How do I add conditional rubygem requirements to a gem specification?

Question

Is it possible to add a gem dependency only if the person is using a certain version of ruby?

Background: I'm working on a fork of a project that uses Test::Unit::Autorunner and the like. They are part of the standard library in ruby 1.8, but aren't part of the standard library in 1.9.1, and is instead in the "test-unit" gem. I want to add a dependency that says that if someone's using ruby 1.9.1 or later, install the "test-unit" gem, but if they're using 1.8 or earlier, they don't need to install anything.

Answer 1

hay ... i'm kind of a ruby newbie ... if this is the best way to do it.

any way ... i wish i can do that using only Ruby .... though u can use your operating system shell to do that, by using this in your program installer, just execute (works for Linux based operating systems):

$ruby --version

(u can execute that from a ruby installer file, just like: ruby --version) and put a possibility according to output, if it's 1.9.1 add an extra line to execute:

$ sudo gem install gem_name

else, just leave it be.

Answer 2

If you look at the gemspec documentation for add_dependency, there isn't an option for a ruby version. Perhaps you could use the post_install_message attribute to tell the user to install the gem if they're using ruby 1.9.

Answer 3

You can't. You need to build two gems, one with

spec.required_ruby_version = '~> 1.8.6'

and one with

spec.required_ruby_version = '~> 1.9.1'
spec.add_dependency 'test-unit', '~> 2.0.5'

Answer 4

Gemspecs are just ruby files anyway, so you can execute any ruby code inside them, so:

spec.add_dependency = 'test-unit', '>= 2.0' if RUBY_VERSION =~ '1.9'

EDIT: Specs run only on the builders machine.

Answer 5

checkout a somewhat tutorial here: http://wiki.github.com/rdp/ruby%5Ftutorials%5Fcore/gem

it shows how to install different versions of dependencies depending on what version of ruby the installee is using.

(short answer--it ain't as easy as it should be)