Ensure Homebrew is installed and up to date.

ensure_homebrew_exists.sh

#!/usr/bin/env bash

# * Ensure Homebrew is installed and up to date.
# ? Reference URL: https://brew.sh/
function ensure_homebrew_exists() {
  echo "Ensuring Homebrew is installed and up to date."

   # * $() is for command substitution, commands don't _return_ values, they _capture_ them
   # ? Reference URL: https://stackoverflow.com/a/12137501/890814

   # * [[ ]] is the newer _test command_ for evaluations, is more literal
   # ? Reference URL: http://mywiki.wooledge.org/BashFAQ/031

   # * the double [[ ]], and $() is important
   # = `command -v brew` will output nothing if Homebrew is not installed

   # * Install Homebrew, if required.
   if [[ $(command -v brew) == "" ]]; then
     echo " => Installing Homebrew.. "
     ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

   # * Update Homebrew, if it exists.
   else
     echo " ✓ Homebrew is installed.  Updating.."
     brew update
   fi
}

ensure_homebrew_exists