brew/Library/Contributions/cmd/brew-switch.rb

require 'formula'
require 'keg'

if ARGV.named.length != 2
  onoe "Usage: brew switch <formula> <version>"
  exit 1
end

name = ARGV.shift
version = ARGV.shift

# Does this formula have any versions?
f = Formula.factory(name.downcase)
cellar = f.prefix.parent
unless cellar.directory?
  onoe "#{name} not found in the Cellar."
  exit 2
end

# Does the target version exist?
unless (cellar+version).directory?
  onoe "#{name} does not have a version \"#{version}\" in the Cellar."

  versions = cellar.subdirs.map { |d| Keg.new(d).version }
  puts "Versions available: #{versions.join(', ')}"

  exit 3
end

# Unlink all existing versions
cellar.subdirs.each do |v|
  keg = Keg.new(v)
  puts "Cleaning #{keg}"
  keg.unlink
end

# Link new version, if not keg-only
if f.keg_only?
  keg = Keg.new(cellar+version)
  keg.optlink
  puts "Opt link created for #{keg}"
else
  keg = Keg.new(cellar+version)
  puts "#{keg.link} links created for #{keg}"
end