don't want to reinstall
that version.

This change allows for a -n option to not install versions that already
exist, while still exiting with a happy status code (for cases where
scripts may be kicking off the rbenv install command where set -e has
been run).

This command stemmed from using rbenv in our CI setup. We wanted to install the ruby version, of each app but each run failed if we already had the ruby version installed. We wound up with a lengthy if-else stanza to try installing the version and ignore errors if they matched the pattern for an already installed version or properly breaking the build if there were installation errors. I realized this was a simpler answer.

If there is another, better, existing solution, I'm all ears. I couldn't figure one out. :\

[kmwhite]

Additionally, I'm open to changing the option's name. I used -n and NOFORCE to indicate the opposite of the existing -f option, but it's an odd name. Maybe -s and SKIP_INSTALLED ?

[mislav]

The following doesn't cut it?

RBENV_VERSION="$version" rbenv which ruby &>/dev/null || rbenv install $version

I would like to avoid adding the option if possible. It just feels awkward.

[sstephenson]

I've wanted this option before. +1

Can we come up with a better name than NOFORCE?

[kmwhite]

@sstephenson Does SKIP_INSTALLED work for you?

[mislav]

How about SKIP_EXISTING? -e/--skip-existing externally

[kmwhite]

@mislav Sounds better. I'd suggest -s for consistency with matching the first character of the long form name like the other options do, but I'm not married to either.

[sstephenson]

-s/--skip-existing sounds great 👍

[mislav]

Do it!

[kmwhite]

updated!

[mislav]

@sstephenson Do we want to run post-install hooks?

[sstephenson]

No need to run post-install hooks if the version has already been installed.

[mislav]

Thanks @kmwhite!