Note: the original post is below for posterity, but installing ROOT has been much simpler for a while now, whether you’re on El Capitan or beyond.

There are only two steps:

1. Install Homebrew.
2. Run this command:

$ brew install root

Boom. Just take care to read the information printed at the end. You can review it later if you missed it:

$ brew info root
==> Caveats
Because ROOT depends on several installation-dependent
environment variables to function properly, you should
add the following commands to your shell initialization
script (.bashrc/.profile/etc.), or call them directly
before using ROOT.

For bash users:
  . /usr/local/bin/thisroot.sh
For zsh users:
  pushd /usr/local >/dev/null; . bin/thisroot.sh; popd >/dev/null
For csh/tcsh users:
  source /usr/local/bin/thisroot.csh
For fish users:
  . /usr/local/bin/thisroot.fish

I recommend installing ROOT via Anaconda as an alternative, which often integrates more nicely in to the reproducible analysis paradigm.

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Original post

I’ve written some lengthy guides on installing ROOT, firstly for OS X 10.8 Mountain Lion and then for OS X 10.9 Mavericks.

They were long because the install process with CMake could be convoluted, requiring several dependencies that are occasionally non-obvious. These days, though, things are extremely simple.

Homebrew

Homebrew is a software management program for OS X. A large group of volunteers maintains a set of so-called formula, recipes that tell your Mac how to build pieces of software.

If you don’t already have Homebrew installed, I highly recommend it. It makes it extremely easy to install software and to keep it up-to-date. To install CMake, for example, you would do:

$ brew install cmake

In addition to the standard repository of formula, which includes formula for programs like CMake and git, there’s a repository dedicated to programs used for science, called Homebrew Science. This repository contains formula for ROOT 5 and ROOT 6.

Installation

By default, brew only knows about the standard repository of formula, so first add the Homebrew Science collection.

$ brew tap homebrew/science

Now we can install ROOT. We’ll go with ROOT 6 here, the latest version.

$ brew install root6

That was easy!

You’ll notice that Homebrew gives you some instructions after you’ve run the installation. You can look at these instructions whenever you like.

$ brew info root6

This tells you to add a line to your shell configuration. For the Bash shell, you can add these lines to the .bashrc file in your home directory:

. $(brew --prefix root6)/libexec/thisroot.sh

Open a new shell and try it out by running root.