Best gem questions in April 2012

Install gem on demand

Asked on Wed, 04 Apr 2012 by Qwe ruby rubygems gem
6 votes

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?

Answered by Tomato

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'