Rubygems: Install if not already installed

One of the things that I’ve needed to do on several occasions is
create scripts that will update an Ruby installation to include all
the gems that a project requires. I first wrote a batch file with
several calls to “gem install”. It worked, but it was somewhat slow,
because it would reinstall all the gems. I really only wanted it to
install a specified gem if it weren’t installed.

I finally went ahead, however, and wrote the code to only install the
gems if necessary. Here it is.

module Gem

Return a list of the versions of the specified gem that are

installed.
def self.installed_versions gem_name
cache.search(gem_name).collect {|x| x.version.to_s}
end

Return the version of the provided gem file name.

@@gem_file_regex = /^(.)-([^-]).gem$/
def self.file_version gem_file
if @@gem_file_regex =~ gem_file
$2
else
nil
end
end

Return the library name of the gem file.

def self.file_gem_name gem_file
if @@gem_file_regex =~ gem_file
$1
else
nil
end
end
end

gem_dir = File.dirname(FILE) + ‘/gems’

desc “Install any gems from the local gems directory that are not
already installed.”
task ‘update-gems’ do
Dir.chdir gem_dir do
Dir[’*.gem’].each do | gem_file |
version = Gem::file_version gem_file
name = Gem::file_gem_name gem_file
desc = “Gem #{name}, version #{version}”
if Gem::installed_versions(name).include? version
puts “#{desc} already installed.”
else
if system “gem.bat install --local #{name}”
puts “#{desc} installed.”
else
raise RuntimeError, “Unable to load #{desc}”
end
end
end
end
end