Unpacking Homebrew

Feb 26, 2022

What is Homebrew?

A package manager for macOS. If you're familiar with linux, it's similar to using apt or yum to install packages. Essentially, it's an easy way to install tools and code libraries through the command line.

Understanding the beer analogy

Ok, I've worked with a package manager before, but what are all these weird terms like "formula", "keg", "rack", "tap"? It'll make much more sense to you if you understand beer culture. Here's a brief and oversimplified introduction:

Beer overview

When you brew your beer you'll start with a recipe (a.k.a. formula).
Once you've brewed your beer (according to the formula) you can put it into a keg.
You can store all these kegs on a rack.
You can keep those racks in a cellar.

Tying it back to package managers

Homebrew overview

All official Homebrew formulas are hosted in this Github repository. When you install a package, you're simply running one of these Ruby scripts.
The default location a package is installed to is /usr/local/Cellar. The package's executable is then sym-linked into /usr/local/bin
Additional Homebrew concepts (e.g. Casks, Taps) found here.

Getting started with Homebrew

# Install Homebrew
$ /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
==> This script will install:
/usr/local/bin/brew
/usr/local/share/doc/homebrew
/usr/local/share/man/man1/brew.1
/usr/local/share/zsh/site-functions/_brew
/usr/local/etc/bash_completion.d/brew
/usr/local/Homebrew
==> The following new directories will be created:
/usr/local/sbin
==> The Xcode Command Line Tools will be installed.

# Install a package
$ brew install jq

# Show jq executable sym-linked into Cellar
$ ls -l /usr/local/bin
lrwxr-xr-x  1 thomasnguyen  admin         jq -> ../Cellar/jq/1.6/bin/jq

A few interesting things to notice:

everything Homebrew-related is installed under /usr/local
all executables installed by Homebrew are under /usr/local/Cellar and /usr/local/Caskroom
these executables are sym-linked into /usr/local/bin so that they can be immediately called from your command line.

Creating our own formula

As we've now learned, brew install is simply Homebrew executing a formula (ruby script) which defines how to install the package.

Let's try writing our own formula!

Identify the .tar.gz executable you want to be able to install using brew. I've decided to create a formula for wget (https://www.gnu.org/software/wget/)

Use brew create to generate a ruby script

brew create --set-name my-wget https://ftp.gnu.org/gnu/wget/wget-1.15.tar.gz

Modify the Ruby script to include instructions on how to install

class MyWget < Formula
  homepage "https://www.gnu.org/software/wget/"
  url "https://ftp.gnu.org/gnu/wget/wget-1.15.tar.gz"
  sha256 "52126be8cf1bddd7536886e74c053ad7d0ed2aa89b4b630f76785bac21695fcd"

  depends_on "gnutls"

  def install
    system "./configure", "--prefix=#{prefix}"
    system "make", "install"
  end
end

Run brew install on the formula we just created. Notice all the dependencies that need to be installed for my-wget and gnutls
```
brew install --build-from-source my-wget
```

Test and check. Notice that even though our formula is called my-wget, the executable we installed from gnu is still called wget.

$ ls /usr/local/Cellar/my-wget/1.15/bin
wget

$ ls -l /usr/local/bin/wget
lrwxr-xr-x  1 thomasnguyen  admin  /usr/local/bin/wget -> ../Cellar/my-wget/1.15/bin/wget

$ wget google.com

Other tidbits

you can use brew edit to locally change an existing formula

References

#linux #tech