Check for Ruby Gem availability

In Ruby 1.9.3 only there is also:

Gem.available?('somegem')

You can use regex expressions too. Handy if I want to allow 'rcov' and GitHub variants like 'relevance-rcov':

Gem.available?(/-?rcov$/)

Since Gem.available? is deprecated (argh!), you have to rescue again (double aaargh). Yes, find_by_name throws an exception if the gem is not found. So to be backwards-compatible with older rubygems, the common solution seems to be :

def gem_available?(name)
   Gem::Specification.find_by_name(name)
rescue Gem::LoadError
   false
rescue
   Gem.available?(name)
end

Note that the new method allows you to pass a specific version to see if that's loaded:

Gem::Specification.find_by_name('rails', '3.0.4')

Looking at the Gem API documentation, using Gem::Specification::find_all_by_name to test for gem availability seems reasonable.

if Gem::Specification::find_all_by_name('gemname').any?
  do stuff
end

find_all_by_name always returns an array (of Specification objects), as opposed to find_by_name which raises an exception if no match is found.

IMHO the best way is to try to load/require the GEM and rescue the Exception, as Ray has already shown. It's safe to rescue the LoadError exception because it's not raised by the GEM itself but it's the standard behavior of the require command.

You can also use the gem command instead.

begin
  gem "somegem"
  # with requirements
  gem "somegem", ">=2.0"
rescue Gem::LoadError
  # not installed
end

The gem command has the same behavior of the require command, with some slight differences. AFAIK, it still tries to autoload the main GEM file.

Digging into the rubygems.rb file (line 310) I found the following execution

matches = Gem.source_index.find_name(gem.name, gem.version_requirements)
report_activate_error(gem) if matches.empty?

It can provide you some hints about how to make a dirty check without actually loading the library.