TL;DR: cd /opt/boxen/rbenv/plugins/ruby-build && git pull origin master
A while ago, I set up my development environment on my two machines using GitHub's Boxen. To be honest, I got it going and moved on. I have not been updating it. Then, I recently started a new Ruby project, and I wanted the latest version of Ruby. I went into rbenv, and it had no recent versions of Ruby - none since I set up Boxen.
How do I update the list of Ruby versions available in my Boxen environment?
I could update Boxen, but that's more than I wanted to bite off. My first guess was to run brew update and brew upgrade rbenv ruby-build, but that failed because rbenv is part of the bootstrap of Boxen and not installed by Boxen's version of Homebrew.
So, I thought I'd need to update rbenv directly. I figured it was installed as a git repo. The question was: where? which rbenv says it's a shell function. Looking at that code, it un-aliases itself to run.
which -a shows that rbenv is in /opt/boxen/rbenv/bin/rbenv. From there I found the ruby-build plugin, which leads to the solution:
git pull origin master
It's not like it's rocket-surgery, but it was a curious puzzle to sort out. Soon I should update my Boxen environment, but that's a story for another day.