Install gem on demand

Question

I would like to install a gem (JSON) on the client side, but only if hasn't been installed already (some 1.9 Ruby distros have JSON bundled).

I couldn't find a clue on how to do that from gem help install. And running gem install json on a Windows system with Ruby 1.9 installed (with JSON bundled) results in

    ERROR:  Error installing json:
    The 'json' native gem requires installed build tools.

-- it tries to install it ignoring the fact that the gem is already there.

And I can't do bash tricks like grepping gem list output because the client might be Windows.

So what's the multiplatform way of installing a gem only if it's not present in the system already?

Answer 1

This may work...

begin
  require "json"
rescue LoadError
  system("gem install json")
end

If you don't want to require "json", you can remove it from $LOAD_PATH.

Or, put as a one liner:

ruby -e 'begin; require "some_gem"; rescue LoadError; system "gem install some_gem"; end'