Installing Ruby with Homebrew

Add this line to your .profile (or .bash_profile, .bashrc, .zshrc, etc):

export PATH=/usr/local/opt/ruby/bin:$PATH

---

This is an up-to-date version of Kyle's answer. As of May 2014, brew info ruby prints:

By default, gem installed executables will be placed into:

  /usr/local/opt/ruby/bin

You may want to add this to your PATH. After upgrades, you can run

  gem pristine --all --only-executables

...to restore binstubs for installed gems.

Install ruby:

brew install ruby

I'd recommend setting $PATH, $GEM_PATH and $GEM_HOME. For latest Ruby it's:

export PATH=/usr/local/opt/ruby/bin:$PATH
export GEM_HOME=/usr/local/opt/ruby/lib/ruby/gems/2.6.0
export GEM_PATH=/usr/local/opt/ruby/lib/ruby/gems/2.6.0

Put them in something like ~/.bash_profile.

And then to verify:

type -a ruby
> ruby is /usr/local/opt/ruby/bin/ruby
> ...

ruby -v
> ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-darwin18]

gem list
> *** LOCAL GEMS ***
> 
> did_you_mean (1.3.0)
> minitest (5.11.3)
> ...

in ~/.bash_profile add the following line

export PATH=/usr/local/Cellar/ruby/1.9.3-p194/bin:$PATH

When you're done, close your terminal and re-open it. You should be fine.

Alternatively, you can execute the follwing in each open shell instead of closing/re-opening:

source ~/.bash_profile

Note: I highly recommend installing ruby via rvm or rbenv so you can manage multiple ruby versions and use gemsets.